GEOLOGY AND PETROLOGY OF THE HAWKS VALLEY-LONE MOUNTAIN VOLCANIC COMPLEX, SOUTHEASTERN OREGON

Located at the transition between the Basin and Range and High Lava Plains provinces, the Hawks Valley - Lone Mountain area (HVLM) of southeastern Oregon exemplifies regional relationships between extensional tectonics and magmatism. Volcanism and faulting since ~16.6 Ma has created a complex region optimal for field-based studies aimed at investigating factors controlling the formation and modification of intracontinental lithosphere. Geologic mapping at the 1:24,000 scale sponsored by the USGS EDMAP Program documents episodes of NW and NNE oriented normal fault development and bimodal volcanism. The NW trending faults produced a large 150 km² graben feature exposing predominantly trachyte to rhyolite lavas. Quaternary basalts erupted from small vents and fissures within this graben and are displaced by NNE-trending faults. These structures locally intersect and displace the prominent NW-striking faults. Collectively these features are associated with mafic and silicic vents thereby providing age constraints on the deformation and evidence of the control exerted by regional stress patterns on magma transport and eruption.

Numerous 16.4±0.2 Ma silicic units (trachyte-trachydacite-rhyolite) emanating from as many as eight local vents are distinguished based on field relationships, petrography, and major/trace element geochemistry. Resorbed alkali feldspar and resorbed and complexly-zoned plagioclase indicate that magma mixing and/or crustal assimilation were important petrogenetic processes. Geochemical parameters support these observations and require a major role for crystal fractionation in the evolution of the HVLM silicic materials. The HVLM’s structural complexities and its proximity to exposures of temporally equivalent Steens flood basalt eruptive loci suggests that basaltic input into the crust stimulated local melt production, the establishment of multiple small upper-level magmatic systems along regional lithospheric weaknesses, and open system differentiation leading to the silicic suite. After a more than 15 m.y. hiatus, local HVLM volcanism resumed with the eruption of 0.9-0.5 Ma low-K, high-Al olivine tholeiite (HAOT) lavas characteristic of the High Lava Plains region and of relatively shallow basalt melt generation processes.