Paleomagnetic Results Of The Middle Permian Lugu Formation Of The South Qiangtang Terrane And Its Paleogeographic Position

The Late Paleozoic and Early Mesozoic were the key periods of the Pangea supercontinent unraveled,the width of Paleo-Tethys Ocean shrunk gradually and the Neo-Tethys Ocean expand gradually.North Qiangtang-Qamdo,South Qiangtang and Lhasa terrane,the core of the Tibetan Plateau hinterland,may also be cracked from Gondwana(Southern hemisphere)and drifted to Laurussia(Northern hemisphere).The paleolocation and movement of these terranes in Late Paleozoic and Mesozoic are great significance to understand major geological scientific issues such as the cracking of Pangea supercontinent and transformation of Paleo-Neo Tethys ocean.The objective of this paper is to obtain the latitude of South Qiangtang terrane in Permian and its relationship with peripheral blocksIn this paper,rock magnetic and paleomagnetic experiments were performed on 131 samples(16 sites),which collected from the Middle Permian Lugu Formation near Rongma Township(Nima County,Tibet),include limestone,basalt,and sandstone.The results show that 1)Most limestone samples cannot separate the high-temperature remanent component(HTC),few limestone samples exhibit two-component behavior and few along a great circle.The data obtained from the limestone samples are too scattered to allow paleomagnetic statistics 2)Both magnetite and maghemite presence in basalt samples and demagnetization curves are mainly two-component.The Characteristic remanent magnetic(ChRM)component in 4 basalt sites(31 samples)is Dg=200.6°,Ig=12.5°,kg=98.5,?95g=93°(in geographic coordinate)and Ds=177.5°Is=39.2°,ks=101.7,?95s=9.2°(in tilt-corrected coordinate).3)Rock magnetic behavior indicate the presence of magnetite and maghemite presence in sandstone samples and the ChRM in 11 sandstone samples which yielded two-component behavior is Dg=364.7°,Ig=7.9°,kg=13.3,?95g=13.0°(in geographic coordinate)and Ds=357.5°Is=-49.3°,ks=13.3,?95s=13.0°(in tilt-corrected coordinate).The ChRM of the sandstone samples which rotates counterclockwise by 105.1° is antipode of the the ChRM of basalt.We calculated the mean direction of the ChRM component of sandstone(after rotation)and basalt,which yielded Dg=193.9°,Ig=11.9°,kg=24.0,?95g=l5.9°,Ds=177.5°,Is=41.3°,ks=95.7,?95s=7.9°.This mean direction can pass the reversal test(C-level)at the 95%confidence and the partial untilting test.The latitude of South Qiangtang terrane is-23.7°N and the paleomagnetic pole is located at ?=33.0°N,?=269.6°E(dp=5.9,dm=9.6).South Qiangtang terrane was located at-23.7°N in Middle Permian.By comparing the paleomagnetic data of the peripheral blocks,South Qiangtang terrane may have begun to separate from the Gondwana continent.It can be concluded that the latitudinal difference between the South Qiangtang and Tarim is 52°,the north-south width of the Paleotethys ocean between South Qiangtang block and Tarim block was more than 5500 km.There is?9° between South Qiangtang and North Qiangtang-Qamdo.,and the Longmucuo-Shuanghu ocean is about 950 km.This article summarizes the existing paleomagnetic,geochemical and paleontological evidence,and draws the map of evolution pattern of the South Qiangtang,North Qiangtang-Qamdo and Lhasa terrane since the Late Paleozoic.