Paleomagnetical Research On Riwancaka Formation In Southern Qiangtang Block Of Early Carboniferous

Qiangtang block is very important to research on evolution and dynamics of Qinghai-Tibet plateau. It has been regarded as entirety since Shuanghu-Longmucuo suture zone was found, which makes Qiangtang block be devided into northern Qiangtang block and southern Qiangtang block. At the same time, boundary of Eurasia and Gondwana was centred in differences between Bangonghu-Nujiang suture zone and Shuanghu-Longmucuo suture zone. Therefore, problem that wehther Shuanghu-Longmucuo suture zone north to southern Qiangtang block belongs to southern Eurasia or not is very critical, that is whether northern Qiangtang block and southern Qiangtang block pieced or not in late Paleozoic. So paleomagnetic research on northern Qiangtang block and southern Qiangtang block is very important. However, paleomagnetic data is sufficient in North Qiangtang but scarce in the South, which is difficult to determine the evolutionary time of Shuanghu-Longmucuo suture zone. Aiming to this problem, paleomagnetism of Riwancaka formation in Southern Qiangtang block in early Carboniferous was studyed viewing to obtain the paleomagnetic pole position and paleolatitude of early Carboniferous, which could provide basis for tectonic evolution of Qinghai-Tibet plateau.12 sites in Gaize region of south Qiangtang were setted and 167 paleomagnetic samples orientated were gotten. Through rock magnetic experiment, we can know the magnetic mineral of the roak mainly include magnetite, goethite and little pyrrhotite. Methods of thermal demagnetization, alternating demagnetization and mixed demagnetization were used to clear the residual magnetic, while principle components analysis and Fisher statistical method were used to analyse the residual magnetism.Then, results of low temperature component and high temperature component could be obtained. According to the results, direction of low temperature component is nearly same to that of modern geomagnetic field, which may be result of remagnetization by modern geomagnetic field. While high temperature component which is approved by strict fold test may represent the original residual magnetism.Direction of high temperature component through strata correction is:Ds=242°, Is=18.8°, Ks=19.9, a95=10°; Paleomagnetic pole position calculated is:Xp=16.9°, ψp=199.1°, dp/dm=10.4/5.4; Paleolatitude is-9.7°.Through the comparison of paleomagnetic pole position calculated in this paper with contemporary northern Qiangtang block, we can reach conclusions that southern Qiangtang block separated with northern Qiangtang block was located in southern hemisphere in Carboniferous and the Tethys Ocean existed between northern Qiangtang block and southern Qiangtang block. And a surmise that northern Qiangtang block and southern Qiangtang block pieced together in the late Permain is gotten combining the previous references. Shuanghu-Longmucuo suture zone was not the southern margin of Eurasia any more which was replaced by Bangonghu-Nujiang suture zone south to Qiangtang block. Southern Qiangtang block had been moving northward accompanying rotation from Carboniferous to Triassic.