Chronostratigraphy and magma genesis of the Ikokuma Andesites in the Northwestern Shimokita Peninsula, Aomori prefecture, Japan

Abstract

The Neogene Ikokuma Andesites (IKA) is widely distributed in the northwestern Shimokita Peninsula, Aomori Prefecture, and consists mainly of andesitic hyaloclastites and lavas, and it has been pointed out that the bulk compositions ( major element ) of the IKA differ between east and west along an estimated fault running north-south through central Okoppe area (Yamazaki and Shuto, 1986). In this study, zircon U-Pb ages of dacitic rocks composed in IKA and bulk compositions (including boron) of IKA are determined to elucidate whether these compositional differences between the east and west are due to 1) differences in magmatic evolution resulted from differences in formation ages, or 2) regional differences in magma compositions. The obtained U-Pb ages show a difference of at least 1 Myr between the east and west. The compositional variations in the eastern IKA are due to the magma mixing of basalt formed by an arc volcanism and dacite with a higher fluid addition rate than the western IKA, whereas the variation of western IKA is suggested to contribution of fluid-poor tholeiitic magma.