| The Late Triassic Yanchang Formation in Ordos Basin is the most important reservoir of oil and gas resources in the whole basin.The determination of its sedimentary boundary plays an important role in the exploration and development of oil and gas.Shigouyi and Ciyaobao are located in the middle of the western margin of the Ordos Basin.The establishment of provenance attributes in this area is of great significance to the restoration of sedimentary boundary in the western margin of the basin and the accurate prediction of oil and gas reservoirs in the western margin of the basin.Previous studies on provenances in the western margin of the basin mainly focused on the Helan Mountains,while the central part of the western margin of the basin lacked systematic provenance studies.On the basis of field outcrop observation and sample collection of the Late Triassic Yanchang Formation in Ciyaobao and Shigouyi,this paper studies the provenance and paleotectonic environment of the Ciyaobao and Shigouyi areas in the western margin of the basin by means of modern high-precision testing methods such as rock slices,heavy minerals,paleocurrent,cathodoluminescence,detrital zircon LA-ICP-MS U-Pb dating,etc.At the same time,this paper attempts to compare the zircon age of the extension period in Helan with that in Ciyaobao and Shigouyi areas on the western margin of the basin,in order to explore whether the central part of the western margin of the basin has the same sediment source area as that in Helan Mountains.Heavy mineral analysis shows that the early assemblage of the western margin of the Ordos basin is garnet+epidote+magnetite,while the middle assemblage is hematite+garnet+ilmenite.The early stage of the parent rock area is magmatic rock and metamorphic rock,and the middle stage is sedimentary rock and metamorphic rock.Paleocurrent analysis shows that the paleocurrent in the study area is mainly from northwest to Southeast and from west to east.The observation of rock slices shows that the debris in sandstone samples are mainly volcanic and metamorphic debris.Through LA-ICP-MS U-Pb dating of Yanchang Formation in Helan and Yanchang Formation in western margin of the basin,three age peaks of late Paleozoic(300 Ma),early Proterozoic(1900 Ma)and Neoarchean(2500 Ma)were obtained.Comparing the test results with the zircon age distribution of the surrounding tectonic units,combining with the paleocurrent,heavy minerals and rock slice observation,it is concluded that the provenance area of the Late Triassic study area has three directions,namely,magmatic rocks?pre-Cambrian metamorphic rocks in Alashan area,metamorphic rocks in Kongzite belt and pre-Mesozoic sedimentary rocks in Bayanhaote area.In the early stage of Late Triassic,Alashan area and Konzite belt are the main provenance areas in the study area,while Bayanhot area is a uplift slope area,which is not denuded intensely and has insufficient provenance supply to the study area,and is the secondary provenance area.In the miaddle stage,the uplift and denudation of Bayanhot area were intense because of the compression of the peripheral tectonic movement,which provided a large number of clastic material sources for the study area and the main provenance area of the study area,while the control of the Alxa Block and Kongzi Rock Belt on the margin of the study area was weakened and became the secondary provenance area of the study area.The heavy mineral assemblages in the study area are very similar to those in Helan Mountains.Garnet-epidote is dominant.From Rujigou in Helan Mountains to the central part of the western margin,the proportion of stable minerals in heavy minerals increases gradually,while the proportion of unstable minerals decreases gradually.At the same time,combined with the NW-SE direction of paleocurrent in Shigouyi area,we conclude that the "Yinchuan paleo-uplift" did not exist in the Late Triassic and that the Helan Mountains area was not separated from the Ordos Basin.The Helan Mountains area is a part of the Ordos Basin. |
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