Basaltic cognate enclaves from Dokdo Island as a window for intraplate mafic alkaline OIB magma dynamics in a back-arc basin

Volcanoes with ocean island basalt (OIB)-like characteristics occur in all settings. Their origin is particularly enigmatic when they occur in close spatial and temporal association with subduction-related volcanism (e.g., back-arc basin basalts; BABB). A set of cognate enclaves and a late-intruding dike from Dokdo Island (Republic of Korea) were used to probe an intraplate mafic alkaline plumbing system developed in a back-arc basin. Detailed petrography, mineral chemistry, and whole-rock major- and trace-element geochemistry were combined with geothermobaro-hygrometric and whole-rock geochemical models. The cognate samples are defined as basalt–trachybasalt and gabbro, while a late-intruding dike has basaltic trachyandesite composition. In general, plagioclase crystals in the samples are classified as labradorite–bytownite, with cores richer in anorthite component compared to rims. Clinopyroxene crystals are sector-zoned diopside characterized by [2 IV Si 4+ + VI Mg 2+ ] basal ↔ [2 IV Al 3+ + VI Ti 4+ ] prism . The prism sector-based thermobarometric models suggest that the cognate enclaves may represent multi-stage magmatic conditions from near-source (> 1250 °C and 1.5 GPa) through underplating (~ 1220 °C at ~ 640 MPa) to intermediate (~ 1180 °C at ~ 400 MPa) stages. Shallow crustal stagnant conditions (~ 1100 °C at ~ 120 MPa) are estimated from the late-intruding dike. Clinopyroxene- and plagioclase-based hygrometric models give a constant H 2 O abundance of ~ 1.5–2 wt% during the Dokdo basaltic magma evolution and ascent. According to our trace-element models, deep near-source parental melt may result from ~ 35% partial melting of hornblendite + spinel-peridotite and the associated mixing with clinopyroxene mush-derived 0.5% partial melt by a ratio of 0.6:0.4. From the parent, crystallization of up to 50% with the fractionating proportions of 0.7 plagioclase + 0.3 clinopyroxene can produce the whole compositional range of the Dokdo mafic alkaline magma. Rare or absent olivine in the early formed enclave may suggest that clinopyroxene and olivine are, respectively, crystallized at high and low pressure due to depressed olivine liquidus via melt compositional effect. We suggest that, during back-arc basin opening, BABB-related melt percolation into the extensional cracks in the lithospheric mantle and accompanying metasomatism provide hornblenditic source for the subsequent OIB magmatism.