APPROPRIATE FILTERING OF ZIRCON GEOCHRONOLOGY DATA: AN EXAMPLE FROM THE CAROLINA TERRANE

Crucial to the effective application of zircon U-Pb geochronology to problems of deformation and sedimentation is the ability to minimize bias while maintaining scrutiny. Indeed, in such pursuits it is simultaneously essential to include (or more appropriately, not exclude) sub-optimal zircon ages that have geologic relevance, while excluding (or not including) apparent ages that result from the significant disturbance of otherwise closed-system behavior. This is truly a challenge. The ability and need to efficiently process and evaluate large datasets has encouraged the use of automatic filters that enable users to systematically exclude or include ages in datasets, ideally allowing expeditious “apples-to-apples” comparisons. Whereas converging upon best practices is incredibly valuable, boilerplate approaches to data filtering can yield discrepant results. This is especially problematic with laser-ablation zircon geochronology, which requires bracketing the analysis of ‘unknown’ zircons with natural zircon reference materials, and which is subject to unpredictable in-run, between-runs, and inter-laboratory analytical variability. Whereas these challenges are often moot for many sedimentary provenance applications, they can present a significant problem for aliquots containing young apparent ages, for those in which metamorphic and maximum depositional ages are particularly valuable or contentious, and for those collected from metamorphic rocks wherein the likelihood for geochemical disturbance is greater. These challenges are further exacerbated by analysis using single-collector instruments, which inhibit the efficacy of common-lead corrections and reliable discordance calculations. Towards addressing these challenges, herein I report the application of different discordance filters to challenging samples from the Carolina Terrane of the southern Appalachian Mountains, whose U-Pb zircon ages fall within the grey area of many conventional discordance filters. In general, the parallel application of appropriate (1) concordance-coded ages, (2) 207/206 vs 207/235 concordance filters, and (3) concordance-based cutoff ages enables a more plausible interpretation of Carolina Terrane zircon geochronology than boilerplate approaches.