Middle Permian and Late Triassic Mafic Magmatism in Nyemo Area of the Eastern Gangdese Belt, Tibet, and Its Constraints on the Subduction Initiation of the Neo-Tethys

Based on the field geology and SHRIMP zircon U-Pb geochronology, we report a new occurrence of Middle Permian ( similar to 266Ma) and Late Triassic (220 similar to 219Ma) protolith age metabasic rocks near Nyemo County within the Gangdese belt. Those Late Triassic metabasic rocks also contain ca. 2. 5Ga zircon xenocrysts. Whole -rock geochemical analyses show that these metabasic rocks have similar geochemical characteristics, including: (1) enrichment in LREE and depletion in HREE and HFSE; (2) relatively consistent Sr and Nd isotope ratios (Sr-87/Sr-86( t) = 0. 703225 similar to 0.703664, epsilon(Nd)(t) = 6. 51 6. 81). In combination with the published geochemical data of the Triassic andesitic-basaltic rocks in the southeastern margin of the Lhasa Block, we conclude that the protoliths of the Nyemo metabasic rocks were formed in a continental arc environment during both the Middle Permian and the Late Triassic, and the magma that formed them originated from 2. 5% similar to 4% partial melting of the spinel-bearing peridotitic mantle and then underwent some degree of crustal assimilation. By comparing the Permian-Triassic magmatic arc records on the southwestern and southeastern margins of the Lhasa Block, we infered that the initial subduction of the Neo-Tethyan ocean beneath the Lhasa Block was diachronous. Subduction of the Neo-Tethys probably began earlier than similar to 266Ma in the eastern segment, and from 255 Ma to 214Ma in the western segment.