Affinity Of Alxa Terrane During Early Paleozoic

Due to the Alxa terrane of the independent bilayer structure development,the discussion of its belonging has a long history,but starting from itself analysis, its east edge that southern part of the Helan study area is located in the Qinling-Qilian orogenic belt combining site,among the Alxa terrane,Qilian terrane,North and South China terrane,study of the key conversion "bridge" the Alxa and neighboring area.But attribution of the Helan study area provenace studies very little,even if it does,its not much for merely analysis of single formation in U-Pb zircon chronology,not from the early Paleozoic overall grasp,doesn’t analyze the basic pattern of sedimentary,lead to the Alxa terrane the evolution process can’t make good answer during early Paleozoic,cannot also fully explain affinity of the Alxa terrane during early Paleozoic, can’t more restrain ocean-land pattern.In this paper,according to a total of seven areas Cambrian Xiangshan Group,Middle Ordovician Miboshan Formation,Upper Ordovician Pingliang Formation in LA-ICP-MS zircon U-Pb dating method,assisted methods including sedimentary framework、structural movements and biological affinity in the study area to track provenance of the entire lower Palaeozoic,the key to probe affinity via the Alxa terrane compares to the Qilian,North and South China terrane during early Palaeozoic, draw the outline of ocean-land pattern. Secondly,compared affinity with the terrance of rest of the world.We obtained the following results:(1) Ages of Xiangshan Group are mainly 2432Ma,1990Ma,1802 Ma, 1405Ma,1082Ma,977Ma,850Ma,655Ma,505Ma.And the Qilian terrane in the magma thermal events are unanimous, for direct content source;the Alxa terrane Neoproterozoic (894 Ma) and Paleoproterozoic (1858 Ma and 1964 Ma) material could supply a large number of source, and the lack of "pan-African period (600-500Ma)", "Grenville orogenic period (1300-900Ma)"、1400-1800Ma in three periods of proterozoic magma-tectonic thermal events, not as content source,speculated that the Alxa terrance for indirect source supply;the North China massif is no heat in Neoproterozoic, own significant> 3.0 Ga thermal events also not reflected in the Xiangshan group, nor is it a content source;the lack of heat in the South China block is 1.8-1.3 Ga,other the peak coincide quite well,deduce indirect provenances. The age range of Miboshan Formation mainly are 1.3-0.8Ga,2.6-2.2Ga,0.8-0.5Ga,secondary age range are 2.2-1.6Ga,1.6-1.3Ga,3.0-2.6Ga,>3.0Ga,the Alxa,Qilian,North and South China terrane in the magma thermal events are not exclusive of provenance district,but combine parts of the provenances.Main age range of Pingling Formation are 1.3-0.8Ga,0.8-0.5Ga and secondary age range 2.6-2.3Ga,2.0-1.6Ga,3.0-2.6Ga, 1.6-1.3Ga,458Ma and 3434Ma,inheritance of Miboshan Formation provenance characteristics,the Alxa、the center Qilian, North and South China terrance in four plots are still the provenance districts.(2)During Cambrian,the Alxa terrane and Qilian terrane are no"touched" in the study area,and North and South China terrance are unified,but the latter than the former strong affinity; Tian jing shan period in the early Ordovician distribution of carbonate stabilize in the study area, marks that the Alxa and Qilian and North China terrance initially collage together;in the middle Ordovician the Alxa terrane Miboshan Formation and Qilian and North China terrane further collage;in the late Ordovician of out Alxa-Qilian-North China terrance realize comprehensive collage.Enter the Ordovician Alxa terrance and the South China terrance familiar gradually collage.During the early Paleozoic, the Alxa terrance、Lhasa block and Australia terrance have strong affinity together, but the Alxa terrance and the North America land affinity is weakening;the Alxa terrance and eastern part India and eastern part Antarctica affinity further reduce, and the Alxa terrance and the South American land is no affinity.(3) During Cambrian, center Qilian terrance from mainland Rodinia splites and gradually close to the Alxa terrance, North Qiiian ocean-North Qin Lingyang (kuanping ocean)-North China sea converge in the south of Helan mountain connected, form the Alxa platform facies sedimentary and the edge of the Qilian continent the ocean and deep-water flysch deposits, and the Cambrian Craton coexistence pattern of the land;Early-middle Ordovician, the North Qilian ocean crust subducted, North Qilian ocean extincted, the Qilian terrance and the Alxa terrance in the Helan mountain "collage", the North Qilian ocean and the North Qin Lingyang (kuanping ocean)sea water have not affected the study area;Midd!e-Late Ordovician,the Yangtze terrance and the Alxa terrance and near North China terrance, Shangdan ocean water inpoured into the area and occured large-scale marine transgression, formed the deep sea structure.