BURIAL AND EXHUMATION OF THE HUMBER ARM ALLOCHTHON CONSTRAINED BY ZIRCON AND APATITE (U-Th)/He THERMOCHRONOLOGY, WESTERN NEWFOUNDLAND

Here, we report new zircon and apatite (U-Th)/He ages from western Newfoundland that provide insights into the timing and rate of cooling during its transition from an active tectonic boundary in the Paleozoic to a modern passive margin. We date nineteen samples from the Cambro-Ordovician Humber Arm Allochthon (HAA), including the Bay of Islands Complex (BOIC) and Coastal Complex (CC) ophiolites and the Humber Arm Supergroup. Additional (U-Th)/He data are presented for the Twillingate Batholith, Fleur de Lys Supergroup in the Baie Verte Peninsula, and the Lower Cambrian Bradore Formation of coastal Labrador.

Zircon (U-Th)/He mean ages from the HAA are constrained to the Devonian and Mississippian periods (402.1 ± 14.3 Ma to 354.5 ± 19.7 Ma (1σ)). These data necessitate the allochthon’s burial and reheating to at least zircon He closure temperatures (~180-200°C) after the allochthon was emplaced on the Laurentian margin in the Early Ordovician. Zircon (U-Th)/He ages do not correlate with effective uranium (eU) concentration, suggesting fast cooling through the zircon He partial retention zone.

The BOIC, CC and Humber Arm Supergroup apatite (U-Th)/He ages indicate cooling through apatite He closure temperatures (~60-80°C) during the Mesozoic (241.4 ± 22.7 Ma to 143.3 ± 62.5 Ma (1σ)). These ages progressively young from west to east across the allochthon. Positive apatite age-eU relationships are sample dependent, supporting slow cooling rates and/or partial resetting in the apatite He partial retention zone. We approximate average cooling rates of 0.65-0.97°C/My for the HAA during the Devonian to Triassic periods by calculating the lag time between zircon and apatite (U-Th)/He mean ages in samples with both thermochronometers.

Combining geologic constraints with HeFTy inverse thermal history models reveals (1) burial and reheating of the HAA sometime after the Taconic Orogeny (2) last cooling through zircon He closure temperatures before or during the Acadian Orogeny, and (3) last cooling through apatite He closure temperatures during Pangea breakup and early Atlantic rifting. We conclude that the HAA cooled slowly since at least the Mesozoic and stabilized at ≤3 km depth by ~200 Ma.