新疆阿合奇地区志留系砂岩地球化学特征及大地构造背景*

塔里木板块西北缘沉积地层的研究,对于恢复南天山洋的演化过程有重要意义。文中利用岩石地球化学的手段,对新疆阿合奇地区志留系砂岩进行物源和构造背景分析。研究表明,阿合奇地区志留系砂岩样品的SiO₂含量范围变化较大,为61.97%~93.91%,平均含量为76.76%;稀土元素球粒陨石标准化配分型式为右倾型,(La/Yb)_N值较高,δEu值较低,Ce异常不明显。中-顶志留统塔塔埃尔塔格组砂岩的成熟度高于下志留统柯坪塔格组。地球化学物源分析图解和大地构造背景判别表明: 研究区沉积物源区逐渐由活动型向稳定型转换,下志留统部分沉积物来自于火成岩物源区,具有主动大陆边缘和大陆岛弧性质;中-顶志留统沉积物全部来自于成熟大陆的石英岩沉积物源区。结合广泛分布的奥陶系-志留系平行不整合,认为晚奥陶世研究区为活动大陆边缘,南天山洋盆向南俯冲到塔里木板块之下;早志留世,向南俯冲结束,研究区大地构造背景开始由活动大陆边缘向稳定的被动大陆边缘转换,来自活动型物源区的沉积物逐渐减少,稳定型物源区的碎屑物质逐渐增多;中-末志留世,研究区构造背景完全转变为被动大陆边缘,碎屑物质全部来自于稳定型物源区。上述成果表明,南天山洋的演化过程中确实存在双向俯冲。

Geochemical characteristics and tectonic history of the Silurian sandstones in Akeqi area,Xinjiang

The South Tianshan Orogenic Belt is located between the Tarim Plate and Yili-Central Tianshan Plate. The tectonic evolution of the South Tianshan Ocean in the Paleozoic has long been debated. Litho-geochemical features of the major and trace elements of the Silurian sandstones from Akeqi area, northwestern margin of the Tarim Plate of Xinjiang,show that Chondrite-normalized REE distribution patterns of the samples are all right skewed, with high(La/Yb)_N value, low δEu value, and negative Ce anomaly. The maturity of the sandstones from the Middle to Top Silurian Tataertage Formation is higher than the samples from the Lower Silurian Kepingtage Formation. The provenance and tectonic setting of the sandstones were evaluated using ternary diagrams that highlight their geochemical characteristics. The results suggest that the provenance of the Akeqi area gradually transformed from active provenance to stable provenance. Some sediments from the Kepingtage Formation are sourced from active continental margin and continental island arc, indicating felsic igneous provenance; Sediments from the Tataertage Formation were sourced from quartzite sediment provenance of mature continent. Combined with widely distributed parallel unconformities between the Ordovician and the Lower Silurian, it can be inferred that the Akeqi area was located in an active continental margin, the South Tianshan Ocean subducted southward beneath the Tarim Plate shortly during the Late Ordovician, which provided the active provenance for the study area. From the Early Silurian, as southwards subduction ended, the tectonic setting of Akeqi area transformed from active continental margin to stable passive continental margin. Sediments from active and stable provenances gradually decreased and increased, respectively. During the deposition of Middle to Top Silurian succession, tectonic setting of the Akeqi area completely transformed to stable passive continental margin, so that sediments were exclusively sourced from stable provenances. In conclusion, this study proposes that bidirectional subduction was developed during the evolution of the South Tianshan Ocean within the Paleozoic.