Archaean crustal thickness of greenstone granite belts of South India

Archaean crustal thickness for the Dharwar craton is estimated using potash index and Rb?Sr crustal thickness grid. The volcanics of the Dharwar greenstone belts appear to have evolved in a less than 20 km thick crust. Whereas the tonalite-trondhjemite pebbles of the Dharwar conglomerates (3250±150 m.y.) were derived from gneisses that evolved in a crust less than 20 km thick, the bulk of the peninsular gneisses and associated granitoids were emplaced in a crust 25 to 35 km thick. The 2000 m.y. old Closepet granite suite was emplaced in a crust thicker than 30 km. It is deduced that the continental crust in the region thickened from 15 to 35 km during a span of about 1000 m.y. between 3250±150 to 2000 m.y. ago. Calculations show that Archaean gecthermal gradients in Dharwar craton were three to four times steeper when compared to the present 10.5°C/km. The thin crust and the steep geothermal gradients are reflected by the emplacement of high magnesia basalts, layered igneous complexes and the strong iron enrichment trend shown by Dharwar metavolcanics.