⁴⁰Ar/³⁹Ar dating of the Skaergaard Intrusion

We report the first ⁴⁰Ar-³⁹Ar data for the Skaergaard Intrusion, based on analyses of hornblende and biotite from a Transgressive granophyre from Upper Zone A (UZa) of the Layered Series. Hornblende separates heated in a resistance furnace and laser-heated single crystals of biotite from the granophyre give plateau ages that are consistent with isochron ages of 55.48 ± 0.30 and 55.40 ± 0.14 Ma respectively (2σ internal uncertainty), with initial ⁴⁰Ar/³⁹Ar ratios of 292.4 ± 19.0 and 291.1 ± 7.2. If uncertainties in the ages of ⁴⁰Ar-³⁹Ar standard fluence monitors are considered (external uncertainties), 2σ errors are 0.76 and 0.72 Ma, respectively. Resistance heating analysis of biotite separated from a gabbro from Lower Zone A (LZa) of the Skaergaard Layered Series indicates that the sample has been extensively contaminated by Ar inherited from old crust and does not constrain the intrusion age, apart from indicating that it is less than approximately 57 Ma old. The Ar cooling ages are consistent within uncertainty with both pluton cooling models presented by Norton and Taylor [1], but the data agree best with their model (S2) which assumes a smaller permeability (10^-13 cm²). Combination of the new ages and the Norton and Taylor cooling models constrains the emplacement and final crystallization of the Skaergaard magma chamber to be at 55.75 ± 0.30 Ma and 55.65 ± 0.30 Ma, respectively. Evidence for late-crystallizing quartz and tridymite near the Sandwich Horizon (SH), combined with phase-equilibria constraints on the near-solidus crystallization temperature of the SH (975 ± 25°C), require the pressure of SH solidification to be 60 ± 15 MPa. This means that the Tertiary basalts and sediments overlying the Kangerdlugssuaq region were 3.2 ± 0.8 km thick at 55.65 ± 0.30 Ma. Assuming that the 6-7 km thick Tertiary section exposed further east along the East Greenland coast is representative of that which accumulated above the Skaergaard, we infer that the Skaergaard emplacement occurred during the Main stage of the East Greenland flood basalt event, after approximately half of the Tertiary section had erupted. The onset of coastal flexure in the Kangerdlugssuaq region was after 55.48 ± 0.76 Ma.