Facies architecture of a subaqueous volcano–sedimentary succession on Bogda Mountains, NW China—Evidence of extension in Late Carboniferous

Abstract

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.

The Pennsylvanian Composite Volcanism in the Bogda Mountains, NW China: Evidence for Postcollisional Rift Basins

“…The sedimentary rocks consist mainly of fine-grade clastic rocks, limestones, and volcanic breccias, with no ophiolites [49]. The present average elevation of the BMs is above 4000 m, which is induced by the late Cenozoic India-Asia collision [50]; the Carboniferous strata were deposited in an intracontinental rift basin [36] or a subduction-related forearc or back-arc basin [13,17,34,35,41]. The Haxionggou section, located in the central peak of the western BMs, is composed of the Mississippian Qierguositao Group, and the Pennsylvanian Liushugou and Qijiagou Formations from bottom to top, with no Devonian or even older strata ( Figure 2(a); [43]).…”

“…A typical Bouma sequence is developed at the bottom, and the whole formation is composed of marine sediments [51]. Notably, in the southern BMs of the Baiyanggou area, the unconsolidated limestone and magma clastic rocks are mixed to form peperites, and the lenticular limestones are distributed in pillow basalts [41].…”

“…The hornblende phenocrysts are few, commonly euhedral, and 0.1-0.5 mm long. The matrix consists mainly of plagioclase, clinopyroxene, volcanic glass, and opaque oxides, some of which have a trachytic texture (Figure 4 Besides, there are also some peperites in the Liushugou and Qijiagou Formations which were formed by magmasediment mingling (magma and unconsolidated limestone, [41]). The main components of the flowing slurry are plagioclases and quartzes, and the host sediments are mainly calcites.…”

“…Due to the controversy on the beginning time of the postcollisional process, the Late Paleozoic (especially the Pennsylvanian) geodynamic mechanism of the CAOB is disputed. The Late Paleozoic Bogda Mountains (BMs) in the southern CAOB have been interpreted to represent a subduction-related island arc [34,35] or back-arc [13,17], mantle plume [36], ridge subduction [37,38], postcollisional extension ( [39]; Zhang et al, 2014; [40]), and local strike-slip faulting [41,42]. These disputes suggest the need to find more solid evidence to interpret the tectonic evolution of the CAOB.…”

“…The Liushugou and Qijiagou Formations of the BMs in the southern CAOB ( Figure 2), belonging to the Late Paleozoic [41,43], could give constraint on this controversial period. The completed successions, exposed along the western and eastern BMs, provided an excellent opportunity to uncover their petrogenesis and geodynamic background.…”