GEOCHEMISTRY, STRATIGRAPHY, AND PETROGENETIC IMPLICATIONS OF THE F BASALT FLOW GROUP, EASTERN SNAKE RIVER PLAIN, IDAHO

The eastern Snake River Plain (ESRP) volcanic basin in southeast Idaho is underlain by ~1 km of dominantly olivine tholeiite basalt and interbedded sediment. This study refines subsurface stratigraphy of the F basalt flow group, a stratigraphically important sequence of uneroded, apparently comagmatic lava flows observed in drill core over an area of ~ 75 km² between ~120-220 m depth. The ~565 ka F flow group erupted during an unusual, short-lived period of reversed magnetic polarity, probably in < 200 years. Detailed lithologic logs for six sections of F flows in drill-core reveal that the thickest (~50-60 m) lie in the southwest part of the study region, which we interpret to be near the vent. In each near-vent section 0.2-0.4 % higher P₂O₅ relative to values observed in the lower flow units characterize the uppermost lava flows. Chemical breaks are also identified in CaO/Al₂O₃ and K₂O/TiO₂ ratios. Preliminary trace element analyses indicate similar breaks at the same stratigraphic interval in incompatible elements. These chemostratigraphic characteristics support correlations between sampled sections. We interpret the F basalt flow group to have erupted from a source which sporadically tapped a periodically replenished magma chamber. Less evolved flows have slightly lower Sr-isotopic ratios than higher flows (0.7068 vs. 0.7071) while all exhibit similar Nd-isotope ratios (~0.5124). We infer that these isotopic signatures reflect magmatic derivation from an isotopically heterogeneous mantle source.