EVALUATING SUBDUCTION CONTRIBUTIONS TO THE SOUTHERN CASCADE ARC: INSIGHTS FROM THE GEOCHEMISTRY OF MAFIC ARC LAVAS AND GORDA PLATE SEDIMENTS

In the Southern Cascade Arc (SCA) constraints on subduction component (SC) and sub-arc mantle contributions are not well known. Here we combine new geochemical analyses (major and trace elements, Sr-Nd-Pb-Hf-O isotopes) of Gorda Plate sediments, offshore of northern California, and of mafic lavas from central and southern Oregon to constrain SC and sub-arc mantle contributions to arc lavas. We also include new ⁴⁰Ar/³⁹Ar geochronology to assess spatial and temporal variations in SC and mantle contributions in the SCA. The lavas from this study range from basalts to basaltic andesites with endmembers including calc-alkaline basalts (CAB), low-K tholeiites (LKT), and intraplate basalts (IPB). As with other studies, LKT and IPB magmas have lower concentrations of fluid-mobile trace elements (e.g., Ba, Sr, Pb) than CAB. Trace element concentrations also reveal variations in mantle source; Nb/Zr ratios are low in southern Oregon and increase in central Oregon (where IPB magmas are found), suggesting a shift from a depleted mantle to a more enriched mantle beneath central Oregon. Radiogenic isotope ratios of the magmas in this study (e.g., ⁸⁷Sr/⁸⁶Sr 0.70351-0.70381; ¹⁴³Nd/¹⁴⁴Nd 0.51286-0.51291) are generally similar to those found elsewhere in the SCA. δ¹⁸O values of olivine (5.1-5.4‰) are similar to mantle values, which suggests small subduction contributions, given the elevated δ¹⁸O values of Gorda sediment (13-18‰). In general, the geochemistry of Gorda Plate seafloor sediments is similar to that of the northern Juan de Fuca, however, Gorda sediments have more radiogenic ⁸⁷Sr/⁸⁶Sr. In order to assess mantle and SC contribution to these arc lavas, primary melt compositions have been calculated through addition of equilibrium phenocryst phases (olivine ± plagioclase). We have modeled SC contributions using Arc Basalt Simulator 5 (Kimura, 2017); models were only completed for southern Oregon basalts as the IPBs of central Oregon appear to be melts of an enriched mantle with little/no SC addition. Our models suggest that the SC contributions to the SCA have become more sediment rich over time: magmas >2 Ma have more sediment-poor (~45-60%) SC, whereas magmas <2 Ma have sediment-rich (~65-70%) SC additions. The one LKT magma in this study has lesser SC additions (~4.5%) compared to the CAB magmas (~7-11%). This work provides the first estimates of SC contributions in the SCA, and indicates that temporal and spatial variations in mantle and subduction inputs to the southern arc are significant.