Transition zone origin of potassic basalts from Wudalianchi volcano, northeast China

摘要

The origin of historic potassic basalts (from the 1719 to 1721 AD eruption) from the Wudalianchi volcanic field, northeast China, is investigated. The samples examined in this study are aphyric, and contain -5 vol.% olivine microphenocrysts. Geochemical analyses of the samples, along with published data for material from the 1719 to 1721 AD eruption, show that the basalts exhibit linear compositional trends in whole-rock variation diagrams, and are characterized by K2O enrichment (>4 wt%) and an EMl-like isotopic signature. Using thermodynamic calculations and the observed penological features of the basalts, the temperature of the magmas shortly before eruption is estimated to have been～1250 °C, and the water content of the magma at depth is estimated to have been >~1.1 wt.%. Because the estimated temperatures are significantly higher than the projected maximum temperature of the lithospheric mantle beneath the Wudalianchi volcanic field, the magmas were likely derived from the asthenospheric mantle. We suggest that both the potassic- and EMl-like natures of the basalts originated from the mantle transition zone, metasomatized by K-rich sediment fluids ～1.5 Ga ago through a stagnation of an ancient slab. The linear whole-rock compositional trends of the basalts primarily reflect the geochemical heterogeneity in the mantle transition zone, with variable but coupled contributions of ～1.5 Ga sediments in the ancient stagnant slab, and recent sediments and peridotites in the stagnant Pacific slab. We infer that the Wudalianchi magmatism was caused by an upwelling of a hydrous mantle plume from the mantle transition zone, which was hydrated through the stagnation of the ancient subducted slab and the recent Pacific slab.