HR: 1330h
AN: V32D-1049    [PDF]
TI: Silicic Eruptions of the Past 50 kyr at the Three Sisters Volcanic Cluster
AU: * Calvert, A T
EM: acalvert@usgs.gov
AF: Volcano Hazards Team, U.S. Geological Survey, 345 Middlefield Rd., MS-910, Menlo Park, CA 94025 United States
AU: Hildreth, W
EM: hildreth@usgs.gov
AF: Volcano Hazards Team, U.S. Geological Survey, 345 Middlefield Rd., MS-910, Menlo Park, CA 94025 United States
AU: Fierstein, J
EM: jfierstn@usgs.gov
AF: Volcano Hazards Team, U.S. Geological Survey, 345 Middlefield Rd., MS-910, Menlo Park, CA 94025 United States
AB: The Three Sisters volcanic cluster in the central Oregon Cascades consists of mafic to intermediate stratovolcanoes surrounded by mafic and silicic flows and domes. The bulk of South and Middle Sister are late Pleistocene while North Sister is middle Pleistocene (Schmidt and Grunder, 2003 GSA abs.). Thick rhyolite and dacite lava flows and domes are rich in potassium, and young glaciation exposes holocrystalline groundmass textures ideal for argon geochronology. Several of these silicic flows bracket stratovolcano growth, and precise geochronology illuminates a rich basalt to rhyolite history in the cluster over the past 50 kyr. Careful step-heating $^{40}$Ar/$^{39}$Ar experiments yield excellent plateau ages with radiogenic yields often above 5% on rocks as young as 20 ka. Most samples have well-determined isochrons with atmospheric (40/36 = 295.5) intercepts, although several have intercepts as low as 286. South Sister is a composite cone with an andesite/dacite base (Hodge Crest) and a young andesite summit sequence. The basal flow of the 300m-thick, youngest conformable stack of andesite lavas at the summit is 27$\pm$3 ka. Unconformably underlying portions date back to at least 50 ka based on ages of overlapping silicic flows. The base of Middle Sister is andesite overlain by $\sim$300m of olivine basalt. Some Middle Sister andesites and all basalts overlie a distinctive dacite agglutinate (20$\pm$2 ka) in the South/Middle Sister saddle. All Middle Sister lavas underlie a thick dacite flow (14$\pm$3 ka) that vented at 8500 ft (2600m) on its S flank. Dacite lava flows erupted from the Middle/North Sister saddle at 27$\pm$2 and 18$\pm$2 ka. Additionally, several $>$100m-thick rhyolite and dacite lavas vented low on the flanks of the cluster. Obsidian Cliff rhyolite (37.8$\pm$1.8 ka) and Lane Plateau dacite (21.4$\pm$1.9 ka) erupted W of Middle Sister, the Dew Lake dacite (32.3$\pm$1.8 ka) located near the locus of the INSAR anomaly W of South Sister flowed around a 148$\pm$4 ka (knob 6482) basalt and near the 149$\pm$5 ka (The Husband), 159$\pm$10 ka (Upper Mesa Creek) and 279$\pm$6 ka (The Sphinx) basalts. The south flank of South Sister is dominated by the late Holocene rhyolites of Rock Mesa and the Devil's Hill chain and about ten 24-50 ka rhyolite and dacite domes and flows. A 100m-thick 35$\pm$2 ka rhyolite lava also extends NE from South Sister into Squaw Creek. In sum, South and Middle Sisters are mafic to intermediate and their youngest activity is coeval. Silicic flows on and around the volcanic cluster are voluminous, abundant and young.
DE: 8439 Physics and chemistry of magma bodies
DE: 8494 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting