GEOCHEMICAL VARIATIONS IN THE BASALTIC VOLCANISM OF THE BRUNEAU-JARBIDGE ERUPTIVE CENTER AND ITS SURROUNDINGS, SOUTHWEST, IDAHO

The Bruneau-Jarbidge eruptive center is a large (95x55 km) structural basin at the southwest end of the Eastern SRP, formed by passage of the Yellowstone hot spot circa 12.5 Ma. Basaltic lavas (8Ma) postdate the rhyolites. NW-trending faults with small displacements are common in the center.

Twenty-four volcanic vents are found within the eruptive center, along with numerous vents that lie outside the structural basin. Most vents are small shield volcanoes that rise a few hundred feet above the basin floor. Vents within the eruptive center are located along three distinct NW trends; vents outside the eruptive center are more scattered.

Sixty-eight samples were analyzed using XRF, microprobe, and ICP-MS. Basalts consist of plagioclase and olivine phenocrysts set in a groundmass of plagioclase, clinopyroxene, opaques, and glass. Basalts from within the B-J eruptive center have Mg#=38.5 to 61.3, with TiO2=1.2 - 2.4 wt%; FeO=10.1 - 14.0 wt%, and P2O5=.06 - .42 wt%. Trace element abundance’s are: Nb=6-16 ppm, Sr=153-356 ppm, Zr=67-261 ppm, LaN=22-65, and (La/Lu)N=2 to 3.5. Basalts from outside the eruptive center have Mg#=38-42, TiO2=3.2-4.2 wt.%, FeO=13.9-16.7 wt.%, P2O5=0.14-0.88 wt.%, Nb=25-38 ppm, Sr=267-373 ppm, Zr=302-396 ppm, LaN=125, and (La/Lu)N=6.7.

The non-centered basalts are distinct from the eruptive center basalts; differences in REE slopes indicate distinct source regions. Compositions for basalts within the eruptive center samples are consistent with ESRP basalts. However there are differences in chemistry within the eruptive center based on the three distinct northwest volcanic trends found within the eruptive center. Basalts which erupted outside the BJ eruptive center are higher in Ti, Fe, and have much higher (La/Lu)N ratios, similar to basalts from the Western Snake River Plain. Forward modeling of the trace elements suggests possible distinct source regions than the eruptive center basalts.