Geochemical and Sr–Nd isotopic characteristics of Cretaceous to Paleocene granitoids and volcanic rocks, SE Tibet: Petrogenesis and tectonic implications

Abstract

Northward subduction of the Neo-Tethyan oceanic lithosphere beneath South Asia gave rise to an Andean-type convergent margin with arc magmatism starting since at least the early Jurassic and lasting until the Eocene. This study reports geochemical and Sr–Nd isotope data of the early Cretaceous (∼133–110Ma) to Paleocene (∼60Ma) magmatic belt that formed in the eastern Lhasa terrane, SE Tibet, including three major Transhimalayan batholiths, namely, Azhagong, Demulha, and Chayu, and an early Cretaceous volcanic succession from Ranwu area. All these rocks show relative enrichments in LILE and LREE, and depletions in HFSE, corresponding to the geochemical characteristics of arc magmas from modern subduction zones. While the Azhagong batholith consists typically of metaluminous I-type granitoids that occasionally contain mafic enclaves, highly fractionated, peraluminous S-type and A-type granites are observed from the Chayu and Demulha batholiths, respectively. These granitoids, as a whole, show heterogeneous Sr and Nd isotope ratios, with εNd(T) varying from −1.5 to −13 and ISr from 0.703 to 0.745. The Ranwu volcanic rocks, which range from basaltic to dacitic compositions, are calc-alkaline in nature. They have εNd(T) from +3 to −6 and ISr from 0.705 to 0.707, overlapping the isotopic range of the Azhagong mafic enclaves. The overall geochemical and Sr–Nd isotope data suggest three magma source components involved in the petrogenesis, which are the mantle wedge, and the lower and upper parts of the Lhasa continental crust. Accordingly, we propose a two-stage petrogenetic model involving (1) deep level differentiation of the mantle wedge-derived mafic magmas associated with assimilation by the lower continental crust of the Lhasa terrane and (2) additional differentiation and upper crustal contamination of the evolved magmas as they rise and install in shallow-level magma chambers. While the first stage can explain how the Ranwu volcanics and Azhagong enclaves formed, the second accounts for the emplacement of the Azhagong and Chayu batholiths. The shallow intrusions, moreover, may have caused low-pressure melting of existing calc-alkaline rocks of Paleoproterozoic Nd isotope model age to produce the early Cretaceous (∼125Ma) A-type granites in the Demulha batholith, a specific rock type that occurs rarely in subduction zones but echoes with the notion for an extension setting in the region. This setting may have taken place after the late Jurassic-early Cretaceous continental collision between the Lhasa and Qiangtang terranes, and interacted with the back-arc evolution in the Neo-Tethyan subduction system.