Formation and evolution of Precambrian granulite terranes: A gravitational redistribution model
Published:February 01, 2011
Leonid L. Perchuk, Taras V. Gerya, 2011. "Formation and evolution of Precambrian granulite terranes: A gravitational redistribution model", Origin and Evolution of Precambrian High-Grade Gneiss Terranes, with Special Emphasis on the Limpopo Complex of Southern Africa, Dirk D. van Reenen, Jan D. Kramers, Stephen McCourt, Leonid L. Perchuk
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This paper proposes a revision of the gravitational redistribution model suggested by Leonid Perchuk for the formation, evolution, and exhumation of Precambrian high-grade terranes (HGTs) located between granite-greenstone cratons. Such HGTs are separated from greenstone belts by crustal-scale shear zones up to 10 km wide and several hundred kilometers long. Pelite samples far (>~50 km) from the bounding shear zones show coronitic and symplectitic textures that reflect a decompression-cooling (DC) pressure-temperature (P-T) path. On the other hand, samples from within ~50 km of the bounding shear zones are characterized by textures that reflect an isobaric or near-isobaric cooling (IC) path. Local mineral equilibria in the schists from the shear zones record hairpin-shaped clockwise P-T loops. The results of a numerical test of the gravitational redistribution model show the following plausible scenario: The diapiric rise of low-density, hot granulite upward in the crust causes the relatively high-density, predominantly mafic upper crust in the adjacent greenstone belt, consisting of metabasalt and komatiite, to move downward (subducted), cooling the base of the granulites along the intervening syntectonic shear zone. This causes (1) the formation of local convection cells that control the movement of some of the ascending granulite blocks near the contact with the cratonic rocks, and (2) near-isobaric cooling (IC) of the granulite blocks in the vicinity of the boundary with the colder wall rocks. Cooling of granulite blocks farther away from the contact is not arrested, and they ascend to the Earth's surface, recording DC P-T paths. In general, the results of numerical modeling provide support for the buoyant exhumation mechanism of granulites owing to gravitational redistribution within the metastable relatively hot and soft early Precambrian crust that was subjected to high-temperature (HT) and ultrahigh-temperature (UHT) metamorphism.