PETROGENESIS OF THE MID-MIOCENE WESTERN SNAKE RIVER PLAIN RHYOLITES

Rhyolites of the Western Snake River Plain (WSRP) are exposed ~60 km southwest of the Boise, ID area along the southwestern margin of the WSRP, and were erupted from 11 Ma to 11.7 Ma. These deposits have “normal” δ¹⁸O values (7‰ - 9‰), but are otherwise physically and chemically similar to the near contemporaneous, though far more voluminous, low δ¹⁸O (≤ 4‰) rhyolites of the Bruneau-Jarbidge region along the Yellowstone Hotspot Track. There are six units in the WSRP with a total eruptive volume of < 80 km³, which consist of large lava flows, small ignimbrites, and a series of small domes. Though all units would be classified as metaluminous, A-Type rhyolites, they show considerable compositional variation. The units in the northwestern part of the field are high temperature (~900° C), low silica rhyolites (≤ 72.5% SiO₂), and the units to the southeast are typically lower temperature (~800° C), and have ≥ 74% SiO₂. The radiogenic isotope signatures of these rhyolites also show a clear geographical distribution, consistent with their location along the well documented “Sr 0.704 -0.706 Line” (e.g. Armstrong et al., 1977; Fleck and Criss, 1985), which likely reflects their differing crustal sources. The lower silica units in the northwest have ⁸⁷Sr/⁸⁶Sr values below 0.706, indicating a source in the Mesozoic accreted terrains, while the higher silica units exposed a few kilometers to the southeast have ⁸⁷Sr/⁸⁶Sr values from 0.707-0.710, indicating a source in the older cratonic crust.

A variety of geochemical modeling tools were applied to whole-rock major and trace element analyses, as well as electron microprobe and laser ablation ICPMS analyses of phenocrysts and glass. Modeling suggests that all of the WSRP rhyolites are the products of crustal melting of granitoid compositions, at depths of 6-10 km, followed by relatively low degrees of fractional crystallization of feldspar (≤ 10%) and/or a trace element sequestering phase (e.g. allanite, chevkenite, etc). Nb/Ta and isotopic ratios imply that most units were likely from distinct sources, though there are two ignimbrites in the southeastern part of the volcanic field that have strikingly similar chemistry, and therefore could have the same source.