Geology of the Binneringie Dyke, Western Australia

Aut?s~itze

Geology of 'the Binneringie Dyke, Western Australia By G. J. H. McCALL, Melbom'ne, and R. PEE~S, Perth *)

With 51 figures and 1 table

Zusammen[assnng

Es wird die Geologie des Binneringie Dykes besehrieben, der eine der grSBten Dyke-Strnkturen der Welt darsteUt. Er erstreekt sieh fiber 820 km L~inge yon E naeh W und hat eine maximale Breite bei Cowan Hill von 8,2 kin. Der Dyke zeiehnet sieh dureh stm'k gefrittete R~nder aus, dagegen sind nur sehr geringe Kontakterseheinungen zu den Naehbarzonen lain zu beobaehten. Die t/andzonen sind rnit Magnesium und Kalzinm angereiehert infolge einer bevorzugten f~ihe- ren Bildung yon Eisemnagnesiummineralien. Die randlichen Cesteine bestehen aus glomeroporphyritisehen und nicht-porphyritisehen Bronziten, Olivinbronziten und Olivin-Bronzit-Quarz-Gabbros. Die Bronzite hal3en AufliSsungsstrukturen, die eine Umwandlung von Magnesium-Pigeoniten widerspiegeln: Die Gesteine gegen das Zentrum des Dykes haben zunehmend einen st~rkeren Ferro-Charak- ter, Auglt-Pigeonit-Gabbros nehmen den Platz von Bronzitgabbros ein (die Ferro-Pigeonite sind davon unberiihrt).

Es gibt ldeinere intermediiire und saure Intmsionen entlang dem Dyke: Sie nehmen die Form yon unregelmiil3igen Grano-Gabbro-FIecken ein, in denen sieh besonders rote Residualgesteine entwiekelt haben und die einem grano- phyrisehen und devitrifizierten sauren Glas entspreehen. Sie k~innen anch die Form pegmatoider Aggregate haben, die mit viel Epidot, Amphibol, Chlorit, Sphen und Prehnit verbunden sind.

Der Groflteil des intermedi/iren und sauren Materials ist dagegen in Form von sdqarf begrenzten Dykes und Adern innerhalb der Gabbros zu linden, wobei diese K~Srper gegen den Gabbro bin gefrittet sind, aUerdings hie iiber die ge- fritteten Riinder des Hauptdykes in die Naehbargesteine eindringen. Sie seheinen eine sp/it magmatisehe Einffillnng von Schrnmpfungsrissen im Gabbro darzu- stellen.

Eine andere VergeseUsehaftung yon Dykes und Adern haben eine basaltisehe Zusammensetzung. Es sind dabei basaltisehe Dolerite mit vulkanitischer Stmk~r vertreten, gekennzeiehnet dureh idiomorphe Olivinelnsehliisse, traehytische Tex- fur, Quarzpalimpseste von ehalzedongefiillten Poren und devitrifiziertem Taehylit- g]as.

Der Dyke hat hiiufig eine magmatische Zonierung nahe den Iliindern, d.h. in einer Zone, die sieh ca. hundert Meter yon den Riindem aus erstreckt. Gebiinder- tes Material weehselt mit massiven Gabbrolagen in einer Weise, die an Skaer- gaard (Griinland) erirmert. Diese magmatisehe Sehiehtung zeigt nut wenigu Ab- weiehungen yon eLner bestiindigen vertikalen Anordmmg, die die Pdehtung des Hauptdykes wider, spiegelt. Darin sind viele rhythmisehe trod eryptisehe Lagemn- gen mit eingeschlossen. Die rhythmisehe Lagerung zeigt gew61mlieh eine Art yon ,,graded bedding" und yon KreuzsebAehtung. Die ,,Gradierung" in den rhyth-

*) Authors' addresses: G.J .H. MCCALL, D. Se., formerly Reader in Geology University of Western Australia, now Director of Exploration, Research and Exploration Management Pry. Ltd., 470 Collins Street, Melbourne. R. PE~s, B. Se., formerly of the University of Western Australia, now Assistant Petrologist, Geological Survey of Western Australia, Mineral House, Adelaide Terrace, Perth.

1174

G. J. H. McCALL and R, PEERS - - Geology of the Binneringie Dyke

misch zouierten Einheiten - - Plagioldase angereiehert gegen den Kontakt des Dykes - - ist vorwiegend in der Binneringie Homestead-Gegend zu finden, aber aueh in anderen Teilen des Dykes l~iBt sich diese ,,Gradierung" beobaehten. Es sind keine Spuren fiJr die Bildung yon Mineralaggregaten zu finden, und der zonierte und poikilitische Charakter der Bronzite sowie die vorwiegende Zonie- rung der Kalziumplagioklase 1N3t die Vermutung zu, dab Aggregatbildungen you fffih ausgeschiedenen Mineralphasen nirgends erhahen sind, da Konvektions- strSmungen in der groBen Magmakammer dies verhindert haben.

Im Tefl II werden ungew6hnliehe Lagerungsstrukturen am Sunday Soak, saure magmatische Brekzien am Cowan Hill und groBe granophyrische und felsitische Intmsionsk6rloer sfidlich von Salt Well beschrieben. Aul3erdem wird die Art des Eindringens des Dykes diskutiert und eine petrogenetische Deutung dieses kom- plexen Dykes gegeben. Die Ergebnisse von 80 Vollanalysen liegen vor sowie eine Diskussion fiber die Differentiationsprozesse. Die gro/3e Verschiedenheit der Ge- steine soll eine fraktionierte Differentiation widerspiegeln, und es ist unwahr- scheinlich, dab irgendwelche Durchrnisehungen stattgefunden haben. Die vertikale Zonierung dieser einmaligen Intrusion wird mit den Randzonen der Skaergaard- Intrusion in Gr6nland verglichen trod l~il3t uns das Ph/inomen der ,,eongelation cumulates" besser verstehcn, ein Phanomen, das wesentlich wichtiger fiir die Natur zonierter Intrusionen ist, als man bisher aunahm. Die randliche Magne- sium- und Kalzinmanreichemng wird auf Diffusionsprozesse zurtiekgefSilart.

Abstract

The geology of the Birmeringie Dyke, one of the largest true dyke occurrences in the world - - it extends for something like 200 miles (820 kin.) from east to west, and has a maximum width at Cowan Hill of two miles (3.2 km.) - - is described. The dyke is characterised by strongly chilled margins, but only meagre contact effects on the country rocks. The marginal zone is enriched in magnesium and calcium, clue to an overpreeipitation of early formed ferromagnesians. The marginal rocks are glomeroporphyritic and non-porphyritic bronzite, olivine-bronzite and olivine-hronzite-quartz gabbros, the bronzite having complex exsolution features reflecting inversion from magnesian pigeonite: the rocks towards the centre of the dyke have a progressively more ferroan character, augite-pigeouite gabhros taking the place of bronzite gabbros (the ferroan pigeonite being univerted). There are minor intermediate and acid phases throughout the length of the dyke: these take the form of irregular granogabbro segregation patches, in which there is an excessive development of residual "red rock", of granophyric and devitrified acid glass composition. They also take the form of pegmatoid clots, associated with much epidote, amphibole, chlorite, sphene and prehnJte. Much of the intermediate and acid material is, however, in the form of sharply bounded dykes and dyketets, within the gabbro: these bodies are chilled against the gabbro, never project through the chilled margin of the main dyke into the country rock and appear to represent infillings by late magmatic phases of shrinkage cracks in the gabhro. Another suite of internal dykes and dykelets has a basaltic character: dykes of basaltic dolerite of volcanic aspect are represented and these reveal idiomorphie olivine phenocrysts, trachytic textures, quartz palimpsests of chalcedony filled vesieles, and devitrified tachylite glass base material.

The dyke shows abundant magmatic layering near to its margins - - that is in a z o n e extending in several hundred feet from its margin. Banded material alternates with thicknesses of massive gabbro in a manner reminiscent of Skaer-

11'75

Aufs~itze

gaard. The magmatic layering shows only minor flexural departures from a consistently vertical attitude, assumed to reflect the attitude of the main dyke walls. There is much rhythmic layering involved and also cryptic layering. The rhythmic layers commonly show sfinulation of graded bedding, and there are also simulations of cross-bedding. The grading in the rhythmic units is consistent - - plagioelase towards the dyke wall - - in the Binneringie Homestead sections, but elsewhere along the dyke departures from this grading sense have been observed. There is no trace of cumulate texture, and the zoned and poikilitie nature of the bronzites, together with the predominantly zoned nature of the calcio plagioclase suggests that cumulate deposition of early formed mineral phases was nowhere involved, though convective stirring of the large magma chamber in all probability was.

In Part 2, unusual layering structures at Sunday Soak, acid breccias at Cowan Hill, and large internal granophyre and felsite bodies south of the Salt Well are described in some detail. The remainder of the text includes a discussion of the manner of emplacement and broader petrogenetic implications of this complex dyke. The results of thirty full chemical analyses are given, and the differentiation trends discussed. The wide variety of rock types represented are considered to reflect ffaetionation, and contamination is considered unlikely to have been of signifieance in their development. The vertical layering of this unique intrusion is compared with the marginal layered zone of the Skacrgaard Intrusion, Greenland, and it is suggested that it provides a better understanding of the phenomenon of "congelation cumulates", which may be a far more significant feature of layered intrusions than has hitherto been realised. The marginal magnesia and lime enrichment is attributed to a diffusion process, akin to adeumulus growth, operating on a congelation cumulus.

R6sum6

Le dyke de Binneringie, un des plus grands du monde, poss6de une longueur de 820 kin et une largeur de 8.2 kin (maximum). Ce eompte-rendu est divis6 en deux parties. La premi6re pattie embrasse ]'introduction g6n6rale, et la p6tro- graphic des roehes dans une localit6 typique proehe de Binneringie Station. Los marges du dyke sent vitrifi6es et enrichies de magn6sium et de calcium, paree que les min6raux ferromagn6siens se sent pr6eipit6s en exc6s. Le processus d'enridzissement est d6crit dans la deuxi6me pattie de ee compte-rendu. Des gabbros ~ norite sent expes6s aux marges, glom6roporphyritiques ou non-porphyritiques, mais los roches int6rieures sent des gabbros '5 angite-pigeonite, non- porphyritiques. Los marginaux gabbros a norite montrent des eomplexit6s d'exsolution dans los eristaux pyrox6niques: le min6ral pigeonite, de composition magn6sienne, est inverse ~ la bronzite: mais los pigeonites fen'cnses de l'int6rieur ne sent pas invers6es.

Les grand et los petits massifs de roches interm6diaires et acides, expos6s partout le long du dyke, faisant de petits dykes int6rieurs ou des massifs irr6gu- liers, sent du type ,< grano-gabbro >,, granophyre ou felsite. Flusieurs montrent un m6sostasis vitreux acide qu'on appelle ,< Red Rock >>. Plusieurs sent pegmatitiques. On laouve aussi de petits dykes int6rieurs de basalte ~ olivine, en partie vitreux et v6siculaire, comme los basaltes volcaniques. Los petits dykes int6rieurs sent enti6rement dans le grand dyke de Birmcringie, et n'ont pas d'extension au dehors du grand dyke. Le grand dyke montre un stratification rhythmique, ainsi qu'une structure comme <<graded bedding>> et une autre comme <<cross- bedding >>. Toutes ces structures sent au plan vertical du dyke, et non proches

1176

G. J. H. McCALL and R. PEERS - - Geology of the Binneringie Dyke

de l 'horizontale eomme la stratification dans les intrusions stratiformes bien con- hues. Le dyke ne montre pas la texture ~< eumul ,,.

La deuxi6me partie du eompte-rendu du <<Binneringie Dyke~> embrasse d 'antres localit6s d'int6rgt sur sa grande longueur. La stratification/L Sunday Soak, les roches aeides br6ehiques g Cowan Hill, et ]es grands massifs int4rieurs de felsite et granophyre an sud du Salt Well sont d6crits en d6tafl. Le reste de eette deuxi6me pat t ie embrasse le mode d ' intrnsion et la p6trologie de ee grand dyke eomplexe. Trentes analyses illustrent le mode de diff6rentiation. La diversit6 de roehes est attribu6e au fraetionnement, et non A la contamination. La stratification vertieale de eette inh~usion unique est compar6e avee le <; Marginal Layered Zone ,, de l ' intrusion de Skaergaard, au Groenland, et d6montre le ph6nom6ne des e o n g 6 1 a t i o n s e u m n l 6 e s , d6crites /L Skaergaard. Ces e o n g 6 1 a - t i o n s c u m u 16 e s o n t , peut ~[re, plus d ' importance qu 'on suppose. L'addit ion de magn6sium et calcium marginal dans le dyke est attribu6e fi la diffusion, nn proeessus eomme << I 'adeumulns eristallisation >>.

I4paTKoe eo~epma,t~e

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Table of contents Part 1

by G. J. H. MCCALL aLd B. PEERS Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1178 Binneringie Peninsula and Island: The nor thern par t of West Ridge,

Birmeringle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1186

Part g by G. J. H. McCALL

Other sections of the Binneringie Dyke . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1215 Sunday Soak . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1215

1177

Aufs~tze

Birmaronca Rock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1218 Cowan Hill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1221 Salt Well . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1284 Murdtmna Hill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1288 Murdunna Lake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1289 The dyke west of Sunday Soak (Bremer Range - - Mt Holland - -

Hyden) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1"240 Structure and maimer of intrusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1242 Petrogenesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1245

Exsolution and inversion relationship in pyroxenes . . . . . . . . . . . . . . . . . 1245 Chilled margins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1246 Primary o~hopyroxene crystallisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1246 Crystallisation history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I247 Geochemical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1248 The late basaltic dolerite dykes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1254 Origin of the acid dykes and segregations . . . . . . . . . . . . . . . . . . . . . . . . 1255 Origin of the pegmatoid segregations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1257

Discussion of the Binneringie Dyke with context of layered intrusions . . . 1257 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1262

Introduction

The Binneringie Dyke is the largest of a swarm of basic dykes, containing minor developments of ultrabasic, intermediate and aeid differentiates, which intrude early Preeambrian metamorphic and igneous rocks of the Western Australian Shield. The entire suite has been called "the Widgie- mooltha Dyke Suite" (SoFouLIS, 1967). There are two immense dykes, several other very large dykes, and numerous smaller dykes, which tend to occur in closely spaced swarms: the smallest members are basaltic dolerite dykes only inches wide. The trend of the suite is broadly east- west, the average being about 15 ~ north of east to 15 ~ south of west. A few minor dykes of the suite display quite aberrant trends, which follow the regional strike of the rocks of the Kalgoorlie System into which the dykes are intruded - - trends approximating to meridional.

The two largest dykes of this suite are situated north and south of Lake Cowan, one of the largest salt lakes in the State. The Binneringie Dyke runs across the north margin of the lake, while the Jimberlana "Dyke", previously mentioned by W. D. CAMPBELL (1906) and BEKKER (1968), and lately studied in detail in the section near to Norseman, by I. H. CAMPBELL (1966): in the Mt Norcott section by I. H. CAMPBELL and McCALL: and in the same section, independently by TY~VCHITT, runs across the south end of the same lake.

The detailed study of the Binneringie Dyke was commenced by the two authors working together in 1965. The initial detailed field study was of a selected area near Binneringie Station HOMESTEAD (Fig. 1 and 2): supporting petrographical and petrological studies were carried out by PEERS (Thesis, 1965). Several more traverses of the dyke by the authors in 1965 were followed by additional traverses by McCALL late in 1965 and in 1966, Petrographic studies of the additional material were carried out by McCALL, and a full analytical programme was mounted covering the entire dyke.

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The Binneringie Dyke has been traced for more than 200 miles (820 km.) from a point four miles (6.7 km.) east of Murdunna Lake to a point on the rabbit proof fence, near Hyden. The evidence of its extension westwards to Mount Holland is to be found in the magnetic anomalies revealed in an aeromagnetie survey carried out by the Bureau of Mineral Resources (Geophysical Bran&, Melbourne)1). To the west of this the trace of the dyke can be faintly detected on air photographs. A large dyke is picked up on the ground at the 192 mile post between Kondinin and Hyden. It may well be a continuation of the same dyke: however, what evidence there is available in published maps indicates that, though the same east- west swarm continues across the Wheat Belt to the vicinity of the Darling Fault, which bounds the Perth Basin (infilled with Phanerozoie rocks), the continuation is in the form of a series of relatively narrow dykes without any giant member such as the Binneringie Dyke. It is noteworthy in this context that the Birmeringie Dyke becomes attenuated where it enters the immense granite "batholith" at Sunday Soak, west of Higgins- ville, and that there seems to be a permissive relationship between really giant dyke intrusions and the Kalgoorlie System metamorphics as country rocks, these rocks allowing increased dilation compared with the granites.

The dyke passes beneath younger Preeambrian sediments (the W o o d- l in e B e d s of SOFOULIS and BecK, 1962) at its eastern termination, which is in the nature of an obscuration, not a real termination.

The very nature of such a giant dyke, 200 miles (820 km.) long and up to 2 miles (8.2 km.) across, provides problems in mapping. Mapping along its entire length is impracticable on account of poor exposure: it is generally not exposed at all or very poorly exposed, and only at certain localities does it rise in low ridges above the sand plain, or even better, reveal low coastal outcrops where it meets the shores of salt lakes. The inland exposures are covered by dense, spiky vegetation and spinifex: and the surface is commonly made up of immense boulders forming a subcrop, not very rewarding to map and extremely difficult to scramble over. Hence, a programme of traverse mapping augmenting the definitive, detailed coverage near Binneringie Homestead, was initiated. The area chosen for detailed study included far more lake shore exposures than anywhere else along the dyke (Fig. 2), and was known to include a complex series of differentiates. In hindsight, it may be said to have been an ideal choice: only at Sunday Soak; south of the Salt Well; and at East Murdunna lake, is anything comparable exposed.

The dyke does show duplication, at Sunday Soak, and possibly also near the Salt Well. There seems to be a tendency to step up into another parallel fracture, just off the original line. In addition, it has a complex internal structure, incorporating many dykelets and minor dykes of acid and basic material within it, as well as ill-defined acid and intermediate segregations. The structure is somewhat like a ladder, since the minor dykes n e v e r extend out from the main dyke as apophyses, the chilled

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and.

Aufs~itze

margins of the main dyke forming a confining boundary. The authors fee! justified in regarding it as a single dyke intrusion in spite of these com- plexities. It is doubtful if many comparable structures are known elsewhere: only the Great Dyke of Rhodesia (WoRsT, 1960) and the associated Jimberlana "Dyke", to the south of Lake Cowan (CAMPBELL, 1966) come to mind, and these two dykes exhibit either a simulated or an actual "lopolithic" character, so far not reeognised in the Binneringie Dyke:, which is, in every sense, a true dyke, although a major intrusion of complex character. Even though no certain occurrences of cumulate have been found, it does reveal complex patterns of differentiation, evidence os eonvection current stirring, and even strnetures resembling magmatic sedimentation structures. The Binneringie Dyke seems to come within the definition of layered igneous rocks accepted by WAGER ~E BROWN (1968), though it does not completely fit the strict definition of "layered intrusions" earlier given by WAGER (1963). There is a suggestion of swells in the pattern of aeromagnetie anomalies, but no evidence on the ground of "canoe" or "basin" structure involving synclinally disposed rather than vertical layering (CAMPBELL, 1966 ) - - what shallow-dipping layering is observed reflects only minor loealised flexures, involving departure from the vertical disposition.

P r e v i o u s W o r k

CLARKE (1925) first mentioned this dyke, "a much more extensive development of the fresh basalt series. . , on the north shore of Lake Cowan, south of the Paris Group"; and FARQtmARSON described a gabbro specimen eolleeted from three miles south of the Paris Mine. CLEVERLY (1955) mentions exposures near Binneringie Homestead in an unpublished account, "the principal interest os this dyke is the number and variety of minor dykes which occur within it". It was CLEVERLY who suggested the present study to the authors. HOOrER (1959) mentioned the dyke more or less in passing, while studying marine Tertiary rocks which abut against it. He gave a brief description of what he called the "M u r d u n n a G a b b r o" (the name B i n n e r i n g i e has been preferred for the entire dyke, for the reason that this is the name in general use: it has been accepted by the Stratigraphie Nomenclature Committee of the Geological Society of Australia).

SOFOULIS & BOCK (1962) and SOFOULIS (1966) have referred to the dyke suite, introducing the name "W i d g i e m o o 1 t h a": and noting the great longitudinal extent of the dyke itself, "at least 120 miles".

M e t h o d s o f s t u d y

F i e l d W o r k .

Aproximately 5 weeks were spent in the field mapping sectors of the dyke, directly on enlargements to 1" = 660' scale os air photographs of the Lands and Surveys Dept., W. Australia, flights of the Widgiemooltha Series. The method was quite satisfactory in view of the meagre topo- graphic relief.

1182

G. J. H. MEC,kLL and R. PEERS - - Geology of the Binneringie Dyke

L a b o r a t o r y

250 thin sections were used as a basis for the study, the serial nmnbers referred to in this text applying to the collections of the Geology Depart- ment, University of Western Australia, in which they are permanently lodged. Detailed petrographic study by PEEaS involved refractive index determinations by oil immersion methods: optic angle determinations (by use of the Universal Stage Microscope for pyroxenes, and simply by noting the degree of curvature of isogyres in centred optic axis interference figures for other minerals): and modal analysis using a Swift point counter (--> 800 points). Comparison of chemical compositions of mineral species were made using the following graphs:

P 1 a g i o c 1 a s e : DuPARc and REINHARD in W~NCHELL & WINCHELL, 1961, pp. 288, 295 and 296: O r t h o p y r o x e n e : POLDERVAART, 1950, !0.1076: C 1 i n o p y r o x e n e : HESS, 1949, p. 684 (nomenclature follows WmsoN, 1954, p. 241): o t h e r m i n e r a 1 s : WINCHELL & WINC~]ELL, 1961, relevant graphs.

