A combined geochronological and paleomagnetic study on ∼1220 Ma mafic dikes in the North China Craton and the implications for the breakup of Nuna and assembly of Rodinia
Jikai Ding, Shihong Zhang, Hanqing Zhao, Hanbiao Xian, Haiyan Li, Tianshui Yang, Huaichun Wu, Wei Wang
Abstract
A new combined geochronological and paleomagnetic study was carried out on the late Mesoproterozoic mafic dikes in the northeastern and central North China Craton (NCC). Three dikes were dated at 1224.2 ± 5.1 Ma, 1219.9 ± 6.0 Ma and 1215.2 ± 7.7 Ma through use of the secondary ion mass spectrometry (SIMS) zircon dating method and one dike was dated at 1219.1 ± 4.3 Ma by using SIMS baddeleyite dating method, indicating that these dikes emplaced at ∼1220 Ma. A total of 286 paleomagnetic core samples were collected from seven ∼1220 Ma dikes and their host rocks, as well as two ∼775 Ma dikes for field tests. All the samples were subjected to stepwise thermal demagnetization. Rock magnetic experiments of representative samples demonstrate that the dominant magnetic carriers of the ∼1220 Ma dikes are single-domain and pseudo-single domain magnetite grains. Two paleomagnetic components were identified from the ∼1220 Ma dikes. The low temperature component is a viscous magnetic remanence acquired in the recent geomagnetic field. The high temperature component (HTC) was isolated between 500 and 580°C, which directs east and down with moderate inclination. A paleomagnetic pole (11.3°N, 175.8°E, *A*~95~ = 7.6°) was determined by averaging seven virtual geomagnetic poles (VGPs), each VGP corresponding to the mean HTC direction of an individual dike. The ∼1220 Ma pole passed a baked-contact test and appeared to adequately average out secular variation. It differs from any younger poles of the NCC, and is thus considered as a reliable pole for the NCC. While our new results place the NCC at intermediate paleolatitudes (∼28.8°N for site located at 36.8°N, 113.4°E) at ∼1220 Ma, the coeval high-quality poles in the global paleomagnetic database, however, indicate that the Australian cratons were positioned near the pole and Laurentia was located around the Equator, depicting a dispersed paleogeographic pattern of the three continents at ∼1220 Ma. The updated paleomagnetic data together with geological evidence may suggest that the NCC separated from northern Australia between ∼1320 and ∼1220 Ma, and later joined northwestern Laurentia between ∼1220 and ∼1110 Ma.