GEOCHEMISTRY OF MAFIC DYKES IN PART OF CHOTANAGPUR GNEISSIC COMPLEX: PETROGENETIC AND TECTONIC IMPLICATIONS

Abstract

Chotanagpur Gneissic Complex basement rocks of the Eastern Indian shield has been dissected by numerous mafic dykes, now occurring as amphibolitic dykes and gneissic amphibolites. These dykes are subalkaline, ranging in composition from basalt through basaltic-andesite to andesite. These rocks have enriched incompatible trace element patterns. These are particularly enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE) with depleted high field strength elements (HFSE; Nb, P, Ti) characterisitcs (i). Negative Sr anomaly is conspicuous. Nb/La and Nb/Ce ratios of the dykes are lower compared to the primitive mantle but these values are closer to average crustal values. Incompatible trace element data suggest enriched source characteristics and influence of crustal contamination in their genesis. Trace element ratios such as Gd/Yb of these dykes indicate at least two different sources. They probably represent Precambrian continental rifting in this region. Copyright © 2007 by The Geochemical Society of Japan.