In the case of the supporting petrographical studies of material obtained on the other traverses, only a general account of the petrography has been attempted by MCCALL: textural descriptions and identification of the mineral species being considered adequate for the purpose of obtaining a picture of the dyke as a who/e.

C h e m i c a l a n a l y s e s .

Five chemical analyses were carried out by Miss PEERS (PEE~S, 1965, pp. 10--11) using standard methods of rapid analysis. Eleven more were carried out by Dr. W. R. O'BEmNE using XRF methods of analysis, and a further fifteen by Dr. J. GRAHAM Of the C.S.I.R.O. Mineral Resources Division, Perth, using the same methods.

P h y s i o g r a p h y

The area lies in the S a l i n a l a n d region defined by JUTSON (1950) comprising Murchison, Kalgoorlie and Wheat Belt divisions. Extensive salt lake development and lack of perennial rivers charaeterise the very level Precambrian shield. Rainfall, limited to 10 inches yearly average, occurs mostly when summer cyclonic storms move inland from the north-west coast, at the end of their path, and virtually the entire annual rainfall may come in one such storm. Drainage over the surface is limited; the area being one of internal drainage, no watercourses carrying run off to the Ocean. The water table lies close to the surface in the salt lake depressions. Topography is controlled by differential weathering of various members of the Kalgoorlie System, and the dyke itself: the other country rock to the dyke, the granite, shows very little relief. The older basic igneous rocks form ridges transverse to the dyke, rising a few hundred feet above the general level of the plain, and the dyke itself forms a chain of hills of similar elevation above lateritised surface, which is mainly covered by sandplain. The lake depressions are scooped out in this surface. The granite forms monotonous sand-plain country of little or no exposure, and the dyke itself is poorly exposed in the extensive granite area west of

1183

Aufs~itze

Sunday Soak. Lake Cowan is filled with reddish silts, and these dry out in summer to a glistening crust of salt crystals, the white surface of which produces an unbearable reflection in the high sunlight. Low outcrop occurs only on the northern and western lake shores (Fig. 8), as in the case with all these salt lakes. The other shores are buried in sand and "kopai" (gypsum) dunes. The country rock is seldom found exposed along the dyke contact, the dyke giving way to sand plain, lake fill or Plantagenet (Eocene) sediments.

Fig. 3. General view looking slightly south of west, along the line of the dyke, over the silt-fiats of Lake Cowan (photo taken in winter) to Birmeringie Island and the West Ridge beyond. The low subcrop type of exposure in the foreground is typical of the dyke inland. Very low cliffs of rock such as fringe part of the

island provide the best exposures.

R e g i o n a l G e o l o g i c a l S e t t i n g

The geology os the region is well documented by SOFOOLIS & Beck (1962) and SOFOULIS (1966), and the map used here (Fig. 1) is adapted from their text figure. East of the Coolgardie-Norseman Highway the dyke cuts a sequence of metasedimentary and meta-igneous rocks of the Kalgoorlie System: these include ophiolite belts (serpentinite, metanorite, metagabbro, metadolerite, pillowed and unpillowed metabasalts and meta- picrites) and belts of quartz-albite-porphyries: metaconglomerates, metasandstones, metasiltstones and fine metapelites. It also cuts the small internal granite body known as the B i n y a r i n y i n n a G r a n i t e (HoorEn, 1959).

The attitude of the dyke is revealed by magrnatic layering, and is clearly steep to vertical throughout. The metamorphic rocks strike north- north-west to south-south-east, and show steep dips (with only local ex- ceptions). Thus the dyke cuts these metamorphic rocks almost at right angles. It is important to note that in the case of the Binneringie Dyke and the Jimberlana Dyke, studies of internal flow structures are all the more rewarding because there is no possibility that attitudes observed

1184

G. J. H. MCCALL aEtd R. PEERS - - Geology of the Binneringie Dyke

have been produced by subsequent movement, there having been no significant tectonic disturbance after the dykes were emplaeed early in the Precambrian. The tensional direction followed by the dyke is also utilised by numerous small dykes of the Widgiemooltha suite and quartz reefs, and is the expected tensional plane of the Kalgoorlie fold belt, transverse to the principal fold axes.

To the west of the highway the dyke enters the vast area of granite relieved only by the Bremer Range metamorphics (= Kalgoorlie System?), and numerous inliers of similar ancient metamorphic rocks enclosed in the granite. This is one of the large granite"batholiths" of Western Australia, and the granite is certainly i n t r u s i v e into the Kalgoorlie System at its margin, for the stratigraphie units wedge out against the granite contact. The "internal" Binyarinyinna Granite is similarly intrusive, and now known to be of like age.

Age Dating (87 Rb/87 St) by Tm~EK (Thesis, 1966) has yielded the following chronology:

(P 1 a n t a g e n e t B e d s : Eocene Limestones and Spongolites) (W i 1 k i n s o n R a n g e B e d s : Permian Tfllites)

W o o d l i n e B e d s 1620(+ o r - - 1 0 0 m , y.) W i d g i e m o o l t h a D y k e S u i t e - - (dated on "Celebration"

and "Jimberlana" dykes, but can be accepted for Binneringie Dyke), 2420 -T- 80 m. y.

G r a n i t e - - (internal and external) 2619~ ( + or - - 18 m. y.) M e t a m o r p h i s m o f K a l g o o r l i e S y s t e m : ages around

9.700 m. y. Among the rock types previously recorded from the Widgiemooltha suite are: norite, gabbro, quartz-dolerite, quartz-diorite, hypersthene-gabbro and peridotite. The present investigation reveals that the Binneringie Dyke alone includes: olivine-bronzite gabbro, olivine-bronzite-quartz gabbro, bronzite- gabbro, augite-pigeonite gabbro, anorthositie gabbros (carrying all three pyroxenes), olivine dolerite, grano-gabbro, grano-phyric differentiates of dioritie, granodioritic, tonalitie and adamellitie composition.

Related swarms of minor dykes are recorded at Norseman, Widgie- mo oltha, Bayley's Workings (Pilbailey Hill), Ive's Peninsula, Camel Hill and Mount Belches in the Lake Cowan-Lake Lefroy region. The suite continues northwards through Kalgoorlie and Leonora, and also southwards of Norseman. SOFOtrLIS & BeCK (1962) regard dykes at Needilup, Ongerup, Jerramongup, Gnowangerup and Borden to be of the same suite, though others dissent from this view. The dykes of the Wheat Belt follow the same general trend, predominantly, and it seems difficult to escape the conclusion that they are, at least in part, of the same age as the Widgie- mooltha Suite.

1185

Aufs~itze

The dykes are easily picked up by virtue of their aeromagnetie anomalies, due to their titanomagnetite (or other opaque mineral) content. The granites contain magnetite as well, and thus the contrast is not great where the dykes cut the granites, as at Binneringie Homestead. Some of the dykes reveal negative anomalies against the large external granite: EVERING- HAM (1965) has discussed this unusual pattern, which does not seem to be simply due to a higher magnetic mineral content in the granite.

The ancient rocks of the Kalgoorlie System form part of the K a 1- g o o r l i e G e o s y n c l i n e (FRIDER, 1952): contemporaneous volcanic activity, and sedimentation along the margin of the basin of deposition was followed by orogenesis, metamorphism and granite intrusion (McCALL, BRAYBROOKE, MIDDLETON ~X: MUHLING, 1967). The older basic igneous rocks show some superficial resemblance to spilite suites, but detailed studies suggest that much of the suite is tholeiitic, including associated ferrogabbros and ferrodolerites (G. D. BARTRAM, Thesis, 1970): however, there are also some rocks of apparently alkalic character (DOEPEL, 1965) near Yflmia. Some of these older rocks are very like the rocks of the FOst-metamorphic dyke suite, especially the bronzitites.

P 1 a n t a g e n e t limestones, of Eocene age, carrying abundant fossil remains; and spongolites, abut on and overlap the dyke on the north shore of Lake Cowan. These, together with the Permian "W i 1 k i n s o n R a n g e B e d s" (tillites) and the mid-Precambrian " W o o d 1 i n e B e d s"

(sandstones) (SoFouLIS, 1966) are the only meagre representatives in the area of the immense time span from 2420 m.y. ago to the present time! Superficial deposits include recent lake deposits, consisting of thick, salt- impregnated, sequences: various types of duricrust and kunkar; eluvia], alluvial and aeolian deposits (the latter including the "kopai" dunes): and residual deposits such as laterite and kaolin.

Binneringie Peninsula and Island

Northern Part of West Ridge (see Fig. 2)

C o u n t r y r o c k a n d c o n t a c t m e t a m o r p h i s m

The dyke cuts the B i n y a r i n y i n n a G r a n i t e at this point, no other country rock being exposed. Contact effects are slight: whereas the grey granite, away from the contact, weathers into smooth, massive, continuous outcrops including extensive rounded pavements, the granite exposed near to the dyke, on its north margin on Birmeringie Peninsula, Binneringie Island and West Ridge, is reddened and fractured, weathering into small angular fragments. There is a small amount of contact metamorphism, of the type known as pyrometamorphism involving fusion, developed very close to the actual contact: but the amount of contact metamorphism is meagre considering the great width os the dyke (about half a mile at this point): this paucity of contact effects is found to be a feature os the dyke throughout its length.

1186

G. J. H. MCCALL and R. ]PEERS - - Geology of the Binneringie Dyke

The Binyarinyinna granite is one of the "internal" granites which according to SOFOULIS & BOCK (1962) "on detailed examination show intrusive relationships but are at a broad scale concordant and follow the same regional trend as the metamorphic belts". Potassic feldspar is usually (though not invariably) in excess of plagioclase in these internal granites, but the Binyarinyinna granite seems to be sodic-potassic, rather than dominantly potassic. The textures are allotriomorphic granular, with coarse replacement perthite textures common. Myrmekite is also not uncommon.

Modal determinations were not attempted, but the rocks seem to be mostly adamellitie, with equal amounts of the two felspars. The plagioelase is oligoelase, varying from AnlT--An~8. The K-felspar may be mieroeline or orthoclase. Accessories include biotite, chlorite and muscovite: apatite, epidote and magnetite. Limonite, kaolinite and sericite are alteration products.

The development of granophyre along the grain boundaries is best seen in specimen 55 245 (Fig. 4). This seems to be a pyrometamorphie effect, since it is only found in granite in close contact with the dyke. Not enough thin sections of the granite have been made to be absolutely certain on this point, but the granophyre is believed to represent limited contact melting along grain margins, followed by fine reerystallisation of the melt.

T h e G a b b r o i c R o c k s

The main rock type is gabbro: there are two varieties,

1. b r o n z i t e - b e a r i n g g a b b r o , which occurs near the dyke margins, and mainly in chilled, contact rocks.

2. a u g i t e - p i g e o n i t e g a b b r o , which occurs in the well-crystallised centre of the dyke.

The first contains a magnesian orthopyroxene within the bronzite composition range, the inversion product of a magnesian pigeonite. The second contains the same augite, which is the dominant pyroxene in both types, but no bronzite: a ferriferous, uninverted pigeonite takes the place of the bronzite.

The plagioelase is mostly labradorite, though andesine is found in the outer zones of zoned crystals: and titanomagnetite is the ore mineral; in both types, hornblende (uraltic), biotite and chlorite form reaction rims around pyroxenes: apatite, sphene and epidote are minor accessories: sericite, tale, bowlingite, bastite, kaolinite and magnetite are alteration products. Prehnite has been introduced interstitially in some rocks, by the agency of late Ca-rich fluids. Granophyre mesostasis, or equant quartz grains, form an ubiquitous mesostasis, indicating excess silica. Olivine is only found in the form of talc-bowlingite-magnetite pseudomorphs, mostly poikilitically enclosed within the bronzite: it is thus restricted to marginal z o n e rocks.

Texture are sub-ophitic, with glomeroporphyritic developments in chilled bronzite-bearing gabbros.

1187

Aufs~itze

Magmatic layering has produced alternation of mafic and felsic layers: this structure is especially well developed just in from the chilled margins: the core of the dyke is mostly massive and coarsely crystallised. The magmatic layering shows only very minor localised departures from a near vertical attitude.

A n o r t h o s i t i e g a b b r o s , in which plagioclase is overwhelmingly predominant occur in layered rocks, and in quite large areas of massive structureless rock in the core of the dyke. The gabbros also grade into

Fig. 4. P h o t o m i e r o g r a p h (55 245). - - Adamellite country rock showing development of mierographic intergrowth along grain boundaries: found strongly developed only in the immediate contact zone, this seems to be a contact pyre-

metamorphic effect (crossed nlcols, X 25).

intermediate rocks in quite another way, with the development of an increasing amount of acid residuum. Transitional types with pyroxenes and labradorite felspar set in an acid residuum base, are referred to as g r a n o - g a b b r o. ROckS entirely devoid of gabbroic character are referred to as i n t e r m e d i a t e and a e i d s e g r e g a t i o n s : they may be pegmatoid, either having the minerals of the gabbros more com'sely developed, together with abundant acid residuum: or with an entirely new set of late stage minerals replacing the gabbro minerals, which are evident only in palimpsest textures.

All these rocks with abundant residuum are considered under grano- gabbros and intermediate to acid segregations.

M i n e r a l o g y o f t h e G a b b r o i c R o c k s Rather than provide repetitions of the mineralogical data, a summary

is given here of the mineral species which occur in the gabbroic rocks of the dyke.

1188

G. J. H. McCALL and R. PEERS - - Geology of the Birmeringie Dyke

Plagioelase - - MAnly of lath-like form, narrower in the chilled gabbros, but broad in the coarser varieties: it also occurs as broad equant crystals and as irregular grains. Modally it ranges from 49~o to 78Yo of the ro&, the lower value applying to bronzite-bearing gabbros and the higher to anorthositic gabbros. Labradorite occurs as unzoned crystals and forms the cores in zoned crystals, which are zoned throughout (? excluding the possibility of cumulate origin) and predominate. It is twinned on the Carlsbad, albite and (less frequently) pericline laws: the outer zones are commonly andesine. Zoned crystals are not well twinned and unzoned crystals show very well developed twimaing. Alteration to serieite is widespread, though patchy: chlorite and kaolinite are other alteration products.

Augite - - This calcie elinopyroxene is present in every rock of gabbroic affinities, other than some acid and intermediate segregations. It tends to be subhedral to anhedral, is colourless, and shows inclusions of plagioelase and titanomagnetite. Twinning parallel to 100 (multiple or simple) is usual. Also, d i a I 1 a g e parting, parallel to 001, produces a herring-bone structure in many grains, and alteration tends to occur preferentially along such diallage partings - - to bastite, chlorite, biotite, hornblende (uralite), tale and limonite.

Pigeonite - - This is the caleinm-poor pyroxene in the normal gabbros of the centre of the dyke. It is antipathetic to bronzite (inversion product of the more magnesian pigeonite of the border zone). The pigeonite reflects the progressive dominance of Fe over Mg in the pyroxene towards the centre of the dyke. An iron-rich variety, it is distinguished by a very low optic angle (2V): it also tends to show lower birefringence than angite, and is more readily altered. Twinning is the same: herringbone effects are produced by a combination of this twinning and cleavage parallel to 001.

Bronzite - - The grains are subhedral, eolourless, and commonly larger than elinopyroxenes (up to 2.Smm. long). Inelusions are tale, magnetite and bowlingite (after olivine). Twin planes of pigeonite are commonly preserved: also 001 exsolution lamellae from the pigeonite, composed of angite. These features testify to its origin by inversion from pigeonite. Augite lamellae due to exsolution below this inversion point may be in the form of irregular vermiform blobs (indicating rapid chilling), or regular lamellae parallel to 100. Alteration is to bastite, chlorite, muscovite, biotite and hornblende.

Titanomagnetite - - Skeletal and massive grains are sparsely scattered through the gabbroic rocks. Alteration to leneoxene reveals their titaniferous nature (? ilmenite-magnetite intergrowth). Triangular and rhombic patterns reveal oetahedral cleavage.

Quartz - - Clear, unaltered, anhedral, interstitial grains are present in virtually all these rocks. Inclusions are apatite and ore: chlorite tends to be in close association. Mierographie aggregates of quartz and K-felspar also commonly form the mesostasis. The mierographie intergrowth may be cuneiform or vermiform.

Potassie felspar - - This shows almost complete kaolinisation throughout, and it is not easy to be certain that albite does also take part in the mesostasis. Staining methods are not practicable on such kaolinised material. However, in the few cases where coarse intergrowth does preserve clear, fresh felspar, it is orthoclase. All the felspar in the micrographic mesostasis is thus taken to be orthoelase, though it is stressed that this remains something of an assumption.

1189

Aufs~itze

Apatite - - Minor rod-like needles with hexagonal section are a common accessory.

Epidote - - Anhedral grains are associated with quartz and felspar in the mesostasis. It is pleoehroic in shades of bright yellowish green, and recognised by virtue of its anomalous birefriugenee.

Sphene - - Euhedral, diamond shaped grains and some subhedral grains are associated with the mesostasis. Relief and birefringenee are extreme.

Hornblende - - Occurs as a marginal fringe to pyroxene (vat: u r a l i t e), it is pleoehroie in shades of green (X = pale green, Y = olive green, Z = dark green). It is almost certainly deuteric.

Biotite - - Occurs with hornblende, usually outside it, fringing pyroxenes. It is pleoehroic: X = pale yellow, Y = brown, Z = dark brown.

Chlorite - - Occurs as an alteration product of pyroxene and plagioelase: also radial aggregates in mesostasis (primary?), It is mostly of a pale green, non- pleochroie variety, showing anomalous blue interference colours.

Bastite - - A form of coarsely crystallised antigorite, occurs as an alteration product of pyroxene: it is pleoehroic: X = pale yellow, Y = yellow green, Z = deep yellow green.

Bowlingite - - Occurs with magnetite and talc as a breakdown product of olivine. It is distinguished from dfiorite and bastite by its higher birefringence.

Tale - - Occurs in fine aggregates, as an alteration product of olivine. Muscovite - - Coarse muscovite has developed as an alteration product of

pyroxene in some rocks, but, mostly, the variety is the fine sericite, occuring as an alteration product of plagioclase felspars.

Kaolinite - - Occurs as a very finely divided aggregate, an alteration product of plagioclase and K-felspar. The latter tends to a greater degree of kaolinisation: mad this can be used to differentiate, it from plagioelase, though the distinction is a fine one. The kaolinised K-felspar tends to have a quite different appearance under the high power objective without the nicols crossed.

Prehnite - - Forms fan-shaped aggregates in cavities, and also occurs in small veins: it is an alteration product of both plagioelase and K-felspar, A late stage mineral, it is thus typical of the grano-gabbros and acid/intermediate segregations, together with K-felspar, quartz, epidote, chlorite, sphene and carbonate.

Hematite, Limonite, Leucoxene - - Occur as alteration products of titanomagne- rite.

"Red Rock" - - T h i s is a term used, following several authorities on gahbroic suites, to denote red-coloured, late-stage material, either occurring as small patchy segregations in the gabbro, or making up large, but hazily defined areas of the main gabbro intrusions, and also forming discrete veins. The term g r a n o p h y r e is inadmissable, for the textllre is not, for the most part granophyric: the term f e 1 s i t e has been used but has other connot- ations. The "r e d r o c k" is partly composed of finely granular material, pardy of eryptocrystalline material, which may show mierolites and crystallites, and seems to represent a devitrified glass component. There may be abundant felspar/atlas in the "red rock", which, however, are invariably diKieult to determine beeanse of extreme alteration. They may be incompletely formed, too. This material is neither completely resolvable megasopieally, nor, usually, mieroscopieally, and the term "'red rock" is convenient to denote this obscure, sflicic, endstage segregation.

1190


D e t a i l e d d e s c r i p t i o n s o f t h e r o c k s p e c i m e n s

1. T h e b r o n z i t e - b e a r i n g g a b b r o s

C h i l l e d b r o n z i t e - b e a r i n g g a b b r o (55224)

Collected two metres from the contact-~) between dyke and granite on Birmeringie Peninsula, this is a fine, grey, chilled gabbro, with intermeshing felspar visible to the naked eye. In the section the texture is seen to be glomeroporphyritic (Fig. 5) groups, of bronzite phenoerysts being set in a sub-ophitic groundmass of plagioclase laths and augite grains. The plagio-

Fig. 5. P h o t o m i c r o g r a p h (55 224). - - Glomeroporphyritic, chilled bronzite- bearing gabbro: grey, eleaved phenoerysts, b r o n z i t e : white laths, p 1 a g i o -

e 1 a s e : small, grey grains in groundmass, a u g i t e (crossed nieols, X 10).

clase (54~o) is irregularly zoned labradori te (core, An56), al tered to sericite and kaolinite: angite (27Yo) (Caz6MgasFe26) occurs as anhedral grains altered to hornblende, bioti te and chlorite at the margins. The bronzite (18Yo) (MgTsFe~2), colourless and altered to chlorite, shows poikilitic pseudomorphs in talc-bowlingite-magneti te aggregates after olivine (Fig. 6). Augite exsolution lamellae are also evident, parallel to 001 of an original pigeonite: and also irregular augite exsolution blebs. Bowlingite (2--1Yo) and dissemin- ated magnet i te grains (1~), together with micropegmati te mesostasis and alteration products make up the remainder of the rock.

