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Processes of silicic melt generation |
1 National Science Foundation, Earth Sciences Division, 4201 Wilson Blvd, Arlington, VA 22330, USA (e-mail: wleeman{at}nsf.gov)
2 Départment de Minéralogie, University of Geneva, 1205 Geneva, Switzerland
3 Department of Earth and Planetary Science, University of California, Berkeley, CA 94720, USA
The origin of large-volume, high-temperature silicic volcanism associated with onset of the Snake River Plain – Yellowstone (SRPY) hotspot track is addressed based on evolution of the well-characterized Miocene Bruneau–Jarbidge (BJ) eruptive centre. Although O–Sr–Pb isotopic and bulk compositions of BJ rhyolites exhibit strong crustal affinity, including strong 18O-depletion, Nd isotopic data preclude wholesale melting of ancient basement rocks and implicate involvement of a juvenile component – possibly derived from contemporaneous basaltic magmas. Several lines of evidence, including limits on 18O-depletion of the rhyolite source rocks due to influx of meteoric/hydrothermal fluids, constrain rhyolite generation to depths shallower than mid-upper crust (<20 km depth). For crustal melting driven by basaltic intrusions, sustenance of temperatures exceeding 900 °C at such depths over the life of the BJ eruptive centre requires incremental intrusion of approximately 16 km of basalt into the crust. This minimum basaltic flux (c. 4 mm year–1) is about one-tenth that at Kilauea. Nevertheless, emplacement of such volumes of magma in the crust creates a serious room problem, requiring that the crust must undergo significant extensional deformation – seemingly exceeding present estimates of extensional strain for the SRPY province.
