Palaeoproterozoic thermal events recorded in the ∼4.0 Ga Acasta gneiss, Canada: evidence from SHRIMP U-Pb dating of apatite and zircon

Abstract

U-Pb isotopes of apatites and zircons extracted from a sample of the Acasta gneisses that occurs along the western margin of the Slave craton in Northwest Territories of Canada have been measured using a SHRIMP II ion microprobe. Eleven apatite analyses (30 μm spot) give 238U/ 204Pb- 206Pb/ 204Pb and 204Pb/ 206Pb- 207Pb/ 206Pb isochron ages of 1905 ± 86 Ma (2 σ) and 1936 ± 28 Ma (2 σ), respectively. Analyses (20 μm spot) on the partly recrystallised central portions of fourteen zircons mostly define a mixing array on a 238U/ 206Pb∗- 207Pb∗/ 206Pb∗ concordia plot with concordia intercepts at 4014 ± 25 Ma (2 σ) and 1967 ± 93 Ma (2 σ). The former agrees with 3.96–4.02 Ga protolith ages obtained on the oldest components of the Acasta gneisses by other workers. The lower concordia intercept for the zircon data is consistent with the U-Pb age of apatites and may be related to a >500°C thermal event, perhaps early in the Palaeoproterozoic Wopmay orogeny.