New Paleomagnetic Results From The Huaibei Group And Neoproterozoic Mafic Sills, North China Craton, And Their Paleogeographic Implications

Mid-Neoproterozoic strata developed successively in the North China platform and suffered from little structural deformation. High quality paleomagnetic results had been obtained from this cover sequence and provided important constraints for the reconstructions of the Precambrian Rodinia and Nuna supercontinents. Huaibei Group presented in the southeastern margin of North China platform is a sequence of stable detritus-marine carbonate strata. Recent geochronology results implied this group was deposited in late Mesoproterozoic- early Neoproterozoic rather than Sinian as formerly considered. New paleomagnetic research in Huaibei Group is significant to determine North China Platform’s positions in the Rodinia supercontinent.1001 cores of 120 sites in Neoproterozoic Huaibei Group, including Wangshan Formation(Fm) thin muddy limestone with ~890 Ma intruding dolerite dykes, Jiayuan Fm sandy marl, Niyuan Fm thin limestone and dolomitic limestone,and ~925 Ma and ~890 Ma intruding sills and Xinxing Fm redbeds of Cangshan region in Shandong Province, were sampled for paleomagnetic research.Rock magnetic experiments(isothermal remnant magnetisation acquisition curves and inverse direction demagnetisation curves measurements, three axis isothermal remnant magnetisation systematic thermal demagnetisation analysis and magnetic susceptibility- temperature change curves) of each rock unit indicated the magnetic carriers as follow: mainly magnetite in Wangshan Fm, magnetite in ~890 Ma and ~925 Ma dolerite sills, magnetite in Jiayuan Fm and heamatite in Xinxing Formation.Most of the samples have two components. The low temperature component(LTC) was isolated before 300℃, which is close to the present geomagnetic field; and the high temperature component(HTC) was isolated between 450℃ and 580℃. The site-mean HTC direction of the Jiayuan Fm is: D = 186.0°, I = 3.8°, a95 = 6.7°; corresponding to a pole located at: 54.0°S, 107.3°E( A95 = 4.0°). The sample-levelaverage HTC direction of the ~925 Ma sill is: D = 162.7°, I = 39.7°, a95 = 3.3°, correspoding to a pole: 30.9°S,136.2°E(A95 = 3.8°). The site-mean HTC directions of the Wangshan formation is: D = 337.2°, I =-44.1°, a95 = 4.8°, and the HTCs could pass fold test, which means it is probably primary, correspoding to a pole: 26.1°N,320.3°E(A95 = 5.2°). The site-mean direction of the ~890 Ma sills is: D = 341.1°, I =1.4°, a95 = 5.0°;and the Ch RMs of the ~890 Ma sills could pass reversal test, fold test and baked contact test, which means it is a primary remanence, defining a key pole located at: 52.6°N, 330.0°E(A95 = 5.3°).This research elucidated NCC located near the equator during the deposition of the middle part of Huaibei group, and Wangshan Fm formed at middle latitude, afterwards, NCC reached the Equator at ~890 Ma. According to the new paleomagnetic results, together with the published poles, a loop-like segment of apparent polar wander path of the NCC was depicted, with an apex at ~925 Ma. It is termed as the “Huaibei loop” herein. The comparison between the “Huaibei loop” and the “Sveconorweigian loop”, aided by the “right-way up” model of Laurentia-Baltica connection, indicates the paleogeographic proximities of the NCC to Laurentia, and probably to Siberia as well, in the supercontinent Rodinia.