2:30 PM - 2:45 PM
[SVC43-14] Geologic and petrologic features of the Goshikidake-nanbu pyroclastics, Zao volcano
Keywords:Zao volcano, Goshikidake, Pyroclastic surge deposit, Pyroclastic dike, Temporal change of magma feeding system
Introduction
Zao volcano is an active stratovolcano in northeastern Japan. The last eruption was in 1940, but many phenomena such as volcanic tremors, slight uplift, and gas emission have been observed since January 2013. It is very important to understand temporal change of the magma feeding system beneath such active volcano. We have performed geologic and petrologic investigations on the youngest edifice, Goshikidake pyroclastic cone (ca .2 ka to present) and concluded (1) these products subdivided into the lava products (Furikodaki lava, Goshikidake-nanpo lava and pyroclastics) and the pyroclastic products (Goshikidake-nanbu pyroclastics, Goshikidake-tobu pyroclastics), (2) all products were formed by mixing between mafic- and felsic-end-member magmas, (3) petrologic features of mafic and felsic end-member magmas were slightly different between the lava and the pyroclastic products, (4) the mixed layer was formed in both cases, but it was more evolved in the lava products, (5) the residence time of felsic-end-member magma-derived orthopyroxene phenocrysts show less than 1 year and more than 5 years respectively. The former is dominant in the pyroclastic products, while the latter is dominant in the lava products.
In this study, we examined the Goshikidake-nanbu pyroclastics in more detail and compared the petrologic features with those of the lava products and the Goshikidake-tobu pyroclastics to understand the temporal change of the magma feeding system. We also examined the pyroclastic dikes included in the Goshikidake-nanbu pyroclastics, because such dikes are rarely observed in volcanoes in NE Japan.
Geologic features of the pyroclastic dikes
The Goshikidake-nanbu pyroclastics distribute in the southeast part of the Goshikidake. The products are composed of piles of pyroclastic surge deposits, and the maximum total thickness is about 12 m, which are intruded by three pyroclastic dikes (pyroclastic dike1~3). The height, lowermost width, uppermost width, and strike of the pyroclastic dikes 1~3 are 12m-7m-7m, 4m-2m-8m, 16m-5m-8m, N50°E, N70°E, N15°E, respectively. Pyroclastic dikes 1 and 2 show an upward-flaring shape, indicating explosive eruption origin. Pyroclastic dikes 1~3 are composed of matrix supported tuff-breccia containing juvenile bombs, lapilli, and small amount of the scoria and lithic fragments. The facies is similar to that of hyaloclastite. These features show the phreatomagmatic eruption origin. The boundaries between pyroclastic dikes and the surge deposits are clear and the surge deposits near the boundaries are reddish-brown colored. Weak stratified structure by horizontally elongated blocks and bombs partially observed in pyroclastic dike indicates possible fallback deposition. Large rock fragments are absent near the boundary, which would show effective fragmentation of the magma.
Comparison of the petrologic features of the geologic units
We compared the petrologic features among the lava products, Goshikidake nanbu-pyroclastics and the Goshikidake-tobu pyroclastics. In conclusion, the Goshikidake-nanbu pyroclastics have intermediate petrological features between the lava products and the Goshikidake-tobu pyroclastics. All rocks are pyroxene andesites and plotted on similar trends in variation diagrams. Looking more closely, the compositional trends of the lava products, the Goshikidake-nanbu pyroclastics, the Goshikidake-tobu pyroclastics shift to higher part in TiO2-SiO2 diagram and lower part in Cr-SiO2 diagram. These features indicate the compositions of the mafic-end member magma changed temporally. The reverse zoned type orthopyroxene phenocrysts showing longer residence times (ca. > 5 years) are abundant in the lava products but rare in the Goshikidake-tobu pyroclastics. In contrast, Mg-rich-rim type orthopyroxene phenocrysts showing shorter residence time (ca. < 1 year) are rare in the lava products but abundant in the Goshikidake-tobu pyroclastics. The amount of both types of orthopyroxene is intermediate in the Goshikidake-nanbu pyroclastics. These features show the mixed layer would be well-developed in the lava products, and that would be poorly developed in the Goshikidake-tobu pyroclastics. The mixed layer would develop intermediately in the Goshikidake-nanbu pyroclastics.
