A Group of Rifting Events in the Terminal Paleoproterozoic in the North China Craton

Abstract

Rifting events of the terminal Paleoproterozoic are well preserved in the North China Craton. They include an aulacogen and cratonic marginal rift basins, mafic dyke swarms, two anorthosite suites and rapakivi granites. All of these events took place after metamorphism, deformation and magmatism of the late Paleoproterozoic between 2.0 Ga and 1.85 Ga (or 1.80 Ga). The rifting events belong to an anorogenic event group, reflecting breakup of a late Paleoproterozoic united North China Craton. An intracratonic rift basin in the center of the North China Craton is named the Yanshan-Taihangshan Aulacogen. A stratigraphic thickness of 9193 m was deposited within the basin, including three groups and twelve formations with time range ca. 1.8 Ga to 0.85 Ga. The oldest (Changcheng) group of the sequence was deposited in a graben and is mainly composed of alluvial basal conglomerates, terrestrial clastic rocks, and shallow marine sandstone and carbonates with super-high-potassium basalt-trachyte. The marginal basins occur around the northern, western and southern margins of the North China Craton, respectively located in Langshan-Baiyun Obo, Helanshan Mountain, and the adjacent regions between Henan, Shanxi and Shaanxi Provinces. The stratigraphic sequences of the basins are generally made up of clastic rocks in the lower parts and carbonates in the upper part, with a thick volcanic sequence with U-Pb age of 1840±14 Ma exposed in the southern marginal regions. Based on stratigraphic sequences, isotopic data and volcanic features, the initial rifts took place at about 1.85 Ga to 1.80 Ga. Mafic dyke swarms are largely developed in central parts of the North China Craton, where they intrude Archean granulite and high-amphibolite rocks. A new single-zircon U-Pb isotopic age of 1769±2.5 Ma has been reported from a mafic dyke in east Henshan Mountain, northwestern Shanxi Province. An anorthosite suite (AGMS) is exposed along two deep fault belts in northern Hebei Province. Isochron Ages of 1683±193 Ma by Rb-Sr and 1735±239 Ma by Sm-Nd were measured from an anorthosite body. The rapakivi granites crop out along the Miyun-Qiangzilu-Xinglong Fault in an E-W direction, and are characterized by megacrysts of alkali feldspars mantled by plagioclase, thus showing many of the petrographic features of classic rapakivi granites worldwide. A typical rapakivi body preserved in Shachang of Miyun County, Beijing, has a U-Pb age of 1716±31 Ma. Super-high-potassium volcanics located in the upper Changcheng Group have been dated at 1683±67 Ma and 1625±6 Ma. The time range for all magmatic events is between 1.72 Ga and 1.62 Ga.