CANNIBALIZATION INTERRUPTED BY ERUPTION OF HOTSPOT RHYOLITE: KIMBERLY RHYOLITE, CENTRAL SNAKE RIVER PLAIN, IDAHO, USA

The 7.7 Ma Kimberly A-type rhyolite formed along the track of the Yellowstone hotspot and was sampled by a Project HOTSPOT drill hole (Shervais et al., 2013). It is a 170 m thick, caldera-filling, high-silica rhyolite lava containing Qz, Pl, Ano, Sa, Aug, Pgt, Mag, Ilm, Zcn, and Ap. δ¹⁸O is low typical of SRP rhyolites (Qz 3.7‰ and Pl 3.0‰). Fe-Ti oxides (926°C) have equilibrated with melt, but other phases preserve strong evidence of disequilibrium. In particular, δ¹⁸O of Zcn is diverse, ranging from 0 to 4.9‰ (Colón et al., 2018). Pigeonite is exsolved (indicating slow cooling as a plutonic rock), resorbed, and mantled by Aug. REE-poor Ap cores are resorbed and oscillatory zones truncated by rims with as much 12.8% SiO₂ and 4.7% LREE_tot. About 15% of the Zcn grains have CL-dark cores enriched in REEs, U, and Th that are mantled by less enriched rims. Also incorporated into the rhyolite were volcanic xenocrysts indicative of assimilation of already solid rhyolite. These include deeply embayed volcanic Qz (unstrained) and composite, three component, feldspar grains (they lack perthite or exsolution). Pl (An₂₀ to An₄₀) cores are rounded and pitted. Ano (Or₂₁Ab₆₄An₁₅) mantles the sieved cores. Sa (Or₄₇Ab₄₈ An₀₅) forms thin, subhedral drapes on the anorthoclase. The matrix has two distinct glasses--light-colored glass (~95%) and dark glass (~5%) enriched in Al₂O₃ , CaO, Na₂O, Sr, and Ba and depleted in Fe₂O₃, K₂O, and Rb. The dark glass composition can be modelled as a melt of clay-altered, low-δ¹⁸O rocks in the Kimberly core.

To explain the variety of disequilibrium textures and compositions, we propose that a solid rhyolite (with composite Pl-Ano grains and Qz) and a slowly-cooled, A-type granite (with exsolved Pgt) were hydrothermally altered in a sub-caldera setting. Both were subsequently intruded by hot rhyolite (the light glass), causing partial melting of the clay-rich portions of the igneous complex to produce the dark glass, resorbed minerals, and incomplete digestion of the granite and rhyolite. The Px, Fsp, Ap, and Zcn xenocrysts were then mantled by new overgrowths. Cannibalization and re-equilibration were interrupted by eruption of the rhyolite lava onto the floor of a slightly older caldera. This scenario outlines the multi-stage genesis of many, if not most, Snake River Plain hot-spot rhyolitic magmas.