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