Petrogenesis of Amphibolites from the Banded Gneissic Complex around Masuda, Rajasthan: Significance to Precambrian Crustal Evolution of the Aravalli Craton

The Banded Gneissic Complex (BGC) occurring around Masuda consists of granitoids, migmatized mica schist, calc-gneiss and amphibolites. The amphibolites (few cm to tens of metre wide) are co-folded with the granitoid and calc- gneisses. The ortho-amphibolites are classified into high-Mg and tholeiitic amphibolites and they have distinct chondrite normalized Rare Earth Element (REE) patterns and Ce/Nd ratios. Modeling of their geochemical data suggests that magmas representing the high-Mg amphibolites were derived by low extents of partial melting of mantle sources more Fe-enriched than lherzolite whereas magmas representing the tholeiitic amphibolites were derived by low-extents of partial melting of sources formed by mixing of high-Mg magmas with lherzolite. The composition of this source is similar to the komatiites. The tholeiitic amphibolites have geochemical and petrogenetic characteristics of the basalts formed in island arcs. They are different from that of amphibolites found in the basal part of Aravalli Supergroup and mafic enclaves found within the BGC occurring east of Nathdwara and Udaipur.