西藏冈底斯带东段尼木地区中二叠世和晚三叠世基性岩浆作用及其对新特提斯洋初始俯冲的约束

野外地质调查和SHRIMP锆石U-Pb年代学研究发现，藏南冈底斯带尼木地区出露有规模较大、原岩形成于中二叠世（~266Ma）和晚三叠世（220~219Ma）的变质基性岩，而晚三叠世变质基性岩中还存在~2.5Ga的捕获锆石。全岩地球化学分析显示，两期基性岩具有相似的地球化学特征，即：（1）相对富集轻稀土元素，亏损重稀土元素和高场强元素；（2）比较一致的Sr和Nd同位素比值（⁸⁷Sr/⁸⁶Sr（t）=0.703225~0.703664，ε_Nd（t）=6.51~6.81）。结合拉萨地块东南缘已发表的三叠纪中-基性岩地球化学数据，发现尼木中二叠世和晚三叠世基性岩皆形成于陆缘弧环境，岩浆来源于尖晶石橄榄岩地幔2.5%~4%的部分熔融，并经历了一定程度的地壳混染。对比拉萨地块西南缘与东南缘二叠纪-三叠纪俯冲相关的岩浆记录，推断新特提斯洋向拉萨地块下的初始俯冲作用具有穿时性，东段俯冲作用在~266Ma已经开启，西段则发生在255~214Ma之间。

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 (~266Ma) and Late Triassic (220~219Ma) 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 (87Sr/86Sr(t)=0.703225~0.703664, εNd(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%~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~266Ma in the eastern segment, and from 255Ma to 214Ma in the western segment.