EARLY OLIGOCENE PARTIAL MELTING AND PROLONGED LOW P/T ANATEXIS OF THE UPPER HIGH HIMALAYA CRYSTALLINE SEQUENCES IN FAR-EASTERN NEPAL

Understanding of the mechanism on partial melting and tectonics on the formation of High Himalayan Discontinuity (HHD) within the High Himalaya Crystalline Sequence (HHCS) are keys to investigate the tectonics of the Tibet-Himalaya orogen. The existent of the HHD suggests that the HHCS evolved as composites of different massifs, rather than a single massif, and it divides the whole HHCS into the upper and lower HHCSs. In this study, the Early Oligocene partial melting and prolonged low P/T anatexis have been investigated from the occurrences of cordierite migmatites, sillimanite-K-feldspar migmatites, and granitic orthogneisses in the upper HHCS, far-eastern Nepal, which have been poorly understood before. The petrological and geochronological studies revealed that these migmatites were formed by the Early Oligocene (ca. 33–25 Ma) biotite dehydration melting (biotite + sillimanite + plagioclase + quartz → garnet + K-feldspar + melt ± cordierite) at ca. 800 °C. The cordierite is only produced in the cases that the reaction occurred in the upper crust level (below 6 kbar). After the Early Oligocene partial melting, the low P/T anatexis lasted up to 17 Ma in the upper crust level during exhumation, but not in the middle crust level. The Early Oligocene biotite dehydration melting in the upper HHCS occurred at distinct period and location from the Early Miocene muscovite dehydration melting in the underlying HHCS, and the metamorphic discontinuity was accompanied by the thrusting of the HHD at ca. 27–19 Ma before activity of the Main Central Thrust. Pervasive partial melting and prolonged low P/T anatexis in the upper HHCS may be preferable to be a lateral southward channel flow of the upper HHCS along the HHD, whereas current channel flow models that the exhumation of the HHCS was driven only by the coupled activity of MCT and STD have limitations to explain the timing of low P/T metamorphism observed in the upper HHCS.