Unspiked K-Ar geochrolonology of Zao Volcano, Northeast Japan: Reconstruction of the volcanic stages

We report new unspiked K-Ar ages for lavas corrected from the Zao volcano, northeast Japan. Unspiked K-Ar dating method (peak comparison method) enables mass fractionation correction of initial Ar ratio, and this method has been applied successfully to young lavas, especially younger than 0.5 Ma, with high atmospheric contamination. Zao volcano is the first volcano that is demonstrated the reliability of the mass fraction corrected ages by Takaoka et al. (1989). They reported 30 ages for central Zao volcano, and However, not all units are covered and some data contradict the stratigraphy probably because of low-K and/or excess Ar contamination.

Based on new geological and geochrolonogical study, the volcanic activity can be divided into six stages. Stage I: subaqueous eruptions of low-K tholleiitic basaltic to andesitic magmas occured in central part at around 1 Ma. Stage II: andesitic northwestern edifice was formed at ca. 0.5 Ma. Stage III: several small to meddle sized andesitic to deictic edifices were formed at western part during ca. 0.35-0.25 Ma. Stage IV: andesitic to dacitic lavas swelled out from central summit area and the main edifice was formed during ca. 0.25-0.20 Ma. Stage V: andesitic lava flows with pyroclastic materials erupted from several vents during ca. 0.13-0.04 Ma. Several lava flows younger than 0.05 Ma were first dated on this study. Stage VI: The most recent stage of Zao volcano began at ca. 35 ka, when the horseshoe-shaped Umanose calderawas formed.