Geochemical characteristics of clastic rocks from the Jurassic Zhiluo Formation in Lingwu, Ningxia and analysis of tectonic background of the source area

This paper is the result of geological survey engineering.

Objective The Zhiluo Formation represents a transitional phase in the lake's advancement and retreat within the broader context of Jurassic geological evolution. Its tectonic background holds paramount importance in exploring the inception timeline of fault-fold belt development along the western margin of the Ordos Basin. This study focuses on the Zhiluo Formation, specifically clastic rocks, from the Jurassic period in the Lingwu area, Ningxia.

Methods We conducted analyses using X-ray and ICP-MS techniques to uncover the geochemical characteristics of clastic rocks and the structural context of the source region.

Results Our results reveal that major element correlation coefficients are generally low, while trace elements and REE exhibit high values. Notably, correlation coefficients between Al₂O₃ and trace elements such as Co, Ni, Cr, V, Sc, Li, Cs, Be, Ga, Tl, Cu, Pb, Zn, and Sn exceed 0.9, and the correlation coefficients between TiO₂ and Nb are at 0.98. This suggests that the primary source of sediment is terrigenous clastic material. Enrichment concentration-type elements (K>1, C_V>1) include Zr, U, and CaO, while enrichment dispersion-type elements (K>1, C_V < 1) consist of Al₂O₃, Fe₂O₃, MgO, Sr, Ba, U, Co, V, Sc, Li, and Pb. Poorly dispersive elements (K < 1, C_V < 1) encompass SiO₂, TiO₂, MnO, Na₂O, K₂O, P₂O₅, Rb, Th, Nb, Ta, Ni, Cr, Cs, Be, Ga, Tl, Cu, Zn, As, Sn, and REE. No elements fall under the poor concentration type (K < 1, C_V>1).

Conclusions The average values for key oxide combinations of main elements are TiO₂ (0.54), TFe₂O₃+MgO (4.58), Al₂O₃/SiO₂ (0.58), and Al₂O₃/SiO₂ (0.18), indicating similarity to an active continental margin. A trace element spider diagram demonstrates the rocks of the Zhiluo Formation's richness in Rb, K, and U, which are large ion lithophile elements, and a significant deficit in Nb, Sr, P, and Ti. The chondrite-standardized REE distribution curve depicts a gentle right-leaning pattern, signifying LREE enrichment and HREE relative depletion. In terms of geochemical indices, the CIA (70.76-81.88) for mudstone exceeds the ICV (1.02-1.6) for sandstone, while mudstone ICV (0.7-1.14) remains mostly less than or equal to 1. This suggests that the Zhiluo Formation in the Lingwu area exhibits little to no recycling and is characteristic of primary sedimentation in the context of tectonic activity with a moderate degree of differentiation. Correlation diagrams indicates that tectonic background of the source region is primarily associated with an active continental margin. Additionally, it exhibits some links to continental island arc systems, suggesting an active continental margin or an active continental margin subduction zone, potentially resembling an Andean-type active continental margin.