Petrogenesis of the Early Jurassic Longtang and Menglong Peraluminous Granites in Tengchong Terrane, and their Tectonic Implication

A comprehensive study of zircon U-Pb geochronology, in situ Hf isotopes, whole-rock major and trace element geochemistry, and Nd isotopes was carried out for two Early Jurassic two-mica granites (Longtang and Menglong) in the southern part of the Tengchong terrane, which is in the northern part of the larger Sibumasu terrane. We assess the origin of the granites and explore their possible genetic relationship to the Paleo-Tethyan regime. LA-ICP-MS zircon U-Pb dating shows that they were simultaneously emplaced in the Early Jurassic (ca. 199 Ma). They have SiO2 contents of 69.7-75.1 wt% and are mainly strongly peraluminous with alumina saturation index (ASI) values ranging from 1.06 to 1.46. They show similar Mg-# (0.29-0.42) to experimental partial melts of metasedimentary rocks under continental pressure-temperature (P-T) conditions. They are enriched in light rare earth elements (LREEs) relative to heavy rare earth elements (HREEs), with moderately negative Eu anomalies and flat HREEs patterns. They show negative epsilon(Nd)(t) values (-9.0 to -12.4) and epsilon(Hf)(t) values (-8.0 to -9.1). Elemental and isotopic data suggest that they most likely to formed by muscovite-dehydration melting of a metapelitic source at lower temperatures in the range of 700 degrees C to 750 degrees C. The granites might represent a post-collisional tectonic setting response to Paleo-Tethyan regime.