Age relations and petrology of alkalic igneous rocks from Oki Dozen, Southwest Japan.

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.

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

“…The major element compositions of the analysed Oki-Dōzen samples form a relatively continuous spectrum from alkalic basalt to trachyte and rhyolite in agreement with previous Page 9 of 21 22 studies (Kaneko et al 1983;Morris 1986;Tiba 1986;Morris and Kagami 1989;Wada et al 1990;Kaneko 1991;Morris et al 1997 ; Fig. 4a).…”

“…Radiometric dating has led to the revision of the interpretation of the caldera formation and central cone trachyte eruption events (Tiba et al 2000). According to K/ Ar dating, the initial "somma" phase of dispersed volcanism occurred between 6.3 and 5.3 Ma with the eruption of the Takuhiyama trachyte at approximately 5.4 Ma (Morris et al 1990(Morris et al , 1997Wada et al 1990;Tiba et al 2000). This implies that a caldera formation occurred prior to 5.4 Ma.…”

“…1). The Takuhiyama trachyte has sub-vertical contacts with a body of syenite (Ōyama syenite), which has yielded K/ Ar ages of 7.4-6.3 Ma (Morris et al 1997;Tiba et al 2000) and is nowhere else exposed other than in the central portion of Oki-Dōzen. The Ōyama syenite intruded a sequence of early to middle Miocene volcanic (calc-alkaline) and sedimentary rocks (Tiba et al 2000;Matsubara et al 2014), which is uplifted and exposed along the ridge to the north of Takuhiyama (Naemura and Shimada 1984).…”

“…Geological contacts exposed at Oki-Dōzen make it ideal for such a study. In particular, the core of Oki-Dōzen consists of a large trachytic body abutting a syenitic intrusion as well as older uplifted basement volcano-sedimentary rocks, thus representing an opportunity to study in situ an exposed magmatic reservoir associated with its consanguineous eruption products (Morris et al 1997). We discuss new chemical, petrological and isotopic data, as well as a new set of chronological constraints for a suite of samples collected from around the island, with emphasis on rocks from the central complex.…”

“…The volcano-sedimentary sequence is folded to a broad NNW-SSE trending anticline and is intruded by NNW-SSE to N-S trending trachytic to rhyolitic dykes (Tiba et al 2000). The syenite is considered the intrusive equivalent of the Takuhiyama trachyte (Morris et al 1997) based on similar trace element and isotopic characteristics.…”