From the area along the Aimagawa river in western Gunma, central Japan, 12 samples were collected for the determination of K-Ar ages and chemical compositions. X-ray fluorescence and instrumental neutron activation analysis methods were applied to determine major and trace elements, respectively. Based on the K-Ar ages, the rock samples were divided into two groups having 4.5Ma and 2.0Ma. Although transformation of magnetite to maghemite, possibly due to high temperature oxidation, was observed among the samples, there is no apparent inconsistency between stratigraphic relation and the K-Ar ages, suggesting that such oxidation gave no effect on the K-Ar ages. No large variation is observed in chemical compositions for both major and trace elements. Among the trace elements, lithophile elements such as rare earth elements and Zn show small variations in their abundances. The samples with K-Ar ages of 4.5Ma yield La/Yb ratios larger than 2.5 as chondrite-normalized values, while the remaining samples (ca. 2.0Ma) give values smaller than 2.0, suggesting a difference in the degree of partial melting between the two stages of magmatism. In contrast to the lithophiles, Cr, Ni, and Co show relatively large variations. Chromium shows a positive correlation with 36Ar. Non-radiogenic Ar must have been trapped in some Cr-bearing mineral(s) (chromite and/or clinopyroxene?).
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