A volcano-plutonic complex in the Susa area, southwest Japan, consists of the Yamashima andesites, the Koyama gabbros and syn-plutonic porphyrite dykes derived from a common basaltic andesite magma. The complex is closely associated with middle Miocene turbidite deposits. The Yamashima andesites are composed mainly of basaltic andesite feeder dykes, massive submarine lavas with hyaloclastites, and their reworked deposits. The lavas and deposits immediately overlie turbidite deposits, indicating submarine volcanic activity. The Koyama gabbros formed hornfels by contact metamorphism of the surrounding turbidites and andesites. Highly purified clinopyroxene and plagioclase mineral separates from the Yamashima andesites were dated by a K–Ar method using an ultra-low blank K analysis procedure. Ages obtained from duplicate analyses are 16.5 ± 1.5, 15.2 ± 1.4, 15.8 ± 1.7, and 16.5 ± 2.0 Ma for clinopyroxene, and 14.2 ± 0.8, 15.2 ± 0.9, and 15.6 ± 0.9 Ma for plagioclase. The clinopyroxene and plagioclase data define a mineral isochron age of 14.7 ± 0.9 (1σ) Ma with an initial 40Ar/36Ar ratio of 297.3 ± 2.4 (1σ), suggesting that clinopyroxene has no excess argon and can be reliably dated by K–Ar. Most of the groundmass ages are considerably younger (12.1–14.6) than the isochron age, perhaps due to argon loss during alteration. The gabbros give ages of 14.2 ± 0.3 and 14.1 ± 0.3 Ma for biotite, and 13.7 ± 0.3 and 13.7 ± 0.7 Ma for green hornblende. The porphyrite dyke yields an age of 12.5 ± 0.3 Ma for the groundmass, and the pelitic hornfels gives a biotite age of 14.8 ± 0.3 Ma. Our new K–Ar ages, together with previous studies, show that a series of geological events took place in the Susa area between 16 and 13 Ma. Conglomerates and sandstones were deposited in the beginning of marine transgression. Subsequent abrupt deepening led to deposition of a thick black shale unit, turbidite deposits and large-scale submarine channel-fill deposits. Coeval igneous activity formed the volcano-plutonic complex. The magmato-tectonic event was synchronous with the opening of the Japan Sea and the associated clockwise rotation of the southwest Japan arc sliver, recording a unique tectonic setting.

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