A trachyte–syenite core within a basaltic nest: filtering of primitive injections by a multi-stage magma plumbing system (Oki-Dōzen, south-west Japan)

Abstract

that distinguish it from the rest of the system. Geochemical modelling indicates that magmatic differentiation through crystal fractionation and minor crustal assimilation occurred in crustal and shallow sub-volcanic magma reservoirs. In the central part of the system, a number of vertically spaced reservoirs acted as a filter, capturing basaltic dykes and hindering their ascent. In the outer region, dykes either reached the surface unhindered and erupted to form the basaltic/trachybasaltic succession or stalled at crustal levels and differentiated to trachyte before forming dispersed domes/flows. The central plumbing system "filter" resulted in a nest-shaped volcano, with a trachytic core surrounded by basaltic products, and stopped direct injection of basaltic magmas into the shallow syenitic magma reservoir, likely preventing its destabilization and explosive eruption.

Oki-Dozen Dike Swarm: Effect of the Regional Stress Field on Volcano-Tectonic Orientations

“…Many radiometric dates have been reported for the OVC. Zircon U-Pb ages of 6.3-6 and 6.2-5.7 Ma have been reported for the subplutonic Oyama syenite and the Takuhiyama trachyte, respectively [15]; an anomalously old age of 7.4 Ma has been obtained for the Oyama syenite [18,19]. Precise K-Ar dating of the majority of the stratocone lavas and associated ODS dikes and sills yields ages of 5.9-5.5 Ma (Figure 3) [16].…”

“…The Oki-dozen islands of southwest Japan consist of the latest Miocene dissected OVC (Figure 1) [15][16][17][18][19][20][21], which is located in the southern Sea of Japan, ~50 km off the north coast of Shimane Peninsula, 15 km southwest of Oki-dogo island. The Oki-dozen islands comprise three main islands: Nishinoshima, Chiburijima, and Nakanoshima (Figures 1 and 2).…”

“…The Oki-dozen islands comprise three main islands: Nishinoshima, Chiburijima, and Nakanoshima (Figures 1 and 2). These islands form part of the Oki-Shimane plateau within the Sea of Japan [22], and parts of these islands have been awarded UNESCO Global Geopark status as a result of the large number of excellent exposures of volcanic deposits [15].…”

“…(1) The ODS formed rapidly (within a million years), as evidenced by precise K-Ar ages of 5.9-5.5 Ma for the majority of the stratocone lavas and associated dikes and sills in this region [16]. In addition (2), the OVC is a volcanic suite that is located ~300 km from the present-day subduction trench in SW Japan, and (3) the coeval magmatism in this region formed ≥200 single, composite, and multiple dikes, pyroclastic cones, and sills [15][16][17][18]. Finally, (4) the ODS has a broad range of magma compositions with SiO 2 concentrations of 44-69 wt% [15][16][17][18].…”

“…The Oki-dozen volcanic complex (OVC) is a volcanic suite located 300 km from the present-day subduction trench in SW Japan (Figure 1) [15][16][17][18][19][20][21]. The complex contains numerous volcanic edifices that define the interior of the complex, including an outer-flank basaltic volcano with lava flows, pyroclastic cones, dikes, and laccolithic sills.…”