Geochronology, geochemistry, and Nd isotopes of early Mesozoic bimodal volcanism in northern Tibet, western China: Constraints on the exhumation of the central Qiangtang metamorphic belt

Abstract: Geochronological (sensitive high-resolution ion microprobe, SHRIMP II, and 40Ar/39Ar), major and trace element, and Nd isotopic data are presented for newly discovered early Mesozoic volcanic rocks in and around the central Qiangtang metamorphic belt on the northern Tibetan Plateau. The data indicate that volcanism/magmatism was active during the Triassic and Jurassic and show a bimodal composition. Mafic lavas are generally similar to oceanic island basalt, characterized by enrichments in Nb and Ta relative to similarly incompatible elements, high rare earth element (REE) concentrations, enrichment in light REEs, absence of Eu anomalies, and positive εNd values. Felsic lavas are characterized by enrichment in large ion lithophile elements and light REEs, depletion in high field strength elements (especially for Nb and P), and very low εNd values (<−10). The trace element and isotopic data suggest that the felsic lavas were derived from partial melting of the continental crust. The volcanism was caused by post-orogenic collapse of the central Qiangtang metamorphic belt due to break-off of the subducting western Qiangtang slab, which may have induced decompressional melting that resulted in the eruption of small volumes of basaltic rocks and the relatively rapid exhumation of high-pressure to ultrahigh-pressure rocks. [Copyright &y& Elsevier]