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[SVC36-P03] Petrological characteristics of the newest stage lavas of Zao Volcano, NE Japan
Keywords:Zao volcano, andestic lava, magma mixing, temporal variation
Zao Volcano is an active stratovolcano in central part of the volcanic front of NE Japan arc. The volcanic activity is divided into six stages and the newest stage is from about 35 ka to present. The eruption products of the newest stage consist of pyroclastics and subsequent lavas. Petrologic studies have been well performed on the pyroclastics. Instead, sparsely distributed lavas have been barely studied. The petrologic study on the lavas is necessary to understand entire magma activity in the newest stage. Here, we report petrographic characteristics and whole-rock chemical compositions of all lave units in the newest stage.
Stratigraphy
Five lava units of the newest stage have been already recognized; Nigorikawa lava (Ngl), Goshikidake-east lava (Gel), Kattadake-north lava (Knl), Furikodaki lava (Fkl), and Goshikidake-south lava (Gsl). Ages were estimated to be ca. 35 ka for Ngl, ca. 30 to 10 ka for Gel and Knl, and younger than ca. 10 ka for Fkl and Gsl. In this study, a lava unit, Nigorigawa-upstream lava (Nul), underlying Gel was newly identified.
Petrographic Characteristics
The newest stage lavas are grey to dark grey basaltic andesite to andesite. The phenocrysts in these lavas comprise 27–37 vol. %. Rocks of Ngl–Nul, Knl, Gel, and Fkl–Gsl are olivine bearing cpx-opx basaltic andesite, olivine bearing opx-cpx basaltic andesite, cpx-opx basaltic andesite, and olivine bearing cpx-opx andesite, respectively. Plagioclase phenocrysts having patchy texture is dominant in Fkl and Gsl, and phenocrysts with patchy texture or dusty zone are remarkable in Nul and Gel. Whereas, patchy texture and dusty zone of phenocrysts are rare in Ngl and Knl. Although cpx reaction rims around opx phenocrysts were observed in Nul, Gel, and Knl but in Ngl, Knl, and Gsl. Opx reaction rims around olivine phenocrysts were remarkably observed in Ngl and Fkl.
Whole-rock compositions
The newest stage lavas have 54–58.5 wt. % SiO2 and belong to medium-K calk-alkaline series. As a whole, the lavas show compositional variation trends of increasing TiO2, Na2O, K2O, Ba, Rb, Y, and Zr and decreasing FeOt, MnO, CaO, and Sr contents with increasing SiO2 content. In detail, the compositional characteristics of lavas in different ages are slightly different. In terms of SiO2 range, the youngest lava units (Fkl, Gsl) show higher (57–58.5 wt. %) than the others (54–57 wt. %). In K2O, and Zr vs. SiO2 diagrams, the oldest unit (Ngl) shows higher trend than middle age units (Nul, Gel, Knl) at same SiO2 content. Among middle age units, Nul and Knl show higher trend than Gel. In these diagrams, the trends converge in silica-poorer part, whereas diverge in silica-richer part. Rocks of the youngest units are plotted on silica-richer part extension of Nul and Knl trend. In some diagrams, such as in FeOt, TiO2 vs. SiO2 diagrams, however, youngest units trend crosscuts the trends of the other ages.
Discussion
Dissolution texture, such as patchy and dusty zone of plagioclase phenocrysts, observed in all units would suggest the mixed origin of all rocks. The trends of oldest and middle age units are different and diverge in richer-silica part in such as K2O, Zr-SiO2 diagrams. Therefore, the felsic end-member magmas of the oldest and middle age units differ in composition. Likewise, the compositions of felsic end-member magmas between Gel and Nul-Knl are slightly different. Furthermore, judging from the crosscut of trends of youngest units and the older units in some diagrams (such as FeOt, TiO2-SiO2), both of mafic and felsic end-member magmas of the youngest units differ from older units.
Stratigraphy
Five lava units of the newest stage have been already recognized; Nigorikawa lava (Ngl), Goshikidake-east lava (Gel), Kattadake-north lava (Knl), Furikodaki lava (Fkl), and Goshikidake-south lava (Gsl). Ages were estimated to be ca. 35 ka for Ngl, ca. 30 to 10 ka for Gel and Knl, and younger than ca. 10 ka for Fkl and Gsl. In this study, a lava unit, Nigorigawa-upstream lava (Nul), underlying Gel was newly identified.
Petrographic Characteristics
The newest stage lavas are grey to dark grey basaltic andesite to andesite. The phenocrysts in these lavas comprise 27–37 vol. %. Rocks of Ngl–Nul, Knl, Gel, and Fkl–Gsl are olivine bearing cpx-opx basaltic andesite, olivine bearing opx-cpx basaltic andesite, cpx-opx basaltic andesite, and olivine bearing cpx-opx andesite, respectively. Plagioclase phenocrysts having patchy texture is dominant in Fkl and Gsl, and phenocrysts with patchy texture or dusty zone are remarkable in Nul and Gel. Whereas, patchy texture and dusty zone of phenocrysts are rare in Ngl and Knl. Although cpx reaction rims around opx phenocrysts were observed in Nul, Gel, and Knl but in Ngl, Knl, and Gsl. Opx reaction rims around olivine phenocrysts were remarkably observed in Ngl and Fkl.
Whole-rock compositions
The newest stage lavas have 54–58.5 wt. % SiO2 and belong to medium-K calk-alkaline series. As a whole, the lavas show compositional variation trends of increasing TiO2, Na2O, K2O, Ba, Rb, Y, and Zr and decreasing FeOt, MnO, CaO, and Sr contents with increasing SiO2 content. In detail, the compositional characteristics of lavas in different ages are slightly different. In terms of SiO2 range, the youngest lava units (Fkl, Gsl) show higher (57–58.5 wt. %) than the others (54–57 wt. %). In K2O, and Zr vs. SiO2 diagrams, the oldest unit (Ngl) shows higher trend than middle age units (Nul, Gel, Knl) at same SiO2 content. Among middle age units, Nul and Knl show higher trend than Gel. In these diagrams, the trends converge in silica-poorer part, whereas diverge in silica-richer part. Rocks of the youngest units are plotted on silica-richer part extension of Nul and Knl trend. In some diagrams, such as in FeOt, TiO2 vs. SiO2 diagrams, however, youngest units trend crosscuts the trends of the other ages.
Discussion
Dissolution texture, such as patchy and dusty zone of plagioclase phenocrysts, observed in all units would suggest the mixed origin of all rocks. The trends of oldest and middle age units are different and diverge in richer-silica part in such as K2O, Zr-SiO2 diagrams. Therefore, the felsic end-member magmas of the oldest and middle age units differ in composition. Likewise, the compositions of felsic end-member magmas between Gel and Nul-Knl are slightly different. Furthermore, judging from the crosscut of trends of youngest units and the older units in some diagrams (such as FeOt, TiO2-SiO2), both of mafic and felsic end-member magmas of the youngest units differ from older units.