Another specimen (55 228), collected from the actual contact on Binne- ringie Peninsula, is even more fine-grained and lacks glomeroporphyrit ic

2) Only the northern eontaet is exposed in the area mapped in detail by McCALL and PEERS" the eontaet is thus the northern contact in all these rock descriptions.

1191

Aufs~tze

texture, being virtually intergranular. Bowlingite patches evenly dis- t r ibuted suggest that olivine grains may have been evenly scattered through the rock, not restricted to poikilitic inclusions within the bronzite.

A n o r t h o s i t i c b r o n z i t e - b e a r i n g g a b b r o (55226)

Collected from 70 metres away from the contact on Binneringie Penin- sula, ths specimen displays crude layering (Fig. 7). The thin section represents the anorthositic material, and shows it to consist of labradorite

Fig. 6. P h o t o m i c r o g r a p h (55 224). - - Talc-magnetite-bowlingite aggregate pseudomorphing olivine, poikilitically enclosed in a bronzite phenocryst: light grey, cleaved, b r o n z i t e : white, uncleaved, bordering it, p 1 a g i o e 1 a s e : black, opaque, t i t a n o m a g n e t i t e : light grey, in inclusion, b o w I i n g i t e

mixed with t a l c (plane polarised light, X 100).

(78Yo) (core An56), augite (11~o) (Ca40Mg~4Fe2~), bronzite (6~) (Mg79Fee_l), micropegmati te mesostasis, minor accessories and alteration products. The texture is sub-ophitic and there is little evidence of chilling.

M o d e r a t e l y c h i l l e d b r o n z i t e - b e a r i n g g a b b r o (55248)

This was collected 160 metres in from the contact on the north-east end of the "West Ridge": it shows finer, though crude flow layering. Felspar is normally zoned labradorite (An56_36): angite (Ca40Mg, sFe.25), and breuzite (Mg75Fee5).

B r o n z i t e - b e a r i n g g a b b r o (55225)

Collected from a layered zone, some distance in from the contact on Binneringie Peninsula, this rock is typical of many from the layered areas of the gabbro, in that layering shows up well on the weathered rock surface, but is only faintly detectable on the fresh surface of a hand-

1192


specimen, being virtually indetectable in thin section. The rock contains bronzite but comes from just outside the chilled zone.

The illustrations of exsolution lamellae and twinning relationships (Figs. 8 and 9) are taken from this rock. The bronzite is (MgrlFe29) and the augite (Ca34Mgz0Fe36), clearly revealing the compositional shift towards iron-rich and end-members of the isomo~-phous series, approaching the centre of the dyke.

Fig. 7. H a n d s p e c i m e n of a n o r t h o s i t i c b r o n z i t e g a b b r o (55 2~96). The crude alternation of felsic and mafle bands is evident, reflecting a

magmatie layering structure.

C h i l l e d b r o n z i t e - b e a r i n g g a b b r o (55258) Another illustration of exsolution rMationships is taken from a specimen

from near the contact on Binneringie Island (Fig. 10). The bronzite pheno- eryst illustrated shows a clear core, a zone of irregular, vermiform augite blebs around it, and a wider zone of angite exsolution lamellae parallel to 100 outside that. The orthopyroxene is (MgrsFee4).

2. T h e a u g i t e g a b b r o s A u g i t e g a b b r o ( 5 5 2 3 5 )

Collected at a distance of 400 metres in from the contact on Binneringie Peninsula, this is typical of the rocks of the centre of the dyke. A medium- grained rock, devoid of flow layering, it reveals a hypidiomorphic granular texture in thin section: plagioclase (58~) ,,(core An~0), augite (30~o) (Ca8sMg~Feaa) and pigeonite are the main compenents: there is minor micropegmatite mesostasis. The augite, a ealcic ferro-augite, shows diallage

1193

Aufs~tze

Fig. 8. P h o t o m i e r o g r a p h. - - Bronz i t e -bear ing gabb ro (55 225) showing b lebs a n d exsolut ion lamel lae of a u g i t e (white), in a b r o n z i t e hos t (grey): c l eaved grey, a u g i t e : wh i t e to black, l amel la r twinned , p 1 a g i o e 1 a s e

(crossed nieols, X 100).

Fig. 9. P h o t o m i c r o g r a p h. - - T h e s ame rock, showing h e r r i n g - b o n e pa t t e rn p r o d u c e d b y aug i t e exsolut ion paral le l to (001) of t he p a r e n t p igeoni te , whid~ reflects the twin compos i t ion p l ane (100) of the p igeoni te . T h e s t ruc ture

indica tes b ronz i te inver ted f rom p igeoni te (crossed nicols, ;K 100).

t194

G. J. H. McCaLL and R. PEERS - - Geology of the Binneringie Dyke

par t ing on 001, combining with twinning on 100 to produce a "herr ing-bone" or "chevron" effect. Alteration of pyroxene to hornblende, biotite and chlorite at the margins is well developed and the felspar is strongly sericitised. The amphibole has properties:

extinction, Z ' A c = 16~ plcochroism, X = pale green, Y = olive green, Z = dark green;

the pigeonite is distinguished only by its low optic angle. Titanomagneti~e

Fig. 10. P h o t o m i c r o g r a p h (55 258). - - A bronzite-bearing gabbro showing a complex orthopyroxene grain: a core of b r o n z i t e (extinguished), without lamellae, is enclosed in a zone of exsolution lamellae, parallel to (001) of the original magnesian pigeonite. A zone of irregular blebs intervenes between clear core and zone of regular larnellae. Light grey, a u g i t e : white to grey, lamellar

twinned, p 1 a g i o e 1 a s e (crossed nieols, X 68).

(lYo) occurs as skeletal grains, altered to leucoxene and limonite: grains of this ore form discontinuous rims around some pyroxene crystals. Alteration products include chlorite (pale green, fibrous and platy varieties) and talc: epidote and apati te are minor accessories.

8. G a b b r o i c r o c k s w i t h s u b s t a n t i a l d e v e l o p m e n t o f a c i d m e s o s t a s i s

These may be divided into: a) Gabbros in contact with minor internal acid dykes and dykelets. b) Grano-gabbro and pegmatoid segregations. a) G a b b r o i c r o c k s i n c o n t a c t w i t h a c i d d y k e s a n d

d y k e l e t s . Intrusion of acid dykes and dykelets into the gabbro produces three

effects on the gabbro immediate ly adjacent to them.

1195

Aufs~itze

(i) Alteration of pyroxenes and plagiodases is enhanced, orthopyroxene being commonly entirely pseudomorphed by chlorite (bastite), though exsolution lamellae still reveal its identity.

(ii) Interstitial "red rock" is developed by the agency of penetrative, acid fluids. These augment the normal residual material of the gabbro, and produce bodies of "red rock" interstitially, chilled against the crystals of the gabbro, and thus revealing a "glassy" fringe. Incipient crystallisation of mierolites and longulites of ore and devitrifieation of the glassy mesostasis have converted the "red rock" to its present cryptocrystalline state. The centres of the "red rock" areas commonly show crystalline quartz and felspar, not due to devitrification, and this suggests that the margins are actually chilled against the large mineral grains of the rock. The distribution of gabbro contaminated in this way, close to contacts of minor internal acid dykes and dykelets, and the progressive increase in the content of such material towards such contacts, leaves no doubt that it is a development related directly to the presence of such minor intrusions: in this case the "red rock", though interstitial like the normal mesostasis, cannot be regarded as entirely segrated from the gabbro in that immediate locality; that is, it has been largely introduced.

(iii) Micropegmatite, again including ore, may take the place of the "red rock" increment described above. The phenomenon is believed to be controlled by temperature differentials between acid melt and gabbro of early crystallisati0n. In some cases, the chilling of the dykelet against the gabbro host has:inhibited the outward movement of material, and no augmentation of "red rock" or micropegmatite mesostasis has occurred.

The felspar in such rocks is no different to that in the normal gabbros, being labradorite: they are minor contact developments of g r a n o - g a b b r o .

C o n t a c t g r a n o - g a b b r o (55264) This specimen shows, in thin section, typical material of the type

referred to above: the interstitial material is a cryptic quartz-felspar intergrowth, very finely granular and heavily impregnated with limonite and kaolinite dust: at the margin of the "red rock" patch, fine spherulitic material occurs, including curved microlites of magnetite. Green chlorite penetrates the intergrowth, and water-clear quartz grains occur in the centre of the patch of intergrowth.

b) G r a n o - g a b b r o a n d p e g m a t o i d i n t e r m e d i a t e - a c i d s e g r e g a t i o n s .

The term "intermediate" is here used to denote rocks essentially gabbroic, but With "red rock" abundantly developed over large areas: not just locally as in the case of the contact developments related to minor dykes and dykelets. Boundaries between such areas and normal gabbro are gradational in the field. The composition of these rocks may be intermediate or acid, overall. The rocks termed segregations are those in which the segregation of late stage fluids and volatiles has produced mineralogical

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G. J. H. McCALL and tl. PEERS - - Geology of the Biuneringie Dyke

assemblages which almost completely or completely obscure the gabbroie character of the rock. They are mostly pegmatoid, showing very coarse crystallation. They tend to form small, isolated patches within normal gabbro.

Most of these rocks are grano-gabbro as defined by JOHANNSEN (1982, VO1. 2, p. 867): more rarely they show oligoclase-andesine as the first formed felspar, and are essentially granodioritic or adamellitic, though

Fig. 11. P h o t o m i c r o g r a p h (55 272). - - Grano-gabbro showing copious development of interstitial "red rock", between p I a g i o c l a s e crystals (clear, white): black microlites, rods and granules of m a g n e t i t e are set in a turbid grey intergrowth of quartz and kaolinised p 1 a g i o c 1 a s e, together with some c h 1 o r i t e : this interstitial material is limonite impregnated, and shows red in

handspecimen (plane polarised light, X 100).

atypical in texture. Exact pigeonholing according to the accepted classi- fications is not easy in view of the cryptic nature of much of the rock.

G r a n o - g a b b r o (55272)

Collected from the edge of a large segregation on Birmeringie Island this rock appears reddened in tile field, on account of the abundant limonite- dusted acid residuum. The affinities to the normal gabbro of the main dyke centre are obvious: labradorite (44~o) (zoned throughout, An~-45, and unzoned Ans~ ) forms large subhedral crystals, and augite (with associated pigeonite, g6~o) occurs as subhedral laths and anhedral grains. The augite composition is (Ca~6Mg~F%l). The "red rock" accounts for 17~o, and the remainder is quartz, chlorite, sericite and titanomagnetite. The interstitial "red rock" is opaque, excessively limonite-stained and kaolinised. Through it are scattered clear quartz grains, curved magnetite microlites and feathery green chlorite (Fig. 11).

77 Geologische Rtmdschau, Bd. 60 11117

Aufsatze

G r a n o - g a b b r o (55280) A similar marginal grano-gabbro from West l~idge, this shows felspar

(zoned, An51--41 ) and augite, possessing remarkably euhedral ouflhaes, having apparently erystallised simultaneously without interference (Fig. 12). The interstitial red rock is nearly opaque, but felspar intergrown with quartz and magnetite can be resolved. The augite shows herringbone structure (Fig. 18).

Fig. 12. P h o t o m i e r o g r a p h (55 280). - - Grano-gabbro showing distribution of "red rock", appearing dark between f e ls p a r s (white), which show a tendency to perfect euhedral form, giving the rock a texture typical of hypabyssal intrusions (small near-surface dykes), although the rock was crystallised at the

same crustal level as the gabbro (plane polarised light X 25).

I n t e r m e d i a t e s e g r e g a t i o n (54112) Collected from the centre of the area of segregation on Bhaeringie

Island, the largest such segregation area so far recognised (from the margin of which specimen 55 272 described above was taken), this is a deep red aphanitic and porphyritic rock: greenish phenocrysts of felspar and dark phenocrysts of pyroxene being set in very fine, deep red base. In thin sections the rock reveals large, partly resorbed, imperfectly formed plagioclase phenocrysts (Fig. 14), and small, less abundant augite grains set in the residuum. The felspar (24~o) is andesine (Ans3) and occurs as:

(i) elongate crystals up to 8.0 mm long, with resorbed embayed boundaries.

(ii) comb-shaped crystals, the "teeth" of which extinguish simultaneously, suggesting that they represent together, a single rapidly crystallised, imperfectly formed crystal.

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G. J. H. MCCALL and R. PEEaS - - Geology of the Binneringie Dyke

Fig. 18. P h o t o m i c r o g r a p h . - - The same rock showing a herringbone pattern produced in au~te by a combination of twinning parallel to (100) and a fine d i a l l a g e parting, Farallel to (001): turbid "red rock" shows to the right of the pyroxene: felspars white and kaolinised bottom left (plane po!arised

light )< 63)

Fig. 14. P h o t o m i c r o g r a p h (54 112). - - A very coarse gabbroic segregation of pegmatoid character, the plagioelase of which is unusual, having the form of imperfectly crystallised laths, "'combs" and longulltes. Titanomagnet/te shows as black rods, etc., and the base is a fine "'red rock" material (plane polarised

light )< 25).

1199

Aufs~itze

(iii) curved longulites, making up much of the groundmass and closely associated with other residual mineral components, altered extensively to sericite and prehnite.

The augite (12Yo) has the composition (Ca~4MgasFeal), and shows extensive alteration to bastite, but herringbone structure is preserved. Titano- magnetite (4.5~o) occurs in the form of massive, leucoxenised and limonitised grains, preserving octahedral partings. Quartz and alteration products (chlorite, hornblende, serfcite and prehnite) are present in minor amounts.

Fig. 15. Handspecimen of a gabbroic pegrnatoid segregation (55 '275), showing p 1 a g i o c 1 a s e (white crystal laths), fringed by c h 1 o r i t e (dark), and set in a base of "'red rock" (grey), whida displays numerous irregular cavities filled with

p r e h n i t e (white, also).

The "red rock" accounts for 59~o of h e rock, being composed of curved felspar laths, quartz grains, and rods of iron ore set in a cryptic limonitised and kaolinised matrix. There is no micropegmatite: the finer matrix appears to be a devitrhqcation product, but glass is not now present, though it once was. The presence of curved magnetite microlites seems to confirm the view that all these fine "red rock" areas represent a primary glassy residuum.

P e g m a t o i d s e g r e g a t i o n (55275)

This also comes from the centre of the large segregation area on Binneringie Island. The appearance of the handspecimen is shown in Fig. 15. Intricately shaped felspars are intergrown with chlorite, which tends to fringe them: the "red rock" base is also inset with cavities, filled with pale greell prehnite. The texture is hypidiomorphie granular but there

1200

G. J. H. MCCALL and 1R. PEERS - - Geology of the Binneringie Dyke

are minor complications: graphic intergrowth between chlorite-pseudomorphed augite and plagioclase (Fig. 16): herringbone relics in chlorite- pseudomorphed augite: epidote-pseudomorphed plagioclase (Fig. 17): fringing zones of cryptocrystalline, spherulitie "redrock" (containingrecognisable quartz and felspar) around the large, altered felspars (suggesting chilling relationships again): palimpsest octahedral cleavage of titanomagnetite retained in fringing chlorite (Fig. 18): and prehnite eoncentrations.

Fig. 16. P h o t o m i e r o g r a p h. - - The same rock, showing graphic intergrowth between a n d e s i n e (white) and c h l o r i t e pseudomorphs of p y r o x e n e

(grey) (plane polarised light X 67).

The mineralogy is andesine (19Yo) (Anal), crystals up to 10 mm long and 4 m m broad: oligoclase (18~o) (An27), small prisms in the groundmass: chlorite (16Yo), fibrous after pyroxene; reticulate, fibrous after fitano- magnetite; spherulitic in the grotmdmass, and arborescent growths in "red rock": quartz (8~o), anhedral, clear grains in the groundmass, and finer grafias within the red ro&: prehnite (6~o), alteration product of plagioelase, vein and cavity filling; typically forms radiating aggregates (Fig. 19) showi_ng bow tie structure; fine polysynthetic twinning is rare; optic angle (2V) = 65 ~ -T- 5 ~ - - (estimated): titanomagnetite (5~o) large, irregular grains altered to leucoxene, limonite and chlorite: epidote (8~o) (pistacite), alteration product of the more calcic plagioclase, and also as cavity filling; euhedral to anhedral grains, up to 0 .4mm diam., irregularly pleochroie in shades of lime green, some twinning on 100; optic angle (2 V) = 75 ~ -T- 5 ~ - - (estimated); birefringenee n)~- -na = 0.080: sphene (1~o), euhedral (some anhedral) grains (Fig. 20), only occurring in cavities: apatite, pyrite and amphibole are trace accessories. Spherulitic and felsitic red

1201

Aufs~itze

Fig. 17. P h o t o m i c r o g r a p h. - - The same rock, showing e p i d o t e pseudomorph ing idiomorphic a n d e s i n e crystals at their margins, and c h 1 o r i t e fringing them in radiating, dark grey aggregates, all three minerals together with grains of q u a r t z (clear, white) being set in turbid grey base of "red rock"

(plane polarised light, X 25).

Fig. 18. P h o t o m i c r o g r a p h. - - The same rock, showing c h l o r i t e pseudomorph ing t i t a n o m a g n e t i t e , and preserving the octahedral lattice pattern.

C h l o r i t e , grey: e p i d o t e , high relief (plane polarised light, X 100).

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G. J. H. MCCALL and tl. PEERS - - Geology of the Birmeringie Dyke

ro& material (22~o) forms much of the cryptocrystalline base material, in which grains of quartz and ore are resolvable: it seems to be a Iimonite- impregnated quartz-felspar-chlorite-ore intergrowth.

4. P e g m a t o i d c l o t s e g r e g a t i o n s

G r a n o - g a b b r o p e g m a t o i d (55254) This is typical of several small clots of pegmatoid grano-gabbro, occurring

on the north side of Bimleringie Island. Such clots are not as red as those

Fig. 19. P h o t o m i c r o g r a p h . - - The same rock, showing a p r e h n i t e aggregate, including rhombic e p i d o t e crystals (high relief) and q u ar t z

granules (elear, white and grey) (crossed nicols, X 25).

previously described, the residuum tending to appear grey or reddish brown in handspecimen. They occur close to swarms of acid dykelets and obviously have some relation to them: this relationship is consistently observed throughout the dyke. The ro& is an aphanite porphyry, lath shaped plagioclase phenocrysts behng set in the residuum, together with pyroxenes (both being up to 1 em long). In thin section the plagioclase (82~o) is found to be labradorite (An56) and the augite (10Yo) (Ca35Mg84F%l) to enclose some bastite pseudomorphs after bronzite (the rock comes from quite close to the north contact). The augite shows diallage parting, producing herringbone patterns, and the orthopyroxene pseudomorphs show structures indicating inversion from pigeonite. Titanomagnetite (2Yo), pretmite (2~o) and quartz (~Yo), apatite, epidote, sphene, sericite and kaolinite are also recognised: the "red rock" accounts for 45Yo of the rock, and is very finely crystalline, composed of quartz, magnetite mierolites,

1203

Aufs~itze

apatite needles, epidote, sphene and chlorite, set in a finer limonitised quartz-felspar matrix.

G r a n o - g a b b r o (55288)

This clot from West Ridge, has a leuco-granodiorite composition, the plagioelase (60Yo) being oligoclase (An29): laths of euhedrat plagioclase are the essential component, being separated by narrow intervening strips and interstitial pools of "red rock". Chlorite is the only other mineral

Fig. 20. P h o t o m i e r o g r a p h. - - The same rock showing euhedral s p h e n e and euhedral to snbhedral e p i d o t e set in a patch of p r e h n i t e (grey base),

(plane polarised light, )< 100).

megascopieally detectable, being sporadically distributed throughout the rock. The texture is almost porphyritic, as there is no intermeshing. The chlorite (9~o) is penninite. Quartz, titanomagnetite, epidote and sphene are also present.

E p i d o t i c s e g r e g a t i o n (55266) This comes from a large clot on the north side of Binneringie Island

in normal marginal gabbro, but near to a swarm of minor acid dykelets. I t is bright pistaccio green, and of saccharoidal texture. Pegmatoid segregations, showing intermeshing felspars set in a greyish brown residuum, grade, towards the centre of the clot, into a rock showing no definite texture, and almost entirely composed of epidote. This specimen does show laths of felspar, reeognisable in handspecimen, but largely altered to epidote. In thin section the texture appears hypidiomorphic granular, epidote pseudomorphing the plagioclase laths, and spherulitic interstitial material fringing them.

1204

G. J. H. MCCALL and R. PEERS - - Geology of the Binneringie Dyke

The mineralogy is: epidote (pistacite) (66~o), anhedral to subhedral grains, in pseudomorphie aggregates and in the interstitial material: quartz (42~o) dear anhedral grains: base material consisting of spherules, 1 mm diameter and imperfectly formed, including epidote, apatite, curved ore microlites and ehlorite: not so limonite impregnated as the typical "red rock", hence the grey-brown eolour in handspecimen. There are traces of hornblende,, sphene and titanomagnetite grains.

In another slide (55 9.76) from the eentre of the clot, epidote replaces most of the plagioclase and also forms most of the interstitial spaces, where it forms spherulites itself.

E p i d o t i e s e g r e g a t i o n (55248) Golleeted from a dyke-like segregation on Binneringie Peninsula, this

consists of a bright green epidote-quartz-felspar rock, associated with a pink eoloured felspar-quartz-ehlorite-epidote rock. There is relict intersertal texture visible, and much of the quartz and felspar are in mierographie intergrowth.

