HR: 09:30h
AN: V41F-07    [Abstracts]
TI: Scope of Silicic Magmatism Associated With the Snake River Plain-Yellowstone (SRPY) "Hotspot" Track
AU: * Leeman, W P
EM: wleeman@nsf.gov
AF: National Science Foundation, 4201 Wilson Blvd., Arlington, VA 22230, United States
AB: Eruptive volumes of silicic volcanic rocks provide indirect albeit minimal indications of the scale of magmatism associated with the SRPY hotspot track. Coherent eruptive centers at Yellowstone produced ca. 6000 km³ of high silica rhyolite over &~2 m.y. whereas the Bruneau-Jarbidge center in the central SRP produced as much as 10,000 km³ between 12.7 and 8 Ma. Total magma volumes could be significantly larger. About half erupted as 'supervolcano class` ignimbrites (i.e., exceeding 10<sup>3</sup> km<sup>3</sup>). Implicitly, such volumes must be on-tap at least periodically during the lifetime of an eruptive center but heterogeneities in mineral populations imply that magma may be derived from a plexus of isolated pockets in the crust (`crustal sponge`) rather than a well-mixed chamber. Magmatism was strongly bimodal - ‘A-type' high silica rhyolite and basalt - intermediate composition lavas are rare. Moreover, each center produced rhyolite for 2-3 m.y. prior to the onset of basaltic volcanism which dominated subsequent activity. Nearly all SRPY rhyolites carry anhydrous mineral assemblages and mineral thermometry indicates high magmatic temperatures (typically >850-900°C). Radiogenic isotopes (Sr-Nd-Pb), trace element patterns, and low ¹⁸O in many SRPY rhyolites implicate a crustal source, although Nd isotopic data preclude large contributions from Archean crust. Silicic volcanism initiated in N-central Nevada ca. 16 Ma and migrated to Yellowstone by 2 Ma. However, the pattern of silicic eruptions was not simply progressive in space and time. Between 11.5-10 Ma major silicic eruptions occurred over a swath of more than 400 km - signifying availability of diverse rhyolite magmas beneath much of the SRP. Assuming that crustal melting was driven by basaltic intrusions, such magmas must have been generated beneath much of the province prior to 10 Ma – in part, well in advance of the postulated position of the Yellowstone hotspot at that time. The quantity of basalt needed to power SRPY silicic magmatism is estimated as equivalent to an underplate of 10-15 km thickness. Available geophysical data suggest that this mass was confined roughly within the physiographic footprint of the province. Because crustal thickness appears to be nearly constant, the crust must have deformed to accommodate emplacement of such large volumes of basalt. It is suggested that deformation was dominantly extensional on Basin and Range style structures. As a consequence, the crust must have become denser and interlaced with significant proportions of ‘juvenile' material over time. It is suggested that crustal evolution influenced eruptive activity, and particularly onset of basaltic volcanism.
DE: 3615 Intra-plate processes (1033, 8415)
DE: 3618 Magma chamber processes (1036)
DE: 8137 Hotspots, large igneous provinces, and flood basalt volcanism
DE: 8415 Intra-plate processes (1033, 3615)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting