HR: 1330h
AN: V42B-0347    [PDF]
TI: Petrology of Holocene Caldera-Forming Eruptions at Ksudach, Kamchatka
AU: * Izbekov, P
EM: pavel@gi.alaska.edu
AF: AVO, Geophysical Institute, 903 Koyukuk Drive, Fairbanks, AK 99775 United States
AU: Gardner, J E
EM: gardner@mail.utexas.edu
AF: Department of Geological Sciences, The University of Texas at Austin,1 University Station C1100, Austin, TX 78712 United States
AU: Andrews, B
EM: ftbja@uaf.edu
AF: AVO, Geophysical Institute, 903 Koyukuk Drive, Fairbanks, AK 99775 United States
AU: Ponomareva, V V
EM: ponomareva@geo.tv-sign.ru
AF: IVGG, Piip 9, Petropavlovsk-Kamcha, 683006 Russian Federation
AU: Melekestsev, I V
EM: ivgg@mail.kamchatka.ru
AF: IVGG, Piip 9, Petropavlovsk-Kamcha, 683006 Russian Federation
AB: Ksudach caldera complex, Kamchatka, appears to represent a case where the mechanism of recurrent caldera-forming eruptions can be inferred using petrological and experimental studies. A sequence of three caldera-forming eruptions occurred at Ksudach during the Holocene: KS-4 (8800 yr. BP), KS-2/KS-3 ($\sim$6000 yr. BP), and KS-1 (1800 yr. BP). This sequence produced a complex of nested calderas, each of which is 2-km to 2x5-km in size. Erupted magmas range from andesite (62.5 wt.% SiO$_{2}$) to rhyolite (72.1 wt.% SiO$_{2}$), though compositional variations within each caldera-forming eruption are smaller. For all erupted magmas the main mineral assemblage is plagioclase, orthopyroxene, clinopyroxene, and Fe-Ti-oxides. We focused first on mineral and glass compositions of the most evolved magmas erupted during each caldera-forming event: the dacite of KS-4 initial fall deposit (67.4 wt.% SiO$_{2}$), the KS-3 rhyolite (70.3 wt.% SiO$_{2}$), and the KS-1 rhyolites (71.5-72.1 wt.% SiO$_{2}$). Magnetite-ilmenite thermometry indicates that the dacite of KS-4 was last equilibrated at 914-924$\deg$C and f$_{O2}$ of NNO+0.4, KS-3 rhyolite at 894-927$\deg$C and f$_{O2}$ of NNO+0.1, KS-1 rhyolite at 870-907$\deg$C and NNO+0.6. It appears that there is a weak overall cooling trend from older to younger magmas. Plagioclase is a dominant phenocryst phase in the studied KS-1, KS-3, and KS-4 products. Oscillatory-zoned plagioclases are most common. Their compositions range from An$_{52.3\pm3}$ in KS-4 to An$_{43\pm6}$ in KS-1, with KS-3 plagioclase compositions being intermediate.In addition to oscillatory-zoned plagioclases, KS-3 magmas contain "dusty-zoned" plagioclases, which resorbed An$_{46-49}$ cores are mantled by An$_{72-75}$ inclusion-rich rims, as well as rare anorthite xenocrysts (An$_{93-96}$). The ongoing petrological experiments and the study of erupted products will be used 1) to investigate whether the erupted magmas represent snapshots of an evolving long-lived magma body, 2) to determine the final storage conditions for the magmas, and 3) to explore possible mechanisms of the recurrent caldera-forming eruptions.
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 8400 VOLCANOLOGY
DE: 8414 Eruption mechanisms
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting