5:15 PM - 7:15 PM
[SVC30-P05] Petrological study of products from Shikotsu caldera and the post-caldera volcanoes
Keywords:Shikotsu caldera, magma mixing, post-caldera volcanism, mafic inclusion
Shikotsu Caldera was formed by a series of explosive eruptions at ~46 ka (Uesawa et al., 2016; Nakagawa et al., 2018). Three post-caldera volcanoes, including Fuppushi (<~26 ka; Furukawa and Nakagawa, 2010; Ishibashi et al., 1973), Eniwa (<~20 ka; Machida and Arai, 2003), and Tarumae (<~9 ka; Furukawa et al., 2006), subsequently formed. We examined thin sections and whole-rock compositions of products from the caldera and the three post-caldera volcanoes to understand the rejuvenation processes of the magma system after the caldera-forming eruption. The data obtained by previous studies (Nakagawa et al., 2006; Hiraga, 2001; Morita, 2023) were also used.
The Tarumae samples are basaltic to andesitic (SiO2=51.6-62.6wt.%), and contain plagioclase, clinopyroxene, orthopyroxene, Fe-Ti oxides, and olivine phenocrysts in mafic samples. The whole-rock compositional trends of the products before Ta-c1(‘Tarume early stage’) and those after Ta-c2(‘Tarumae late stage’) are distinct in Harker diagrams. The Tarumae early-stage samples show the low-K series with varied trends, while the Tarumae late-stage samples show the medium-K series. In the historical period, the compositional range is narrowed and becomes more mafic. The Eniwa samples are andesitic to dacitic (SiO2=54.8-63.6wt.%) and the phenocryst assemblages are similar to those of the Tarumae products, but hornblende is found in some lavas. In addition, the amount of olivine increases with volcanic activity. The Fuppushi samples are andesitic to dacitic (SiO2=59.6-64.4wt.%) with phenocrysts of plagioclase, clinopyroxene, orthopyroxene, and Fe-Ti oxides; some samples contain quartz and hornblende as well. Quartz phenocrysts are anhedral, and some samples contain both olivine and quartz phenocrysts. Mafic inclusions in Eniwa and Fuppushi were porphyritic, with plagioclase, clinopyroxene, orthopyroxene, and olivine phenocrysts. The composition of the mafic inclusion from Eniwa (SiO2=54.7 wt.%) is on the extrapolation of the trend formed by the main samples in each Harker diagram, whereas that of Fuppushi (SiO2=52.5 wt.%) is within the range of the Tarumae early stage. The Shikotsu samples, consisting of rhyolitic (A-type) and andesitic-dacitic (P-type), exhibit distinct trends in Harker diagrams (Nakagawa et al., 2006). The whole-rock compositional trends among the different groups are distinct. For the Eniwa products, the rates of the increase in the Ba, Zr and V contents with increasing SiO2 are smaller and the rate of the decrease in MgO is larger than those of the other groups. The compositions of the Fuppushi products are characterized by decreasing trends in Y, CaO, and P2O5 contents with increasing SiO2. The Ba/Rb ratios tend to decrease with increasing SiO2 contents in all the volcanoes.
The anhedral outlines of some phenocrysts and the coexistence of olivine and quartz phenocrysts, and disequilibrium phenocrysts in the Tarumae products (Nakagawa et al., 2011) and reverse zoning of pyroxene phenocrysts in the Eniwa lavas (Masuda, 1990) indicate magma mixing in all the post-caldera magmatic systems. The decrease in the SiO2 contents and the increase in olivine in lavas with eruption ages suggest that the mixing of mafic magma progressed in the Tarumae and Eniwa magmatic systems. The similar whole-rock composition of the mafic inclusion from Fuppushi to those of the Tarumae early-stage products implies a common mafic magma source. The mafic inclusion from Eniwa volcano is thought to represent the mafic end-member magma in the Eniwa magmatic system. The distinctive compositional characteristics among the Shikotsu and the post-caldera volcanic products suggest that they had different magma systems. The variations in some incompatible element ratios of the products in each group imply that the felsic end-member magmas were not derivatives of the mafic end-member magmas. Further research is needed to understand the origins of felsic end-member magmas and the evolution of mafic magmas.
The Tarumae samples are basaltic to andesitic (SiO2=51.6-62.6wt.%), and contain plagioclase, clinopyroxene, orthopyroxene, Fe-Ti oxides, and olivine phenocrysts in mafic samples. The whole-rock compositional trends of the products before Ta-c1(‘Tarume early stage’) and those after Ta-c2(‘Tarumae late stage’) are distinct in Harker diagrams. The Tarumae early-stage samples show the low-K series with varied trends, while the Tarumae late-stage samples show the medium-K series. In the historical period, the compositional range is narrowed and becomes more mafic. The Eniwa samples are andesitic to dacitic (SiO2=54.8-63.6wt.%) and the phenocryst assemblages are similar to those of the Tarumae products, but hornblende is found in some lavas. In addition, the amount of olivine increases with volcanic activity. The Fuppushi samples are andesitic to dacitic (SiO2=59.6-64.4wt.%) with phenocrysts of plagioclase, clinopyroxene, orthopyroxene, and Fe-Ti oxides; some samples contain quartz and hornblende as well. Quartz phenocrysts are anhedral, and some samples contain both olivine and quartz phenocrysts. Mafic inclusions in Eniwa and Fuppushi were porphyritic, with plagioclase, clinopyroxene, orthopyroxene, and olivine phenocrysts. The composition of the mafic inclusion from Eniwa (SiO2=54.7 wt.%) is on the extrapolation of the trend formed by the main samples in each Harker diagram, whereas that of Fuppushi (SiO2=52.5 wt.%) is within the range of the Tarumae early stage. The Shikotsu samples, consisting of rhyolitic (A-type) and andesitic-dacitic (P-type), exhibit distinct trends in Harker diagrams (Nakagawa et al., 2006). The whole-rock compositional trends among the different groups are distinct. For the Eniwa products, the rates of the increase in the Ba, Zr and V contents with increasing SiO2 are smaller and the rate of the decrease in MgO is larger than those of the other groups. The compositions of the Fuppushi products are characterized by decreasing trends in Y, CaO, and P2O5 contents with increasing SiO2. The Ba/Rb ratios tend to decrease with increasing SiO2 contents in all the volcanoes.
The anhedral outlines of some phenocrysts and the coexistence of olivine and quartz phenocrysts, and disequilibrium phenocrysts in the Tarumae products (Nakagawa et al., 2011) and reverse zoning of pyroxene phenocrysts in the Eniwa lavas (Masuda, 1990) indicate magma mixing in all the post-caldera magmatic systems. The decrease in the SiO2 contents and the increase in olivine in lavas with eruption ages suggest that the mixing of mafic magma progressed in the Tarumae and Eniwa magmatic systems. The similar whole-rock composition of the mafic inclusion from Fuppushi to those of the Tarumae early-stage products implies a common mafic magma source. The mafic inclusion from Eniwa volcano is thought to represent the mafic end-member magma in the Eniwa magmatic system. The distinctive compositional characteristics among the Shikotsu and the post-caldera volcanic products suggest that they had different magma systems. The variations in some incompatible element ratios of the products in each group imply that the felsic end-member magmas were not derivatives of the mafic end-member magmas. Further research is needed to understand the origins of felsic end-member magmas and the evolution of mafic magmas.