ALLANITE AND CHEVKINITE AS ABSOLUTE CHRONOMETERS OF RHYOLITE DIFFERENTIATION

Allanite [(Ca, LREE, Th)₂(Mg,Fe)₃Si ₃O₁₂(OH)] and chevkinite [(Ca, LREE, Th)₄Fe²⁺(Al, Mg, Fe,Ti)₂Ti₂Si₄O₂₂] are accessory minerals which can be particularly useful for delimiting the evolution of rhyolitic magmas because they often contain significant compositional zoning that is sensitive to changes in magmatic conditions and is amenable to geochronology due to high Th concentrations. Both minerals are relatively common in metaluminous rhyolites from a variety of tectonic settings, with allanite generally occurring in hosts that crystallized at lower magmatic temperatures than those hosting chevkinite. Reconnaissance experiments indicate that allanite saturation occurs in high-silica rhyolites between 760-770°C; at ≥780°C, anhydrous chevkinite replaces allanite. The compositions of allanites from rhyolites erupted at Long Valley, Mono Craters, and Coso, USA, and Toba, Indonesia, follow a similar compositional trend of decreasing La, Ce, Ti, Mg and increasing Mn, Y, and Th, which mirrors the variability of compatible and incompatible elements, respectively, during fractionation of rhyolitic magma containing LREE-rich phases. Single crystals of chevkinite from caldera-forming and postcaldera rhyolites at Yellowstone, USA, may contain significant zoning in LREE, Th, Mg, and Y due to differentiation. However, parental melt compositions are difficult to calculate because partition coefficients between chevkinite and metaluminous melts are poorly delimited. For young chevkinites and allanites (<350 ka), the compositional variations in single crystals can be linked to absolute age through ²³⁸U-²³⁰Th dating via ion microprobe analysis. Compositional zoning in Toba allanites records crystal growth from rhyolitic melts related by up to 45% fractionation, which when keyed to ²³⁸U-²³⁰Th crystallization ages, dates the accumulation and differentiation of a voluminous silicic magma over an approximately 150,000 year interval. Chevkinites from postcaldera rhyolite at Yellowstone yield apparent ²³⁸U-²³⁰Th ages that 1) fall within range of crystallization ages for coexisting zircon and 2) correlate with the compositional variations recorded by single crystals.