Paleoproterozoic orogenic event in the western North China Craton: Insights from zircon U-Pb-Lu-Hf isotopes and geochemistry of meta-supracrustal rocks in the Beidashan complex, Alxa block

Abstract

Recent studies have highlighted Paleoproterozoic magmatic and metamorphic events within the Alxa block. Although these geological records are generally believed to be associated with Paleoproterozoic orogenic events, the nature and location of the potential orogenic belt remains poorly understood. In this paper, we systematically investigate U-Pb-Lu-Hf isotope analyses of detrital zircons, alongside whole-rock major and trace element analyses from a suite of Paleoproterozoic meta-supracrustal rocks in the Beidashan complex, to provide new insights into this enigmatic orogenic event.

The formation age of the upper sequence of the meta-supracrustal rock suite is constrained to 1.89 ∼ 1.83 Ga, with detrital zircons exhibiting a unimodal distribution centered at ∼ 1.96 Ga and Hf model age peaks at ∼ 2.4 Ga and 2.6 ∼ 2.7 Ga. In contrast, the deposition age of the lower sequence is constrained to 2.08 ∼ 2.05 Ga, with detrital zircons show age peaks at ca. 2.20 Ga, 2.30 Ga and 2.41 Ga, and a Hf model age peak at ∼ 2.95 Ga. Geochemical analyses reveal that amphibolites from the upper sequence are enriched in light rare earth elements (LREE) and show negative Nb and Ta anomalies. Conversely, amphibolites from the lower sequence display relatively low REE differentiation and are depleted in high field strength elements (HFSE). Both sequences exhibit Nb-Ta troughs, characteristic of typical continental arc basalts and andesites. The amphibolites from the upper sequence are more enriched in LREE, which likely indicates their formation in a late-orogenic environment.

The detrital zircons from the Beidashan complex exhibit age distributions and Lu-Hf isotope compositions that align with the Archean-Paleoproterozoic magmatic rocks exposed in the Alxa block. This alignment suggests that the Beidashan, Longshoushan and Bayanwulashan regions to the south of the Alxa block likely served as the source area for the lower sequence, while the Diebusige region to the north may be the provenance for the upper sequence. This provenance shift around ∼1.9 Ga likely reflects the uplift of an accretionary orogen north to the research region, which not only introduced new juvenile material into the sedimentary record but also stopped the original material supply from the south. Therefore, we propose that the Alxa block experienced an orogenic event north to the Beidashan complex during 2.1 to 1.8 Ga, potentially related to the subduction and collision between an unknown continent and the Alxa block along its northern margin, contributing to the formation of the Columbia supercontinent.