High mantle heat flow in a Precambrian granulite province: evidence from Southern India

Abstract

Twelve new heat flow values determined at nine sites and heat production estimated from radioelemental measurements at 330 sites in the southern granulite province (SGP) bring out contrasting crustal and subcrustal thermal characteristics between the SGP and the adjacent Archaean Dharwar greenstone-granite-gneiss province (DP) in south India. A two-layer granulitic crust of Late Archaean charnockites and gneisses characterizes the northern block (NB), north of the Palghat-Cauvery lineament (PCL). The heat production of the upper, 7-10 km thick, metasomatized granulitic layer ranges between 0.2 and 0.75 μW m^-3 (mean 0.5 ± 0.3 (SD) μW m^-3). This layer overlies radioelement-depleted granulites characterized by very low heat production ranging from 0.14 to 0.2 μW m^-3 (mean 0.16 ± 0.07(SD) μW m^-3). In a large sector of the NB, erosion of the upper metasomatized granulite layer has laid bare the depleted granulitic rocks, which represent one of the lowest heat-producing crustal sections. The mean heat flow in the NB is 36 ± 4 mW m^-2 (N = 10). The southern block (SB), south of PCL, in contrast to the NB, comprises complexly interlayered charnockites, gneisses, granites, khondalites, and leptynites, which have variable and much higher levels of heat production ranging between 1.11 and 2.63 μW m^-3. The heat flow in the SB is 47 ± 8 mW m^-2 (N = 3). Overall, the range of heat flow values in the SGP is within the range for the DP. Mantle heat flow in the NB, both from the lowest heat-producing sector and other areas, is deduced in the light of heat production and heat flow data, at 23-32 mW m^-2, whose values are distinctly higher than 11-16 mW m^-2 for the adjacent DP. The higher mantle heat flow in the NB appears to be a consequence of higher heat production in the subjacent mantle.