The Petrology and Geochemistry of Oto-Zan Composite Lava Flow on Shodo-Shima Island, SW Japan: Remelting of a Solidified High-Mg Andesite Magma
Yoshiyuki TatsumiToshihiro SuzukiHiroshi KawabataKeiko SatoTakashi MiyazakiQing ChangT. TakahashiKenichiro TaniTomoyuki ShibataMasako Yoshikawa
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Abstract:
The Oto-Zan lava in the Setouchi volcanic belt is composed of phenocryst-poor, sparsely plagioclase-phyric andesites (sanukitoids) and forms a composite lava flow. The phenocryst assemblages and element abundances change but Sr–Nd–Pb isotopic compositions are constant throughout the lava flow. The sanukitoid at the base is a high-Mg andesite (HMA) and contains Mg- and Ni-rich olivine and Cr-rich chromite, suggesting the emplacement of a mantle-derived hydrous (� 7w t %H 2O) HMA magma. However, Oto-Zan sanukitoids contain little H2O and are phenocryst-poor. The liquid lines of descent obtained for an Oto-Zan HMA at 0� 3GPa in the presence of 0� 7–2� 1w t %H 2O suggest that mixing of an HMA magma with a differentiated felsic melt can reasonably explain the petrographical and chemical characteristics of Oto-Zan sanukitoids. We propose a model whereby a hydrous HMA magma crystallizes extensively within the crust, resulting in the formation of an HMA pluton and causing liberation of H2O from the magma system. The HMA pluton, in which interstitial rhyolitic melts still remain, is then heated from the base by intrusion of a high-T basalt magma, forming an H2O-deficient HMA magma at the base of the pluton. During ascent, this secondary HMA magma entrains the overlying interstitial rhyolitic melt, resulting in variable self-mixing and formation of a zoned magma reservoir, comprising more felsic magmas upwards. More effective upwelling of more mafic, and hence less viscous, magmas through a propagated vent finally results in the emplacement of the composite lava flow.Keywords:
Phenocryst
Felsic
Igneous differentiation
Magma chamber
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Melt inclusions
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Melt inclusions
Igneous differentiation
Fractional crystallization (geology)
Incompatible element
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Detailed study of the texture and chemical zoning of phenocrysts in a plinian fall deposit (Yunokuchi Pumice) from Akagi Volcano, Japan, permits reconstruction of the subvolcanic magma chamber. Hypersthene phenocrysts are complexly zoned; some have spongy cellular textures whereas others exhibit simple reverse zoning. Augite phenocrysts have texturally similar cores but Al and Ti vs mg-number zoning patterns are correlatable with hypersthene cores. Most plagioclase phenocrysts have a uniform calcic core enclosed by a normally zoned rim; others have variable An contents and mottled textures. These characteristics and melt (glass) compositions suggest a complex, multistage mixing of lot-T (940–960°C) highly silicic and high-T (980–1060°C) less silicic magmas. Stratigraphic variations of the phenocryst types show that the earlier erupted magma inherited phenocrysts from more high-T magma components. We propose the presence of an inhomogeneous mushy chamber filled with a highly silicic magma which was recharged periodically by less silicic magmas. The large viscosity contrast and density balance of the new magma relative to the mush controlled the efficiency of mixing and the position of emplacement of the high-T magma in the mush. Tapping the chamber first expelled the fluidal mixed magma. Followed by progressive erosion of the mush as the eruption continued.
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Detailed study of the texture and chemical zoning of phenocrysts in a plinian fall deposit (Yunokuchi Pumice) from Akagi Volcano, Japan, permits reconstruction of the subvolcanic magma chamber. Hypersthene phenocrysts are complexly zoned; some have spongy cellular textures whereas others exhibit simple reverse zoning. Augite phenocrysts have texturally similar cores but Al and Ti vs mg-number zoning patterns are correlatable with hypersthene cores. Most plagioclase phenocrysts have a uniform calcic core enclosed by a normally zoned rim; others have variable An contents and mottled textures. These characteristics and melt (glass) compositions suggest a complex, multistage mixing of lot-T (940–960°C) highly silicic and high-T (980–1060°C) less silicic magmas. Stratigraphic variations of the phenocryst types show that the earlier erupted magma inherited phenocrysts from more high-T magma components. We propose the presence of an inhomogeneous mushy chamber filled with a highly silicic magma which was recharged periodically by less silicic magmas. The large viscosity contrast and density balance of the new magma relative to the mush controlled the efficiency of mixing and the position of emplacement of the high-T magma in the mush. Tapping the chamber first expelled the fluidal mixed magma. Followed by progressive erosion of the mush as the eruption continued.
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