Zircon U-Pb geochronology, geochemistry and Sr-Nd-Hf isotopes of the Eocene alkali -rich magmatic rocks in the Jianchuan area of western Yunnan and their implications for petrogenesis

Located in the southern section of the Sanjiang region on the southeastern margin of the Qinghai-Tibet Plateau, alkali-rich magmatic rocks in the Jianchuan area of western Yunnan are important part of the Jinsha River-Red River alkali-rich magmatism belt. These alkali-rich magmatic rocks include granites and syenites. The former includes granite porphyry and quartz-monzonite porphyry, and the later consists of syenite porphyry and trachyte. In this paper, we report major and trace elemental composition, zircon U-Pb geochronology and Sr-Nd-Hf isotopic composition of these alkali-rich magmatic rocks. The zircon U-Pb dating indicates that the granites and syenites formed at 35. 1 similar to 36. 1Ma and 35. 7 similar to 35. 8Ma, respectively, consistent with the Cenozoic alkali-rich magmatic rocks in western Yunnan. Geochemical results showed that the granite porphyry and quartz-monzonite porphyry belong to high-K calc-alkalic series, with Si02 contents of 67. 92% similar to 69. 93% and K2O/Na2O ratios of 0. 86 similar to 1. 22. The syenite porphyry and trachyte belong to shoshonite series, with Si02 contents of 53. 94% similar to 63. 51% and K2O/Na2O ratios of 1. 20 similar to 2. 68. They are both enriched in alkali, K2 0, light rare-earth elements ( LREEs) and large ion lithophile elements ( LILEs), but depleted in high field strength elements ( HFSEs) relatively. The granite porphyry and quartz-monzonite porphyry geochemically show adakitic affinity, characterized by high Sr contents and high ratios of Sr/Y, La/Yb, but low Y and Yb contents. These geochemical signatures, along with Sr-Nd-Hf isotopic compositions, reveal that the granites were product of partial melting of thickened mafic lower crust and the syenites were derived from direct partial melting of metasomatized lithospheric mantle with various degree of fractionation. These alkali-rich magmatic rocks in the Jianchuan area formed during the late stage of collision between Indian and Eurasian continents. The Cenozoic alkali-rich magmatism is the response to asthenospheric upwelling, following the thinning of mantle lithosphere in western Yunnan.