GEOCHEMICAL MODELING TO CONSTRAIN THE SOURCE OF THE BROWNS CREEK RHYOLITE: WESTERN SNAKE RIVER PLAIN, IDAHO

Geochemical modeling was performed in order to investigate potential source rocks for the Browns Creek rhyolite (BCR) of the western Snake River Plain (WSRP). The BCR lava erupted during the mid-Miocene and is contemporaneous with the widespread rhyolite volcanism in the Owyhee Front and central Snake River Plain. The BCR is a good candidate to investigate the plausibility of a granitic source for WSRP melts due to the presence of granitic lithics in proximal ash deposits and an exposure of a granitic body directly beneath the unit. To accomplish this, modeled compositions were compared to whole rock data and microprobe data from phenocrysts. Various source compositions, taken from whole rock data on Idaho Batholith granites of Cretaceous and Eocene age, were allowed to crystallize to roughly 40%-50% crystallinity. Liquid compositions at various melt fractions can be considered the same as partial melts in a system moving up the temperature path. The major element chemistry of the BCU can be broadly modeled as a melt of locally exposed granites of the Idaho Batholith. Trace element chemistry is more complex, and indicates that a mafic component is likely present in the system.

Volcanologically, the Browns Creek rhyolite is unusual relative to the bulk of rhyolitic units in the western and central Snake River Plain. A proximal vent location is particularly well exposed in the BCR, and appears as an extended topographic “dome” with widespread, disordered, and near-vertical flow banding, along with ramping structures and breccias. These flow features are also visible on the petrographic scale in the form of flow banding around most crystals. The BCR lavas can contain up to 40% phenocrysts, with some crystals larger than 2 cm in diameter. Two populations of feldspar (oligoclase and sanadine) exist in the BCR along with quartz, clinopyroxene, Fe-Ti oxides, zircon, and apatite. The BCR is among many units in the Snake River Plain that shows evidence of contact with ancient Lake Idaho and the unconformity with the sedimentary rocks of the lake and its feeder streams, is particularly well preserved in the Brown’s Creek rhyolite location.