ABSTRACT

K/Ar age data, whole-rock major, trace and rare earth element geochemistry, ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd isotope chemistry, and computer modelling have been used to clarify the timing and petrogenesis of late Miocene igneous activity on Oki Dozen Island in the Japan Sea. Commencing at about 7 Ma, quartz syenite was intruded on the southeastern part of present-day Nishinoshima. At the same time, compositionally similar magma (trachyte) was erupted as lava and pyroclastic flows. At approximately 6 Ma, alkali olivine basalt-trachyandesite was erupted, forming a volcanic shield (somma phase). Following this, the locus of volcanic activity, and the composition of the magma changed, with eruption of trachyte and rhyolite from satellite vents (parasitic phase). Dikes, compositionally similar to extruded magma, were intruded throughout this volcanic episode. K/Ar dating cannot resolve the individual phases of volcanic activity, which partly overlaps with late Miocene volcanism on Oki Dogo. The composition of somma and parasitic phase volcanics were controlled by fractionation of observed phenocryst phases, but decoupling of trace elements and small isotopic shifts indicate the transition from somma to parasitic volcanism was not a closed-system process. Syenite and comagmatic central cone trachyte are compositionally similar to anorogenic granitoids, and may result from melting of a felsic lower crust by upwelling basaltic magma. Despite weak age overlap, divergent trace element and isotopic trends, and small differences in rare earth element chemistry indicate that alkalic volcanic rocks on Oki Dozen and Oki Dogo tapped different mantle sources.