CONSTRAINTS ON THE NATURE OF THE KAPUSKASING STRUCTURAL ZONE FROM THESTUDY OF PROTEROZOIC DYKE SWARMS

Petrochemical, paleomagnetic, and structural studies on Early Proteroz oic dyke swarms show that the rocks of the Kapuskasing structural zone (KSZ), central Superior Province, were upthrust about 10-15 km along the Ivanhoe Lake fault after intrusion of 2.04 Ga Kapuskasing dykes. T his uplift was part of a more widespread deformation in flanking terra nes that involved sinistral motion along north-north-northwest-trendin g faults, dextral displacement along east-northeast-northeast-trending faults, and a distortion of the 2.45 Ga Matachewan swarm to the west and northwest of the Ivanhoe Lake fault. The most spectacular demonstr ation that the KSZ is largely a product of Proterozoic deformation is that Matachewan dykes change their magnetic polarity on crossing the z one, a consequence of remanence acquisition at deep crustal levels bei ng delayed until after a reversal of the earth's magnetic field and pr ior to uplift of the dykes. Matachewan and Kapuskasing dykes within th e amphibolite- to granulite-grade rocks of the KSZ are relatively fres h and contain a high-alumina green amphibole and feldspar laths that e xhibit tea-coloured clouding due to the presence of submicroscopic inc lusions of magnetite. These features may be diagnostic of crystallizat ion at deeper crustal levels because they are found in regions where R b-Sr biotite ages from tonalites suggest broad-scale uplift of the cru st after about 2 Ga, About 50 km west of the KSZ, dykes exhibiting clo uded feldspar and high-alumina amphiboles occur on the upthrown side o f the Budd Lake fault zone, which has a trend similar to that of the I vanhoe Lake fault. Therefore a second thrust block may exist, raising the possibility that the KSZ represents the basal member of a series o f imbricate thrust slices that extends westwards from the Ivanhoe Lake fault. Radiometric age data suggest that the region affected by this Early Proterozoic deformation continues to the northwest for a further 300 km.