New Paleomagnetic Results From The Ordovician Carbonate Rocks In The South China Block And Their Tectonic Implications

During the development of plate tectonics theory,a lot of early Paleozoic reconstruction models were proposed to restore the global plate pattern.Most of these models share a common problem,thus the paleoposition of South China varies,which is a problem that cannot be ignored in the plate reconstruction at the Early Paleozoic.As a unique research method that can quantitatively constrain the paleolatitude of the plate,paleomagnetism plays an important role in solving this problem.There are contradictions between the existing South China paleomagnetic data in the Early Paleozoic Ordovician.It is necessary to clarify the source and accuracy of different paleomagnetic data to improve the temporal and spatial information of the South China Block.The Ordovician Pagoda Formation limestone and Dawan Formation limestone between Zhangjiajie City,Hunan Province and Yichang City,Hubei Province,in the north margin of the Yangtze Block,stored great number of paleontological fossils,which can constraint the relatively accurate age.In order to obtain higher quality Early Paleozoic paleomagnetic data and better determine the temporal and spatial location of the South China block,this research carried out a comprehensive paleomagnetism and rock magnetism study on the 170 Ordovician Pagoda Formation limestone and Dawan Formation limestone samples from 5 sections.The results of K-T curve experiment,isothermal remanence acquisition experiment and coercivity spectrum analysis show that the main magnetic mineral of the Dawan Formation limestone sample is hematite with a small amount of magnetite,and high temperature component is not isolated after systematic thermal demagnetization,Dawan Formation limestone sample only carries the modern geomagnetic field component;the main magnetic mineral of the Pagoda Formation sample is magnetite and a very small amount of hematite.After the systematic thermal demagnetization,two completely different high temperature components?HTC?A(Ds=328.9°,Is=59.4°,?₉₅=2.7°)and HTC-B(Ds=100.6°,Is=-35.9°,?₉₅=8.3°)are isolated.The HTC-A is close to that of the Late Jurassic,not the primary remanence,but whether it is remagnetization needs further study.The HTC-B is different from the paleomagnetic data in the later period,highly likely the primary remanence.HTC-B yields paleomagnetic pole B?18.6°S,213.7°E,dp=5.6°,dm=9.6°?,corresponding paleolatitude of South China Block in Ordovician is 19.9°S,located near the tropical latitudes of the southern hemisphere.Combined with the existing South China Block paleomagnetic data and compared with the Australian block paleomagnetic data of the in the same period,the South China block has a more direct relationship with Australia in early Paleozoic,rather than India and Pakistan.