Characteristics And Tectonic Significance Of The Volcanic Rock Inordovician Koumeizi Formation,North Qilian Orogen

The Ordovician Koumenzi Formation is located in South Wusongtala of Qilian County, Qinhai province, which unconformably overlay the Cambrian strata but was later deformed to be bounded by ductile or brittle faults.The lower part of this formation is composed of a set of intermediate-mafic volcanics manifesting a clear eruption sequence, the lithologies of which are majorly andesite, basalt and lesser dacitic, andesitic and basaltic pyroclastic rocks. Geochemical analyses of andesites indicate a high aluminum & sodium alkaline series accordant to chracteristics of the island-arc basalt type(VAB) in terms of the chondrite-normalized REE distribution pattern implying a relatively high abudance of the REE and an obvious fractionation of the LREE, and the spider diagram of trace element ratio displaying the enrichment of Rb, Ba, K, U, La, Nb and Sm and the depletion of Sr, Zr, Nd. A zircon U-Pb age of 442.9±5.2 Ma using the method of LA-ICP-MS was obtained, which implies that the subduction of the North Qilian Ocean and the relevant volcanism along the active continental margin ended until the Late Ordovician.Basalts show geochemical characteristics of aluminum-low & sodium-rich alkaline series which is analogous to the MORB type in aspects of the chondrite-normalized REE distribution pattern characterized by a low REE content, a weak fractionation of HREE and a slightly depletion of Eu, and the fluctuated curves of the spider diagram of trace element ratio chacracteristic of the enrichment of Ba, U, the depletion of Rb, K, the enrichment of Sm of LREE and Ta, Hf of LILE, and the poorness of Rb, K, Ti. Besides, the trace elements present a mantle origin but also assimilated and contaminated by the crustal materials. A concurrent zircon U-Pb age of 442.8±7.6 Ma using the method of LA-ICP-MS was also accquired constraining the closure time of the North Qilian Ocean.Both the andesites of the island-arc basalt type(VAB) and the basalts of the MORB type reflect a portion of the subduction and closure process of the Early Paleozoic North Qilian Ocean. The volcanic eruption cycle a recorded in the middle-late ordovician qilian ocean subduction during subduction, in mainland near the subduction zone inside direction formed high aluminium, alkaline basalt. Wusongtala fore-arc basins in the south of Zoulang island-arc.After the arc-continenral collision in the Silurian, post-collison granites extensively intruded along the both margins of the subduction zone formed collision type granite. age of 427.1 ± 2.6 Ma.After the collision, the North China terrane, the Central Qilian terrane and the South Qilian block were in order collaged with each other respectively by the activites of arc-continental collision and contimental-continental collision. The Qinling-Qilian-Kulun archipelagic ocean eventually closed, and the main part of China including the Qinghai province was incorporated with the part of China continent, which then experienced the evolution of the intraplate orogeny.There are differences in East and West as zoning in South and Notrh as two way Subduction of North Qilian archipelagic ocean. There are characteristics of tectonic evolution from multiple islang-arc-basin and trench-atc-backare to conversion of ocean-land in North Qilian archipelagic ocean of Early Paleozoic.