Petrogenesis and geochemistry of the Late Carboniferous rear-arc (or back-arc) pillow basaltic lava in the Bogda Mountains, Chinese North Tianshan

• The Bogda pillow basaltic lavas were erupted at ~ 311 Ma.
• Different characteristics of the pillow core and rim suggest post-magmatic seafloor hydrothermal alteration.
• The Bogda pillow lavas were sourced from a dry and depleted mantle source metasomatized by sediment-derived melts.
• The Bogda Late Carboniferous basalts were likely generated at a rear-arc or back-arc environment.

The tectonic nature of the Chinese Tianshan Orogen during the Late Paleozoic has been long disputed. With aims of providing constraints on this issue, an integrated study of geochronology and geochemistry has been carried out on the Late Carboniferous pillow basaltic lava of the Qijiagou Group from the Bogda Mountains, Chinese North Tianshan. Zircon SHRIMP U–Pb dating of a dacite ignimbrite, which is in conformable contact with the pillow lava, suggests that they were erupted at ~ 311 Ma. The pillow cores and rims show different petrological and geochemical characteristics, suggesting post-magmatic seafloor hydrothermal alteration. Nevertheless, both pillow cores and rims have the MORB-like Sr–Nd–Hf isotopes and arc-like trace element compositions. Clinopyroxene and plagioclase from the pillow lavas are compositionally different from those of the mafic rocks related to the Tarim mantle plume. These observations, together with the tholeiitic index (THI > 1) and the Fe/Mn ratios (53–57) of them, indicate that the Bogda pillow lavas may have been generated from a dry and depleted mantle source metasomatized by sediment-derived melts. Compared with basalts of the Izu–Bonin arc-back-arc system, the Bogda Late Carboniferous basaltic lavas show great resemblance to the Izu–Bonin rear-arc basalt (including the arc-like back-arc basalt) in terms of major and trace element and mineral compositions. It suggests that these basalts were likely formed in a rear-arc or back-arc environment.