T h e d y k e r o c k s w i t h i n t h e g a b b r o

Two suites of dyke rocks are reeognised within the main gabbro dyke: 1. Acid dykes and dykelets. "2. Basaltic olivine dolerite dykes and dykelets. It is not eertain whieh is the first intruded suite, but relations at Bin-

neringie Peninsula suggests that a large internal acid dyke is truncating a swarm of internal basaltic dolerites. However, CLEVERLY (1955) believed that the reverse relationship could be established, and evidence from Murdunna Hill (part 2) also suggests this. The two dyke suites have never yet been found by the authors exposed in intersecting relationship.

T h e a c i d d y k e s These are clearly related to the late stage residuum segregations in the

gabbro. They show no obvious preferential trend, though transverse dykes following the principal eontraetion jointing direction of the main dyke are common. They occur throughout the area mapped in detail, varying from 8 metres wide to less than two centimetres wide. They can rarely be traced inland for any distance but show up well in coastal sections, where they commonly form ribs projecting up from the lake silts bordering the main dyke. They never pass out of the dyke into the country rock. Swarms are associated with oceurrenees of pegmatoid dots and red rock segregations in the gabbro, though there is no direct connection between the two (except in the case of the limited contact developments of grano-gabbro already described).

Granophyre is the name given to these dykes, although granophyre is defined as "'quartz porphyry or fine grained porphyritic granite eharacterised by a groundmass with mierographic texture" (Dictionary of Geological Terms, 1962), and it must be noted that many of these rocks do not exactly fit the definition, because many show little micrographic texture

1205

Aufs~itze

or none at all: however, all are essentially composed of a fine quartz- plagioclase-K-felspar intergrowth, and the use of the term to embrace the entire suite is considered reasonable.

Mineralogically these dykes are simple: quartz, plagioclase and K-felspar are the major components, and sphene, epidote and apatite the consistent accessories: chlorite and hornblende may also be present, and even may be visible in handspecimens. Pretmite commonly fills irregular cavities. In contrast to the simple mineralogy there is a wide variation of texture. It is worth noting that some spheroidal granophyric dykes are chemically very deficient of potassium - - e. g. (55 238).

C h l o r i t e - b e a r i n g g r a n o p h y r e ( m i c r o - a d a m e l l i t e ) (55 249) Collected from a dykelet 8 centimetres wide on the West Ridge, it has

the character of a pink and grey microgranite in handspecimen, texturally, it is porphyritic and hypidiomorphic granular. The mineralogy is oligodase- andesine (80Yo) (An~0), eqnant and lath-shaped phenocrysts (up to 6 mm long): quartz (25~o), irregular, interlocking grains (up to 0.5 mm diam.): orthoclase (25~o) anhedral, kaolinised grains in the groundmass only: chlorite (10~o), scattered pale green fibrous aggregates, associated with muscovite (5~o) and small amounts of a green amphibole; apatite is a minor accessory.

H o r n b l e n d e - b e a r i n g g r a n o p h y r e ( m i c r o - t o n a l i t e ) (55 278) A fine pink and grey rock with sparse felspar phenocrysts, taken from

a dykelet 10 centimetres wide on the West Ridge, this reveals a base of interlocking amphibole and plagioclase needles in handspecimen. In thin section the texture appears hypidiomor-phie granular, and there is a complete absence of mieropegmatite. The mineralogy is: oligoclase-andesine (60~o) (Ane7): hornblende (20Yo), pleochroic, X = pale yellow green, Y = olive green, Z = dark green; extinction, Z^ c = 13~ elongate crystals: quartz (10Yo), anhedral, interstitial grains: titanomagnetite (10~o), massive grains, lencoxenised and limonitised, commonly associated with the hornblende: chlorite, a trace: sphene, apatite, epidote, sericite, kaolin, also in trace amounts: some K-felspar may be present in the fine base material, but it conld not be positively determined.

A similar rock from the "West Ridge" (55 277) shows prehnite mad epidote replacing the felspars: another rock from Binneringie Island (55 278) (Fig. 21), closely resembles 55 278.

F e l s i t i c g r a n o p h y r e ( m i c r o a d a m e l l i t e ) ( 5 5 2 6 5 )

From Binneringie Island, this rock is only very sparsely porphyritic, the plagioclase being set in a pink, aphanitic groundmass. In thin section it is seen to be andesine (An~l) (max. length, 1 mm): quartz, orthocIase and chlorite form the fine groundmass, while prelmite, sphene and epidote arc minor components.

1206


A similar rock from nearby (55 264) shows irregular cavities infilled with quartz, prehnite and some epidote. This rock also shows fine granophyre along the plagioelase margins.

M i e r o g r a p h i e g r a n o p h y r e ( m i c r o a d a m e l l i t e ) ( 5 5 2 6 8 ) Collected from a complex of dykelets on Binneringie Island, this dyke

is unusual in not showing a well-defined contact with the gabbro; in handspecimen it appears salmon pink, and shows scattered inclusions of

Fig. 21. P h o t o m i c r o g r a p h (55 278). - - Taken from a homblendic acid dykelet on Binneringie Island, this rook shows very little granophyrie intergrowth. It is of mierotonalite composition, and plagioclase (oligoclase-andesine JAn30] ), forms small, subhedral Fhenocrysts (centre). The groundmass is a fine aggregate of h o r n b l e n d e , c h l o r i t e , f e l s p a r , q u a r t z and t i t a n o m a g n e t -

i t e (crossed nicols X 25).

ore. The thin section (Fig. 22) reveals the predominant texture to be a mierographie intergrowth of quartz and felspar: there are some stubby, well-formed, sericitised laths of oligoelase (An~9) set in this granophyre. Orthoelase, heavily kaolinised, rims the plagioelase and forms the granophyric intergrowth, with quartz: the intergrowth varies from irregular to cuneiform and "myrmekitie". Titanomagnetite in irregular grains, fibrous and platy hornblende grains formed by alteration of this ore mineral, chlorite, apatite, serieite and kaolin make up the remainder of the rock.

A similar rock from nearby (55 '262) contains brown biotite (an alteration product) and aceessory sphene.

S p h e r u l i t i c g r a n o p h y r e ( m i e r o a d a m e l l i t e ) (55282) Taken from the margin of an acid dyke on Binneringie Peninsula, nearly

two metres wide, this is a fine-grained, pink aphanite, with little mega-

1207

Aufs~itze

Fig. 22. P h o t o m i c r o g r a p h (55 268). - - Granophyre with abundant micrographic intergrowth, from a complex of dykelets on Binneringie Island. Of micro- adamellitie composition: q u a r t z - - white, o r t h o e l a s e - - grey, and

p 1 a g i o c 1 a s e twinned grey (crossed nicols, X 68).

Fig. 28. Spherulitic granophyre from the largest internal dyke (55 284): handspecimen showing the radiating, fibrous internal structure of the spheroids.

1208

O. J. H. MCCALL and R. PEEItS - - Geology of the Binneringie Dyke

scopie evidence of spherulitie texture. The thin section revealed radiating spherules up to 8 m m diameter. The mineralogy is: oligoclase (AnzT), anhedral grains forming interlocking mosaics with quartz and K-felspar; kaolinised serieitised and dusted with Iimonite: quartz, anhedral grains (0.Smm diam.): orthoelase in irregular areas, associated with quartz: muscovite, chlorite (some spherulitic aggregates). The spherules are not themselves easy to determine mineralogically: however, X-ray diffraeto-

Fig. ~4. P h o t o m i e r o g r a p h (55 2.38). - - Showing a single spherule in the spherulitie g-ranopbyre. The radiating fibres (of a I p h a q u a r t z ?) are somewhat hazily outlined and show undulose extinction. The finely granular ground-

mass is a mixture of q u a r t z and o 1 i g o e I a s e (crossed nieols, X 10).

metric study suggests that they are alpha quartz. This study was based on a speeimen from the centre of the same dyke (55 288) (Fig. "24) whi& appears much more spherulitie in handspecimen. The cores of the spherules tend to weather out (Fig. 28) leaving small circular voids. In places tile spherules have weathered out of the dyke, and are scattered all over the surface.

M i c r o g r a p h i c s p h e r u l i t i e g r a n o p h y r e (55287) This specimen was collected from the largest granophyre dyke on

Birmeringie Peninsula, more than 8 metres wide. It appears in handspecimen as a salmon pink, aphanitie rock, devoid of spherulitie texture. In thin section (Fig. 25) it shows the same mineralogy as the spherulitie

dykes ; not surprisingly for it seems to represent a westwards continuation of the dyke from which 55 288 and 55 282 were collected. The textm-e is, however, quite different, compatible with the extreme width of the dyke. Frond-like spherules, showing imperfeet form due to interference by neigh-

1209

Aufsiitze

bouring spherules, are set in an irregularly micrographic base, which locally has a regular cuneiform character.

B a s a l t i c o l i v i n e - d o l e r i t e d y k e s These are found on Binneringie Peninsula (Fig. 9.6), where a swarm cuts

the gabbro against which they are chilled. Plagioelase is the dominant mineral in these dyke rocks, bowlingite pseudomorphs after olivine are common, especially as phenocrysts in the chilled phases, and ferroaugite is the usual pyroxene, occurring in varying amounts. I t is altered to

Fig. 25. P h o t o m i c r o g r a p h (55 287). - - Very coarse material from the core of the largest internal dyke, granophyre with a texture verging on granitic,

though crude spheroids are evident (crossed nieols, X 68).

chlorite, hornblende, biotite and bastite. Titanomagnetite is ubiquitous accessory. Textures vary considerably according to degree of &filing, those of the chilled reeks being of trachytie and porphyritic character and the unchilled reeks subophitie.

P o r p h y r i t i c , c h i l l e d b a s a l t i c o l i v i n e - d o l e r i t e (55280) Taken from the margin of the largest of these dykes on Binneringie

Peninsula (8 metres wide), this specimen includes a &flied zone of dark grey aphanite 2 mm wide and the gabbro country rock, which is in no way abnormal or altered. The thin section reveals phenoerysts of plagioelase (10~o) (Ana,) ( lmm long), showing flow alignment: euhedral olivine and a groundmass, consisting of intergranular pyroxene, plagioclase laths, chlorite and ore: the extreme margin is brownish coloured, and slightly translucent: it seems to be a devitrified basaltic glass, now converted to a eryptocrystalline aggregate of extreme fineness of texture.

1210

G. J. H. MCCALL and IR. PEERS - - Geology of the Binneringie Dyke

Fig. 26. Basaltic dolerite dyke normal to the trend of the main dyke, cutting gabbro on Binneringie Peninsula (small island, south side). The tensional cross- jointing due to cooling shrinkage is clearly seen, running normal to the dyke

walls (scale in inches).

Fig. 27. P h o t o m i c r o g r a p h (55 288). - - A porphyritic, fine-grained basaltic dolerite from a small dyke, showing a b o w 1 i n g i t e pseudomorph of an euhedral o 1 i v i n e phenocryst: also a rounded vesicle infilled wi th chalcedony. The laths are p 1 a g i o c 1 a s e (grey), and a u g i t e forms the small interstitial grains (grey), associated wi th opaque t i tanomagneti te grains (plane polarised

light, X 68).

1211

Aufsiitze

Fig. 28. Vertical layering in gabbro, showing swirling deviations, on the north side of Binneringie Island, near to die northern contact with the granite country

lOOk,

Fig. 29. Layering structure from the same locality, showing complications simulating cross bedding of festoon type. The contact is behind the hammer.

1212


F i n e - g r a i n e d b a s a l t i c d o l e r i t e (55229) This specimen comes from the centre of the same dyke: it has a hypidio-

morphic granular texture, not ophitic: there are some pyroxene phenocrysts but this rock is not markedly porphyritic. Andesine (88~o) (Ana4) forms

Fig. 80. Vertically disposed layering (locality as above) showing grading structure, simulating graded bedding. Several of the bands show clark minerals (py-roxenes) concentrated towards the left hand side of the band, and grading into a plagioclasie layer of anorthositic composition towards the right. The sharp cut-off between the binary, graded units is apparent, separating each unit from a repetition of the same graded sequence. The north contact of the dyke is towards the right: the plagioclasic layer, which is thinner than the mafic layer, is thus

consistently towards the contact.

anhedral and subhedral grains: ferroaugite (Ca3~Mgz3Fe~4) occurs as anhedral grains: both felspar and pyroxene have a maximum dimension of I mm. Chlorite (25~o), t i tanomagnetite (9~o), altered to leucoxene: hematite and limonite: quartz (8~) clear interstitial grains including apatite needles: biotite (lYo) and hornblende (1~o), both marginal alteration products of the pyroxene, make up the remainder of the rock.

78 Geologigche Rundschau, Bd. 60 1213

Aufs~itze

P o r p h y r i t i c f i n e - g r a i n e d b a s a l t i c o l i v i n e - d o l e r i t e (55 23s) Collected from one of the small dykes on an island on the south side

of Binneringie Peninsula (Fig. 26), one of the group of three transverse to the main dyke, this is a dark grey, aphanitic rock. Phenocrysts of euhedra] olivine (9~o), completely pseudomorphed by bowlingite, are revealed in thin section (Fig. 27): they are set in a hypidiomorphic granular base, of labradorite laths (42~o) (An~), altered to chlorite and kaolinite: ferroangite

Fig. 81. Repeated flow layering showing extraordinary regularity: this type of structure is typical of the extreme marginal development of flow layering. This was photographed in exposures on West Ridge, on the south side in the contact

z o n e .

(28Yo) (Ca3~Mg32Fe32), lath shaped grains, altered to bastite and hematite. An unusual feature of this rock is the presence of spherical vesicles, infilled with chalcedony (lYo), which also fills some irregular, interstitial spaces.

L a y e r i n g s t r u c t u r e s . N e a r B i n n e r i n g i e P e n i n s u l a a n d I s l a n d

Though the rocks of the dyke reveal little trace of cumulate textures, there are layering structures developed in them very similar to those associated with layered intrusions (WAcEa and BROWN, 1968): near to the margins of the dyke there is a broad zone just inside the chilled margin which displays an alternation between rhythmically layered rocks and massive unlayered rocks - - an association just like that described by those authors from Skaergaard, but the laye14_ng is vertically disposed except for localised swirling flexures of no great amplitude (Fig. 28). This layering

1214


appears to be a kind of rhythmic layering - - there is an alternation of pyroxenie and felsic layers, many of which show graded transitions (Fig. (80), simulating sedimentary graded bedding, and there are some occurrences of a "cross-bedding" simulation structure (Fig. 29). The graded units may, near the dyke margin, be very fine and regular, individual units less than two centimetres apart having a felsie component which stands up in finely ruled ridges, being more resistant to weathering (Fig. 81). The grades in the exposures near Birmeringie Station Homestead are consistently in the sense felspathie portion of the grade towards the margin,

There is also a form of cryptic layering in the dyke as the mineralogy changes insensibly by virtue of progressive variation in the members of isomorphic series present from wall to centre of the dyke. The only form of phase layering is represented by the transition from bronzite to ferroan pigeonite bearing rocks, but the analogy is in this case strained, while the analogy between rhythmic and cryptic layering seems well made.

Part 2

Other sections of the Binncringie Dyke

The dyke in the vicinity of Binneringie Station Homestead has been covered by reasonably detailed petrographic descriptions in Part 1. Other localities along the length of the dyke, in whi& traverses across it were made, either by both authors of Part 1 or by McCALL independently, are shown in Fig. 1. The brief descriptions given below of these traverses, taken from Sunday Soak progressively eastwards along the eastern half of the dyke, to the most easterly outcrops of all at Murdnnna Lake, intro- duce some quite new features. Petrographic detail is kept to a minimum, only unusual features being described. Little can be said about the very poorly exposed western extension through the great granite "batholith" west of Sunday Soak. The geochemical data following these descriptions cover samples from these additional traverses as well as those from the vicinity of Binneringie Peninsula and Island, described in Part 1.

S u n d a y S o a k

In the vicinity of the derelict homestead of Sunday Soak, situated at an old staging post, an excellent lake shore section was discovered. The dyke has, at this point, split into two unequal branches (Fig. 82). The wider of the two, the northern arm, is very poorly exposed where the track crosses it west of the homestead, and the only rock outcrops are of massive augite-pigeonite gabbro, with abundant mesostasis. In contrast, the southern branch, only 200 metres wide, shows complex structure, chilled margins and pronounced layering. Intermediate and acid differentiates are abundant within it. The rock at the dyke margin, in contact with the granite host rock - - the main granite of the "batholith" - - is very clark and chilled on the north side. In thin section, it reveals glomerocrystic aggregates of brnnzite, much of it inverted from magnesian pigeonite. The

1215

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O. J. H. MCCALL and t{. PEEt~S - - Geology of the Binneringie Dyke

bronzite displays talcose palimpsests of poikilitic olivine and a wavy extinction. Moving southwards through the dyke, the gabbro displays vertically disposed layering with only minor departures from the vertical attitude, in the nature of small, sinuous flexures. Layered gabbro alternates with bands of massive, unlayered gabbro. On the south margin of the outcrop of the dyke, on the shore of a small lake, the layering is associated with mineralogical grades (Fig. 88). The sense of the grading is not entirely consistent, the felsie concentration being mostly towards the centre of the

Fig. 88. Vertical, graded, rhythmic layering in the gabbro near to the south margin of the southern bran& dyke, at Sur;day Soak.

dyke (the reverse of the case near Binneringie Station Homestead), though grades in the reverse sense were noted. A most unusual feature of the Sunday Soak outcrops is the splaying out and fading of the layering, passing into massive gabbro along the strike: for a distance of about 10 metres the layering is absent, only to reappear in a complementary splaying facing in the reverse direction (Fig. 34).

Granophyre dykelets are quite numerous along the lake shore exposure of this southern bran& of the dyke, and there is also development of irregular and poorly deigned grano-gabbro, reddish coloured due to copious "red rock" residuum, as on Binneringie Island.

The chilled margins are exposed on both sides of this southern bran&, and the granite contiguous to the dyke is converted to a fine hornfels, while the chilled gabbro of the dyke contains xenolithie clots and xeno- erysts of quartz and felspar.

The exposures at Sunday Soak reproduce all the features of the exposures near Birmeringie Station Homestead, except for the basaltic dolerite internal dykes. It is obvious that, considering the narrow nature of the southern arm of the dyke at Sunday Soak, the great width of the dyke

1217

Aufs~itze

further east has no direct bearing on the development of minor internal intrusions of intermediate and acid composition.

B i n n a r o n c a R o c k

At Binnaronca Rock (Fig. 85) immediately east of tile Coolgardie- Norseman Highway at the 411 mile peg, there is a continuous exposure of the dyke across its width, where it forms a smooth pavement, not unlike the familiar granite pavements that provide catchments throughout the

Fig. 34. Flow banding {arming out and fading into massive gabbro in a pavement exposure near the south contact at Sunday Soak.

south-western part of Australia. The gabbro is exfoliated to form a thin skin, separated from the pavement beneath and mostly removed. The rock composing the residual areas of this skin is very weathered, whereas the rock of the pavement is fresh, but difficult to sample. Layering is meagrely developed in this section, the predominant rock type being massive, coarse gabbro with individual grains up to 5 mm long. The marginal rocks tend to be porphyritic, with phenoerysts up to 1 mm long and a fine, holo- crystalline groundmass. The dyke here cuts dark green metabasalts of the Archaean K a 1 g o o r 1 i e S y s t e m , unusually coarsely reerystallised.

Thin sections reveal a marginal zone of chilled brortzite gabbro on the north side, passing into coarse gabbro: the marginal phases carrying bronzite extend quite a distance into the dyke at the southern margin, and include a bronzite-gabbro preserving fresh olivine, described below. Two granophyre dykes were noted. Intermediate and acid segregations are, however, rather scarce here. The contrast between the vegetation on and off the dyke is never better seen than at Binnaronea rock. The tall eucalypt trees form a wall along the contact, cutting out abruptly and completely over the gabbro of the dyke, which is covered only by bush vegetation.

1218

G. J. H. MCCALL and R. Pszas - - Geology of the Binneringie Dyke

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but is of a typically marginal character, strongly chilled. It is seen, in thin section, to contain bronzite phenocrysts up to 1 mm long, many forming

1219'

Aufsatze

glomerocrystic clusters and showing wavy extinction. Enclosed in them poildlitically are inclusions of olivine (Fig. 86), only partly converted to aggregates of talc and specks of opaque iron oxide. Some fresh olivine does actually project out from the bronzite, so it cannot be regarded as having been entirely protected from reaction with the excess silica, represented in abundant quartz-felspar granophyric intergrowth in the

Fig. 86. P h ~ t o m i e r o g r a p h (56 032). - - Olivine-bronzite-quartz nofite- gabbro fromBinnaronca Rock, showing unaltered l~oikilitic olivine (darker grey) included in b ronzite glomeroerysts (lighter grey), set in a sub-ophitie groundmass of lamellar twinned calcie plagioclase laths (whitish grey) and minor inter-

stitial granophyre (X 60, crossed nicols).

finer base, by "armour plating". This is a disequilibrium assemblage, produced in a rock that has suffered moderately rapid cooling. The plagioclase in this rock is within the andesine range and is strongly zoned. The groundmass has a finely sub-ophitie texture. There is a trace of uralitisation evident, biotite is also aggregated around the pyroxenes, and magnetite is an accessory mineral component. Rocks of this unusual character are of academic interest because the mineral paragenesis virtually reproduces the reaction series of BOWEN, modified by BAXaTH (1962), within the limits of a single thin section.

The coarse gabbros forming the centre of the dyke at Binnaronea Rock tend to be plagioclasic, and approach the character of anorthosites. The felspar is labradorite zoned to andesine or, more rarely, andesine: but,

1220

G. J. H. McCALL and R. PEEns - - Geology of the Binneringie Dyke

even when the latter is the case, the character of the rock is not characteristically dioritic, primary hornblende being scarce or entirely lacking and the chemistry being basic rather than intermediate.

