Silurian-Devonian provenance changes of South Qinling basins: implications for accretion of the Yangtze (South China) to the North China cratons

Abstract

The North China and Yangtze (South China) cratons are the most important tectonic units of China. The two cratons finally coalesced through collision with the formation of the E-W-trending Qinling Mountains that are considered to be the eastern extreme of the Paleotethys. The North Qinling and South Qinling belts constitute the southern active margin of the North China Craton and the northern passive margin of the Yangtze Craton, respectively. We have studied chemical compositions of 573 fine-grained clastic sedimentary rocks of all the Early Proterozoic to Neogene ages from the North China Craton, the Yangtze Craton, and the North and South Qinling orogenic belts, as well as their ten tectonostratigraphic subunits. The purpose of this study is to investigate possible provenance changes that are expected to have been recorded in the Phanerozoic clastic sediments of the South Qinling basins, if North and South China had approached and accreted, because the North China Craton/North Qinling Belt and Yangtze Craton have distinct upper crustal and provenance compositions. Both the North China and Yangtze cratons and the North Qinling Belt are characterized by a relatively uniform, non-evolutionary composition of clastic sediments in LaThScCo systematics during post-Archean times. They can be, however, distinguished from each other in provenance in terms of the La/Th, Sc/Th, Th/Co, as well as K 2O/Na 2O ratios. In contrast, there is a significant provenance change in the Silurian-Devonian clastic sediments from the Baiyun-Zhashui, Liuba-Yunxian and western Xichuan subunits of the South Qinling Belt. The late Sinian (700-600 Ma), Cambrian and Ordovician pelites show characteristics that are consistent with the Yangtze Craton, being high in La/Th (3.5–10) and Sc/Th (1.4–2.4) and low in Th/Co (0.2–1.0) and lacking La/Th ratios < 2.0, whereas pelites and graywackes of Silurian, Devonian, Carboniferous and Permian ages have characteristics in these features that are nearly identical to the North Qinling sediments. They have low La/Th (1.0–3.5), Sc/Th (0.6–1.5) and high Th/Co (0.2–1.8) and only few samples show La/Th > 3.5. These are accompanied by the increasing immaturity of the Silurian-Devonian clastic sediments illustrated by the decreasing K 2O/Na 2O ratio, which is attributed to the near source of the North Qinling Belt dominated by low K 2O island arc igneous suites and immature metagraywackes. We propose that the South Qinling basins, except the eastern Xichuan subunit, had been in a close proximity and accreted to the North Qinling Belt in Silurian-Devonian times. The result agrees well with recent studies of benthic faunas, granite distribution, age of granulite and clastic provenance directions of the South Qinling Devonian sediments. But it is apparently inconsistent with the age of coesite- and diamond-bearing ecologite in the Dabie Mountains to the east and with paleomagnetic data, both suggesting an important Triassic-Jurassic collision event between North and South China. It is likely that the Silurian-Devonian and the Mesozoic represent two different episodes/phases of accretion and collision between North and South China. New models regarding the Phanerozoic tectonic evolution of East China are needed to reconcile the contrasting evidence.