Evidence of High-Pressure Metamorphism Along the Mahanadi Shear Zone in the Eastern Ghats Province, Eastern India: Implications on Tectonics and Continental Assembly Involving India and East Antarctica

Abstract

A suite of mafic granulite enclaves within mylonitised felsic gneiss occurring along the E-W trending Mahanadi Shear Zone of the Eastern Ghats Province preserves evidence of high-pressure metamorphism. Garnet-clinopyroxene-bearing mafic granulite contains a mineral assemblage of garnet + clinopyroxene + plagioclase + quartz + rutile which was formed after dehydration melting of a hornblende-bearing protolith during M₁ metamorphism that peaked at 1.1–1.4 GPa, 760°C–840°C. The retrograde stage (M_1R) is marked by the formation of hornblende and symplectic intergrowth of clinopyroxene + plagioclase + orthopyroxene after garnet at 0.8–0.9 GPa, 760°C–810°C, suggesting an isothermal decompression type P–T path. The whole rock trace element and REE characteristics suggest a MORB-OIB protolith for the mafic granulites. The host felsic gneiss has a granitic protolith which was emplaced in an arc setting. The rocks exposed south of the Mahanadi Shear Zone in the Phulbani domain are represented by granulites with contrasting metamorphic characteristics. The garnet-orthopyroxene-bearing mafic granulite within coarse-grained charnockite and the aluminous granulite within felsic gneiss show evidence of biotite dehydration melting. The peak M₁ assemblage in the aluminous granulite is represented by the assemblage spinel + garnet + quartz + plagioclase + K-feldspar which was stable at 0.70–0.74 GPa, 904°C–935°C. M_1R in this rock is characterised by coronas of garnet and sillimanite over spinel and the formation of matrix biotite at 707°C–806°C by near-isobaric cooling. Similar isobaric cooling has been documented from the formation of garnet, clinopyroxene and quartz coronas on orthopyroxene in mafic granulite and garnet and quartz coronas on clinopyroxene, wollastonite and calcite in calc-silicate granulite. The juxtaposition of lower crustal rocks showing clockwise and counterclockwise P–T paths across the Mahanadi Shear Zone implies a paired metamorphic character in a subduction–collision setting. Zircon U-Pb and monazite U-Th-total Pb data show a complex history of the rock suite. The enclave suite of rocks within the Mahanadi Shear Zone underwent peak M₁ metamorphism at ca. 980–960 Ma which was followed by decompression to a shallower level by ca. 960 Ma when the host granitic magma crystallised. Rocks occurring in the Phulbani domain (southernly placed crustal domain), on the other hand, underwent ultrahigh temperature metamorphism at shallower crustal levels broadly at the same time. We argue that the southern Phulbani domain of the Eastern Ghats Province, India, collided with the Angul-Prydz domain of the Rayner Province, East Antarctica which eventually caused underthrusting of the former below the latter across the Mahanadi Shear Zone. In the context of the Eastern Ghats-Rayner reconstruction, this indicates the closure of the intervening Mawson Sea. A second metamorphic event (M₂) reworked the exhumed deep crustal rocks at ca. 900 Ma during the final docking of the Eastern Ghats-Rayner belt against cratonic India. Our results clearly show that the Angul domain is an exotic block, and the Mahanadi Shear Zone is a terrane boundary shear zone suturing discrete domains of the Rayner-Eastern Ghats orogen.