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Department of Geology & Geophysics, Institute of Technology, Kharagpur-721303, West Bengal, India
A pressure-temperature-time synthesis and a thermal model are presented for the anorthosite-massif-granulite interface near Bolangir, Orissa, India: a type area of the Eastern Ghats metamorphic belt. While the anorthosite pluton can be shown to have been forcefully intruded as a schlieren dome into the granulite cover, the calculated mineral equilibria of the interface suggest a P-T convergence at 620–740°C and 4.7–7 kbar and a nearly isobaric cooling in this pressure range under a cratonic overburden between 15 and 25 km. Parameterized model calculations suggest that a noritic anorthosite pluton of 7.5 and 10 km radius with 50% initial crystallinity and a lower crustal contact temperature in the region of 800°C would cool from the initial temperature of 1200°C to 850–900°C in about 20 Ma and then the cooling rate would fall dramatically to the order of 0.75°C Ma–1. Finite difference calculations of the heat conduction from the equilibrated anorthosite-granulite interface suggest that for some combinations of the thermal properties of the medium and the heat generation and uplift rates, boundary conditions based on an initial transient elevated geotherm produce a nearly isobaric cooling path. The initial prograde metamorphism of the granulites, the anorthosite invasion and the final cooling and P-T convergence of the anorthosite- granulite interface have resulted in a composite overturned (anticlockwise) Y-shaped P-T-t configuration. It is suggested that this configuration is characteristic of the Eastern Ghats type metamorphic belt, representatives of which seem to be widespread in the Archaean to Proterozoic continental segments of North America, Scandinavia and parts of Africa, Western Australia and Antarctica. In contrast to the Alpine type belts, the Eastern Ghats type belts are much wider and thickly cratonized with low isostatic resilience (low uplift/exhumation rate) and have anorthosite intrusive cores acting as ‘exotic’ heat sources, resulting in characteristic anticlockwise overturned and forked Y-shaped P-T-t loops.
