P-T -time history and thermal modelling of an anorthosite-granulite interface, Eastern Ghats metamorphic belt, India

Abstract

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 anorthositegranulite 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 Yshaped P-T-t loops.

Humite‐ and scapolite‐bearing assemblages in marbles and calcsilicates of Dronning Maud Land, Antarctica: new data for Gondwana reconstructions

“…Newton, 1983; Drury et al ., 1984; McLelland & Isachsen, 1985; Sandiford & Powell, 1986; Sandiford, 1989) and a model based on ‘magmatic underplating’ (e.g. Wells, 1980; Bohlen, 1987; Mukherjee, 1989; Emslie & Hunt, 1990). We interpret the occurrence of anorthosites and the lack of compelling evidence for a profoundly tectonically thickened crust to favour the latter model for the formation of the granulites in CDML.…”

“…We interpret the occurrence of anorthosites and the lack of compelling evidence for a profoundly tectonically thickened crust to favour the latter model for the formation of the granulites in CDML. The anorthosites are believed to form during fractionation of large masses of tholeiitic magma at the base of thick (50 km) continental crust ( Mukherjee, 1989; Emslie & Hunt, 1990). The heat supply from the possibly underplated magma caused melting in the lower parts of the crust under water‐deficient conditions.…”

Emplacement of Proterozoic massif-type anorthosite during regional shortening: evidence from the Bolangir anorthosite complex (Eastern Ghats Province, India)

“…Subsequent investigations largely concentrated on petrological, geochemical or geochronological aspects (Tak 1972(Tak , 1983Mukherjee et al 1986Mukherjee et al , 1999Mukherjee 1989;Raith et al 1997;Bhattacharya et al 1998;Krause et al 2001), although some publications briefly describe and discuss structures in the anorthosite massif and neighbouring country rocks. In contrast, the present contribution is only concerned with structural and tectonic aspects.…”