Paleomagnetism Of The Early Devonian Pingshagou Formation In The Qiangbei Block Of The Qinghai-Tibet Plateau

The Northern Qiangtang block is the key area to solve the problems of the boundary of the margin of Gondwana and the tectonic evolution of the Pro-Tethys Ocean.Quantitative descriptions of the palogeographic and kinematic processes since the Paleozoic era can provide important foundations and key constraints for studying the paleostructural pattern of the Tibet Plateau,the Phanerozoic Tethys' evolution and the reconstruction of paleogeography.We sampled 92 Early Devonian Pingshagou Formation limestone samples from the Mayer Kangri area(N33.8°,E86.6°)of the Northern Qiangtang block,and we also conducted on the systematic rock magnetic and paleomagnetism studies to these samples in this paper.The results of rock magnetism show that the magnetic minerals of the most of samples are dominatede by magnetite in the limestone samples of the Pingshagou Formation.Through thermal demagnetization,alternating demagnetization and mixed demagnetization experiment,we obtain stable characteristic remanent component from these samples.The demagnetization curves of most samples show two-component characteristics:Low-temperature component is viscous remanence of modern geomagnetic field.The high temperature component passed the reversal test(C class,95% confidence level),consistent with primary remmance.The tilt-corrected mean direction for the Early Devonian rocks in the North Qiangtang Block is Ds=16.6°,Is=-17.7°,Ks=23.5,?95=5.3°,which yields a corresponding paleomagnetic pole at ?=44.3°N,?=243.4°E(A95=4.0),and a paleolatitude of 9.1°S.Our results,together with previously reported paleomagnetic data,indicate that:during the Early Devonian to Early Permian,The Northern Qiangtang Block may be affected by the expansion of the Paleo-Tethys,southward migration and collage with The Lhasa block,become the northern margin of Gondwanaland block.At the same time,the paleomagnetic poles of the Early-Middle Devonian in Yangtze and Qiangtang Block are basically consistent,in the Early Paleozoic-Middle Devonian,they were all located in the mid-low latitudes of the Southern Hemisphere.There is a large difference between the polar position of the North China block and the Qiangtang block,Late Devonian it is located in the mid-low latitudes of the Northern Hemisphere and away from Gondwana as a discrete block of Tethys ocean.In Devonian,the ancient latitudes of Lhasa and Himalayan blocks are all at 20-30°S,showing the good relevance to The Gondwana.