EVOLUTION OF SNAKE RIVER PLAIN (SRP) SILICIC MAGMAS – THE MAGIC RESERVOIR ERUPTIVE CENTER

The SRP is characterized by time-transgressive bimodal rhyolite-basalt magmatism. Early voluminous rhyolitic lavas and ignimbrites (Idavada fm.) migrated from SW (~14 Ma) to NE (~2 Ma) parts of the province. This phase of activity generally persisted in local areas for ~2 Ma, and was followed by predominantly basaltic magmatism of varied duration – ranging to Holocene time in the central and eastern parts of the SRP. This pattern of activity reflects the effects of migration of North America past the Yellowstone hotspot coupled with regional extension. An important exception to this pattern is manifest by the Magic reservoir eruptive center (north-central SRP) where Idavada magmatism (10-8 Ma) was followed by rejuvenated silicic eruptions (~6 to 3 Ma) of fundamentally different character (comprising caldera-related pyroclastics and domes and hybrid basalt-rhyolite mixtures), then by intermittent basaltic activity characteristic of other parts of the SRP.

Whereas the early Idavada rocks are H₂O-poor, hot pyroxene-bearing rhyolites, the younger silicic activity evolved to cooler, hydrated amphibole ± biotite-bearing high-silica rhyolite. Differences in trace element and Sr-Nd-Pb compositions preclude a simple petrogenetic evolution from older to younger silicic magmas, and indicate that different source materials were involved. Early rhyolites appear to represent melts of high-grade lower crustal rocks, whereas later rhyolites involve melting of distinct hydrated crust – most likely dominated by hydrated rocks of the Idaho Batholith. The fundamental driving mechanism appears to be injection of hot basaltic magma into the crust at progressively shallower levels with time, leading to shoaling of the silicic magmatic system within the crust. Uncharacteristically prolonged silicic magmatism in this region may reflect a combination of lateral differences in crustal history and composition, coupled with tectonic factors favoring a high and long-lived flux of basaltic magma.