G o w a n H i l l

The Binneringie dyke attains its greatest width of more than 9. miles (8.9, kin) at Cowan Hill (Fig. 87). The dyke here is not simple, and appears to be either:

a) bifurcating westwards, with the southern arm immediately tailing off, whereas the northern arm continues westwards to Binnaronea rock; or

b) swelling at this point, and incorporating a substantial horse of country rock.

Either explanation is compatible with the existence of a considerable area of granophyre, granophyre breccia and weathered metasediments within the thickness of the dyke, hemmed in by an arcuate chilled margin of the gabbro at its eastern termination. The western termination of this horse is not exposed, being obscured by sand plain cover. To the east of this horse the gabbro extends across the full width of two miles (8.2 kin).

The outer chilled margins and contacts with the country rock are not exposed, coarse gabbro being the first rock encountered on both sides of the dyke, but the abrupt vegetation change delineates the contact. The chilled margin against the internal horse has the normal character of chilled zones throughout the length of the dyke, displaying bronzite glomero- trysts (inverted from the magnesian pigeonite), set in a finely sub-ophitie groundmass. Magmatic layering is abundantly developed, being mostly vertically disposed. It is mostly developed close to the contact, but throughout this section occurs at intervals in the body of the dyke, forming narrow zones separated by bands of massive gabbro. Localised departures from the vertical attitude of the layering are recorded, having the form of sigmoid flexures. The attitude of the layering is not consistent with a canoe-shaped internal structure, such as is characteristic of the Jimberiana Dyke to the south of Lake Cowan (CAMPBELL, 1966), though the manner in which the layering curves around the termination of the horse does produce a form of canoe-shaped structure. The nature of the horse suggests that it is not an uppermost, acid, canoe-shaped phase layer.

The gabbro at the dyke centre is plagioclasic, approaching anorthositic composition, and is extremely coarse textured. A very few intermediate and acid dykes and dykelets, some quite coarse textured, and also one basaltic olivine-dolerite dykelet, were recognised in this section, within the coarse gabbro. Irregular patches of finer dolerite are also present near to the south margin, about one hundred metres into the dyke from that margin.

In view of the unusual character of the dyke at Cowan Hill, the petrography of the component rocks is described and discussed in some detail:

1221

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C. J. H. MCCALL and R. PEERS - - Geology of the Binneringie Dyke

G a b b r o i e R e e k s 56006, a n o r t h o s i t i c g a b b r o . Collected about one hundred metres from tile north contact, this consists

of calcie plagioclase laths, mostly labradorite (c. An55), zoned and unzoned, and considerably serieitised: augite, and ehloritie (partly bastitie?) relict bronzite. The texture is coarsely sub-ophitic, the plagioelase crystals being up to 2.5 mm long. The pyroxene occurs as grains interstitial to and moulded onto the plagioelase laths: it is only very meagrely represented in this plagioelasic rock. Pigeonite has not been recognised. Granophyre mesostasis is abundant, and skeletal titanomagnetite a sparse accessory,

56007, a u g i t e - p i g e o n i t e g a b b r o Differing from the specimen described above in the abundance of

pyroxene, and presence of pigeonite, this rock, collected from a point further into the dyke, shows unusual vermiform inclusions of pyroxene in the plagioelase: a texture suggestive of exsolution. There are traces of altered bronzite showing vermiform exsolution inclusions of angite. The felspar is zoned, where it is in contact with the granophyre mesostasis: where it is not situated close to this mesostasis it tends to be unzoned. This relationship, characteristic of rocks throughout the Binneringie Dyke, suggests that the zoning is a product of some interaction between the mesostasis and unzoned ealcie plagioelase. The augite shows d i a 11 a g e parting. The mineralogy of this rock does suggest the possibility that pigeonite and bronzite are not completely antipathetic (see Part 1). However, optical recognition of pigeonite is not ahvays eonfirmed by more detailed mineralogical study, for some sections of augite give optical figures resembling those of pigeonite. If all three pyroxenes a r e present, it would imply disequilibrium within the limits of a single thin section, for, according to the theory of pigeonite-bronzite inversion, the three mineral phases should not coexist.

56 012 and 56 089, norite-gabhros from the marginal zone, close to the central horse, show no unusual features for such contact gabbros, but another such specimen, 56 088, displays abundant red-brown biotite flakes, peripheral to the marginal ttralite zone of the clinopyroxenes.

56 016, a dolerite, comes from the irregular patch close to the southern margin of the dyke. It is not unlike the material of the chilled margins of the dyke, but does not display glomerocrystic textures and is separated from the dyke margin by a tract of coarse anorthositic gabbros. Sparse bronzite phenoerysts (N 1.5 mm long), showing traces of augite exsolution and fringed by augite, are set in a sub-ophitic groundmass composed of poorly formed augite grains, labradorite (An~4 in the core of zoned laths), titanomagnetite grains, and interstitial quartz grains. Traces of bowlingite suggest that this was an olivine dolerite, but no fresh olivine is preserved. The pyroxenes are marginally uralitised.

No trace of cumulate texture has been reeognised in the gabbros of Cowan Hill, despite the suggestion of eanoe-shaped structure of the contact between the gabbroie rocks and those of the central horse.

1223

Aufs~tze

G r a n o p h y r e d y k e s a n d d y k e l e t s Three occurrences of granophyre dykelets were sampled from the

northern branch of the dyke.

56009, f e l s i t i c g r a n o p h y r e This is taken from a fine granophyre dykelet, which, like the white

felsite of the Salt Well section (p. 1286), represents the extreme albitie end stage differentiates: poorly formed, kaolinised plagioclase grains up to 0.5 mm long, are aggregated with anhedral quartz and granophyre mesostasis of quartz and kaolinised K-felspar. The plagioelase shows traces of albite twinning, and is identified as albite (An~). A few larger laths of orthoelase showing Carlsbad, but not albite twinning are also present. Chlorite, epidote, sphene, biotite, serieite, a trace of a grennish amphibole, and specks of titanomagnetite, complete the mineral assemblage. A similar rock, 56 048, differs only in being cut by veins of prehnite and fan-shaped aggregates of chlorite.

B a s a l t i c d o l e r i t e One small body of basaltic dolerite was reeognised in the form of a

boulder train across the surface of the gabbro, to the west of Cowan Hill summit beacon.

56048, b a s a l t i c o l i v i n e - d o l e r i t e A dense ro& (S.G. = 8.05), this is a black aphanite porphyry, dis-

playing obscure ferromagnesian phenocrysts (<--1 mm diameter). In thin section, olivine (var. chrysolite) is seen to form euhedral to subhedral phenoerysts (partly altered to tale, bowlingite), set in a fine base of long, narrow dinopyroxene laths, many curved, tubular and of branching, swallow tailed (arborescent) form (Fig. 88) - - evidence for supercooling. Traces of green uralite form rims to these pyroxenes, which are augite. There are subordinate, long, narrow laths of labradorite (Ans0) and skeletal, rounded or octahedral magnetite grains associated with the pyroxene in the fine base. Despite the modal predominance of olivine and the lack of modal plagioclase, the chemical composition is not picritic (Table 1).

R o c k s of t h e c e n t r a l h o r s e The rocks of the horse, enclosed by the slightly erenulate chilled margin

of the gabbro, include: a) fine grained granophyre, similar to the rock composing many minor

internal dykelets within the Binneringie Dyke (see Part 1). b) xenolithic granophyre, containing inclusions ranging from faint pat-

ches and spots, through faint nebulitie streaks, to distinct angular or rounded fragments of metasediments and altered gabbro (Figs. 89, 40). Even the distinct fragments are in a highly altered state, largely homogenised to the composition of the enclosing granophyre and partly assimilated. Many rounded and marginally resorbed fragments of gabbro display a concentric zonation, similar to that displayed by

1224

C. J. H. MCCALL and B. PEERS - - Ceology of the Binneringie Dyke

partly assimilated norite-gabbro xenoliths in the felspathie pyroxenite above the magmatic unconformity 'in 'the Jimberlana Dyke at Mt Noreott (CAMPBELL, MCCALL & TYltWHITT, 1970). Whether the gabbro xenoliths are derived from the Binneringie Dyke itself or from the older metagabbro country rock of the greenstone belts is uncertain. These breccias have been named the Cowan Hill Breccia. The fragments display swirling train patterns indicative of mobilisation.

Fig. 88. P h o t o m i c r o g r a p h (56 048). - - Pasaltic olivine dolerite from an internal dykelet. Phenoerysts of olivine are set in a felted base of swallow-tailed and tubular laths of clinopyroxene and ~ subordinate plagioelase (X 60, plane

polarised light).

e) finely and regularly laminated, hornfelsed metasediments, vertically disposed and striking along the elongation of the dyke.

d) very weathered outcrops of coarser banded metasedhnents, probably metasandstones. Traces of pebbles suggest that conglomeratic layers are also represented. The banding appears to be of a secondary Liesegang's Ring type, related to weathering. These rocks are, in fact, so weathered that microscopic examination is completely unrewarding. Some exposures of these rocks display a reticulation of thin stringers of felspar, a net-veining pattern attributed to outlying soaking effects related to the granophyre nearby.

P e t r o g r a p h y of t h e C o w a n H i l l B r e c c i a This is an extraordinary occurrence: unfortunately, the homogenised and

partly assimilated nature of the xenoliths renders study in thin section

1225

Au~s~tze

Fig. 89

Fig. 40

Fig. 39 & 40. The Cowan Hill Breccia: the rock outcrop showing angular, partly assimilated xenoliths of metasedimentary material, rounded, partly assimilated xenoliths of gabbroie or metagabbroic material, and swirling trains of vaguely defined nebulites. The white host material is granophyre (scales are given by the

camera lens cover, 2.8 inch diameter).

unrewarding. Though the fragments may show up well cn wet rock surfaces (Figs. 89, 40), the boundary between xenolith and host granophyre is difficult to detect under the microscope. About hal~ the xenoliths are metasedimentary, for their laminations are picked out by trains of opaque

1226

G. J. H. MCCALL and R. lvl~Ells - - Geology of the Binneringie Dyke

Table 1. Chemical analyses - - Rocks from the 13inneringie Dyke.

1. Binneringie Island and Peninsula

Analyst: tl. PEERS and C. E. S. DAws

Method: Standard rapid and atomic absorption (alkalis)

Serial No . . 1 2 8 4 5 Serial No . . 55 284 55 285 55 238 55 275 55 2,33

Chilled Basaltic Gabbro Rock type . bronzite Normal Spheru]itic gabbro gabbro olivine pegmatoid

dolefite segregation granophyre

Si02 . . . . TiOe A120s �9 Fe~Os . FeO MnO MgO CaO Na20 K.zO . . . . H20+ H~O-

53.9 5S,5 52.1 61.0 78.0 1.8 0.4 1.0 1.7 0.4

11.8 15.0 12.6 11.7 11.6 2.5 2,1 7.7 4.5 0.6 5.6 5,5 6.6 6.8 0.9

9.1 6.9 6.7 8.9 2.2 11.6 9.9 9.0 4.4 1.2

2.5 2,7 2.8 4.2 8.8 0.6 1.2 0.4 0.2 4.4

0.2 0.4 0.3 0.6 0.4

Total 99.6 97.6 99.2 98,5 98.0

Q . . . . 4.54 3.29 8.17 20.27 81.29 Or . . . . 8.56 7.07 2.62 1.17 26,09 Ab . . . . 21.14 22.87 28.71 35.56 27.90 An . . . . 19.20 25.26 20.51 12.49 4.I5 N e . . , , . . . . . .

Di . . . . 80.48 19.26 19.28 7.57 1.42 W o . . . . - - . . . .

Hy . . . . 13.48 15.58 11.66 11.08 5.81 O 1 . . . . - - . . . .

Mt . . . . 3.64 8.06 11.16 6.58 0,88 H m . . . . - - . . . .

I1 . . . . 3.41 0.76 1.90 8.28 0.76 MgO/MgO

+FeO .519 .745 ,842 .719 .988

minerals (it seems unlikely that they are banded rhyolites - - the only other possibil i ty - - for these are absent f rom this genera l area - - McCALL, in the press). These lamina ted enclaves tend to be of angular outline. In contrast the gabbroic enclaves t end to be rounded, and their coarse igneous texture is p reserved on wet ro& surfaces, though difficult to de tec t in thin section. Despi te the un reward ing na tu re of these thin sections, petro-

122~

Aufsiitze

2. Cowan Hill (Continuation of Table I)

Analyst: W.R. O'BEIRNE J. GRAHAM Method: X. Pt. F. X. It. F.

Serial No . . . 6 7 8 9 10 Rock No. , . 56 088 56 010 56 014 56 048 56 009

Basaltie Granophyre Chilled Normal Cowan olivine Rock type. bronzite (internal

gabbro gabbro Breccia dolerite dyke,) (internal dyke)

SiO2 . . . . 5"2.80 TiO~ o.48 AlzOz 18.06 F%Oa 1.00 FeO 7.18 MnO 0.17 MgO 7.50 CaO 9.74 Na~O 2.58 K20 . . . . 1.07 H~O + 0.58 H20- 0.08

54.86 65.65 50.88 67.15 0.81 0.72 0.68 0,48

19.58 15.20 19..54 18,70 1.81 0.84 1.9'8 ,59 6.89 4.42 8.87 2,85 0.13 0.08 0'.17 0.05 8.56 2.82 18.85 1,88 8.74 8.17 8.10 2,48 8.24 4.07 1.60 6.80 1,22 8.07 0.45 0.08 0.81 1.53 1.48 2.42 0.14 0.15 0.12 0.!.4

100.19 I00.78 100.72 99.62 98.29

Q . . . . - - 8.78 17.21 - - 21.82 Or . . . . 6.82 7.28 18.20 2.66 0.71 Ab . . . . 21.80 27.88 84.8.9 18.52 58.24 An . . . . 84.51 85.23 14.10 25.67 020 N e . . . . - - . . . .

Di . . . . 11.28 6.79 1.89 11.69 2.51 W o . . . . - - . . . .

Hy . . . . 22.10 15.98 11.90 87.16 7.87 Ol . . . . 2.20 -- -- 8.28 --

Mt . . . . 1.45 1.90 0.50 2.87 0,85 H m . . . o - - . . . .

t l . . . . 0.91 1.5.5 1.87 1.19 0.91

MgO/MgO .675 .529 .581 .760 .599 q- F e O .

graphic descriptions are g iven be low to provide some sort of p e r m a n e n t record of these unusual rocks.

56014, g r a n o p h y r e b r e c c i a

A granoblast ie aggregate , of a seriate character, of quartz , kaolinised orthoclase and subordinate ol igoclase-andesine (Ana,); contains chlorite

1228


3. Binnaronca rock (Continuation of Table 1)

Analyst: W. 19t. O'BEIRNE J. GRAHAM Method: X . R . F . X . R . F .

Serial No . . 11 12 18 14 15 Rock No, . 56 024 56 021 56 026 56 082 56 019

Ro& t y p e .

Olivine- Country rock Chilled Normal bronzite Granophyre

, metabasah bronzite- gabbro quartz (internal Archaean gabbro gabbro dyke)

(marginal)

SiOz 46.42 50.38 52.71 53.33 72.18 TiOe 1.92 0.62 0.98 0.53 0.'25 AleO 3 15.52 14.90 18.86 12.69 13.05 Fe.203 4.21 0.22 0.74 0.62 0.69 FeO . . . . 9.99 8.29 8.50 8.80 1.81 MnO 0.80 0.15 0.14 0.15 0.04 MgO 6.94 9.09 4.54 10.14 1.00 CaO 10.59 9.04 8.67 8.34 1.14 Na.zO 2.68 2.32 3.38 2.10 4.04 KeO . . . . 0.28 4.51 1.10 0.86 3.30 H2 O§ 1.09 0.52 1.07 1.24 0.99 H20- 0.17 0.07 0.17 0.11 0.16

100.11 100.11 100.86 98.91 98.65

Q . . . . - - - - - - 1.98 81.04 *) Or . . . . 1.66 26.69 6.54 5.08 19.51 Ab . . . . 22.65 4.76 28.56 17.74 34.14 An . . . . 29.45 16.88 82.97 22.62 5.65 Ne . . . . - - 8.04 - - - - - - Di . . . . 18.89 22.79 8.40 15.25 - - W o . . . . - - . . . .

Hy . . . . 5.90 - - 18.58 82.95 4.91 Ol . . . . 10.53 18.86 1.64 - - - - Mt . . . . 6.11 0.32 1.07 1.95 1.60 H i T I . . . . - - . . . .

I1 . . . . 3.65 1.18 1.87 0.87 0.47 MgO/MgO

+ F e O . .651 .677 .528 .690 .577

*) C 0.74

a f t e r h o r n b l e n d e ( the out l ines of which are p r e s e r v e d in sparse pa l impses ts ) , carbonate a n d sericite. In a n o t h e r t h in sec t ion the l a rge ly h o m o g e n i s e d xenol i th ie f r a g m e n t s show t races of an or ig inal semischis t character , al-

t h o u g h t hey a re n o w of g r a n o p h y r e compos i t ion : q u a r t z is m o r e a b u n d a n t ,

79 G e o l o g i s c h e R u n d s c h a u , B d . 60 | 2 2 9

Au~s~itze

4. Murdunna Hill and E. Murdunna Lake (Continuation of Table 1)

A n a l y s t : W . R . O ' B E I I ~ N E

Method: X. R.F. X . R . F .

Serial No . . 16 17 18 19 20 Rock N o . . 55 989 55 977 55 998 55 997 A 55 997 B

Normal Chilled Granophyre Rock type . bronzite Basaltic Normal gabbro

gabbro dolerite gabbro (contaminated?) (internal dyke)

sio.~ TiO~ A120a Fe~O3 FeO , MnO MgO CaO N%O K~O . . . . H20 +

H . 2 0 -

58,16 47.50 58.2,8 58.80 64.28 0,59 2.59 0.82 0.91 1.12

16.21 14.85 16.88 18.08 18.86 1.45 4.88 2.88 8.08 2.57 6.62 9.71 6.90 7.55 3.79 0.15 0.20 0.15 0,19 0.07 8.29 6.67 5.89 4.70 2.65 9:59 8.42 8.44 5.54 5.58 2.26 3.50 2.89 3.38 4.74 1.82 0.24 1.29 0.29 0.09 0.98 2.06 1.18 8.07 1.80 0.22 0.54 0.20 0.29 0.15

100.79 101.11 100.10 100.88 100,60

Q . . . . 1.09 - - 8.41 9.47 *) 21.52 Or . 7.80 1.48 7.66 1,75 0.55 Ab . . . . 19.10 29.58 24.42 28,56 40.06 An . . . . 80.15 24.08 27.71 27.54 16.24

Ne . . . , . . . . . Di . . . . 14.11 14.42 11.57 - - 9.06

Wo . . . . - - . . . . Hy . . . . 24.17 10,44 18.85 21.95 5.36 O1 . . . . - - 6 . 6 6 - - - - - -

Mt . . . . 2.11 7.00 4.10 4.40 3.73

H m , , . o . . . . .

I1 . . . . 1.12 4.92 1.55 1.73 2.14

MgO/MgO .727 .689 .677 .603 .738 -}- FeO . .

*) C 2.09

a n d flakes of a l t e r ed b r o w n b io t i t e are p r o m i n e n t . A t h i r d sl ide shows a coarse pa tch iness , areas of i l l -def ined finely g r anu l a r ma te r i a l (homo- gen i sed xenol i ths?) b e i n g enc losed in a l ac ing n e t w o r k of m u c h m o r e coarsely crys ta l l i sed g r a n o p h y r e ; e u h e d r a l s p h e n e gra ins are p r o m i n e n t . A n o t h e r

t h i n sec t ion shows fea tu res typ ica l of t h e m i n o r ac id a n d i n t e r m e d i a t e

1230


5. Salt well section (Continuation of Table 1)

Analyst: J, GRAHAM Method: X. R. F.

Serial No . . 21 22 28 24 25 Rock N o . . 62 280 62 287 62 299 62 304 62 285

Chilled Normal Pink White Granophyre Rock type . bronzite (internal

gabbro gabbro granophyre felsite dyke)

SiO_~ 52.76 56.11 61.00 67.18 67.55 TiO2 0.35 0.52 1.42 0.27 1.06 A120~ 17.27 15.67 12.64 14.88 13.83 Fe2Oz . 0.97 1.34 4.49 1.18 4.85 FeO . . . . 5.64 7,34 5.82 1.04 3.29 MnO 0.14 0.17 0.16 0.05 0.11 MgO 7.16 6.69 1.73 2,46 1.38 CaO 11.43 9.56 3.89 0.12 2.59 NaeO 1.62 2.20 1,74 7.00 5.00 K~O . . . . 1.07 1.76 2.88 1.41 2.30 H~O + 1.60 1.84 1.59 1.89 1,80 HzO- 0.09 0.18 0.18 0.27 0.18

100.10 100.88 97.52 97.75 102.9,0 **)

Q . . . . 4.16 4.98 26.95 16,92 *) 23.15 Or . . . . 6,82 10.41 17.08 8,34 18.60 Ab . . . . 18.69 18.59 14.70 59,16 42.25 An . . . . 86.64 27.68 18.18 0.59 7.10 N e . . . . - - . . . .

Di . . . . 16.26 16.19 0.97 - - 4.37 W o . . . . - - . . . .

