Geochemistry and Nd isotopes of Lower Permian siliciclastic rocks from central Tibet, western China: Evidence for Early Permian initiation of the rifting between the Lhasa and Qiangtang blocks?

Abstract

Fifteen Lower Permian siliciclastic rocks from central Tibet were analysed for the major and trace elements, and ten samples among them were chosen for determination of Nd isotopic compositions. They are characterized by low concentrations of SiO2 and low ratios of SiO2/Al2O3, moderate abundances of Fe2O3 + MgO (mudstones, 7.28–9.72 wt.%; sandstones, 4.62–10.02 wt.%) and TiO2 (mudstones, 0.55–0.95 wt.%; sandstones, 0.53–0.99 wt.%), significant enrichments of light rare‐earth elements (LREEs) relative to the heavy REEs (HREEs; average (La/Yb)N = 9.8 [mudstones] and 9.48 [sandstones]), flat HREE distribution, obvious negative Eu anomalies (average Eu/Eu* = 0.68), higher Th/Sc and La/Sc ratios, and negative initial εNd values (on average −7.2 and −8.5 for sandstones and mudstones, respectively). The mixing modal analysis based on some elemental pairs and Nd isotopic composition, respectively, Nd depleted mantle model age (TDM) and initial εNd values, uniformly indicates that about 30–40% mafic materials could have been added into these sedimentary rocks. These siliciclastic rocks could have formed in a setting of rifting between the Lhasa and Qiangtang blocks, when a 280 Ma mantle plume upwelling event was occurring in northern Gondwana.