| The Upper Carboniferous Benxi Formation in the Ordos Basin is the first set of sedimentary strata after a 150 Ma sedimentary discontinuity between the early and late Paleozoic.Due to the effect of strong tides on the transformation and redistribution of sediments in the pre-existing source-sink system,the conventional provenance analysis indicators of the Upper Carboniferous Benxi Formation in the South Ordos Basin have almost been destroyed,leaving the southern sediment sources still unsolved puzzle.In this paper,through the outcrop and core observations and related petrology,detrital zircon chronology,and trace element analysis and testing,the remaining delta and tidal environmental indicators and their symbiotic relationships were found,and the origin of the Benxi Formation sediments in the southern Ordos Basin was traced.,The nature and tectonic background of the source area were clarified,and the origin of the South Provenance of the Benxi Formation in the Late Paleozoic was limited,and then the tectonic paleogeography of the North China Plate and the Qinling Microplate in the critical period of the Late Carboniferous was discussed.The provenance analysis of the whole rock geochemical analysis results of the Benxi Formation shows that the sediments in the northern Ordos Basin are mainly related to the lithology formed by the upper crust felsic and the mixed source region from the deep water-rich mantle.The provenance area contains materials with various tectonic backgrounds,such as passive continental margins and active continental margins,and belongs to the orogenic belt formed by hyperplastic collisions.The sediment originates from the relatively stable source area,rather than the product of the source-sink system with strong structural activity.Samples in the southern part of the basin revealed that the source of their sediments was mainly medium-acid sources and some old passive continental margins.The structural background of the source area is relatively complicated,mainly the passive continental margin,the continental island arc and the Andean-type active continental margin,suggesting that the source area is mainly a complex orogenic belt formed by arc-land collision.The zircon age spectrum shows that there are obvious differences between the south and the north of the basin,suggesting that the sediments come from different provenance areas.The clastic zircon age values in the north of the basin are distributed in three concentrated areas of 290-319 Ma,1600-2500 Ma,and 3200-3300 Ma.The distribution of clastic zircon age values in the southern basin is mainly in 340-418 Ma,424-521 Ma,675-992 Ma,1010-1556 Ma,and 1656-3057 Ma.The comparison of the age spectrum with the peripheral plate shows that the source in the northern part of the basin is mainly from the northern margin of the North China Craton;the source in the south is mainly from the North Qinling and South Qinling on the south side.Zircon trace elements reveal that the zircon source rocks in the north of the basin are mainly hot,low-diversity metamafic magma,indicating that the zircon comes from intraplate magmatism in the background of regional extension;the south of the basin comes from cold,high-diversity Acidic magma comes from the magmatic arc or orogenic belt tectonic background.Combining sedimentary characteristics such as the sand body dispersion system of the Benxi Formation and the distribution of conglomerate,it is revealed that in addition to the northern Xingmeng orogenic belt previously thought,the source area of the Benxi Formation also provides provenance in the southern Qinling orogenic belt.It shows that Qinling Mountain has become the source area of the orogenic belt in the Late Carboniferous.Moreover,the South Qinling Mountains and the North Qinling Mountains have been united together at this time to provide detrital sources for the Ordos Basin.The south-north orogenic belt and the tidal-delta composite sedimentary system in the basin constitute the source-sink system of the Late Carboniferous Ordos Basin. |
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