Geochemistry, Geochronology Of The Dandeer Intrusive Rocks In The Middle Section Of The Cenral Qilian Mountain And Their Tectonic Significance

Located in Qilian County, Qinghai Dandeer area, the study area belongs to Qin-QiKun giant construction in western China part of the magmatic belts, with very unique geological significance. The characteristics of major element show that the Dandeer rocks are granitoids of alkaline and peraluminous type. The study of REE and trace elements indicate that the rocks are rich in Rb, Th, Ba, K and other large LILE but depleted in high field strength elements such as Sr, Ti, P, Nb, Ta which suggest that amphiboles and plagioclases might exist in the residual phase of melting. The high potassium calc-alkaline characteristics and other seven characteristics confirm that those rocks belong to S-type granites.LA-MC-ICPMS zircon U-Pb dating of the Dandeer granitoid shows there are one peak period and two sub-peak periods. In combination of the local tectonic evolution, the forming age of the granitoid is confirmed to be 438 Ma, Late Ordovician. The other two age perieds are given by inherited zircons.Material sources of these rocks were determined by four determination diagrams. The Si O2-Zn diagram shows fractional crystallization characteristics for them, which are similar to that of felsic igneous rocks. In Rb/Y-Nb/Y diagram, it shows a trend of enrichment in subduction zone or crustal contamination. The Th/Yb-Ta/Yb diagram also suggests an evolutionary trend of fractional crystallization. The Th/Y-Nb/Y diagram indicates that the Mingdandeer granitoids were derived from the crust. Combining with the Sr and Yb contents, we conclude that the residual phase of the granite source region is composed of plagioclase and garnet.The rocks are Zhemin and Nanling type transition type according to the Sr-Yb contents, which reflects low pressure and thin crust during their formation. We therefore speculate that the study area was subsiding caused by horizontal movement at that time. In addition, the granitoids are related to the tungsten minerals.The study of 38 representative rock masses shows that the North Qilian Early Palaeozoic ocean experienced southward subduction and then northward subduction during early palaeozoic, and the Dadeer granitoid formed in post-collisional orogenic stage.We employed Pearce JA granite discrimination diagrams, Brown GC granite discrimination diagrams and R1-R2 tectonic discrimination diagrams to comprehensively demonstrate that the rocks formed in extensional setting with collisional orogenic characteristics. Therefore, the forming age of the Dandeer granitoid marks the end of the collision orogenesis stage after the northward subductin of the Qilan ocean.