A Study On Tectonic Evolution During The Period Of Nanhua To Devonian At The North And South Of Qaidam Basin

Northwest China is the key area of China continental geology and dynamics, as an important part of the area the south and north Qaidam have been greatly concerned by researchers. This article focuses on the diabase selected from the south Qaidam area of Xiaomiao, Jinshuikou, Qingshuiquan and clastic rock from Maoniu Moutain in the north Qaidam, and their Chronology and geochemistry were studied in detail. Meanwhile the Quanji Group, Early Paleozoic ophiolite, igneous rocks, high pressure-ultra high pressure metamorphic rocks and the nature of the Devonian basin of South and north Qaidam were targeted to studied systematicly. At last, relatively fine chronology framework from Nanhua period to Devonian was established in the study, and the regional geological evolution was analyzed, then knowledge was obtained as follows:1. After geology, geochemistry and geochronology of basic dykes from Xiaomiao area and blasto-gabrro from Jinshuikou area being detailed researched, it was ascertained that the diabase was enriched in large ion lithophile elements and LREE, loss of high field strength elements such as Nb and Ta, the blasto-gabrro was flat REE pattern, enriched in large ion lithophile elements, loss of high field strength elements such as Nb and Ta. They all formed in continental rift tectonic setting, crystallization ages of rocks were 733.6±6.6Ma and 796±41 Ma respectively measured by zircon U-Pb isotopic LA-ICP-MS, after that sedimentary characteristics in extensional tectonic environment from the sedimentary-volcanic Quanji Group nouth Qaidam was detailed researched, then a conclusion can be made that all these precisely confirmed the existence of the Rodinia continent breakup during Nanhua period.2. Qingshuiquan diabase was choiced to major concern, it was confirmed that its geochemistry was LREE enrichment type, significantly enriched in large ion lithophile elements such as Nb and Ta, loss of high field strength elements, the diabase derived from the active continental margin arc tectonic setting, and its crystallization age is 436.4±1.2Ma measured by zircon U-Pb isotopic LA-ICP-MS. On this basis, combined with sedimentary characteristics and the detailed analysis of the features of Early Paleozoic ophiolite, igneous rocks and high pressure-ultra high pressure metamorphic rocks, a conclusion can be made that the north Qaidam Ocean formed in 447-521 Ma, then the subduction occurred and the UHP eclogite formed in 486-445Ma, deep subduction occurred in crust during the period of 437-422Ma; back-arc oceanic basin in Qimantage formed in the Early and Middle Ordovician, eastern Kunlun oceanic basin formed in 522-460Ma, then subduction occurred at 455-436Ma, at last continental collision occurred in 427-410Ma, at the same time the corresponding arc basin and the foreland basin were formed.3. after detailed study of the field geology and regional comparative on Devonian in the area researched, and selecting geochronology study of the Upper Devonian clastic rock, a conclusion can be made that the Upper Devonian Maoniushan group was not earlier than the 430-407Ma, Maoniushan group deposits was a stretch molasse, Early and Middle Devonian was foreland basin, Late Devonian was extensional basin.4. The key geological events to the timing and qualitative, combined with previous research results were been to determine the tectonic evolution framework from Nanhua period to Devonian in the area researched.(1) Early Neoproterozoic(950-982Ma)existed an ocean in the area studied, the ocean was closed during the period of 830-927Ma.Ancient continent started to crack and discrete to become an ocean during the period of 796-733Ma in the north and south Qaidam, there formed Quanji Group with extensional environment sediments in the nouth, and Xiaomiao basic dykes and Jinshuikou blasto-gabrro derived from cracking tectonic environment in the south.During the period of Late Cambrian to Early Ordovician(467-522Ma), the area researched evolved into an ocean basin, Qimantage orogenic belt may be a rift.North Qaidam ocean may start subducting during the Early to Middle Ordovician(496-445Ma), Tanjianshan Group deposited island arc magmatic rocks and sediments from back-arc basin, with the increase of the subduction, the UHP eclogite was formed in 490Ma-450Ma, this stage may be ocean subduction, eastern Kunlun ocean occurred subduction toward nouth during the period of Middle to Late Ordovician(455-436Ma), so Early Paleozoic island arc magmatic belt came into being in the nouth of eastern Kunlun ocean, at the same time, the Qimantage small ocean occurred subduction toward nouth, then it formed arc volcanic sediments from Back-arc basin existing in the north Tanjianshan Group.When all the subduction of oceanic crust reduced off during the Late Ordovician to Early Silurian, the continental crust occurred deep subduction under the drag of the oceanic crust, then eclogite was formed with the age of 437-422Ma, Class of flysch from Saishiteng group may be the sediments from retroarc foreland basin, eastern Kunlun ocean and Qimantage ocean was closed in the Late Silurian, the flysch deposit in Baiganhu Group may retroarc foreland basin sediments.During the period of Middle Silurian to Middle Devonian, with the increasing of collision orogeny, north Qaidam, eastern Kunlun, and Qimantage graduateed into suture zone, Early and Middle Devonian deposits became sedimentary gap due to the orogenic uplift, collision orogeny was finished until Middle Devonian.(3) Nouth and south Qaidam began to form extensional tectonic evolution in Late Devonian, and there deposited a series of extensional molasse.