Rock Magnetism Of The Late Paleozoic Carbonate Rocks In The Lhasa And Baoshan Terranes And Its Paleomagnetic Implications

Late Paleozoic is the key period for the breakup of Gondwana.The Lhasa and Baoshan terranes are the essential tectonic parts originating from Gondwana,paleomagnetic study of which terranes is helpful to clarify the configuration and breakup of Gondwana.However,the paleomagnetic results from the late Paleozoic carbonate rocks in Lhasa terrane are contradictory and scarce in the Baoshan terrane.In view of the complex remanent magnetic signal of carbonate rocks,in-depth rock magnetism study is the premise of obtaining reliable paleomagnetic results.In this study,we collected paleomagnetic samples from the Middle Permian carbonate rocks within Luobadui Formation in the Lhasa terrane.Rock magnetic analyses were performed on representative samples and stepwise thermal demagnetization was performed on all 54 samples from 4 sites.The rock magnetic results show that pyrrhotite is the major magnetic carrier.Thermal demagnetization results show that the mean direction of low temperature components is close to the recent geomagnetic field in this region,while no stable high temperature component can be separated from these samples.In order to compare their results,we also collected paleomagnetic samples from the Upper Devonian limestone of Dazhaimen Formation in the Baoshan terrane,which also belongs to the Tethyan tectonic domain similar with Lhasa terrane.Representative samples were selected for rock magnetic analyses and all 105 samples from 9sites were conducted on systematic thermal demagnetization.The rock magnetic analyses demonstrate magnetite is the major magnetic carrier,and a few samples contain a small amount of hematite.The high temperature component passed a fold test and the site-mean direction is D_g=250.8°,I_g=-27.3°,k=23.9,?₉₅=9.53°in geographic coordinates,and D_s=245.9°,I_s=-32.3°,k=35.1,?₉₅=8.8°in stratigraphic coordinates.A paleopole at-28.2°N,0.6°E,(A₉₅=7.7°),is obtained by averaging all virtual geomagnetic poles calculated for each site.The paleolatitude of the reference site(25.0°N,99.1°E)is constrained at 18.2±7.7°S.In the Luobadui reservoir profile,pyrrhotite is the major magnetic carrier on Middle Permian carbonate rocks of Luobadui Formation in Lhasa Terrane and failed to retain stable remanent magnetism,which suggests this profile is not suitable for paleomagnetic research.Magnetite is the major magnetic carrier on Upper Devonian limestone of Dazhaimen Formation in Baoshan terrane,which records stable remanent magnetism.The significant difference of paleomagnetic results is thus strongly associated with the different types of magnetic carrier of Late Paleozoic carbonate rocks in Lhasa Terrane and Baoshan terrane.Our study strengthens the necessity of detailed rock magnetism investigation in order to find suitable unites for paleomagnetic research in Lhasa terrane.