Revisiting the Yejishan Group of the Lüliang Complex, North China: Implications for a Paleoproterozoic active continental marginal basin in the Trans-North China Orogen

Abstract

Widespread Paleoproterozoic supracrustal rocks in the Trans-North China Orogen (TNCO) contain important information for the tectonic evolution of the North China Craton (NCC). Here, we report new in situ LA-ICP-MS detrital zircon U-Pb ages, Lu-Hf isotopic data, and whole-rock geochemical data for the Yejishan Group in the Lüliang Complex. The group is a well-preserved volcanic-sedimentary assemblage and can be subdivided into the Qingyangshuwan, Bailongshan, and Chengdaogou formations from bottom up. The Bailongshan Formation consists dominantly of meta-basalts interbedded with several layers of clastic rocks, and the other two formations are composed exclusively of sedimentary rocks. In this study, samples of fourteen clastic rocks and thirteen basalts from the Yejishan Group were collected for measurements based on detailed fieldwork. Detrital zircon U-Pb dating of six clastic samples from the three formations display similar age distributions and give youngest ages between ca. 1870 and 1988Ma. The youngest age peak of 1870Ma obtained from the Qingyangshuwan Formation is regarded as the maximum depositional age of the Yejishan Group for the first time. The ca. 1779Ma mafic dyke intruding into the Yejishan Group constrains the depositional age between 1870 and 1779Ma. Clastic interlayer in the Bailongshan basalts has a youngest age peak of ca. 1988Ma, suggesting that the basalts should erupt after 1988Ma. The time constraint of the Bailongshan basalts in this study disapproves the previous view that the Bailongshan Formation should be removed from the Yejishan Group and assigned to the Lüliang Group. According to the geochemical features, the Yejishan clastic rocks had a chemical immature source mixed of felsic and basaltic magmatic rocks with weak weathering, and suggestive of an active tectonic setting. The basalts show variable LREE enrichment, prominent Nb-Ta, Ti, P anomalies, probably derived from a subcontinental lithospheric mantle metasomatized by subduction-related fluids. Taken together the geochemical and chronological data, we propose that the Yejishan Group were formed in an active continental marginal basin at ca. 1870–1780Ma. The conclusion is consistent with model of the ca. 1850Ma collision in the TNCO.