SNAKE RIVER PLAIN-YELLOWSTONE SILICIC VOLCANISM: IMPLICATIONS FOR MAGMA GENESIS AND CRUSTAL EVOLUTION

Energetics and physical processes associated with bimodal basalt-rhyolite magmatism of the Snake River Plain-Yellowstone (SRPY) hotspot track are investigated from the perspective of the voluminous rhyolites that comprise the initial eruptive phase.

Because rhyolite compositions correlate strongly with geography and the nature of the underlying basement, and considering strong crustal affinity of O-Sr-Pb isotopic compositions, it is evident that these magmas are dominantly of crustal origin. Yet Nd isotopic data preclude wholesale melting of the underlying Archean basement and point to involvement of a juvenile component - taken here to be derived by fractionation or remelting of contemporaneous basaltic magmas. Following the work of Annen and coworkers, we assume that energy for melting is provided by massive input of basalt into the crust. Several lines of evidence suggest that this occurs in the upper crust (<20 km depth) and involved a relatively infertile protolith.

Published and new Ti-in-quartz thermometry indicates that temperatures of the rhyolite magmas commonly approached 1000°C. To sustain such high Ts in the upper crust for several m.y. requires intermittent intrusion of a large volume of basaltic magma, effectively equivalent to at least 10 km thickness. In this type of scenario, remelting of earlier mafic intrusions is inevitable. Melting of crustal lithologies depends in detail on frequency and depths of basaltic intrusion, but locally approximates a fractional fusion process - leading to eruption of progressively less evolved rhyolite over the duration of a specific magma system. This behavior is exemplified by products of the large Bruneau-Jarbidge center that erupted ca. 10,000 cubic km of rhyolite between 12.7-8.0 Ma in the central SRP (Bonnichsen, Leeman et al., 2007). This scale of magmatic activity requires significant modification of the underlying crust.