Abstract

A tea-coloured cloudiness in groundmass feldspars characterizes dykes of two Early Proterozoic swarms (2.45 Ga Matachewan; 2.04 Ga Kapuskasing) that intrude amphibolite to granulite host rocks within the Kapuskasing Structural Zone (KSZ) of northern Ontario. Outside the KSZ the dykes occur in lower-grade terranes and contain unclouded feldspars. Matachewan dykes within the KSZ are also fresh, contain an aluminous, blue–green amphibole, have fewer plagioclase phenocrysts, and on average are thinner compared with their counterparts outside, which are more hydrously altered, contain a green, fibrous amphibole, and are locally abundant in felspar phenocrysts.Within the Matachewan swarm, magnetic polarity domains — areas of dominantly normal or reversed magnetization — have been identified; the boundaries of these domains correspond closely to major faults. Two of these faults form the boundaries of the KSZ and separate normal dykes within from dominantly reversed ones outside.The foregoing petrological and paleomagnetic observations are related to differential vertical crustal movements that have raised younger, normal-polarity Matachewan dykes (as exemplified by those within the KSZ) from a deeper crustal level and juxtaposed them against older, reversely magnetized dykes that were intruded at shallower crustal levels.Paleomagnetic baked-contact tests on Kapuskasing dykes suggest that they were intruded both before and after the main period of crustal uplift and tilting along the KSZ and that dykes within 4 km of the southeastern boundary fault were raised from depths where temperatures exceeded 580 °C, the Curie-point isotherm for magnetite.

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