5:15 PM - 6:30 PM
[SCG49-P01] Magma plumbing system of the eruption products in West Chokai caldera, Chokai volcano
Keywords:Chokai volcano, magma plumbing system, hornblende
Chokai volcano is an active stratovolcano in rear arc side of NE Japan and has a long-eruption history of ca. 0.6 million years. West Chokai volcano started at about 0.16 Ma after the activity of old Chokai volcano and continued to the Holocene age. Whereas East Chokai volcano started at about 0.02 Ma and continued to present. During the activity of West Chokai volcano, a horseshoe shaped caldera,West Chokai caldera, was formed at about 0.12 Ma and following activity has occurred inner part of the caldera. In this study, the stratigraphy of the inner caldera eruption products was re-examined and the magma plumbing system of the products was revealed based on petrologic data. We found that the hornblendes are one of main phases crystallizing at several depths in middle crust during ascent of the mafic magmas, which triggered eruptions.
The eruption products of inner part of the caldera can be divided into eight units. These are Nakanosawa lower lavas, Sensumoriin dome lava, Nakanosawa upper lavas, Torinoumi scoria, Nabemorishitsugen agglutinate, and three lava units (Nabemoriyama dome lava, Maenabemori dome lava, Mansukedou lava) in ascending order. The order of formation of the last three lavas is unknown. The total volume of the products is ca. 0.8 km3 and about 90 percentage is of the Nakanosawa lower lavas.
The rocks are dark gray to gray lavas or black scoria, having plagioclase, clinopyroxene, orthopyroxene, olivine, and amphibole as phenocryst. The samples from older than Torinoumi scoria and SiO2 poorer samples of Nabemorishitsugen agglutinate are rich in olivine and the others are rich in amphibole. Mafic inclusions showing dikty-taxitic texture are observed in almost all units. Two types of mafic inclusions can be observed. Type A includes olivine and amphibole phenocrysts and type B does not include these.
As a whole, all samples are plotted on same trend in the variation diagrams. These belong to high-K calc-alkaline series. Taking a closer look, two variation trends are recognized. One is defined by Nakanosawa lower lavas and Sensumoriin dome lava, and the other is by Nakanosawa upper lavas, Torinoumi scoria, Nabemorishitsugen agglutinate (poorer SiO2). Nabemorishitsugen agglutinate (richer SiO2), Nabemoriyama dome lava, Maenabemoriyama dome lava, and Mansukedou lava plot on near the silica richer extension of either of the two trends. The type A mafic inclusions are plotted on the host rocks trend. On the other hand, type B ones are not. The trend of the type B is tholeiitic.
Low-Mg clinopyroxene (Mg#=71-75) and orthopyroxene (Mg#=64-67), and high-Fo (~85) olivine are observed in all samples. The high-Mg clinopyroxene (Mg#=~85) are rarely observed in some samples. Some of the low-Mg pyroxenes have ~30µm higher-Mg zone (Cpx: Mg#=~83, Opx: Mg#=~74) near the rim. The amphiboles are magnesio hornblende, pargasitic hornblende, or pargasite. The former is observed in all units except for Nakanosawa lower lavas and Sensumoriin dome lava. The latter is observed in all units. The pyroxene thermometry applied to low-Mg pyroxene core pairs gives 830 to 890 degrees C. The hornblende thermobarometry applied to magnesio hornblendes, pargasitic hornblendes, and pargasites gives ca. 840-860 °C and 1.2-1.5 kb, ca. 940-960 °C and 2.7-3.3 kb, and around 1000 °C and 4.5-6.0 kb, respectively. Nakanosawa lower lavas and Sensumoridome lava lack magnesio-hornblendes. The pargasitic hornblendes are observed in most of units and are usually large but are small in Nakanosawa upper lavas and Torinoumi scoria. The pargasites are observed in Nabemoriyama dome lava, Maenabemori dome lava, Mansukedou lava, and richer SiO2 samples of Nabemorishitsugen agglutinate.
