Middle Ordovician Intra‐Oceanic Arc Magmatism in the Northern Qilian: Evidence From Gabbro and Basalt in the Huangcaogou Area, Qilian County

ABSTRACT To investigate the tectonic evolution of the Proto‐Tethys Ocean, this study focuses on ophiolites that record the evolution of the North Qilian Ocean basin—a key representative remnant of the Proto‐Tethys Ocean. It aims to provide critical constraints on the tectonic history of the North Qilian Ocean by constraining the petrogenesis of magmatic rocks in the Huangcaogou area, North Qilian orogenic belt. We conducted a systematic campaign of petrological, LA‐ICP‐MS zircon U–Pb geochronological, whole‐rock major and trace element geochemical, and clinopyroxene mineral chemical analyses, coupled with zircon Hf and whole‐rock Sr–Nd–Pb isotopic analyses on gabbros and basalts from the Huangcaogou area. LA‐ICP‐MS zircon U–Pb dating yields a crystallization age of 463 Ma for the gabbros, indicating their emplacement in the Middle Ordovician. Whole‐rock compositions of the gabbros and their hosted clinopyroxenes both exhibit tholeiitic affinity, whereas the basalts display calc‐alkaline affinity. Both rock suites are characterized by right‐skewed rare earth element (REE) patterns with light REEs (LREEs) enriched relative to heavy REEs (HREEs), enrichment in large‐ion lithophile elements (LILEs; e.g., Ba, Th, U), and depletion in high‐field‐strength elements (HFSEs; e.g., Nb, Ta, Ti), collectively indicating a prominent subduction‐zone magmatic affinity. Zircon εHf(t) values of the gabbros range from +9.2 to +14.5, and their whole‐rock εNd(t) values vary between +3.82 and +5.56. The basalts have whole‐rock εNd(t) values of −0.57 to +1.83. Whole‐rock Pb isotopic compositions of both rock suites exhibit an Indian‐type mantle affinity. The gabbros and basalts in the Huangcaogou area were both derived from a depleted mantle that had been metasomatized by subduction‐related components. Integrated results suggest that these rocks represent arc magmatic products formed by partial melting of a subduction‐modified depleted mantle during the northward subduction of the North Qilian Ocean.