Mid-Mesoproterozoic bimodal magmatic rocks in the northern North China craton: Implications for magmatism related to breakup of the columbia supercontinent

摘要

Large volumes of diabase sills occur within the late Paleoproterozoic to Mesoproterozoic sedimentary rocks in the northern part of the North China Craton (NCC) and previous geochronological studies suggest some of these were emplaced in the Mid-Mesoproterozoic (Ectasian). Here we present new zircon and baddeleyite SHRIMP, LA-ICP-MS and SIMS U-Pb analyses on representative samples of the diabase sills emplaced into the Wumishan, Tieling and Xiamaling Formations. The results show emplacement ages ranging from 1325±5Ma to 1316±37Ma, suggesting voluminous magmatism leading to the formation of the diabase sill swarms (the Yanliao mafic sill swarms) in the Mid-Mesoproterozoic. Zircon LA-ICP-MS U-Pb dating of the Shangdu-Huade granitic pluton in the northern NCC yielded similar emplacement ages ranging from 1331±11Ma to 1313±17Ma. These ages are comparable with the 1.4-1.2Ga rift-related anorogenic magmatism recorded elsewhere on the globe such as North America, Greenland, Baltica, and Siberia, associated with the breakup of the Columbia (Nuna) supercontinent. The Mid-Mesoproterozoic magmatic rocks in the northern NCC constitute a typical bimodal magmatic suite. The geochemical and Nd-Hf isotopic data suggest that the diabasic rocks were likely generated by partial melting of the depleted asthenosphere mantle coupled with slight crustal assimilation in a continental rift setting; however, the granitic rocks were generated mainly through partial melting of the ancient continental crust, probably induced by the upwelling of hot asthenosphere mantle during continent rifting processes. The recognition of these bimodal magmatic rocks indicates that the northern NCC experienced Mid-Mesoproterozoic rifting events that are considered to have led to the final breakup of the Columbia supercontinent. The breakup of the NCC from the Columbia supercontinent occurred during Mid-Mesoproterozoic time, probably at around 1.33Ga. The carbonatite rocks and rare earth element-niobium mineralization in the giant Bayan Obo ore deposit in the northern NCC were also probably related to the continental rifting event in the Mid-Mesoproterozoic.