Oregon’s High Lava Plains Province (HLP) has strongly bimodal basalt and rhyolitic volcanism. The Province caps the northern margin of the Basin and Range Province and serves as a transitional region between westward extension of the Basin and Range Province and unextended crust to the north . The High Lava Plains overlap an area dominated by abundant minor northwest-striking faults of the Brothers Fault Zone. About 10 million years ago, silicic volcanism began at the eastern end of the HLP and spread west, younging towards the Cascade arc. Basalts erupted along the HLP are not age progressive. Most work in the High Lava Plains and Blue Mountains has left the interface between the two provinces as a nondescript region with few data capable to provide insight into processes active within this transitional region. This thesis focuses on the transition near Hampton Butte. New 40Ar-30Ar ages for Hampton Butte and adjacent Cougar Butte definitively assign both to volcanism during the emplacement of the John Day Formation and indicates a common origin. These units underlie basalts and an andesite that have ages consistent with a pulse of basaltic volcanism along the High Lava Plains around 8 Ma. An ignimbrite with composition and age (3.8 ± 0.6 Ma) consistent with outcrops at Espeland Draw, near the town of Hampton, has a probable origin at Frederick Butte and is grouped here as the Hampton Tuff. Further east, along the northern margin of the HLP, an age of 4.18 (± 0.14 Ma) was obtained for the basaltic andesite scoria cone of Grassy Butte. At Dry Mountain a summit sample produced an age of 7.91 ± 0.12 Ma and is basaltic, a contradiction with previous reports. A sample of rhyolite from Potato Hills, adjacent to Hat Butte, yielded an age of 6.52 ±.07 Ma.
Basaltic melts of this study appear to be near direct partial melts of the mantle and are part of the volcanism of the High Lava Plains related to fault propagation and extension. More evolved magmas of the High Lava Plains originate from isobaric fractionation in isolated small magma chambers with little if any assimilation. The eruption of these lavas may be a result of the timing between northeast to southwest extension and Basin and Range east to west extension. The older (> 20 Ma) high silica rocks of Hampton Buttes are the result of diverse fractional crystallization paths from a common basaltic source with little or no assimilation.