SOURCE AND PROCESS EFFECTS ON BASALTIC VOLCANISM IN THE JORDAN VALLEY VOLCANIC FIELD, SOUTHEASTERN OREGON

The Jordan Valley Volcanic Field (JVVF) is situated at the northernmost end of the Owyhee Plateau and provides an excellent opportunity to investigate small-volume basaltic volcanism. Geomorphic characteristics and K-Ar and Ar-Ar ages suggest that the 12 constituent monogenetic volcanoes range in age from Pliocene to Recent. The JVVF is characterized by considerable between-vent chemical and isotopic heterogeneity. Within- vent heterogeneity also is documented, particularly in the younger eruptive systems. Two overall chemostratigraphic groups are identified. One group includes the younger (<250 ka), mildly alkaline vents whose products illustrate considerable scatter in element-element plots. This, in conjunction with their Sr, Nd, and Pb isotope ratios and high La/Yb ratios, hints at derivation from a distinct, albeit heterogeneous source. In addition, the rare earth element patterns likely reflect varying degrees of partial melting, and observed positive correlations of MgO with ⁸⁷Sr/⁸⁶Sr are suggestive of assimilation and fractional crystallization. The latter possibility is interesting given the short life spans and presumed ephemeral nature of such volcanoes. The other group is defined by older (ca. 0.5-5.5 Ma) vents that erupted a continuum of tholeiitic basalt compositions from low-K, high-alumina olivine tholeiite to Snake River olivine tholeiite endmembers. Within-vent geochemical heterogeneity is less apparent in this group, but the between-vent chemical and Sr, Nd, and Pb isotope variations are far more systematic, suggestive of a dominant role for melt or source mixing. Superimposed upon the primary source controls are the effects of minor upper-level differentiation as inferred from small-scale, within-vent geochemical heterogeneities. The isotope data indicate a greater role for heterogeneous lithospheric mantle in the generation of the tholeiites versus the mildly alkaline basalts. JVVF basalt geochemistry correlates poorly with location, whereas there is a general shift toward less radiogenic Sr isotope ratios and increasing alkalinity through time. The observed spatio-temporal characteristics clearly reveal the complexity of monogenetic basalt fields and offer insights into small-scale mantle heterogeneity, differentiation, and melt segregation and transport.