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 system 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 1.72 ± 0.04 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 fo 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) mid mid- to lower-crustal segments (Wawa Domal Gneiss Terrain) plot on a 2.48 ± 0.02-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 ( 206 Pb 204 Pb = 14.02, 207 Pb 204 Pb =14.88, 208 Pb 204 Pb = 33.9 ), and the ratio 232 Th 238 U (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.