Petrology And Geochronology Of HP Pelitic Granulites Of The Yadong Area In The Central Himalayan Orogen

The Higher Himalayan Crystalline Sequences(HHS) in the core of the Himalayan orogenic belt were produced by the Cenozoic collision of Indian and Asian plates, representing an excellent natural laboratory to study continental subduction, crustal melting and tectonic evolution of collisional orogen. Although many studies about the Himalayan orogeny have been carried out, the metamorphic conditions, metamorphic time, P-T-t path of the HHS is still controversial. Here we present a detailed petrological and zircon U-Pb geochronological study of the meta-pelitic granulites from the Yadong area, the central Himalayan orogen. The results show that the meta-pelitic granulites are composed of quartz, garnet, biotite, sillimanite, plagioclase, K-feldspar, kyanite, cordierite in variable proportions, and with minor rutile, ilmenite and zircon as accessory minerals. Garnet occurs as porphyroblasts and contains abundant mono- or poly-mineral inclusions in their cores. Some biotite, sillimanite and cordierite occur as corona around garnet, representing the retrograde minerals. The granulites underwent complex metamorphic evolution and associated partial melting, with three stages of mineral assemblage. The early prograde metamorphic mineral assemblage is garnet + biotite + plagioclase + K-feldspar + muscovite + quartz, the peak-metamorphic mineral assemblage is garnet + kyanite + biotite + plagioclase + K-feldspar + quartz + rutile + ilmenite, and the retrograde metamorphic assemblage is garnet + sillimanite + biotite + plagioclase + K-feldspar + quartz ± cordierite + ilmenite. Phase equilibria modeling shows that the granulites underwent high-temperature and high-pressure peak-metamorphism under conditions of 825~835 °C and 12.0~14.5 Kbar. Connecting the P–T fields constrained by the prograde, peak and retrograde mineral assemblages, a clockwise P–T path was constrained, characterized by heating and burial prograde, and early isothermal decompression and subsequent isobaric cooling retrogression. The anatexis of the HHS occurred through muscovite- and biotite-dehydration melting during the prograde and peak metamorphic process and with a melt modal content of up to 18~32 vol.%, providing a potential source for the Himalayan leucogranites. The zircon U-Pb dating and trace element analysese show that the dated zircon grains display weak regular zoning, and have low HREE contents and flat or even depleted HREE patterns with weak negative Eu anomalies, and low Th/U ratios. Thus features are of typical metamorphic zircon that grew coevally with garnet and feldspar during granulite-facies metamorphism and partial melting. The obtained ages of 31.3~16.7 Ma represent the granulite facies metamorphic and anatectic time. This study shows that the upper structural level of the HHS experienced high-temperature and high-pressure granulite facies metamorphism, rather than high-temperature and low-pressure metamorphism proposed by previous investigations, and providing a new insight into tectonic evolution of the Himalayan orogen.