Geochemistry Of Intrusive Rocks In The Kekesentao Area From Northern Xinjiang (China) And Their Tectonic Settings

There is still controversy on tectonic nature and evolution stages for the Ertix fault zone which can be attributed to its special location and evolution complexity,and it is a collision suture zone between Siberian plate and Kazakhstan-Junggar plate.The Ertix fault zone cross the Kekesentao of Buerjin of Xinjiang in the direction of northwest-southeast,which is belonged to the joint of Sawur-Ertai late Paleozoic island arc zone at the Junggar micro-plate and Chaerkesi-Qiaoxiahala suture zone.The Ertix fault zone has experienced long periods of plate subduction,collision and accretion during the Late Paleozoic that resulted in forming multistage eruption-invasion.This thesis-selects the Devonian-Permian intrusive rocks as the research object,and these rocks are divided into 3 types according to contact,space-time distribution and isotope dating of intrusive rocks:Late Devonian,Late Carboniferous and Early Permian.On the basis of this,the goal of this article is to determine magma sources and petrogenesis by petrologic and geochemical methods.Results of this thesis would provide important data support for us to better understand the tectonic feature of the Ertix fault zone or even Altai orogenic belt,and make it possible to study the age of subduction closure of paleo-Asian oceanic.The Late Devonian intrusions are located between ophiolite melange belt and Middle Devonian strata.The emplacement is closely related to the structure zone and the rocks are developed gneissicly.The lithology,in this area,is mainly composed of tonalite,quartz monzodiorite,quartz diorite and diorite.Besides that,the dating of zircon from tonalite by La-ICP-MS is 361±2Ma.This result indicated that the tonalite was formed in the Late Devonian.Comprehensively,these rocks of late Devonian belong to calc-alkaline series as the content of SiO₂ is 65.10-69.56%and the content of Na₂O+K₂O is 4.45-6.35%.These rocks also belong to peraluminous since the aluminum saturation is greater than one for each kind of them.The rocks there are enriched in LREE while relatively flat in HREE.And the ratios of the Zr/Hf and Nb/Ta remain to the ratios of these elements in the primitive mantle.Combining the evidences that were produced geologically and geochemically,this research presented that the rocks were formed by mixing the mantle derived magma with crustal rocks.Besides that,the rocks were formed between the period of pre-collision and collision.The intrusions in Late Carboniferoushave been widely distributed in this area and the latest invaded strata is Lower Carboniferous Tongjimunai Formation.The main rocks are porphyrite,granodiorite,monzonitic granite,biotite monzogranite,biotite syenogranite,granodiorite porphyry,monzogranite porphyry and granite.Subsequently,the dating of these zircons shows that age for biotite monzogranite is 305±2 Ma and the value of granodiorite porphyry is 305±1 Ma.These results indicate that the intrusive rocks were formed during the Late Carboniferous.The intrusion sequence is characterized by continuous evolution beginning from quartz diorite to granodiorite?granodiorite porphyry?to monzogranite?monzogranite porphyry?and then reach syenogranite.The evidences of the former conclusion are as follows.Initially,the Ritmann index,DI and the total REE increase gradually.Then,the anomaly of Eu is gradually decreasing.Last but not least,the losses of Nb and Ta are continuous reducing.The geochemical characteristics of these rocks indicate that the source of magma is comprehensively influenced by mixing,differentiation,contamination,and partial melting of the lower crust which is the main cause of formation of these rocks.The rocks were formed in the period between collision and post-collision.The distribution of early Permian intrusive sequence is relatively small and the latest invaded strata belong to Lower Permian.The lithology of this sequence mainly composes of granite,monzonitic granite,quartz monzonite and granite porphyry.The result of Zircon U-Pb dating yielded an age of 285±3Ma for quartz monzonit,which indicates that this intrusion was formed during the Early Permian.Quartz monzonite is characterized by potassium basalt series as the relatively high total alkali content of it.Besides that,these rocks enrich Ba,K,La and Ce and loss Rb,Th,U,Nb,Ta,P and Ti.The appearance of alkaline-rocks represents that these intrusions formed in the extensional setting.Therefore,the intrusions were confirmed that it formed between the periods of post-collision and intracontinental development stage.Based on the above study,the study shows that the time of closed of the oceanic basin in the northeast of Junggar is the late Middle Devonian,and along with large-scale compressive-torsional reverse faults and the intrusion of calc-alkaline series magma,results in the formation of the late Devonian sequence intrusions.By the end of the Late Carboniferous,the Late Carboniferous sequence intrusions are formed by strong orogeny.Finally,in the late Early Permian,the Ertix fault zone should be in the post-collision-intracontinental development stage,and the Early Permian sequence intrusions were formed under the extensional system.