Geological Characteristics and Tectonic Implication of the Xiaozhai Sedimentary Assemblages in the North Qinling

Key words: 
• the North Qinling /
• sedimentary setting /
• back-arc basin /
• tectonic evolution

Abstract: Based on the studies of geology and geochemistry,the Qinling orogenic belt is characterized by three plates subduction and collision along the Shangdan suture zone and MianLue suture zone during major orogenic period.The Danfeng arc volcanic rocks and the Erlangping ophiolite indicate the existence of trench-arc-basin system along the North China plate.The Erlangping Group is composed of the northern Sishuyuan clastic sedimentary assemblages.intermediate ophiolite and the southern Xiaozhai clastic sedimentary assemblages,and represents the relicts of the Erlangping back-arc basin which was an important part of the active continental margin of the North China Plate in the Palaeozoic. The Xiaozhai metamorphic sedimentary assemblages consist of two different rock associations,including mica-schist,mica-quartz-schist and mica-schist,issite rocks.Based on the analysis of protolithes and geochemistry of the Xiaozhai metamorphic assemblages,the most of the two rock associations are sandstones and mud stones except part of the issite rocks.Research on the texture of sandstones show that the clast is mainly made up of feldspar, lithic fragment and little quartz,and the matrix content in sandstones is high,Both the compositional maturity and the textural maturity in the sandstones are low,so the rock types belong to detritic sandstone and feldspathic graywacke.The Xiaozhai sedimentary assemblages exist a series of sedimentary structures such as cross-beddings,horizontal beddings,evenly beddings and graded beddings,these different structures and rock types consist of Bouma sequence and can be clearly observed in weak deformation regimes.The study of sedimentary setting indicates that the Xiaozhai sedimentary assemblages set their position in sea-trench or trench-slope,which is related to the subduction of the Erlangping back-arc basin.The paleowater direction and sediment analysis verified that the Xiaozhai sedinentary assemblages clastics derived from the southern Qinling island arc. The structural deformation analysis have revealed that the Xiaozhai sedimentary assemblages had undergone four different deformation stages.These stages represented the tectonic evolution of the Erlangping back-arc basin.Firstly,fold deformation at deep-middle tectonic level,resulted in isoclined foldings,taking the primary bedding as deformation stratification and axial-plane clevages is rigional penetrative schistosity. Secondly,thrust ductile deformation formed in the deep-middle tectonic level.and show the deformation characteristic of major subduction stage.Finally,brittle shearing and strike-slip shearing dominated the sedimentary assemblages in the intracontinental stage.The subductional granites,tectonic melange and high pressure metamorphic zone in the south of the Xiaozai sedimentany assemblages which were formed in Hercynian-Indosinian epoch indicate that the Erlangping bock-arc basin subducted beneath the Qinling island arc during the major orogenic period. In the light of the comprehesive study of the Xiaozai sedimentary setting,metamorphism,deformation and magmatism,the evolution processes of Erlangping back-arc basin can be recognized as three stages,i.e., spreading stage(Pt 2-O 2),subduction-collision stage(O 2-T 2) and intracontinental stage (T 3-Q).