CANNIBALIZATION AND MIXING PROCESSES IN HOTSPOT RHYOLITES: ANOMALOUS TEXTURES AND GEOCHEMISTRY OF THE KIMBERLY MEMBER OF THE CASSIA FORMATION, YELLOWSTONE HOTSPOT TRACK, IDAHO

The 7.7 Ma Kimberly Member of the Cassia Formation is part of a succession of A-type rhyolites associated with the Yellowstone hotspot track, sampled by the Kimberly core, drilled on the Snake River Plain as part of Project HOTSPOT (Shervais et al., 2013). The Kimberly Member is a 170 m thick high-silica rhyolite with Qz, Pl, Ano, Sa, Aug, Pgt, Fe-Ti oxides, Zrn, and Ap. There are three chemically distinct feldspars: anorthoclase, sanidine, and a continuum from oligoclase (An₆₆Or₂₆Ab₈) to andesine (An₆₁Or₃₄Ab₅). Anorthoclase is rare in Snake River Plain rhyolites. Pl, Ano, and Sa each lie on solvus lines with different temperatures. These feldspars also have a variety of textures. Complex intergrowths and overgrowths between Pl, Ano, and Sa are common. Pl is commonly surrounded by pitted Ano that is mantled by Sa. In addition, feldspars of the three compositions are found in complex clots with pyroxenes and oxides. Unmantled Pl grains are extremely rare or nonexistent, while single crystals of unmantled Ano and Sa are common. Most feldspar grains and clots are quite rounded. Pgt (Mg # 0.15) is complexly mantled by Aug (Mg # 0.19). In contrast with the feldspars, these pyroxenes form single populations. Qz is intensely resorbed. The Kimberly Member has low δ¹⁸O values (3 per mil in feldspar, 0-3 per mil in zircons; Colon et al., 2017). U/Pb zircon ages, acquired through TIMS and LA-ICPMS span one million years. εHf values in zircon are likewise diverse and range from -8 to 1.

The unique textures and geochemistry of the Kimberly rhyolite may be caused by cannibalism of a previous generation of rhyolite or granite and magma mixing. Cannibalization is indicated by the range of Zrn ages and the possibly remnant Pl. Magma mixing is consistent with the intensely resorbed Qz, diverse feldspar compositions, multiple generations of pyroxene, and feldspar resorption and mantling. The range of δ¹⁸O values and diverse εHf values are consistent with either process. A possible scenario is an older granite or rhyolite with Pl and Zrn was partially digested by an Ano- and Pgt-bearing magma. It then mixed with a magma crystallizing Sa and Aug, causing Ano to resorb, Sa to mantle Ano, and Aug to mantle Pgt. These processes of cannibalization and mixing may be important in many hotspot settings.