A compilation of thermobarometric data from the Metasedimentary Belt of the Grenville Province, Ontario and New York State

A new compilation of regional metamorphic temperature-pressure data in the Metasedimentary Belt (MB) of the Grenville Orogen, as exposed in Ontario and New York, shows the effect of major zones of ductile shear on metamorphic gradients. In addition to other thermometers, available data from garnet-biotite and two-feldspar thermometers were recalculated with a single calibration to permit comparisons among different field areas. Temperatures in the MB range from <500 degrees C near Madoc, Ontario, in the Elzevir domain, and are in excess of 700 degrees C in the western Gneiss Belt (GB) and the eastern Frontenac domain. Offsets in temperatures are observed across the Metasedimentary Belt Boundary Zone (MBBZ), the Sharbot Lake Shear Zone (SLSZ), and perhaps the Carthage-Colton Shear Zone (CCSZ). Pressures range from <6 kbar (<600 MPa) in the Elzevir domain near Madoc, Ontario to 8 kbar (800 MPa) in the westernmost part of the Frontenac domain. There is a 1-2 kbar (100-200 MPa) discontinuity in pressure across the Metasedimentary Belt Boundary Zone (MBBZ) and at the Robertson Lake (RLSZ) and Sharbot Lake (SLSZ) shear zones. However, some shear zones in the MB predate the peak of metamorphism, and do not seem to offset metamorphic gradients in the field (e.g., the Mooroton shear zone). Comparing temperatures and pressures across these boundaries requires knowledge of the timing of peak metamorphism. Geochronological studies indicate that some boundaries juxtapose domains that do not have the same metamorphic histories. In the Grenville Orogen, such integrated data suggest that zones of ductile shear variably represent tectonically active terrane boundaries and intra-terrane zones of displacement.