The Paleozoic To Mesozoic Magmatism And Tectonic Evolution Of The Central Altay Tectonic Belt, Xinjiang, China

The Altay orogenic belt is located in the core area of the northern part of the ancient Asian orogenic system,and the collage joint between the Siberian plate and the Junggar plate,mainly formed at the oceanic-continental evolutionary stage of the Irtysh Ocean.The Irtysh suture zone may be the final closed position of the ancient Asian Ocean in the north,so the Altay orogenic belt occupies a vital position in the exploration of the tectonic evolution of the northern Asian orogenic system.Based on the scientific problem of the Irtysh oceanic subduction and closure time limit,this paper selects the main area of the Altay Mountains with low degree of work,weak research and development of magmatic rocks as the study area,and takes the magma arc on the metamorphic basement of the Altay micro-continent,the granite and volcanic rocks in the rift volcanic basin as the research objects.By the systematic studies of field investigation,zircon U-Pb geochronology,main-trace rare earth elements geochemistry and Sr-Nd isotope,multi-method interpretation of genetic structure and multi-evidence comparison,the oceanic-continental evolution of the Irtysh Ocean and the intraplate stretching process are discussed here.And the progress and preliminary conclusions have been achieved as follows:1.The framework of the stratigraphic and tectonic-magmatic evolution in this area has been established by the systematic research of zircon U-Pb isotopic geochronology.The newly discovered marine volcanic rocks(482-457Ma)are determined as the Ordovician East Xileke Group,and the shallow metamorphic flysch of the proto-Middle-Upper Ordovician Habahe Group which lie with a unconformity under the East Xileke Group are classified as the Sinian-Lower Cambrian Kanas group.Migmatite and gneiss of the Habahe Group unconformity beneath the flysch formation are classified as metamorphic basement of the Altay micro-continent-the Middle Proterozoic Supute Group.Based on the newly obtained data of 421-402 Ma from the Middle Devonian Altay Formation volcanic rocks,the Middle Devonian Altay Formation and the Middle-Upper Silurian Kulumuti Group are combined and classified as the Upper Silurian-Lower Devonian Kangbutiebao Formation.According to the field macroscopic characteristics,interpenetrating relationship,microscopic identification and geochemical data,the primary Variscan intrusive rocks are disintegrated into three stades:Middle-Late Ordovician(464-451Ma),Middle Silurian-Middle Devonian(425-387Ma),Late Triassic(222-202Ma).2.The research summarized the macroscopic,microscopic and geochemical characteristics of each period of magmatic rocks,and analyzed the genetic type,provenance characteristics and formation environment of magmatic rocks,and determined the internal relationship between tectonic evolution process and magmatic event response.The Ordovician is dominated by the subduction-continental margin arc syntectonic calc-alkaline type I diorite-granite assemblage,which is the product of the Irtysh oceanic subduction.Middle Silurian-Middle Devonian is a combination of high-K calc-alkaline granodiorite-muscovite monzonite-syenite-alkali feldspar with syn-collisional syntectonics,which has a tendency to transform from I-S-A2 type to A1 type and is formed in a tectonic environment such as subduction,paleo-block disintergration,micro-block collision and Irtysh Ocean closure.Late Triassic is a combination of shoshonite series S-A1-type alkaline granites such as muscovite monzonitic granite-syenite granite-alkali granite,which is formed in the extensional environme-nt of non-orogenic intraplate.3.Based on the restoration of tectonic environment of magmatic rocks,new evidence constraints and time limits for the Irtysh ocean subduction,paleo-block disintergration-collision,ocean basin closure and intraplate evolution in various periods in the region have been reconstructed,and also the regional tectonic evolution history has been reconstructed.The study concluded that the volcanic rocks with 482 Ma in the area started from the early north subduction of the Irtysh Ocean.The emplacement of the 387 Ma muscovite monzonitic granite marks the closure and oceanic-continental conversion of the Irtysh Ocean within the study area.