Simultaneous growth and reworking of the Lhasa basement: A case study from Early Cretaceous magmatism in the north-central Tibet

Abstract

Magmatic rocks preserve a lot of information about the formation and evolution of continental crust, and they play an important role in understanding the growth and reworking of ancient continent. Cretaceous magmatic rocks occurring in the Lhasa terrane are a window to investigate magmatic processes and continental evolution of the Tibetan Plateau. In this study, we report zircon UPb ages and Hf isotopic data, as well as whole-rock major and trace elemental compositions for Early Cretaceous magmatic rocks, including andesite and dacite from the Zenong Group, and associated diorite and granite in the Xinji area, north-central Tibetan Plateau. Zircon from andesite and dacite yielded weighted-mean 206Pb/238U ages of 120–108 Ma and variable zircon εHf(t) values between −12.3 and +2.2. Geochemical compositions suggested that they both displayed high-K calc-alkaline features. Andesite was probably formed by mixing between crust- and mantle-derived melts, whereas dacite was most likely generated by partial melting of ancient crust with input of mantle-derived material. Dioritic and granitic intrusions were intruded into the Precambrian strata and Zenong Group, respectively. Zircon from diorite gave a weighted-mean 206Pb/238U age of ca. 116 Ma and zircon εHf(t) values of −0.5 to +7.3. Diorite displayed medium-K calc-alkaline series signatures and was most likely formed by partial melting of mantle wedge peridotite accompanied by crustal assimilation. Granite yielded zircon weighted-mean 206Pb/238U ages of 107–104 Ma, and positive zircon εHf(t) values of +1.5 to +6.7, indicating a major source of juvenile crust. Granitic samples show irregular REE distribution with variable Ce anomalies, which were probably inherited from subducted pelagic sediments. The Early Cretaceous magmatic rocks in the Xinji area records distinct formation processes involved partial melting of mantle, re-melting of crust, and mixing between mantle and crust. Such processes, however, had a complicated source from mantle, ancient and juvenile continental crust, and recycled pelagic sediments, suggesting simultaneous crustal growth and reworking of the Lhasa basement during the Early Cretaceous.