6:15 PM - 7:30 PM
[SVC47-P10] Petrologic characteristics of early part of the Okama-Goshikidake activity of the Zaovolcano.
Keywords:zao volcano, andestic lava, pyroclastic surge, agglutinate, calc-alkaline
Stratigraphy: The eruption products can be subdivided into Furikodaki lava, Goshikidake-nanpo lava and pyroclastics, Goshikidake-nanbu pyroclastics, Goshikidake-toubu pyroclastics. The Furikodaki lava flowed down from the northeastern base of the Goshikidake cone along a stream. The length and width of lava are ca. 750m and 20-30m, respectively. The Goshikidake-nanpo lava and pyroclastics cropped out in a narrow area of ca. 650m south from the summit of Goshikidake. This unit is composed of upper brecciated lava with coarser lateral and finer vertical joints, and lower hyaloclastite-like tuff breccia. Goshikidake-nanbu pyroclastics distributes ca. 500m southward from the summit, consisted of pyroclastic surge deposit and vent breccia. The former consists of lapilli tuff to volcanic breccia with ca. 10m in thickness. The latter consists of three veins elongated from northeast to southwest with 2-8m width and 5-8m length. These intrude nearly vertically into surge deposits. They are poorly sorted tuff breccia with minor amount of ~20cm rounded bombs. The Goshikidake-toubu pyroclastics distribute in ca. 300m eastward from the summit. The thickness is ca. 6-35m. They are fairly stratified tuff - lapilli tuff - tuff breccia including various amounts of volcanic bombs.
Petrography: All rocks are cpx-opx andesites. Most of plagioclase phenocrycrysts has dissolution textures such as dusty zone and/or patchy zoning. Plagioclase of the Furikodaki lava, the Goshikidake-nanpo lava and pyroclasts lack dusty zones. Opx and cpx phenocrysts show usually euhedral to subhedral texture except for some rounded cpx phenocrysts. About half of the pyroxene phenocrysts include sparsely glass inclusions. Cpx content in the bombs of the vent breccia of the Goshikidake-nanbu pyroclastics is higher than other products.
Whole rock chemistry: All products belong to medium-K, calc-alkaline series, with 56 to 58% SiO2. Looking at in detail, the range of SiO2 contents of Furikodaki lava, Goshikidake-nanpo lava and pyroclasts is 57.5-58wt%, while that of the other products is 56-57.7wt%. Most of the products are plotted in same variation trends in silica variation diagrams, but the bombs of the vent breccia have higher FeO, TiO2 contents and lower MgO contents than the other products.
Mineral chemistry: In Furikodaki lava, Goshikidake-nanpo lava and pyrocrastics, the core composition of opx phenocrycrysts is 64-65 Mg#. In the pyroclastic surge deposits, that is around 65 Mg#. In the bombs, that has a range of Mg#62-69. The cpx core composition shows a range of ca. 64 to 70 Mg# in all products. The core compositions of plagioclase phenocrycryst show wide range of An62-92. In Furikodaki lava, Goshikidake-nanpo lava and pyrocrastics, the main peak compositions are in An68-70 and around An78, and subordinate peak is in An90. The pyroclastic surge deposits have main peak compositions of An64-66 and An76-78, subordinate peak composition of An90. The bombs have main peak compositions of An62-66 and An74-76.