RESTRICTIONS POSED BY (U-TH)/HE AND APATITE FISSION TRACK THERMAL MODELLING OF A LATE CRETACEOUS/EARLY TERTIARY CRUSTAL HEATING EVENT IN THE SOUTHERN CANADIAN ATLANTIC REGION

Apatite fission track (AFT) data of sandstones from productive zones in the Scotian Basin, offshore Nova Scotia, indicate that rocks now at depths of 3-4 Km were considerably hotter than at present (up to 40^oC hotter) until some time in the Paleogene. AFT measurements also detect this late cooling in exposed crystalline basement rocks, the South Mountain Batholith, and Triassic and Permian sandstones in Nova Scotia and Prince Edward Island, indicating that it was a regional effect. AFT models compute that rocks now near the surface experienced temperatures of ca. 50^oC to 60^oC in Late Cretaceous to Early Tertiary time. (U-Th)/He ages have been obtained from 5 samples from sandstones and granitoids also analyzed for AFT. Corrected (U-Th)/He ages are significantly younger than both the respective stratigraphic ages and the corresponding AFT ages. (U-Th)/He ages obtained for the Bay of Fundy region are consistent with AFT time-temperature models which infer a period of elevated temperatures (exceeding 70^oC) associated with the development of the rift system from 230 Ma to 200 Ma. These temperatures would have caused complete diffusive helium loss and resetting of the (U-Th)/He system. The (U-Th)/He data for sandstones are also consistent with the AFT model temperature predictions of ca. 50^oC during Late Cretaceous to Early Tertiary time, which would produce only moderate helium diffusion (40-80^oC). Samples farther from the Bay of Fundy rift system were likely less affected by the Jurassic heating. However, the Late Cretaceous to Early Tertiary 60^oC temperatures predicted by the AFT models for these granitoid samples appear to be too high to produce the observed (U-Th)/He ages, since at that temperature significant diffusion of helium, and a high degree of rejuvenation of the (U-Th)/He ages would be expected. Several possible explanations for this apparent inconsistency are being investigated.