Late Ordovician to Early Silurian Volcanic Activity in the Eastern Section of the North Qilian Orogenic Belt, China: Constraints on the Evolution of the Proto‐Tethys Ocean

ABSTRACT The conjunction of the Qinling‐Qilian Orogenic Belt is a critical area for understanding the tectonic relationship between the eastern and western sections of the Central China Orogenic Belt. It serves as an important region for studying the tectonic framework of the northern margin of the Proto‐Tethys Ocean. This paper identifies a series of meta‐volcanic rocks in the Fanjiaqu area of Fenggeling Town, situated in the eastern section of the North Qilian Orogenic Belt. To determine the formation age, petrogenesis, and tectonic environment of these volcanic rocks, we conducted systematic studies that include petrology, geochemistry, isotope geochronology, and zircon Lu‐Hf isotope analysis. Petrographic analysis reveals that the protoliths of the meta‐volcanic rocks comprise an assemblage of basalt, andesite, dacite, and rhyolite. LA‐ICP‐MS zircon U–Pb dating indicates that the protoliths of the meta‐basaltic andesite and meta‐dacite were formed at 441.9 ± 4.1 Ma and 443.9 ± 2.6 Ma, respectively. Geochemical characteristics show that the meta‐basalt‐andesite has medium SiO 2 contents (47.10% to 59.64%), higher TFe 2 O 3 and TiO 2 contents, lower Cr and Ni concentrations, Mg # values ranging from 43 to 53 and La/Nb ratios between 4.53 and 5.97. In contrast, the meta‐dacite and meta‐rhyolite exhibit high SiO 2 (67.62% to 75.18%) and Al 2 O 3 contents, along with low TFe 2 O 3 , TiO 2 and P 2 O 5 contents, as well as Mg # values ranging from 42 to 55. Overall, the meta‐volcanic rocks display notable negative Eu anomalies, with Eu/Eu* values varying from 0.76 to 0.83 for basic rocks and from 0.47 to 0.79 for acidic rocks. The meta‐volcanic rocks are enriched in large‐ion lithophile elements (LILE), such as Th and U, while being depleted in high‐field‐strength elements (HFSE) like Nb, Ta and Ti, indicating geochemical characteristics similar to those of arc magmatic rocks in subduction zones. The zircon ε Hf (t) values of the meta‐dacite range from +4.5 to +13.1, while the two‐stage model ages (T DM2 ) range from 1135 to 588 Ma. These characteristics suggest that the meta‐basalt‐andesite derived from partial melting of the mantle, metasomatised by subduction fluids, whereas the meta‐dacite and meta‐rhyolite are products of crust–mantle mixing. Integrating regional geological data, this study proposes that the meta‐volcanic rocks in the Fenggeling area formed in a continental margin arc environment, while the eastern section of the North Qilian Orogenic Belt was in the subduction setting of back‐arc oceanic crust during the Late Ordovician to Early Silurian.