Petrogenesis of the Jiaoziding granitoids and associated basaltic porphyries: Implications for extensive early Neoproterozoic arc magmatism in western Yangtze Block

Abstract

Middle Neoproterozoic (ca 860–750Ma) granitoids are widely distributed in the western margin of the Yangtze Block, China, yet their magma sources and tectonic settings are unclear. The geochronology and geochemistry of the granitoids and associated basaltic porphyries, which intruded the ~970Ma Tongmuliang arc volcanic rocks in the Jiaoziding area (east of Pingwu county), were investigated in this study. LA-ICP-MS zircon U-Pb dating indicates that the Jiaoziding granitoids and basaltic porphyries were formed at 795±6Ma and 790±20Ma, respectively. The granitoids have high SiO2 (69.2–76.9wt%), K2O (2.3–5.6wt%), and Na2O (3.2–5.1wt%) contents, and a low Al2O3 (12.4–14.5wt%) content. The basaltic porphyries contain high concentrations of TiO2 (~3wt%) and high field strength elements, have steep rare earth element patterns, and are depleted in Nd and Hf isotopes. Batch partial-melting modelling indicates that the Jiaoziding granitoids could have been derived by ~5% and 50-70% partial melting of Tongmuliang mafic rocks and quartz-keratophyres, respectively. Formation of the basaltic porphyries by melting of upwelling asthenospheric mantle would have been facilitated by extensive lithospheric delamination during the Neoproterozoic. This study established a link between mid-Neoproterozoic granitic magmatism and ~970Ma juvenile arc crust, indicating that extensive early Neoproterozoic juvenile arc crust, and partial melting of this crust in an extensional setting, favoured the formation of middle Neoproterozoic granitic rocks along the W–NW margin of the Yangtze Block.