VARIATIONS IN MANTLE COMPOSITION AND SUBDUCTION CONTRIBUTIONS TO MAFIC MAGMAS IN THE SOUTHERN CASCADE ARC

In the Cascade arc, basaltic magmas reflect a range of origins, from subduction-derived to decompression melts. Here we use magma geochemistry (major and trace elements, volatiles, isotopes) and ⁴⁰Ar/³⁹Ar geochronology to investigate spatial and temporal variations in mantle source and subduction contributions to mafic magmas erupted in the Cascade arc between northern California and central Oregon. Samples include whole rock and olivine-hosted melt inclusions from monogenetic basaltic volcanoes and from flows on Mt. McLoughlin, the southernmost stratovolcano in Oregon. New ⁴⁰Ar/³⁹Ar geochronology reveals a long history of volcanism in the Mt. McLoughlin region; the oldest flows are ~6 Ma and the most recent are only ~93 ka. The magmas in this study include calc-alkaline (CAB), low-K tholeiite (LKT) and intraplate-type (IPB) basalts. We find H₂O contents of olivine-hosted melt inclusions from the three magma types are uniform and low (≤2.6 wt%); however, S and Cl concentrations are higher in CAB and LKT magmas, suggesting a greater subduction influence. Similarly, other “subduction tracers” such as Pb/Ce and Sr/Nd are highest in the CABs and lowest in the IPBs. Although CABs have more radiogenic ⁸⁷Sr/⁸⁶Sr than LKTs and IPBs, Pb isotopes are nearly identical for the three magma types and most δ¹⁸O_olivinevalues overlap with mantle values. Combining our results with previously published geochemistry, we investigate temporal and spatial variations in mantle compositions and subduction contributions to basaltic magmas erupted in the southern Cascade arc. Similar to previous studies, we find that the mantle beneath the arc becomes progressively more enriched (e.g., higher Nb/Zr) and subduction contributions decrease northward along the arc. Although geochronology data are sparse, there are no clear variations in mantle source or subduction contribution over time in the southern arc. Our preliminary models of subduction contributions to the magmas suggest that CABs likely have very small amounts of sediment-rich subduction component added to a slightly enriched mantle source and LKTs have little/no subduction contribution to a more depleted mantle, whereas IPB magmas erupted in central Oregon likely originate from decompression melting of an enriched mantle source.