Magmatic and metamorphic evolution of a layered gabbro-anorthosite complex from the Coorg Block, southern India: Implications for Mesoarchean suprasubduction zone process

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As one of the oldest crustal blocks in Southern Peninsular India, the Coorg Block has been the focus of investigations related to crustal evolution in the early history of the Earth with implications on Mesoarchean plate tectonic processes. The Coorg Block is dominantly composed of arc magmatic rocks and bordered on the north by the Mercara Suture Zone, a Mesoproterozoic subduction-collision zone hosting extruded high-pressure and ultra-high temperature metapelitic and meta-mafic rocks. Here, we report the occurrence of a dismembered gabbro-anorthosite complex corresponding to a layered intrusion from the northern margin of the Coorg Block. We present results from an integrated study on the petrology, mineral chemistry, P-T phase equilibria, whole-rock geochemistry, and zircon and monazite U–Pb geochronology to understand the magmatic and metamorphic evolution of the layered intrusion and associated rocks. The geochemical data suggest that the parental magma was generated within a suprasubduction zone setting. Phase equilibrium modelling of garnet-bearing metagabbro from the Coorg Block suggests metamorphic P–T range of 10.5–11 kbar at 1000–1100 °C corresponding to ultra-high temperature conditions. The zircon U-Pb data from the various rock types of the mafic–ultramafic suite yield weighted mean ages at 3176 Ma, 3174 Ma, 3143 Ma, and 3124 Ma whereas the associated charnockite shows a slightly older age of 3319 ± 12 Ma. Metamorphic zircon from charnockite yield an age of 3101 Ma, close to the monazite age of 3110 ± 24 Ma from the same rock, constraining the timing of metamorphism as Mesoarchean, and marking the collisional event between the Coorg Block and the Dharwar Craton along the Mercara Suture Zone. We propose a tectonic model involving southward subduction of the Western Dharwar Block beneath the northern margin of the Coorg Block that can explain the extensive arc magmatism and suprasubduction zone rock suites, followed by ocean closure and collision along the Mercara Suture Zone, accompanied by high P-T metamorphism.

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Gondwana Research