Preliminary Study On Paleogeomagnetics Of The Early-Middle Silurian Sedimentary Strata In The Weibei Block Of The Qinghai-Tibet Plateau

The North Qiangtang terrane,a stable block in the Tibet Plateau with a largest amount of area,an old basement and a relatively developed Paleozoic strata,is a most pivotal area to solve scientific problems of the boundary of the northern margin of Gondwana and the tectonic evolution of Pro-Tethys Ocean.A preliminary paleomagnetic research on the Early-Middle Silurian sedimentary strata in the North Qiangtang terrane has been done and a corresponding paleogeographic position has been obtained which helps to sovle the geological problems above.In this paper,the works we have done and the results obtained are as follow:10 sampling sites were set in the Early-Middle Silurian limestone in the north margin of Longmu Co,Dongru Village,Rutog County with a collection of 98 orientated core samples.The cores were cut and machined in to 130 standard specimens,6 and 116 of which were respectively conducted for the rock magnetism experiment and demagnetization experiment and the rest for the reservation.Rock magnetic data reveals that magnetite and maghemite with low coercivity and high unblocking temperature are the main magnetic carrier minerals,besides,there is less pyrrhotite in very few specimens.Statistical analysis of remanence components yields a low temperature field strength component which may be the remagnetization of PDF and two high temperature/field strength components B1,B2 with two significantly different directions.The paleopole of B1 component is 48.9°N,186.1°E(A95=8.7°)with a paleolatitude of 16.5°N,which may be a result of the Early-Middle Creaceous remagnetization.B2 component passes a reversal test(C class,95%confidence level)and the calculated paleopole is-43.2°N,172.8°E(A95=5.7');the corresponding paleolatitude is 24.5°S,which is different from the results of Permian-Mesozoic.So,we believe that B2 component represents the original residual magnetism.Our results together with the previously published paleomagnetic data demonstrates that the northern Qiangtang terrane stayed relatively stable in the middle-low latitude of South Hemisphere during S1-2 to C3-P1,but a significant northward motions happened in P1-P2 with the paleolatitude up to 14.5°S.Then,the blok stayed stable again till P3.In addition,the North China Block and Tarim were both located in the lower latitudes of North Hemisphere in Silurian,while the Yangtze block hovered at the vicinity of the equator and it might have affinity to the Yangtze block because of the close paleomagnetic pole and the similar experience of the tectonic movements(such as:the late Caledonian tectonic movement).In combination with the previously qualitative research on the Early Palaeozoic tectonic framework in the north margin of Gondwanaland,paleogeographic recovery of northern Qiangtang terrane and its adjacent blocks in Silurian the was done in the paper.