Radioelements and heat production of an exposed Archaean crustal cross-section, Dharwar craton, south India

Abstract

The Dharwar craton (DC) in south India exposes a tilted ∼30-km-thick crustal section equivalent to upper, middle and lower Archaean continental crust. The DC is composed of poly-magmatic–metamorphic terrains such as the Western Dharwar Craton (WDC) and Eastern Dharwar Craton (EDC) whose contact is marked by the Closepet Granite batholith (CG). In-situ gamma-ray spectrometric analysis has been carried out to measure K, U and Th abundance at 1023 sites covering all major rock formations of the DC. The data show that the greenschist and amphibolite facies tonalite–trondhjemite–granodioritic (TTG) gneisses of the WDC are poorer in radioelements compared to granodioritic gneisses of the EDC. Similarly, the WDC granites of the amphibolite facies region have lower abundance indicating their derivation from a depleted lower crust, which had suffered an earlier crustal differentiation. The elemental data on the gneisses in the WDC and EDC do not show a difference between the greenschist and amphibolite facies, but there is a marked depletion in the granulite facies. The depletion reaches a maximum in the high- P granulites in the southernmost part of the DC. The elongate CG is an I-type, calc-alkaline, metaluminous granite, which exposes a ∼12-km-thick batholith, where K is remarkably uniform but U and Th show several fold increase with decreasing crustal levels. Heat production was calculated from the radioelemental data. For estimating the radiogenic heat contribution of crust to heat flow, we arrive at a present-day crustal configuration comprising four northerly dipping metamorphic facies layers: greenschist, amphibolite, metasomatized granulite and depleted granulite. Gross heat production of each layer is computed from heat production of constituent rocks and their abundance. The crustal contribution is found to decrease, from greenschist to granulite facies regions, from 23 to 18 mW m −2 in the WDC, 27 to 7 mW m −2 in the EDC. Surface heat flow and the crustal contribution models indicate that mantle heat flow of the WDC is lower (7–10 mW m −2) compared to the EDC (17–24 mW m −2) in the greenschist and amphibolite facies regions. High mantle heat flow of 29 mW m −2 beneath the depleted granulite facies region of the EDC appears anomalous in the Dharwar craton.