SILURIAN-DEVONIAN PROVENANCE CHANGES OF SOUTH QINLING BASINS - IMPLICATIONS FOR ACCRETION OF THE YANGTZE (SOUTH CHINA) TO THE NORTH CHINA CRATONS

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 elastic 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 an expected to have been recorded in the Phanerozoic elastic 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 elastic 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 K2O/Na2O ratios. In contrast, there is a significant provenance change in the Silurian-Devonian elastic 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 elastic sediments illustrated by the decreasing K2O/Na2O ratio, which is attributed to the near source of the North Qinling Belt dominated by low K2O island are igneous suites and immature metagraywackes.