Provenance  and Parentrock Types  of Member 7 of Yanchang Formation(Triassic), Ordos Basin

Key words: 
• light and heavy mineral

Abstract: The research shows that there are 5 directions of provenances existed in Chang 7 period of Ordos Basin, that the northeast and southwest are main provenances, northwest, south and west as the secondary provenances. The lithological characteristics of parent rock from synthetic analyzing the light and heavy mineral characteristics and debris feature are inferred. Meanwhile, combining with the method of recovering relationship among heavy minerals reveals the feature of  parent rocks.
  According to the research, the basic magmatic rock and the highgrade metamorphic rock are main part in the rocks of the northeast area; The sedimentary rock and the lowgrade metamorphic rock are main part of the rock in the southwest area; The lowgrade metamorphic rock, dolomite rock and sedimentary rock are main part in the rocks of northwest area; The sedimentary rock and the lowgrade metamorphic rock are main part in the rocks of north area; The sedimentary rock is main part in the rocks of west area. The REE pattern is similar to upper crust and the geochemical diagrams of HfLa/Th, La/ScCo/Th and REELa/Yb indicate that the mudstones come from felsic rock area with less bedrock and andesite. (Gd/Yb)N>2 shows that the provenance of northeast should be from Archean strata, and (Gd/Yb)N<2 shows that the provenance of other areas should be from the strata above Archean.
  On this basis, by arranging the lithology of bedrock in the margin of Ordos Basin, it recovers that granulitefacies metamorphite series are outcropped in the northeast area. The degree of metamorphism can be greenschistfacies to amphibolitefacies of lowmedium pressure type in the southwest area. The metamorphism has reached granulitefacies to amphibolitefacies and greenschistfacies in the bedrock of northwest area. While greenschist facies to amphibolite facies of low pressure is the important metamorphism in the south area, and granulitefacies on local area.
  Conclusion demonstrated that the parent rock can correspond to the bedrock in the northeast area, southwest area and west area. The highgrade metamorphic bedrock is developed in the northwest area and south area which were not found at the parent  rock in these deposition areas. Therefore, it was inferred that the old bedrock in these areas has not uplifted at Chang 7 period, and not providing source to basin.