Paleogeographic Reconstruction Of Yangtze Block In The Ordovician:Constraints From Paleomagnetism And Detrital Zircon Geochronology

The relationship between South China(Yangtze and Cathaysia blocks)and Gondwana has been of considerable controversy.This paper presents paleomagnetism and detrital zircon U-Pb geochronology on the Ordovician strata of Yangtze Block in order to constrain the paleoposition of South China within Gondwana.The paleomagnetic study of the Lower Ordovician Hungshihyen Formation in the Wuding-Luquan area of Yunnan province in the western margin of Yangtze Block has yielded a characteristic remanent magnetization(ChRM)likely carried by hematite.The direction of ChRM is D/I=326.1°/54.0°(k=21.4,?95=5.4°,N=34)before tilt corrections,and D/I=318.4°/60.3°(k=67.3,?95=3.0°,N=34)after bedding correction,passing fold test and yielding a paleomagnetic pole at-52.3°N,228.2°E(A95=4.3)which is obviously different from the poles since Silurian.These suggest that the remanent magnetization was probably acquired during the earliest stage of sedimentation.The paleomagnetic study of the Upper Ordovician Pagoda Formation in the Wangcang area of Sichuan province in the northern margin of Yangtze Block has yielded two components by detailed stepwise thermal demagnetization.The direction of a low-temperature component is D/I=3.9°/54.0°(k=362.8,?95=1.1°,N=45)before tilt corrections,which falls close to the local present Earth field direction.The direction of a high-temperature component is D/I=137.8°/-36.3°(k=8.7,?95=10.2°,N=26)in in situ coordinates,and D/I=132.6°/-35.2°(k=61.7,?95=3.6°,N=26)after bedding correction,which was of bipolarity and passed reversal and fold tests,yielding a paleomagnetic pole at 45.8°S,191.3°E(dp=2.4°,dm=4.2°)that was significantly distinguishing from the poles since Silurian obtained from the Yangtze Block.Magnetic stratigraphic studies carried on three short sections of about 10-15 meters show a succession of at least 7 normal and reversed magnetic intervals of each section.These results prove that the remanent magnetization is of primary nature.The paleomagnetic results of the early-middle Floian and the mid-Caradoc of Yangtze Block show that the great-circle angular separation is 29.2±5.4°.The coeval poles of Baltica reveal about 25° of great-circle angular separation.The comparison of great-circle angular separation between Baltica and Yangtze Block shows that they have nearly identical looping,maybe implying that true polar wander occurred during Ordovician.Whether identical looping exists in other major continents(Siberia,Gondwana and Laurentia)remains enigmatic,because their paths are less constrained due to lack of reliable coeval Ordovician poles.The problem of true polar wander occurring during Ordovician needs to be sovled by more paleomagnetism.Combined with the perevious Cambrian and Silurian poles,our new Late Ordovician pole falls within the coeval pole of Gondwana after rotating around an Euler pole at 10.4°N,140°E with an angle of 71°,implying that South China was situated to the northwest Australia during the early Paleozoic.The study of detrital zircon in situ U-Pb geochronology of the Lower Ordovician Hungshihyen Formation in the western margin of Yangtze Block reveals that the sample(G3-1)from the lower strata of this formation has four major age peaks:?528 Ma,?586 Ma,?784 Ma,and?974 Ma,while the sample(D4-1)from the upper stata of this formation has five major age peaks:?524 Ma,?590 Ma,?830 Ma,?977 Ma and?2500 Ma.The presence of Pan-African age population suggests that Yangtze Block had a strong affinity with Gondwana during the Early Ordovician.The age distribution of detrital zircons was obviously similar among Yangtze Block and Tethyan Himalaya,Qiangtang,western Australia,and Sibumasu,revealing that they might share a common source provenance and have adjacent relationships.Combined the previous research results,there was a close proximity of South China and Australia during the Ordovician.Based on the paleomagnetism and detrital zircon U-Pb geochronology on the Ordovician strata,South China occupied a position on the west Australia and was adjacent to Sibumasu in the Ordovician.