A Paleomagnetic And Geochronologic Study On The Late Mesoproterozoic To Early Neoproterozoic Rocks In The Eastern North China Craton,and Its Implications For The Reconstruction Of Rodinia

The interval from the late Mesoproterozoic to early Neoproterozoic is generally considered as a critical time for the amalgamation of supercontinent Rodinia.The location of the North China Craton(NCC)in Rodinia remains contentious.A geochronologic and paleomagnetic study was conducted on the early Neoproterozoic mafic sills and late Mesoproterozoic to early Neoproterozoic strata in the eastern NCC.In the Xuzhou and Dalian regions,three sills were dated at-945 Ma and one at-920 Ma by using the zircon U-Pb secondary ion mass spectroscopy method.Rock magnetic results indicate that the main magnetic carriers in the sills are pseudo-single domain(PSD)magnetite grains.Paleomagnetic investigation revealed no significant discrepancy between these sills.A positive baked-contact test,a secular variation test and the presence of reversals together support the primary origin interpretation for the remnant magnetization.A high-quality pole at(28.2 °S,141.9 °E,A95=10.4°)was thus obtained by averaging the new results and a virtual geomagnetic pole previously reported for a-920 Ma sill in the region.These Neoproterozoic sills intruded the successions that contain correlative strata that are named Nanfen Formation(Fm)in Benxi and Dalian regions,Xinxing Fm in the Xuzhou region and Liulaobei Fm in the Huainan region.The Nanfen Fm and its equivalents are constrained between?1120 and 945 Ma by detrital zircons and the mafic sills.Rock magnetic results indicate that the dominant magnetic carriers in those rocks are magnetite or a mixture of hematite and magnetite.The anisotropy of magnetic susceptibility data imply that those rocks of the Nanfen Fm still preserve the primary sedimentary magnetic fabrics.The paleomagnetic inclinations observed from the lower parts of the Nanfen,Xinxing and Liulaobei formations are notably steep.The corresponding poles from these rock units are consistent and averaged at 38.6°N,136.7°E(A95=3.2°).The quality of this pole is strengthened by a positive reversal test and its distinctiveness from the younger poles of the NCC.In the middle part of the Nanfen Fm,however,the paleomagnetic directions are characterized by moderate inclinations,being significantly different from those in the lower part of the Nanfen Fm and its equivalents.The calculated pole for the upper part of the Lower Member of the Nanfen Fm is at 8.0°N,128.5°E(A95=7.9°).Another pole obtained from the Middle Member of the Nanfen Fm is at 112°S 127.7°E(A95=8.5°).These two poles also differ from the younger poles of the NCC and likely represent the primary remanences.This study indicates that the NCC located near the polar region during the deposition of the lower part of Nanfen Fm(?1100 Ma).The poles from the younger rock,however,indicate that the NCC wandered in the low latitudinal regions in the early Neoproterozoic(-950-890 Ma).The paleomagnetic data obtained thus far may suggest a large scale equator-ward drift of the NCC occurred in the transition between Mesoproterozoic and Neoproterozoic.The new results in combination with the existing global paleomagnetic data,aided by the "right-way-up" connection model between Laurentia and Baltica in Rodinia,support a NCC-NW Laurentia connection between?1120 and 890 Ma.The new reconstruction model is supported by detrital zircon procenance analyses also.