TIMING AND THERMAL INFLUENCE OF LATE OROGENIC EXTENSION IN THE LOWER CRUST - A U-PB GEOCHRONOLOGICAL STUDY FROM THE SOUTHWEST GRENVILLE OROGEN, CANADA

In the southwestern Grenville Province, the Central Gneiss Belt consis ts of a bell of parautochthonous rocks in the north and a collage of a llochthonous lithotectonic domains in the south. Near Pointe-au-Baril, Ontario: the allochthon-parautochthon boundary is marked by the Shawa naga shear zone, a northwest-directed thrust zone between the Britt an d Shawanaga domains that was reactivated during ductile, top-to-the-so utheast extensional shearing. U-Pb ages of 1042 + 4/-2, 1019 +/- 4 and 988 +/- 2 Ma for granitic pegmatite dykes that are pre-kinematic, lat e syn-kinematic, and post-kinematic with respect to top-side-down disp lacement 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 th e dykes. Recognition of the inherited nature of these grains comes fro m the multi-dyke dating approach employed here, and illustrates a pote ntial pitfall in dating shear zone movement using only single dykes in terpreted 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 titani te grain size or vary systematically from domain to domain. However, w hen sample microtexture, strain stale, structural position, titanite m orphology, and pegmatite U-Pb data are considered together, the titani te ages can be reasonably inferred to date: (1) regional metamorphism (1049-1045 Ma); (2) cooling below the titanite isotopic closure temper ature (similar to 600 degrees C) during extensional unroofing (1028-10 18 Ma); (3) recrystallization-controlled titanite growth and (or) isot opic resetting in high-strain zones (1008-1000 Ma); (4) post-kinematic recrystallization, most likely in the presence of a late fluid phase (967-956 Ma). Each titanite age group is made lip of samples from both the Britt and Shawanaga domains, indicating that these processes were not restricted to a single domain bur rather occurred within localize d regimes, some which were clearly structurally controlled. The 93 m.y . spread of concordant titanite ages within a 20 x 20 km area demonstr ates that cooling through isotopic closure is only one of several poss ibilities to be considered when interpreting metamorphic titanite ages in high-grade orogenic terranes. (C) 1998 Elsevier Science B.V.