The role of radiogenic heat production in the thermal evolution of a Proterozoic granulite-facies orogenic belt: Eastern Ghats, Indian Shield

摘要

The Eastern Ghats Belt (EGB) is a deeply eroded Proterozoic orogenic belt juxtaposed against the Archaean Dharwar-Bastar-Singhbhum cratons in the Indian Shield. The tectono-metamorphic history of this belt is broadly similar to the Grenvillian orogenic belts. The EGB exposes dominantly the granulite-facies rocks such as charnockites, khondalites and migmatitic gneisses with less abundant intermediate to mafic granulites. The granulites are intruded by small plutons of granites, syenites and anorthosites. The northern and southern parts of the belt (NEGB and SEGB) have distinct crustal histories, but 1600-1000-550 Ma old tectonometamorphic events are common to both the belts. In this research, we attempt to understand the present-day and past thermal state of this orogen on the basis of our radioelement (K, U and Th) measurements at 562 sites using in-situ gamma-ray spectrometry, with other geological and geophysical constraints. The dominant rock types of the NEGB and SEGB have similar radioelement abundances (> 3% K, 3 ppm U and 30 ppm Th) and heat production. Heat production of chamockites, gneisses, khondalites and intermediate granulites of the NEGB is 2.9, 2.8, 2.9 and 1.1 mu W m(-3), respectively. In the SEGB, they are 2.7, 2.5, 2.8 and 0.6 mu W m-3, respectively, and the mafic granulites are the lowest in heat production (0.3 mu W m(-3)). On the basis of the heat production data and crustal petrologic models, we show that the crustal contribution of the NEGB and SEGB is 54 and 45 MW m(-2), respectively, which are broadly in good agreement with the available surface heat flow data. Thermal models of the present-day EGB crust indicate that the temperature at the Moho is similar to 550 degrees C. Radioelements and heat production of the Eastern Ghats granulites are higher than the other granulite belts in the Indian shield. The crustal radiogenic heat contribution and Who temperatures of the EGB are higher than the adjoining Archaean cratons. This could also be responsible for the development of inverted metamorphic isograds in the tectonic boundary between the EGB and the adjoining cratons during a collisional orogenic event at about 550 Ma ago. (c) 2006 Elsevier B.V. All rights reserved.