Spatial distribution of ～ 1950-1800 Ma metamorphic events in the North China Craton: Implications for tectonic subdivision of the craton

摘要

Great progress has been made in the last twenty years in understanding the unification of the North China Craton. According to a prevailing model it developed by amalgamation of four sub-blocks by three spatially and temporally separate orogenic belts: the first at ～1950 Ma formed the Western Block, the second at ～1900 Ma created the Eastern Block, and the third at ～1850 Ma unified the Eastern and Western Blocks. However, new data show that the 1950 Ma orogen was affected by granulite fades reequilibration at 1800 Ma, and parts of the 1850 Ma orogen have evidence of metamorphism at 1950-1900 Ma New zircon U-Pb ages enable the Paleoproterozoic metamorphic events to be statistically sub-divided into four: at ～1955 Ma (M1), ～1920 Ma (M2), ～1885 Ma (M3) and ～1850 Ma (M4). The M_(1-2) events always appear together, mainly in two belt areas including the Helanshan-Qianlishan, northern Ordos, Yinshan, Liangcheng, Huai'an, Lushan, Jiaobei and Liaodong regions; whereas the M_(3-4) events are distributed over larger areas and were superimposed on the M_(1-2) activities. This age distribution is inconsistent with previous models of three separate orogens. It should also be noted that the igneous rocks contemporary with the regional (ultra-)high-temperature/high-pressure granulite fades metamorphisms (M_(1-2)) were previously described as arc-related series, i.e., the gabbro-norites, I-/S-type granites and trimodal volcanics. Based on the spatial distribution of the Paleoproterozoic metamorphic events (M_(1-4)), and of the coeval igneous rocks, we speculate that the assembly of the NCC was created by one orogen, which formed between the Eastern and Western Cratons in two tectonic stages: two marginal arcs, the Korean and Xuwujia, developed against both eastern sides of the two sub-cratons, and were both accreted and deformed at 1965-1900 Ma (M_(1-2)). Subsequently the amalgamation of the two sub-cratons resulted in metamorphism on the western side of the Eastern Craton, and in metamorphism that was superimposed on both accreted arcs at 1900-1790 Ma (M_(3-4)).