TIME-SPACE PATTERNS OF BASALTIC VOLCANISM ON THE OWYHEE PLATEAU, OR-NV-ID

The Owyhee Plateau (OWP) is a complex subprovince of the northwestern US flood basalt province, located at the intersection of the Snake River Plain, Oregon Plateau, Basin and Range, Northern Nevada Rift, and Oregon-Idaho Graben. Basaltic volcanism on the OWP initiated concurrently with the main pulses of the Columbia River and Steens basalts, ~17 Ma, with the eruption of strongly differentiated basalts to basaltic andesites. Beginning ~11 Ma, small volumes of less fractionated olivine tholeiites were erupted from discrete vents throughout the OWP. These basalts represent a spectrum of chemical types from primitive low-K, low-Ti, high alumina olivine tholeiites (HAOT) to high-K, high-Ti Snake River Plain-type olivine tholeiites (SROT). These endmembers cannot be related by simple differentiation processes, but must reflect the interplay of heterogeneous mantle source materials. This scenario is further complicated by the presence of chronologically indistinguishable, overlapping shield vents, apparently aligned along the dominant regional structural trends, which erupted these dissimilar basalt compositions.

Basalts erupted between 11-6 Ma are characterized by similarly high ⁸⁷Sr/⁸⁶Sr (0.7065-0.7075), regardless of chemical type; subsequent pulses of eruptive activity between 6-3 Ma and 3-0 Ma are characterized by wider ranges of ⁸⁷Sr/⁸⁶Sr which generally trend toward lower values. The change ~11 Ma to relatively undifferentiated, small-volume basalt eruptions was followed by an apparent cessation of volcanism along a northward-migrating front. Basaltic volcanism ceased on the southern OWP by 5 Ma (coincidentally, with the eruption of the most primitive HAOT identified on the OWP to date). The most recent eruptions on the OWP occurred near the mapped southern margin of the Oregon-Idaho Graben, and include the only alkali olivine basalts observed on the OWP. We suggest that these phenomena are related to (1) variable mixing of melts from discrete, geochemically distinct layers of a complexly interthrust lithospheric mantle, and possibly from the sublithospheric mantle; (2) the exhaustion of easily fusible components from the lithospheric mantle; (3) the geometric properties of the lithosphere in this region; and (4) diffuse extension across the Oregon Plateau/northern Basin and Range.