New paleomagnetic results from the Huaibei Group and Neoproterozoic mafic sills in the North China Craton and their paleogeographic implications

Abstract

New paleomagnetic pole positions are obtained from the Neoproterozoic sills and carbonate rocks of the Huaibei Group in the northern Jiangsu and Anhui Provinces (Xuhuai region), North China. A total of 240 samples are used in statistics after stepwise thermal demagnetization and principal component analysis, including 85 samples from three ∼890Ma mafic sills, 27 samples from the ∼890-Ma-sill-baked rocks of the Wangshan Formation (Fm), 50 samples from unbaked rocks of the Wangshan Fm, 60 samples from unbaked rocks of the Jiayuan Fm and 18 samples from a ∼925Ma mafic sill. For most of these samples, after removing a low unblocking temperature component (LTC) of viscous magnetic remanence acquired in recent geomagnetic field, high unblocking temperature components (HTCs), likely carried by magnetite for the sill and unbaked carbonate samples and by partly oxidized magnetite for baked carbonate samples, were isolated. The HTCs of the ∼890Ma sills passed a reversal test, a fold test, and a baked contact test. It is interpreted as a primary remanence, defining a key pole (Q=7) at 52.6°N, 330.0°E (A95=5.3°) for the North China Craton (NCC). The HTCs of the unbaked carbonate rocks of the Wangshan Fm passed a fold test, the corresponding pole being at 26.1°N, 320.3°E (A95=5.2°). The sample-level mean direction of the ∼925Ma sill provides a virtual geomagnetic pole (VGP) position of 30.9°S, 136.2°E, dm/dp=4.0°/2.4°, being antipodal to the pole of the Wangshan Fm. We interpreted that the two rock units have the same age and their mean pole likely represents the primary remanences. A paleomagnetic pole position of 54.0°S, 107.3°E (A95=4.0°) is obtained from the Jiayuan Fm, by averaging the site-level VGPs. This pole is of dual polarity and consistent with those published poles from this formation and other three carbonate formations in the middle part of Huaibei Group, but differs from any younger poles obtained in the region. We also interpreted it as of a primary remanence. Our new data, together with the published poles, depicted a loop-like segment of apparent polar wander path of the NCC, with an apex at ∼925Ma. 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.