Geological Characteristics And Later Reformation Of The Permo-Carboniferous Basin In Yingen-Ejina Area,NW China

The Yingen-Ejina area,located at the middle segment of the southern Central Asian Orogenic Belt,is the transitional zone between the Tarim Plate and the North China Plate,and it also occupy the key junction between the Altyn Tagh Fault to the southwet and the East Gobi Fault to the northeast.Due to the special tectonic location of the Yingen-Ejina area,the tectonic setting and geological characteristics of the Permo-Carboniferous basin are highly controversial,and the later reformation processes of the Permo-Carboniferous basin are quite complex.This dissertation is aimed to clarify geological characteristics and tectonic attributes and unravel later reformation processes of the Permo-Carboniferous basin in the Yingen-Ejina area.To address these academic problems,an integrated study synthesizing field geology,geochemistry,geo-thermochronology and subsurface geology was carried out in this dissertation.The following conclusions can be drawn:The zircon U-Pb dating results show that the Zhusileng-Hangwula tectonic belt in the northern Yingen-Ejina area exists Precambrian rocks with an age of ~0.9 Ga,and the two-stage zircon Hf model ages are between 2.0 and 1.6 Ga,which provides conclusive evidence for the existence of the Precambrian terrane in this area,and it should be the extension of the southern Mongolian microcontinent in China.Compiling this result with a published dataset from in the Yingen-Ejina area,the basement of the Permo-Carboniferous basin can be divided into three distinct units from south to north,and all of them have Precambrian crystalline rocks.In detail,the Yabulai-Nuoergong tectonic belt to the south and the central Zongnaishan-Salazha tectonic belt been amalgamated at ~0.9 Ga,and this amalgamation event linked the North China Plate to the Rodinia supercontinent.The central Zongnaishan-Salazha tectonic belt and Zhusileng-Hangwula tectonic belt to the north have a relatively late collage timing,and the Engger Us fault zone between them acts as the suture.In general,the Yingen-Ejina area shows a geodynamic system during the Permo-Carboniferous times,which closely connected the sedimentary basins with Precambrian stable blocks and active tectonic belts.Sedimentological and stratigraphic studies show that that during the Late Carboniferous-Early Early Permian(i.e.,the sedimentary period of the Amushan Fm.),the southern area of Engger Us developed littoral-neritic sediments,while the Engger Us area developed deep sea sediments.During the Middle-Late Permian(i.e.,the sedimentary period of the Maihanhada Fm.,Aqide Fm.and Haersuhai Fm.),the deep sea has been disappeared,and the littoral-neritic sediments developed widespread in the Yingen-Ejina area.Specially,the Amushan Fm.and the overlying Maihanhada Fm.shows an angular unconformity contact relationship in the Engger Us area,and this unconformity also coincided with the transformation from deep sea sediments to littoral-neritic sediments,which indicates that the regional tectonic regime has undergone tremendous changes in the Yingen-Ejina area.Petrographic and petrochemical analysis indicates that the Permo-Carboniferous clastic sedimentary rocks in the Yingen-Ejina area show low textural and compositional maturity,and the source rocks experienced relatively weak chemical weathering,sedimentary sorting and diagenesis,which suggests that these rocks were deposited in active tectonic setting.In addition,the petrochemical data suggest that the source material of the Permo-Carboniferous clastic sedimentary rocks in the Yingen-Ejina area was dominated by the felsic arc igneous rock.Detrital zircon U–Pb geochronology data indicate that the Amushan Fm.shallow marine facies sediments in the southern area of Engger Us has a unified source-sink system,showing a bidirectional provenance,indicating that there was no oceanic basin between the Yabulai-Nuoergong tectonic belt and the Zongnaishan-Salazha tectonic belt during the Late Carboniferous-Early Early Permian;and the detrital zircon age spectrum pattern of these sediments has a high similarity to typical back-arc rift basin.By