Abstract Zircon crystals frequently comprise discrete growth zones, each of which may preserve temporal information on different geological processes. However, discrete growth domains can be below the spatial resolution of the applied analytical technique, resulting in composite isotopic ratios when mixed during sampling. Here, we develop a technique to rapidly extract meaningful core model ages, including propagated uncertainties, from core–rim mixtures when the rim age is constrained. A Python-based program with a graphical user interface is provided to facilitate easy application of this approach. We demonstrate the efficacy of this procedure on a detrital sample from the East Greenland Caledonides, where >90% of analyses were unavoidable core–rim mixtures. Metamorphic rim analyses yield a 206Pb/238U age of 418 ± 7 Ma (2σ), overlapping within error of the crystallization age of nearby granitoids (426 ± 1 Ma; 2σ). Using this rim age, we recover dates of detrital cores from core–rim mixtures. Model assumptions include original core concordance and a lack of isotopic disturbance at any time other than the rim age, but such assumptions can be validated and a metric of model reliability is provided. Reconstructed detrital core age populations are statistically indistinguishable from those of nearby equivalent metasedimentary rocks. We caution against the use of this technique without checking the appropriateness of the underlying assumptions but provide chemical means to evaluate the assumptions (e.g., alpha dose). The unmixing approach provides a mechanism to determine core ages from core–rim mixtures to recover additional information for addressing geological questions.