华夏早古生代俯冲作用(Ⅱ)：大爽高镁-镁质安山岩新证据
Early Paleozoic Subduction in Cathaysia (Ⅱ): New Evidence from the Dashuang High MagnesianMagnesian Andesite

早古生代加里东期造山运动(广西运动)的性质是华南大地构造演化长期争议的重大地质问题.桂东南岑溪大爽一带出露一套早古生代玄武安山岩-安山岩火山岩系.玄武安山岩-安山岩SiO2含量为47.8%～58.8%，Al2O3含量为13.35%～14.25%，CaO含量为3.79%～5.61%，TiO2含量为0.46%～0.56%，而MgO(4.39%～9.96%)、Ni(141×10-6～210×10-6)和Cr(427×10-6～750×10-6)的含量明显偏高，属钙碱性到过渡系列的高镁-镁质玄武安山岩-安山岩.轻稀土富集，轻重稀土分异明显,δEu弱负异常(0.73～0.92)，(La/Yb)N为3.4～7.9，(La/Sm)N为3.15～4.95.大离子亲石元素Rb、Ba、Th、U、K、Pb明显富集，高场强元素Nb、Ta、Ti相对亏损，具较低Ti/V(14.7～21.8)，表现为高镁-镁质玄武安山岩-安山岩(赞岐岩)的典型特征.镁质安山岩由LAICPMS锆石UPb法获得的成岩年龄为443±4Ma，表明这套火山岩系形成于晚奥陶世-早志留世.上述研究表明，大爽玄武安山岩-安山岩系为晚奥陶世-早志留世形成于洋壳俯冲岛弧弧前环境的高镁-镁质玄武安山岩-安山岩系，其发现为华南扬子与华夏之间存在早古生代洋壳俯冲作用提供了重要证据.
The nature of early Paleozoic Caledonian orogeny (Guangxi movement), as one of the key geolocial problems of tectonic evolution in South China, has been controversial for a long time. In this study we report a suite of early Paleozoic basaltic andesiteandesite volcanic rocks located at the Dashuang in the Cenxi area of southeastern Guangxi in South China. Petrogeochemical data of the basaltic andesiteandesite show the contents of SiO2=47.8%-58.8%, Al2O3=13.35%-14.25%, CaO=3.79%-5.61% and TiO2=0.46%-0.56%. It also shows high contents of MgO=4.39%-9.96%, Ni=141×10-6-210×10-6 and Cr=427×10-6-750×10-6, indicating that they belong to high magnesian (HM)magnesian (M) basaltic andesiteandesite from calcalkaline to transitional series. The volcanic rocks exhibit enrichedLREE, weak negative δEu abnormally (0.73-0.92), (La/Yb)N=3.4-7.9, (La/Sm)N=3.15-4.95. Basaltic andesiteandesite is characterized by pronounced enrichment of LILEs (e.g. Rb, Ba, Th, U, K, Pb), relative depletion of HFSEs (e.g. Nb, Ta, Ti), and low Ti/V=14.7-21.8. All these show typical characteristics of high magnesian (HM)magnesian (M) basaltic andesiteandesite (sanukite). The LAICPMS zircon UPb age of the magnesian andesite is 443±4Ma, showing that the formation ages of the volcanic rock series should be between the Late Ordovician and Early Silurian epoch. In conclusion, we suggest that the Dashuang volcanic rocks are the suite of high magnesian (HM)magnesian (M) basaltic andesiteandesite series formed in the forearc tectonic environment in Late OrdovicianEarly Silurian epoch, which provides key evidence for Early Paleozoic oceanic crust subduction between Yangtze Craton and Cathaysia block along QinzhouHangzhou tectonic suture belt in South China