Metamorphic petrology of siliceous marbles and associated gneissic rocks in the Grenville Province of southeastern Ontario

The marbles of the Carleton Place area are described within the CaO-MgO-SiO 2-KAlSi3O8-CO2-H2O system. An invariant point involving the phases calcite, dolomite, quartz, potassium feldspar, phlogopite, tremolite, diopside, and fluid in pressure-temperature space exists at ∼625 °C and 6.0 kbars. This invariant point controls the phase relations of the polybaric T-XCO2 diagrams, resulting in significantly different diagrams for conditions above and below this invariant point.; The following isograds were mapped in the study area: (1) Talc-out: 2 talc + 3 calcite = tremolite +dolomite + H2O + CO2; (2) Tremolite-in: 5 dolomite + 8 quartz +H2O = tremolite + 3 calcite + 7 CO2; (3) Tremolite + K-feldspar-in: 24 quartz + 6 calcite + 5 phlogopite = 3 tremolite + 5 K-feldspar + 2 H2O + 6 CO2; (4) Diopside-in: tremolite + 3 calcite + 2 quartz = 5 diopside + 3 CO2 + H2O; (5) Diopside-dolomite-in: tremolite + 3 calcite = dolomite + 4 diopside + H2O + CO2 ; (6) Forsterite-in: tremolite + 11 dolomite = 8 forsterite + 13 calcite + 9 CO2 + H2O.; Evidence from the petrology of the marbles indicates fluid composition was controlled by both internal and external buffering.; Thermobarometric data obtained from the various gneissic and plutonic rocks associated with the marbles suggest different metamorphic conditions prevailed on either side of the Clayton Lake shear zone. Relatively high temperature (554 °C) and low pressure (3.3 kbars) conditions existed in the west based on the Lavant Gabbro Complex. High temperature and high pressure regional metamorphism are suggested by the Wolf Grove Structure which recorded a mean temperature and pressure of 713 °C at 7.8 kbars. Gneisses of the Pakenham Dome experienced P-T conditions of 598 °C and 8.5 kbars. These data were used to calculate approximate field gradients for the study area. A gradient of ∼ 47 °C/km was obtained for the rocks west of the Clayton Lake shear zone and ∼ 26-32 °C/km for rocks to the east.; Based on the differences in mineralogy, metamorphic grade, structural deformation, and associated rock types for the Carleton Place area marbles, it is apparent that two marble units are present in the field area juxtaposed against each other by the Clayton Lake shear zone. Rocks to the west of the shear zone are characterized by marble intercalated with metavolcanic rocks and intruded by the Lavant Gabbro Complex, with evidence of both contact metamorphism and low grade regional metamorphism in the presence of water-rich fluids. East of the shear zone, marble appears to be an integral part of a metasedimentary package containing various gneisses, and quartzite, intruded by granitic to granodioritic plutons. This package has experienced complex deformation and prolonged high grade metamorphism with dominantly CO2-rich conditions.; This evidence supports the regional thrust model of Hildebrand and Easton (1995) in which the high grade rocks of the Frontenac terrane were thrust over the low grade rocks of the Sharbot Lake terrane. However, the presence of two marble units indicates that marbles are present in the footwall as suggested by Davidson and Carmichael (1997) and the terrane boundary coincides with the Clayton Lake shear zone rather than the Maberly shear zone.