A POSSIBLE DISLOCATED TRACK OF THE YELLOWSTONE MANTLE PLUME IN OREGON

Many features of the Yellowstone mantle plume, such as its connection with the Siletzia Terrane and the Columbia River Basalts have been recently proposed; one feature that has had regional consideration is the presence of a continuous hotspot track from the Oregon accreted continental margin to Yellowstone. We propose new detailed locations, geochemical variations, and refinement of the radiometric age range for this westerly segment of the Yellowstone Hotspot track west of McDermitt Caldera that has not been previously studied because tectonic deformation has displaced the eruptive centers along this portion of the track. Northerly and westerly movements and clockwise rotation of the continental crust have obscured the track, which places it now about 660 km northwest of its 50 Ma point of continental overrun. From here the track extends southeastward through eastern Oregon to its recognized track at the 16.1 Ma western end of the Snake River Plain.

Four eruptive centers can be distinguished from Cascade arc and other Oregon volcanic deposits on the basis of titanium, total iron, trace element content, lead, neodymium and oxygen isotopes, and radiometric (Ar/Ar) age ranges. East of the mouth of the Columbia River, the plume erupted its first products on the North American continent, a cluster of four high-TiO2 and high-FeO basaltic lava deposits in the lower Columbia River area—Tillamook, 42.0-40.3 Ma, in the northern Coast Range of Oregon; Grays River, 42.0-36.9 Ma, in the southern Coast Range of Washington; Waverly Heights, 44.4-39.9 Ma, south of Portland; and Kalama River, 39.2-37.4 Ma, west of Mt. St. Helens. The second eruptive center in the Warm Springs Reservation, to the southeast, is a possible caldera of lower John Day Formation, containing as much as 1,000 m of rhyolitic tuff. The third center, further southeast, is the Crooked River caldera, which erupted basalt and rhyolite lava flows and domes between 36.0 and 28.8 Ma of the John Day Formation. And the fourth center is Steens Mountain, which erupted Steens Basalt, 16.7 Ma, forming the lower member of the Columbia River Basalt Group. South of Steens Mountain the dislocated track connects with the western end of the Snake River Plains track at McDermitt Caldera.