Early Devonian To Late Carboniferous Paleomagnetic Results From The Qaidam Block In The Qinghai-tibetan Plateau And Their Tectonic Implications

The Qinghai-Tibetan Plateau is located in the eastern part of the Tethys tectonic domain.Its formation and evolution are the result of the collision and amalgamation of blocks in the plateau with the southern edge of Eurasia in different periods since Phanerozoic.This process also involves the opening and extinction of several ancient oceans（e.g.,the Proto-Tethys,Paleo-Tethys and Neo-Tethys）.Devonian-Carboniferous is a key stage of the transformation of the Proto-and Paleo-Tethys Ocean and the evolution of the Paleo-Tethys Ocean,has attracted geoscientists’extensive attention.Qaidam block is located in the northeast of Qinghai-Tibetan Plateau and sandwiched between the Proto-and Paleo-Tethys Ocean suture zone.It is an ideal area to study the above issues.However,the paleolocation of the Qaidam block during Devonian-Carboniferous is still controversial at present,limiting the understanding of the paleogeographic pattern and the evolution of the Proto-and Paleo-Tethys Ocean.To solve the above scientific problems,this thesis focuses on the Devonia-Carboniferous strata of the Qaidam block and exploits tectonic paleomagnetism to analyse the movement history of the Qaidam block and further discuss its tectonic attribution and the evolution process of the Proto-Paleo Tethys Ocean between the Qaidam block and North Qiangtang Block.The main achievements are as follows:（1）Rock magnetism and petrography show that the type of magnetic minerals in the volcanic rocks of Devonian Maoniushan Formation（Fm）in Qaidam block are magnetite and hematite.The LA-ICP-MS zircon U-Pb dating of the Andesite from Maoniushan Fm shows an age of 416.2±6.0 Ma,which confirm that the age is Early Devonia.The stable characteristic remanent magnetization（Ch RM）(Dg=4.2°,Ig=-65.4°,kg=6.1,N=44,α₉₅=9.6°;Ds=28.4°,Is=-35.6°,ks=85.6,N=44,α₉₅=2.3°)which obtained from volcanic rocks of Devonian Maoniushan Fm（44 sites,354 samples）pass the fold and C-class reversal test,the corresponding paleomagnetic pole is:-27.5°N,67.9°E（A95=2.6°）.（2）Rock magnetism and petrography show that the types of magnetic minerals in the sedimentary rocks of Early Carboniferous Chengqianggou Fm in Qaidam block are magnetite.The stable Ch RM(Dg=353.9°,Ig=-26.9°,kg=11.4,N=36,α₉₅=7.4°;Ds=332.8°,Is=-37.9°,ks=41.3,N=36,α₉₅=3.8°)which obtained from sedimentary rocks of Early Carboniferous Chengqianggou Fm（36 sites,268 samples）pass the fold test,the corresponding paleomagnetic poles is:-25.7°N,125.1°E（A95=3.9°）.（3）Rock magnetism and petrography show that the types of magnetic minerals in the limestone of Late Carboniferous Keluke Fm in Qaidam block are magnetite.The stable Ch RM(Dg=331.6°,Ig=-9.0°,kg=25.4,N=10,α₉₅=9.8°;Ds=321.1°,Is=-38.5°,ks=100.8,N=10,α₉₅=4.8°)which obtained from limestone of Late Carboniferous Keluke Fm（10 sites,75 samples）pass the fold test,the corresponding paleomagnetic poles is:-20.2°N,134.7°E,（A95=4.3°）.（4）At the same reference point（36.8°N,98.0°E）,the paleolatitude of the Early Devonian,Early Carboniferous and Late Carboniferous of the Qaidam block is 21.4°±2.6°N,22.5°±3.9°N and 23.3±4.3°N respectivel.This indicates that the Qaidam block did not experience significant latitudinal motion from the Early Devonian to the Late Carboniferous,while took a 67.3°clockwise rotation.Comparing with the published paleomagnetic results of surrounding blocks,we proposed that the Qaidam block had similar paleolatitude and kinematic characteristics to the North China block,and maintained a latitudinal distance about15°（40°）from the Tarim block（North Qiangtang block）.Taking the existing Paleozoic paleomagnetic results and geological evidence into consideration,we suggest that the Qaidam were closely related to the North China block and independent from the Tarim block.Moreover,the width of the Tethys Ocean between the Qaidam block and North Qiangtang block maintained a latitudinal distance about～4400 km during the Latest Early Paleozoic to Early Permian.This indicats that the evolution of Proto-Tethys to Paleo-Tethys Ocean is likely a continuous process,and then the Paleo-Tethys Ocean gradually vanished with the northward drift of the North Qiangtang block in the Middle Permian and finally closed in the Triassic.