Abstract
Oki-Dozen (Japan) is a Late Miocene (7-5 Ma) intraplate alkalic volcano composed of a central trachytic pyroclastic complex surrounded by a ring-shaped succession of basaltic to trachybasaltic lavas and pyroclastic rocks and dispersed trachytic bodies. The central trachytic complex is in contact with a syenite that was intruded into the basement early Miocene volcano-sedimentary succession. In the centre of the system there are no alkalic basaltic rocks that are correlative of the outer flank ring. We present whole-rock major and trace element chemistry, Sr-Nd-Pb isotopic compositions and petrological data from the central trachytic volcanic complex and the intrusive syenite body, as well as from the outer ring basaltic succession. We also present and discuss a new set of zircon U/Pb ages collected from the central trachyte and syenite bodies. All the eruptive products of Oki-Dozen, as well as the syenite, plot on a single liquid line of descent initiated from a mantle-derived alkalic basaltic parent. A younger (2.8 Ma) basaltic eruption (Uzuka basalt) has isotopic compositions 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.