Zircon U-Pb Dating And Hf Isotopes Of The Yaxi Gabbro From Central Tianshan In Xinjiang And Its Tectonic Significance

The Central Asian Orogenic Belt is a giant suture zone formed by subduction of the Ancient Asian Ocean and the collision between plates during the Late Paleozoic.During this period,extensive magmatism and metallogenic metallization occurred.It is of great scientific significance to explore the tectonic evolution and magmatic activity of the Central Asian Orogenic Belt during the Late Palaeozoic era.Therefore we select Yaxi gabbro located in the south of the Aqikekuduke-Shaquanzi deep-seated fault as the object of research in this paper.Based on field investigation,chronology,petrography,mineralogy,geochemistry and zircon Hf isotopic tracing,we analyze the geological features of the Yaxi gabbro,the nature of the parent magma and the magma source area,the diagenetic age and the tectonic setting.And combined with previous research results,the chronological summary and Hf isotopic mapping have been carried out on the mafic ultramafic rocks in the Middle Tianshan massif and its adjacent areas and the tectonic evolution process of the Middle Tianshan massif and its adjacent area during the late Paleozoic era has been discussed.Based on these researches we get some achievements as follows:1.The main lithofacies of the rock mass are gabbro,containing a small amount of pyroxenite and diorite,and there are obvious vanadium and titanium magnetite mineralization.The minerals are plagioclase and common pyroxene,followed by olivine and biotite,and less plagioclase and metallic minerals.2.LA-MC-ICP-MS zircon U-Pb date of the Yaxi gabbro intrusion have been itroduced in this easy.The results show that the gabbro intruded during the Late Carboniferous period with the ²⁰⁶Pb/²³⁸U weighted average age of 311.7±3.5 Ma?MSWD=0.104,n=14?.This is not the Triassic period previously considered,which is earlier than the age of the gabbro formed in the study area.3.EPMA data display of the clinopyroxene:The vast majority of clinopyroxene is common pyroxene with a few diopside;The Mg^#value is higher?68⁸1?,with an average of 77.4,which comes from the lower evolution of magma;casting SiO₂^Al₂O₃ and Al^IVSi diagram,the Yaxi gabbro primary magma is tholeiitic basaltic magma.4.We present LA-MC-ICP-MS Lu-Hf isotopes analyses of the Yaxi gabbro intrusion,and positive?_Hf?t?values of+2.1⁺8.1,and average value is+3.93.Lu-Hf mantle mode ages vary from 622 to 865 Ma,and mean age?802 Ma?is older than U-Pb age.These characteristics indicate that the parental magma derived from depleted mantle which has subjected to subduction-related modification process.It provides evidence for the southern subduction of the Junggar ocean during the Late Paleozoic.5.Comprehensive comparison,from Bogda Harlik to Jueluotage and the Central Tianshan to the Beishan,?_Hf?t?value decreased in volatility,zircon Hf model ages of also showed a decreased characteristic from the north to the south.Combined with the regional tectonic evolution and the age of zircon Hf model,it indicates that the Middle Tianshan massif and its adjacent areas have been rebuilt due to the subduction,resulting in the heterogeneous modification of the mantle in the late Paleozoic and the different degree of modification in different areas.It suggest that Bogda Harlik and Jueluotage and Central Tianshan had a south-directed subduction most likely from the Junggar ocean.