Zao volcano is an active stratovolcano in northeastern Japan. The last eruption was in 1940, but many phenomena such as volcanic tremors, slight uplift, and gas emission have been observed since January 2013. It is very important to understand temporal change of the magma feeding system beneath such active volcano. We have performed geologic and petrologic investigations on the youngest edifice, Goshikidake pyroclastic cone (ca .2 ka to present) and concluded (1) these products subdivided into the lava products (Furikodaki lava, Goshikidake-nanpo lava and pyroclastics) and the pyroclastic products (Goshikidake-nanbu pyroclastics, Goshikidake-tobu pyroclastics), (2) all products were formed by mixing between mafic- and felsic-end-member magmas, (3) petrologic features of mafic and felsic end-member magmas were slightly different between the lava and the pyroclastic products, (4) the mixed layer was formed in both cases, but it was more evolved in the lava products, (5) the residence time of felsic-end-member magma-derived orthopyroxene phenocrysts show less than 1 year and more than 5 years respectively. The former is dominant in the pyroclastic products, while the latter is dominant in the lava products.
In this study, we examined the Goshikidake-nanbu pyroclastics in more detail and compared the petrologic features with those of the lava products and the Goshikidake-tobu pyroclastics to understand the temporal change of the magma feeding system. We also examined the pyroclastic dikes included in the Goshikidake-nanbu pyroclastics, because such dikes are rarely observed in volcanoes in NE Japan.
Geologic features of the pyroclastic dikes
The Goshikidake-nanbu pyroclastics distribute in the southeast part of the Goshikidake. The products are composed of piles of pyroclastic surge deposits, and the maximum total thickness is about 12 m, which are intruded by three pyroclastic dikes (pyroclastic dike1~3). The height, lowermost width, uppermost width, and strike of the pyroclastic dikes 1~3 are 12m-7m-7m, 4m-2m-8m, 16m-5m-8m, N50°E, N70°E, N15°E, respectively. Pyroclastic dikes 1 and 2 show an upward-flaring shape, indicating explosive eruption origin. Pyroclastic dikes 1~3 are composed of matrix supported tuff-breccia containing juvenile bombs, lapilli, and small amount of the scoria and lithic fragments. The facies is similar to that of hyaloclastite. These features show the phreatomagmatic eruption origin. The boundaries between pyroclastic dikes and the surge deposits are clear and the surge deposits near the boundaries are reddish-brown colored. Weak stratified structure by horizontally elongated blocks and bombs partially observed in pyroclastic dike indicates possible fallback deposition. Large rock fragments are absent near the boundary, which would show effective fragmentation of the magma.
Comparison of the petrologic features of the geologic units
We compared the petrologic features among the lava products, Goshikidake nanbu-pyroclastics and the Goshikidake-tobu pyroclastics. In conclusion, the Goshikidake-nanbu pyroclastics have intermediate petrological features between the lava products and the Goshikidake-tobu pyroclastics. All rocks are pyroxene andesites and plotted on similar trends in variation diagrams. Looking more closely, the compositional trends of the lava products, the Goshikidake-nanbu pyroclastics, the Goshikidake-tobu pyroclastics shift to higher part in TiO2-SiO2 diagram and lower part in Cr-SiO2 diagram. These features indicate the compositions of the mafic-end member magma changed temporally. The reverse zoned type orthopyroxene phenocrysts showing longer residence times (ca. > 5 years) are abundant in the lava products but rare in the Goshikidake-tobu pyroclastics. In contrast, Mg-rich-rim type orthopyroxene phenocrysts showing shorter residence time (ca. < 1 year) are rare in the lava products but abundant in the Goshikidake-tobu pyroclastics. The amount of both types of orthopyroxene is intermediate in the Goshikidake-nanbu pyroclastics. These features show the mixed layer would be well-developed in the lava products, and that would be poorly developed in the Goshikidake-tobu pyroclastics. The mixed layer would develop intermediately in the Goshikidake-nanbu pyroclastics.