HR: 0800h
AN: V51A-1472    [Abstracts]
TI: Youngest volcanism about 1 million years ago at Kaho`olawe Island, Hawai`i, was not part of rejuvenated-stage volcanism
AU: Sano, H
EM: sanoubb@ybb.ne.jp
AF: Division of Earth and Planetary Sciences, Kyoto University, Kyoto, 606-8502 Japan
AU: * Sherrod, D R
EM: dsherrod@usgs.gov
AF: Cascades Volcano Observatory, U.S. Geological Survey 1300 SE Cardinal Court, Vancouver, WA 98683 United States
AU: Tagami, T
EM: tagami@kueps.kyoto-u.ac.jp
AF: Division of Earth and Planetary Sciences, Kyoto University, Kyoto, 606-8502 Japan
AB: To understand rejuvenated-stage volcanism we must delimit its extent. Kaho`olawe island's youngest eruptions left five small deposits of dike-fed cinders and sparse lava flows, previously undated but thought part of a rejuvenated stage on the basis of a substantial erosional unconformity. The dikes cut colluvial breccias on the east wall of Kanapou Bay, and the lava and cinders discordantly mantle the flat-lying lava flows of the Kanapou caldera. Using peak-height comparison methods and mass fractionation correction procedures, we obtained K-Ar ages and 2-sigma error of 1.41ñ0.45 Ma (KHLW-1, lava flow) and 0.97ñ0.20 Ma (KHLW-2, dike) from the southernmost site and 1.19ñ0.24 Ma (KHLW-4, dike) from the next site northward. These three ages overlap at their 2-sigma uncertainty. Normal polarity magnetization for the southerly dike suggests it was emplaced during the Jaramillo Normal-Polarity Subchron, between about 0.98 and 1.04 Ma. We extend this interpretation to the other dike and the volcanic deposits, although we lack exhaustive magnetization results. For comparison, the emplacement of Kaho`olawe's shield and postshield lava flows encompasses the time from about 1.4 to 1.04 Ma or only slightly older, on the basis of seven previously published ages and our own new age from a caldera-filling lava. The only magnetization data available is the reversed-polarity determination we obtained from a caldera-filling lava flow. The dating results indicate that very little time intervened between the filling of the Kanapou caldera and the development of a substantial erosional unconformity now flooded by Kanapou Bay-perhaps as little as 0.05 m.y. and probably not more than 0.1-0.2 m.y. Given such a brief intervening period, we assign the young volcanic rocks to Kaho`olawe's late-shield transition or postshield-stage volcanism. Kaho`olawe joins the ranks of Wai`anae and Haleakala, other volcanoes in the island chain that lack rejuvenated-stage volcanism but that experienced substantial short-lived erosional events within volcanic periods.
DE: 1105 Quaternary geochronology
DE: 1115 Radioisotope geochronology
DE: 8137 Hotspots, large igneous provinces, and flood basalt volcanism
DE: 8486 Field relationships (1090, 3690)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005