Isotope and rare earth element chemistry of carbonatite–alkaline complexes of Deccan volcanic province: implications to magmatic and alteration processes J.S. Ray a , R. Ramesh a, *, K. Pande a , J.R. Trivedi a , P.N. Shukla a , P.P. Patel b a Physical Research Laboratory, Ahmedabad, 380 009, India b M.S. University of Baroda, Vadodara, 390 002, India Accepted 3 December 1998 Abstract Results of dierent isotopic and trace element studies on three carbonatite–alkaline complexes (Amba Dongar, Mundwara and Sarnu-Dandali) of the Deccan flood basalt province, India, are presented. The Amba Dongar (Ambadungar) complex has been dated precisely to 65.0 20.3 Ma by the 40 Ar– 39 Ar method. The minimum initial Sr isotopic ratio of alkaline rocks of Amba Dongar is found to be same as that of the coexisting carbonatites, suggesting their derivation from a common parent magma, probably through liquid immiscibility. The rare earth element abundance in these rocks also supports the liquid immiscibility hypothesis. Further investigation revealed that the parent magma of this complex has been contaminated ( 0 5%) by the lower crustal material, which is clearly reflected in the initial 87 Sr/ 86 Sr variation of alkaline rocks but not in the carbonatites. Sr study also suggests that the mantle source of Amba Dongar like the other two complexes is a Rb/Sr enriched source. The temporal and spatial relationships of all the three complexes with the Deccan flood basalts support the hypothesis of reunion plume origin for these. Fractional crystallization and subsequent hydrothermal/meteoric alteration are found to have controlled the stable carbon and oxygen isotopic variations in carbonatites. This study suggests that all the complexes have been derived from isotopically average mantle except for a particular batch of parent magma at Amba Dongar, which appears to have incorporated recycled crustal carbon. In a plume origin scenario such incorporation indicates the entrainment of 13 C-enriched subcontinental lithospheric mantle by the plume. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Carbonatite; Deccan volcanic province; Isotope chemistry; Amba Dongar; Mundwara; Sarnu-Dandali 1. Introduction Carbonatites are mantle derived igneous rocks, which contain more than 50% by weight of carbonate minerals (Wooley and Kempe, 1989). Though they represent only <1% of all the magmatic rocks, they have attracted much attention because of their unusual physical and chemical properties. Carbonatites range in age from Archean to recent and are mostly observed in continents. A few have been reported from oceanic areas as well. Continental carbonatites are commonly associated with alkaline (nephelinitic/kimberlitic) igneous rocks. Ca-rich carbonatites are the most abun- dant carbonatites world wide, whereas the only known active carbonatite volcano, Mt. Oldoinyo Lengai, erupts Na-rich variety (see Bell, 1989). Small occur- rences of Mg-rich and Fe-rich carbonatites also have been reported (Woolley and Kempe, 1989). Carbonatite studies have contributed to the under- standing of the secular evolution of the mantle because of their large variation in age (Bell and Blenkinsop, 1989). Being rich in carbonates, these are probably the best samples to study the mantle carbon reservoir and Journal of Asian Earth Sciences 18 (2000) 177–194 1367-9120/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S1367-9120(99)00030-9 * Corresponding author at Physical Research Laboratory, Earth Science and Solar Science Division, Oceanography and Climate Studies Group, Rm 265, Navangpura, Ahmedabad 380 009, India. Tel.: +91-79-646-2129; fax: +91-79-630-1502. E-mail address: [email protected] (R. Ramesh).

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Page 1: Isotope and rare earth element chemistry of carbonatite ...library/Ray_JS_2000_1abst.pdfCarbonatites are mantle derived igneous rocks, which contain more than 50% by weight of carbonate

Isotope and rare earth element chemistry of carbonatite±alkalinecomplexes of Deccan volcanic province: implications to magmatic

and alteration processes

J.S. Raya, R. Ramesha,*, K. Pandea, J.R. Trivedia, P.N. Shuklaa, P.P. Patelb

aPhysical Research Laboratory, Ahmedabad, 380 009, IndiabM.S. University of Baroda, Vadodara, 390 002, India

Accepted 3 December 1998

Abstract

Results of di�erent isotopic and trace element studies on three carbonatite±alkaline complexes (Amba Dongar, Mundwaraand Sarnu-Dandali) of the Deccan ¯ood basalt province, India, are presented. The Amba Dongar (Ambadungar) complex hasbeen dated precisely to 65.020.3 Ma by the 40Ar±39Ar method. The minimum initial Sr isotopic ratio of alkaline rocks of

Amba Dongar is found to be same as that of the coexisting carbonatites, suggesting their derivation from a common parentmagma, probably through liquid immiscibility. The rare earth element abundance in these rocks also supports the liquidimmiscibility hypothesis. Further investigation revealed that the parent magma of this complex has been contaminated (05%) by

the lower crustal material, which is clearly re¯ected in the initial 87Sr/86Sr variation of alkaline rocks but not in the carbonatites.Sr study also suggests that the mantle source of Amba Dongar like the other two complexes is a Rb/Sr enriched source. Thetemporal and spatial relationships of all the three complexes with the Deccan ¯ood basalts support the hypothesis of reunion

plume origin for these. Fractional crystallization and subsequent hydrothermal/meteoric alteration are found to have controlledthe stable carbon and oxygen isotopic variations in carbonatites. This study suggests that all the complexes have been derivedfrom isotopically average mantle except for a particular batch of parent magma at Amba Dongar, which appears to haveincorporated recycled crustal carbon. In a plume origin scenario such incorporation indicates the entrainment of 13C-enriched

Keywords: Carbonatite; Deccan volcanic province; Isotope chemistry; Amba Dongar; Mundwara; Sarnu-Dandali

1. Introduction

Carbonatites are mantle derived igneous rocks,which contain more than 50% by weight of carbonateminerals (Wooley and Kempe, 1989). Though theyrepresent only <1% of all the magmatic rocks, theyhave attracted much attention because of their unusualphysical and chemical properties. Carbonatites range

in age from Archean to recent and are mostly observedin continents. A few have been reported from oceanicareas as well. Continental carbonatites are commonlyassociated with alkaline (nephelinitic/kimberlitic)igneous rocks. Ca-rich carbonatites are the most abun-dant carbonatites world wide, whereas the only knownactive carbonatite volcano, Mt. Oldoinyo Lengai,erupts Na-rich variety (see Bell, 1989). Small occur-rences of Mg-rich and Fe-rich carbonatites also havebeen reported (Woolley and Kempe, 1989).Carbonatite studies have contributed to the under-standing of the secular evolution of the mantle becauseof their large variation in age (Bell and Blenkinsop,1989). Being rich in carbonates, these are probably thebest samples to study the mantle carbon reservoir and

Journal of Asian Earth Sciences 18 (2000) 177±194

PII: S1367-9120(99 )00030 -9

* Corresponding author at Physical Research Laboratory, Earth

Science and Solar Science Division, Oceanography and Climate

Studies Group, Rm 265, Navangpura, Ahmedabad 380 009, India.

Tel.: +91-79-646-2129; fax: +91-79-630-1502.

E-mail address: [email protected] (R. Ramesh).