Middle Triassic volcanic rocks from the Gangdese belt, southern Tibet: petrogenesis and implications for Tethys tectonic evolution

The voluminous Middle Triassic to Early Jurassic magmatic rocks in the southern Gangdese belt of Tibet have received wide attention in tracing the subduction history of the Yarlung-Zangbo Neo-Tethys Ocean (YZNTO) and Bangong-Nujiang Tethys Ocean (BNTO). Here we report a suite of volcanic rocks from the Gangdese arc belt and present results from petrological, geochemical and zircon U-Pb-Lu-Hf isotopic studies with a view to constrain their timing and magma source in relation to the Tethys tectonic evolution. Zircon U-Pb data show ages in the range of 237.2-235.0 Ma for the andesitic rocks, representing their eruption ages during the Middle Triassic. Zircon Lu-Hf data display markedly positive epsilon(Hf)(t) values of 11.44-15.99 with the two-stage model ages (T-DM (C)) of 535-241 Ma, suggesting that the magma was derived from depleted mantle or juvenile crust components. Geochemically, the andesitic rocks show low Sr/Y (20.26-43.74) and La/Yb ratios (8.52-14.21) in the absence of Eu anomalies, enrichment in LILEs (K, U, Rb, Ba) and LREEs, and depletion of HFSEs (Nb, Ta, Ti, Zr) and HREEs, suggesting normal arc andesite with emplacement in a continental island arc setting. Accordingly, we infer that these rocks were formed by partial melting of a young oceanic crust and sediments in the transitional zone between plagioclase and garnet stability within subduction-related setting, followed by fractional crystallization of amphibole and apatite. In conjunction with previous studies on the Tethys tectonic evolution, we argue that the southern margin of the Gangdese belt during the Middle Triassic might be unrelated with the northward subduction of the YZNTO and southward subduction of the BNTO. Our results suggest that the Middle Triassic magmatic suites in the southern margin of the south Gangdese belt were generated through the southward subduction of the Zhikong-Sumdo Tethys Ocean.