Timing and thermal influence of late orogenic extension in the lower crust: a UPb geochronological study from the southwest Grenville orogen, Canada

Abstract

In the southwestern Grenville Province, the Central Gneiss Belt consists of a belt of parautochthonous rocks in the north and a collage of allochthonous lithotectonic domains in the south. Near Pointe-au-Baril, Ontario, the allochthon-parautochthon boundary is marked by the Shawanaga shear zone, a northwest-directed thrust zone between the Britt and Shawanaga domains that was reactivated during ductile, top-to-the-southeast extensional shearing. UPb ages of 1042+4/−2, 1019±4 and 988±2 Ma for granitic pegmatite dykes that are pre-kinematic, late syn-kinematic, and post-kinematic with respect to top-side-down displacement constrain major extensional transport on the Shawanaga shear zone to ca. 1020 Ma. However, nearly all the zircons in the dykes are inherited from a pre-Grenvillian, ca. 1460 Ma granitic host rock to the dykes. Recognition of the inherited nature of these grains comes from the multi-dyke dating approach employed here, and illustrates a potential pitfall in dating shear zone movement using only single dykes interpreted as syn-kinematic. UPb ages of metamorphic titanite from 14 samples collected in a transect across the Shawanaga shear zone span a 93 m.y. segment of concordia. These ages do not correlate with titanite grain size or vary systematically from domain to domain. However, when sample microtexture, strain state, structural position, titanite morphology, and pegmatite UPb data are considered together, the titanite ages can be reasonably inferred to date: 1. (1) regional metamorphism (1049-1045 Ma); 2. (2) cooling below the titanite isotopic closure temperature (∼600°C) during extensional unroofing (1028-1018 Ma); 3. (3) recrystallization-controlled titanite growth and (or) isotopic resetting in high-strain zones (1008-1000 Ma); 4. (4) post-kinematic recrystallization, most likely in the presence of a late fluid phase (967-956 Ma). Each titanite age group is made up of samples from both the Britt and Shawanaga domains, indicating that these processes were not restricted to a single domain but rather occurred within localized regimes, some which were clearly structurally controlled. The 93 m.y. spread of concordant titanite ages within a 20 × 20 km area demonstrates that cooling through isotopic closure is only one of several possibilities to be considered when interpreting metamorphic titanite ages in high-grade orogenic terranes.