The discovery of the Early Permian adakitic rock in the Hongji Mountain area within central and western Qiangtang, Tibet Plateau, and its geological implications

The adakitic rock was recently found in Permian ophiolite complex of the Hongji Mountain within central and western Qiangtang Block. It intruded into the Permian ophiolite in the form of veinlets. The authors studied petrology, zircon LA-ICP-MS U-Pb age and whole-rock geochemistry of the adakitic rock, with the emphasis placed on its petrogenesis and geological implications. The adakitic rock is muscovite granite. Its high Th/U (0.22~0.63) and clear zoned zircons indicate magamatic origin. Zircon LA-ICP-MS age of muscovite granite is 271.7Ma±2.3Ma, indicating that the crystallization of adakitic rock took place in Early Permian. The adakitic rock is characterized by high SiO2 and Al2O3 and low K2O/Na2O, thus belonging to aluminous-peraluminous granite. The REE model is characterized by enrichment of light REE, moderate fractionations in HREE and small Eu anomalies. The values of Y, Yb are low and the content of Sr is moderate. The primitive mantle normalized trace element spider patterns show that they are enriched in Rb, U, K and have negative anomalies of Nb, Ta, Ti. Geochemical characteristics are similar to features of the adakite formed by partial melting of the subducted oceanic crust. The Hongji Mountain adakitic rock was most probably generated by partial melting of the subducted Paleo-Tethys oceanic crust under the amphibolite-eclogite facies condition, whereas the residual facies were hornblende and garnet. The earlier adakitic melt sufficiently replaced the mantle peridotite, whereas the later melt weakly replaced mantle peridotite. The plagioclase experienced fractional crystallization during the upward migration of the melt. The discovery of Hongji Mountain adakitic rock indicates that the Paleo-Tethys oceanic crust subducted in Early Permian and entered into the regression stage.