THE UTILITY OF LREE-RICH ACCESSORY MINERALS IN CHARACTERIZING AND DATING PLEISTOCENE SILICIC TEPHRAS AND LAVAS OF THE WESTERN UNITED STATES

Calvin Miller’s contributions to understanding the evolution of granites and rhyolites include highlighting the importance of light-rare-earth-element (LREE)-rich accessory minerals in controlling and recording the LREE evolution of felsic magmas. He recognized that when coupled with geochronology, LREE-rich minerals could yield unique time-compositional information about magmatic evolution. LREE- and Th-rich minerals, particularly allanite and chevkinite, occur in many Pleistocene rhyolites from the western United States. Ion and electron microprobe analyses reveal that microphenocrysts of these minerals have intra- and inter-crystal compositional diversity, with variability in REE, Th, U, Mg and other trace element concentrations that reflect the diversity of their parental silicic magmas. In addition, these accessory phases are characterized by relatively low U/Th, which when dated in conjunction with a U-rich co-crystallizing phase such as zircon, can link precise ²³⁸U-²³⁰Th geochronology with magmatic evolution. For example, allanite + zircon U-Th dates for tephras from the high-silica rhyolite Mono Craters of eastern California delimit an eruption sequence between ~65 and ~20 ka. Allanite compositions, including REE ratios such as La/Nd, can be distinctive between different Mono tephras, aiding tephrochronology and reflecting compositional heterogeneity in parent rhyolites. Approximately 200 km to the south, the youngest group of rhyolites from the Coso volcanic field contain allanites of relatively restricted chemical composition, which when coupled with co-crystallizing zircons yield U-Th dates consistent with near-eruption crystallization from ~70 to ~100 ka. Coso allanites with outlier compositions are rare, and are commonly in U-Th secular equilibrium, likely indicating recycling of old wall-rock crystals. Chevkinites from rhyolitic lava at Yellowstone caldera, Wyoming, record secular trends for Zr, Nb, and Mg concentrations during the interval between ~290–115 ka, with U/Th mirroring the trend for the same ratio in co-existing zircons. These cases demonstrate that allanite and chevkinite compositions are commonly diagnostic, and their unique combination of high Th and low U/Th enables precise dating of the varied pathways along which young silicic magmas evolve.