33 Magmatism in the Cascades: Variations in Space and Time

Friday, 17 May 2019
8:00 AM - 12:00 PM
Room C124

DIVERGENCE OF GEOCHEMICAL TRACERS IN ANDESITIC SUITES FROM AMBIENT BASALT OVER ~3 M.Y. OF ACTIVITY AT THE GOAT ROCKS VOLCANIC COMPLEX, CASCADE ARC, WASHINGTON

Paper number: 33-5
Program: Theme Sessions
Time: 9:15 AM – 9:35 AM

Authors

Abstract

Age and compositional data reveal ~3 m.y. of evolution of the recently extinct Goat Rocks volcanic complex, a long-lived cluster of broadly andesitic composite cones that was built in four stages. New 40Ar/39Ar ages on groundmass are more precise than, and typically consistent with or slightly younger than, previously determined zircon U/Pb ages (Wall et al., 2018). The productive Lake Creek volcano was active until at least 559 +/-5 ka, and thus a maximum ~119 k.y. elapsed before the onset of Old Snowy Mountain stage volcanism.

Exploratory isotopic data of Goat Rocks area calcalkaline basalts (CABs) yield changes in time. In general, primitive basalts erupted from peripheral vents (Devils Washbasin, Hogback Mountain, and Walupt Lake volcanoes) have isotopic compositions similar to other CABs in the southern Washington Cascades. These CABs align with the High Cascades array in 87Sr/86Sr-143Nd/144Nd as well as 208Pb/204Pb-206Pb/204Pb space, and are displaced toward the Adams-Simcoe intraplate basalt array (or toward average Rainier basement) in 207Pb/204Pb-206Pb/204Pb space (Mullen et al., 2017). In detail, the Goat Rocks area primitive basalts record less radiogenic 208Pb/204Pb and 207Pb/204Pb through time, and the youngest sample has less radiogenic 87Sr/86Sr and 143Nd/144Nd than the oldest sample, though basalts of intermediate age have variable Sr and Nd isotope ratios.

The intermediate to felsic magmas erupted at Goat Rocks exhibit temporal patterns in trace element ratios that in some cases oppose those of the peripheral basalts. While the basalts display decreasing Ba/Nb and Sr/Y through time, Goat Rocks andesites and dacites display broadly increasing Ba/Nb and Sr/Y, as well as increasing Ba/Th and La/Nb and decreasing Th/La and Hf/Pb. CABs of the southern Washington Cascades generally overlap in trace element concentrations and ratios, but intermediate compositions of the major arc volcanoes occupy different fields, and at Goat Rocks they changed systematically through time from more Adams-like to more Rainier- or St. Helens-like. Based on the decoupled basaltic and andesitic behavior, we infer that over time, andesite genesis at Goat Rocks became more strongly controlled by crustal factors (e.g. magma reservoir size and depth, wall rock) than by input from mantle magma source(s).

Geological Society of America Abstracts with Programs. Vol. 51, No. 4

doi: 10.1130/abs/2019CD-329705

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