The U–Pb isotopic ages of metamorphic zircons from deformed amphibolitized metadiabase dykes and associated unstrained anatectic pegmatites have been determined to constrain the times of postdyke ductile flow related recrystallization and deformation. Samples from the Central Gneiss Belt of the Grenville Province in the southern Georgian Bay region provide a means of linking areas formed during common episodes of ductile flow. Three zircon fractions from one attenuated dyke in the Moon River domain gave identical concordant ages of 1062, 1065, and 1064 ± 2 Ma, while those from another gave identical concordant ages of 1052, 1051, and 1050 ± 2 Ma for the time of metamorphism. Zircons from a folded dyke from the Go Home domain gave identical concordant ages of 1049, 1047, and 1046 ± 2 Ma, while a single overgrowth fragment from an unstrained anatectic pegmatite in the fold hinge gave a concordant age of 1047 ± 2 Ma, as if folding, metamorphic zircon growth and postfolding pegmatite are part of the same process. An anatectic pegmatite in a metadiabase boudin neck in the Ahmic domain gave two zircon fractions with identical concordant ages of 1079 and 1080 ± 2 Ma that match the concordant 1078 ± 2 Ma age of a single overgrowth type grain from the host migmatite. Zircons from the metadiabase have similar but less precise ages due to a low level of uranium and a low abundance. U–Pb results for the Love Lake granite situated at the Parry Sound–Ahmic domain boundary are equivocal but may imply folding of this structure after 1068 Ma, the age of contained titanite. Either the upper or lower end of the ca. 1050–1372 Ma zircon discordia may correspond to granite emplacement. Zircons from a dyke that postdates a granulite-grade migmatization in the Parry Sound domain record a metamorphism at 1114 ± 2 Ma but older inherited grains are present in other fractions. Ductile flow in the Go Home and Moon River domains occurred in the interval 1047–1064 Ma during which the latter was probably emplaced, whereas earlier (1080 Ma) ductile flow in the Ahmic domain may relate to the time of overthrusting of the Parry Sound domain. Parry Sound domain dyke metamorphic ages imply a lack of post 1114 Ma migmatization, and a predyke migmatization consistent with a more exotic derivation of this domain. A [Formula: see text] Ma age for a postmigmatite crosscutting pegmatite dyke sets an upper limit on the age of a late buckling event in the Ahmic domain.

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The horizontal opening of vertical fractures during emplacement of pegmatite-dyke swarms is an important mid-crustal mechanism of large-scale horizontal extension. This is documented in the southwestern Grenville Province, a deeply eroded part of the collisional Grenville orogen. In the Georgian Bay region of central Ontario, Grenville gneisses host c. 990 Ma old, angular dykes which attest not only to horizontal extension but also to vertical thinning. The original dilation dykes probably varied in strike and were statistically vertical. However, many dykes had subhorizontal or inclined segments that were oblique or quasi-concordant to the gneissic foliation in the host rocks. The number of dykes exposed per 0.25 krn2 varies on the scale of a few kilometres, and this is indicative of heterogeneous late orogenic extension of the Grenville gneisses. The apparent absence of regional gradients of peak palaeo-pressure, at the present erosion level, suggests that the extension was horizontal and initially unaccompanied by vertical contraction of the host gneisses. Subsequent buckling of the pegmatite dykes led to gentle, open or close folds with vertical enveloping surfaces. The geometric effects of gentle buckling of pegmatite dykes can be difficult to recognize in the field, especially where the late-stage vertical thinning is relatively weak. Among the geometric indicators of buckle-shortened dykes, the characteristic deflection ('fanning') of coplanar, inherited folia in gneissic host rocks is most sensitive. Systematic changes in the local degree of vertical shortening are indicative of heterogeneous vertical thinning, and may be associated with pull-apart structures at the horizontal scale of several kilometres.