A Combined Geochronological And Paleomagnetic Study On ?1220 Ma Mafic Dykes In The North China Craton And Its Implications For The Transition From Nuna To Rodinia

In order to better understanding how did the North China Craton(NCC)correspond to the transition of Nuna-Rodinia supercontinent,a new combined geochronological and paleomegnetic study is carried out on 10 mafic dykes in the western Liaoning Province and Taihang area in NCC.New SIMS(Cameca)zircon datings on three dykes in western Liaoning Province yield weighted mean 207Pb/206 Pb ages of 1226.9 ± 3.4 Ma(95% confidence,MSWD = 1.9),1222.9 ± 6.1 Ma(95%confidence,MSWD = 1.02),1217 ± 13 Ma(95% confidence,MSWD = 1.4);baddeleyite dating on one dyke in Taihang area yealds age of 1219.1 ± 4.3 Ma(95%confidence,MSWD = 1.2);respectively,they indicate the emplacement age of the dykes was at ~1220 Ma.Rock magnetic experiments indicate that the dominant magnetic carriers are SD+MD magnetite or titanomagnetite.All samples were subjected to step-wise thermal demagnetization and two components were isolated.The low temperature component(LTC)was identified in general below 300 oC.Their directions are around the present geomagnetic dipole field(PDF)and are thus interpreted as the recent viscous magnetization.The high temperature component(HTC)was isolated from 309 samples of all the 10 dykes,with unblocking temperatures up to 580 oC.The HTC directs east and down with moderate inclination for the Jianping(western Liaoning Province)and Pingshan(Taihang area)dykes.It passes a positive baked-contact test and a secular variation test,suggesting its primary in origin.Paleomagnetic pole calculated from the HTC is at(16.0 oN;180.3 oE),A95 =6.6 o,being not similar to any younger published poles of the NCC.We thus consider it as a key pole of the NCC at ~1220 Ma.Using the reliable Precambrian poles of the NCC that had been published and the new reliable pole,the apparent polar wander path(APWP)was proposed for the NCC during ~1780-890 Ma;and the paleolatitudes and orientations of NCC at different times were also suggested.Comparing with the high quality,coveal poles from Australia and Laurentia,it suggested that the connection of NCC-Australia-Laurentia in Nuna had ruptured at ~1220 Ma.The ~1320 Ma bimodal magmatic actives in the northern NCC and the coeval magmatic actives in northern Australia may lead to the depature of the NCC from Australia.And the deep-water lithofacies in the Pinguila Group in Yukon area of northwestern Laurentia provides evidence for the rupturebetween Laurentia and Australia.Combined with the relationship between Laurentia-Siberia-Baltica and NCC during early Neoproterozoic which is constrained by the similar age and shape of the APWPs of NCC and Baltica,the NCC was on the pathway to dock with Siberia,when the Rodinia supercontinent finally formed.