SPATIO-CHEMICAL ANALYSIS OF MAFIC LAVAS OF THE CENTRAL OREGON HIGH CASCADES

The central to northern portion of the Oregon Cascade arc shows the highest vent density within the contemporary arc. Lavas erupted from the latest Miocene through the Quaternary have been dominantly mafic (SiO₂ < 57 wt.%), with basaltic andesites (52 to 57 wt.% SiO₂) being especially prevalent. The central Oregon High Cascades exhibit an especially high concentration of small to medium volcanic edifices typically aligned on north-south lineaments parallel to the strike of the High Cascades Graben. This study examines the chemical similarities of distinct volcanic centers in close spatial proximity, particularly where vents are clustered or aligned.

Previous investigations in the central Oregon Cascades, such as those conducted on the Sand Mountain Volcanic Field and the Wuksi Volcanic Chain, have shown subtle but undeniable chemical variations between lavas erupted from clustered vents which erupted within a short timeframe. Here, we combine this understanding with spatial analysis methods employed in the Chichinautzin Volcanic Field of Mexico (Robidoux et al., 2020) to examine spatial magma heterogeneity within this unique arc segment. Data for this study include new bulk-rock analyses alongside published and previously unpublished data for mafic volcanic centers in the Oregon High Cascades.

Early results show that chemical heterogeneities within lavas sourced from clustered or aligned vents are common in the study area. This is the most evident in the rare earth elements (REE) where discrepancies, namely crossing chondrite-normalized REE patterns, are difficult to reconcile with fractionation within magmatic reservoirs. Although complete prior REE data are currently limited, more widely available elements and element pairs mirror the REE behavior (i.e., variable Ba/Nb and/or Sr/Y at the same wt.% MgO). These findings illustrate the complex petrogenetic processes underlying the extensive mafic magmatism in the central Oregon Cascade arc.