Geochemistry Of Aqishan Granite In Jueluotage Orogenic Belt And Its Tectonic Significance

The tectonic evolution of the Tianshan orogenic belt and its adjacent areas is an important stage in the evolution history of the continental crust in the central Asian orogenic belt.Most people tend to think that the Tianshan orogenic belt experienced the ancient Tianshan orogenic process formed by the subduction,collision and proliferation of different blocks in the Paleo-era and the reconstruction process of the Meso-Cenozoic orogenic belt affected by the Tethys tectonic domain,respectively.the Tianshan orogenic belt experienced the ancient Tianshan orogenic process formed by different blocks through subduction,collision and proliferation respectively.During the Mesozoic period,Tianshan Mountain and its adjacent areas have entered the stage of intracontinental tectonic evolution,and the existence and activity of the Tethys Ocean on the south side have a great influence on the intracontinental tectonic evolution in this area.Therefore,in this paper,the Aqishan granite body exposed in Xinjiang is selected as the research object.Through the study of petrography,geochemistry,chronology and Hf isotopic tracer,the geological characteristics of the Aqishan granite body are clarified.The nature of parent magma and magmatic source area,diagenetic age and geological significance;Combined with the previous research results,it is considered that the magmatic activity in East Tianshan area may be the process of transformation from the post-collision stage of Hercynian to intraplate magmatic activity.This is of great significance for further understanding the process of tectonic evolution after collision and intracontinental tectonic evolution,the origin of intraplate magmatic activity and the regional dynamic background of the Tianshan orogenic belt.The main research results are as follows:?1?The Aqishan granite body is a complex rock mass composed of fleshy red syenite granite and grayish biotite monzonitic granite,in which the main components of syenite are quartz,plagioclase,biotite,hornblende and so on.The main components of biotite monzonitic granite are quartz,plagioclase,striped feldspar,biomica,common pyroxene and so on.Accessory minerals are secondary minerals such as sericite.?2?LA-MC-ICP-MS zircon U-Pb isotopic dating results:the weighted average age of zircon ²⁰⁶Pb/²³⁸U from syenite is 271.7±3.1Ma?MSWD=0.71,n=14?.The weighted average age of zircon ²⁰⁶Pb/²³⁸U from monzonitic granite is 251.0±3.7Ma?MSWD=0.16,n=11?,which represents the emplacement age of rock mass,indicating that they were formed in Middle Permian and early Triassic,respectively.?3?The results of petrogeochemistry data show that it is a complex rock mass with same original magmatic sources and different time invasion located in the same tectonic unit,belonging to high potassium calc-alkaline series,quasi-aluminite-weak peraluminous granite.Rich in Rb,Th,U,K and poor in Sr,Ba,Nb,Ta,P,Ti,Zr.All of these reflect the characteristics of crust-derived rocks.?4?The Hf isotopic composition of zircon is uniform,the zircon ¹⁷⁶Hf/¹⁷⁷Hf?t?value is 0.282761⁰.282952,the corresponding_?Hf?t?value is 4.63¹1.65,and the average value is 8.46.the calculated zircon second stage Hf model age T_DM2M2 is 548⁹81 Ma.This reflects that the source rocks of the Aqishan granite come from the younger crust.