Late Cretaceous (ca. 95 Ma) magnesian andesites in the Biluoco area, southern Qiangtang subterrane, central Tibet: Petrogenetic and tectonic implications

Abstract

The tectonic evolutionary history of the Lhasa and Qiangtang collision zones remains hotly debated because of the lack of pivotal magmatic records in the southern Qiangtang subterrane, central Tibet. We present zircon U–Pb dating, whole-rock major and trace-element geochemical analyses, and Sr–Nd isotopic data for the newly discovered Biluoco volcanic rocks from the southern Qiangtang subterrane, central Tibet. Zircon U–Pb dating reveals that the Biluoco volcanic rocks were crystallized at ca. 95 Ma. The samples are characterized by low SiO2 (50.26–54.53 wt%), high Cr (109.7–125.92 ppm) and Ni (57.4–71.58 ppm), and a high Mg# value (39–56), which plot in the magnesian andesites field on the rock classification diagram. They display highly fractionated rare earth element patterns with light rare earth element enrichment ([La/Yb]N = 21.04–25.24), high Sr/Y (63.97–78.79) and no negative Eu anomalies (Eu/Eu* = 0.98–1.04). The Biluoco volcanic rocks are depleted in Nb, Ta and Ti and enriched in Ba, Th, U and Pb. Moreover, the eight samples of Biluoco volcanic rocks display constant (87Sr/86Sr)i ratios (0.70514–0.70527), a positive εNd(t) value (2.16–2.68) and younger Nd model ages (0.56–0.62 Ga). These geochemical signatures indicate that the Biluoco volcanic rocks were most likely derived from partial melting of the mantle wedge peridotite metasomatized by melts of subducted slab and sediment in the subducted slab, invoked by asthenospheric upwelling resulting from the slab break-off of the northward subduction of the Bangong–Nujiang oceanic lithosphere. Identification of ca. 95 Ma Biluoco magnesian andesites suggests they were a delayed response of slab break-off of the northward subduction of the Bangong–Nujiang oceanic lithosphere at ca. 100 Ma.