Geochemical Characteristics And Tectonic Settings Of Carboniferous Volcanic Rocks From Junggar Basin, China

The Junggar Basin is an important part of the Central Asia petroliferous area. Carboniferous volcanic rocks are widing rang in the basin,which records the information about the Neopaleozolc geotectonic evolution of Junggar Basin. This thesis focus on building intrabasinal Carboniferous stratigraphic time framework, trying to dissect genetic mechanism and their tectonic setting of Carboniferous volcanic rocks from Junggar Basin,and analyzing eruption time and fill characteristic,in order to lay a foundation for oil-gas exploration activity of Junggar Basin. This thesis focus on the core-watching,petrography,zircon U-Pb dating,rock geochemistry and Lu-Hf isotopic element geochemistry investigation of the Carboniferous Batama- yineishan formation volcanic rocks in the type well from the different tectonic unite of the Junggar Basin,in order to define Carboniferous volcanic rocks chronology framework and age and reveal the source region,genetic mechanism and crust-mantle function in the form process of rocks. Zircon U-Pb ages reveals that Carboniferous volcanic rocks have a range between 397.9 Ma and 289.5 Ma,the Early Carboniferous volcanic rocks are characterized that LILE(Rb,Th, Ba et al) is relatively enrich, while HFSE(Nb,Ta),particular Sr,P and Ti element is relatively depleted;the Late Carboniferous volcanic rocks enrich HFSE,which reveals that they maybe originstem from depleted mantle. On the base of this,in combination with regional tectonic setting,we argue Junggar Basin Carboniferous structural evolution. This thesis yielding the following results and understanding:(1)Junggar Basin Carboniferous volcanic rocks are divided to five stages,including three stages in the Early Carboniferous and two stages in the Late Carboniferous. The limit-time of the five stages is respectively:①Tournaisian;②Visean;③Serpukhovian;④Bashkirian and Moscovian;⑤Kasimovian and Gzhelian。(2)Batamayineishan formation in Junggar Basin is divided to three parts,such as upper volcanic rocks series,middle sedimentary rock series and lower volcanic rocks series,Lower volcanic rocks series characterized by intermediate-acidity rock,while upper volcanic rock series characterized by intermediate-basic rock,there is a dormant period between them. at the present,Using SHRIMP U-Pb zircon dating we obtain that the age of lower volcanic rocks series is 334Ma which belongs to Early Carboniferous.(3)The two sides volcanic rocks formed in the different tectonic setting. Lower volcanic rocks series formd in active continent margin,while upper volcanic rocks series were the products of post-collision extend tectonic setting.(4)Ludong area had already entered intracontinental evolution estate in Late Carboniferous , and Luxi area was in island arc setting yet , these made Batamayineishan formation have diachronous feature.(5)Luliang uplift southern and northern Carboniferous volcanic rocks all had the feature of active continent margin arc volcanic rocks,and then it formed following continent- continent collision. Therefore,Kelameili-Sangequan-Darbut may be a continued suture zone,at least at the belt had oceanic crust subducting in the Junggar.(6)Carboniferous deep andesitic tuff from Moshen 1 formed island arc setting,and we infer that there was the oceanic crust subduction function in the south of Junggar Basin,which provides lower limit value for South Junggar ocean close time. Addition,from a 1243.2±18.1Ma zircon we think Junggar Basin has Precambrian crystalline basement,at least there exists Proterozoic microcontinental block in Junggar Basin.(7)Through the division of single well volcanic cycles and the contrast of the well-well profile, we establish the Junggar basin filling sequence. Lower Carboni- ferous volcanic rocks and sedimentary rocks are distributed widely,in addition to Mosuowan overhang,Shiyingtan sag,eastern of Dibei overhang and southern of Kelameili,they filled the whole basin.;Upper Carboniferous volcanic rocks have smaller distribution,only seen in Mahu sag,Xiayan overhang,Dishuiquan sag,Suosuowan sag,Wucaiwan sag,Shishugou sag,Shiqiantan sag,Beisantai sag,Jimusaer sag and Wutongwozi sag.