PETROLOGY OF THE RHYOLITES IN THE KIMBERLY DRILL CORE, PROJECT HOTSPOT, SNAKE RIVER PLAIN, IDAHO

Disparate models have been proposed for the origin of rhyolites associated with hotspots like those on the Yellowstone hotspot track. In the central part of the track, study of these large magma systems has been hindered because the eruptive sources are buried by basalt. The 2 km Kimberly core helps fill that gap; it penetrates deep into the rhyolitic underpinnings of the southern margin of the province. The core is dominated by rhyolite lava and welded ignimbrite, with basalt and sediment between 241 m and 424 m depth.

Our initial chemical, petrographic, O-isotopic, and paleomagnetic measurements suggest that there are three major rhyolite units in the core. Rhyolite 3, the uppermost, is nearly 130 m thick. It contains phenocrysts of plag-san-px-zcn-ap-ilm-mt. Rhyolite 2 is the most highly evolved unit with ~75% SiO₂; it has anorthoclase and quartz, in addition to the minerals in Rhyolite 3. Rhyolite 1, the base of which is not exposed, is nearly 1,340 m thick, has no apparent flow contacts or cooling breaks and may represent a single, thick intracaldera ignimbrite. The ignimbrite has the same minerals as the upper unit and is a low-silica rhyolite with high concentrations of Fe₂O₃ and TiO₂; it is not obviously zoned—TiO₂ ranges from 0.75 to 0.84% and Nb from 39 to 41 ppm.

All units have the characteristics of A-type rhyolites and, compared to typical arc rhyolites, they are ferroan, Sr-and Al-poor, dry, and hot (~900°C). They have low La/Nb and Pb/Ce ratios and their trace element patterns have only small negative Nb anomalies. Initial analyses show that all three units are low δ¹⁸O rhyolites; feldspar ranges from 1‰ in Rhyolite 1 to 3‰ in Rhyolites 2 and 3. In the thick lower ignimbrite, whole-rock δ ¹⁸O increases systematically from the base upward (0.5 to as much as 9‰ in the altered top).

We suggest that many of these characteristics were imposed by rhyolite derivation from low degrees of partial melting of tholeiitic basalt lodged in a mid-crustal sill. The low δ¹⁸O values reflect recycling of hydrothermally altered crustal rocks. The isotopic data also indicate progressive recycling of more hydrothermally altered material into the younger magmas. More work is needed to establish correlation with regional units, understand the volcanic settings, and ascertain the origin of these distinctive low ¹⁸O, A-type rhyolites.