Study On Lithological Characteristics And Tectonic Setting Analysis Of Carboniferous Volcanic Rocks In Wulungu Region

Junggar Basin is a Late Carboniferous-Quaternary sedimentary basin developed onthe Junggar block. Carboniferous-Early Permian was a important period of structuralchange from Paleoasian system to Intracontinental deformation system in Junggar region.As a result, the research of Carboniferous volcanic petrographic and structuralenvironment in the northern Junggar is significant for the acquaintance of Carboniferousvolcanic activity patterns, tectonic movement and exploration of oil and gas storage.With the Carboniferous volcanic rocks of Bashan group in Wulungu Depression asresearch object and based on the previous research achievement, this thesis uses theoriesand methods of Volcanology, Petrology, Geochemistry etc. to analyze the geologicalcharacteristics of Bashan group. The main research results are as follows:(1)The volcanic rocks in the study area are mainly volcanic lava and volcanic clasticrock, followed by pyroclastic sedimentary rock. Three stages were clarified; The understage was characteristic of developing andesite, basaltic andesite, intermediate basicvolcanic agglomerate and breccia, and it was a symbol of volcanic tuffs and tuffaceoussedimentary rocks in the middle stage. While in the upper stage, those rocks as andesite,dacite and volcanic tuffs developed.(2)Volcanic rocks in the study area are mainly composed of neutral andesite, inacidic dacite, middle basic basaltic andesite, and there are little basalt. Most of rock wasmiddle or weak calcium basic. Separation and crystallization obviously exist in themagma evolution according to the huck graphics. Based on the feature of rare elementand rare earth element, it could be found that the chosen different lithologic samples wereproduct differentiation of the same magma, and the magma may be affected by crustalcontamination in rising process.(3)In the study area, volcanic rock lithofacies were eruption facies, overflow faciesand volcanic sedimentary facies, and they can be divided7subfacies. Volcanic activitywas central type eruption, and the eruption is frequent, with strong ability. They could bedivided into three stages and six volcanic traps. The under stage contained explosivefacies of early phase and effusion facies of late phase, and can be dicided into three traps.The middle stage was composed by volcanogenic sedimentary facies in the dormant ofvolcanic activity. While the upper stage can be divided into two traps, the second trapcontained explosive facies and effusion facies, effusion facies for the first trap. The zonaldistribution of volcanic rock lithofacies was contolled by NW-SE faults. (4)The structure background of the carboniferous volcanic rocks in Wulungu wasvolcanic arc in the active continental margin. The magma was formed after the uppermantle replaced by the fluid from dehydration of subduction oceanic plate, which wasrich in water and LILE. Magma evolution was characteristic of fractional crystallization,which in the early phase mafic minerals fractional crystallization was primary, while inthe late phase the fractional crystallization of plagioclase played a leading role.