DETRITAL ZIRCON (U-TH)/HE ANALYSIS OF THE LARAMIDE MACKENZIE MOUNTAINS, NWT, CANADA

Due to its high radiogenic concentration and ubiquity in the sedimentary rock record, zircon is a common mineral utilized in detrital (U-Th)/He thermochronometry. In the case of sedimentary strata that have been cooled through the mineral’s closure temperature, the reset (U-Th)/He ages are useful in discerning the timing of deformation and exhumation. However, this interpretive ability has so far been marred by the natural variation in helium diffusion between detrital grains, creating a wide range of single crystal closure temperatures within a single rock, and consequently a large spread in cooling ages within a sample. The strong kinetic control that radiation damage in zircon has on helium diffusion suggest the intra-sample scatter in zircon cooling ages may reflect, not only cooling rate and grain size, but also multiple kinetic populations within a single sample. The siliciclastic Neoproterozoic strata of the Mackenzie Mountains, the northern-most extension of the Canadian Cordillera, are an appropriate natural laboratory to test these relationships; the rocks possess a Neoarchean to Mesoproterozoic provenance and have experienced repeated episodes of burial and exhumation through the Phanerozoic. Samples were collected at regular intervals along a strike-perpendicular 130 km traverse through the Mackenzie Mountain fold-thrust belt. To assess grain-age relationships, single grain (U-Th)/He analysis was performed on 12 zircon grains/sample of varying size and morphology. Effective uranium concentration (eU = U + 0.235 • Th) was used as a proxy for radiation damage. While intra-sample variation in ages was as great as 300 m.y., all but one of the samples record Late Cretaceous and Cenozoic cooling ages indicative of Laramide deformation. The lone sample that does not record a Laramide signature instead has an average age of 225 Ma, supporting a regional Triassic exhumation event modeled by AFTA in the adjacent basin. In all samples, grains with the largest eU concentrations yield the youngest ages, and grains with low eU values have older ages, potentially reflecting partial retention of relict thermal histories. Thermal modeling of these relationships has significant utility in the Mackenzie Mountains, discerning candidate thermal histories from otherwise broadly scattered detrital age populations.