METAMORPHISM OF MARBLES IN THE MORIN TERRANE (GRENVILLE PROVINCE, QUEBEC)

The Morin terrane (Grenville Province, Quebec) is dominated by the 1.15 Ga Morin anorthosite and related granitic rocks, all of which have experienced granulite-facies metamorphism and deformation. Anorthosite-suite rocks surrounded by marble in roadcuts near the village of St. Jovite show a post-intrusion granulite-facies overprint where intrusions and their skarns are deformed. Skarn minerals are consumed by garnet-forming reactions (e.g. An + Wo=Gr + Qtz) that record granulite-facies pressures and temperatures. Carbon isotopes in Morin terrane marbles preserve high-temperature fractionations between calcite and graphite (ΔCc-Gr=3.38±0.25‰; n=21) which correspond to peak temperatures of 755±38°C. There is no correlation between calcite-graphite temperatures and proximity to intrusions, and delaminated graphite crystals from marbles show negligible or no isotope zoning. Caclite-graphite thermometry from the Labelle and Morin shear zones yields more variable and generally lower temperatures. Other samples from shear zones contain dull and pitted graphite suggestive of retrograde (and disequilibrium) growth. Peak metamorphic conditions (ca. 750°C and 7-8 kbar) and the retrograde path in the Morin terrane (Indares and Martignole, CJES 1990) are very similar and probably correlative to metamorphic conditions in the Adirondack Highlands caused by the 1.07 Ga Ottawan orogeny. Metamorphism and deformation of anorthosite-suite rocks and marbles of the Morin terrane are consistent with initial anorthosite intrusion followed by a distinct granulite-facies overprint.