Tectonic Evolution Of The Central North China Craton Since The Late Paleozoic

The Qinshui Basin is the largest coalbed methane producing basin in China. The Upper Paleozoic is the main coal-bearing strata. Research on the basic geology of the Late Paleozoic strata is of great significance to coalbed methane exploration. Detrital zircon U-Pb, Lu-Hf, and apatite fission track (AFT) analysis techniques were used to investigate the soure provenance of the Upper Paleozoic and subsequent thermal evolution, respectively.U-Pb geochronologic and Hf isotopic results of seven sandstones collected from Late Carboniferous through Early Triassic strata record a dramatic provenance shift near the end of the Late Carboniferous. Detrital zircons from the Late Carboniferous sandstones are dominated by the Early Paleozoic components with positive εHf(t) values, implying the existence of a significant volume of juvenile crust at this age in the source regions. Moreover, there are also three minor peaks at ca.2.5 Ga,1.87 Ga and 1.1-0.9 Ga. Based on our new data, in conjunction with existing zircon ages and Hf isotopic data in the North China Craton (NCC), Central China Orogenic Belt (CCOB) and Central Asian Orogenic Belt (CAOB), it can be concluded that Early Paleozoic and Neoproterozoic detritus in the south-central NCC were derived from the CCOB. By contrast, detrital zircons from the Permian to Triassic sandstones are characterized by three major groups of U-Pb ages,2.6-2.4 Ga,1.9-1.7 Ga and Late Paleozoic ages. Specially, most of the Late Paleozoic zircons show negative εHf(t) values, similar to the igneous zircons from intrusive rocks of the Inner Mongolia Paleo-Uplift (IMPU), indicating that the Late Paleozoic detritus were derived from the northern part of the NCC. This provenance shift could be approximately constrained at the end of the Late Carboniferous and probably hints that tectonic uplift firstly occurred between the CCOB and the NCC as a result of the collision between the South and North Qinling microcontinental terranes, and then switched to the domain between the CAOB and the NCC.Additionally, on the basis of Lu-Hf isotopic data, we reveal the pre-Triassic crustal growth history for the NCC. The crustal growth curve indicates that 70% of crust had been formed before 2.5 Ga, and old components reworking dominated the crust growth process since then. In comparison among the three crustal growth curves obtained from modern river sands, our samples, and the Proterozoic sedimentary rocks, we realize that old components are apparently underestimated by zircons from the younger sedimentary rocks and modern river sands. Hence, cautions should be taken when using this method to investigate growth history of continental crust.The AFT ages range from the Early Cretaceous to Eocene, which are much younger than the stratigraphic age (Late Paleozoic), indicating that these apatites had been totally annealing before exhumation. However, the AFT ages show clearly unrelated relations with the elevation and Dpar values, respectively. Based on the analysis of the regional geological background, elevation-AFT age decoupling model was proposed. This model provides reasonable mechanism that caused these discordances. The inverse modeling results of six AFT samples indicate that the eastern part of the Qinshui Basin had experienced a very slowly cooling event from the Late Jurassic to Miocene. By contrast, a very fast cooling event occurred after the Late Miocene.