The Late Paleozoic Tectonic Framework And Evolution In Southern Beishan: Constraints From Paleomagnetism And Magmatism

Currently,There are still ongoing debates with respect to Late Paleozoic geoscience issues in southern Beishan orogen,NW of China,which is an excellent area to study tectonic evolution of southern Central Asian Orogenic Belt?CAOB?.Due to unclear sequence of multiple phases of magmatic activity and geodynamic backgrounds or only paleomagnetic data in Late Paleozoic not giving enough evidences for tectonic setting and evolution issue,this study focuses on both magmatic activity and paleomagnetic data to reveal the magmatic sequence and relevant tectonic settings in Late Paleozoic in Southern Beishan area.Meanwhile,combining the new reliable paleomagnetic data in Late Paleozoic in the southern Beishan area with other geological study?e.g.,ophiolite,biogeography,stratigraphic and sedimentary features?,more detail study on tectonic setting,evolution and closure time of the Paleo-Asian Ocean?PAO?was done,which provide more theoretical and practical guides on mineral and petroleum exploration in Beishan and adjacent area.A combined study of petrography,multi-element geochemistry,whole rock Sr-Nd and zircon U-Pb-Hf isotopes has been carried out for volcanic and a few intrusives in Late Paleozoic in the southern Beishan area with published magmatic age data,geochemical data and other geological data,magmatic activity can be subdivided into three phases which include Early-Middle Devonian?420³90Ma?,Late Devonian?375³60Ma?and Late Carboniferous-Middle Permian?305²60Ma?.Most of the Early-Middle Devonian magmatic rocks in the southern Beishan area are intermediate-acid igneous rock with a small amount of alkaline basalt.The basalts are similar characteristic of WPB?within-plate basalt?and the acidic igneous rocks are A-type granite which has characteristic of post-collisional granite.Combined with other geological evidence,we suggest that the Early-Middle Devonian magmatic event is associated with a slab break after closure of Huitongshan Ocean.The Late Devonian magmatic rocks are mainly subalkaline acid igneous rock.The rocks in the earlier stage are Adakites and A-type granitic rocks in late stage which are related to the delamination of thickened lithosphere.Late Carboniferous-Middle Permian magmatic rocks are mainly subalkaline with a small amount of alkaline rocks which are characterized by bimodal volcanic rocks assemblage.Subalkaline intermediate-basic volcanic rocks are mostly tholeiite series.The basalts which is derived from asthenosphere hybridized by subduction material are similar characteristic of WPB and MORB?mid ocean ridge basalt?in discrimination diagrams.During this period,the A-type granite is common and the acid rocks show the features of post-collisional granitoids which indicates the Late Carboniferous-Middle Permian magma related to post-collisional extensional setting.In this paper,we present the Carboniferous-Permian paleomagnetic studies in the southern Beishan terrane.The corresponding pole positions are at-33.8°N,115.3°E,A₉₅=18.6°for the Early Carboniferous,-0.2°N,168.4°E,A₉₅=2.9°for the Late Carboniferous?300Ma?,and 74.5°N,268.5°E,A₉₅=1.6°for the Early Permian?284²81Ma?.The results indicate that the southern Beishan terrane was located at 13.1°N±23.6°,11.7°N±3.1°and25.1°N±1.2°?reference points 32.9°N/83.5°E?during the Early Carboniferous,Late Carboniferous,and the Early Permian respectively.The Upper Paleozoic strata of the southern Beishan Region is divided into five megasequences?MS?by four major unconformities and huge sedimentary hiatus based on newly obtained stratigraphic,paleontological and isotope dating data.MS1 consists of Sangejing formation which is made up of continental deposits formed during Early-Middle Devonian?420³90Ma?.This formation mainly exposes in the west but very limited exposure in east of study area.MS2 consists of Dundunshan group which is made up of continental acid volcanic rocks formed during Late Devonian?371³67Ma?with very limited outcrops.MS3 consists of Hongliuyuan,Shibanshan and Jijitaizi formations in Weixian Age to Moscow Age in Carboniferous?346³10Ma?in which magmatic activity is scarce and strata shows large-scale regression.MS4 is made up of transgressive marine sedimentary strata consisted of Ganquan,Shuangbaotang,Jushitan and Jinta formations formed during Late Carboniferous-Middle Permian?302²59Ma?.MS5 consists of Fangshankou formation made up of continental clastic rocks without volcanic formed during Late Permian.MS5 is top Late Permain strata with unconformity.This study provides the first Map of basement geochronology based on Hf-Nd isotope study in Beishan region which shows that prominent areas of the continental crust growth are on both northern and southern sides of the Hongshishan-Baiheshan-Pengboshan ophiolitic mélange belt.Meanwhile,significant differences in northern and southern sides of ophiolite mélange belt may represents major suture of PAO.Subduction of PAO in Paleozoic both northward and southward formed large-scale island arc and accretionary wedge in northern PAO while continental arc in northern Hanshan terrain in southern PAO.Further southern Beishan area mainly consist of the lower and middle Proterozoic crustal basement which possibly was part of the Colombian and Rodinia supercontinent.Combined all geological evidence in the Beishan region,the Ordovician-Permian tectonic evolution for the southern Beishan area is subdivided into five stages:?1?O-S?485⁴20Ma?plate subduction stage;?2?D₁-D₃?420³60Ma?post-collision tectonic stage;?3?D₃-C₂?360³05Ma?intracontinental stable stage;?4?C₂-P₂?305²60Ma?post-collisional extensional stage;?5?P₃?260²50Ma?the final closure of the PAO.Using both reliable paleomagnetic data and paleobiogeograpohic information,the schematic Late Carboniferous?300Ma?and Early Permian?280Ma?tectono-paleogeographic reconstructions of the PAO is proposed.In this reconstruction,the southern Beishan block may have already been amalgamated with North China-Alxa and Qaidam blocks during Late Carboniferous-the Early Permian,but was separated from the Tarim block by a small ocean?here named Qiemo-Xingxingxia paleo-ocean?,and from Mogolia-Tuwa by the PAO at this time.In summary,the closure times of the west and east part of the PAO are different.In the northern Xinjiang area,the ocean basin was closed between Late Early Carboniferous and Early Late Carboniferous at North Tianshan or South Tianshan suture while the ocean,in this study area and NE China,was closed during the Late Permian at Hongshishan-Enger Us-Solonker-Xar Moron River suture.