Hy . . . . 19.24 20.60 8.75 6.78 1.87 O l . . . . - - . . . . Mt . . . . 1.40 1.94 6.15 1.71 7.08 Hm . . . . - - . . . . II . . . . 0.66 0.98 2.69 0.51 1.01 MgO/MgO

+ F e O . .719 .651 .587 .933 .903

*) C 1.61 **) F oor summation, specimen to be re-analysed.

segregat ions ha the dyke at Binner ingie Is land; a few large crystals are set in a t lne granophyr ie base, p roduc ing a porphyr i t ic texture; the large crystals are, however , the stable phases of the gabbro not these daaracteris- tie of the granophyre : in this case they are serieit ised laths of labradori te up to 2 m m long. The host g ranophyre consists of fresh ol igoclase-andesine and al tered K-felspar laths (up to 0.5 m m long), which are in themselves

I231

Aufsiitze

6. Sunday Soak (Continuation of Table 1)

Analyst: J. GRAHAM Method: X. R. F.

Serial No. . 26 27 28 29 30 81 Rock N o . . 62 310 62 823 62 324 62 315 62 314 62 312

Chilled Chilled Chilled Gabbro Grano- Rock type . bronzite bronzite bronzito Normal pegmatoid phyre

gabbro gabbro gabbro gabbro segrega- (internal tion dyke)

SiO2 . . 53.98 55.79 54.87 65.02 68.04 67.29 TiO 2 . . 0.46 0.55 0.55 0.56 1,19 0.61 AleO 3 . . 15.84 13.80 13.69 16.15 13.11 12.65 F e c Q . 1.35 1.84 1.17 1.76 4.88 1.22 FeO . . . . 6.46 6.78 7.18 6.80 5.32 2.94 MnO . . 0.15 0.15 0.15 0.14 0.12 0.04 MgO 6.73 7.9'5 7.80 5.27 1.37 1.43 CaO . . 10.17 7.65 8.04 8.22 3.55 2.37 Nar 2.30 2.40 2.20 3.10 1.79 4.80 K20 . . . . 1.01 1.40 1.34 1.76 3.57 2.40 HeO + . . 1.64 1.39 1.57 2.00 1.41 1.21 H20- 0.Ii 0.16 0.II 0.16 0.2.5 0.14

100.15 99.90 98.17 101.44 99.58 96.60 *)

Q . . . . 4.60 6.79 7.07 4.90 27.60 2,5.12 Or . . . . 5.97 8.28 7.92 10.41 21.11 14.19 Ab . . . . 19.48 20.28 18.59 26.20 15.12 36.84 An . . . . 29.87 22.71 28.48 24.91 17.14 8.09

Di . . . . 16.76 12,35 18,87 12.99 0.81 8.02 W o . . . . , ~ . . . . .

Hy . . . . . . . 18.90 24.17 23.29 16.24 7.29 5.52 O1 , . . . - - . . . . . Mt . . . . 1.95 2.66 1.69 2.55 7.00 1.76 H m . . . . - - . . . . .

I1 . . . . 0.87 1.04 1.04 1.06 2.26 1.15

MgO/MgO .684 .718 .675 .648 .528 .577 q- F e O .

*) Poor summation, specimen to be re-analysed.

inse t in a f inely g ranob las t i c b a s e of t h e same minera ls , quar tz , f ine g ranophyre , chlorite, c a r b o n a t e a n d sphene . T h e la rge l a b r a d o r i t e insets are r e g a r d e d as xenocrys ts de r i ved f rom pa r t l y ass imi la ted g a b b r o mate r ia l : t h e y a re la rge ly a l t e red to c a r b o n a t e a n d p r e h n i t e .

I n a n o t h e r spec imen , 56 084, t he ba se is a n a g g r e g a t e of ep ido te g ranu-

1232


les, pools of chlorite associated with euhedral sphene, prehnite, patches of finely granoblastic quartz and kaoliil_ised felspar, and patches of quite coarse granophyre.

In specimen 56 087, circular pools of granoblastic, even-sized (~< 1 mm diameter) quartz grains are set in a base of felspar (mainly altered plagioclase), aggregates of opaque minerals resembling chromosomes, sphene, chlorite, epidote and biotite. Traces of laminated xenoliths are also visible - - ghosts picked out by trains of opaque granules.

M e t a s e d i m e n t s o f t h e h o r s e

Unrewarding as the granophyric breccias are for microscopic study so far as the xcnotithic material is concerned, it is apparent that a considerable amount of metasedimentary material is included in them. That the horse is not entirely composed of intermediate acid magmatic roe_ks with xenolithie enclaves is confirmed by the identification of considerable outcrops of metasediments in the centre of the horse. The granophyric material appears to be less xenolithie close to the contact with the gabbro, whereas further in to the horse it becomes xenolithie to an increasing extent. The extreme centre of the horse displays very scattered and poor outcrops of net-veined and unveined rnetasediments. Two specimens of these metasediments were studied in thin section:

62330, a g r e y , f l i n t y , b a n d e d h o r n f e l s

This rock is very regularly and finely laminated and there is just a suggestion of graded bedding. The microtexture is not unlike that of many fine metasiltstones of the older Archaean metamorphic sedimentary series, quartz and oligoclase being identifiable, though very finely erystallised, and considerably kaolinised: bleached biotite is also a prominent constituent. The texture is not like that of any igneous rock, not even resembling that of a devitrified, banded rhyolite, which the rock does slightly resemble in handspeeimen. Though this rock-type forms lengthy outcrops, recent careful re-examination suggests that these hornfelses form xenoliths in the granophyre.

62881, k a o l i n i s e d m e t a s a n d s t o n e (?)

Collected from a point to the south of the median line of the horse (where the specimen described above was collected), this is a greyish, medium- to fine-textured rock, preserving minute elastic granules ~) and traces of larger pebbles, faintly discernible. In spite of the intense kaolinisation and iron oxide staining, the character of a pebbly porphyroid conglo- merate, of a type widely developed near Widgiemooltha (McCALL, 1969) is recognised.

T h e o r i g i n of t h e C o w a n H i l l B r e c c i a The nature of the horse is complex - - far more so than the similar

enclosure of acid rocks within the gabbro in the Salt Well section (described below). The following facts seem to be established:

~) In the sedimentologieal sense.

1233

Aufs~itze

a) the gabbro is chilled against the granophyre, which must have e i t h e r been solidified prior to this chilling o r was a cooler molten mass when the initial main-stage solidification of the gabbro occurred: o r e 1 s e, the chilling was against metasedimentary country rock, later invaded by mobile residual acid phases of the gabbro, intimately along fractures: and affected by penetrative metasomatism moving outwards from these fractures, and even partly mobilised.

b) The horse consists partly of metasediments. c) The granophyre invasion aeeompanied by assimilative effects and

mobilisation spread inwards into the country rock horse from the margins.

d) Net-vMning by a quartz-felspar pegmatite fluid (soaking) was the immediate forerurmer of this metasomatism: the extreme, outlying effect was hornfelsing.

e) The breccia is at least in part a mobilisation breecia (i. e. magmatic), otherwise the xenoliths could not show alignment in swirling trains.

f) Besides sedimentary material, gabbro or metagabbro was incorpor- ated.

T h e S a l t W e l l s e c t i o n

An excellent section across virtually the entire width of the dyke was discovered south of the Salt Well, on the track south from the Paris Mine, on the shore of Lake Gowan (Fig. 4i). Here, the greater part of the thickness of the dyke is composed of acid rocks, in spite of the fact that un- relieved gabbro is exposed across it less than a mile to the west. The eoun- try rocks are tremolitie ultrabasie rocks (metamorphosed pieritie voleanies?), forming part of the ophiolite belt that passes through the Paris Mine. Jasper bars (opalised fine pelitie metasediments) are also represented in the country rock.

The south contact of the gabbro is well-exposed on a small island 9,00 metres off the coast. Extreme chilling at the contact and the development of very regular layering in the gabbro are features of this contact. Across the gap on the mainland coast, the gabbro is mostly massive, but further north it is again chilled, against an internal mass of pink granophyre. At both these chilled contacts the usual glomeroporphyritie bronzitie norite-gabbro marginal rock type is developed. At the internal &flied contact the pink granophyre sends out apophyses into the chilled gabbro, and these are chilled against the gabbro. The contacts on both sides of the internal area of acid reeks are parallel to the elongation of the dyke, and there is a further contact trending the same direction within the acid rocks, separating a mass of pink granophyre to the south from a mass of white felsite to the north. The pink granophyre shows ovoid xenolithie patches of dark rock near its contact with the gabbro and also coarse felsie dots: the nature of this xenolithie zone suggests limited assimilative relationships, but there is no expanse of granophyre breccia as at Gowan Hill.

Though it may have had its origins in a bifureation of the dyke, the enclosure of acid rocks in the Salt Well section seems to be largely in the

1234

G. J. H. McCALL and R. PEERS -- Geology of the Binneringie Dyke

nature of an internal concentration of magmatie acid rocks, rather than of brecciated and metasomatieally soaked, and mobilised metasediments as seem to be the case at Cowan Hill.

The petrography of the gabbroie rocks is similar to that of previously described suites. The aeid rocks are however of interest:

INFERRED POSITION OF DYKE M A R G I N y

.;'7~-!F:4).. ~ - j ~ E OABBRO

+ ~ '

-F 4- + I~HITE STRUCTURELESS FELSITE

§ + +

. . . . . . . -+---I%RoPT C0.TACT

, Jr + ,~ffPINK ORANOPHYRE

~- ~- _~'g~i~OARK ENCLAVES

CHILLED AGAINST THE OABBRO />/~ ~ ,~PROSRESSIVE CHILLING

T i

i'

MASSIVE COARSE eABBRO

"! / r / \'.~/

i, PEe MATOI D CLOT (/V

Ac~o DYKES 'v

CHILLED MARGIN

--TRAVERSE SKETCH MAP- ~ OJAGP, A~.~MATIC SECTION-

Fig. 41. Sketch map and section of the dyke in the Salt Well section, south of the Paris Mine.

P i n k g r a n o p h y r e

This forms a mass of quite uniform, massive reeks, of monotonous mineralogy and showing only slightly patchiness and textural variations. Of the four specimens examined, 62 299, the coarsest textured, is the most typical of the mass as a whole (Fig. 42). The microtexture is holoerystalline, hypidiomorphie granular. There is a decussate arrangement of stubby laths of albite (Ana) up to 8 mm long: these do not mesh and are set in a fine,

1235

Aufs~itze

allotriomorphie granular base of granophyre (quartz/K-felspar intergrowth); augite showing chevron twinning but largely altered to a brownish uralite; biotite, chlorite, epidote, sphene, prehnite and flmcnite (?). The rock is punctuated by coarse patches of quartz, prehnite and chlorite, in which the quartz shows good crystal form approaching dipyramidal. The felspar in this rock is considerably saussuritised, the saussurite containing fine grains of epidote, quite distinct from the larger grains not associated with saus-

Fig. 4g. P h o t o m i c r o g r a p h (62299). Pink granophyre, showing albite (turbid) fringed by chlorite (dark), in turn haloed by granophyre, consisting of a quartz/K-felspar intergrowth. Salt Well section (;K 60, plane polarised light).

suritisation. 62 800 shows fine areas in which vestiges of laminae now picked out by opaque granules can be detected: these areas are set in a coarser aggregate of normal granophyre, they may represent nebulitie metasedimentary xenoliths, in an advanced state of assimilation. 62 802 displays felspars concentrated in pools, suggestive of cavity fillings. The opaque minerals in this thin section show considerable marginal alteration of the skeletal aggregates to chlorite. 62 801 displays within the thin section the sharp contact between the pink granophyre and the white felsite. Though the pink granophyre at the contact is not as coarse as in some outcrops, it is not noticeably chilled.

W h i t e f e l s i t e This is a fine aphanite. A typical specimen 62 803 (Fig. 48), displays a

fine, allotriomorphic granular texture: the main components are quartz and albite. Pools of chlorite, specks 9f opaque minerals and minute traces

1236


of other ferromagnesian minerals make up the remainder, except for pools of a khaki-green isotropic mineral (possibly devitrified glass) inset with minute spheroids (or "clocks") of chlorite. This specimen was probably hemierystalline, containing subordinate interstitial glass, but 62 804 is holoerystalline. It has a fine, seriate texture, being composed mostly of equidimensional grains. Larger, sub-rounded grains of microeline and plagioclase (An~), up to 0.15 mm diameter, are set in a finer granoblastie

Fig. 48. P h o t o m i c r o g r a p h (62 808). - - White felsite, showing the fine allotriomorphie granular texture. The component minerals are albite, (ragged, lamellar twinned laths), quartz and minute, equidimensional grains of mierocline

(streaky grey) (X 60, crossed nicols).

base of similar material and quartz (0.5--0.02 mm diameter), which is the most abundant component of this rock. Unlike the granophyre, K-felspar is dominant over plagioelase in the felsite. There is interstitial chlorite and brownish uralite present in the fine base. Coarser patches of quartz, kaolinised K-felspar and fresh plagioclase associated with subordinate granophyre punctuate this felsite specimen.

The felsite from the contact with the pink granophyre seen in 69, 801, is fine-textured and consists of an allotriomorphic, equigranular aggregate of kaolinised felspar grains and subordinate quartz. This fine base is inset with a few poorly formed grains of ehequerboard albite (Anl4) and micro- cline. The albite is up to 8 mm diameter, and is sieved by triangular areas of chlorite, reminiscent of textures in volcanic rocks. Chlorite and amphibole are subordinate interstitial accessories in the fine base.

The phenomenon of double chilling, at the contact between the pink granophyre and the gabbro, is difficult to explain, especially in the light

1237

Aufsiitze

of the lack of sedimentary material or breceias in the acid rock enclave (in contrast to the ease at Cowan Hill). Another explanation besides that invoking an area of metasedimeuts or meta-igneous country rocks against which the gabbro was chilled before segregation of the granophyre and felsite must perhaps be= sought. So far, no answer other than the possibility that the gabbro was chilled against a coder, but as yet uncrystalhsed acid magma, has been found. The phenomenon of double chilling relations displayed by gabbroic rocks and their residual acid differentiates is not well documented, though chilling relationships in such associations are discussed by KING (1965).

Minor internal intrusions of granophyre that cut the gabbroic rocks in this section are not described in detail, as they are similar to those profus- ely developed near Binneringie Station homestead (see Part 1). Tke north contact og the gabbro in this section is obscured by sand plain.

The area between this section and Cowan Hill is extremely poorly exposed and inaccessible, but, where the dyke forms a great west facing Y-pattern as it opens up around yet another acid rock enclosure within it there is a large tract of granophyres, felsites and breceias. Unfortunately, though this is probably the largest and most complex of these acid rock enclosures, situated between very narrow arms of the gabbro, the lack of outcrop would require very detailed mapping before the exact boundaries between the various rock types could be delineated.

M u r d u n n a H i l l

Murdunna Hill is an area of rough, inland outcrop, with poor lake shore exposures on the south margin of the dyke. The only place where a good section across the entire dyke is exposed is along a sand-ttoored creek that cuts right through the dyke (Fig. 44). Neither margin is exposed here, the northern being obscured by massive P 1 a n t a g e n o t (Eocene) spongolites, capped by lateritic ironstone, and the southern by fossiliferous limestones of the same sequence (Hoo~En, 1959; Cocx~taN, 1968). The gabbroie rocks are similar to those at Binneringie Peninsula and Island (see Part 1), but angite-pigeonite gabbros outcrop throughout virtually the entire section. There are many coarse, pegmatoid dots, closely associated with minor internal intrusions of granophyre as on Binneringie Island. There is also a quite wide body of dolerite cutting the gabbro: while this could be regarded as simply a fine area in the gabbro, the field evidence suggests that it is a wide internal dyke and the major element geochemistry supports this view. There are no grano-gabbro segregations exposed in this section. Layering is pronounced near both margins and intermittently developed throughout the thickness of the dyke: it maintains a steep to vertical attitude with only minor deviations.

The only unusual rock type is the dolerite mentioned above.

55975, d o l e r i t e A fine grained, non-porphyritic, phaneritic rock, this shows, in thin

section, an ophitie to sub-ophitie texture: laths of plagioclase (Ar157_49) are

1238


intergrown with augite and pigeonite. There is a considerable development of a greenish alteration product (bowlingite?) in the section, but no palimpsest olivine could be detected. Quartz makes up 5Yo (modally), and titano-magnetite 5~o. Granophyre mesostasis is also sparsely evident.

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W

M u r d u n n a L a k e

The dyke continues eastwards from the Murdunna Hill section until it meets the shore of a northwards embayment of Murdurma Lake (Fig. 45). Here an excellent section is exposed, including the &filed rocks at the northern contact. Layering, with finely spaced graded rhythms, is well- developed near to this eontaet. The country rocks are exposed a few hundred feet north of the contact, being meridionally trending and steeply disposed schists (Hooper records them as staurolite schists). As one moves

1239

Aufs~itze

into the dyke a very similar set of reeks to those exposed on Birmeringie Peninsula and Island are seen, the same differentiation pattern from the margin inwards being recognised in microscopic study. There are, many small acid dykelets, and one of these was seen to be composite - - about a foot wide, it consists of a dioritie outer zone and a finer felsitie core, the two rock types being separated by an abrupt contact. No basaltic doterite dykes were reeognised in this section. Some pegmatoid segregations in this section display spectacular textures and mineralogy, but no description of them can be given here because of lack of space.

On the east side of the embayment the dyke outcrops again in a peninsula. Extensive grano-gabbro segregation is developed here, as usual showing red coloured on aeeount of the copious red rock mesostasis. The dyke continues eastwards across the lake floor in low reefs, the last exposure being very poor outcrops on the extreme east shore of Murdurma Lake. No further exposures are known to the east of this point, and, in any case, the dyke soon becomes obscured by the Proterozoie quartzites of the Woodline Beds (SoFouL~S, 1966).

The only other point of interest is the presence of a very striking body of felspar porphyry to the north of the dyke on the peninsula. It contains a green amphibole of a type similar to the hornblende in the dioritic ("porphyrite") minor intrusions of the Kalgoorlie area, and is believed to be pro-metamorphic country rock. The felspars of this porphyry show a strikingly developed flow alignment.

T h e d y k e w e s t o f S u n d a y S o a k ( B r e m e r l : t a n g e - M t H o l l a n d - H y d e n )

Though SOFOULIS (1968) notes outcrops of the dyke in the vicinity of the Bremer Range metamorphies, it was found impossible, with the resources available, to locate these outcrops in featureless hummocky sandplain overlying the granite "batholith". The continuation of the dyke, albeit eonsiderably narrowed, through the granite to Mt Holland is however confirmed both by air-photography and by an aeromagnetic survey - - these dykes produce a strong positive anomaly in most cases, though some negative anomalies follow zones with the same alignment through the granite. There is, in fact, evidence from small scale generalised maps of the eastern Wheat Belt and air-photography that the dyke continues much further west, passing just north of Hyden, and outcropping strongly at the 192-mile peg on the Hyden-Kondinin road. The gabbro specimens collected here (62 274 from the margin, 62 275 from the centre of the dyke) are rather more uralitised than most of the gabbros of the dyke in its eastern exposures, but no more so than some of the granophyre-rich specimens from those exposures. The specimen taken from the marginal zone is fresh, and shows traces of early-crystallised bronzite, now entirely decomposed. The rock specimen from the centre of the dyke shows the same characteristic brown uralite that is a eommon but minor deuteric (?) constituent of the gabbros in the eastern exposures. Both specimens contain

1240

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abundant interstitial granophyre. The character of these rocks is consistent with them being part of the same early Proterozoic suite as the Birmeringie Dyke: in all probablility the dyke does continue to this point, but further to the east the granite is cut by a swarm of small short dykes on the same trend, and there is no trace of any large "master" dyke (though an immense dyke is recorded a long way to the west on the same trend line, meeting the Darling Fault near Cookernup). Most of the Wheat Belt dyke swarm is probably of this early Proterozoic age; the fact that a north- westerly trend comes in prominently at the longitude of York is not necessarily evidence of a different age but rather of a different fracture pattern control.

Structure and manner of intrusion of the Binneringie Dyke

The dyke is about 100 times as long as it is wide (at the point of maximum width). There are no features suggesting that it is not a true dyke. Though layering is present, it is vertically disposed (but for local swirls) and there is no trace of eumulate texture associated with the layering, despite the fact that graded rhythms and cross-bedding simulations testify to the action of convection currents. The attitude of the dyke walls is far too steep for magmatic sedimentation to have deposited cumulus as the curent moved down the dyke margins. The only suggestion of cumulate texture is found in the clustering of orthopyroxenes in the marginal rocks, in the form of glomerocrysts, but the poikilitic nature of many of these crystals seems to discount cumulus origin, even though the texture resembles that of certain high-level "cumulates" of the Jimberlana Dyke (GAMFBELL, 1966). The systematic variations shown by the Harker Diagrams (see below) provide evidence against cumulate deposition - - Harker Diagrams of the Jimberlana Dyke cumulates show no distinct trends. As has been noted above the zoning of some of the felspars in all the gabbroic rocks is regarded as less conclusive evidence against cumulate origin.

We seem to be seeing the result of magmatie injection of a consistent basaltic magma stem, of tholeiitic character, along a fracture more than 200 miles (820 km) long. The stem magma was apparently available throughout this are of the crust. Tile exact preeess that produced the layering, the: graded rhythms, and the cross-bedding simulation is obscure, but it seems to involve convection stirring (WAcEn & DEER, 1989), and the layering is preferentially developed in the marginal zone. I t is extremely regular here and less so as one moves into the dyke. There is no consistent sense to the graded rhythms throughout the dyke. Some process of elutriation has been considered, but it might rather be a process involving simply rhythmic alternations in the mineral phase being precipitated in conjunction with current movement down the dyke wall and possibly intermittent cessation of precipitation, marked by the abrupt boundary between successive graded rhythms. These rhythmic alternations could be related to pressure fluctuations coupled with supersaturation rhythms (an idea that was finding favour with WAeER (1963) just prior

1242

G. J. H. MCCALL and t{. P~ERS - - Geology of the Binneringie Dyke

to his death). There does not appear to be any record of su& complex rhythmic layering patterns from other large dykes. The immediate cause of the cross-bedding simulation must be a current scouring effect. The bifurcation and splaying out patterns, together with the transition along the strike into normal gabbro, at Sunday Soak must be explained in any satisfactory resolution of this anomaly. The regularity of the layering close to the contact and development of swMs as one moves into the dyke could be related to the slackening of convection current velocities, allowing greater turbulence, but it may simply be due to the loss of proximity of the regular contact in itself, slight irregularities in the newly formed wall of gabbro allowing more turbulence, without change in the current velocity.

