摘要:
出露于中天山马鞍桥、巴仑台北的石炭纪地层属于大塘阶晚期至威宁期碳酸盐和碎屑岩的混合沉积。沉积特征和微量元素标志显示该套地层主体为潮坪—泻湖相沉积,仅顶部为河湖相沉积。砂岩的元素地球化学特征以及骨架颗粒的百分含量均显示,该套地层的蚀源区为岛弧火山岩系和中酸性花岗岩系的中天山微板块。有足够的证据表明该套地层形成于中天山微板块与南天山弧后盆地碰撞的挤压背景,即前陆盆地中,构造和海平面变化是该套充填物的主要控制因素。
Abstract:
The Carboniferous strata outcropped in Maanqiao and northern Baluntai area in Middle Tianshan include Maanqiao Formation (late Datang stage) and Sangshuyuan Formation (Weining stage).The strata belong to the mixed sediments of carbonate and clastic rocks. Maanqiao Formation, most of which is carbonate tidal-flat facies, consists of two depositional cycles. Cycle one shows peritidal cycle, but cycle two shows intertidal-subtidal cycle, which indicate a deepening of sea water from cycle one to cycle two. The Sangshuyuan Formation is composed of three depositional cycles, which mainly show subtidal cycles. The fluvial and lacustrine facies on the top of Sangshuyuan Formation indicates the retreating of sea water. The trace element composition shows that the depth of sea water is very shallow, but the major element composition indicates that most samples fall into the region of island arc. The QFL , QmFLt and QpLvLs diagrams for Carboniferous sandstones give the same results as the major element composition plots. It can be seen that the source area of the Carboniferous strata is from underlying middle Tinshan microplate composed of a great deal of volcanic rocks of island arc type and intermediate-acid granite. So, the Carboniferous strate are formed in the foreland basin resulting from the collision between middle Tianshan microplate and southern Tianshan back-arc basin. The evolution of Tianshan microplate shows that the opening of paleo-Tianshan Ocean starts from Sinian-early Ordovician. With the opening of paleo-Tianshan Ocean, the middle Tianshan microplate shifts from the Tarim plate. The opening of southern Tianshan back-arc basin results from the subduction and collision in the northern margin of middle Tianshan microplate. The collision between Tarim plate and middle Tianshan microplate results in the closure of the southern Tianshan back-arc basin. The collision ,also called as soft collision ,does not make the southern Tianshan uplift abruptly. The sea water in the research area and southern Tianshan does not retreat until the early Carboniferous. So, there is devoid of sediments during the early Yanguan stage. With the transgression of Carboniferous, southern Tianshan accepts the sedimentation during the late Yanguan stage , but the research area during the late Datang stage. After a short time of sedimentation , the research area uplifts.
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