Tectonic Evolution Of Maza-Kangxiwar Melange Belt In Southern Margin Of West Kunlun

The West Kunlun Orogenic Belt,is located in the northwest of Tibetan Plateau,at the junction of tectonic domain of Paleo-Asian and Tethys.It is the western extension of the Central Orogenic Belt,and adjacent to the Tarim plate in the north and the Qiangtang block in the south.It is the critical region for studying the evolution of the Tethys,as well as the hot spot and focus of global geoscience research.The Maza-Kangxiwar Tectonic Belt,as an important boundary fault in the southern margin of West Kunlun Orogenic Belt,has undergone evolution of the Proto-Tethys and the Paleo-Tethys,which is the key to understand the evolution of the West Kunlun Orogenic Belt.Guided by the accretionary orogenic theory,this thesis is focused on the tectonic evolution of Maza-Kangxiwar Tectonic belt from Early Paleozoic to Late Triassic,Combined with multi-disciplines of through field geological survey,tectonic geology,sedimentology,petrogeochemistry and chronology,this thesis conducts field geological survey,tectonic deformation analysis,sedimentary sequence restoration,major and trace geochemistry,zircon U-Pb chronology and other research methods,analyzed the structures of the accretionary wedge in the south and magmatic arc in the north of the Maza-Kangxiwa tectonic belt,found out the tectonic attributes of Maza-Kangxiwar Tectonic Belt as well as the orogenic mechanism of the West Kunlun Orogenic Belt,discussed the evolution of Maza-Kangxiwar Tectonic Belt.The main achievements of this thesis are as follows:1.We newly discovered a seamount formation of basalt and limestone from Ordovician Dongguashan Group in the south of Maza-Kangxiwar Tectonic Belt.Pillow basalts has typic geochemical characteristic of oceanic island basalt(OIB),with zircon U-Pb age of 465.1±6.6Ma.Meanwhile,the age of hornite and tricobite from the limestone were identified in the Sandbian to Katian stages of the Late Ordovician,indicating that seamounts were formed in the Proto-Tethys during the Late Ordovician.Detrital zircons from the upper sandstones of the Seamount formation reveal that the detrital rocks were mainly derived from the West Kunlun Orogenic Belt and a small amount from the Tianshuihai block,suggesting that the Proto-Tethy Ocean was closed to a limited ocean basin during the Late Ordovician.The determination of seamounts indicates that the Early Paleozoic to Triassic strata in the southern of the Maza-Kangxiwar Tectonic Belt are accretionary complexes of different ages,which formed a giant accretion wedge in the southern margin of the West Kunlun orogenic belt during the northward subduction of the ocean crust.2.Based on the study of composition and deformation of the accretion complexs in the south side of Maza-Kangxiwar Tectonic Belt,the accretion wedge is mainly composed of Late Cambrian carbonate platform formation,Late Ordovician seamount formation,Silurian trench-bathyal formation,as well as Permian-Triassic for-arc basin formation.The huge accretionary wedge in the southern margin of west Kunlun Orogenic Belt is formed by blocks and nappes(slab)which composed of accretion complexes in different ages,in the way of collage and superpose along the Maza-Kangxiwa Tectonic Belt,Heiqia-Guozhacuo Tectonic Belt as well as southern Aktahe River Belt from north to south.3.Based on the analysis of the Early Paleozoic magmatic arc in the North of Maza-Kangxiwar tectonic belt,we found: firstly,the Early Paleozoic magmas can be divided into two periods of 512-483 Ma and 467-443 Ma.The arc-diorites with age of 512-483 Ma was formed in the subduction of oceanic crust.The paraluminous,high-k,calc-alkaline granites with age of 467~443 Ma was formed during the accretion orogeny.Secondly,Field geological feature of the Saitula group has been investigated,mica-quartz schist formed in the Neoproterozoic active continental margin are identified,and three stages of deformation in Saitula group are analyzed.Finally,the Early Paleozoic strata distributed along the Wuyibuke belt are determined to be intra-arc deposits,and restored to be the sedimentary sequence of near margin rapid accumulation.The deposition age of intra-arc basin is limited to 483Ma-467 Ma,meanwhile,indicating transient extensional environment during the transition from Proto-tethys subduction to collision.4.The monzonites intruded into Bayankalashan group are high-k,calc-alkaline and peraluminous granites,and the magmatic source is from partial melting of upper crust sediments,which is formed in the process of accretion orogeny.The zircon U-Pb age of the monzonites is 214.8±1.6Ma,which limited the closure time of the Paleo-Tethys.5.The evolution of the Proto-Tethys and Paleo-Tethys,in the southern margin of west Kunlun orogenic belt from Early Paleozoic to Late Triassic is explored.The 6 important evolution stages of Proto-Tethys subduction(512Ma-483Ma),Proto-Tethys oceanic crust roll-back(483Ma-467Ma),Proto-Tethys accretion orogeny(467Ma-D),Paleo-Tethys expansion(C1-P1),Paleo-Tethys subduction(P1-T1)and Paleo-Tethys collision-accretion orogeny(T1-T3)are discussed in detail.