Tectonic Characteristics Of Pingquan-Yingzi Area And Provenance Analysis Of Triassic Sedimentary Succession In The Middle Of Yanshan Belt

Yingzi Basin is located in the central section of Yanshan Intraplate Orogenic Belt, within which the Triassic terrestrial clastic succession is characterized by Syntectonic Sedimentation. Stratigraphy of Yingzi Basin is composed of Liujiagou Formation, Heshanggou Formation and Ermaying Formation cross-bedding sandstones. Direction of paleocurrent indicates that the provenance of sandstones were mainly coming from the south and southeast of the basin. Fold-thrust deformation are the predominantly structures in the surrounding areas of basin.The tectonic settings of provenance of Triassic sandstones deposited in Yingzi Basin are analyzed through systematic sampling, counting of quartzs, feldspars and lithic fragments using microscope, as well as the Dickinson QFL/QmFLt diagram plotting for discrimination of tectonic settings. It is concluded that (1) provenance of the First Member of Liujiagou Formation sandstones belongs to Dissected Arc, reflecting a magmatic arc developed during the convergence between North China Craton and Mongolian Fold Belt; (2) provenance of the Second Member of Liujiaogou Formation sandstones is plotted into the transitional area between Magmatic Arc and Orogenic Belt, representing the ending episode of orogenesis dominated by the deformation of thin-skinned thrusts; (3) provenances of the Third and Fouth Member of Liujiagou Formation are classified into Recycled Orogenic Belt environment, implying the stop of collision and convergence between two plates and the formation orogenic belt on the north margin of North China Craton; (4) provenances of Heshanggou and Ermaying Formation belong to Within-plate Basement Uplift setting, demonstrating the Indosinian Intraplate Orogenesis in the Yanshan Belt, which is characterized by a series of thin-skinned and thick-skinned structures. The Archean basement and other old stratigraphic units were thrusted and uplifted to the ground and were exposed under the surface erosion. Finally, the uplifted strata were unconformably covered by Jurassic volcanics.The Syntectonic Sedimentation Analysis for the Yingzi Basin identified an important tectonic event-Indosinian Intraplate Orogenesis in the middle of Yanshan Belt, which resulted in a series of thin-skinned and thick-skinned thrusts under the roughly N-S compression. On the basis of the results on previous isotopic geochronology and geochemistry research, it is inferred that the duration of the Indosinian Tectonic Event in the middle of the Yanshan Belt happened and evolved during the period of 222~191Ma.