AGE RELATIONS AND PETROLOGY OF ALKALIC IGNEOUS ROCKS FROM OKI-DOZEN, SOUTHWEST JAPAN

K/Ar age data, whole-rock major, trace and rare earth element geochemi stry, Sr-87/Sr-86 and Nd-143/Nd-144 isotope chemistry, and computer mo delling have been used to clarify the timing and petrogenesis of late Miocene igneous activity on Oki Dozen Island in the Japan Sea. Commenc ing at about 7 Ma, quartz syenite was intruded on the southeastern par t of present-day Nishinoshima. At the same time, compositionally simil ar magma (trachyte) was erupted as lava and pyroclastic flows. At appr oximately 6 Ma, alkali olivine basalt-trachyandesite was erupted, form ing a volcanic shield (somma phase). Following this, the locus of volc anic activity, and the composition of the magma changed, with eruption of trachyte and rhyolite from satellite vents (parasitic phase). Dike s, compositionally similar to extruded magma, were intruded throughout this volcanic episode. K/Ar dating cannot resolve the individual phas es of volcanic activity, which partly overlaps with late Miocene volca nism on Oki Dogo. The composition of somma and parasitic phase volcani cs were controlled by fractionation of observed phenocryst phases, but decoupling of trace elements and small isotopic shifts indicate the t ransition from somma to parasitic volcanism was not a closed-system pr ocess. Syenite and comagmatic central cone trachyte are compositionall y similar to anorogenic granitoids, and may result from melting of a f elsic lower crust by upwelling basaltic magma. Despite weak age overla p, divergent trace element and isotopic trends, and small differences in rare earth element chemistry indicate that alkalic volcanic rocks o n Oki Dozen and Oki Dogo tapped different mantle sources.