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

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).