Combined apatite fission track and (U–Th)/He thermochronometry in a slowly cooled terrane: results from a 3440-m-deep drill hole in the southern Canadian Shield

Combined apatite fission track (AFT) and (U–Th)/He (AHe) thermochronometry can be particularly valuable for deciphering the low temperature thermal history of slowly cooled terranes, especially in those parts where Phanerozoic structural or stratigraphic geological history constraints are absent. We evaluate the results of such a combined approach through the study of 19 samples from a range of lithologies (e.g., granophyre, quartz gabbro, norite) over different depths in a 3440-m-deep drill hole in the 1.85 Ga Sudbury Igneous Complex (SIC), southern Canadian Shield. AFT ages range from ∼360 Ma at the surface to ∼140 Ma at the base of the well, with the age–depth profile following a sinuous form. This shape is largely attributed to variability in the fission track retentivity due to changing apatite chlorine content between the different lithologies. In contrast, AHe ages range from ∼210 Ma at a depth of 600 m to ∼20 Ma at 2800 m and generally show a steady decrease of age with depth. The most consistent and reproducible AHe results are obtained from the relatively low U and Th apatites in mafic rock types. Due to the strict geometrical constraints imposed by the vertical profile, forward modeling of data derived from both methods provides a well-constrained Phanerozoic thermal history. The models reveal a history of heating and cooling during two cycles in Late Silurian to Early Devonian and Permian to Jurassic time, which are attributed mainly to burial beneath foreland sediments derived from the Appalachian Orogen to the southeast and their subsequent erosion. Below ∼1000 m, AHe samples also record a ‘weak’ Late Cretaceous–Early Tertiary heating event not recorded by AHe data from near-surface samples or AFT samples at all. The combined results provide complementary insights into the lowest part of the thermal history, which would not have been recognised using either method alone.