Modal and Geochemical Compositions of the Lower Silurian Clastic Rocks In North Qilian, Nw China: Implications For Provenance, Chemical Weathering, and Tectonic Setting

Abstract: The North Qilian orogenic belt records the transition from Ordovician marine back-arc basin to Early Devonian terrestrial molasse basin as the Central Qilian Block was sutured to the North China Craton. The detrital framework and geochemical composition of Lower Silurian sedimentary rocks within the North Qilian Belt record the provenance, tectonic setting, and a variety of sedimentary processes that operated during transport from source to sink. The petrological and geochemical characteristics of sandstones, siltstones, and shales in the western segment of the North Qilian Belt indicate a generally low degree of chemical weathering of source area(s) and limited sorting prior to deposition. These characteristics suggest an active tectonic setting. In contrast, samples studied from the eastern part of the North Qilian Belt indicate low to moderate chemical weathering and low to moderate hydraulic sorting. Such characteristics imply a more stable tectonic setting. Geochemical data including Eu/Eu*, Ti/Nb, Th/Sc, and Co/Th ratios suggest contrasting source regions for the western and eastern segments of the North Qilian Belt. Sources in the west included basement rocks of the Central Qilian Block and the North China Craton. In addition, western sources may have included mafic rocks located within the North Qilian arc. In contrast, data derived from eastern samples suggest a felsic provenance that probably formed part of the basement complex in the Central Qilian Block and the North China Craton. Overall, detrital and geochemical data, combined with the regional geology, suggest a foreland basin setting that formed during collision of the Central Qilian Block with the North China Craton.