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Potassium-argon geochronology of a basalt-andesite-dacite arc system: The Mount Adams volcanic field, Cascade Range of southern Washington

¹ U.S. Geological Survey, Menlo Park, California 94025

High-precision K-Ar dating and detailed mapping have established an eruptive chronology for a Cascade stratovolcano and its surrounding array of coeval basaltic centers. Mount Adams is a 200 km³ andesitic cone that stands at the center of a coeval 1250 km² Quaternary volcanic field that contains >60 discrete vents. K-Ar ages were measured for 74 samples from 63 of the 136 volcanic units defined in the course of the mapping. Within analytical error, there are no discrepancies between K-Ar ages and stratigraphic sequence. Major activity began in the area ca. 940 ka, and inception of the central stratovolcano took place at ca. 520 ka. A plot of cumulative volume erupted versus time shows that between 940 and 520 ka the eruptive rate was <0.04 km³/k.y. and 80% of the products were basaltic. Andesites are volumetrically dominant and were emplaced in three main cone-building episodes centered at 500, 450, and 30 ka—at eruptive rates of 1.6-5 km³/k.y. At a lower rate of 0.05-0.1 km³/k.y., the magmatic system remained almost continuously active between the main pulses—although breaks in activity as long as 30 k.y. are permitted by the K-Ar data. Andesitic-dacitic activity in the focal region and dominantly basaltic activity on the periphery have coexisted for 520 k.y., and their products are interstratified. The last main episode of cone construction occurred ca. 40-10 ka, the oldest andesite identifiably derived from within the present-day edifice having an age of 33 ± 14 ka. Andesites forming the south-summit rim and the true summit have ages of 13 ± 8 ka and 15 ± 8 ka, respectively.

The time-volume-composition data bear upon several fundamental questions concerning the long-term behavior of arc volcanoes. Stratovolcanoes commonly grow in spurts but can stay active between the widely spaced episodes of peak productivity. Large stratocone systems may remain active for half a million years. Subdivision of complex stratovolcanoes into eruptive or constructional "stages" without detailed mapping, geochronology, and compositional data should be treated skeptically. Discussion of volumetric eruptive rates can be misleading without an adequate time scale. Stratovolcanoes need never develop large upper-crustal magma chambers. Basalt erupts peripherally, but its ascent is suppressed beneath stratovolcanoes by deep-crustal domains of magma storage and hybridization that form where concentrated injection of basalt amplifies crustal melting.

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