ON THE RELATIONSHIP BETWEEN THE OREGON HIGH LAVA PLAINS AND YELLOWSTONE-SNAKE RIVER PLAIN VOLCANIC SYSTEMS

The High Lava Plains province (HLP) is a Late Tertiary-Quaternary bimodal volcanic field in southeastern Oregon. Rhyolitic volcanism of the HLP is age progressive, producing a trend that co-originates with, and coarsely mirrors, the Yellowstone-Snake River Plain (YSRP) trend. We outline similarities and differences between the provinces and consider the ramifications of this relationship. STUCTURAL SETTING: HLP truly marks the northwestern termination of the Basin and Range; no significant difference in extension across much of the YSRP. AGE PROGRESSION: Both HLP and YSRP trends originate in a broad region of 17-14 Ma silicic volcanism on the axis of flood basalt vents. Rates: HLP, 35 km/m.y. from 10 to 5 Ma then 15 km/m.y.; YSRP system was organized by about 12 Ma, rate of about 40 km/m.y (or 30 km/m.y. since 10 Ma). RHYOLITES: YSRP rhyolites mostly large volume (>1,000 km³) tuffs; HLP rhyolites mostly erupted as domes with several tuffs of up to 300 km³. Most HLP rhyolites are similar in isotopic composition to basalts, lying on or near the mantle array; YSRP rhyolites highly isotopically evolved. BASALTS: Basalts are not age progressive in either province, and basaltic volcanism continues in the wake of both trends. Basalts in both systems are predominantly tholeiitic; wider compositional array of basalts occurs in the YSRP. Primitive HLP basalts have incompatible element enrichments in only a few fluid-mobile elements (Ba, Sr, Pb), while YSRP basalts are more broadly enriched. HLP basalts have isotopic compositions on the mantle array (but more evolved than MORB); YSRP basalts are much more isotopically evolved. DISCUSSION: The time-space relationships between the provinces clearly suggest a genetic relationship. The general petrologic similarities reflect a basalt driven (and hence bimodal) magmatic system. Differences in composition and volumes of volcanic products may be understood as the result of the contrasting lithosphere on which the provinces have developed. That they be related does not require that they were the result of the exact same process, but suggests that the processes were related. The existence of the HLP does not disprove the plume model for the YSRP. We feel that the plume model, including complex plume-lithosphere interactions, is the best model for the YSRP and may, in part, explain the HLP.