Screening Apatite for Potential Use as a U-Pb Geochronometer in Zircon-Poor Magmatic Rocks: Age of Late-Stage Alkaline Magmatism in the Bolnisi District, Georgia

Abstract The timing of Late Cretaceous shoshonitic magmatism in the Georgian Bolnisi district within the Lesser Caucasus is poorly understood due to the absence of zircon for reliable age determination. This study addresses this gap by reporting new ID-TIMS U-Pb apatite ages, along with their textural and chemical characteristics analyzed by EPMA and LA-ICP-MS. Both magmatic and hydrothermal features were identified and we determined suitable apatite crystals for reliable radiogenic dating. Additionally, we report new LA-ICP-MS zircon U-Pb ages from stratigraphically older rhyodacite samples to constrain the temporal framework for magmatic events in the Bolnisi district. Hydrothermal apatite grains from stratigraphically older rhyodacite exhibit pale yellow to yellow luminescent colors, with low concentrations of light rare earth elements (LREEs) (e.g., La, Ce, Pr, Nd), U, Th, and Pb, alongside a depleted Sr/Y ratio. These apatite grains are primarily hosted by plagioclase and hornblende crystals, having undergone low-temperature fluid alteration, resulting in a low Pb*/Pbc ratio, unsuitable for reliable radiogenic age determination. Magmatic apatite grains from the rhyodacite display dark yellow to brown luminescent colors, enriched in LREEs and Cl, but low in U, Th, and Pb. Their Pb*/Pbc ratio suggests slight thermal overprint from Eocene magmatic activity or late-stage hydrothermal fluids, while retaining a magmatic origin. In contrast, apatite grains from alkaline trachyandesite exhibit higher concentrations of Sr, SiO2, and LREEs, distinguished by a blue luminescent color. These apatite grains have a higher Pb*/Pbc ratio, suitable for determining radiogenic magmatic ages within the study area. Uranium-lead zircon LA-ICP-MS geochronology yielded weighted mean ages of 82.9 ± 0.4 Ma to 81.6 ± 0.8 Ma for stratigraphically older rhyodacite, consistent with previous studies in the Bolnisi district. The stratigraphically younger trachyandesite yielded a U-Pb apatite weighted mean ID-TIMS age of 80.4 ± 2.0 Ma, thus providing a robust temporal framework for the alkaline magmatic event, necessary for understanding the geological evolution of the Bolnisi district.