MANTLE SOURCES INTERPRETED FROM MAFIC VOLCANOES ADJACENT TO THE ANDESITIC GOAT ROCKS VOLCANIC COMPLEX, SOUTHERN WASHINGTON CASCADES

In the Southern Washington Cascades (SWC), broadly distributed mafic volcanoes provide information about the mantle compositions and conditions that underlie the arc. Basalts and basaltic andesites erupted on the periphery of the Goat Rocks volcanic complex, a long-lived, recently extinct andesitic center between Mt. Adams and Mt. Rainier, between about 3 Ma and 200 ka. Vent areas include Devils Washbasin, Hogback Mountain, and the Walupt Lake area. The most primitive lavas from each of these suites are calc-alkaline basalts, with an array of trace element and radiogenic isotope compositions that point to repeated tapping of multiple mantle sources. The Devils Washbasin volcano, one of the oldest mafic centers, erupted basalts distinct from others in the Goat Rocks area and from the SWC as a whole: high in Ca at given Mg, depleted in HFSE and HREE, and containing the most refractory spinel (Cr# 0.5-0.6), but isotopically similar to other Goat Rocks area basalts. Broadly, basalts from the periphery of the Goat Rocks complex fall within two groups: 1) more LILE-enriched, higher Ba/Nb, more fractionated HREE, and more radiogenic ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd (including Devils Washbasin); and 2) less LILE-enriched, lower Ba/Nb, less fractionated HREE, and less radiogenic ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd. Notably, lavas from both groups erupted in close spatial and temporal proximity, e.g. at Hogback Mountain and Walupt Lake. Our preliminary interpretation is that the basaltic melts are derived from repeated tapping of both a deeper, more subduction-influenced source (Group 1, with a larger degree of flux melting yielding the unique compositions of Devils Washbasin) and a shallower source with a weaker subduction component (Group 2). Isotopically, the Goat Rocks area basalts are intermediate between Mt. Adams CAB (closer to Group 1) and Mt. Adams IPB (closer to Group 2), and along with basalts from other SWC vents, form an array that is slightly oblique to the High Cascades and subparallel to the Adams Array in Pb-isotopes (Mullen et al., 2017). We infer that the HIMU-like mantle component that contributes to some Mt. Adams basalts is also distributed further north and west beneath the SWC, including the Goat Rocks area, where its contribution is diluted by a greater subduction component.