M. Satyanarayanan

Archean TTG Magmatism in the Aravalli Craton, NW India: Petrogenetic and Geodynamic Constraints

The Archean Banded Gneissic Complex (BGC) of the Aravalli Craton (NW India) is volumetrically most important basement complex scattered along the northwestern margin of the Indian craton. It is mainly comprised of grey gneisses, undeformed granitoids and volcano-sedimentary sequence. Amongst various lithocomponents, the grey gneisses represent the oldest component of the BGC. Rock association comprising tonalite, trondhjemite and granodiorite (TTG) form a major component of the grey gneisses. Geochemically, the TTG are essentially sodic in nature (avg. K2O/Na2O = 0.31) and characterized by high Al2O3 (avg. 15.5 wt%), Na2O (avg. 5.3 wt%) and Sr (avg. 413 ppm), and low Y (avg. 10 ppm) contents. They are characterized by enrichment of large ion lithophile elements (LILE) and light rare earth element (LREE) contents. On chondrite- and primitive mantle normalized trace element diagrams, the TTG exhibit: (i) highly fractionated rare earth element (REE) patterns; and (ii) negative anomalies of Nb and Ti. These geochemical characteristics of the TTG suggest that they were not produced from partial melting of mid-oceanic ridge basalts/normal oceanic crust. Instead we propose that the TTG rocks of the Aravalli Craton were formed by partial melting of an enriched source (oceanic plateau) in a subduction setting; with melting taking place at variable depths.