Multi‐stage metamorphism of ultrahigh‐temperature Mg‐Al granulites during Gondwana assembly: Evidence from southern India

Abstract

The Southern Granulite Terrane (SGT) in India hosts granulite facies rocks metamorphosed at ultra-high temperature (UHT) conditions in the various crustal blocks, as well as within the Palghat-Cauvery Suture Zone (PCSZ), that is considered as a trace of the Late Neoproterozoic-Cambrian Gondwana suture. Here we investigate UHT granulites from the northern margin of the Madurai Block adjacent to the PCSZ where Mg-Al-rich granulites are exposed. We identify sodic gedrite + kyanite in these rocks as the high-pressure prograde stage assemblage, followed by sillimanitegarnet-orthopyroxene that formed during pressure decrease and temperature increase. The rare remnant gedrite is also stable at the near-peak UHT metamorphism until it was replaced by sapphirine. The rocks subsequently underwent decompression that formed sapphirine + cordierite and sapphirine + plagioclase symplectite around sillimanite. Dehydration during decompression generated orthopyroxene-sillimanite-quartz assemblage with the appearance of sapphirine, defining the diagnostic mineral assemblage indicative of peak UHT metamorphism (T > 900 C) at relatively high-pressure (P > 9 kbar). The UHT peak metamorphism in this region is consistent with the results of P-T calculations using conventional geothermometers and phase equilibrium modelling (T up to 1,050 C, P over12 kbar). Zircon and monazite geochronology on the UHT metapelites indicate distinct stages. Detrital zircon grains in the metasediments indicate protolith from ca. 2.5 Ga igneous source and the metamorphic overgrowths yield 206 Pb/ 238 U mean ages concentrated at ca. 550-520 Ma. Monazite ages define another younger group 206 Pb/ 238 U mean ages at ca. 450 Ma. The prograde high-pressure granulite-facies metamorphism and following UHT event correlate with the subduction-collision tectonics at 550-500 Ma associated with the final stage of amalgamation of the Gondwana supercontinent, while the 420-460 Ma monazite age records a later hydration at the postorogenic stage, possibly associated with deep shearing and fluid influx.