Trace element geochemistry of Amba Dongar carbonatite ...library/Ray_JS_2004_abst.pdf ·...

Trace element geochemistry of Amba Dongar carbonatitecomplex, India: Evidence for fractional crystallization

and silicate-carbonate melt immiscibility

Jyotiranjan S Ray∗ and P N Shukla

Physical Research Laboratory, Navrangpura, Ahmedabad 380 009, India.∗e-mail: jsray@prl.ernet.in

Carbonatites are believed to have crystallized either from mantle-derived primary carbonate mag-mas or from secondary melts derived from carbonated silicate magmas through liquid immiscibilityor from residual melts of fractional crystallization of silicate magmas. Although the observed coex-istence of carbonatites and alkaline silicate rocks in most complexes, their coeval emplacement inmany, and overlapping initial 87Sr/86Sr and 143Nd/144Nd ratios are supportive of their cogenesis;there have been few efforts to devise a quantitative method to identify the magmatic processes. Inthe present study we have made an attempt to accomplish this by modeling the trace element con-tents of carbonatites and coeval alkaline silicate rocks of Amba Dongar complex, India. Trace ele-ment data suggest that the carbonatites and alkaline silicate rocks of this complex are products offractional crystallization of two separate parental melts. Using the available silicate melt-carbonatemelt partition coefficients for various trace elements, and the observed data from carbonatites, wehave tried to simulate trace element distribution pattern for the parental silicate melt. The resultsof the modeling not only support the hypothesis of silicate-carbonate melt immiscibility for theevolution of Amba Dongar but also establish a procedure to test the above hypothesis in suchcomplexes.

1. Introduction

Carbonatites have long been recognized as mag-matic rocks (e.g., Bell 1989). Very low silica andhigh incompatible trace element contents makethem unique amongst igneous rocks. In spite of thefact that they represent a very small fraction of allthe magmatic rocks, they have attracted consid-erable attention because of their unusual physic-ochemical properties. The study of carbonatiteshas substantially improved our understanding ofmany important mantle processes such as mantlemetasomatism, melt extraction, recycling of crustalmaterial into the mantle and mantle degassing.Also because of their large age distribution (fromArchean to present) they provide us probably thebest samples to study the secular evolution of

the Earth’s mantle (Bell and Tilton 2002). Evenafter a considerable amount of research some ofthe fundamental aspects of the origin and evolu-tion of carbonatites still remain elusive. There isdebate about the nature of the primary carbon-atite melts, their mantle sources and the magmaticprocesses that precede their final emplacement intothe crust. It is still not clear whether the carbonatemelts represent direct silica-undersaturated mag-mas from mantle (e.g., Harmer and Gittins 1998)or are products of magmatic differentiation, thelatter being either fractional crystallization (e.g.,Petibon et al 1998) carbonate-silicate melt immis-cibility of carbonated silicate melts (e.g., LeBas1989). Experimental findings appear to support allthree possible scenarios (e.g., Sweeney 1994; Leeand Wyllie 1997; Veksler et al 1998a). However,

Keywords. Carbonatite; Amba Dongar; India; liquid immiscibility; trace elements; REE.

Proc. Indian Acad. Sci. (Earth Planet. Sci.), 113, No. 4, December 2004, pp. 519–531© Printed in India. 519