桂西南早中生代酸性火山岩年代学和地球化学:对钦-杭结合带西南段构造演化的约束

Geochronology and geochemistry of Early Mesozoic acid volcanic rocks from Southwest Guangxi: Constraints on tectonic evolution of the southwestern segment of Qinzhou-Hangzhou joint belt

出露于钦-杭结合带西南段的早中生代酸性火山岩,主要沿着桂西南十万大山中新生代盆地两侧分布。其中出露于盆地北西侧的上-中三叠统北泗组火山岩主要由长英质熔岩(玄武安山岩、英安岩和流纹岩)夹火山碎屑岩(主要为集块熔岩、角砾熔岩、凝灰熔岩、熔结角砾凝灰岩和熔结凝灰岩)等岩石组成,获得英安岩锆石SHRIMP U-Pb谐和年龄为246±2Ma;而出露于盆地南东侧的三叠系板八组火山岩主要由流纹岩夹珍珠岩、凝灰熔岩、集块熔岩和流纹质凝灰岩组成,获得流纹岩锆石SHRIMP U-Pb谐和年龄为250±2Ma。主量、微量元素及Sr-Nd同位素地球化学研究表明,这些火山岩属于典型的过铝质高钾钙碱性火山岩系,表现出富集大离子亲石元素(如K、U、Ba、Rb和Th)和轻稀土元素,而Nb、Ta、P和Ti等高场强元素和重稀土元素明显亏损,并具有较高的锶同位素初始比值((^(87)Sr/^(86)Sr)i=0.713929～0.722178)和较低的ε_(Nd)(t)值(-10.33～-9.02),反映其具有俯冲消减作用形成的岛弧型火山岩地球化学特征。结合本区存在有早古生代MORB型变质基性火山岩和二叠纪弧后扩张中心环境形成的E-MORB型玄武岩的资料表明,扬子板块和华夏板块结合带(称之为钦-杭结合带)西南段有古生代洋盆的存在,该洋盆的俯冲消减过程一直延续至中三叠世的印支运动导致扬子板块和华夏板块发生碰撞才终止。

Early Mesozoic acid volcanic rocks are exposed dominantly in the southwestern segment of Qinzhou-Hangzhou joint belt, and are mainly distributed in both sides of the Shiwandashan Mesozoic-Cenozoic basin. The volcanic rocks of Lower and Middle Triassic Beisi Formation are exposed in northwest side of the basin, which consists of alternating volcanic succession of felsic lavas （basaltic andesite-dacite-rhyolite） interlayered with pyroclastic rocks （dominantly agglomerate lava, breccia lava, tuffolava, breccia ignimbrites, ignimbrites） and a dacite sample yielded a zircon SHRIMP U-Pb concordia age of 246±2Ma. The volcanic rocks of Triassic Banba Formation are exposed in southeast side of the basin, which composed mainly of rhyolite interlayered perlite, tuffolava, agglomerate lava and rhyolite tuff and a rhyolite sample yielded a zircon SHRIMP U-Pb concordia age of 250±2Ma. Major- and trace-elements as well as Sr-Nd isotope geochemistry researches show： The volcanic rock association belongs to a typical peraluminous high-K calc-alkaline series and are characterized by relative enrichment in LILEs （e.g., K, U, Ba, Rb and Th） and LREEs and pronounced depletion in HFSEs （e.g., Nb, Ta, P and Ti） and HREEs. They also have relatively high （87Sr/86Sr）i （0.713929~0.722178） and low εNd（t）（-10.33^-9.02）. Such signatures display geochemical characteristics of typical subduction-related arc volcanic rocks. Together with early Paleozoic MORB-type metabasic volcanic rocks and Permian E-MORB type basalts were formed in an back-arc spreading center setting in the region, appears to support a development of a Paleozoic oceanic basin within the southwestern segment of the joint belt between the Yangtze and Cathaysian blocks （so-called Qinzhou-Hangzhou joint belt）, which suggests that the oceanic basin had not been closed until the Indosinian orogeny at middle Triassic resulted in the final collision between the Yangtze and Cathaysian blocks.