THE TECTONOMAGMATIC DEVELOPMENT OF A MID-MIOCENE SILICIC ERUPTIVE CENTER NEAR THE OREGON PLATEAU – BASIN AND RANGE BOUNDARY, SE OREGON

Located just north of the Oregon-Nevada border and southwest of Steens Mountain, the Hawks Valley – Lone Mountain eruptive center (HVLM) provides an excellent location to investigate the relationship between tectonism and silicic volcanism, and the geochemical and petrologic relationships between closely spaced silicic vents. Mid-Miocene to Recent extension in southeastern Oregon is manifest in three prominent structural trends: 1) a N-S trend, 2) a NW-SE trend well expressed by the Eugene-Denio and Brothers Fault Zones, and 3) a NNE-SSW trend of regional Basin and Range faulting. The influences of the latter two are apparent in the HVLM where at least six silicic vents (domes and a dome complex) are identified based on field observations including dike orientations and locations, lava flow patterns, and the presence of near vent products. The silicic eruptive loci are associated with HVLM structural trends, including syn- and post-eruption structural influences and modifications. These observations suggest that the forces responsible for the NW-SE and NNE-SSW structural trends were active during HVLM magmatism and that the resulting crustal weaknesses acted as catalysts and conduits for magma generation, ascent, and eruption. Preliminary K/Ar ages indicate that these linked tectonomagmatic processes occurred in the HVLM from approximately 16 to 14.5 Ma, followed by continued structural modifications influencing small-volume basaltic volcanism.

Petrographic and major and trace element geochemical data define the mid-Miocene HVLM materials as trachydacites and rhyolites. Resorbed and step-zoned plagioclase and the presence of small mafic inclusions in some feldspars indicate that magma mixing or/and crustal assimilation took place. Geochemical parameters support these observations but also point to a major role for crystal fractionation in the evolution of the HVLM silicic materials. The close spatial and temporal link to voluminous Steens flood basalt eruptive loci suggests that basaltic input into the crust may have stimulated local melt production and open system differentiation within crustal magma chambers. Structural disruption of these crustal-level magmatic systems catalyzed and focused the ascent and establishment of the observed silicic vents that together comprise the HVLM.