A Late Carboniferous volcano–sedimentary complex outcrops on the southwestern flank of the Bogda Mountains, NW China. The volcanic architecture, spatial relationships, and emplacement mechanisms of the lithofacies were studied in detail, providing a lithofacies context for previously published structural and geochemical studies and constraining the reconstruction of palaeo‐environments. Four distinct volcano–sedimentary facies are identified in the Qijiagou Formation: close‐packed pillow basalts, pillow basalts with interstitial sediments, hyaloclastites, and peperites. Textural features of the lithofacies, especially hyaloclastites and peperites, provide clear evidence for in situ fragmentation of lava flows, co‐volcanic sedimentation of limestone, and intimate interaction between lava, water, and sediments. They demonstrate autochthonous, subaqueous origin of the succession. Discovery of peperites and stratigraphic transitions of the related lithofacies indicate a progressively deepening subaqueous environment, resembling a volcanic evolution from early stage of eruption at a shallower level to increasingly subsiding basin with increasing eruption frequency of basaltic lava at greater water depth. Vesicularity of basalts, ambient pressure, and peperitic features indicate eruption depth at between 1,500 and 2,500 m. The deep extensional basin in the Baiyanggou area was coeval with bimodal volcanism, uplift, and granite intrusion along strike in the region, all indicating continental collision and post‐collisional extension. The substantial depth of the Baiyanggou Basin indicates that it may have been a back‐arc or even a residual oceanic basin.
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