Geochronology and Geochemistry of Early Cretaceous Granitic Rocks in the Dongqiao Area, Central Tibet: Implications for Magmatic Origin and Geological Evolution

摘要

We present here new geochronological and whole-rock geochemical data for the Dongqiao intrusive rocks. Six samples yield zircon U-Pb ages of 111–115 Ma, indicating that the Dongqiao granitic rocks are contemporaneous with an Early Cretaceous (115–110 Ma) magmatic flare-up event in central Tibet. On the basis of geochemical data, granitic rocks from the Dongqiao area can be divided into I- and A-type granites. The A-type granites contain higher Na_(2)O + K_(2)O, Zr, Hf, and Zr + Ce + Nb + Y contents than the I-type granites. The results indicate that I-type granites were formed by partial melting of a juvenile mafic lower-crustal source, while A-type granites were derived from a middle-crustal source, followed by fractional crystallization. The formation of A-type granites requires broad thermal anomalies, indicating that Early Cretaceous magmatism occurred in a postcollision extensional setting, likely related to the upwelling of hot asthenospheric mantle triggered by detachment and sinking of the Bangong-Nujiang oceanic lithosphere.