Extreme thermal metamorphism associated with Gondwana assembly: Evidence from sapphirine-bearing granulites of Rajapalayam, southern India

Abstract

Abstract The Madurai block is the largest composite crustal block in the Southern Granulite terrane of India, where granulite-facies rocks metamorphosed at ultrahigh-temperature (UHT) conditions occur in several localities. Here, we investigated UHT rocks from Rajapalayam, in the southern domain of the Southern Granulite terrane, using integrated thermobarometry and in situ monazite geochronology to precisely constrain the nature and timing of this extreme metamorphism and its implications for regional tectonics. Conventional thermobarometry and petrological phase equilibrium modeling reveal prograde pressure-temperature (P-T) conditions at 0.75–1.2 GPa and <900 °C, followed by peak/postpeak UHT metamorphism at 0.72–0.82 GPa and 1025–1050 °C, and retrograde reequilibration at 0.72–0.80 GPa and 875–895 °C. The granulites thus record a clockwise P-T path defining geothermal gradients of 1200–1500 °C/GPa at peak metamorphism, indicating the presence of an extreme thermal perturbation in the middle to lower continental crust. In situ monazite dating indicates prograde metamorphism at 607–585 Ma, peak metamorphism at 546–543 Ma, and retrograde cooling and exhumation at 539–483 Ma. As such, the entire tectonothermal cycle was complete within ∼120 m.y., although temperatures exceeding 900 °C were likely sustained for at least 30 m.y. Such extreme thermal events preserved in geological terranes worldwide are commonly associated with lithospheric extension, although our data show that prolonged heating can occur during continental convergence instead, supporting inferences made by thermomechanical models. Thus, supercontinent formation may act as a driver for spatially distributed UHT tectonometamorphism, as shown by the episodic records in geological history. The age of peak metamorphism constrained here was synchronous with UHT metamorphism in other localities in the Southern Granulite terrane, Sri Lanka, Madagascar, and Antarctica, indicating their correlation with the final amalgamation of eastern Gondwana at ca. 550 Ma.