Xingxingxia Late Paleozoic Intrusive Rocks And Rocks Again Xingxingxia Miscellaneous Determined - Discussion Of The Tianshan Property And Late Paleozoic Tectonic Framework

This thesis carries out systematic geological field investigation, petrography observation, zircon geochronology and whole rock geochemistry on para-metamorphic rocks of the Xingxingxia complex and intrusions within it. The study area is in XingXingxia region and adjacent area near the Xinjiang-Gansu provincial boundary, in the east part of Tianshan orogen and to the north of Tarim Basin.(1) Constraining late Paleozoic the tectonic framework of Xingxingxia area of Central Tianshan Block in and the final closure time of South Tianshan Ocean. The Late Paleozoic magma activity in Xingxingxia region are mainly divided into three period. The first period is intrusive magma activities under the collision background between late Ordovician to the late Devonian; the second period is continental arc intrusive magma activities under the subduction background between304±3Ma and278±3Ma; the third period is intracontinental intrusive magma activities under the collision and post-collision background between268±5Ma and259.9±2.6Ma based on this and previous work. The final suture zone of South Tianshan ocean should be between Central Tianshan Block and South Tianshan accretion complex belt, depending on previous work both the first period magma activities (i.e. intrusive magmatic activities between late Ordovician to late Devonian) and the516±7Ma(cumulate gabbro zircon U-Pb age) Hongliuhe Ophiolite complex intruded by404±5Ma (zircon U-Pb age) granite indicate a close event between Central Tianshan Block and South Tianshan Accretion Complex;304±3Ma dioritic cataclastic rock of XingXingxia complex and278±3Ma diorite are all the products of Continental margin arc magmatism,289±3Ma gabbro are formed by magma from metasomatic enrichment mantle, all these indicate the second period is continental margin arc intrusive magma activities under the subduction background; the third period granitic magma activities could be mainly under the the post-collision extensional tectonic setting, through the second period and the third period magma activities is a transitional tectonics from compressional tectonic setting to post-collision extensional tectonic setting; Between268±5Ma and259.9±2.6Ma the final closure of South Tianshan ocean finish and Tianshan orogen come into the intra-continental evolution stage, during and after the closure process, a wide range of metamorphism and large dextral strike-slip faults develop.(2) Zircon U-Pb age data provide constraints on the crustal evolution of XingXingxia complex. The spatial distribution of gneiss and migmatic rocks in XingXingxia complex is limited, in constrast, the Xingxingxia complex is mainly cosisit of para-metamorphic rocks including quartzite, mica quartz schist, the minimal concordant age of detrital zircons of Sample XX21, XX22are265±2Ma,246±2Ma respectively which provide upper limit of its deposition time of protolith in the late Permian to early Triassic; but the major age populations of detrital zircons of sample XX21is between265±2Ma and379±3Ma, one secondary age peak is about1.6Ga and there are also some sporadic age record near0.67Ga, and3.3Ga. Age distribution of sample XX22is relatively complicated, about half of U-Pb ages of zircons is in Late Paleozoic, the rest age distribute in populations near0.6Ga-0.8Ga,1.2Ga-1.5Ga and1.6Ga; The age of sample XX23range from595Ma to380Ma or so.the zircon age peak distribution of the para-metamorphic rocks of Xingxingxia complex is in good consistence with previous studies, indicating that the main parametamorphic rocks of Xingxingxia complex (samples XX21and XX22) deposited in Late Permian to early Triassic contain age information of the Tianshan Precambrian tectonic evolution, sedimentary the provenance is mainly Central Tianshan block, a small amount of provenance may come from South belt of Beishan orogen and Tarim craton; part of the formation age of metamorphic may be relatively old (sample XX23), but still later than Middle Paleozoic, with the characteristics of all age between380Ma and595Ma, the provenance may be mainly island arc complex in Central Tianshan Block, this may indicate that the Central Tianshan Block Island Arc begin at least about595Ma.In conclusion, XingXingxia complex in Xingxingxia region Central Tianshan Block is formed by regional metamorphism at the end of the late Paleozoic to early Mesozoic and mainly metamorphosed into schist facies for parametamorphic rock, some intrusive rocks metamorphosed into gneiss facies as a minor part of XingXingxia complex. The high metamorphic Precambrian rock in Xingxingxia region Central Tianshan Block is exposed discontinuously and strongly reformed by the late magmatic activities, but the Central Tianshan Block still show the typical dual cover-and-basement structure for a typical block. XingXingxia complex contain a little high metamorphic Precambrian rock and the age is1.4Ga-1.8Ga in Mesoproterozoic and0.9Ga in Neoproterozoic respectively, The age of Precambrian rock and zircon age information from Paleozoic rock in Xingxingxia region Central Tianshan Block is very similar and mainly focused on0.8-1.OGa,1.3-1.6Ga and1.8Ga which represent Precambrian geological evolution of Xingxingxia region Central Tianshan Block; However the main rock records in Xingxingxia region should be Paleozoic island arc complex, the arc might start at around595Ma, and develop to the end of Permian with partly gneiss facies metamorphism. The Central Tianshan Block is considered to be a Paleozoic Arc Complex due to the main rock records in Xingxingxia region, while in view of The Precambrian rock age and zircon age information from Paleozoic rock, The Central Tianshan Block is a Micro-Block (Central Tianshan Micro-Block)with Mesoproterozoic and Neoproterozoic continental residual crust substance. Widespread post-collisional granite develop accompanied by large dextral scale strike-slip faults indicating Xingxingxia region Central Tianshan Block entere the intracontinental stage in the late Permian with strong metamorphism and magmatic activity.(3)Through geological field investigation, zircon chronology analysis on parametamorphic rocks of the BeiShan Orogen South Belt, BeiShan Orogen South Belt exist proterozoic (its formation between0.9-1.6Ga) remant basements, as schist facies parametamorphic rocks with descending age signal of1.7Ga-2.1Ga,2.3Ga-2.7Ga,2.8Ga; late Paleozoic quartzite marble combination also develop in the BeiShan Orogen South Belt, and the protolithes represent a relatively stable continental margin sedimentary environment, the minimum effective age of the late Paleozoic quartzite is389±3Ma, the main age peak and Mesoproterozoic-Neoproterozoic age signal of the late Paleozoic quartzite are very similar with parametamorphic rocks sample of XingXingxia complex, in conclusion, BeiShan Orogen South Belt exist proterozoic (its formation between0.9-1.6Ga) remant basements with Paleoproterozoic-Neoarchean age signal which doesn’t exist in Mesoproterozoic-Neoproterozoic remant basements of Xingxingxia region in Central Tianshan Block, in late Paleozoic BeiShan Orogen South Belt develop relative-stable continental margin sedimentary environment, BeiShan Orogen South Belt and Xingxingxia region in Central Tianshan Block has a similar Mesoproterozoic-Neoproterozoic remant basements and the Paleozoic arc age signal.