ABSTRACT

In order to define the contaminant and the contamination process, strontium isotopic compositions were determined on thirty-six rock samples from lava flows of the Quaternary Kinpo volcano in the Beppu-Shimabara Graben, Southwest Japan. Forty-two rock samples were also analyzed for major chemical compositions. The rocks are all subalkalic. All but a few tholeiitic basalts are calc-alkalic with abundant phenocrysts. The rocks are characterized by rather high Sr contents, ranging from 387 ppm to 1106 ppm. ⁸⁷Sr/⁸⁶Sr ratio ranges from 0.70377 to 0.70537. Marked linear relationships are found between ⁸⁷Sr/⁸⁶Sr and 1/Sr of the rocks. Repetitive occurence of basalts and basaltic andesites in the stratigraphic succession and lines of petrographic evidence for magma mixing suggest the operation of a periodically refilled magma chamber under the Kinpo volcano. The linear relationships between ⁸⁷Sr/⁸⁶Sr and 1/Sr, and also between some major oxide components are interpreted as the manifestation of eruption of lavas during the repeated magma mixing. The increase of ⁸⁷Sr/⁸⁶Sr ratio is postulated to be due to the contamination by melts derived from the granulitic basic lower crust during the upward migration of the chamber to attain the heat balance.