Cordilleran Section - 115th Annual Meeting - 2019

Paper No. 33-2
Presentation Time: 8:35 AM


JOHNSON, Emily R.1, SHAFFER, Jamie2, COLE, Meredith A.3, RAMOS, Frank C.1, BINDEMAN, Ilya N.4 and CALVERT, Andrew T.5, (1)Department of Geological Sciences, New Mexico State University, PO Box 30001, MSC 3AB, Las Cruces, NM 88003, (2)Department of Geological Sciences, New Mexico State University, Dept. Geological Sciences/MSC 3AB New Mexico State University, P.O. Box 30001, Las Cruces, NM 88003, (3)Department of Geology, University at Buffalo, Buffalo, NY 14260, (4)Earth Sciences, University of Oregon, Eugene, OR 97403, (5)US Geological Survey, 345 Middlefield Rd, MS-937, Menlo Park, CA 94025

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 δ18Oolivinevalues 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.