Detrital zircon U-Pb ages of Middle Ordovician flysch sandstones in the western ordos margin: New constraints on their provenances, and tectonic implications

摘要

The southwestern Ordos Margin is located between the Alax massif to the northwest, the North China Craton (NCC) to the east, and the North Qilian Orogenic Belt (NQOB, part of the Chinese Central Orogenic belt) to the southeast. Controversy surrounds the tectonic setting of the Western Ordos Margin in the Early Paleozoic; key questions are whether the Alax massif connected to the NCC in the Early Paleozoic, when exactly this event did happen, where the Lower Paleozoic sedimentary rocks along the southwestern Ordos had their provenances, and whether the so-called Helan aulacogen formed in the Ordovician. Laser-ablation inductively coupled plasma mass spectroscopy U-Pb dating of detrital zircons in four samples from Middle Ordovician sandstones in the Xiaoluo Shan and Niushou Shan located in the Western Ordos yield a majority of~ (206)Pb/~(238)U ages between 800 and 1000 Ma, a smaller group yields ~(206)Pb/~(238)U ages between 500 and 600 Ma; there are a few zircons with other ages, including some older than 2400 Ma. The youngest ~(206)Pb/~(238)U ages of the four samples are similar and range from 533 ± 3 Ma to 556 ±4 Ma. These zircon U-Pb ages combined with several lines of sedimentary evidence imply that the so-called Helan aulacogen did not exist in the Ordovician; a peripheral foreland basin related to the NQOB developed instead. The detritals mainly came from the Alax massif, the North Qilian Arc, and the Dunhuang massif; and the NCC can be excluded as an important provenance. This study also suggested that the Alax massif did not connect with the NCC before the Middle Ordovician, and that the Alax is similar to the Yangtze Craton or the Tarim Craton with a majority of intrusions in the range of 800-1000 Ma and containing Precambrian tillites, which are characteristics that differ from the NCC.