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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SiO2含量为47.8%~58.8%,Al2O3含量为13.35%~14.25%,CaO含量为3.79%~5.61%,TiO2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,δEu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±4Ma,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 the Early Paleozoic Caledonian orogeny (the Guangxi movement) is a major geological problem that has long been disputed in the tectonic evolution of the South China terrane. A set of Early Paleozoic basaltic andesite-andesite volcanic rocks was exposed in the area of Cenxi, southeastern Guangxi. The content of CaO is 3.79% ~ 5.61%, the content of TiO2 is 0.46% ~ 0.56%, while the content of MgO is 4.39% ~ 9.96% -6 ~ 210 × 10-6) and Cr (427 × 10-6 ~ 750 × 10-6), which are high-Mg-Mg basaltic andesite-andesite of calc-alkaline transitional series.Light REE (La / Yb) N ranges from 3.4 to 7.9, and (La / Sm) N ranges from 3.15 to 4.95. The large ion lithophile elements Rb, Ba, Th, U, K and Pb are obviously enriched. The high field strength elements Nb, Ta and Ti are relatively depleted and have lower Ti / V (14.7 ~ 21.8), showing high-Mg-Mg basaltic andesite- ). The magmatic andesite obtained by LA-ICP-MS zircon U-Pb method has a diagenetic age of 443 ± 4Ma, indicating that this volcanic series was formed in the Late Ordovician-Early Silurian.The above studies show that large Shuang basaltic andesite - Andean series for the late Austrian World - Early Silurian subduction of oceanic crust formed at the former island-arc environment of high magnesium - magnesian basaltic andesite - andesite line, which findings provide important evidence for the existence of the Early Paleozoic subduction of oceanic crust between Hua Nanyang child and China.