Provenance of Ediacaran (Sinian) sediments in the Helanshan area, North China Craton: Constraints from U-Pb geochronology and Hf isotopes of detrital zircons

- The Neoproterozoic zircons with peaks at 818 ± 4 and 905 ± 8 Ma from Ediacaran system and Early Cambrian series was found for the first time in the Helanshan area, even in the western part of NCC.
- According to the U-Pb age geochronology and Hf isotope characteristics of detrital zircons in this study, the Neoproterozoic igneous or sedimentary rocks in the Alxa block may provide the clastics for the Neoproterozoic system and Early Cambrian series in the Helanshan area.
- The Alxa block and NCC had joined with each other by the late Neoproterozoic, even earlier.

The tectonic relationship between the Alxa Block and the North China Craton has long been controversial. The Helanshan area lies at the western margin of the Ordos Block and to the east of the Alxa Block, and it contains Eidacaran Zhengmuguan Formation and Tuerkeng Formation. The Zhengmuguan Formation is made up of tillites, limestone and dolomine, and the Tuerkeng Formation consists of silty slate. A parallel unconformity marks the boundary between the Tuerkeng Formation and the Early Cambrian Suyukou Formation, which is composed mainly of phosphatic pebbly sandstone and sandstone. U-Pb dating ages of detrital zircons in the Zhengmuguan and Suyukou Formations reveal two Neoproterozoic age peaks of 818 ± 4 Ma (n = 88) and 905 ± 8 Ma (n = 20). Neoproterozoic magmatic events have yet been reported in the basement of the Ordos Block, while some Neoproterozoic igneous rocks have been found in the basement of the Alxa Block. Moreover, the two Neoproterozoic age peaks correspond well with the ages of Neoproterozoic igneous rocks in Alxa Block. The Hf isotopic characteristics of a part of Neoproterozoic zircons from the Ediacaran Zhengmuguan Formation in the Helanshan area (εHf(t) = -7.812 to 3.274, TDMC = 2211-1578 Ma, n = 10) are similar to those Neoproterozoic igneous zircons from the Langshan area (εHf(t) = -1.105 to 5.928, TDMC = 1.75-1.38 Ga, n = 23) and Bayinnuoergong area in the Alxa Block. The parental magmas had a long residency time in the crust. It seems, therefore, that Alxa Block provenance provided Neoproterozoic clastics for Helanshan area in Ediacaran, and Alxa Block had already been a part of the North China Craton by the late Neoproterozoic, even earlier.