Petrology, phase equilibria modelling, and in situ zircon and monazite geochronology of ultrahigh-temperature granulites from the khondalite belt of southern India

Abstract

The granulite facies metapelite (khondalite) belt in the Trivandrum Block of southern India has been central to investigations on extreme crustal metamorphism associated with the final assembly of the Gondwana supercontinent during late Neoproterozoic-Cambrian. Here we investigate garnet-sillimanite-cordierite-spinel-bearing metapelites from this khondalite belt using state-of-the-art petrologic, mineral phase equilibria modelling, and coupled zircon and monazite U-Pb geochronology to characterize the nature and timing of metamorphism and their tectonic implications. From textural studies and mineral phase equilibrium modelling, we infer that Sill + Grt + Crd representing prograde metamorphism was stable at 6–9 kbar and 760–790 °C. Equilibrium spinel-quartz assemblage suggests peak metamorphic conditions of 6.5–7 kbar and 1010–1030 °C consistent with ultra-high temperature metamorphism in the Trivandrum Block. Growth of cordierite and biotite at the expense of garnet correlates with retrograde metamorphism at 4.5–6.5 kbar and 770–950 °C. Our results allow an interpretation of the prograde, peak, and retrograde P–T conditions of the khondalites, where from the peak ultrahigh-temperature stage of >1000 °C, the rocks underwent isothermal decompression as indicated by garnet breakdown to cordierite, followed by isobaric cooling, corresponding to an overall clockwise P–T evolution. We present results from U-Pb geochronology of zircon as well as LA-ICPMS data on monazite grain separates. The zircon grains show two distinct age peaks, with Paleoproterozoic cores surrounded by Late Neoproterozoic-Cambrian rims or recrystallized domains. The younger ages around 550–560 Ma are similar to the lower intercept age, whereas the wide range of Paleoproterozoic ages fall along a discordia. The LA-ICP-MS monazite U-Pb data also show two distinct age populations at Paleoproterozoic and latest Neoproterozoic-Cambrian, although the Paleoproterozoic population is scarce. We correlate the late Neoproterozoic-Cambrian ages to the ultrahigh-temperature metamorphic event, associated with the assembly of the Gondwana supercontinent. The wide range of monazite ages indicate that high temperature metamorphism was long-lived.