HOT BEFORE COLD: OROGENIC PULSES OF THE GRENVILLE PROVINCE, WESTERN QUEBEC

Convergence during the Grenville orogeny led to crustal thickening and juxtaposition of the currently shallow southeast-dipping imbricated terranes of the Central Metasedimentary Belt (CMB) in eastern Canada. Subsequent crustal thinning during post-orogenic collapse was likely accommodated along shear zones that were reactivated during extension. The Mont Laurier terrane of the CMB (Quebec) contains intercalated Mesoproterozoic gneiss, marble, amphibolite, and quartzite units that originate from upper amphibolite to granulite facies metamorphism of a back-arc sequence. In the Otter Lake region, the terrane is divided into the Marble (west) and Quartzite (east) Domains, separated by southeast dipping Heney deformation zone (HDZ), where we performed ⁴⁰Ar/³⁹Ar thermochronology and LA-ICPMS U-Pb geochronology on a suite of samples. Mineral separates were obtained from garnet-biotite, biotite-plagioclase, and potassium feldspar gneisses, amphibolites and metagabbros, and a marble containing subvertical relict sedimentary bedding. Most samples are pervasively deformed, with fine- to medium-grained biotite defining the foliation fabric; amphiboles exhibit little to no alignment, and are relatively inclusion-free. Deformational microstructures include sutured and bulging quartz grains, subgrain development, and occasional brittle deformation of garnet. In addition to an inherited age component, migmatites yield a bimodal U-Pb zircon and monazite age population at c. 1230 Ma and c. 1160 Ma, the latter age populated by leucosomal minerals. ⁴⁰Ar/³⁹Ar analyses yield generally well-behaved age spectra with antithetic Ca/K spectra exhibiting full or near plateaux. In proximity to the HDZ, hornblende ages are c. 1145 Ma and (euhedral) phlogopite ages are c. 1150 Ma, whereas west of the shear zone hornblende (c. 980-950 Ma) and biotite (c. 890-900 Ma) ages are markedly younger. Peak metamorphism with attendant partial melting and subsequent rapid cooling affected the CMB at 1160-1140 Ma, likely during the "hotter" Ottawan orogeny; the Marble Domain resided at midcrustal conditions until it was cooled >200 m.y. later, possibly during crustal stacking of the small, cold and short duration of the Rigolet orogeny.