Radioelement abundances and heat production in the Paleoarchaean and early Neoarchaean granitoids of the Singhbhum Craton, eastern India: Thermal and geodynamic implications

Distributions of long-lived radioelements (Th, U and K) in the upper crust play a vital role in lithospheric thermal modelling and understanding of underlying geodynamic processes. In the present study, we report abundances of Th, U, K and radiogenic heat production (A) in Paleoarchaean and early Neoarchaean granitoids from the Singhbhum Craton, located in the eastern part of the Indian shield, for the first time in a systematic way, using laboratory gamma-ray spectrometric set-up. We have studied 204 samples consisting of Paleoarchaean gneiss, three phases of Paleoarchaean Singhbhum Granite and early Neoarchaean granitoid. Th, U, K and A in the Paleoarchaean Singhbhum Granite are found to be marginally higher (10.8 ppm, 1.4 ppm, 2.4 per cent and 1.3 mu Wm-3) than the Paleoarchaean gneiss (9.6 ppm, 1.7 ppm, 1.8 per cent and 1.3 mu Wm-3). In comparison, such values are much higher in the early Neoarchaean granitoid (31.1 ppm, 4.2 ppm, 3.9 per cent, and 3.6 mu Wm-3). The surface heat production (1.36 mu Wm-3) of this craton is comparatively low compared to most cratons worldwide. In addition, it has the lowest average crustal heat production (0.42 mu Wm-3), due to which it will contribute little to the observed surface heat flow. Moreover, Th is a dominant component in heat production, followed by U and K in these granitoids. Spatially, radioelement ratios Th/U and K/U show similar trends as the radioelements, indicating systematic depletion of Th and U, compared to K. The study also indicates that the spatially separated coeval granitoids are formed from different magma sources in the heterogeneous crust that prevailed during the Archaeans. The lower radioelement abundances in the Paleoarchaean granitoids mostly resulted from the partial melting of mafic sources, whereas higher radioelement abundances in the early Neoarchaean granitoids resulted from the partial melting of the felsic source.