Paper No. 184-5
Presentation Time: 9:00 AM-6:30 PM
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.