Sapphirine + quartz corona around magnesian (XMg ~ 0.58) staurolite from the Palghat-Cauvery Suture Zone, southern India: Evidence for high-pressure and ultrahigh-temperature metamorphism within the Gondwana suture

We report the first finding of equilibrium sapphirine + quartz assemblage from the Palghat-Cauvery Suture Zone (PCSZ) in southern India providing unequivocal evidence for extreme crustal metamorphism at ultrahigh-temperature (UHT) conditions associated with the collisional assembly of the Gondwana supercontinent in the Late Neoproterozoic–Cambrian. The sapphirine and quartz occur as coronas around Mg-rich (XMg ~ 0.58) staurolite within poikiloblastic garnet in an Mg–Al-rich rock, suggesting the progress of the prograde dehydration reaction: staurolite + garnet → sapphirine + quartz + H2O. Although the occurrence of Mg-rich staurolite was previously noted from some localities in the PCSZ, no sapphirine + quartz direct association has yet been reported. The available experimental studies on Mg-rich staurolite indicate that the mineral is stable at P > 15 kbar, and suggest that the complex texture that we report here might correspond to a peak high-pressure regime prior to the UHT event. The symplectic sapphirine + quartz around staurolite probably implies decompression from P > 15 kbar toward the stability of sapphirine + quartz at temperatures of c. 1000 °C along a clockwise P–T path. The prograde high-pressure metamorphism and following UHT event correlate with the subduction–collision tectonics associated with the final stage of amalgamation of Gondwana supercontinent.