Chemical Evolution of Peninsular Gneiss in the Western Dharwar Craton, Southern India

Abstract

The major rock type of Peninsular India consists of deformed, sialic rocks broadly termed "Peninsular Gneiss." Suites investigated in the western part of the Dharwar craton have a tonalitic/trondhjemitic composition and have isotopic ages generally in the range of 3,400 to 3,000 m.y. The last event recorded by numerous whole-rock isotopic systems was 3,000 m.y. ago, coincident with the emplacement of diapiric trondhjemite plutons. The gneissic suite appears to consist of a large number of separate magmatic bodies, some of which were equilibrated at very low pressures and may be extrusive. Low initial $$^{87}Sr/^{86}$$Sr ratios indicate derivation of the melts from the mantle or from materials that had acquired crustal Rb/Sr ratios only a short time prior to partial melting. Some of the ages represent metamorphism of rocks emplaced not long before the metamorphic event. The gneisses contain very little sedimentary component, as shown by the absence of rounded zircons, low Cr contents, and low (or zero) normative corundum contents. Some compositional modification occurred after initial emplacement of the gneisses. Considerable addition of U, and some Th, took place about 3,000 m.y. ago. Other lithophile elements may have been mobilized at the same time. There is no direct evidence for metasomatic addition of elements to the gneisses during the granulite-facies metamorphism that occurred in deep levels of the crust about 2,500 m.y. ago, despite depletion of the highest-grade charnockites in lithophile components.