Petrogenesis of submarine volcanic rocks dredged from Dokdo, Ulleungdo, and the neighboring seamounts in the East Sea: Constraints from mineral chemistry, geochemistry, and 40Ar/39Ar ages

Abstract

Several seamounts, including the Dokdo (DD), Ulleungdo (UD) Anyongbok (AYB), Simheungtack (SHT), and Isabu, were formed from intraplate volcanism in the southwest of the East Sea back-arc basin during the Quaternary. In the present study, mineralogy and geochemistry data generated using submarine volcanic rocks collected from the DD, UD, AYB, and SHT locations were utilized to investigate characteristics of the source, evolution, and storage conditions of the associated magmas. Analyses of representative samples from the DD, UD, and AYB using the 40Ar/39Ar method yielded plateau ages of 1.566–2.658, 0.533–0.714, and 1.736 Ma, respectively. The dredged samples consist of trachybasalt, phonotephrite, trachyandesite, and trachyte and have oceanic island basalt (OIB)-like characteristics. According to numerical modeling using trace element data, the primary magma linked to the formation of these samples can originate from ≤5% partial melting of an enriched garnet–peridotite facies metasomatized by melt derived from a subducted slab. The composition of major oxides, from the trachybasalts to trachytes, and mass balance calculations suggest that the trachyandesites and trachytes were formed via fractional crystallization from a basaltic melt. These diverse rock compositions probably highlight pulses of magma that separated from the main reservoir in the shallow crust. Results from mineral–melt thermometry indicate that the olivine and plagioclase in these basaltic rocks crystallized at temperatures of 1213 ± 56 °C and 1163 ± 23 °C, respectively. Thermobarometric calculations using clinopyroxene–melt pairs indicate that the Quaternary volcanism was associated with two magma storage regions that correspondingly involved pressures of 791 ± 253 MPa (1098 ± 34 °C) and 187 ± 123 MPa (938 ± 77 °C). The primary magma was likely present in a reservoir beneath the Moho (17–32 km), and it evolved by fractional crystallization to produce rocks, such as trachybasalts and phonotephrites. Crystallization and differentiation of the basaltic magma that was transported to crustal reservoirs explain the evolved compositions. The trachytic magma generated were present at depths of 2–11 km, and crystallization produced a paragenetic assemblage comprising clinopyroxene, amphibole, plagioclase, biotite, apatite, and titanomagnetite.