Eastern Helanshan Late Triassic Sedimentary Provenance And Its Significance In Ancient Tectonic Environment

Helanshan is located in the northwestern margin of the Ordos Basin, which is in the north section of the important geological structural boundary between Eastern and Western China. The present structural features are complex in Helanshan because it was experienced multiphase tectonic movements. The Late Triassic strata widely distribute in Helanshan area, which have certain similarities with Ordos Basin. The research of Late Triassic sedimentary and tectonic environment of Helanshan has got some achievements, but there is still controversial on sedimentary provenance. The provenance of Late Triassic in eastern Helanshan is an important part for determining the original sedimentary boundary and environment in northwestern Ordos Basin in Late Triassic. it can also provide an important basis for establishment and improvement of the evolution of space-time coordinates of the Ordos Basin. Meanwhile it is of great significance to comprehensively evaluate sedimentary reservoir in Late Triassic of the northern margin of Ordos Basin.Based on the field geological observations and sample collection of Late Triassic sedimentary outcrops in eastern Helanshan, it uses a variety of modern high-precision testing and analysis techniques, such as the analysis of detrital composition, the components of the pebbles, the paleo-currents, heavy mineral composition, constant, trace and rare earth elements features and detrital zircon U-Pb dating. Provenance feature and tectonic environment is researched in eastern Helanshan. At the same time, together with the Late Triassic sedimentary strata distribution and source characteristics of western Helan and of the western Ordos Basin, the paleo-tectonic environment of Helanshan is discussed.A single factor source analysis and multifactor comprehensive source analysis is proposed from the five aspects of general direction of provenance, tectonic setting, source rock properties, source age dating, source correlation analysis of Helanshan and adjacent areas to carry on the provenance discussion. It is revealed that the Late Triassic sources of east Helanshan were to the west and northwest of Helanshan mainly with metamorphic rocks, sedimentary rocks and magmatic rocks. The sedimentary environment of Late Triassic in eastern Helanshan was semi-hypoxic to hypoxia weakly reduction environment and terrestrial freshwater environment, Yanchang Formation with the five sections formed a fining upward sedimentary cycle with depositional environment changing from alluvial fan to braid fluvial deposit to fan delta front and then to semi-deep lake and deep lake. Regionally it was in a compressive environment with local extensional setting. The result of detrital zircon U-Pb dating shows the provenance of Triassic Yanchang Formation in western was coincident with eastern of Helanshan with strong similarity in age distribution, which suggests that there was no source to the eastern region. Helanshan Yanchang Formation had three major source areas: the northwestern source was Indosinian-Hercynian magmatic rocks and Precambrian metamorphic rocks in Alxa area, the western source was Pre-Mesozoic sedimentary rocks in the present Bayanhaote area and the northern source was Paleo-Proterozoic metamorphic rocks (khondalite series) in Hetao area. There was no provenance from east to west in the eastern Helanshan in Late Triassic. Therefore, the so called "Yinchuan Paleo-uplift" did not exist to the eastern Helanshan in Late Triassic.