The Geochemical Characteristics,chronology And Significance Of Igneous Rocks In The Aqishan Formation In The Aqishan Area Of Xinjiang

The eastern Tianshan Jueluotage tectonic belt in Xinjiang is an important polymetallic metallogenic belt,located at the junction of the Tarim plate and Junggar plate,and adjacent to the Junggar plate in the north by the Kangur fault.The Aqikekuduk fault in the south is connected with the Middle Tianshan block.For a long time,the tectonic evolution background in this area has been controversial,and the orogenic action in the tectonic belt is strong,so it is one of the ideal areas to study the East Tianshan orogenic belt.The Aqishan formation is an important part of the Jueluotage belt,and the Aqishan formation in the Aqishan area is chosen as the main research object.On the basis of collecting the previous research data systematically,the strata of the Aqishan formation are systematically divided and the rhythm of volcanic eruption is divided.The paleovolcanic mechanism of the Aqishan formation in the Aqishan area is studied for the first time,and the petrology is used.A systematic study of magmatic rocks(intrusive rocks,volcanic rocks,pyroclastic rocks)from the Aqishan formation in the Aqishan area,including detailed field work and petrographic studies,was carried out in lithofacies,geochemistry and geochronology.The geotectonic framework in this area is discussed on the basis of accurate geochronological study,whole rock main trace element and U-Pb isotopic analysis.The following conclusions are obtained:1.The Aqishan formation belongs to the intermediate acid volcanic rock-fine clastic rock formation and is divided into three lithologic sections according to the lithologic assemblage characteristics and volcanic eruption rhythm.The first lithologic segment can be divided into three rhythms,the second lithologic section can be divided into 9 rhythms,and the third lithologic section can be divided into three eruptive rhythms.2.Five paleovolcanoes were divided into five ancient volcanoes in the Aqishan formation in the Aqishan area,and 16 paleovolcanic mechanisms were determined.3.The volcanic rocks of the Aqishan formation are characterized by low titanium(mean 0.72)and high magnesium(average 3.23).The alkali-rich rocks are mainly calc-alkaline rocks with high potassium and medium potassium.It is rich in large ion lithophile elements,such as Rb?K?Ba and so on,and is relatively depleted in high field strength elements,such as Nb?Ta?P,etc.,and it has the characteristics of magmatic rocks in subduction zone.It is assumed that its formation background is an active continental margin(continental margin arc)environment.4.LA-ICP-MS zircon U-Pb dating was performed on 5 samples of the Aqishan formation.The granodiorite(438.1±5.1Ma)and quartz diorite porphyrite(447.3±5.9Ma)are late Ordovician-early Silurian,which may indicate the time of subduction of the Paleo-Tianshan Ocean towards the Tarim block andesite(332.0±5.1Ma)and a large number of 336.2Ma zircon ages in pyroclastic rocks indicate that the volcanic rocks of the Aqishan formation were formed at an early Carboniferous age of(332 ~ 336.2Ma),diorite-porph(316.6±5.1Ma)age data indicate that the magmatic activity in this area during the late Carboniferous.The age data of(711.8Ma,888.8Ma)and some ancient zircon indicate that there may be an ancient proterozoic basement in the Aqishan Yamansu island arc belt.5.The volcanic rocks of the Aqishan formation were formed in the subduction background and the magmatic evolution was mainly characterized by crystallization differentiation and partial melting.It has the characteristics of melt metasomatism enrichment and fluid metasomatism enrichment,and is contaminated by crustal materials in the evolution process.6.This article holds that the Palaeo-Tianshan Ocean between Junggar-Kazakhstan and Tarim plates in the East Tianshan region formed before The ancient Tian shan oceanic basin from the late Ordovician and Early Silurian(438.1 ± 5.1Ma,447.3 ± 5.9Ma)to bidirectional subduction,in southern subduction formed Aqishan-Yamansu island arc,the northward subduction of the ocean crust formed intensified Dananhu-Tou Suquan arc.The AQishan formation is an continental margins volcanic rock formed in the Ancient Tianshan ocean subduct to northern margin of the Tarim plate from the early Carboniferous.The bidirectional subduction of the Paleo-Tianshan Ocean lasted until the end of the early late Carboniferous.The late Carboniferous basin closed into the continental collision stage,the late of early Permian area entered the post-collision stage,and the middle Triassic entered the intraplate evolution.