The Greater Himalayan Crystalline Sequence (GHS),located in the core of the Himalayan orogen,was derived from metamorphism of the subducted Indian continent,representing the ideal object to study the formation and tectonic evolution of collisional orogens.Here we present a detailed petrological,phase equilibria modeling and zircon and monazite U-Pb geochronological study of the orthogneiss from the upper structural level of the GHS in Cona region,eastern Himalaya.The results show that these rocks have the peak assemblage of garnet + plagioclase + K-feldspar + biotite + muscovite + quartz + ilmenite,and anatectic textures.The garnet grains preserve growth zoning.Phase equilibria modeling reveals that the orthogneisses have peak-metamorphic conditions of 710 ～750℃ and 9.0 ～ 10.5kbar,and records a clockwise P-T path with a prograde process of both temperature and pressure increase and partial melting,and subsequent retrogression of cooling and decompression.The zircon and monazite U-Pb dating indicates that the orthogneiss have protolith ages of 510 ～490Ma,and retrograde process between ～27Ma and 11Ma.The present study shows that the upper structural level of the GHS underwent upper amphibolite-facies metamorphism and partial melting,providing a key constraint on the tectonic evolution of the Himalayan orogen.%位于造山带核部的高喜马拉雅结晶岩系是由印度大陆俯冲到亚洲大陆之下经历变质作用的产物,是研究喜马拉雅造山带形成与演化过程的理想载体.本文对造山带东段错那地区高喜马拉雅结晶岩系上部构造层位的正片麻岩进行了岩石学、相平衡模拟,锆石与独居石U-Pb年代学研究.研究结果表明这些岩石的峰期矿物组合为石榴石+斜长石+钾长石+黑云母+白云母+石英+钛铁矿,保留有深熔作用的结构特征.岩石中的石榴石具有生长成分环带.相平衡模拟表明,岩石的峰期变质条件为710 ～ 750℃和9.0～ 10.5kbar,具有一个顺时针型变质作用P-T轨迹,其进变质过程以升温、升压和部分熔融为特征,退变质作用为降温、降压过程.锆石与独居石U-Pb定年表明,这些正片麻岩具有510 ～ 490Ma的原岩年龄,和27～11Ma的退变质时间.本研究表明高喜马拉雅结晶岩系的上部构造层位经历了高角闪岩相变质作用与部分熔融,为造山带的构造演化提供了重要信息.
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