Based on the estimated pressure condition of the hornblendes, the depth of shallowest magma chamber is considered to be 4-5 km. Low-Mg pyroxenes would also precipitate in this chamber. On the other hand, high-Fo olivine precipitated from much hotter mafic magmas at greater than 20 km depth. These magmas ascended and injected to the shallow magma chamber, that triggered the eruptions. The mafic magmas entrapped at several depths in the middle crust to differentiate and precipitate pargasitic hornblendes or pargasites. In other words, plutonic rocks having hornblendes were formed in middle crust during the volcanic activity. We note the compositions of both of shallow magma and deeper mafic magmas changed their composition after Sensumori dome lava formation, because the compositional trend changed from L to U trend.
The eruption products of inner part of the caldera can be divided into eight units. These are Nakanosawa lower lavas, Sensumoriin dome lava, Nakanosawa upper lavas, Torinoumi scoria, Nabemorishitsugen agglutinate, and three lava units (Nabemoriyama dome lava, Maenabemori dome lava, Mansukedou lava) in ascending order. The order of formation of the last three lavas is unknown. The total volume of the products is ca. 0.8 km3 and about 90 percentage is of the Nakanosawa lower lavas.
The rocks are dark gray to gray lavas or black scoria, having plagioclase, clinopyroxene, orthopyroxene, olivine, and amphibole as phenocryst. The samples from older than Torinoumi scoria and SiO2 poorer samples of Nabemorishitsugen agglutinate are rich in olivine and the others are rich in amphibole. Mafic inclusions showing dikty-taxitic texture are observed in almost all units. Two types of mafic inclusions can be observed. Type A includes olivine and amphibole phenocrysts and type B does not include these.
As a whole, all samples are plotted on same trend in the variation diagrams. These belong to high-K calc-alkaline series. Taking a closer look, two variation trends are recognized. One is defined by Nakanosawa lower lavas and Sensumoriin dome lava, and the other is by Nakanosawa upper lavas, Torinoumi scoria, Nabemorishitsugen agglutinate (poorer SiO2). Nabemorishitsugen agglutinate (richer SiO2), Nabemoriyama dome lava, Maenabemoriyama dome lava, and Mansukedou lava plot on near the silica richer extension of either of the two trends. The type A mafic inclusions are plotted on the host rocks trend. On the other hand, type B ones are not. The trend of the type B is tholeiitic.
Low-Mg clinopyroxene (Mg#=71-75) and orthopyroxene (Mg#=64-67), and high-Fo (~85) olivine are observed in all samples. The high-Mg clinopyroxene (Mg#=~85) are rarely observed in some samples. Some of the low-Mg pyroxenes have ~30µm higher-Mg zone (Cpx: Mg#=~83, Opx: Mg#=~74) near the rim. The amphiboles are magnesio hornblende, pargasitic hornblende, or pargasite. The former is observed in all units except for Nakanosawa lower lavas and Sensumoriin dome lava. The latter is observed in all units. The pyroxene thermometry applied to low-Mg pyroxene core pairs gives 830 to 890 degrees C. The hornblende thermobarometry applied to magnesio hornblendes, pargasitic hornblendes, and pargasites gives ca. 840-860 °C and 1.2-1.5 kb, ca. 940-960 °C and 2.7-3.3 kb, and around 1000 °C and 4.5-6.0 kb, respectively. Nakanosawa lower lavas and Sensumoridome lava lack magnesio-hornblendes. The pargasitic hornblendes are observed in most of units and are usually large but are small in Nakanosawa upper lavas and Torinoumi scoria. The pargasites are observed in Nabemoriyama dome lava, Maenabemori dome lava, Mansukedou lava, and richer SiO2 samples of Nabemorishitsugen agglutinate.
Based on the estimated pressure condition of the hornblendes, the depth of shallowest magma chamber is considered to be 4-5 km. Low-Mg pyroxenes would also precipitate in this chamber. On the other hand, high-Fo olivine precipitated from much hotter mafic magmas at greater than 20 km depth. These magmas ascended and injected to the shallow magma chamber, that triggered the eruptions. The mafic magmas entrapped at several depths in the middle crust to differentiate and precipitate pargasitic hornblendes or pargasites. In other words, plutonic rocks having hornblendes were formed in middle crust during the volcanic activity. We note the compositions of both of shallow magma and deeper mafic magmas changed their composition after Sensumori dome lava formation, because the compositional trend changed from L to U trend.