Early Cretaceous volcanic rocks in Yunzhug area, central Tibet, China, associated with arc–continent collision in the Tibetan Plateau?

Abstract

The Lhasa-Qiangtang collision prior to the Cenozoic collision of India and Asia is a key process for the early stage of tectonic history of the Tibetan Plateau; however, when and how the Lhasa-Qiangtang collision is less well known. In this paper, we present an analysis of petrological, geochemical, Sr–Nd–Hf isotopic data, and zircon LA-ICP-MS UPb data for the volcanic rocks from the Yunzhug area, northern Lhasa. Our new data reveal an important, previously unrecognized stage of tectono-magmatic event in northern Lhasa subterrane. Zircons from the andesites and rhyolites yield concordant ages of 120.24 ± 0.79 Ma (mean square of weighted deviates (MSWD) = 0.35) and 120.1 ± 1.1 Ma (MSWD = 1.18), respectively, which are younger than the age of Jurassic intra-oceanic magmatism and older than the 112–110 Ma Lhasa-Qiangtang collision. The Yunzhug volcanic rocks are composed of andesites and A-type rhyolites of high-K affinity. The andesites originated mainly from a mixed mantle source with variable degrees of contribution of continental crust, and the rhyolites were derived mainly from melting of lower continental crust. Addition of subducted flysch or oceanic sediments resulted in the high-K affinity. Fractional crystallization of plagioclase, K-feldspar, amphibole, and minor microlitic apatite and FeTi oxides minerals in the shallow level is mainly responsible for variable major and trace element contents of the andesites and rhyolites. Our results show that the early Cretaceous Yunzhug volcanic rocks can represent the early stage magmatism recording the entrance of subducted continental crust into the subduction zone, agreement with processes of arc-continent collision from intra-oceanic subduction to continental collision. The presence of oceanic sediments or monazite residual in magma source under the arc-continent zone can cause LREE enrichment to exceed the continental crust, making arc magmatic compositions close to average continental values.