THE HAMPTON TUFF, EASTERN OREGON: EVIDENCE FOR WESTWARD AND TEMPORAL DECLINE IN RHYOLITIC ACTIVITY ON THE HIGH LAVA PLAINS

The Hampton Tuff is one of at least six ignimbrites on the High Lava Plains (HLP), an area of late Tertiary bimodal volcanic activity located at the northwestern margin of the Basin and Range Province in southeastern Oregon. Silicic dome complexes and ignimbrites decrease in age to the NW, starting as early as 12 Mya. The 9.6 Ma Devine Canyon Tuff, the 8.4 Ma Prater Creek Tuff, and the 7.1 Ma Rattlesnake Tuff have distributions indicating sources in the eastern and central High Lava Plains. The 3.8-Ma age of the Hampton Tuff and correlation to a source near Frederick Butte (4 ± 0.4 MA) in the western HLP make it part of the westward age progression.

The estimated volume of the Hampton Tuff is ~42 km³ (DRE of ~30 km³) using a radius of 30 km and a thickness of 15 m. It has a volume about 10 times less than the 280 km³ (DRE) of the Rattlesnake Tuff, which has a similar volume to the Devine Canyon Tuff, based on similar outcrop area.

Based on age and distribution, the Hampton Tuff includes a welded tuff banked on the south flank of Hampton Butte, the tuff at Hampton, poorly welded outcrops north of Brothers and the more extensive Tuff of Espeland Draw, which is attributed to a source at the semicircular Frederick Butte dome complex (Johnson, 1998; Macleod et al., 1975). Pumices vary in color from white to tan and brown to black and may be oxidized to pink and range from coarsely vesicular to having finely lineated vesicles. In thin section, the pumices vary from <1.0% to ~ 5% crystals composed of plagioclase, lesser sanidine, sparse quartz, hedenbergitic clinopyroxene, fayalite, and opaque oxides. Mineralogy and preliminary geochemistry of the Hampton Tuff, ranging from dacite to high-silica rhyolite, further the tie to the Tuff of Espeland Draw.

Petrographic and lithologic similarities to the Rattlesnake Tuff suggest the Hampton Tuff had similar petrogenesis. The much smaller volume supports the conjecture that the thermal anomaly that supports silicic magmatism in the HLP is waning in time as it migrates to the NW.