U-Th-Pb isotope study of mafic dykes in the superior Province, Ontario, Canada: uniformity of initial Pb isotope ratios of the Hearst dykes

Abstract

The U-Th-Pb systems of minerals and whole rocks from Hearst and Kapuskasing mafic dykes of the eastern Superior Province, Canada, were investigated to determine their suitability as chronometers for primary and secondary events. Minerals from the dykes all register secondary events ranging from 1.9 to 2.2 Ga coinciding with large-scale crustal deformation associated with uplift of the Kapuskasing Structural Zone (KSZ). Plagioclase phenocrysts and megacrysts from both sets of dykes indicate a later alteration event at 1.720.04 Ga. Dyke whole rocks have a restricted range of μ-values from 4.4 to 21 and present-day isotopic compositions that scatter on a Pb isochron plot. For the Hearst dykes, the scatter is primarily the result of resetting effects which gave rise to multistage evolution of the samples' isotopic systems. Two- and three-stage models of the U-Th-Pb systems of the disturbed dykes suggest that the U/Pb and Th/Pb ratios of the rocks were reduced during these events. There is little evidence for large variations among the isotopic composition of these dykes at the time of their crystallization. Pb isotope data for undisturbed whole rocks from Hearst dykes from both upper-crustal segments (Hornepayne) and mid- to lower-crustal segments (Wawa Domal Gneiss Terrain) plot on a 2.480.02-Ga isochron. We interpret this to indicate that these rocks were derived from a homogeneous (mantle) source and that significant in situ country-rock contamination did not occur during emplacement. The initial Pb isotope ratios ( 206Pb/ 204Pb= 14.02, 207Pb/ 204Pb= 14.88, 208Pb/ 204Pb=33.9), and the ratio 232Th/ 238U (3.6) for this source suggest an undepleted (or, more contaminated) source relative to the mantle which produced later-formed lamprophyric dykes and carbonatites of the KSZ. The Pb isotopic signatures of the lamprophyres and carbonatites indicate that neither of their sources were as depleted as that of widespread komatiitic and basaltic volcanism in the Wawa and Abitibi belts at 2.7 Ga.