Evolution of Quaternary tholeiitic basalt eruptive centers on the eastern Snake RiverPlain, Idaho

The tectonic and magmatic evolution of Quaternary olivine tholeiites in the eastern Snake River Plain (SRP) are evaluated by their spatial distribution and geochemi- cal signatures. Individual lava-flow groups and their as- sociated shield-building eruptive centers are either ex- posed at the surface or inferred to exist beneath overly- ing volcanic layers. Stratigraphy and dimensions of over- lapping subsurface flow groups rely heavily on current well and core-hole correlations interpreted from natural gamma logs of numerous wells in conjunction with pet- rographic, paleomagnetic, geochemical, and radiometric age investigations. Magmatic sources inferred from vent distribution are dispersed over a wide expanse of the sub- continental mantle beneath the eastern SRP. Major and trace element variations depict at least four general types of sources, although each flow group has a distinct chemi- cal signature representing a separate zone of melting. Basaltic shield dimensions, geochemical signatures, and Sr-Nd isotopic analyses support magma genesis charac- terized by low-volume batches of melt extracted from local reservoirs within EM2-like upper mantle. The source region was variably enriched in incompatible elements, and perhaps compositionally stratified, by previous sub- duction and within-plate processes. The wide variation of trace elements observed within some flow groups sug- gests that, for each monogenetic shield volcano, either (1) a heterogeneous batch of magma is derived by vari- able low degrees of partial melting and erupted with mini-