Ultrahigh-temperature metagabbros from Wynad: Implications for Paleoproterozoic hot orogen in the Moyar Suture Zone, southern India

Abstract

The Wynad region is located along the confluence of the Mercara and Moyar Suture Zones in southern India which mark major zones of amalgamation of microcontinental blocks during Neoarchean to Paleoproterozoic. Here we report garnet- and clinopyroxene-bearing metagabbros from this region and present petrological, geochemical, and zircon U-Pb and Lu-Hf isotopic data. The prograde, peak, and retrograde mineral assemblages of the metagabbro are garnet+clinopyroxene+plagioclase+ilmenite+hornblende+quartz, garnet+clinopyroxene+plagioclase+ilmenite, and garnet+clinopyroxene+plagioclase+ilmenite+hornblende+orthopyroxene, respectively. The geochemical data showing nearly flat or slightly depressed LREE and constant HREE patterns, with the absence of negative Nb anomaly in primitive mantle-normalized plots suggest N-MORB affinity for the protolith, indicating remnants of oceanic lithosphere accreted during the subduction-collision event associated with the microblock amalgamation. The metamorphic conditions of the rock were estimated using phase equilibria modeling in the system NCFMASHTO which yield P-T conditions of >960°C and >12.8kbar, suggesting high-pressure and ultrahigh-temperature conditions. The prograde P-T conditions are inferred as 7.8–8.3kbar at 600°C and 10.6–11.8kbar at 900°C, whereas the retrograde P-T conditions computed based on hornblende- and orthopyroxene-bearing mineral assemblages show <660°C and <10.4kbar. A hairpin-type clockwise P-T path is inferred from the P-T phase equilibria modeling, suggesting rapid heating and cooling in the lower crustal level. This P-T path is in contrast to those reported from other meta-mafic rocks in southern India. The absence of any decompression texture involving garnet also supports near-isobaric cooling of the rock. Our zircon U-Pb data suggest Neoarchean (2.51Ga) magmatism and Early Paleoproterozoic (2.36–2.22Ga) high-grade metamorphism. Zircon Lu-Hf data indicate that the protolith magma was derived from Meso- to Neoarchean juvenile components with very limited reworked crustal materials. Our study reveals Paleoproterozoic ultrahigh-temperature metamorphism associated with the hot orogen formed during the collision between the Coorg and Nilgiri Blocks.