Depositional Characteristics of Carboniferous and Its Tectonic Significance in Maanqiao Area, Middle Tianshan

Key words: 
• Carboniferous /
• tidal-flat and lagoon facies /
• major element and trace element composition /
• framework grain /
• foreland basin /

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 , Q_mFLt and Q_pL_vL_s 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.