新疆阿克苏地区新元古代冰成沉积地球化学研究

A geochemistry study on Neoproterozoic glaciogenic sediments in Aksu area,Xinjiang

新疆塔里木盆地西北缘阿克苏地区出露有2套与新元古代冰期相关的杂砾岩沉积,通过对其进行元素地球化学分析,讨论了沉积物的构造背景和古气候风化条件。化学蚀变指数(CIA)指示出本区的2套新元古代杂砾岩代表了2次寒冷的冰期记录,分别为巧恩布拉克冰期和尤尔美那克冰期,其中在巧恩布拉克冰期中出现了2次小冰期旋回,而且本区从早新元古代开始一直处于较为寒冷干燥的环境下,直到晚新元古代才逐渐变暖。通过K2O/Na2O-SiO2图解和La-Th-Sc、Th-Sc-Zr/10、Th-Co-Zr/10三角图解讨论了沉积构造背景从早新元古代岛弧环境向晚新元古代被动大陆边缘环境演化的趋势,而主元素含量也体现了从不成熟向成熟演化的特征,与本区存在的阿克苏群变质岩基底吻合。

The Neoproterozoic strata in the Aksu region, NW China, which lies unconformably on the Precambrian Aksu Group basement, is comprised of the Qiaoenbrak, Yuermeinak, Sugetbrak and Chigebrak formations from the bottom to the top. The Qiaoenbrak, Yuermeinak and Sugetbrak formations were analyzed for major and trace elements （including REE） to identify the paleoclimate and tectonic setting. The chemical index of alteration （CIA） values indicate that the Qiaoenbrak and Yuermeinak formations represent two distinct episodes of glaciations, and the cold paleoenvironment began to get warm at late-Neoproterozoic （Sugetbrak Formation depositing）. Diagrams of K2O/Na2O-SiO2, La-Th-Sc, Th-Sc-Zr/10 and Th-Co-Zr/10 and some element ratios indicate that the tectonic setting of Qiaoenbrak Formation was an arc at active continent margin, and the tectonic setting of Yuermeinak and Sugetbrak formations was passive margin. It indicates the transition from active to passive continental margin, which is the same as the sedimentary sequences.