Tracking C-O-H fluid-rock interactions in reworked UHT granulite: Tectonic evolution from ca. 990 Ma to ca. 500 Ma in orogenic interior of Eastern Ghats Belt, India

Abstract

A suite of high-grade granulite rocks occurring close to one of the earliest reported ultra-high temperature rocks (“Paderu” rocks) from the central part of the Eastern Ghats Belt, India has been studied. Here we present a detailed pressure-temperature-time-fluid history of the suite of aluminous granulite and associated gneisses from a single exposure, which were injected by granitic aplite veins at a high angle at a ductile depth of the host gneissic rocks. Detailed studies of the sapphirine-spinel-quartz-bearing aluminous granulite reveal a peak metamorphic condition of ~1000 °C at ~8 kbar. Porphyroblastic garnet and quartz in the peak assemblage have mono-phase, high-density, CO2-rich primary fluid inclusions (~1 g/cm3). Orthopyroxene-garnet-sillimanite forming corona textures in the aluminous granulite and the presence of cordierite-K-feldspar-quartz intergrowths imply post-peak cooling and subsequent decompression down to ~6 kbar. Secondary fluid inclusions in quartz grains of the aluminous granulite have the signature of bi-phase CO2-H2O fluids with a comparatively low estimated density (0.8 g/cm3). The CO2 isochore plots suggest a pressure drop from 7 to 8 kbar to 4–5 kbar (simultaneous or after the cooling from 1000 °C to 800 °C). Texture-constrained monazite dating using U-Th-total Pb EPMA technique reveals that there are strong peaks at ~940 Ma, ~900 Ma, and ~ 800 Ma with several smaller and younger peaks. On the other hand, the UPb SHRIMP dating of zircon from the same rock reveals discordant ages with upper and lower intercepts at ~1700 Ma and ~ 990 Ma. Late pegmatite dykes and aplite veins were emplaced cutting across the dominant structural fabric of this litho-assemblage at a high angle. The asymmetric dragging of the foliation of the host leptynite possibly implies a shear-induced intrusion. The fluid inclusion study of the aplite reveals that the quartz grains contain numerous primary and pseudosecondary fluid inclusions (CO2 monophase to biphase). The calculated density is in the range of 0.71–0.82 g/cm3, suggesting the entrapment of carbonic fluid at <3 kbar. Monazite dating yields a strong single peak at 497 ± 4 Ma, one of the rare dates from the orogenic interior of the Eastern Ghats Belt. The studied suite of deep crustal granulites with aplite veins preserves a history of at least two events of fluid-rock interactions during its residence at the extremely hot deep crustal level until its exhumation to shallower levels where a magmatic fluid is trapped, through a time corridor between the Grenvillian and Kuunga orogenies.