TWO YOUNG SILICIC SISTERS AT THREE SISTERS VOLCANIC FIELD, OREGON

Combined with detailed remapping of Middle and South Sister volcanoes, we have undertaken a comprehensive effort to calibrate their time-volume-compositional growth by 40Ar/39Ar at Menlo Park. Dozens of thin, crystal-rich olivine-plagioclase mafic lavas (52% SiO2) that form the upper half of the Middle Sister cone are dramatically exposed in the glaciated vertical E wall of the edifice. The severe glaciation is young, however, as this stack is sandwiched between the dacitic agglutinate complex of Chambers Lake that outcrops widely in the saddle between Middle and South Sisters (65% SiO2; 20 ± 2 ka) and a thick dacite coulee (64% SiO2; 14 ± 3 ka) that vented high (8500 ft) on the S flank of Middle Sister (10,047 ft). These lavas overlie stacks of crystal-poorer pyroxene-andesite lavas that make up the base of Middle Sister, exposed adjacent to Diller and Hayden Glaciers, and are currently being dated. Other silicic units around Middle Sister include: (1) dacite lavas of Renfrew Glacier that extend 3 km NW from the summit (18 ± 2 ka); (2) the dacite of Lane Plateau, 3.5 km WNW of the summit (21 ± 2 ka); and (3) rhyolite of Obsidian Cliff, ~3 km NW of the summit (76.5% SiO2; 38 ± 2 ka).

High on the E shoulder of South Sister, the intrusive andesitic core of the Hodge Crest vent yields an age of 17 ± 2 ka. Attempts to date the still-younger summit-forming andesitic agglutinate complex (56% SiO2) have so far failed. In addition to the well-known late Holocene rhyolites of Rock Mesa and the Devils Hill chain, silicic lava flows that erupted high on the flanks of South Sister include: (1) dacite of Moraine Lake (38 ± 2 ka); (2) rhyodacite S of Lewis Glacier that vented at 8600 ft (24 ± 5 ka); (3) a small rhyodacite dome S of Clark Glacier (50.5 ± 9.5 ka); (4) rhyodacite of Mesa Creek (47 ± 8 ka); and (5) rhyodacite of Carver Glacier (not yet dated). In addition, a major rhyolite lava complex as thick as 300 m that erupted near Carver Lake and flowed 3 km E down the headwaters of Squaw Creek yields an age of 36 ± 2 ka.

In the area of the INSAR anomaly centered 6 km W of the South Sister summit, dated units include: (1) dacite of Dew Lake (32 ± 2 ka); (2) basalt of vent knob 6482 (148 ± 4 ka); and (3) basalt of The Husband (149 ± 5 ka). Such peripheral vents as The Sphinx and The Wife significantly predate growth of the surprisingly young and clearly coeval South and Middle Sister edifices.