Geochemical evidence for the Proterozoic tectonic evolution of the Qinling orogenic belt and its adjacent margins of the north China and Yangtze cratons

The North China and Yangtze (South China) cratons are two important tectonic units of China. They finally came together due to collision during the Late Palaeozoic to Middle Triassic with the formation of the E-W-trending Qinling Orogenic Belt. Proterozoic rocks were well developed in the area investigated and span nearly the entire Proterozoic. The Proterozoic tectonic evolution of the region was studied, based on rock associations and chemical composition (particularly Ti, Cr, Zr, Y and REE) of volcanics, together with the provenances of elastic sedimentary rocks, from major Proterozoic suites in eight tectonostratigraphic subunits. The results imply a three-stage Proterozoic tectonic development. (1) Palaeo- and early Mesoproterozoic intracontinental rifting, characterized by alkaline bimodal volcanics and graywackes derived from the North China Craton and North Qinling Belt. (2) Late Meso- to early Neoproterozoic continental convergence, represented by a tholeiite-turbidite sequence from a marginal sea basin setting, also involved peak metamorphism in the North Qinling Belt, thick keratophyre/quartz-keratophyre dominated volcanosedimentary sequences with are affinities in the eastern South Qinling Belt, and co-magmatic volcanic-intrusive suites in the Yangtze Craton and western South Qinling Belt. Both the volcanic and intrusive suites in the Yangtze Craton show the clear systematic temporal and spatial variations in rock associations and chemical compositions which characterize circum-Pacific island arcs or active continental margins. (3) Early Sinian (800-700 Ma) continental rifting is indicated by either bimodal volcanics or alkali basalt/trachyte and associated alkaline gabbro and A-type granites, overlain by continental tillite. This was followed by rapid subsidence and the formation of passive margins, which is testified by thick, Late Sinian-Cambrian successions of platform carbonates in the North China and Yangtze cratons and South Qinling Belt, with widespread, unique phosphorite deposits and black shales formed in the latter two areas.