Provenance and tectonic setting of the Upper Paleozoic Shihezi Formation in the northern Ordos basin

This study investigates the provenance and tectonic setting of the Upper Paleozoic Shihezi Formation in the northern Ordos basin using a comprehensive suite of analytical techniques. These include thin section petrography, heavy mincrals analysis, cathodoluminescence imaging of quartz grains, geochemical analysis of major and trace Clements, and zircon U-Pb geochronology (LATCP-MS). Our rcsults show that the detrital composition, heavy mineral assemblagcs, and cathodoluminescence characteristics of quartz grains from the He8 Mcmbcr of the Shihezi Formation point to a northern provenance. These characteristics display distinet west-to-east variations with obvious zoning on the plane, suggesting the presence of multiple parallel ancient water Systems transporting sediment. Chondrite-normalizcd REE patterns from both the northern outcrop arcas and basin samplcs arc consistent, further supporting a northern sourec. The REEs patterns displayed an obvious fractionation of light and heavy REEs, with a notable light REE enrichment and a moderate negative Eu anomaly. Furthermorc, major elements (Si02-K20/Na20, K20/Na20-Si02/Al203, TFe203 + MgO-TiO,) and trace element ratios (Th/Sc, Th/U), as well as the geochemical discrimination diagram of the tectonic settings, indicated a complex tectonic setting for the sourec area, encompassing active continental margin, passive Continental margin, and island arc. This aligns with the tectonic history of the northern Ordos basin margin during its collision with Yinshan block. Subsequent cyclic uplifts in the northern uplift zonc (Yinshan and Daqingshan) generated a distinet cyclic sourec, cxplaining the multi-cyclic characteristics observed in the detrital component discrimination patterns. Zircon U-Pb dating of metamorphic and igneous rocks in the northern outcrop area, as well as detrital zircons from basin sandstones, reveals two distinet age populations: 2. 0~1. 8 Ga and 2. 4 ~ 2. 2 Ga, both within the Proterozoic. The youngest detrital zircon age of 295.8 + 3.6 Ma indicates Sedimentation oecurred aftcr the Early Pcrmian, eoineiding with the uplift and denudation stage following the closurc of the paleo-Asian Ocean along the northern margin. The 2. 0~1. 8 Ga and 2. 4~2. 2 Ga zircon ages arc consistent with ages from the khondalite belt in the western block, indicating the provenance mainly originated from the khondalite belt and intrusive magmatic rocks in the north. Our comprehensive analysis demonstrates multiple parallel ancient water Systems transported sediment from the northern crosion area to the Upper Paleozoic Shihezi Formation in the northern Ordos basin. © 2024 Geological Society of China. All rights reserved.