Petrological implications of strontium isotope compositions of the Kinpo volcanic rocks in Southwest Japan

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 1,106 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 periodicaly 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. (author)