Existence of andesitic primary magma: An example from southwest Japan |
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| Authors: | Yoshiyuki Tatsumi Kyoichi Ishizaka |
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| Institution: | Earthquake Research Institute, University of Tokyo, Tokyo 113Japan;Department of Geology and Mineralogy, Faculty of Science, Kyoto University, Kyoto 606Japan |
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| Abstract: | A high-magnesian andesite (SiO2 58.50%, MgO 9.47%) occurs at Teraga-Ike in southwest Japan. It belongs to the Setouchi volcanic rocks of middle Miocene age and carries olivine and bronzite as phenocrysts (4.2 and 1.4 modal percent, respectively). This andesite is characterized by Mg-values as high as 75, suggesting that it may be a primary andesite. Olivine phenocrysts (Fo87–91) are in equilibrium with the groundmass (= liquid) on the basis of Fe-Mg exchange partitioning between olivine and liquid, and they have high NiO contents (up to 0.45%). Chromite inclusions in olivine and rarely bronzite have high Cr2O3 contents (max. 54.87%). These features strongly suggest that the Teraga-Ike andesite keeps the chemical composition of the primary magma generated in the upper mantle, and therefore verify the existence of primary andesite magmas. |
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