Continental basalts of the Boise River Group near Smith Prairie, Idaho
Authors: Vetter Scott K. , Shervais John W. 
Published: February 2008

The Boise River Group of late Cenozoic age consists of basaltic lavas and intercalated fluvial and lacustrine sediments deposited in drainages of the Boise River and Boise River South Fork from 1.8 to 0.2 m.y. Basalts of the Boise River Group can be divided into two groups based on major and trace element chemistry. Boise River Group 1 basalts (BRG 1), which comprise the three oldest flows in the Smith Prairie area, are silica‐saturated olivine tholeiites characterized by low alkalis, Mg/Fe, and Ni, high concentrations of high field strength elements (HFSE), and enrichment in the heavy isotopes of Sr, Nd, and Pb. They are chemically similar to basalts of the Snake River Plain, and both units probably derive from a similar, enriched mantle lithosphere source. Some BRG 1 flows (Long Gulch, Rock Creek) record the affects of crustal assimilation and fractional crystallization. Boise River Group 2 basalts (BRG 2) are all younger than BRG 1 (< 0.7 m.y.) and are transitional between olivine tholeiites and alkali olivine basalts. BRG 2 basalts are characterized by high alkalis, Mg/Fe, and Ni, lower HFSE concentrations, and isotopic compositions of Sr and Nd near bulk earth. These lavas have distinct geochemical characteristics and are not cogenetic with the older, BRG 1 flows or with coeval lavas of the Snake River Plain. Chemical variations within individual flows are consistent with low‐pressure crystal fractionation of the observed phenocryst phases (olivine + plagioclase). Chemical variations between different flows within the same group cannot result from low‐pressure fractionation but can be modeled by combined high‐pressure pyroxene fractionation and low‐pressure olivine + plagioclase fractionation. The high Mg numbers and Ni of the younger BRG 2 basalts are not consistent with enrichment of the light field strength elements by crustal assimilation; this enrichment must reflect a mantle source region characteristic. The trace element and isotopic systematics of this source region are similar (but not identical) to the asthenospheric source inferred for ocean island basalts. In contrast, chemical and isotopic systematics of the BRG 1 basalts imply derivation from ancient subcontinental lithosphere which has been isolated from the asthenosphere for at least 1.5 eons. The subcontinental lithosphere was affected by an early enrichment in Rb/Sr which supported growth of high 87Sr/86Sr. A second enrichment event, resulting in Fe‐Ti metasomatism of the lithosphere, occurred later in response to intrusion of partial melts from a mantle plume rooted in the underlying asthenosphere. This is second enrichment event is required by the high Nb/Rb ratios of the BRG 1 lavas, and of similar lavas of the Snake River Plain. The transition from saturated olivine tholeiites of Boise River Group 1 to transitional‐alkalic lavas of BRG 2 in the Smith Prairie region implies a time‐dependent change in mantle source region, with early magmas derived from a shallow lithospheric source giving way to younger magmas derived from a deeper asthenospheric source. The eruption of these younger lavas along the flanks of the Snake River Plain coeval with eruption of saturated tholeiites of the Snake River Group to the south further implies an axial zonation to magmatism in the Snake River Province. The nature of this axial zonation suggests that the deeper asthenospheric melts were preferentially tapped along the margins of the Snake River Plain but were blocked from reaching the surface in the axial zone. These time‐space relationships are consistent with a model in which thinned lithosphere near the rift axis undergoes extensive partial melting which overwhelms any asthenospheric melts that may attempt passage. The lithosphere is too cool to melt along the rift margins (where pressure release by extensional thinning is minor); subsequent small volumes of asthenospheric melt are able to traverse this region more easily and erupt with little or no lithospheric contamination.