Implication Of The Depositional Environment And The Tectonic Setting Of The K-Bentonite-bearing Black Shales In The Xiamaling Formation, Early Neoproterozoic, North China

The Neoproterozoic of North China is a significant interval in the stratigraphic study of China. Recently, numerous K-bentonites have been recognized in the black shales of the Early Neoproterozoic Xiamaling Fm. in Western Hill of Beijing. The present study is focused on the depositional environment and the related tectonic setting of the black shales and interbedded K-bentonite layers, with an integrated approach of sedimentology, petrology and petro-geochemistry.Four sections, a south-north direction research corridor for this study, have been observed. The four sections are the Zhaojiashan section in Huailai County, Heibei Province, the Yanchi section and Xiaweidian section in West Hill of Beijing, as well as the Yesanpo section in Laishui County, Heibei Province, respectively. Among which, the Zhaojiashan section is the standard section with the most complete and the thickest strata sequence. From the south to the north, both the thickness of the third member of the Xiamaling Fm. and the number of K-bentonite layers increase gradually.Preliminarily, the analysis of the petro-geochemistry has shown that the black siliceous shales (slates) of the Yesanpo section should deposit at a shelf area, which was resulted in partly by the hot water. By the contrast, the ones of Zhaojiashan section should be at the margin of out shelf, which was influenced by the hot water more evidently. On the other hand, the K-bentonite layers originated mainly from the intermediate-to-silicic series of trachyandesitic, rhyodacitic/dacitic and rhyolitic magmas, which show the tectonic setting of synCOL-VA-WP, closely related to the explosive volcanic eruptions from the collision and subduction zone of the relevant paleo-continental blocks/arcs. Further, according to the distribution of the K-bentonite layers and other regional data, the original volcano belt should locate in the present northern margin of the North China Plate.Stratigraphically, the depositional succession of the Xiamaling Fm. and its contacts both at the base and the top horizon can be interpreted as a continuous tectonic process that is in good agreement with the orogeny-originated model of the Paleozoic black-shale deposit of North America by Ettensohn. Consequently, with the mentioned-above analysis of the interbedded K-bentonite layers, it may suggest that both the formation of the K-bentonites-bearing black shales of the Xiamaling Fm. should originate from the plate tectonic movements, including the subduction and the accretion, between the North China Plate and the unnamed plate/arc in the north area of the present North China, which may be regarded as the depositional correspondences of the assemblage of the Neoproterozoic Supercontinent Rodinia.