新疆西天山阿拉套山南缘阿克沙克组变质碎屑岩地球化学特征及其对物源区和构造环境的制约

Geochemistry characristics of metasedimentary rocks from Akeshake Formation in the southern Alataw Mountains,western Tianshan Mountains,Xinjiang: Constraint on the provenance and tectonic environment

新疆阿拉套山南缘出露的下石炭统阿克沙克组沉积地层,是西天山石炭纪构造演化的重要物质记录。阿克沙克组第一段以碎屑岩发育为特点,在泥质粉砂岩中含少量珊瑚、腕足及苔藓虫化石;阿克沙克组第二段受后期晚石炭世花岗岩侵吞,呈残留状出露,以碎屑岩角岩化和含板岩为特征。阿克沙克组变质碎屑岩岩原岩为泥质岩,高Si O2含量(63.91%~64.58%),高场强元素Nb、Ti、Ba强烈亏损,Sr/Ba值为0.13~0.16,Th/Sc值为0.97~1.02,La/Sc值为2.33~2.38,稀土总量为181.37×10^(-6)~186.67×10^(-6),轻重稀土比值为7.2~7.23,(Ce/Yb)N值为5.18~5.31,(La/Yb)N值为7.49~7.95,轻稀土元素富集,Eu亏损明显(δEu值为0.59~0.60),Sm/Nd值为0.188~0.191,物质来源于上地壳;碎屑岩碎屑颗粒沉积速度较快,属于近源沉积,沉积物来源于活动性质的构造环境。通过沉积物源及岩石地球化学特征,综合认为阿克沙克组蚀源区构造环境为活动大陆边缘,且物源区中存在较多的岛弧成分。

The Lower Carboniferous Akeshake Formation exposed in the southern margin of Alataw Mountains,Xinjiang,is an important material record of Carboniferous tectonic evolution in Western Tianshan. The first member of Akeshak Formation is characterized by clastic rock development,with a small amount of coral,brachiopod and bryozoan fossils in argillaceous siltstone;the second member of Akeshak Formation is invaded by late Carboniferous granite and exposed in residual form,characterized by clastic rock hornification and slate. The protolith of metamorphic rocks in Akeshake Formation is ferruginous argillaceous rocks,with high Si O2 content( 63.91%-64.58%),high field strength elements such as Nb,Ti,Ba and strong loss,Sr/Ba value is 0.13-0.16,Th/Sc value is 0.97-1.02,La/Sc value is 2.33-2.38,rare earth total amount is 181.37×10^(-6)-186.67×10^(-6),rare earth ratio is 7.2-7.23,( Ce/Yb)Nvalue is 5.18-5.31,( La/Yb)Nvalue is 7.49-7.95,light rare earth elements are enriched,Eu depletion is obvious( δEu value is 0.59-0.60),Sm/Nd value is 0.188-0.191,and the materials come from the upper crust. The clastic grains of clastic rocks are deposited rapidly,belonging to near source deposits,and the sediments are derived from active tectonic environment. According to the characteristics of sediment source and rock geochemistry,it was considered that the tectonic environment of Akeshake Formation erosion source area is active continental margin,and there are more island arc components in the provenance area.