Identification and origin of the Late Oligocene to Miocene pyroclastic rocks in the Lunpola Basin and link with deep geodynamics in the Lhasa terrane, Tibetan Plateau

Tephrochronology provides a powerful tool to date and correlate the sedimentary sequence. However, the recognition of tuff and tuffinite in Cenozoic sequences is challengeable because of modification and alteration during diagenesis. In this contribution, we studied samples from the Cenozoic Lunpola basin on the central Tibetan Plateau to explore the potential provenance and distinction of pyroclastic materials from clastic sediments and to recognize the source-to-sink process of pyroclastic rocks and their deep geodynamic implications. The bulk mineralogical compositions and major and trace elements and Sr-Nd-Hf isotopic compositions of the pyroclastic rocks and clastic sediments in the Lunpola Basin were determined and compared with those of contemporary volcanic rocks in the Lhasa terrane. It is suggested that Th/U-Rb/Sr and (La/Yb)N-Na2O/Al2O3 discrimination plots might be used to distinguish pyroclastic from clastic sedimentary rocks in terms of source, transportation and diagenesis. According to the trace element and Sr-Nd and Hf isotope signatures, the 23.5 Ma pyroclastic rocks in the Lunpola Basin may originate from the Xungba ultrapotassic magmatism, and the - 21-15 Ma pyroclastic rocks may have been supplied by corresponding adakitic magmatism. The spatial distributions of the ultrapotassic and potassic as well as adakitic volcanic rocks in the Lhasa terrane may reveal that the west Lhasa terrane mantle lithosphere was detached and the east Lhasa terrane lower crust was thickened along with the India-Asia plate convergence.