The petrogenesis and tectonic significance of the late Early Cretaceous granodiorites in the Nyixung area, western Lhasa block, Xizang

The Cretaceous magmatic rocks widely exposed in the central and western Lhasa block, Xizang may provide important clues to the study of the pre-Cretaceous evolutionary history of the Qinghai-Xizang Plateau. However, there is still considerable debate concerning the petrogenesis and tectonic significance of the Cretaceous magmatic rocks in the restoration of the evolutionary processes of the Plateau. The emphasis in this study is placed upon the late Early Cretaceous granodiorites in the Nyixung area, western Lhasa block, Xizang. The petrographic and whole-rock geochemical studies have disclosed that these rocks have SiO₂ contents from 65.24% to 67.61%, Na₂O/K₂O ratios of 0.91 to 3.93, and aluminium saturation index (A/CNK) values of 0.94 to 1.14%, indicative of I-type calc-alkaline granitic rocks. The Nyixung granodiorites are enriched in LREEs and LILEs (e.g. Rb and Th) and depleted in HREEs and HFSEs (e.g. Nb and Ta), with remarked LREE and HREE fractionation(La/Yb)N=7.6-12.2 and slightly to moderately negative Eu anomalies (δEu=0.6-0.9). The measured two granodiorite samples give the LA-ICP-MS zircon U-Pb ages of 119.7 Ma and 120.5 Ma, respectively, which are dated at the late Early Cretaceous. The results of research in this study have reflected that the granodiorites were derived from the partial remelting of ancient lower crust, and associated with the mingling and mixing of the contemporaneous mantle-derived magmas. It is inferred that the granodiorites may be the products of the northward subduction of the Yarlung Zangbo Neo-tethyan oceanic crust.