1:45 PM - 2:00 PM
[SVC34-01] Evolution of a multicyclic caldera volcano inferred from lithic analyses of deposits (Kp IV) of caldera-forming eruption - Kutcharo volcano, Hokkaido
Keywords:Kutcharo volcano, Caldera, Caldera Cycle, Lithic component
Reconstruction of evolutionary processes of calderas that have experienced multiple caldera-forming eruptions (multicyclic) is generally difficult due to repeated volcanic and structural overprinting by later eruptions. This issue can be solved by using geologic and petrologic information on lithic fragments in the caldera-forming eruption deposits that were destroyed and incorporated by the eruption, and provides a useful method for evaluating pre-existing geology (Cole et al., 1998).
Kutcharo volcano in eastern Hokkaido is a multicyclic caldera that formed after the pre-caldera volcanic activity between 1.84 and 0.87 Ma. The caldera-forming activity consists of at least nine cycles of large-scale eruptions from FWT (ca. 400 ka), Kp VIII (ca. 200 ka) to Kp I (ca. 40 ka) (Hasegawa et al., 2016). The Kp IV eruption (120 ka) is the most voluminous (175 km3). We analyzed the componentry and chemistry of lithic fragments in the Kp IV eruption deposits in order to evaluate vent geology and to discuss the evolutionary process at Kutcharo caldera.
Bulk samples were collected from pumice flow unit of Kp IV at nine outcrops, which is the main body of the eruption deposits. The proportion of lithic fragments for bulk samples averaged around 35 wt.%. More than 300 lithic fragments (32 to 4 mm) per outcrop were observed and counted for component calculation. Lithic fragments were classified into six types (obsidian fraction was removed from the component analysis due to the similar chemistry as co-existing pumice). Lithic components do not differ significantly among samples collected from nine outcrop. The most common lithic type is (i) fresh volcanic rock (VOL) that accounts for approximately over 50 wt. %. (ⅱ)Consolidated sedimentary rock (SED) and two altered rock types (white/green volcanic rocks, i.e., lithic component types (iii) and (iv)) can be correlated with the Miocene basement of Kuctharo volcano as described by Katsui (1962). The remaining two types of lithic components ((v) and (vi)) do not correspond to any yet reported geological formations in this area.
We determined the petrography and whole-rock geochemistry of VOL samples for correlation with pre-caldera lava (PRE) samples. Although the phenocryst assemblage and modal composition of VOL is similar to those of PRE, their whole-rock compositions are distinctive. VOL is predominantly dacitic ranging from basaltic andesite to rhyolite, whereas PRE is mainly basaltic to andesitic (Fig.1). In addition, VOL has lower Sr concentrations (32 to 304 ppm) and more LREE enrichment than PRE. These petrological features of VOL, including major and trace elements, are consistent with those of Kp Ⅳ juvenile materials (scoria and pumice), suggesting that VOL originated from the magma related to Kp IV caldera-forming eruption rather than pre-caldera activity.
Several post-caldera volcanic edifices, that formed between nine cyclic caldera-forming eruptions, have been reported in Kutcharo caldera (Hirose and Nakagawa, 1995). One of them, Birao volcano (300±20 ka) formed after the 400 ka FWT eruption, has geochemical composition closely matching that of VOL. Our future work will include radiometric dating of VOL samples to evaluate the origin of the volcanic rocks in detail.
The absence of PRE and abundance of VOL in the lithic components in the Kp IV eruption deposits suggests that VOL are likely derived from caldera volcanic edifices that existed within the caldera (post-caldera formations) after the former (Kp V) large-scale eruption. Similar cases, where lithic fragments in caldera-forming eruption deposits have similar characteristics with juvenile material rather than pre-caldera volcanic rocks have been reported in other multicyclic caldera volcanoes, such as Santorini caldera (Druitt, 2014). We need further investigation to fully understand the geologic and magmatic evolution of multicyclic caldera volcanoes.
Kutcharo volcano in eastern Hokkaido is a multicyclic caldera that formed after the pre-caldera volcanic activity between 1.84 and 0.87 Ma. The caldera-forming activity consists of at least nine cycles of large-scale eruptions from FWT (ca. 400 ka), Kp VIII (ca. 200 ka) to Kp I (ca. 40 ka) (Hasegawa et al., 2016). The Kp IV eruption (120 ka) is the most voluminous (175 km3). We analyzed the componentry and chemistry of lithic fragments in the Kp IV eruption deposits in order to evaluate vent geology and to discuss the evolutionary process at Kutcharo caldera.
Bulk samples were collected from pumice flow unit of Kp IV at nine outcrops, which is the main body of the eruption deposits. The proportion of lithic fragments for bulk samples averaged around 35 wt.%. More than 300 lithic fragments (32 to 4 mm) per outcrop were observed and counted for component calculation. Lithic fragments were classified into six types (obsidian fraction was removed from the component analysis due to the similar chemistry as co-existing pumice). Lithic components do not differ significantly among samples collected from nine outcrop. The most common lithic type is (i) fresh volcanic rock (VOL) that accounts for approximately over 50 wt. %. (ⅱ)Consolidated sedimentary rock (SED) and two altered rock types (white/green volcanic rocks, i.e., lithic component types (iii) and (iv)) can be correlated with the Miocene basement of Kuctharo volcano as described by Katsui (1962). The remaining two types of lithic components ((v) and (vi)) do not correspond to any yet reported geological formations in this area.
We determined the petrography and whole-rock geochemistry of VOL samples for correlation with pre-caldera lava (PRE) samples. Although the phenocryst assemblage and modal composition of VOL is similar to those of PRE, their whole-rock compositions are distinctive. VOL is predominantly dacitic ranging from basaltic andesite to rhyolite, whereas PRE is mainly basaltic to andesitic (Fig.1). In addition, VOL has lower Sr concentrations (32 to 304 ppm) and more LREE enrichment than PRE. These petrological features of VOL, including major and trace elements, are consistent with those of Kp Ⅳ juvenile materials (scoria and pumice), suggesting that VOL originated from the magma related to Kp IV caldera-forming eruption rather than pre-caldera activity.
Several post-caldera volcanic edifices, that formed between nine cyclic caldera-forming eruptions, have been reported in Kutcharo caldera (Hirose and Nakagawa, 1995). One of them, Birao volcano (300±20 ka) formed after the 400 ka FWT eruption, has geochemical composition closely matching that of VOL. Our future work will include radiometric dating of VOL samples to evaluate the origin of the volcanic rocks in detail.
The absence of PRE and abundance of VOL in the lithic components in the Kp IV eruption deposits suggests that VOL are likely derived from caldera volcanic edifices that existed within the caldera (post-caldera formations) after the former (Kp V) large-scale eruption. Similar cases, where lithic fragments in caldera-forming eruption deposits have similar characteristics with juvenile material rather than pre-caldera volcanic rocks have been reported in other multicyclic caldera volcanoes, such as Santorini caldera (Druitt, 2014). We need further investigation to fully understand the geologic and magmatic evolution of multicyclic caldera volcanoes.