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Geology editing sample

Before 

Abstract: The southwestern Ordos is located between the Alxa 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 in the Early Paleozoic, the key questions are whether the Alxa massif connected to the NCC in the Early Paleozoic or not, when this happened, where the provenances of the Lower Paleozoic sedimentary rocks along the southwestern Ordos were, and whether the so called Helan aulacogen formed in the Ordovician or not. The LA-ICP-MS U-Pb dating of the detrital zircons of four samples from the Middle Ordovician sandstones in the Xiaoluo Shan and Niushou Shan located in the Western Ordos shows that majority of zircon grains yield 206Pb/238U age between 800 and 1000 Ma, the less group yields 206Pb/238U age between 500 and 600 Ma, zircons with other ages are few, and some zircons older than 2400 Ma are also found. The youngest 206Pb/238U ages of the four samples are similar and range from 533±3Ma to 556±4Ma. These zircon U-Pb ages combined with several lines of sedimentary evidence imply that the so called Helan aulacogen in the Ordovician did not exist, a peripheral foreland basin related to the NQOB developed instead. The detritals mainly came from the Alxa massif and the North Qilian Arc and the Dunhuang massif, the NCC can be excluded as the important provenance. And this study also suggested that the Alxa massif did not connect with the NCC at least before the Middle Ordovician, and the Alxa massif shows many similarities to the Yangtze Craton or the Tarim Craton such as more intrusions of 800-1000 Ma in age and pre-Cambrian tillites, which are different from the NCC.

After

Abstract: The southwestern Ordos is located between the Alxa 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 Alxa 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 206Pb/238U ages between 800 and 1000 Ma, a smaller group yields 206Pb/238U ages between 500 and 600 Ma; there are a few zircons with other ages, including some older than 2400 Ma. The youngest 206Pb/238U 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 Alxa 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 Alxa massif did not connect with the NCC before the Middle Ordovician, and that the Alxa 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.