PATTERNS OF RHYOLITIC VOLCANISM ALONG THE TRACK OF THE YELLOWSTONE HOT SPOT REVEAL SUBSURFACE CONDITIONS

The track of the Yellowstone hotspot is represented by a bimodal volcanic province and developed over the last 16 m.y. in response to the SW movement of the North American (NA) plate over a thermal anomaly. Volcanism dominated by explosive activity includes some of the largest eruptions of high-silica rhyolite on Earth. Basaltic eruptions signal final stages of volcanism and cap the voluminous rhyolite. Volcanism progressed from SW to NE in successive volcanic fields comprised of nested caldera complexes. Most caldera-forming eruptions within a field are separated by 0.2 to 1 m.y. Volcanic fields may be separated in time and space by as much as 2 m.y. and 50-150 km, respectively. Thus, passage of the NA plate over a melting anomaly resulted in regional uplift and tectonism, voluminous explosive rhyolitic eruptions and caldera formation, followed by basaltic volcanism. A series of 4.5- to 12-Ma caldera-forming ignimbrites from the eastern and central Snake River Plain (SRP) have been analyzed for major, minor, and trace elements, and for ä18O to examine how individual ignimbrites and volcanic fields have varied chemically in space and time. In addition, 45 samples of Lava Creek Tuff (LCT), which erupted from the 0.64-Ma Yellowstone caldera, also were analyzed. Rhyolitic SRP ignimbrites have whole rock δ18O values ranging from 0.4-7.5 per mil (average 3.8). Quartz separates from SRP ignimbrites have δ18O values that range from 1.9-9.4 per mil (average 4.7). SRP ignimbrite samples are not visibly or chemically altered, suggesting low δ18O magmas. As in Watts et al. (this session), a clear pattern for the younger Heise and Yellowstone Plateau volcanic fields emerge. The youngest ignimbrite in the Heise field and the older post-caldera lavas from Yellowstone have the lowest δ18O values indicating the crustal source materials for these eruptions were hydrothermally altered. However, ignimbrites from the central SRP all have relatively low δ18O values suggesting that the crustal source materials derived for their magmas were affected by more extensive hydrothermal alteration. Large caldera-forming eruptions and development of individual volcanic fields are related to the rate of movement of the NA plate, volume of fusible material available in the crust, and melt flow dynamics in the upper crust.