Multiple stages of metamorphism from the Eocene to Miocene in the Yardoi gneiss dome, eastern Himalaya: constraints from P-T-t paths

An integrated study of petrography, mineral chemistry, phase equilibrium modelling, geochronology, and geochemistry of zircon and monazite was carried out for three orthogneisses and one paragneiss from the Yardoi gneiss dome in eastern Himalaya, southeastern Tibet. The results indicate that the Yardoi gneisses underwent three metamorphic stages from the Eocene to Miocene and shared similar clockwise P-T paths. First, the paragneiss experienced the early high-pressure Barrovian type metamorphism under P-T conditions of similar to 11-12 kbar and similar to 600-625 degrees C at ca. 44.8 Ma in the middle Eocene, corresponding to thermal gradients of 15.2-17.2 similar to C/km within the Barrovian facies series. This indicates that the crustal rocks were metamorphosed at 36-40 km in eastern Himalaya where crustal thickening is prominent during the continental collision. This is different from UHP eclogites in western Himalaya that were metamorphosed at mantle depths of > 100 km through deep subduction of the continental lithosphere. Afterwards, the first episode of decompressional exhumation is recorded by the paragneiss as occurring from similar to 11-12 kbar to similar to 6-8 kbar at elevated temperatures to similar to 650-675 degrees C during 44.8-22.3 Ma, corresponding to increases of thermal gradients to 24.6-34.1 degrees C/km. Garnet and monazite have a large variation of Eu anomalies that are related to the behavior of plagioclase during this period. Then, the second episode of decompressional exhumation is recorded in the orthogneisses as taking place from 7-10 kbar to 3.5-5.5 kbar at 585-640 degrees C during 37.8-17.8 Ma, which corresponds to significant increases in thermal gradients from 15.5-26.4 to 32.2-55.4 degrees C/km. There have been progressive increases in metamorphic thermal gradients during two episodes of decompressional exhumation, indicating secular changes in both thermal state and dynamic regime in the collisional orogen. Therefore, the Yardoi gneiss dome provides important information on the collision and exhumation of the Indian continental crust from the middle Eocene to Miocene at the convergent plate margin.