Tectonic-lithological Assemblages Of Southern Qinling Accretionary Complex And Their Formation Process

The 'Baishuijiang Group' in the western part of the southern Qinling(Kangxian, Huixian counties of Gansu province,and Lueyang,Mianxian and Liuba counties of Shannxi province) consists mainly of fine-grained clastic rocks,carbonate blocks and volcanic rock blocks with metamorphism at different degrees,and has been traditionally inferred as a stratum of the passive margin of northern Yangtze plate. Based on the primary field investigation and mapping,and integrative analysis of sedimentology,paleontology,structure geology and geochemistry,the 'Baishuijiang Group' and the Mian-Lue belt together composed an accretionary complex of the sourthern Qiling.The accretionary complex belt of southern Qinling is divided into three subbelt according to the characteristic of the matrix and blocks.The northern subbelt(the Dahedian formation) is composed of abyssal mudstone and carbonaceous siliceous slate, and carbonate,chert and volcanic blocks;the central subbelt(The Zhouqu formation and Diehu formation)contains huge thick turbidit which formed the trench-depositional environment and a few limestone blocks;the southern subbelt (The Dabao formation and Sanhekou group)comprised abyssal mudstone and carbonaceous and siliceous slate,and limestone,chert and volcanic blocks.The analyse of petrology,heavy mineral assemblage,geochemistry and clastic zircon dating of the matrix shows that they originated from active continental margin,rather than from a passive continental margin.In southern subbelt and northern subbelt,the volcanic rock blocks include Ultrabasic,basic,and intermediate volcanic rocks,and have undergone different degree of metamorphism in different structural position.Geochemical characteristics indicate that the igneous rocks formed within-plate setting and from the mantle,which is similar to oceanic island arc and seamount.SHRIMP U-Pb analyses for zircon grains showed that the mafic rock blocks in the Loufanggou area of the northern subbelt were formed in the Mesozoic(210±5Ma).SHRIMP dating results yielded ages of 667～778 Ma to the mafic blocks and gabbro and diabase blocks was dated at 445Ma and 450Ma in Yuntai and Fotianban in southern subbelt.The ages of mafic and basic rock blocks and the data of ophiolitic and mafic blocks of Mian-Lue subbelt in the early papers,indicate that Qinling ocean was a proterozoic ocean basin,rather than the so-called Devonian regenerated ocean basin. In accretionary complex of the southern Qiling,many fossils were found in matrix and rock blocks,the fossils contain the Cambrian small shell fossils and middle Devonian Coral fossils in limestone and chert blocks and Permian radiolarian in mudstone and argillite slate of Matrix in northern subbelt,and Silurian to middle Devonian Coral and conodonts in limestone blocks and Silurian chitinozoas, scolecodonts and spores in clastic rocks in central subbelt,and Devonian Coral fossils in limestone blocks and graptolites in carbonaceous and siliceous slate in southern subbelt.La-ICP-MS clastic zircon analyses in matrix obtain the age of early Devonian.In the region,late Paleozoic radiolarian were found in San hekou group and Xixiang group in cherts.Therefore,according to ages of the fossils found in matrix and blocks,the formation age of accretionary complex of southern Qinling is the Permian-Triassic,rather than the the Silurian as previously inferred.Based on this research results and comprehensive analysis of regional data,the Qinling orogenic belt includes the island-arc belt of northern Qinling,the forearc basin of middle Qinling and an accretionary complex of sourthern Qinling from Shangdan fault to Kangxian-Mianlue fault.The ages of volcanic rocks,matrix and limestone blocks in accretionary complex indicate the Qinling ocean existed from proterozoic to trassic,and the subduction occurred to north in Ordovician and south in Devonian.The closure of the whole Qinlng ocean basin was late Trassic.