Ultrahigh-temperature metamorphism and decompression history of sapphirine granulites from Rajapalaiyam, southern India: implications for the formation of hot orogens during Gondwana assembly

Abstract

AbstractSapphirine-bearing Mg–Al granulites from Rajapalaiyam in the southern part of the Madurai Block provide critical evidence for Late Neoproterozoic–Cambrian ultrahigh-temperature (UHT) metamorphism in southern India. Poikiloblastic garnet in quartzo-feldspathic and pelitic granulites contain inclusions of fine-grained subidioblastic to xenoblastic sapphirine associated with quartz, suggesting that the rocks underwent T > 1000°C peak metamorphism. Quartz inclusions in spinel within garnet are also regarded as clear evidence for a UHT condition. Inclusions of orthopyroxene within porphyroblastic garnet in the sapphirine-bearing rocks show the highest Al2O3 content of up to 10.3 wt%, suggesting T = 1050–1070°C and P = 8.5–9.5 kbar. Temperatures estimated from ternary feldspar and other geothermometers (T = 950–1000°C) further support extreme thermal metamorphism in this region. Xenoblastic spinel inclusions in sapphirine coexisting with quartz suggest that the spinel + quartz assemblage pre-dates the sapphirine + quartz assemblage, probably implying a cooling from T ~ 1050°C or an anticlockwise P–T path. The FMAS reaction sapphirine + quartz + garnet → orthopyroxene + sillimanite indicates a cooling from the sapphirine + quartz stability field after the peak metamorphism. Corona textures of orthopyroxene + cordierite (± sapphirine), orthopyroxene + sapphirine + cordierite, and cordierite + spinel around garnet suggest subsequent near-isothermal decompression followed by decompressional cooling toward T = 650–750°C and P = 4.5–5.5 kbar. The sapphirine–quartz association and related textures described in this study have an important bearing on the UHT metamorphism and exhumation history of the Madurai Block, as well as on the tectonic evolution of the continental deep crust in southern India. Our study provides a typical example for extreme metamorphism associated with collisional tectonics during the Late Neoproterozoic–Cambrian assembly of the Gondwana supercontinent.