1:45 PM - 3:15 PM
[SVC36-P02] Componentry of lithic fragments in the 120 ka Kutcharo caldera-forming eruption deposit (KpIV), eastern Hokkaido, Japan
Keywords:Caldera, Kutcharo volcano, Componentry of lithic fragments, Magma plumbing system
Geologic and petrologic information on lithic fragments composed of accessory and accidental materials from pre-existing rocks provides a powerful method to understand subsurface geology and evaluate source vents of caldera-forming eruptions (Suzuki-Kamata et al., 1993; Hasegawa et al, 2022). To investigate the spatio-temporal evolution of vent/conduit and magma plumbing systems of caldera-forming eruptions, we carried out componentry studies of lithic fragments of pyroclastic deposits from the largest caldera-forming eruption (KpIV: VEI>7) at Kutcharo volcano, eastern Hokkaido, Japan.
The eruption deposits of KpIV can be divided into 4 units (Fig. 1). In ascending order, Unit 1 is widely dispersed and consists of silt-sized ash. Unit 2 is a moderately sorted multiple pumice fall deposit. Unit 3 consists of widely distributed ignimbrite and is the most voluminous. It can be subdivided into Unit3-L (lower) and Unit3-U (upper). Lithic concentration zones (LCZ) are observed at the base of Unit3-L and Unit3-U. Unit 4 is composed of scoria flow deposits. Its distribution is limited to the northeast (NE) side of the caldera.
Based on distribution and lithofacies, we divided the area sampled into northwest (NW), northeast (NE), and south (S) of Kutcharo caldera (Fig. 1). Bulk samples were collected from each unit (except Unit 1) by passing through 4 mm field sieves to separate the fine component. Lithic fragments were picked and classified from dried bulk samples in the laboratory, and representative lithic fragments were cut for thin sectioning to verify observations made by eye and hand lens. Lithic fragments were divided into volcanic rocks (VOL), sedimentary rocks (SED) and altered rocks (ALT). VOL is characterized by clear (fresh) phenocrysts, and composed of 8 subtypes varying from porphyritic to aphyric, and from pyroxene basalt to dacite. ALT mainly consists of 3 subtypes of highly altered white (bleached), red (oxidized) and green (chloritized) volcanic rocks. Proportions (%) of these lithic types were estimated by grain counting. Based on the geologic maps/cross-sections of Kutcharo volcano, we infer that VOL is derived from early Quaternary pre-caldera volcanoes, whereas SED and ALT are from the Neogene basement (Yahata, 1989).
All bulk-sampled units contain more or less juvenile materials (pumice and scoria). Proportions of lithic fragments in the bulk samples were estimated to be 44%~54% in Unit 2 (pumice fall), while Units 3 and 4 (pyroclastic flows) have 14%~90% lithic fragments (the maximum value is from LCZ of Unit 3). The most common lithic fragment type in all units is VOL, but we clearly recognized a spatio-temporal change in proportions (percent lithic component) of ALT and SED. The lithic components for lower pumice fall of Unit 2 is richer in ALT (43%) than in the upper one (13%). VOL increases upward from 47% to 73% in Unit 2. Dominant lithic fragment in Unit3-L is VOL (66%~74%) followed by ALT (19%~33%) in the NE and S areas. ALT is characteristically rich in NW area’s Unit 3-L (50~59%). Of all the units, NE area’s Unit 3-U contains the largest amount (19%) of SED. Unit 4 is the richest in ALT (58%).
Previous geologic and petrologic studies (Hasegawa et al., 2016) suggest that at least 3 independent mafic magmas successively injected into a shallow large silicic magma body during KpIV eruption. These mafic injections triggered the ejection of each unit (Mastumoto et al., 2018). Our componentry results show that ALT which is likely from a deeper level is rich in the lower parts of Units 2, 3-L (only NW area) and 4. These data confirm previous findings that three mafic injections from a deeper level triggered the generation of Units 2, 3 and 4. Spatially biased abundance of ALT in NW area’s Unit 3-L and NE area’s Unit 4 suggest that injections occurred at NW and NE parts of Kutcharo caldera.
The eruption deposits of KpIV can be divided into 4 units (Fig. 1). In ascending order, Unit 1 is widely dispersed and consists of silt-sized ash. Unit 2 is a moderately sorted multiple pumice fall deposit. Unit 3 consists of widely distributed ignimbrite and is the most voluminous. It can be subdivided into Unit3-L (lower) and Unit3-U (upper). Lithic concentration zones (LCZ) are observed at the base of Unit3-L and Unit3-U. Unit 4 is composed of scoria flow deposits. Its distribution is limited to the northeast (NE) side of the caldera.
Based on distribution and lithofacies, we divided the area sampled into northwest (NW), northeast (NE), and south (S) of Kutcharo caldera (Fig. 1). Bulk samples were collected from each unit (except Unit 1) by passing through 4 mm field sieves to separate the fine component. Lithic fragments were picked and classified from dried bulk samples in the laboratory, and representative lithic fragments were cut for thin sectioning to verify observations made by eye and hand lens. Lithic fragments were divided into volcanic rocks (VOL), sedimentary rocks (SED) and altered rocks (ALT). VOL is characterized by clear (fresh) phenocrysts, and composed of 8 subtypes varying from porphyritic to aphyric, and from pyroxene basalt to dacite. ALT mainly consists of 3 subtypes of highly altered white (bleached), red (oxidized) and green (chloritized) volcanic rocks. Proportions (%) of these lithic types were estimated by grain counting. Based on the geologic maps/cross-sections of Kutcharo volcano, we infer that VOL is derived from early Quaternary pre-caldera volcanoes, whereas SED and ALT are from the Neogene basement (Yahata, 1989).
All bulk-sampled units contain more or less juvenile materials (pumice and scoria). Proportions of lithic fragments in the bulk samples were estimated to be 44%~54% in Unit 2 (pumice fall), while Units 3 and 4 (pyroclastic flows) have 14%~90% lithic fragments (the maximum value is from LCZ of Unit 3). The most common lithic fragment type in all units is VOL, but we clearly recognized a spatio-temporal change in proportions (percent lithic component) of ALT and SED. The lithic components for lower pumice fall of Unit 2 is richer in ALT (43%) than in the upper one (13%). VOL increases upward from 47% to 73% in Unit 2. Dominant lithic fragment in Unit3-L is VOL (66%~74%) followed by ALT (19%~33%) in the NE and S areas. ALT is characteristically rich in NW area’s Unit 3-L (50~59%). Of all the units, NE area’s Unit 3-U contains the largest amount (19%) of SED. Unit 4 is the richest in ALT (58%).
Previous geologic and petrologic studies (Hasegawa et al., 2016) suggest that at least 3 independent mafic magmas successively injected into a shallow large silicic magma body during KpIV eruption. These mafic injections triggered the ejection of each unit (Mastumoto et al., 2018). Our componentry results show that ALT which is likely from a deeper level is rich in the lower parts of Units 2, 3-L (only NW area) and 4. These data confirm previous findings that three mafic injections from a deeper level triggered the generation of Units 2, 3 and 4. Spatially biased abundance of ALT in NW area’s Unit 3-L and NE area’s Unit 4 suggest that injections occurred at NW and NE parts of Kutcharo caldera.