【摘 要】
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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
【机 构】
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State Key Laboratory of Lithospheric Evolution,and Institutions of Earth Science,Institute of Geolog
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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～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℃.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 XGrs isopleth in the pseudosections yield peak P-T conditions of～12 kbar and ＞900℃.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 206pb/238U 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 ～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.
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