A laser 40Ar–39Ar study of minerals across the Grenville Front: investigation of reproducible excess Ar patterns
40Ar–39Ar ages have been determined on single grains of biotite and amphibole from rocks on a 40 km traverse across the Grenville Front (GF) in the Temagami area, Ontario. Minerals in the vicinity of the GF contain excess Ar in varying amounts characterized by both anomalously high integrated and plateau ages. The excess Ar in biotite varies with distance from the GF, such that the overall age distribution along the GF forms an asymmetric wave-like pattern (an argonami) with two age maxima, one on either side of the GF. The integrated age maxima are 2.34 Ga for biotite and 3.21 Ga for amphibole located 6 and 0.2 km north of the GF, respectively, whereas maxima are 1.33 Ga for biotite and 1.50 Ga for amphibole 7 km south of the GF. This wave-like pattern of apparent ages shows a remarkable similarity with that seen in an earlier K–Ar study across the GF at Chibougamau, Quebec, and also with a small-scale pattern of excess Ar associated with the emplacement of a dyke. Variations in the magnitude of excess A...