Zircon fission-track ages from Newfoundland—A proxy for high geothermal gradients and exhumation before opening of the Central Atlantic Ocean

Abstract

Following Appalachian orogenesis, metamorphic rocks in central Newfoundland were exhumed and reburied under Tournaisian strata. New zircon fission-track (ZFT) ages of metamorphic rocks below the Tournaisian unconformity yield post-depositionally reset ages of 212–235 Ma indicating regional fluid-absent reheating to at least ≥220°C. Post-Tournaisian sedimentary thicknesses in surrounding basins show that burial alone cannot explain such temperatures, thus requiring that palaeo-geothermal gradients increased to ≥30–40°C/km before final late Triassic accelerated cooling. We attribute these elevated palaeo-geothermal gradients to localized thermal blanketing by insulating sediments overlying radiogenic high-heat-producing granitoids. Late Triassic rifting and magmatism before break up of Pangaea likely also contributed to elevated heat flow, as well as uplift, triggering late Triassic accelerated cooling and exhumation. Thermochronological ages of 240–200 Ma are seen throughout Atlantic Canada, and record rifting and basaltic magmatism on the conjugate margins of the Central Atlantic Ocean preceding the onset of oceanic spreading at ~190 Ma.