Cenozoic ultrahigh-temperature metamorphism in pelitic granulites from the Mogok metamorphic belt, Myanmar

Ultrahigh-temperature (UHT) metamorphism is critical for understanding the most extreme thermal evolution of continental crust. However, UHT metamorphism predominantly occurred in the Precambrian and is rarely observed in the modern Earth. Here, we report the discovery of similar to 25 Ma UHT granulites from the Mogok metamorphic belt (MMB) in Myanmar via a combined study of petrology and geochronology. The studied pelitic granulites well preserve a peak mineral assemblage of garnet + sillimanite + plagioclase (antiperthite) + K-feldspar + quartz + Ti-rich biotite + rutile + ilmenite. Pressure (P)-temperature (T) pseudosections and conventional geothermobarometry data only constrain the P-T conditions of the peak stage to <12 kbar and 780-890 degrees C. However, high Zr contents in the matrix rutile (3005-4308 ppm) and high Ti contents (up to 9.2 wt% TiO2) in the biotite demonstrate that the Mogok granulites may have experienced UHT metamorphism. The Zr-in-rutile thermometer and X-Grs isopleth in the pseudosections yield peak P-T conditions of similar to 12 kbar and >900 degrees C. In situ SIMS and LA-ICP-MS U-Pb dating and trace element analyses show that both metamorphic zircon cores and rims have flat heavy rare earth element (HREE) patterns with negative Eu anomalies. The metamorphic zircon rims show the lowest HREE contents and yield Pb-206/U-238 ages of 24.9 +/- 0.5 and 25.4 +/- 0.6 Ma, respectively, representing the timing of UHT metamorphism. Our results indicate that the central MMB underwent similar to 25 Ma UHT metamorphism, which is possibly induced by continental rifting along the thinned orogenic lithosphere. Our data, as well as reported Cenozoic UHT events, further suggest that UHT metamorphism can be produced in the modern plate tectonic regime by lithospheric extension.