Geochemistry of the Eocene Felsic Porphyric Rocks and High‐Mg Potassic Rocks along JARSZ: Implication for the Tectonic Evolution in Eastern Tibet

Abstract

Abstract:Eocene felsic porphyric rocks and the high‐Mg potassic volcanic rocks (HMPR) occur along the Jinshajiang‐Ailao Shan‐Red River shear zone (JARSZ) in eastern Tibet. Compared with the HMPR, which are generally believed to be sourced from an enriched mantle, the felsic porphyric rocks show similar K2O contents, enrichment in LREE and LILE, particularly radiogenic isotope (e.g. Sr and Nd) features much similar to the former, implying generation of the felsic porphyric rocks most likely related to the HMPR, although they both have clearly different major and trace element compositions. The close relationship in spatial‐temporal distribution and similar Sr‐Nd characteristics between the felsic porphyric rocks and HMPR in eastern Tibet indicate that both of them were possibly formed by a similar tectonic process (event). Combining the basic dikes in southern and eastern Tibet, we suggest that the break‐off of north‐dipping Neo‐Tethyan slab in southern Tibet during 50–40 Ma, triggered formation of high‐Mg potassic magma. This led to developing felsic porphyric magma production by partial melting of underplating HMPR in the lower crust, or fractionation crystallization of the high‐Mg potassic magmas. The break‐off of slab in the Eocene may also have contributed to the abundant ore‐forming material related to earlier subduction events, resulting in formation of the porphyric deposits along JARSZ in eastern Tibet.