The Discovery And Tectonic Implications Of Early Carboniferous Arc Magmatism In Central Qliangtang, Tibetan Plateau

The Longmu Co–Shuanghu suture zone (LSSZ), which lies between the southernQiangtang–Baoshan block and northern Qiangtang–Qamdo block, representsremnants of the Paleo-Tethys Ocean on the Tibetan Plateau. Previous investigationshave focused on the ophiolite, high-pressure zone, and metamorphic belt, whereas fewstudies have considered the arc magmatism of the LSSZ. The present study examinesa suite of Carboniferous arc-magmatic rocks in the western region of the LSSZ thatare composed of acidic to basic volcanic rocks, including rhyolite, dacite, andesite,basaltic andesite, basalt, and some pyroclastics, which named as WangguoshanFormation. Zircon U–Pb dating of three samples (two andesites and one dacite) bylaser ablation–inductively coupled plasma–mass spectrometry demonstrates that theserocks were emplaced at ca.351–346Ma. The basalts have low Cr and Ni abundances,indicating that they were generated by varying degrees of partial melting of alithospheric mantle that was metasomatized by subduction-derived components.Zircons in the felsic rocks have positive εHf(t) values of0.8–14.8, indicating an originby varying degrees of partial melting of juvenile basaltic lower crust, consistingmainly of underplated magmas with similar compositions to the basalts. Geochemicaldata show that the volcanic rocks are enriched in light rare earth elements and largeion 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 Al2O3and low Zr concentrations,and are calc-alkaline. These magmas are subduction-related volcanic arc rocks,indicating that Paleo-Tethys subduction began in the early Carboniferous.