Tectonic and metamorphic events in the westernmost Grenville Province, central Ontario: new results from high-precision U–Pb zircon geochronology

Abstract

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.

The Parry Sound domain: a far-travelled allochthon? New evidence from U–Pb zicon geochronology

“…Metamorphism of rocks structurally beneath the Parry Sound domain as early as ca. 1080 Ma(Bussy et al 1995) is consistent with transport of the Parry Sound domain over the Central Gneiss Belt at about this time. Thrusting documented in the possibly correlative Central Metasedimentary Belt boundary thrust zone at 1080-1060 Ma (e.g., McEachern and van Breemen 1993) may be related to the same tectonic episode.…”

Post-convergent structures in lower parts of the 1090–1050 Ma (early-Ottawan) thrust-sheet stack, Grenville Province of Ontario, southern Canadian Shield

“…3), in which mesoscopic boudinage of amphibolite dikes was associated with migmatization and occurred under peak metamorphic conditions at ca. 1080 Ma, implying boudinage took place during thrusting at mid-crustal levels (Bussy et al 1995;Klemens 1996;Schwerdtner and Klemens 2008). On this basis, mesoscopic boudins in peak assemblages elsewhere in the CGB, locally with wide gaps attesting to >500% bulk extension of competent layers, are also inferred to have formed at mid-crustal levels.…”

“…4a-4f;Schwerdtner 1987;Bussy et al 1995;Schwerdtner et al 1998), and the recent interpretation of low-order (crustal-scale) boudinage in the western Grenville Province (Rivers 2012), no intermediate-order counterparts on the km-scale have been recognized. The thicknesses of such boudinaged layers might correspond, for example, to those of individual lithostructural domains delineated in the CGB ( Fig.…”

“…4;Schwerdtner 1987;Bussy et al 1995;Schwerdtner et al 1998). Wherever exposed in three dimensions, these boudinage structures prove to be non-cylindrical, but the length ratios of the major and intermediate axes of individual boudins are highly variable.…”

Emplacement and deformation of granite pegmatite dykes in a mid-crustal regime of late-orogenic extension, southwest Grenville Province, Ontario, Canada

“…l), where the tectonics of Approximately vertical, c. 990 Ma old pegmatite dykes seem to occur throughout the c. 1.6-1.7 Ga and 1.35-1.45 Ga old Proterozoic gneisses of the Georgian Bay region, central Ontario (Fig. lb;van Breemen et al 1986;Krogh, Culshaw & Ketchum, 1993: Tuccillo et al 1992Bussy et al 1995). The dykes are virtually undeformed at many localities, and known as "postkinematic pegmatites" (Culshaw et al 1994, p. 168).…”