Low-temperature thermochronology of Anticosti Island: A case study on the application of conodont (U-Th)/He thermochronology to carbonate basin analysis

The Ordovician to Silurian carbonate succession preserved on Anticosti Island is a near continuous record of the changing depositional and tectonic history of the Early Paleozoic Laurentian margin. These strata are a prime natural laboratory to test the applicability of conodont apatite (U-Th)/He thermochronology (CAHe) for carbonate basin analysis in a setting with a well-understood burial history and abundant paleotemperature constraints. We report 63 CAHe dates from seven samples collected from two boreholes. X-ray micro-computed tomography (XR-μCT) was used to assess individual conodont element size, volume and alpha ejection correction. CAHe dates are negatively correlated with effective uranium concentration (eU), with single conodont dates enigmatically older than the age of deposition to conodont dates as young as 2.7 ± 0.2 Ma. We rationalize this dispersion as a product of several factors, including diagenetic processes and pervasive conodont crystalline porosity and permeability. Additionally, one basement and two sandstone samples were collected for apatite and zircon (U-Th)/He (AHe, ZHe) and apatite fission track (AFT) thermochronology. In the Precambrian basement, ZHe dates range from 581.7 ± 46.5 Ma to 761.2 ± 60.9 Ma (n: 4) and AHe dates range from 26.7 ± 1.6 to 48.4 ± 2.9 Ma (n: 4). AHe dates from two Lower Ordovician sandstones (n: 8) are positively correlated with eU, increasing from 13.7 ± 0.8 Ma to 105.2 ± 6.3 Ma over a range of 186 ppm eU. Thermal history modeling of these data suggests two plausible thermal histories, both of which are novel as they imply that far-field effects related to the opening of the Atlantic Ocean have influenced the thermal history of Anticosti Island. Ultimately, these data highlight the complexities associated with, and necessity of, low-temperature thermochronology in carbonate basins.