EVALUATING SUBDUCTION COMPONENT CONTRIBUTIONS TO THE SOUTHERN CASCADE ARC: INSIGHTS FROM GEOCHEMICAL ANALYSES OF MONOGENETIC VOLCANO MAFIC LAVAS AND GORDA PLATE SEDIMENTS

Subducted sediment at convergent margins plays a crucial role in worldwide arc magmatism by fertilizing the mantle and introducing volatiles and other elements to the mantle wedge (Plank and Langmuir, 1998). In the Cascadia Subduction Zone, sediment subduction may increase from the north to the south; this hypothesis is based on more radiogenic ⁸⁷Sr/⁸⁶Sr and less radiogenic ¹⁴³Nd/¹⁴⁴Nd ratios in basalts erupted in the southern arc (Green and Harry, 1999; Schmidt et al., 2008; Jicha et al., 2009). Furthermore, subduction signatures in the northern Cascades (Garibaldi Belt) have been shown to be negligible (Mullen and Weis, 2013). To constrain subducted sediment contributions in the southern Cascades, this research uses whole-rock major and trace elements, whole-rock radiogenic isotopes, and olivine δ¹⁸O from basaltic monogenetic volcanoes from the southern Cascade arc and ten drill core sediment samples from the Gorda Plate. Concentrations of fluid-mobile trace elements in primary melt compositions (re-equilibrated with Fo₉₀) indicate a subduction component contribution similar to the northern Cascade arc (Ba 175-360 ppm in the south, 150-675 ppm in the north (Mullen and Weis, 2013); Pb 3-9 ppm in the south, 1-6 ppm in the north (Mullen and Weis, 2013)). However, ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd signatures are similar to other southern Cascade arc lavas (⁸⁷Sr/⁸⁶Sr 0.70354± 0.014 - 0.70375±0.014, ¹⁴³Nd/¹⁴⁴Nd 0.51285±0.01 - 0.51291±0.01); these lavas have lower ¹⁴³Nd/¹⁴⁴Nd and higher ⁸⁷Sr/⁸⁶Sr ratios than northern Cascades basalts. Pb isotope signatures (²⁰⁸Pb/²⁰⁴Pb and ²⁰⁶Pb/²⁰⁴Pb) of lavas and Gorda Plate sediments from this study overlap, which may indicate a contribution from Gorda Plate sediments to the arc lavas. Preliminary δ¹⁸O olivines are similar to mantle δ¹⁸O, which may indicate that at least some magmas are derived primarily from decompression melting of a minimally hydrated mantle. Additional ¹⁸O of olivine phenocrysts, as well as further trace element and radiogenic isotopes of basaltic lavas and Gorda Plate sediments, will be used to model sediment input into the sub-arc mantle, and to investigate along-arc trends in sediment subduction.