| As an important continental oil-bearing basin in China and even the world,sedimentary process of Songliao Basin has always been the key point of research.With the development of oil and gas exploration,the exploration in this basin has been gradually oriented to the deep.Previous studies suggest that basement of Songliao Basin is mainly composed of Caledonian and Hercynian folded metamorphic rock series and magmatic rocks.Basin also have life cycle,and study of basin basement is the beginning of the cognition of basin life cycle.Songliao Basin,located in the east of the Central Asian Orogenic Belt and in the center of northeast China.Borehole SK2,one of the scientific boreholes of International Continental Scientific Drilling Project of the Songliao Basin,located in Xujiaweizi fault depression of Songliao Basin,was successfully completed in2018 with a drilling depth of 7108 m.Through systematic zircon U-Pb isotopic dating of the drilling cores,a set of Triassic volcanic-sedimentary strata in the interval of 5960.05-6958.60 m of SK2 has been discovered by our research team.Taking this discovery as a starting point,this thesis analyzes petrogenesis and Precambrian zircon age clues,and discusses their implications for regional tectonic problems in the Songliao Basin and its adjacent areas based on Triassic basement cores identified from borehole SK2 and other deep exploration wells in the Songliao Basin.(1)Geochronology of Triassic basement of the Songliao BasinBased on LA-ICP-MS zircon U-Pb isotopic dating of cores collected from borehole SK2 in Xujiaweizi Fault Depression,Songliao Basin,Triassic volcanic-sedimentary sequence consisting of intermediate-acid volcanic rocks,sandstones and conglomerates were identified between the depth of 5960.05 m and the depth of 6958.60 m.Taking this discovery as a starting point,LA-ICP-MS zircon isotopic U-Pb dating of 16 basement cores collected from 12 oil and gas exploration wells in Songliao Basin was carried out.As a result,monzogranite with the formation age of 242±2Ma,diorite with the formation age of 243±2Ma,quartz monzoite with the formation age of 250±2Ma,dacite with the formation age of 244±6Ma and andesite with the formation age of 245±2Ma are revealed by well FS4,SJ23,LT1,LS1 and LS3,respectively.Though dating of the basement cores in the Songliao Basin,early to middle Triassic intermediate-acid igneous rocks,sandstones and conglomerates were revealed by the six drilling boreholes of "three fault depressions and one uplift" in the Songliao Basin,among which volcanic rocks are distributed in uplift and intrusive rocks are distributed in fault depressions.(2)Petrology of Triassic basement of the Songliao BasinAccording to assemblage of explosive facies and effusive facies in different depth ranges of the Triassic sequence revealed by borehole SK2,volcanic rocks between the depth of 5960.05 m and the depth of 6958.60 m can be divided into products of four volcanic eruption stages,indicating eruption intensity has undergone “strong”,“weak”,“strong”,“weak” changes.According to the sedimentary rocks assemblage and detrital structure characteristics of the Triassic sedimentary rocks revealed by borehole SK2,sedimentary rocks between the depth of 6137.35-6423.85 m can be divided into products of fan delta front in the lower part and fan delta plain in the upper part.Analysis of detrital compositions of sedimentary rocks indicating that magmatic arc at convergent plate margin is the main provenance of Triassic sedimentary rocks revealed by borehole SK2.Analysis of geochemical characteristics of Triassic adakitic igneous rocks revealed by borehole SK2 and other deep exploration wells indicating that they were formed by partial melting of subducted oceanic crust at convergent plate margin.Analysis above suggests that potential development area of Songliao Basin was in the tectonic setting of convergent plate margin during early to middle Triassic.(3)Tectonic implications of petrology and geochronology studies of Triassic basement of the Songliao Basin(1)Comparison of clastic composition of sedimentary rocks and geochemical types of igneous rocks of Triassic basement and syn-rift sequence in the Songliao Baisn shows that clastic of Triassic sedimentary rocks are mainly from magmatic arc provenance,while there are more clastic from continental blocks of sedimentary rocks of Shahezi Formation formed at syn-rift period;igneous rocks of Triassic basement are mainly formed by partial melting of oceanic crust,while igneous rocks of Yingcheng Formation formed at syn-rift period are mainly formed by partial melting of continental crust.Evidences above indicate that tectonic settings of area where Songliao Basin developed changed from plate margin tectonic setting to intracontinental tectonic setting before and after basin developed.(2)Comparison of zircon U-Pb dating data of basement rocks in the Songliao Basin obtained from previous studies and this study shows that different from data of basement samples published at previous studies where there are few Precambrian zircons,Precambrian zircons also developed in cores collected from wells located in the north of the Songliao Basin of this study,suggesting there may also be Precambrian basement in the northern part of the Songliao Basin.(3)Comparison of adakites formed by partial melting of oceanic crust and detrital compositions of sandstones in the Songliao Basin and its adjacent area shows that late Permian-middle Triassic adakites formed by partial melting of oceanic crust and middle Triassic sandstones with framework compositions and detrital zircon age spectrums similar to sandstones at plate margin tectonic settings distributed in the north part of Solonker-Xar Moron-Changchun-Yanji Suture,spreading from west to east,indicating that potential development area of the Songliao Basin located at convergent plate margin tectonic setting during early-middle Triassic and early-middle igneous rocks and sedimentary rocks of Triassic basement of the Songliao Basin are products of magmatism and sedimentation during northward subduction process of Paleo-Asian Ocean beneath the Songliao-Xilinhot block. |
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