Pressure-temperature-fluid evolutionary history of the polymetamorphic Sandmata granulite complex, Northwestern India

Abstract

An assemblage of granulite facies metamorphosed pelitic, quartzofeldpathic, basic and calc-silicate rocks is exposed within amphibolite facies tonalitic gneisses, amphibolites and pelitic schists, belonging to the Banded Gneissic Complex, at Sandmata, northwestern India. The granulite facies rocks are bounded by a ductile shear zone and are intruded by enderbite and metanorite. Reaction textures, geothermobarometric data and phase equilibria considerations show that (a) the granulite facies rocks were metamorphosed at ca. 8 kbar, 820°C during an early phase of metamorphism, (b) enderbite and metanorite were emplaced subsequent to this metamorphism, (c) all the rocks, including the surrounding amphibolite facies rocks, were subsequently metamorphosed at ca. 6.5-7 kbar, 650-700°C, when coronal garnet appeared in pyroxene-bearing lithologies and (d) the rocks were affected by non-pervasive hydrous fluid flux either prior to or during the later metamorphism. It is suggested that the deep crustal granulite facies rocks were juxtaposed against the amphibolite facies rocks along the ductile shear zone during this later phase of metamorphism. Interpretation of available geochronologic data suggests that the granulite facies metamorphism occurred prior to 1.7 Ga and that the entire terrain was possibly reworked at ca. 1 Ga (Grenvillian orogeny).