Composition,Geochronology And Mesoproterozoic-Neoproterozoic Tectonic Evolution Of The Central-Southern Altyn Tagh

Altyn Tagh area is located in the northern margin of Qinghai-Tibet Plateau,sandwiched between Tarim and Qaidam block,which has important geological significance.The Precambrian geological bodies in Altyn Tagh area are widely distributed,but the research on the tectonic evolution history of the Altyn Tagh in Meso-Neoproterozoic is relatively weak,which restricts the understanding of the relationship between the Altyn Tagh and global supercontinent events.In this paper,the Changcheng System Bashikuergan Group,the Jixian System Taxidaban Group and the Qingbaikou System Suoerkuli Group defined by predecessors and the Neoproterozoic magmatic rocks in Altyn Tagh,as the research objects,are selected.Based on a large number of field geological investigations,the formation time,sedimentary environment,clastic provenance and tectonic background of the Bashikuergan Group,Taxidaban Group and Suoerkuli Group are confirmed,the paper determined the geometric and kinematic characteristics of the above-mentioned strata and discussed its dynamic processes,through lithology,zircon chronology,geochemistry and structural deformation analysis.At the same time,through petrology,geochemistry and zircon LA-ICP-MS U-Pb dating of Neoproterozoic magmatic rocks,the chronological framework of Mesoproterozoic Neoproterozoic magmatic events in the Altyn area is established and the characteristics of magma source region and tectonic setting of each stage of igneous rocks are discussed.Finaly,based on the above research and combined with the previous research results,we discussed the tectonic evolution history and block affinity of the Middle-Neoproterozoic of the Altyn region.The main achievements are as follows:1.The Taxidaban Group is a suit of parametamorphic rocks with low greenschist facies,original formation of the metamorphic rocks may be a flysch with turbidite characteristics in a deep-water reducing environment.The provenance area is dominated by felsic continental arc and upper crust.The Taxidaban Group was formed in the tectonic environment of active continental margin and its formation age is between 1087 ? 945 Ma.The greenschist facies parametamorphic rocks in the Altyn complex and the middle Altyn Tagh Taxidaban Group have the same material composition,formation age,detrital zircon spectrum and zircon Hf isotope,indicating that they belong to the same set of strata.2.The tuff interlayer was first discovered in the Luanshishan Formation of the Suoerkuli Group and its formation age was limited to 936 Ma.The Suoerkuli Group is a suit of clastic rocks and carbonatite construction which is formed in the shallow sea-tidal flat environment.The provenance areas are mainly derived from the recycle orogenic belt,a few from the craton.The Suoerkuli Group formed in the extensional tectonic environment.The composition of conglomerates and lithic sandstones of the Suoerkuli Group is similar to that of the Taxidaban Group and they have similar detrital zircon spectrum,which indicates that the Taxidaban Group is the source of the Suoerkuli Group.3.Several Neoproterozoic granitoids have been identified in the Altyn complex and their diagenetic ages range from 997 to 901 Ma.Geochemical characteristics show that they are S-type and I-S transitional type granites.The magma originated from partial melting of the Taxidaban group and the South Altyn metamorphic supracrustal rocks.Three magmatic records of the Middle-Late Neoproterozoic were disintegrated from the Changcheng System Bashikuergan Group previously divided: 825 Ma A-type granites,779 Ma high fractionated granites,758 Ma high fractionated granite and contemporaneous diabase,these magmatic rocks were mainly formed in the intraplate extensional environment.4.It is revealed that the Changcheng System Bashikuergan Group,which was defined by predecessors,is not a set of continuous sedimentary strata,but mainly composed of Neoproterozoic-Paleozoic blocks in different genetic environments,which are disorderly patched together and are tectonic melanges.In this paper,the bashikurgan group is redefined as a set of volcanic-clastic rock series with rift basin characteristics in the late Neoproterozoic(Nanhua period).Other material components should be separated from the bashikurgan group.5.The analysis of structural deformation shows that the Taxidaban Group experienced at least three stages of tectonic deformation.The first stage of tectonic deformation may be related to the Neoproterozoic supercontinent aggregate.The Suoerkiqun Group was deposited at least after the first stage of deformation of the Taxidaban Group.The Taxidaban Group,Suoerkiqun Group and the southern part of the Suoerkiqun Group experienced the compression from SSW to NNE,the northern part of the Bashikuergan complex developed SSE thrust at the same time,both of which may be related to the subduction of the North Altyn Ocean.The late phase NW-SE compression of the Taxidaban Group,Suoerkiqun Group and the Suoerkiqun Group may be related to the formation of left-lateral strike-slip fault system in the Altyn.6.The Middle-South Altyn,East Kunlun and Qaidam block have similar Late Meso-Neoproterozoic magmatic-sedimentary records and constitute the Precambrian basement of Qaidam block.7.Combined with the similarity of the eastern margin of Laurentia,the southwestern margin of Baltica and the Amazoina in sedimentary records and magmatic activities,this paper provides a new model for the reconstruction of tectonic palaeogeography and suggests that Altyn may be located between Laurentia,Baltica and Amazoina,the core area of Rodinia supercontinent.