广西南部凭祥中三叠世沉积盆地构造环境——来自岩相学和碎屑岩地球化学的证据

岩相学特征表明,广西凭祥盆地砾岩可分为颗粒支撑和基质支撑2种,砾石成分主要为生物碎屑灰岩,其次为砂岩和泥岩。盆地砂岩主要由成分成熟度和结构成熟度较低的亚岩屑(杂)砂岩和岩屑(杂)砂岩组成,物源区为碰撞造山带或再旋回造山带。碎屑岩地球化学特征表明,凭祥中三叠世盆地中的砂岩和泥岩样品SiO2含量为61.71%~74.85%,接近于上地壳的平均值。具有高的K2O/Na2O值(6.50~0.51)和高的TFe2O3+MgO含量(7.29%~10.31%),TFe2O3/K2O值为2.05%~5.54%,矿物稳定性较差。稀土元素标准化配分曲线呈现出轻稀土元素富集、重稀土元素平坦和明显的Eu、Ce负异常特征,类似于上地壳和典型的太古宙页岩,具有海相沉积的特征。砂岩风化蚀变指数CIA高(71~88),Th/U值为3.68~9.53,表明砂岩和泥岩经历了较强的风化作用。砂岩物源区判别图表明,凭祥盆地物源与酸性岛弧具明显的亲缘性,增生楔和活动陆缘是主要的物源区。这些特征综合表明,凭祥盆地是一个伴随古特提斯分支洋盆闭合、经强烈构造改造的残余弧前盆地。

Petrography and geochemistry of clastic rocks and their constraints on the tectonic setting of the Middle Triassic Pingxiang basin, Southern Part of Guangxi

Petrographic characteristics of Pingxiang basin show that the conglomerates can be divided into clast-support and matrix-support types, which consist of bioclast-dominated limestones, sandstones and mudstones. Petrographic features show that the sandstones, which have low compositional maturity and textural maturity, were derived from recycled orogen or collisional orogen. The geochemistry of sandstones and mudstones are characterized by moderate SiO2 values (61.71%~74.85%) , close to the value of the upper crust; in addition, they have high K2O/Na2O ratios (6.50~0.51) and high TFe2O3+MgO values (7.29%~10.31%). The TFe2O3/K2O ratios (2.05%~5.54%) show that the stability of the minerals is low. The REE patterns, which are similar to patterns of the upper crust and typical Archean shales, are characterized by LREE enrichment, flat HREE and obvious Eu and Ce anomalies, suggesting marine sediments. CIA indexes (generally 71~88) and Th/U ratios (3.68~9.53) imply that all sediments have undergone high degree of weathering. Various diagrams for sedimentary provenance using major and trace elements show that all the sediments studied are obviously associated with felsic volcanic rocks. The sediments were mainly derived from accretionary prism or active continental margin. Geochemical data of the clastic rocks suggest an active continental margin setting. The results achieved by the authors suggest that Pingxiang basin, considered to be a synchronal product with Early Triassic arc-type rhyolite around the basin, is an intensively modified sutural forearc basin that resulted from the closure of the Paleo-Tethys ocean.