Apatite fission track (AFT) analyses for granitoid and metamorphic bedrock samples from the Western Superior Province (Ontario), the Churchill-Rae Province (Melville Peninsula and Southampton Island, Nunavut), and the Slave Province (Northwest Territories) show a broad range of single grain effective uranium concentrations (eU) (<1 to ~300 ppm) and some of the oldest reported AFT ages in North America. Although most of our samples are characterized by near-endmember fluorapatite composition with implied low track retentivity (<0.1 apfu Cl, rmr0 ~0.85-0.82), single-grain AFT ages are statistically overdispersed and ages decrease with increasing eU content. This eU-age relationship is resonant of the Hendriks and Redfield (2005) Earth Planet. Sci. Lett. 236 (443-458) argument for α-radiation enhanced fission track annealing (REA) and is analogous to the negative age-eU correlations observed in published zircon and titanite (U-Th)/He data from slowly-cooled cratonic rocks. In all cases, the samples fail the canonical χ2 test (<5%), generally considered to indicate that the ages are unlikely to be drawn from a single Poissonian distribution with a discrete mean value and may represent multiple populations. The high intra-sample age variability for low-Cl bedrock apatites with protracted histories (>200-500 m.y.) at <100°C since the Precambrian suggests strong REA control on AFT ages. Conversely, some low Cl AFT samples with a narrower eU range show less age dispersion and a weak apparent age-eU correlation. A complex trade-off between radiation damage, chemical composition (e.g. low Cl and REE enrichment), and thermal history is implied when eU and rmr0 are positively correlated. Previous assessments of the influence of REA on AFT age were based on evaluating central age and mean track length, which potentially mask high single-grain age scatter and REA effects due to the modal nature of central age determination. REA is also supported by and compatible with materials science and nuclear waste studies of radiation damage in different apatite groups, therefore it is crucial that bedrock samples exhibiting high age scatter are evaluated in terms of intra-sample compositional heterogeneity. AFT samples with relatively low Cl concentrations are especially prone to greater REA control of cooling ages and this underscores the need for routine acquisition of compositional data for AFT datasets. Our broad range in single-grain AFT ages (with no other clear, strong compositional controls) supports the notion that radiation damage affects both the AFT and (U-Th)/He thermochronometers in slowly-cooled settings and must be accounted for during thermal history modeling and interpretation.

New and recently published U-Pb, muscovite-biotite 40Ar/39Ar, K-feldspar MDD 40Ar/39Ar, zircon and apatite (U-Th)/He, and apatite fission-track data were compiled and inverted for a comprehensive, thermal history of southern Baffin Island, Canada. This work is a contribution to the Geo-mapping for Energy and Minerals (GEM) Baffin Island initiative and Trans-GEM synthesis of the Phanerozoic exhumation history of the Canadian Shield. Southern Baffin Island is comprised of Archean plutonic basement metamorphosed during the Trans-Hudson Orogeny. Monazite U-Pb dating on the Hall Peninsula suggest peak metamorphic conditions were at ca. 1850-1820 Ma and remained at >550ºC ca. 100 My after the thermal peak [1], while 40Ar/39Ar hydrous mineral ages and modeling suggest temperatures remained at >420-450ºC ca. 150-200 My after peak conditions [2]. New apatite U-Pb age populations are in agreement and range from 1674 ± 35 Ma to 1796 ± 75 Ma (2σ), suggesting elevated post-THO temperatures at ~450ºC. During the Meso- to Neoproterozoic the Hall Peninsula region experienced prolonged slow cooling on the order of ≤0.5ºC/My until ca. 1000 Ma when cooling accelerated to ~1ºC/My due to supercontinent Rodinia assembly. Sedimentary sequences place minimum timing constraints on basement rocks being at near-surface conditions in the early Paleozoic. Preliminary results from apatite fission-track data suggest that southwest Baffin (Meta-Incognita and Hall Peninsula) was fully exhumed by Paleozoic time during basement uplift that likely exploited preexisting, regional structures. Nearby Foxe Basin sediments suggest this region of the Canadian Shield was exhumed by the Late Ordovician (ca. 450 Ma) and either remained topographically high, or experienced minor burial during subsequent continental-wide transgression and shallow marine carbonate deposition in the Silurian-Devonian. AFT data from a >1890 Ma volcanic tuff cutting the Paleoproterozoic Hoare Bay Group sediments on the easternmost Cumberland Peninsula record rapid cooling in the Jurassic. The cooling signal recorded along Cumberland Peninsula is likely due to early crustal thinning related to rifting of Greenland from mainland Canada during Pangaea breakup and aligns with a model of rift-flank uplift. AFT models are in agreement with ages of dike swarms in West Greenland given as evidence by [3] for the onset of rift extension. The summarized cooling history of southern Baffin Island suggests post-THO cooling rates of ~1-3ºC/My from ca. 1700-1500 Ma, followed by slow cooling and Mesoproterozoic cooling pulses at ca. 1300 Ma and ca. 1000-950 Ma, likely due to Rodinia assembly. Rocks have been at temperatures <100ºC since ca. 500 Ma. [1] Skipton et al., 2016, J. of Petrology, v.57(8); [2] Skipton et al., 2017, Lithos, v.284; [3] Larsen et al., 2009, J. Geol. Soc., v.166.