Study On Geochronology And Geochemistry Of Paleozoic Magmatism In West Junggar Area, Xinjiang

West Junggar area has undergone long and complex tectonic evolutions of multi-period and multi-stage, accompanied by the corresponding granitic magma activities. The genesis of granite and provenance relates to the formation of West Junggar orogenic belt, especially the geodynamic process and evolution of each stage of magmatic activities. This paper focuses on the Paleozoic West Junggar area of magmatite (volcano rock and intrusive rock), carries studies and analysis of petrology, geochemistry, zircon morphology, zircon LA-ICP-MS U-Pb geochronology and Hf isotope geochemistry on the selected representative rock bodies,then has determined the time spans of different tectonic events, the intrusion periods of various magmatite in the research area and petrogenesis, then inverts the western Junggar orogenic belt tectonic evolution.By the Zircon LA-ICP-MS U-Pb dating method,a detailed chronology study is carried on seven granites from this area for recognition of granite intrusive age of each stage. The granite in Sugensala is at the age of 415.6±1.1Ma of the late Silurian. The gabbro of Yushenkala is at the age of 364±1Ma.of the Late Devonian. The quartz monzodiorite of Heiziletawudinghaore is at the age of 364.7±3.0Ma of Late Devonian. The monzonitic granite of Mt.Musidao is at the age of 335.9±0.9Ma of the Early Carboniferous. The granite porphyry of Jiaerluapu is at the age of 305±2Ma of Late Carboniferous. The diorite of Kuoyitasi is at the age of 297± 2Ma of Late Carboniferous. The K-feldspar granite of Qiaqihai is at the age of 280.1±3.2Ma.of Early Permian. The ages abtained provide high precision dating data for the construction of tectonic magmatic events of the western Junggar area. Based on statistical data and previous research, the magmatite stages in the study area is divided into 4 stages:the Early Silurian-Early Devonian(436-405Ma), Early Carboniferous (350-320Ma), the Late Carboniferous -early Permian (318-270Ma) and Late Permian (270-263Ma). From the view of spatial and temporal distribution of magmatic rocks, the ages of magmatic rocks can be concluded that it’s older in both North and southland newer in the middle.The study of rock geochemistry of intrusive rocks shows that, the rock in this area of various stages are mainly A type and I type granite. The granite of west section of Mt.Xiemisitai is A2 type granite and I type granite with weak alkaline and peraluminous differentiation. Early Carboniferous granite is dominated by I type, and distributed mainly in Sawuer area in a belt shape which was formed in continental subduction. The granites of the Late Carboniferous and Early Permian are mainly A type, widely distributed in a planar distribution with geochemical characteristics of alkaline and metaluminous, and was probably formed by fluid metasomatism related with subduction, part of magma source area owns significant characteristics of arc magmatism,and the chemical characteristics indicate that it was formed in an extensional environment after the collision. The gabbro of Yushenkala of Late Devonian was formed from lithospheric mantle depleted by subduction fluids, and the magma has not been contaminated by crustal materials during ascent. Early Permian mafic intrusive rocks in the source region owns the prints of the subduction and involves with mantle components. It can be inferred that these rocks may be the partial melted product of mantle peridotite replaced by the fluid released from subducted slab or subtractive sediment.The newly abtained zircon Hf isotopic compositions seven intrusive rocks in western Junggar area show that intrusive rocks in the area of various periods share similar zircon Hf isotope,which are positive εHf(t) value and the positive value is high, indicating that the granites of various periods are mainly the product of partial melting of the new continental crust, reflecting that new continental crust has been forming continuously since Middle Proterozoic to early Paleozoic Ordovician in the western Junggar area. This agrees with the facts that the Central Asian orogenic belt has been forming new continental crust since the Neoproterozoic, arc continent collisions and collage and continental accretion.Based on the above research, combined with the regional geological background and tectonic evolution characteristics of the western Junggar orogenic belt, the time of the paleo Asian ocean closure and post collision have been discussed. The results show that there have been multiple collisions and subductions, and the oceanic basins were repeatedly formed and subducted one after another. The paleo Asian Ocean was very wide, and the closure time may vary in different regions. This article speculates that in the northern region of Western Junggar (near Eerqisi suture), Late Devonian oceanic subduction event has basically ended, the closure of the paleo Asian Ocean was no earlier than 364Ma. For the post collision period, combined with regional geological data, we believe that the deep magmatic activities in West Junggar area after the collision started from the Early Carboniferous, and ended by the end of Early Permian,namely the magmatic activities after collision occurred in 340-270Ma.Through the research of magmatic rock petrogenesis and the relationship between Hf isotope tracing and tectonic evolution, it’s believed that Paleozoic especially the late Paleozoic is an important period for continental crustal accretion of the Central Asian orogenic belt, the accretion includes and lateral accretion related to subduction, vertical accretion related to mantle source underplating.The continental crustal accretion is a multi-stage multi-cycle process,and exists throughout the whole formation and evolution process of the Central Asian orogenic belt.