The Advent of the Proterozoic - a Trigger for Extensive Intracrustal Processes in the South Indian Shield?

Abstract

Geochemical, Sr-isotopic and geochronological data on the major granitic activity in the south Indian shield suggest a significant event of intracrustal melting and differentiation processes, particularly at middle- and lower crustal levels, resulting in irreversible geochemical differentiation of the crustal column into a large ion lithophile (LIL) element - depleted anhydrous lower crust and an upper crust characterised by copious granitic activity, marking the Archean-Proterozoic boundary. A perceptible link between the formation of granulites in the lower crust and the granitic activity in the upper crust is recognised in this region. Major element geochemistry of the granites suggests an anatectic origin for these granites. The geochemical signatures of the granites in the Closepet, Hampi (Karnataka), Lepakshi, Kadiri, Tanakallu and Hyderabad regions (Andhra Pradesh) suggest that the source regions for the granitic melts were mainly the juvenile crustal additions of the tonalilte-trondhjemite-granodiorite (TTG) types at lower crustal levels. Sources which had little crustal residence time are indicated by relatively low initial strontium isotopic ratios. This granitic activity was mostly of the granodiorite, quartz-monzonite and granite types which exhibit I-type character. These granites usually have highly fractionated REE patterns, mostly with negative Eu anomalies, implying significant fractionation of palgioclase into the residue and the strong effects of intracrustal differentiation processes. The heat required for melting might have been provided by crustal thickening and radioactive decay processes, with possible involvement of minor amounts of mantle input at times. CO 2- streaming, which reduces considerably the melting temperatures and helps in the removal of LILE into the melt, may also have played an important role. Most of the granites have geochemical characteristics of origins in volcanic arc or syn-collisional settings, suggesting the operation of plate tectonic processes and crustal accretion mechanisms during the Archaean-Proterozoic transition in the south Indian shield. Proterozoic crustal accretion and the granitic crustal differentiation contributed to relative stability of the continental crust and resulted in cratonisation process.