The Morin terrane (Grenville Province, Quebec) is dominated by the 1.15 Ga Morin Anorthosite Massif and related granitic intrusions, all of which exhibit granulite-facies mineral assemblages. Anorthosite-suite rocks are deformed both in shear zones and in the interior of the terrane and show intrusive contact relations with marble along road cuts near the village of St. Jovite. Intrusive rocks exposed in these road cuts have well-developed skarns, which were deformed with the intrusions after emplacement. Skarn minerals are consumed by garnet-forming reactions (e.g., An + Wo = Gr + Qtz) that preserve granulite-facies temperatures and pressures. Calcite–graphite thermometry of Morin terrane marbles records temperatures of 755 ± 38 °C (n = 21), independent of proximity to anorthosite-suite plutons. Preserved metamorphic conditions and the retrograde pressure–temperature (P–T) path in the Morin terrane are very similar to conditions during the 1.07 Ga Ottawan orogeny in the Adirondack Highlands. Metamorphism and deformation of anorthosite-suite rocks and marbles of the Morin terrane are consistent with anorthosite intrusion followed by a distinct granulite-facies overprint.

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The kinetics of the breakdown reaction dolomite = periclase + calcite + CO 2 were investigated using cores of dolomitic marble. Two samples of Reed Dolomite from southwestern Nevada were cut into cylinders approximately 4 × 6 mm in size. The cores were sealed in gold capsules with isotopically enriched water (H 2 18 O or HD 18 O 0.5 16 O 0.5 ). The samples were heated in a cold-seal hydrothermal apparatus to 650-750 °C at 100 MPa for durations ranging from 2-59 days. The cores were then sectioned and examined by EPMA, XRD, and SIMS techniques. All experiments showed some amount of reaction regardless of duration or temperature. Reaction products occurred mainly along grain boundaries, fractures within grains, and along sample edges. Ion images and isotope-ratio analysis indicated that reaction products exchanged with infi ltrating fl uids. Reaction rates were calculated from measured extents of reaction, which were determined from automated EPMA modes. At 700 °C, we measured reaction rates ranging from 3.8 × 10 -14 to 2.3 × 10 -12 mol/mm 2 ·s. The extent of reaction is proportional to the square root of time, suggesting a diffusion-controlled process. A shrinking-core model for the dolomite breakdown reaction fi ts the grain-size data, suggesting that diffusion of H 2 O and CO 2 through the mantle of reaction products controlled the rate. Apparent activation energies for that diffusion are ~283 ± 32 kJ/mol for coarse-grained samples and ~333 ± 36 kJ/mol for fi ne-grained samples. Initial reaction occurred relatively fast near the surface of dolomite grains, but continued diffusion through the reaction products ultimately controlled the rate of dolomite breakdown.

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