REGIONAL SCALE INVESTIGATION OF WIDESPREAD BASALTIC ANDESITES OF THE CENTRAL TO NORTHERN OREGON CASCADE ARC

The central to northern Oregon portion of the Cascade Arc is anomalous within the range due to high Quaternary vent density and overwhelmingly mafic (SiO₂ < ~57 wt.%) character of volcanic products. Extensive work has been done to classify primitive basalts in the region and to explain chemical diversity via mantle source conditions and dominant tectonic drivers. Whereas these studies have been integral for understanding the generation of the most primitive lavas, true basalts (< 52 wt.% SiO₂) are far subordinate volumetrically to basaltic andesites (52 to 57 wt.% SiO₂). Although a few studies have investigated basaltic andesites at individual volcanic centers, there has not been a thorough investigation of basaltic andesites on a regional scale and what variations thereof imply for crustal processing of basaltic mantle inputs.

This project includes new XRF, ICP-MS, and petrographic data for previously unsampled mafic centers in the central to northern Oregon High Cascades in tandem with prior published and unpublished collections. Additionally, our ongoing sampling campaign includes re-sampling centers where data are incomplete. The ultimate goal is to generate a comprehensive geochemical database for mafic lavas within this portion of the Cascade Arc. These data will be used for reevaluating basaltic andesite types, their primitive lineages, and crustal inputs.

Currently available data show that basaltic andesites have a narrower range in K₂O as compared to basalts (0.19 to 1.27 wt.% and 0.09 to 2.28 wt.%, respectively), while many are still relatively primitive (Mg# > 55; Ni > 100 ppm). Basaltic andesites are variably enriched in large ion lithophile elements (LILE) and depleted in high field strength elements (HFSE), with Ba/Nb values more akin to more calc-alkaline High Cascade basalts (Ba/Nb = 16 to 69 and 21 to 187, respectively) as opposed to regional basalts with lesser subduction signatures (Ba/Nb = 7-20). Diversity in basaltic andesites is most notable in trace elements, for example with concentration ranges of: ~2 to 24 ppm La, ~4 to 19 ppm Nb, and ~1 to 5 ppm Th at the same wt.% MgO. Distinct differences in slopes of REE diagrams are also notable. All signatures bear on the question of why and how basaltic andesites are by far the most voluminous magma type within this segment of the Cascade Arc.