Middle Triassic ultrapotassic rhyolites from the Tanggula Pass, southern Qiangtang, China: A previously unrecognized stage of silicic magmatism

Abstract

There is a Mid–Late Mesozoic (Jurassic–Cretaceous) arc magmatic belt in southern Qiangtang subterrane associated with the north-dipping subduction of the Bangong–Nujiang Tethyan Ocean. Early Mesozoic (Triassic) magmatism, tectonic evolution of the southern Qiangtang subterrane, and the mechanism of its crustal growth, however, remain unclear. This paper reports zircon U–Pb age, whole-rock major, trace elemental, and Sr-Nd-Hf isotopic components for the Mesozoic Tanggula Pass rhyolites in southern Qiangtang subterrane. Our new data reveal a significant, previously unrecognized stage of magmatism in southern Qiangtang subterrane. The Tanggula Pass rhyolites are moderately-strongly peraluminous (A/CNK=1.09–1.54), highly fractionated (SiO2>73%) and ultrapotassic (K2O/Na2O>44%). Zircon U–Pb dating by LA-ICP-MS yields a concordant age with a weighted mean 206Pb/238U age of 235.9±0.86Ma (n=18, MSWD=0.25). The large variations of major and trace element concentrations are mainly attributed to fractional crystallization of plagioclase, hornblende, Fe-Ti oxides, and accessory minerals such as apatite, allanite, monazite, and zircon in the shallow level. K-feldspar mainly occurs as cumulated mineral, resulting in high K2O contents and K2O/Na2O ratios. The rhyolites were derived by partial melting of mixed source rocks including older sedimentary rocks in the upper crust and juvenile materials, triggered by north-dipping subduction of the Bangong-Nujiang Tethyan oceanic lithosphere. The eruption age of Middle Triassic suggests that the initiation of northward subduction of the Bangong-Nujiang Tethyan ocean took place earlier than is currently assumed. Whole-rock Nd (ca. 1.55 to 2.34Ga) and zircon Hf (ca. 1.77 to 2.33Ga) data reflect the presence of the Proterozoic basement in the southern Qiangtang subterrane. Basaltic magma underplating and accretion of intra-oceanic arc complexes and oceanic plateau contributed to vertical crustal growth and lateral crustal growth, respectively.