Late Paleozoic Magmatism And Crustal Growth In West Junggar

The West Junggar,which is located in the southwestern part of the Central Asian Orogenic Belt,with the intense hypogene magma in the later Paleozoic,occurred the tectonic transition from oceanic evolution to intracontinental evolution.In this transition,crustal growth and evolution have multiple stages,with the strong crust-mantle interaction and the significant vertical and lateral proliferation of the crust.The predecessors have done a lot of research in petrology,chronology,geochemistry and isotope,but there are still controversies about the time of ocean-continent transformation,chronology framework,crustal growth and crustal growth pattern of the area.In this thesis,focusing on the problems above,we carried out the research work with the combination of key areas and key points,aiming at perfecting the sequence of Later Paleozoic magmatism in the West Junggar area,reshaping the material structure of crust and mantle,exploring growth pattern of crust,and analyzing the tectonic background.This is of great significance for deepening the tectonic evolution of the West Junggar.The Jietebudiao rock mass in the southwestern part of West Junggar have high-precision zircon U-Pb age of 287-250 Ma,which is mainly composed of alkali-feldspar granite,monzonitic granite and granodiorite.It has the characteristics of high SiO₂ and potassium calc-alkaline,poor Mg and Fe,quasi-Al-weak Al and low LREE?Eu?Ba?Nb?Sr?P?Ti.There are different types and sizes of mafic microgranular enclavein the monzonitic granite and granodiorite,which belong to the I-type granite of mixed crust-mantle derivation.There is no mafic microgranular enclave inside the alkali feldspar granite,which is derived from the partial melting of the new crust.It belongs to the high fractionated I-type granite.In the Jietebudiao area,it is in an intracontinental environment with high temperature,low pressure and crustal thinning and crustal extension in the Early Permian-Early Triassic,and there are multiple stages of plutonic magma intrusion.The monzonite granite invaded by the Early Triassic?250.4Ma?is the youngest granite ever discovered in Western Junggar,and the intrusion marks the end of the plutonic magma intrusion in the Western Junggar.The newly discovered columnar joint rhyolite on the northwest side of the Miaoergou batholith is formed at 303.6-294.5 Ma.It has the characteristics of high SiO₂ and potassium calc-alkaline,poor Mg and Fe,quasi-Al-weak Al,low Eu,rich LREE?Rb?Th?K?Pb,low HREE?Ba?Nb?Sr?P?Ti.During the diagenesis process,it has experienced sufficient magma crystallization differentiation,belonging to the A2-type granite of the depleted mantle derivation.Which formed in the post-orogenic extension of the environment,produced in the continental slope phase strata.The new material is involved in the sharp decline of diagenetic components in the West Junggar region,which has similar petrological and geochemical characteristics to the columnar jointed rhyolite developed by the ocean-continental transition zone in the western Pacific tectonic domain,both occur in continental marginal environments.It is believed that the western junggar region was in the stage of transcontinental evolution when the columnar jointed rhyolite was formed.Therefore,the time limit of the ocean-continent conversion in the western junggar region is 303.6-294.5 Ma.It found that the western Junggar region had high positive?_Hf?t?,high positive?_Nd?t?,low(⁸⁷Sr/⁸⁶Sr)i and young crust model ages by the Sr-Nd-Pb-Hf isotope mapping study,and the deep crust did not exist.The ancient crystalline basement is dominated by the Paleozoic new crust,and the Neoproterozoic"old"base may exist locally.During the Carboniferous-Permian period,about 85% of the western Junggar has a crustal growth of 75%-95%,mainly in the lateral growth and material recycling of the crust;about 15% of the area?from the Jietebudiao area?the amount of crustal growth is 50%-75%,which is dominated by vertical growth of the crust.Junggar Ocean was in the stage of ocean basin evolution before early carboniferous epoch.In early carboniferous epoch?360-320 Ma?,intra-oceanic subduction occurred,forming an intra-ocean arc represented by the arch of Gubaotu Island,with the lateral crustal growth.It is the first stage of the development of the orogenic belt.In the early stage of Late Carboniferous and Early Permian?320-294 Ma?,the Junggar Ocean subduction was gradually weakened and the west Junggar turned into a tectonic setting of the post-orogenic extension.The sedimentary construction gradually changed from abyssal-bathyal facies to continental slope facies,and magmatism reached its peak stage,forming granitic batholith,mid-subsiliceous dike swarm,and small and medium-sized rock stock.The crustal growth may be dominated by the recycling of the Paleozoic oceanic crust or the oceanic island arc,and there may be a small amount of new material directly formed by the mantle magma underplating.The crust growth in most areas is as high as 75% or more.It is the second stage of the development of the orogenic belt and the formation of columnar jointed rhyolite marks the formal transition from oceanic evolution to intracontinental evolution in the western Junggar region.During Early Permian-Early Triassic period?272-250 Ma?,the area of the Tebubu area is still in the extended environment of high temperature,low pressure and crustal thinning.The presence of the Early Triassic?250.4 Ma?monzonitic granite marks the end of plutonic magma intrusion in the West Junggar area.The West Junggar stoped at the second stage of the development of the orogenic belt.