Geochemistry and petrogenesis of Quaternary volcanic rocks from Ulleung Island, South Korea

Abstract

Ulleung Island is a Quaternary intraplate alkaline volcano located in the central-western part of the East Sea (Sea of Japan) back-arc basin. Previous studies have divided the geological history of the volcano into five stages: Stage-1 trachybasalt, Stage-2 and -3 trachytes, Stage-4 phonolite, and Stage-5 leucite-bearing trachyandesite. This study presents geochemical compositions, including Sr–Nd–Pb–Li isotopes, for the volcanic rocks. The trachytes and phonolite were formed by fractional crystallization of trachyandesitic magma at different depths. The trachybasalts have highly fractionated REE patterns with (La/Yb)N values of 23.6–30.7 and lack Eu anomalies. In a primitive-mantle-normalized multi-element distribution diagram, the trachybasalts show OIB-like LILE enrichment patterns without HFSE depletion. However, they show negative K anomalies, suggesting hydrous residual potassic phases in the source. The lithology and physical conditions of the mantle source for the Ulleung basalts are estimated to have been a pyroxenite fraction of 4 ± 2%, a mantle potential temperature of 1303 ± 8 °C, a minimum pressure of basalt segregation (Pmt) of 2.5 ± 0.0 GPa, a temperature of the melt at Pmt of 1327 ± 8 °C, and a degree of melting of 1.9 ± 0.6 wt%. The basalts have EM1-type OIB-like Sr–Nd–Pb isotopic compositions (87Sr/86Sr = 0.70472–0.70507; εNd = −4.5 to −1.8; 206Pb/204Pb = 17.95–18.09). The Stage-5 trachyandesites lack pronounced negative K anomalies and are characterized by slightly more depleted Sr–Nd–Pb isotopic compositions (87Sr/86Sr = 0.70445–0.70453; εNd = −1.3 to −1.2; 206Pb/204Pb = 18.06) compared with the Stage-1 trachybasalts, implying their origin from a relatively dry source. δ7Li values of the basalts range from −8.8‰ to +1.9‰, but the trachyandesites are relatively heavy, having δ7Li values ranging from +3.5‰ to +6.3‰, suggesting differential loss of low-temperature 7Li from a recycled oceanic slab in the source. The differentiated Stage-2 to −4 trachytes/phonolite have Sr–Nd–Pb–Li isotopic systematics resembling those of the trachyandesites, but the effects of assimilation of thinned lower continental crust during magma evolution are observed in some samples. The Ulleung magmatism was likely generated by wet upwelling from the stagnant slab in the mantle transition zone in combination with small-scale/edge-driven convection in the asthenosphere.