Fore-arc volcanism in NE Japan during the Miocene opening of the Japan Sea

Geochemical features of the Shiogo Aphyric Andesite, Ibaraki Prefecture

Abstract

During the back-arc opening of the Japan Sea between 21 and 15 Ma, volcanic activity increased in range to the oceanic side, erupting a variety of magma types with different origins. We obtained a new K‒Ar age of 16.40±0.10 Ma for an aphyric volcanic rock intruding Jurassic sedimentary rocks in Shirosato Town, Ibaraki Prefecture, which indicates that this rock is a product of Early Miocene fore-arc volcanism. Samples of this volcanic rock show intermediate whole-rock geochemical compositions with high Fe and low Al. Trace-element compositions and Sr‒Nd isotopic ratios indicate that the volcanic rocks were derived from a residual melt generated by crystal fractionation of a mafic magma similar in composition to the nearby Motokosawa Basalt. This basalt may have been derived from lithospheric mantle melted by the injection of upwelling high-temperature asthenospheric material involved in the opening of the Japan Sea.