Geologic Mapping and Petrologic Modeling of the Green Mountain Shield Volcano, Oregon

Abstract

The Green Mountain Shield Volcano (GM)–dated at 740,000 Ka–is located in Eastern Oregon at the intersection of the Oregon High Lava Plains Province and the northernmost Basin and Range Province. In a preliminary study, Friedemann and Friedrichs (2015) observed minimal variation in geochemistry across the suite of GM lavas. In this study, we refine our petrologic model of the GM Shield Volcano through detailed geologic mapping as well as additional geochemical analysis using XRF spectroscopy on a more complete suite of GM basalts. We compare the GM lavas to an additional, yet much older, volcanic province in western Oregon: the Siletzia terrane. Our geochemical results confirm that the Mg# of the GM suite of lavas remains constant suggesting that all GM magmas experienced a similar degree of crystal fractionation. Our results also show that the GM basalts are geochemically distinct from the Siletzia basalts as well as others. Additionally, we have procured a radiometric age (40Ar/39Ar) for the basal sequence of the GM shield volcano by collecting samples from the ‘lava lakes’ in the center of one of the erupted maars: 891.7 ± 183.5 Ka.