Early Jurassic highly fractioned rhyolites and associated sedimentary rocks in southern Tibet: constraints on the early evolution of the Neo-Tethyan Ocean

Abstract

An integrated investigation of Lower Jurassic Jialapu Formation (including U–Pb dating of igneous and detrital zircons, geochemical analysis of igneous rocks, and analysis of detrital modes of sandstone interbeds) in the southern margin of the Lhasa terrane, Tibet, provides new constraints on the early evolution of the Neo-Tethyan Ocean. Our new data reveal a stage of silicic magmatism, with an arc affinity along the southernmost Eurasian margin at 193–190 Ma, which is characterized by high SiO2 (76.5–80.0 wt%), Na2O (5.30–7.22 wt%) and low K2O (0.07–0.55 wt%), MgO (0.16–0.54 wt%), Cr (1.44–4.37 ppm), and Ni (0.67–1.62 ppm) contents with low eNd(t) values of + 1.21 to + 2.33. These silicic volcanic rocks are enriched in large ion lithophile elements (LILEs) and light rare earth elements (LREEs) and can be classified as highly fractionated I-type rhyolites. They show a wide range of zircon eHf(t) values from − 11.7 to + 9.4, suggesting variable contributions from both juvenile and ancient crustal sources. It is concluded that the Jialapu Na-rich rhyolites were derived by the partial melting of a highly differentiated residual melt separated from the K-rich magma, triggered by upwelling asthenospheric mantle. Detrital zircons in the sandstones from the Lower Jurassic Jialapu Formation are dominantly the Mesozoic ones, which are likely derived from Lhasa terrane sources. Considering the associated sedimentary records as well as literature data, it is suggested that an Early Jurassic back-arc basin developed along the southern Lhasa terrane in response to the northward subduction of the Neo-Tethyan Ocean.