LA-ICP-MS zircon U-Pb dating for gneissic garnet-bearing biotite granodiorite in the Yadong area , southern Tibet, China and its geological significance

Yadong Formation-complex is an important precambrian metamorphic strata of the Yadong area in Tibet. LA-ICP-MS method is used to determined the zircon U-Pb isotope age for gneissic biotite granodiorite with garnet insert the Yadong Formation-complex of the Yadong area,Tibet. It′s magmatic crystallization age of (499.2±3.9) Ma (Middle Cambrian), provides a Pan-African tectonic - magmatic event record. Geochemical analysis shows that SiO2 content of 68.04%~ 68.92%, K2O+Na2O-SiO2 diagram into granodiorite region of the sub-alkaline series, A/CNK index greater than 1.1, Shand index diagram in the area fall into peraluminous series. REE patterns show light rare earth enrichment right-inclined curve, and slightly negative Eu anomalies (δEu=0.62~0.79); Trace elements showed the features of Ba, Nb, Ta, Sr loss, and Rb, Th and U relative enrichment .It is considered that the gneissic biotite granodiorite with garnet insert the Yadong Formation-complex of the Yadong area formed in the post-collisional stage though Rb-(Y+Nb) and Rb-Hf-Ta discriminant diagrams.