Geochemical Characteristics Of Volcanic Rocks In The Hashan Region Of The Junggar Basin And The Geological Significance

The strata in the northwestern part of the Junggar Basin in the Hararat Mountains have certain hydrocarbon generation capacity.The volcanic rock development section of this formation system,as an important hydrocarbon reservoir,has unclear stratigraphic delineation and complex tectonic action,which seriously affects further exploration and development.In this dissertation,based on core description,thin section identification,comprehensive seismic and logging data,combined with analytical tests such as main trace elements,Sr-Nd and U-Pb isotope dating,the lithological characteristics,geochemical characteristics,chronological characteristics and geological significance of the volcanic reservoirs in the study area are analyzed and studied.The research results are of great significance for guiding the next step of oil and gas exploration.The lithological characteristics show that volcanic lava,volcanic clastic lava,volcanic clastic rock and sunken volcanic clastic rock are the four main types of volcanic rocks developed in the Hashan area,and basal shallow-forming intrusive rocks can be seen locally.The eruptive phase,overflow phase and sunken volcanic phase are identified as three phases and six subphases.The eruptive environments and phase development patterns of Carboniferous and Permian volcanic rocks are different,with the former mainly erupting on land,and the eruptive mode being central bursting medium-acidic jet type;the latter both on land and underwater,and the eruptive mode being central-fracture transition medium-basic magmatic type.The Rittman assemblage index is basically greater than 4 for the alkaline-subalkaline series,and the alkaline content of the medium-basic rocks is higher than that of the medium-acidic rocks,and the K2 O content is lower for the low-potassium labyrinth-neutral calcium-alkaline series.The rare earth elements show medium enrichment of LILE and HREE,loss of HFSE and LREE,etc.The 87Sr/86 Sr values are moderately low and the radioactive Nd is high.U-Pb dating shows that the Carboniferous age is 304.4Ma±5.4Ma-333.4Ma±8.6Ma,according to which it can be further divided into two early and late ages.The comprehensive analysis suggests that the volcanic rocks in the study area are not primary magma formed by the mantle,and may have been mixed with the preexisting magma during the eruption process.The overall Carboniferous is a tectonic environment of ocean basin subduction-island arc uplift,influenced by the tectonic action,the stratigraphic superposition is complex,the early Carboniferous strata are overlain by the late Carboniferous strata;the Permian,on the other hand,from the subduction island arc into the relaxation stage of collisional stress,the early island arc collisional development of part of the middle basaltic rocks,at the end of the ocean basin subduction closure,the subduction plate retreat folded back,the development of the middle basaltic rocks indicate the intra-land extensional environment.