Abstract We investigated alkali series volcanic rocks newly discovered in the southern Mongolia volcano-plutonic belt of the Central Asian Orogenic Belt (CAOB) and classified as basalt, basaltic trachyandesite, andesite, trachyandesite, and trachyte, mainly showing andesitic-dacitic compositions. In Harker diagrams, the TiO2, FeO, MnO, Al2O3, CaO, and MgO contents generally decrease with increasing SiO2, representing a series of differentiated volcanic rocks. The primitive mantle-normalized trace element spider diagram is characterized by incompatible element-rich patterns and negative Nb anomalies. The tectonic discrimination diagrams show that the volcanic rocks were formed by arc-related volcanism rather than within-plate volcanism. K-Ar dating of feldspars from six volcanic rocks yields ages ranging from 270 to 239 Ma, indicating that andesitic and dacitic magmatism occurred during this period. These geochemical and geochronological data indicate that the differentiated volcanic rocks intruding middle to late Palaeozoic volcanic sediments were formed by back-arc type subduction ca. 270–240 Ma. Based on detrital zircon U-Pb ages, the terrigenous sandstones underlain by the volcanic sediments record a maximum depositional age of Late Triassic (ca. 205 Ma), and abundant detrital zircons of ca. 250–210 Ma imply the Early Triassic felsic volcanism near this region. They also include a main cluster of Early Devonian ages (ca. 440–380 Ma) and Late Carboniferous to Early Permian (ca. 330–280 Ma), possibly derived from the Devonian-Carboniferous arc complex and Early Permian granites observed surrounding this study area. These results from the volcanic and sedimentary rocks in this study, combined with investigations in previous studies, imply that subduction beneath the eastern segments of the CAOB to consume the Palaeo-Asian Ocean continued until Late Permian to Early Triassic.

中文翻译：

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中亚造山带蒙古南部火山-深成岩带晚二叠世至早三叠世弧后型火山作用：古亚洲洋最终闭合时间的启示

摘要 对中亚造山带（CAOB）蒙古南部火山-深成岩带新发现的碱系火山岩进行了研究，分类为玄武岩、玄武质粗面岩、安山岩、粗面岩和粗面岩，主要为安山-英安岩成分。在 Harker 图中，TiO2、FeO、MnO、Al2O3、CaO 和 MgO 的含量普遍随着 SiO2 的增加而降低，代表了一系列分化的火山岩。原始地幔归一化微量元素蜘蛛图的特征是不相容的富元素模式和负 Nb 异常。构造判别图表明，火山岩是由弧相关火山作用形成的，而不是板内火山作用。来自六种火山岩的长石的 K-Ar 定年产生的年龄范围为 270 至 239 Ma，表明这一时期发生了安山质和英安质岩浆作用。这些地球化学和年代学数据表明，侵入中晚古生代火山沉积物的分化火山岩是由弧后型俯冲形成的。270-240 毫安。根据碎屑锆石 U-Pb 年龄，火山沉积物下的陆源砂岩记录了晚三叠世（约 205 Ma）的最大沉积年龄，以及丰富的约 200 年的碎屑锆石。250-210 Ma 暗示该地区附近存在早三叠世长英质火山作用。它们还包括一个主要的早泥盆世时代（约 440-380 Ma）和晚石炭世至早二叠世（约 330-280 Ma），可能源自泥盆纪-石炭纪弧复合体和围绕本研究观察到的早二叠世花岗岩区域。