contrast,detrital zircon U–Pb geochronology data of the Amushan Fm.turbidite sediments in the Engger Us area show a unidirectional source provenance,and the age pattern has a high similarity to typical trench or fore-arc basin.Detrital zircon U–Pb geochronology data suggest that the Middle-Upper Permian sediments in the Yingen-Ejina area has a unified source-sink system with a bidirectional provenance,which accept contribution from the Yabulai-Nuoergong and the Zongnaishan-Salazha tectonic belts to the south as well as contribution from Zhusileng-Hangwula tectonic belt and southern Mongolia terrane to the north.In addition,the detrital zircon age spectrum pattern of the Middle-Upper Permian sediments in the Yingen-Ejina area has a high similarity to typical rift basin or peripheral foreland basin.Based on the above results,combined with the latest published data from petrotectonics,paleomagnetism and tectonobiogeography,this dissertation proposes that the final closure of the Paleo-Asian Ocean in the Yingen-Ejina area occurred in the late Early Permian,i.e.after the deposition of the Amushan Maihanhada Fm.but prior to the deposition of Maihanhada Fm..During the Late Carboniferous-early Early Permian,the Paleo-Asian Ocean subducted southward and formed a trench-arc-basin system in the Yingen-Ejina area.In detail,the Engger Us area developed a trench or fore-arc basin,and the southern area of Engger Us developed rift basin due to back-arc extension.During the Middle-Late Permian,the Yingen-Ejina area developed a marine intracontinental rift basin due to post-collisional extension.Using structural deformation,fault chronology,magmatic-thermal event dating,and low temperature thermochronology data,this dissertation proposes that the Yingen-Ejina area has undergone 10 stages of tectonism and later reformation events since the Permo-Carboniferous times.Specifically,the late Early Permian event had a great influence on great influence on the Amushan Fm.,the lastest Permian-early Late Triassic event had strongest reformation effect on the Upper Paleozoic,and the late Middle Jurassic-earliest Cretaceous event and the Early Cretaceous event also had a strong reformation effect on the Upper Paleozoic in the Yingen-Ejina area,while the rest of the tectonism and later reformation events had a relatively weak reformation effect on the Upper Paleozoic.The form of later reformation in the Yingen-Ejina area mainly includes structural deformation,uplift and erosion,superimposed deep-burial,dismemberment,and thermodynamic reformation.These 5 types of later reformation in the Yingen-Ejina area differentially superimposed with different intensity in different regions asynchronously.In terms of the effect of later reformation,the superimposed deep-burial is favorable for the preservation of the Upper Paleozoic,the rest reformation forms are unfavorable to the preservation of the Upper Paleozoic in the Yingen-Ejina area.In addition,the geodynamic background of each reformation events in the Yingen-Ejina area is quite different.In detail,the late Early Permian event was affected by the closure of the Paleo-Asian Ocean.The several Mesozoic events were sequentially influenced by the evolution of the paleo-Tethys Ocean to the south,the Mongol-Okhotsk Ocean to the north,and the paleo/morden Pacific Ocean system to the east.The Cenozoic event was closely related to the episodic sinistral strike-slip activities of the the Altyn Tagh Fault to the southwet and the East Gobi Fault to the northeast.According to the basin proto-type,hydrocarbon source conditions and later reformation of the Permo-Carboniferous basin in the Yingen-Ejina area,this dissertation proposes that there are 3 different levels of hydrocarbon preservation-accumulation units and prospective areas.And the most favorable hydrocarbon preservation-accumulation unit and prospective target for Permo-Carboniferous hydrocarbon exploration is the underlying Permrian rocks in the Mesozoic Guaizihu-Hari Sag.In addition,the uplifting Upper Paleozoic strata in the Cretaceous faulted-sag may form buried-hill hydrocarbon play,which is also an important target for Permo-Carboniferous hydrocarbon exploration in the Yingen-Ejina area.