Paleomagnetic Result Of The Silurian From North Qiangtang Terrane And Its Tectonic Implications

The North Qiangtang terrane,located in the central part of the Qinghai-Tibet Plateau,is one of the largest block with stable crystalline basement and relatively complete stratigraphic development since the Paleozoic.Existing geological data show that there are records of early Paleozoic and Late Paleozoic Tethys ocean in the north and south of the North Qiangtang terrane.Due to the lack of complete and reliable paleomagnetic data,the paleogeographic location of the North Qiangtang terrane during the Paleozoic are still unclear,and the affinitie and discrete convergence process between the North Qiangtang terrane and Gondwana or Eurasia are still unclear,and the evolution process of the proto-Paleotethys ocean on the north and south sides of the North Qiangtang terrane is controversial.In order to obtain reliable paleomagnetic data of the early to Middle Silurian and Late Silurian of the North Qiangtang terrane and reconstruct the palaeogeographic position of the North Qiangtang terrane during this period,a detailed paleomagnetic study was carried out on the Silurian limestone and sandstone in the north of Longmu Co,Ngari,Tibet.Combined with the existing paleomagnetic data of late Paleozoic in the North Qiangtang terrane,the paleogeographic evolutions during the Paleozoic in the North Qiangtang terrane are restored,which provide a basis for discussing the evolution process of the proto-Paleotethys ocean in the region.In this paper,two sections were selected to collecte samples in the north of Longmu Co,277 limestone samples were collected from the Early-Middle Silurian Longmu Co Upper Formation.159 sandstone samples were collected from the Late Silurian Longmu Co Lower Formation.The results of rock magnetism show that magnetite is the main magnetic mineral in the limestone samples,together with a subsidiary fraction of pyrrhotite.Magnetic minerals in sandstone samples of Longmu Co Upper Formation are complex,likely both magnetite and iron sulfide（such as pyrrhotite）.The demagnetization results show that most limestone samples from the Longmu Co Lower Formation show two-component characteristics,the low-temperature component is close to the direction of the present-day dipole field,the high-temperature（high-field）component C is Dg=304.7°,Ig=31.3°,Kg=12.8,α₉₅g=12.6°（in geographic coordinate）and Ds=320.0°,Is=40.6°,Ks=30.7,α₉₅s=8.0°（in stratigraphic coordinate）,which can pass the fold test and Classification B reversal test.Combining various evidences,we believe that component C is probably the direction of primary remanence.Most sandstone samples from the Longmu Co Upper Formation also show dual-component characteristics.The low-temperature component is the viscous remanence direction of modern geomagnetic field,and the characteristic remanence component is obtained at 200-400℃.Combined with other evidences,we therefore interpreted that these sandstone samples contain remagnetized magnetization directions.In conclusion,the direction（component C）of the Early-Middle Silurian primary remanence in the North Qiangtang terrane has been obtained.Based on this,the positions of the early-middle Silurian paleo-geomagnetic poles in the North Qiangtang terrane are calculated asλ=53.5°N,φ=343.3°E,dp=10.5°,dm=6.4°,and the corresponding paleo-latitude is 23.2°S.Using this data,we reconstructe the palaeogeographic position of the North Qiangtang terrane during the Early-Middle Silurian by combining with geological evidences,and concluded that the North Qiangtang terrane was relatively independent in the Tethyan Ocean during the Early-Middle Silurian,probably not connected to gondwana continent.They are separated by the Tethys Ocean（Longmu Co-Shuanghu Ocean）,the Longmu Co-Shuanghu Ocean evolved continuously during Paleozoic.