Pressure‐temperature‐time evolution of ultrahigh‐temperature granulites from the Trivandrum Block, southern India: Implications for long‐lived high‐grade metamorphism

Abstract

The Southern Granulite Terrane in India comprises a number of Archean to Neoproterozoic magmatic arcs and supracrustal units that underwent latest Neoproterozoic to Cambrian high‐grade metamorphism. Here, we present a new petrological and geochronological data from khondalites in the western part of the Trivandrum Block and discuss pressure‐temperature‐time (P–T–t) path of the block for unravelling the duration and heat source of high‐grade metamorphism. Phase equilibria modelling of the khondalite indicates peak P–T condition of 920°C–1,030°C and 6.0–7.6 kbar, suggesting ultrahigh‐temperature (UHT) metamorphism. Prograde and retrograde P–T conditions of ~750°C/~7 kbar and ~750°C/~4 kbar, respectively, were also obtained, based on which a clockwise P–T path with geotherm‐parallel slow cooling is inferred. Zircon and monazite U–Pb geochronology and rare‐earth elements (REE) patterns suggest that heavy rare‐earth elements (HREE)‐depleted zircons grew together with garnet during prograde partial melting at >810°C or during fluid activity at around 582 ± 17 Ma, which was followed by peak UHT metamorphism at 555.1 ± 8.1 Ma as inferred from the dominant monazite ages. Relatively HREE‐enriched zircons (527.3 ± 8.0 Ma) and monazites (501.9 ± 8.5 Ma) were probably formed by garnet breakdown during retrograde metamorphism. The growth of HREE‐enriched zircons at 489 ± 12 Ma might be related to later fluid infiltration and hydration of garnet to form biotite at <770°C/~4 kbar. Our results suggest that high‐grade metamorphism continued at least 90 Myr, from 582 to 489 Ma, suggesting a long‐lived thermal event possibly related to the input of radiogenic heat from the crust and/or magmatic heat from syntectonic to post‐tectonic intrusions.