ORIGIN OF MIOCENE ACIDIC VOLCANIC ROCKS IN SETOUCHI AREA

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Regional volcanic activities of high magnesian andesite during the late stage of Japan Sea opening were widely extended in the circum Seto Inland Sea area. In the northwest of Shikoku, calcalkaline volcanism which was related to the magmatism of Ishizuchi cauldron spread thick dacitic pyroclastic flow westward followed by calcalkaline Kuromoritoge andesite. The dacitic pyroclastic flow called Takano pyroclastic rock comprises lower pumice and fragment rich layr and upper fine ash rich sediments.Various rhyo-dacitic intrusion such as pottasium rich Taisanji-type acidic volcanic rocks, per-aluminous Myojinyama-type acidic volcanic rocks and K-feldspar bearing Misakatoge rhyolite are also exposed in the area. Rhyo-dacitic pyroclastic volcanism was also antecedent to high magnesian magmatism in Goshikidai.Chondrite normarized REE patterns of rhyo-dacitic to rhyolite rocks of the area exhibit LREE enrichment and steep inclination to HREE side and the most of the pattern exhibit negative Eu anomaly. Specific patterns which reflect magma source are also present. The potassium rich Taisanji-type rocks are enriched in REE and indicate relative mild inclination, whereas Misakatoge rhyolites are relatively poor in HREE and show steep inclination. Not distinctive negative Eu anomaly in the Kuromoritoge andesite suggests a least fractionation of magma. Variation of Inital Sr isotope ratios of these rocks are accordant with REE characteristics.