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

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, A₉₅ = 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.