Thermochronology and geochronology of the Otter Lake region, QC, Central Metasedimentary Belt, Grenville Province
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High grade metamorphism preserved in the Grenville Province of eastern Canada resulted from multiple orogenic events (Elzevirian, Shawinigan, and Grenvillian) between ca. 1250 and 980 Ma. The Mont Laurier terrane of the Central Metasedimentary Belt (Quebec) contains intercalated gneiss, marble, amphibolite, and quartzite units that originated as a Mesoproterozoic back-arc sequence. Northwest of Ottawa in the Otter Lake region, the terrane is divided into the Marble (west) and Quartzite (east) domains, separated by the easterly dipping Heney deformation zone (HDZ). Both domains consist of pervasively deformed upper amphibolite/lower granulite facies rocks (at peak equilibrium conditions of 6 ± 1 kbar and 650-700°C). Fine- to medium-grained biotite define the foliation fabric in garnet-biotite, biotite-plagioclase, and potassium feldspar gneisses, amphibolites and metagabbros; amphiboles exhibit little to no alignment, and are relatively inclusion-free. Deformational microstructures include sutured and bulging quartz grains, subgrain development, and the rare brittle deformation of garnet.
LA-ICPMS U-Pb geochronology on select gneisses/migmatites from both domains yielded bimodal U-Pb zircon and monazite age populations at ca. 1230 Ma and ca. 1160 Ma in addition to an older inherited age component of ca. 1380-1370 Ma. 40Ar/39Ar mineral analyses yielded generally well-behaved age spectra with antithetic Ca/K spectra exhibiting full or near plateaux. In proximity to the HDZ, hornblende and phlogopite ages are ca. 1145 Ma and ca. 1150 Ma, respectively, whereas to the west in the Marble Domain hornblende ages are markedly younger (ca. 970-950 Ma). Biotite ages of 905-885 Ma are similar to the majority of biotite Ar/Ar ages reported across the southern Grenville Province.
The Marble and Quartzite domains record similar higher temperature U-Pb mineral ages indicating initial Elzevirian age (ca. 1245-1225 Ma) metamorphism followed by Shawinigan age (ca. 1190-1140 Ma) upper amphibolite to lower granulite regional metamorphism and migmatization. Rapid cooling of the Quartzite Domain at the end of the Shawinigan Orogeny together with the complete lack of evidence for any metamorphic overprinting in the Otter Lake region during the long-lived Grenville Orogeny (1095-980 Ma) is consistent with this region residing in the mid-crust or Orogenic Lid of the Grenville Orogen (Rivers, 2008, 2009). Cooling of Marble Domain rocks through 500°C nearly 200 m.y. after cooling of the Quartzite Domain suggests considerable late normal displacement across the HDZ analogous to that proposed for terrane bounding shear zones like the Robertson Lake and Bancroft shear zones in Ontario, perhaps related to vertical collapse or lateral stretching of the Orogenic Lid. Relatively uniform biotite Ar/Ar ages across the province suggest orogen-wide thermal stabilization at ca. 900 Ma.
Document number: kent1333295679
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