Sapphirine plus quartz assemblage from the Southern Granulite Terrane, India: diagnostic evidence for ultrahigh-temperature metamorphism within the Gondwana collisional orogen

A synthesis of the petrological characters of granulite facies rocks that contain equilibrium sapphirine + quartz assemblage from three localities (Rajapalaiyam, Ganguvarpatti and Panangad) in the Southern Granulite Terrane (SGT), India, provides unequivocal evidence for extreme crustal metamorphism associated with the collisional assembly of the Gondwana supercontinent during the Late Neoproterozoic to Early Cambrian. The reaction microstructures associated with sapphirine + quartz vary among the samples, probably suggesting different tectonic conditions during the metamorphic evolution. Sapphirine and quartz in pelitic granulites from Rajapalaiyam in the Madurai Block occur as an intergrowth in poikiloblastic garnet, whereas the two minerals show host-inclusion relationship (quartz inclusion in coarse-grained sapphirine) in a Mg-Al-rich rock from Ganguvarpatti in the same block. The sapphirine quartz association and related microstructures from the two localities are consistent with isobaric cooling from sapphirine + quartz to orthopyroxene + sillimanite + quartz fields, possibly along a counterclockwise P-T path. In contrast, sapphirine + quartz occurs as coronas around Mg-rich (X-Mg similar to 0.58) staurolite within poikiloblastic garnet in an Mg-Al-rich rock from Panangad in the Palghat-Cauvery Suture Zone (PCSZ). The texture suggests decompression from the stability field of Mg-rich staurolite (P > 15 kbar) at T > 1000 degrees C possibly along a clockwise P-T trajectory. Such contrasting P-T paths are consistent with a recent tectonic model of the SGT, according to which the metamorphism in the Madurai Block is related to heat (and fluid) input from the mantle, whereas that in the PCSZ is related to the subduction-collision tectonics between the Madurai Block and the Dharwar Craton, where high-pressure and ultrahigh-temperature (UHT) metamorphic rocks were extruded along the subduction channel. Copyright (C) 2010 John Wiley & Sons, Ltd.