Petrogenesis and tectonic setting of Early Cretaceous magmatism in the Jiwa area, central Lhasa Terrane, Tibet

Abstract

ABSTRACTNew zircon LA-ICP-MS U–Pb ages, Sr-Nd isotopic data, and whole-rock major and trace element data from Early Cretaceous volcanic rocks are reported for the Jiwa area in the southern central Lhasa Terrane of Tibet. These mainly silicic volcanic rocks and subordinate intermediate-basic volcanic rocks have long been considered to be Pliocene (Wuyu Group) or Eocene (Pana Formation) in age. However, our new zircon U–Pb ages constrain the timing of eruption to the Early Cretaceous (124.6 ± 1.6–126.1 ± 1.1 Ma); thus, we have redefined these volcanic rocks as the Lower Cretaceous Zenong Group. The silicic volcanic rocks feature high-K calc-alkaline to shoshonitic compositions and are mostly strongly peraluminous, rich in Rb, Th, and light rare earth elements (LREEs), and depleted in Nb, Ta, P, and Ti. They are also characterized by negative whole-rock εNd(t) values (–9.1 to –13.1) and variable 87Sr/86Sr ratios. Thus, the geochemical and zircon U–Pb age data of the Jiwa volcanic rocks suggest that these rocks are associated with a continental arc and are mostly likely derived from anatexis of ancient continental crustal material and minor basalt-derived melts. The discovery of Early Cretaceous volcanic rocks in the southern central Lhasa Terrane extends the duration of magmatism triggered by southward subduction of the Bangong-Nujiang oceanic lithosphere from the Late Jurassic to the Early Cretaceous. The spatial distribution of magmatism is also extended 70–80 km to the south. The Lower Cretaceous volcanic rocks in the Jiwa area are proposed to be a result of bi-directional subduction, with southward subduction of the Bangong-Nujiang oceanic crust and northward subduction of the Yarlung Zangbo oceanic crust. The bi-directional subduction of the oceanic lithosphere and gravitational sinking led to slab retreat at ca. 125 Ma. The roll-back of the slab would have then led to back-arc extension and asthenospheric upwelling. The subduction-induced decompression melting of the mantle led to the generation of widespread rhyolitic volcanism with continental arc geochemical signatures.