Carboniferous arc magmatism in the Qiangtang area, northern Tibet: Zircon U-Pb ages, geochemical and Lu-Hf isotopic characteristics, and tectonic implications

The Longmu Co-Shuanghu suture zone (LSSZ), which lies between the southern Qiangtang-Baoshan block and northern Qiangtang-Qamdo block on the Tibetan Plateau, represents remnants of the Paleo-Tethys Ocean. Previous investigations have focused on the ophiolite, high-pressure zone, and meta-morphic belt in this zone, whereas few studies have considered the arc magmatism. The present study examines a suite of Carboniferous arc-magmatic rocks in the western region of the LSSZ that consist of acidic to basic volcanic rocks, including rhyolite, dacite, andesite, basaltic andesite, basalt, and pyroclastics. Zircon U-Pb dating of three samples (two andesites and one dacite) by laser ablation-inductively coupled plasma-mass spectrometry demonstrates that these rocks were emplaced at ca. 351-346 Ma. The basalts have low Cr and Ni abundances, indicating that they were generated by varying degrees of partial melting of an enriched lithospheric mantle that was metasomatized by subduction-derived components. Zircons in the felsic rocks have positive epsilon(Hf)(t) values of 0.8-14.8, indicating an origin by varying degrees of partial melting of juvenile basaltic lower crust, consisting mainly of underplated magmas with similar compositions to the basalts. Geochemical data show that the volcanic rocks are enriched in light rare earth elements and large ion lithophile elements (e.g., Th, U, and Pb) but are strongly depleted in high field strength elements (e.g., Nb, Ta, and Ti), have high Al2O3 and low Zr concentrations, and are calc-alkaline. These magmas are therefore subduction-related volcanic arc rocks, indicating that Paleo-Tethys subduction began in the early Carboniferous. (C) 2015 Elsevier Ltd. All rights reserved.