Accretionary Orogenesis Of Carboniferous-Triassic In The Southern Belt Of East Kunlun Orogen

The southern belt of the East Kunlun Orogen is located in the northern margin of the Paleo-Tethys tectonic domain,retaining a series of structural,sedimentary,metamorphic,and magmatic records related to the subduction and accretionary tectonics of the Paleo-Tethys Ocean.It is an ideal area for reconstructing the accretionary orogenic process of the East Kunlun Orogen.The Carboniferous-Triassic is the key period from episodic accretion to closure in the tectonic evolution of the Paleo-Tethys Ocean.The sedimentary filling sequence of the basin records the time,pattern and polarity of the tectonic events,and holds the key for reconstructing the accretionary orogenic process.This paper takes the Buqingshan ophiolitic mélange and the Carboniferous-Triassic strata that widely developed in the south Kunlun Belt as the research object.In this paper,on the basis of detailed field observations and geological sections,comprehensive research of petrology,sedimentary facies analysis,provenance analysis,zircon U-Pb dating and whole-rock major and trace elements analysis have been carried out,which help us reconstruct the formation process of the Buqingshan ophiolitic mélange and the tectonic evolution of the Carboniferous-Triassic basin in the South Kunlun Orogenic Belt.（1）The Buqingshan Complex,an imbrication of five east-west-oriented litho-structural slices separated by N-dipping thrusts,is characterized by“block-in-matrix”structures with several blocks of mafic-ultramafic rocks in a deformed clastic matrix.It could be interpreted as a part of the accretionary complex.The serpentines show similar geochemical characteristics to typical abyssal peridotites,belonging to the residual mid-ocean ridge upper mantle after melt extraction.The mafic rocks have geochemical affinities to N-MORB and OIB,which are formed in middle ocean ridge and mantle plume-related seamount environments,respectively.The diabase pf N-MORB affinity yield a Late Carboniferous age at 303±3 Ma.The cover bioclastic limestones of the seamount were demonstrated to be Early Permian in age based on paleontological data,which constrains the formation age of the seamount to the Early Permian.（2）The results of stratigraphic correlation show that the Carboniferous-Triassic strata in the South Kunlun Orogenic Belt are characterized by multiple sets of clastic rock-carbonate-volcanic rock assemblages separated by multiple unconformities,which include,from the bottom to top,the Halaguole Formation（C₁h）,the Haoteluowa Formation（C₂P₁h）,the Gequ Formation（P₃g）,the Hongshuichuan Formation（T₁h）,the Naocangjiangou Formation（T₂n）,the Xilikete Formation（T₂x）and the Babaoshan Formation（T₃b）.（3）Five unconformities,the unconformity between the Haoteluowa Formation and the underlying strata,the unconformity between the Gen Formation and Haoteluowa Formation,the disconformity between Hongshuichuan Formation and the Gen Formation,the unconformity between the Babaoshan Formation and the underlying strata,and the disunconformity between the Yangqu Formation and the Babaoshan Formation,separate the Carboniferous-Triassic into three main tectonosynthems,revealing the episodic tectonic-sedimentary evolution history of the south Kunlun Belt.（4）The Lower Carboniferous-Lower Permian is characterized by multiple sets of intermediate-acid volcanic rocks sandwiched by clastic rocks and carbonate rocks and developed five depositional systems which include volcanic-sedimentary,fan delta,barrier shore,barrier-free shore,and shallow sea shelf.It is characterized by multiple sedimentary cycles,of which the paleocurrent direction is dominated by S,and the clastics mainly come from the south and central Kunlun Belt in the north.It is a forearc basin of the erosion continental margin,which is associated with the northward subduction of the Paleo-Tethys Ocean.（5）The Upper Permian unit consists of the fan delta system at the bottom and the shallow sea shelf system at the top.The clastic fillings mainly come from the South and Central Kunlun Belts in the north,and partly come from the Buqingshan-Anemaqen ophiolitic mélange zone in the south.The Middle-Lower Triassic succession,which is characterized by a sequence of clastic rock and carbonate rocks,with minor acidic volcanic-pyroclastic-rocks,from bottom to top,is composed of fan delta（barrier-free shore）,shallow sea shelf,and bathyal slope facies.The dominant direction of paleocurrent is S,and the clastic sediments mainly comes from the underlying South Kunlun Belt and the Central Kunlun Belt in the north.（6）The Late Permian-Middle Triassic was the depositional stage of a forearc basin in the accretionary continental margin,and the regional extension and continuous subsidence of the basin may be controlled by the roll-back of the subducting slab.In the Late Triassic time,the area translated into fault depression basin basins and developed a set of continental sedimentary systems consisting of alluvial fans,rivers,deltas,lakes facies.（7）Based on the comprehensive analysis of the sedimentary evolution in the Southern Belt of East Kunlun Orogen,the formation processes of the Buqingshan Complex and the evolution of regional magmatism,the accretionary orogenesis of Carboniferous-Triassic in the Southern Belt of East Kunlun Orogen could be divided into four major stages,they are normal subduction（C₁-P₁）,formation of the accretionary wedge（P₂-P₃）,steep subduction corresponded to slab roll-back（P₃-T₂）,and intracontinental evolution（T₃）.