This non-cumulate layering contrasts with the type represented by the Jimberlana Dyke (CAMI~ 1966), which may have suffered gravitational differentiation. Perhaps the most important lesson to be learnt from the Binneringie Dyke is that rhythmic layering, even accompanied by grading and cross-bedding simulation, does not necessarily imply gravitational sorting.

The presence of layering and the evidence of convective effects indicates a quite fluid magma at the time of emplacement. An initial squeezing up of a vast volume of magma through a tensional fracture is envisaged, the walls gradually separating until they were up to two miles (g.2 km) apart. The alternative concept of an open fracture up to miles apart waiting to receive the uprising magma seems untenable. Similarly, the fact that convective effects were operative during the first stage of erystallisation from the margin inwards suggests that there was already a wide body of magma emplaeed, and that emplacement was not piecemeal, as the erystallisation process continued. Any theory involving numerous small pulses of magma represented by the rhythmic layers is also quite untenable - - whereas a limited degree of multiple injection seems to have been the case in the Jimberlana Dyke (CAMI'BELL, 1966) and several sills of the older Archaean metamorphic, sequence in this area, the gabbroie rocks of the Binneringie Dyke appear to represent a single injection of magma. Only the granophyre and felsite masses internal to the dyke, and the minor basaltic dolerite and granophyre internal dyke and dykelet bodies appear to represent later magmatic injections.

The extension of the Widgiemooltha dyke suite over thousands of square miles of the Western Australian Shield clearly shows that the emplacement closely followed a period of tension in a regional scale, a crustal deformation of geotectonie rather than purely local significance. Why the giant Birmeringie Dyke adopted a quite normal form and its giant neighbour, the Jimberlana Dyke, adopted an internal lopolithic form is quite unexplained - - both were emplaced up regional tension fractures of the same system. Possible explanations have been suggested by CAMPBELI~, MCCALL & TYltWI-IITT (1970).

The internal dyke and dykelets are believed to have filled openings resultant on cooling, contraction of the main dyke. The abundance of transverse dykes normal to the elongation of the main dyke supports the

1243

Aufs~tze

idea that cooling contraction produced many of the joints utilised by the minor internal intrusions, for dykes tend to produce such transverse joints on cooling because of the pattern of the retreating isotherms. How- ever, many of the minor internal dykes and dykelets are oblique and longitudinal, not transverse, normal to the elongation of the main dyke.

These minor intrusions were emplaced under the same depth of burial conditions as the coarse gabbros. Whether they represent feeders to an elongated volcanic province that at one time existed at the surface, displaying volcanoes and fissure vents erupting basaltic, intermediate and acid lavas, is not certain. The whole dyke complex may conceivably have been "blind". However, such a relationship to surface volcanics seems very likely, for these minor intrnsives display characteristic volcanic textures, vesicles, devitrified glass, axiolites, longulites and spherulites. Of course, if such a volcanic field ever existed, it has long since been removed during the immense span of denudation (>-- 2400 m. y.) since the early Proterozoic. This complex dyke certainly does give us an inkling of the probable nature of the deep roots of some volcanic fields, displaying a contrast of eruptive types within a small area, derived from differentiation of a single tholeiitie stem magma. Great dykes such as the Birmeringie Dyke cannot conceivably have broken surface bodily - - before the magma reached the surface it will have commenced to cool and crystallise, and stresses consequent on contraction during crystallisation and slow uprise of the magma through the near-surface zone of brittle fracture will have produced rupturing of the already crystallised hood to the rising pluton and also of the roof of country rocks. Such fracturing will have allowed magma to escape rapidly to feed surface volcanics through narrow conduits: the resultant dykes will have had the narrow dimensions and fine, aphanitic textures characteristic of volcanic feeders. This fracturing will have introduced an entirely new eruptive regime: instead of the great mass of magma being pushed slowly, bodily upwards, it will have been arrested and completed its solidification while static: the continuing magmatic pressures from beneath will have been dissipated by periodic spurts of the last part of the melts rapidly up the narrow conduits to the surface. In this conjectured relationship we may find an important principle involved: that 1 a r g e p 1 u t o n s a r e e s s e n t i a l l y a r e f l e c t i o n o f a p r e c u r s o r p h a s e of b o d i l y u p w a r d m o v e m e n t , p r i o r t o d i s p e r s e d e r u p - t i o n t h r o u g h s m a l l s c a l e c o n d u i t s t o s u p p l y s u r f a c e v o l c a n i c s : t h e y a r e s e l d o m , i f e v e r , t h e d i r e c t f e e - d e r s o f s u r f a c e v o l c a n i c s .

The presence in these minor internal intrusions of vesicles, devitrified glass, and typically volcanic textures has an important bearing on the problem of controls on development of phaneric ("plutonic") and aphanitic ("hypabyssal" volcanic) textures, in igneous rocks. The type of texture displayed by the olivine basalt internal dykes is one that has customarily been explained in volcanic rocks by invoking the transport of intratellurie- ally-crystallised phenocrysts of olivine rapidly to the surface, the remainder of the rock solidifying under quite different conditions of rapid &filing.

1244

G e o l o g i s c h e lk t~ndsc} iau~ B a n d 6 0 G . J . t t . I 'vlCGAI ,L c~ I~. P L E I ' , S - f e x l l a fe l

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Here we have such textures in rocks that cannot have suffered transport to quite a different physical environment. The possibility remains that the olivines crystallised deep within the magma reservoir, and the remainder of the magma was chilled as it was squirted into narrow fracture conduits. But we must not lose sight of the alternative possibility: that the control on such textures is purely physico-chemical and there is no involvement of two environments of crystallisation. In this case, it may be that the contrast between phenocrysts and finely crystalline or partly glassy base stems from cooling in one single environment, that is cooling sufficiently rapid for there to be a passage over a critical threshold between the metastable and labile zones before the entire melt has crystallised under metastable conditions to produce a coarse-grained even-textured gabbro. It seems that the threshold between the metastable and the labile zones, respectively characterised by sparse and prolific nucleation is very abrupt, at least in the case of basaltic melts, and in this fact lies the key to the origin of many porphyritic textures in basalt, not a two stage crystallisation history involving two crystallisation environments.

Petrogenesis

E x s o t u t i o n a n d i n v e r s i o n in p y r o x e n e s

E x s o 1 u t i o n may occur in any pyroxene, converting it to a more ordered structure (POLI)E~VAA~T and HESS, 1951, pp. 481--8). The process can only go to completion with slow cooling, and is not reflected in the pyroxenes of "hypabyssa!" or volcanic rocks, characterised by rapid cooling. Complete separation of the (Ca, Mg, Fe)~Si~O6 and (Mg, Fe)~Si.20~ components is never attained. Such umnixing tends towards minimising the energy of the surfaces of separation between the phases: the orientation and thickness of exsolution lamellae are influenced by the ease of access of the ions to the exsolution plane. The lamellae are usually narrow, and consist of pyroxenes in the diopside-hedenbergite range.

A u g i t e e x s o l u t i o n b e h a v i o u r

Magnesian angites may exsolve pyroxene lamellae of pigeonite composition above the point of intersection of the pigeoniteJorthopyroxene inversion curve (DEER, HOWIE • Z U S S M A N , 1968, p. 149). The lamellae are parallel to (001), and are easily distinguished on account of their different orientation to orthopyroxene exsolution lamellae, which are parallel to (100) below the point of intersection. Hypersthene (or bronzite) may be exsolved parallel to (100) by augite that has initially suffered pigeonite exsolution. This exsolution in augite is not recognised in gabbros and associated rocks of the Binneringie Dyke: there is no trace of either lamellae or graphic intergrowths in the augite, despite it being a major component of all the gabbroie rocks. Diallage structure is recognised, a fine parting parallel to (001), and this ceuld represent a sub-microscopic lamellar exsolution. The apparent inhibition of exsolution in augite in rocks of the Binneringie Dyke is unexplained.

Aufsiitze

P i g e o n i t e e x s o l u t i o n b e h a v i o u r Magnesian pigeonite crystallised just above the pigeonite-orthopyroxene

inversion temperature tends to invert to orthopyroxene (provided the cooling rate is moderately slow). Pigeonite contains about 9.5~o CaO, two thirds of which is accomnted for by exsolution of augite lamellae parallel to (001) just prior to inversion, leaving the orthopyroxene inversion product capable of holding the remaining one third of the lime content of the pigeonite in its lattice structure. However, with slightly more rapid cooling, the readjustment cannot take place in a regular fashion, and the orthopyroxene exsolution takes the form of irregular blebs of the augite, instead of regular lamellae - - the "graphic" intergrowth of WALKER • POLDERVAART (1949): it may also take the form of lamellae along the (001) cleavage of the pigeonite, but with the preservation ef the original clinopyroxene twin composition planes of the pigeonite, producing an orthopyroxene in which the herringbone twin pattern of the pigeonite is preserved in palimpsest. This condition is only possible because the b and c crystallographic axes of the exsolved lamellar clinopyroxene and the orthopyroxene are common to one another after inversion. In rare cases augite is exsolved parallel to (100) of the orthopyroxene, after inversion. Inversion of pigeonite generally cccurs at a Mg/Fe ratio of 70:80. Inversion above and below this ratio have been recorded (volatiles and trace components can affect the value?), but this value does fit in well with the composition of the inverted pyroxenes present in the ro&s of the Binneringie Dyke. Inversion from magnesian pigeonite to bronzite is the only relationship recoguised. It is noted in the marginal bronzite-bearing noritie gabbros, and also in some intermediate grano-gabbro segregations. Near the dyke margin, irregular blebs of diopsidic-augite indicate exsolution under conditions of comparatively rapid cooling in the zone of chilling. Further in to the dyke, cooling was somewhat slower and the exsolution lamellae are regular, and parallel to (001). The pigeonite from the normal gabbro of the central area of the dyke is more ferroan, and thus not subject to lamellar exsolution, having crystallised too far above the inversion curve for exsolution to have occurred: it has thus remained stable in its uninverted form.

C h i l l i n g

The term chilling is used throughout this account in a relative sense, for the really strongly chilled rocks, the basaltic dolerites, are much more strongly chilled than the rocks from the extreme margin of the dyke, which do show exsolution effects despite the fact that they are clearly the chilled phases of the coarse gabbro.

P r i m a r y o r t h o p y r o x e n e c r y s t a l l i s a t i o n

The orthopyroxene phenocrysts that show cores of clear orthopyroxene devoid of exsolution lamellae, and marginal zones eharacterised by exsolution lamellae, can only be reasonably interpreted as due to primary orthooyroxene crystallisation, direct from the melt, though in some cases,


they may show evidenee of reaction between the residual melt und early formed divines, as is shown by the tale-bowlingite-magnetite polkilitie inclusions, pseudomorphing olivine, and rare preservation of fresh, poikilitie inclusions of olivine itself. The rim of orthopyroxene displayhag exsolution lamellae is consistent with the normal order of {erromagnesima erystallisation, olivine, then bronzite, and then pigeonite being the successive stable mineral phases, the last having suffered inversion exsolution.

C r y s t a l l i s a t i o n h y s t o r y

The nature of the chilled margin zones and the layered structure leaves no doubt that this dyke was emplaeed in an almost wholly liquid state. The olivine could conceivably have erystallised prior to emplacement of the magma. The bronzite crystallisation seems to have been largely controlled by physical conditions pertaining dose to the dyke margin (evidence of exsolution patterns - - vermiform close to the margin, regular further in). Though the clear cores to some bronzite crystals could, like the olivines, have commenced to erystallise prior to emplaeement of the magma, in all probability the entire crystallisation sequenee eommeneed after emplacement of the great mass of tholeiitie melt pushed apart the margins of the dyke to their present separation.

The mineral paragenesis may be broadly represented:

Olivine Labradorite Bronzite

tqgeonite (magnesian ~ bronzite) Augite

Hornblende Biotite Residua Oligoclase Orthoclase

Titanomagnetite Chlorite Uralite Epidote Sphene Quartz Pretmite

What alteration processes are evident in the gabbroic rocks are believed to be due to:

a) Deuteric processes: pyroxene to uralite, biotite and bastite. Chloritisation. Sericitisation (especially of felspars). Olivine to tale, bowlingite and opaque iron oxides, Veining by quartz, epidote, prehnite, and

carbonate. Part of the kaolinisation (?).

Aufsatze

b) Weathering: Part of the koalinisation (or all?). Oxidation of opaque minerals to hematite and

limonite.

There is no evidence at all of regional metamorphism affecting the rocks of the dyke, which are accepted as postmetamorphic.

The uralitisation and other deuteric processes may be closely related to the concentration of acid residuum, for rocks containing copious granophyre mesostasis characteristically display advanced uralitisation etc., and it is noted that gabbroie fraction of the grano-gabbros characteristically shows similar alteration patterns.

G e o c h e m i c a l d a t a

The results of 80 major element chemical analyses are given in Table 1. Five are by standard rapid methods of analysis with alkali oxides determined by atomic absorption: twenty five are by X-ray fluorescent spectro- metly. C.I.P.W. norms are given with the results obtained by both methods.

Trends of differentiation from margin to centre of the dyke are shown in Figs. 46 and 47, Harker Diagrams of oxides plotted against silica percentages, and other diagrams derived from these, Harker Diagrams. The same essential trends are seen to be reproduced throughout the length of the eastern section of the dyke studied in detail (65 miles = 104 km). The intermediate and acid segregations and internal dykes and dyke!ets of the same composition, show consistent differentiation trends suggesting that they are fractionation products of the tholeiitic parent magma. The most siliceous roeks show rather more variation than the gabbroie rocks, but this is consistent with end-stage effects related to the using up of all but the Si, A1 and Na ions at slightly different stages in the fractionation sequence, consequent on slightly different initial crystallisation patterns and initial percentages in the parental magma. As already noted, the Harker Diagrams show consistent trends with increasing silica percentage, in striking contrast to the diagrams obtained by CAMPBELL (1966, p. 158) for the Jimberlana Dyke. The reason for the disparity is that cumulate sequences of layered intrusions are essentially sequences of rocks composed of cumulus and intercumulus minerals collected at the bottom of a magma chamber, and variation diagrams of this sort depend, not only on the composition of the magma from which the entire mineral assemblage was crystallised, but also on such factors as:

a) The number of cumulus minerals. b) The proportion of the cumulus mineral phases where more than one

is present. e) The variation within isomorphous series of the cumulus minerals. d) The proportion of intercumulus material (severely influenced by

adeumulus growth).

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Aufs~itze

T O T A L A L K A L I E S / S I L I C A D I A G R A M

o ~ ~ ~ o i

~o , , , , ; , , , , , , , , , ~ . . . . = , . . . . , , , ,

Fig. 48. Total alkali/silica diagram.

1 2 5 0

I \ I o / o \~%

�9 / �9 o o\oa

/ • \~t +

/~ + o *

/ �9 /

/

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B~S~Trr D~E~JT~ | C,NTER~ D*K~S~

*H,TE F~S,T~ 1 I

+ ~R~,NAL CCHrLLED~ G~S~

Fig. 49. AFM triangular diagram for the Binneringie Dyke (F = total iron as FeO).


e) The nature of the intercumulus material (dependent on the composition of the residual magma).

In the type of wall-to-centre differentiation of the Binneringie Dyke, the first two have no application, as there are no cumulus mineral phases: but, although there is no cumulus and intercumulus, very similar controls to those due to the last three factors are involved in this type of differentiation - - namely, the variation in the isomorphous series of the minerals

f

/ .,/ / / / '

js

\

Fig. 50. Differentiation trends on the AFM triangular diagram used in Fig. 49, compared for the Hawaiian, Tasmanian, Skaergaard, Dillsburg Sheet, Jimberlana

Dyke and Binneringie Dyke differentiated suites.

from margin to centre, the proportion of mesostasis to ferromagnesians and plagioelase, and the nature of this mesostasis. These effects are not, however, nearly as significant as the disturbing effects applicable to the Jimberlana Dyke cumulate sequence, and they produce only minor dis- turbanees of the progressive trends in the Harker Diagrams for the Binneringie Dyke.

The total alkalies/silica plot, following MACDONALD and KATSURA (1964) is given in Fig. 48. All but two of the analyses fall below what Yoder regards as the position of the trace of the plane of silica saturation, and these results are consistent with a tholeiitie suite. Such plots are known to be far from perfectly discriminatory between tholeiitie and alkalie basalt suites (MA.~'SON, 1967, p. 244), there being a limited amount of overlap.

1251

Aufs~itze

They have, however, proved satisfactory in the case of the Hawaiian basaltic suites, and this particular suite shows only two anomMous analyses, falling above the line. Of these, one is an anomalous analytical result the potash value is far too high for a gabbroie rock, and there is nothing to indicate it ir~ the~mode, yet it was obtained again on a check analysis, and some sort of contamination is suspected: the other is a basaltic dolerite, with very high total iron and low silica, and a high modal magnetite content.

The AFM triangular plot is given in Figs. 49 and 50. The method of plotting follows MACDONALD & KATSURA (1964) in that total iron is plotted as FeO. This seems to be more realistic than plotting only the FeO percentage. This plot displays the following characterstics:

a) The origin of the curve produced to intersect the FM line is very near to the composition of the Skaergaard olivine.

b) There is a marked correspondence between the early part of this curve and that representing the gabbroie rocks of the Lower Layered Series of the Jimberlana Dyke (CAMFZELL, 1966). The downwards extension of the curve towards the FM line, present on CAMPBELL'S curve, but absent on the Binneringie curve, reflects the lack of gravity-separated ultramalqc rocks - - picrites and bronzities - - in the Binneringie differentiation sequence.

c) There is a marked shift towards the F corner of the curves for the Upper Layered Series of the Jimberlana Dyke and for a line joining the positions of two of the three basaltic dolerites of the Binneringie Dyke, and one from the Jimberlana Dyke. These are rocks derived from late pulses of magma, and the geochemical variation could be due to a number of eanses:

(i) a new partial melting episode under slightly different physical conditions at upper mantle depth (unlikely?).

(ii) a pulse stemming from a continuously active reservoir in depth, of magma impoverished in alkalies due to the removal of volatiles.

(iii) a pulse stemming from a continuously active reservoir in depth, of magma enriched e i t h e r by sinking of ferromagnesian minerals o r , possibly, by overpreeipitation of such minerals, compensated by ionic diffusion of unwanted ions or mechanical expulsion of pore-fluids.

d) The intermediate and acid segregations, together with the internal dykes and dykelets of acid composition, all plot consistently in the manner of simple products of fraetionation. There is no indication of any complication by assimilation.

e) The differentiation curve lies below the curve for the very similarly differentiated Dillsburg tholeiitie sheet (HoTz, 1953), and those for the Hawaiian and Tasmanian tholeiitic suites (MACDONALD & KAT- SU~, 1964; EnwAm~S, 1942). The latter suite does display fayalitic and hedenbergitie granophyre differentiates, and these are not recognised in the Binneringie differentiation s equence - - thus, in the case of the Tasmanian suite of dolerites there appears to be a slightly

1s

G. J. H. MCCALL and R. I%E]RS - - Geology of the Binneringie Dyke

greater tendency towards differentiation of iron-rich residua [such as is reflected in its extreme development in the Skaergaard differention curve, with its sharp, high apex near F corner (WAGE• & D E E I %

1989)]: than in the case of the Binneringie Dyke. The apparent contrast between the Binneringie curve and the rapid tailing off of the Jimberlana Dyke curve towards the A corner may not be real

- - CAMPSELL had only 15 analyses available, and this curve relies excessively on a single anorthosite analysis: a programme of 100 major element analyses of rocks from the Jimberlana Dyke is pro- posed, and this will produce a much more realistic curve.

M a g n e s i a a n d l i m e e n r i c h m e n t o f t h e m a r g i n a l r o c k s Enrichment of the marginal rocks of this dyke in magnesia and lime

(and very slight FeO, but not total iron, enrichment) can be detected from the Harker diagrams and the diagrams constructed from them (Figs. 46 and 47). The magnesia enrichment is the most pronounced effect and can be related to two factors:

a) The modal development of magnesian olivine and orthopyroxene in the marginal rocks. It is apparent from a study of the marginal rocks that the ferromagnesians predominate modally over the felspars, and this is reflected in the contrast between the thick ferromagnesian bands and very thin felspar bands in the graded rhythmic layering: it is also evident in the textures of marginal gabbros, glomerocrysts of ferromagnesians forming the greater part of the rock. This effect is a phase proportion effect.

b) The gradational change within isomorphous series of the pyroxenes and felspars from margin towards the centre of the dyke. This effect is a cryptic effect.

