ULTRAHIGH-TEMPERATURE METAMORPHISM OF SAPPHIRINE-BEARING GRANULITES FROM RAJAPALAIYAM, SOUTHERN INDIA, AND THEIR CORRELATION WITH GONDWANA ASSEMBLY

The Madurai Block is the largest composite crustal block in the South Granulite Terrane of India where granulite-facies rocks metamorphosed under ultrahigh-temperature (UHT) conditions occur in several localities. Here we investigate UHT rocks from Rajapalaiyam in the southern domain of this block using modern tools in petrology to precisely constrain the nature and timing of the extreme metamorphism and determine its implications for regional tectonics. Phase equilibrium modeling reveals prograde P–T conditions of 7.5–12 kbar and <900 °C, followed by peak/post-peak UHT metamorphism at 7.2–8.2 kbar and 1025–1050 °C, and retrograde metamorphism at 7.25–8.0 kbar and 875–895 °C. The granulites thus record a clockwise P–T path with geothermal gradients ranging from 1200–1500 °C/GPa for peak metamorphism followed by near-isobaric cooling. In-situ monazite U–Pb dating indicates prograde metamorphism at 607–585 Ma, peak at 546–454 Ma, and retrogression at 539–483 Ma. Similar UHT rocks in fragments of eastern Gondwana, including Sri Lanka and the Lützow-Holm Complex of East Antarctica, also formed at c.550 Ma, suggesting that the extreme thermal event was associated with supercontinent assembly during late Neoproterozoic–Cambrian.