Geochemistry and zircon U-Pb dating of the Saizhuo granodiorite in Tibet and their tectonic significance

This paper reports chronological and geochemical studies on the Saizhuo granodiorite in the Risong area, western Tibet. Zircon U-Pb dating yields an age of 106.0±1.0 Ma, indicating that the Saizhuo granodiorite emplaced in Early Cretaceous. The granodiorite has A/CNK ratios of 0.96～1.02 and K₂O/Na₂O ratios of 1.06～1.30. Compared with the CIPW compositions, most samples contain hypersthene and part of samples contains diopside. A large amount of mafic microgranular enclaves occur within granodiorites, suggesting that the Saizhuo granodiorite belongs to metaluminous to weakly peraluminous high-K calc-alkaline series I-type granitoid. The rocks have a total REE content of(97.84～143.44)×10^-6 and distinct enrichment of LREE, showing a distinct right-oblique feature. The negative Eu anomalies are obvious, with δEu values of 0.63～0.73. They are also enriched in Rb, Th and U, but depleted in K, Ba, Sr, Nb, P and Ti. Considering regional geology of the Risong area, the Saizhuo granodiorite is inferred to form in an island arc setting related to the southward subduction of the Bangonghu-Nujiang oceanic crust, which provides new evidence for the tectonic evolution of the Bangonghu-Nujiang Tethys Belt.