This enrichment is n o t simply a h e t e r o m o r p h i c effect, due to cooling rate contrasts. If it were so, the overall chemistry of border and central rocks would not differ, despite modal contrasts. The fact that the overall chemistry d o e s differ, and significantly so, can only be due to an over-precipitation of magnesia and lime-rich minerals, ions being brought in from the still molten centre of the dyke. The most likely introduction mechanism is ionic diffusion towards the border zone. The alternative process, mechanical concentration of ferromagnesians at the dyke margins during each successive convection current flow seems unlikely, because it is quite contrary to the expectation of fluid solid suspension mechanics. If ionic diffusion did take place, there must have been a compensating process taking away unwanted ions from the border zone. Ionic diffusion is again possible, but the compensating process may have been simply one of mechanical expulsion of pore-fluid, while the magnesia and lime rich minerals over-precipitated (the process being similar to adcumulus growth). That the ions lost in this compensating were aluminium, titanium and silicon, is apparent from Figs. 46 and 47. This ionic diffusion mechanism seems to be the only answer to this anomaly other than mechanical

1253

Aufs~itze

concentration of early crystals: it is suggested purely on the basis of the field and laboratory evidence, and its theoretical feasibility remains qnes- tionable. It is regarded as the best of two unlikely explanations!

An alternative to increment from the centre of the dyke of certain ions to allow overprecipitation would be loss of certain ions to the country rocks: but this alternative seems quite untenable because of the degree of magnesia enrichment of some of the marginal rocks.

A similar marginal enrichment effect has been noted in certain small olivine-tholeiite dykes near B~,~LE~'S Workings (in the Widgiemooltha area) by MUHLING (1965). Some of these dykes measure less than one metre across and could not conceivably be convection stirred. There seems to be a pronounced modal enrichment in olivine at the margins of these small, melanocratic dykes and overprecipitation accompanied by diffusion of ions seems to be involved. However, the conclusions drawn must be regarded as tentative at this stage, for the problems of sampling accurately across the dykes are bedevilled by the bouldery state of the sub-crop, and the problem calls for the use of drill-core material, at present unavailable.

The marginal enrichment phenomenon must be of fundamental interest to petrologists, who commonly assume that chilled margins represent undifferentiated parental magma. There is evidence from both rapidly cooled tholeiitic dyke rocks at BAYLEY'S Workings and from more slowly cooled tholeiitic gabbros of the Binneringie Dyke that this is by no means a universally valid supposition. The Binneringie Dyke comprises a large volume of normal, not-enriched gabbroic material, a relatively small volume of marginal, magnesia and lime enriched gabbroic material, and a small volume of acid/intermediate differentiate material (not more than 5Yo). The composition of the parent melt cannot, in this case, be close to either marginal rocks or acid/intermediate differentiates, for the bulk of the rocks of the dyke fall between these two extremes. The "normal" augite-pigeonite gabbro that forms the greater part of the dyke, does clearly have an average major element geochemistry (Figs. 46 and 47), and must be compositionally very close to that of the parent magma of the differentiated dyke.

It is interesting to note, in conclusion, that the author's suggestion that gravitational sorting is not the whole answer to early ferromagnesian (olivine and bronzite) concentration from basic magmas, as supposed by BOWEN and accepted by petrologists, without question, the world over, is echoed in a paper by DaEVEn (1956). DREVEa, like the present author, finds the accumulation theory unsatisfactory and considers that certain picritic rocks in Greenland cannot be explained by such a process.

T h e l a t e b a s a l t i c d o l e r i t e d y k e s a n d d y k e l e t s

The late basaltic dolerite dykes and dykelets might reasonably be supposed to represent late spurts of undifferentiated parental melt from a continuously active source below, but this is not the case, whatever

1254

G. J. H. McCALL and R. P~ERS - - Geology of the Binneringie Dyke

their origin, for the geochemieal results and the nature of the pyroxenes show that they are, in faet, significantly differentiated. Silica impoverish- ment, magnesia enrichment, and total-iron enrichment are manifested by them to varying degrees. Possible modes of origin have already been suggested under geochemistry.

T h e o r i g i n o f t h e a c i d d y k e s a n d s e g r e g a t i o n s

The Binneringie Dyke eontains numerous late stage acid and intermediate segregations and acid dykes and dykelets, and it was noted prior to the present investigation that these seem to be prolifically developed where the dyke cuts the Binyarinyinna granite close to Binneringie Station homestead. The possibility that eontamination of the gabhro by granite was responsible for the presenee of these internal bodies was suggested. In faet, there appear to be three possible explanations of these bodies:

a) Assimilation of granite (and elsewhere silieeous metasediment) xenoliths, directly produced the segregations and dykes.

b) Assimilation of granite and other siliceous rooks at depth, well below that represented by the present erosion level, enhanced the amount of potential aeid residuum in the parental magma, and thus the intermediate and acid bodies refleet contamination, but indirectly - - i. e. through a process of fraetionation.

e) Simple concentration of residual material, the result of fraetionation of an uneontaminated tholeiitie melt, has been effeeted by a form of filter pressing etc., in patches and along fraetures produced during contraetion shrinkage of the main mass of gabbro.

The eomplete absence of xenoliths (excapt at Cowan Hill and the Salt Well Section) seems to rule out d i r e e t contamination. Additional mapping, sinee 1965, has revealed that there are seetors of the dyke that display a proliferation of late phase acid and intermediate segregations and dykes: and that there are others that do not. There seems to be no systematic relationship of these sectors to aeidity and basieity of the country rooks, and, indeed, the largest segregations, near the Salt Well, are situated in a seetor eharacterised by ultrabasie and basic metamorphic rooks! Likewise, the West Ridge sector shows a paueity of minor intermediate and acid bodies, yet the dyke cuts granite at this point! It must be concluded that the evidence strongly favours simple residual concentration from an uneontaminated melt, and there is nothing in the volumetrie relationships to preelude this. The small volume of intermediate and aeid rooks in the Binneringie Dyke is in aceord with the conclusion of WAGER & DEER (1989), that the maximum change in eomposition due to differentiation of basaltic magma is brought about only after erystatlisation of 75~o to 80Yo of its volume. Though there are some apparently extensive intermediate and acid concentrations such as those at the Salt Well, these have no con- tinuity along the dyke, and it is also probable that they have no significant continuation in depth, and so are volumetrieally quite insignificant eompared with the volume of gabbro.

1255

Aufs~itze

The geochemical evidence (Fig. 49) seems to be entirely compatible with a residual origin, unrelated to contamination. However, deep level contamination, though not supported or refuted by the evidence cannot be entirely discounted - - it is certainly acceptable on experimental and theoretical grounds (BowEN, 1928, p. 199).

The acid dykes and dykelets characteristically show chilled margins, indicating later intrusion than the gabbro. Except in the case of the Cowan Hill Breccia, which shows chilling of the gabbro against it, and the chilling of the gabbro against the intermediate and acid segregations in the Salt Well section, the gabbroic rocks do not show any chilling at their contacts with the acid and intermediate bodies.

The acid dykes tend to run across the length of the dyke or along it, but many are oblique. The high number of transverse dykes normal to the main elongation does suggest that the host fractures are shrinkage fractures, formed during the main stage cooling: for this is consistent with the isotherm pattern in dykes. The mechanism of emplacement is not easy to envisage, but autointrusion into a still slightly plastic host rock by the product of filter pressing is favoured. Actual tectonic squeezing, externally derived, is not required, for a dyke of this size must, at a late stage in its solidification, have set up within it many internal differentials of pressure (both hydrostatic and directed), able to initiate migration and concentration of mobile, residual pore-space fluids.

The dykes never extend beyond the chilled margin of the host dyke, a fact which emphasises the internal nature of the stresses that produced the host fractures.

The spherulitic granophyres were probably wholly liquid at the time of magma emplacement up the fractures. They and the white granophyre of the Salt Well section are unusual, and are clearly the extreme differentiates of the sequence.

The contact effect of the acid dykes may be of two kinds: a) They may be intruded into gabbro nearly completely crystallised at

the time, and so suffered pronounced chilling at their margins, and supplied no increment of mobile acid fluids to the gabbro country rock. Thermal effects caused reerystatlisation of the granophyre already present as mesostasis to this gabbro close to the acid dyke, and thus such proximal gabbros show more spectacular micrographic intergrowths in their mesostasis than is usual. These enhanced crystallisations of the mesostasis may show chilled margins against the enclosing gabbro, having apparently been heated and melted to form a small molten pocket within the gabbro ?

b) They may be intruded into gabbro that was more plastic, that is less completely crystallised at the time. Fluids emanating from the acid dyke penetrated this gabbro mixing with the existing mesostasis, which was melted.. This effect produces considerable increase in the volume of mesostasis in the gabbro, and again it is much more coarsely crystallised and marginally chilled against the enclosing gabbro.

1256

G. J. H. MCCALL and It. PEERS - - Geology of the Binneringie Dyke

The pegmatoid segregations have been discussed elsewhere, their origin being considered to be largely residual, and unrelated to contamination. They are believed to reflect patchy concentration of acid residuum rather than concentration as discrete fracture fillings (producing dykes). The trapping of the mobile, volatile-rich residual fraction may reflect uneven erystallisation of the gabbro, early areas of erystallisation forming a roof beneath which the mobile residuum is trapped and then concentrated by the filter pressing mechanism already described. Many of the smaller pegmatoid segregations are distributed close to acid dykes, but there seems to be no possibility that they are the source of the dyke material, which is of quite different character and is chilled against them, so the conclusion is drawn that they are due to either mobile fractions moving out from the dykes into the gabbro or mobilisation of the mesostasis of the gabbro by the thermal effect of the dykes. Thus, they are patchy concentrations of material emanating from the dykes or mobilised by the contact effects of the dykes described above. In contrast, the larger segregations show no association with internal acid dykes, and appear to have an origin quite apart from the eontact effect of these dykes. The presence of the ealeic plagioelase of the gabbros in many of the large segregations, in the form of large inset laths, does suggest that these segregations commenced erystallisation at much the same time as the host gabbro, not much later. They have the character of variants of the primary gabbro crystallisation, highly modified by deuterie effects related to an excess of residual fluid, not of intermediate segregations introduced into the gabbro host discretely, and long after it crystallised.

The Binneringie Dyke dicussed in the context of layered intrusions

The Binneringie Dyke is a dyke intrusion of unusual dimensions that displays a number of unusual variations on a rather eommonplaee theme - - that is to say, the suite of rocks contains no uncommon minerals and has the tholeiitie composition characteristic of innumerable intrusions throughout the world. Perhaps the greatest significance of this study is to be found in the fact that we have in the Binneringie Dyke layering patterns very similar to those of the Skaergaard Intrusion, Greenland ( W A G E R ~Xl DEER, 1989; W'AGER • BROWN, 1968), the type example of magma &amber differentiation - - but in the ease of the Binneringie Dyke these structures are entirely in the vertieal dimension. We can be certain that this dyke has suffered, at the most, rotation of a degree or so during recent epeirogenic downwarping of the continental surface towards the south, and thus the vertical attitude of the layering is the attitude in which it was formed.

The Binneringie Dyke not only shows layering, but it reproduces the rhythmic mineralogical grades and cross-bedding simulation characteristic of crystallisation in a convection current stirred magma chamber such as Skaergaard. Sub-horizontal layering is predominant in Skaergaard, and is, in fact an integral part of WACER'S definition of layered intrusions

1257

Aufsatze

(1968). Yet it seems that the Binneringie Dyke must be regarded as a layered intrusion and the definition requires some modification.

Layered intrusions are those which are large enough to have undergone convective effects during crystallisation. This seems to be one feature that is shared by them all, apart from layering. They include large shallow and steep funnel shaped lopoliths, large sills, smaller funnel shaped intrusions, large dykes with a very steep funnel shaped cross section, and keeled-ship intrusions of the type of the Muskox Intrusion, Canada (IRvINE & SMITH, 1967), which combine features of large dykes and those of shallow-funnel shaped lopoliths.

The Skaergaard Intrusion displays a marginal zone that is charaeterised by an external tranquil zone, representing magma chilled prior to the onset of differentiation and thus of parental composition: and an irmer banded zone, characterised by steeply inclined rhythmic banding. It is to this inner, banded marginal zone, and not to the subhorizontally layered inner part of the Skaergaard Intrusion (forming the bulk of the complex) that the analogy must be drawn. There is no sign of an appreciable tranquil zone in the ease of the Binneringie Dyke - - the layering extends to within a few feet of the contact - - but the Binneringie Dyke, like the Skaergaard Intrusion, is believed to have crystallised its entire layered sequence from a single emplacement of magma. In the banded, marginal zone of the Skaergaard Intrusion, WAGER & BROWN recognise suspended crystals caught up in the eongelation process, to which no connotation of gravitational accumulation can be applied: they call these early crystals p r i m o- c r y s t s. The layering, which in part of the marginal banded zone of Skaergaard is near-vertical, is attributed by these authors to congelation of successive samples of the residual magma, moved down the margin by eonvedtion currents. The rocks of this banded division are not cumulates, but like cumulates they do not in their composition reflect the composition of the contemporaneous liquid, because they contain varying amounts of primocrysts. These rocks are referred to as congelation cumulates: in the formation of such rocks the crystal/liquid ratio was not a function of packing or adcumulus growth, but complexly related to heat loss from the eonvecting magma.

In the case of the Binneringie Dyke, we appear to be dealing with similar effects. Whether the terms p r i m o c r y s t and e o n g e 1 a t i o n e u m u 1 a t e s should be applied remains an open question, but we appear to be dealing with erystallisation of rare olivine, more abundant bronzite and even more abundant magnesian pigeonite crystals at an early stage in the congelation of marginal eonveetion currents in situ, as they sweep down the interface between already congealed rock and residual liquid in the centre of the magma chamber. It may be that these early crystals should be regarded as primocrysts and the much finer sub- ophitic gronndmass representative of the liquid fraction. Further into the dyke either plagioclase felspar alone or felspar and clinopyroxene may be regarded as primocrysts, and the liquid fraction crystallised much granophyric mesostasis. Though the analogy with the marginal banded

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G. J. H. McCALL and R. PEERS - - Geology of the Binnefingie Dyke

zone of Skaergaard seems close; especially as there is a similar alternation of rhythmically layered zones and massive zones (reflecting, according to WAGER & BROWN, 1968 rhythms of convection alternating with static conditions) it seems very doubtful, in the case of the Binneringie Dyke, whether there was any in e c h a n i c a 1 enrichment of certain mineral phases at the dyke margin - - rather, a mechanism of overprecipitation and diffusive ionic adjustment, combined with mechanical expulsion of pore- space fluid seems applicable to explain the composition of the magnesia and lime enriched marginal rocks, (though admittedly, there seem to be theoretical objections to such a process).

WAGER • BROWN (1968) stress the complex role of marginal heat loss in producing the layered rocks that they call congelation cumulates. Such complex factors may have operated during the congelation of the rocks of the Binneringie Dyke. Indeed, the margin to centre crystallisation characteristic of all layered intrusions may be directly related to the pattern of heat loss. In a conveeting magma chamber, the congelation takes place only in the marginal zone and never in the heart of the residual liquid reservoir: the marginal congelation may occur at the lower part of the lateral wall, on the floor or at the roof, but always it is marginal, and tends to move progressively inwards, as the volume of the residual liquid reservoir diminishes. In other words, the convection seems to set up a pattern that keeps the centre of the liquid reservoir above freezing point of any of the equilibrium mineral phases corresponding to its composition, whereas the marginal zone is continuously (though intermittently) brought just below this critical temperature. This regime operates throughout the entire sequence of magma chamber crystallisation of a single magmatic injection system such as Skaergaard or the Binneringie Dyke, despite the fact that the composition of the residual liquid (the "contemporaneous" liquid of WAGER & BROWN, 1968) is progressively changing, consequent on the progressive removal of crystals, in the marginal zone. The thermal conductivity of the country recks may well have a considerable role in determining the pattern of crystaflisation in a convecting magma chamber - - indeed the rare roof-downwards crystallisation (recognised at Skaergaard and in the Golden Mile Dolerite~ a layered sill at Kalgoorlie, Western Australia: G. D. BARTRAM, verbal communication) may owe something to this factor.

In the case of the Binneringie Dyke the heat loss must have been outwards through the contact. The crystallisation took place within vertically moving convection currents, and all the rhythmic and cryptic effects of the Skaergaard complex were simulated, even to the rhythmic grading. The control on this structure appears to have been entirely convective and related to the pattern of heat loss, and the rhythms appear to owe nothing to gravitational sorting of heavy and light pyroxenes and felspars. The origin of the rhythmic alternations must be sought in some form of complex physico-chemical periodicity, producing simply rhythms of precipitation within the moving currents. Pressure rhythms and supersaturation

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rhythms seem most likely to provide the answer to this enigmatic pattern of vertically disposed, non-cumulate graded and "cross-bedded" rhythms.

The existence of such rhythms, which in the ease of Skaergaard appear to be less well developed, and to have been overshadowed by the predominance of sub-horizontal graded rhythms attributed to convection and gravitational sorting, does, perhaps, suggest that gravitational sorting may have been overemphasised in studies of some layered intrusions that display graded rhythms. This ds also suggested by the recognition of consistently reversed plagioelase-pyroxene rhythmic grades in the Lower Layered Series of the nearby Jimberlana Dyke (CAMPBELL, McCALL & TYRWItlTT, 1970).

The author is not, of course, discounting accumulation. The textures of the lower pyroxenitic layers of the Jimberlana Dyke are quite obviously due to a form of magmatie sedimentation. But the actual grading within rhythmic layers may be due to physico-chemical controls on precipitation rather than density contrasts between the mineral phases. It is significant that the norite-gabbros of the Binneringie Dyke resemble closely, mineralogically, texturally and chemically, the upper cumulates of the Jimberlana Dyke, part of a basin shaped layered complex. WAOER & BROWN (1968)stress the fact that l a c k of m a n i f e s t c u m u l a t e t e x t u r e m u s t n o t b e t a k e n as e v i d e n c e a g a i n s t a c c u - m u 1 a t e o r i g i n. The senior author, working on the Jimberlana Dyke, has observed that, where three or more cumulus minerals have come out together, the textures are virtually indistinguishable from those of the non-cumulate rocks of the Binneringie Dyke. The suggestion is advanced that many of these "high level cumulates" may, in truth, be co n- g e 1 a t i o n c u m u 1 a t e s, formed of three or more primocrysts mad a liquid fraction, but not true cumulates at all. Crystallisation of three or more of the main mineral phases with which the contemporaneous liquid was saturated may have occurred simutaneously in a shallowing residual liquid reservoir, less convection stirred than before (when it was larger and the lower layers of the complex crystallised out). Coming out together, these phases may have rapidly formed a mush and suffered no significant gravitational sorting: though the inhibition in gravitational sorting may have been due to the increasing viscosity of the contemporaneous liquid. Marginal crystallisation would still be operative, and limited convective effects. Under these conditions, additional growth of primocrysts in a very similar manner to adcumulus growth and development of intercumulus margins (CAMPBELL, 1966) could occur under conditions of slow precipitation.

This suggestion, admittedly tentative, may have a considerable application to the problem of great layered intrusions which display convective effects but little or no trace of cumulate textures. In such com- plexes, gravitational sorting may be of little significance, perhaps because of the viscosity of the melt or the rapidity of simultaneous precipitation of mineral phases. The controls on the magma chamber differentiation would be the pattern of convection and heat loss, and the progressive change in

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composition of the residual liquid as layered sequences of eongelation cumulates, consisting of primoerysts and congealed liquid solidified. Gravit- ational sorting would be of little significance; rhythmicity would be due to physicochemical causes.

Patches of granophyric rocks in the Skaergaard Intrusion have been variously attributed to simple residua of gabbroie fractionation and to assimilation of gneisses. Triangular diagram plots of quartz, albite and

.............................. / / .......... DESCEND FROM BASTC MAGMAS {af~ Borth, 1962, pl231 ~ MARGINAL GAgBROS

sJo~ ~o x

~ R d I �9 CO'NAN HiLL ~ECCtA +r ~ ~ in �9 GR~NOPHYRES 8r FELSITES

. , , o , o ' , ; : = : 2 & ... . . . . . . . . . . . . TM~d volley ot 1000 kg ~n~ os ~ bY

. " ,'~O O Tuff2: : : : r i27Fe ~'d;2;0 } . . . . . .

NaAisioa K A~S~O, 6 60 L:~ ~o o P ~o

5oe.*o

WT. PER CENT

Fig. 51. Triangular diagram for normative quartz, albite and orthoclase.

orthoclase normative values suggest that these granophyres are simple residua (WhoER & BROWN, 1968) - - they are very similar to those from the Binneringie Dyke (Fig. 51) - - but isotopic studies are conflicting: oxygen isotope studies tend to support this interpretation and strontium values to suggest considerable assimilation. Preliminary isotopic studies on reeks from the Binneringie Dyke have been carried out by Dr. W. COMPSTON at the Department of Geophysics and Geochemistry, Australian National University, Canberra, but the results are slightly anomalous. Most of the reeks fit the 249.0 m.y. isochron (corresponding to the age of the Widgiemooltha Dyke suite derived from other members) well, but there are anomalous granophyres which plot as if of younger age. This anomaly is unexplained, to date, but it may be due to collection of not absolutely fresh samples. There is nothing in these results to indicate contamination. In the light of conflicting evidence from Skaergaard, one may question whether reliance on initial STSr/S6Sr ratios is even justified to discount simple fraetionation

81 Geologische Rundschau, Bd. 60 12111

Aufsatze

to derive granophyric rocks forming minor associates of layered intrusions. The author strongly favours an origin in simple fractionation, and this may yet prove to be the ease for Skaergaard, despite the strontium isotope evidence.

References

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MANSON, V.: Geochemistry of Basaltic Rocks: Major elements. - - In "'The Polder- vaart Treatise on Rocks of Basaltic Composition. - - (Eds. Hess, H.H., and Poldervaart, A.A.) New York (John Wiley) 1967.

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Documents

Geology of the Binneringie Dyke, Western Australia