Geochronology and Geochemistry of the Xianshuihu Formation Volcanic Rocks in the Beishan Orogenic Belt, Northwest China: Implications for the Early–Middle Ordovician Tectonic Setting

The Beishan orogenic belt, in the southern margin of the Central Asian Orogenic Belt (CAOB), records a complex Paleozoic tectonic history, yet the nature and timing of its early magmatism remain debated. This study presents new petrological, geochemical, zircon U–Pb geochronological, and Lu–Hf isotopic data, combined with fossil biostratigraphic constraints, for volcanic rocks of the Xianshuihu Formation to constrain their formation age, petrogenesis, and tectonic setting. LA-ICP-MS zircon U–Pb dating of rhyolitic tuff yields a weighted mean age of 472.6 ± 3.7 Ma, indicating Early to Middle Ordovician volcanism and falling within the range of published ages (490.2 ± 2.9 Ma and 462 ± 3 Ma) for related volcanic units. Zircons from the rhyolitic tuff have 176Hf/177Hf ratios of 0.283077–0.283371, and positive εHf(t) values of +19.7 to +28.2, plotting in the depleted mantle field. The volcanic rocks have mostly primitive mantle-like Zr/Hf (33.40–39.13) and Nb/Ta (7.53–18.87), but low Nb/U (3.81–9.29), Ce/Pb (4.03–7.91), and Mg# (52–62). They are enriched in LILEs (e.g., Ba, U, Pb) and LREEs, depleted in HFSEs (Nb, Ta, P, Ti), with limited HREE fractionation. These geochemical fingerprints indicate arc-related magmatism and, when integrated with the deep-marine depositional environment indicated by graded and rhythmic bedding and graptolite fossils, are consistent with a continental arc setting. Our data do not support a direct link between these volcanic rocks and the ophiolite-bearing ocean basins in the Beishan area. Instead, we tentatively interpret them as products of southward subduction of the Zoolen Ocean beneath the Hanshan Block, which induced continental arc magmatism during the Early–Middle Ordovician.