Late Paleozoic Tectonic Framework And Evolution For The Dashizhai Area Inner Mongolia, China

On the basis of geological mapping in Dashizhai area, eastern Inner Mongolia, together with detailed observations on strata sequence and lithology, analysis of sedimentary facies and sedimentary environment, tectonic deformation, geochemical studies and isotopic geochronology, main achievements have been made as follows:1. Stratigraphic sequence and sedimentary facies analysis indicate that the depositional sequences of the study area represent an integral evolution process from marine facies to terrestrial facies. A trench-island arc-back arc basin system is inferred to developed in early Permian, which consititue a complex marginal sea basin on the active continental margin, in which an offshore sub-deep-marine-deep-marine volcanic rocks-clastic rocks construction formed. During Middle Permian the study area evolved into intracontinental residual basin or residual backarc basin, filled with marine limestone, fine-grained clastic rocks to paralic fine clastic sediments. Late Permian sequence is chacterized by dark to light-colored fined grained clastic rocks deposited in terrestrial locastrain environment.2. Dashizhai tectonic melange belt is suggested to be the east extension of Hegenshan ophiolitic melange belt. Melange mainly composed of the Wujiatun Formation of intensive deformed muddy, silty slate, and the ultramafic rocks (287～300Ma) with chromium iron, unconformly overlied by Middle Permian Zhesi Formation Ultramafic rocks mainly derived from the lower part of SSZ typy ophiolite complex, which is thought to be a relict of a new formed oceanic crust following an early Permian Hegenshan-Dashizhai ocean.3. The volcanic stata sequences of Early Permian Dashizhai Formation (294±2Ma) reestablished in this paper. Petrologic and geochemical features of the rocks in this Formation suggested that it formed within an island arc associated with the subduction of Hegenshan-Dashizai oceanic crust rather than formed within an intracontinental rift or taphrogenic trough. Dashizhai Formation volcanic rocks consisting of andesite, dacite, tuff lava, and a small amount of basalt, belong to the rich silicon and aluminum poor titanium calc-alkaline volcanic rock series. A polarity from tholeiitic through calc-alkaline series to rich potassium calcium series toward paleocontinent has been identified. Both Dashizhai Formation and the plutonic rocks of Late Carboniferous-Early Permian (302～296Ma) along Huho Racecourse-Hada -Majiawopu are inferred to be an important part of Inner Mongolia magmatic arc.4. Lines of evidence indicate that so-called Wujiatun Formation, Dashizhai Formation and Shoushangou Formation are synchronologic counterparts deposited in different specific enviroment (290-300Ma). These formations constitue an Early Permian island arc-basin system instead of forming a unified sucssesive strata sequence. Wujiatun Formation formed within trench-forearc basin. Dashizhai Formation consist of island arc volcanic rocks. Shoushangou Formation was formed in the back-arc basin.5. Hegenshan-Dashizai ocean basin is inferred to be an independent ocean basin, with Xilinhaote Paleocontinent lying between it and southern Xar Moron Ocean. Symetric distribution of subducton relatedisland arc volcanic and intrusive rocks relative to the melange implies that the Hegenshan-Dashizhai oceanic crust experienced double directed subduction.