ON THE METAMORPHIC EVOLUTION OF ECLOGITIC ROCKS IN THE WESTERN CANADIAN GRENVILLE PROVINCE

The spatial distribution and metamorphic evolution of eclogite within orogenic belts can provide valuable information on the position of major structural discontinuities and the geodynamic processes occurring prior to or during continental collision. In the western Central Gneiss Belt (CGB) of the Canadian Grenville Province high pressure eclogite is most commonly found in close proximity to boundary zones between the structurally lowest parautochthonous rocks (reworked Laurentian margin) and overlying mid-level allochthonous domains. Existing zircon TIMS ages are mostly in the 1080-1100 Ma range, although it remains unclear whether this represents the timing of eclogite facies metamorphism or exhumation to mid-crustal levels. Along the boundary between the upper and lower Go Home domains meter-scale pods of variably retrogressed eclogite are wrapped by the high-strain foliation. One retro-eclogite pod contains large garnet porphyroblasts with primary quartz, rutile, and zircon inclusions and pargasite-labradorite symplectitic coronas. Cores preserve a uniform Mg and Ca richer composition that decreases at the margins with increasing Fe and Mn. Along the pod margin, epidote is common in symplectitic intergrowths with quartz around hornblende and locally as euhedral overgrowths on small allanite cores. A mylonitic gneiss sample within a meter of retro-eclogite lacks symplectitic reaction textures and preserves prograde-type garnet zoning patterns, with bell-shaped decrease in Mn from core to rim. LA-ICPMS zircon analyses yield ²⁰⁷Pb/²⁰⁶Pb spot ages dominantly between 1080-1180 Ma for the interior and marginal retro-eclogite samples, with spot age probability peaks at ca. 1130 Ma and anchored discordia intercepts at ca. 1140 Ma. Spot ages from the mylonitic gneiss sample mostly fall between 1070-1150 Ma, with probability peak at ca. 1090 Ma and discordia intercept at 1106±12 Ma. Petrological modeling of garnet zoning profiles and symplectite-forming reactions, as well as new chronological data from a number of eclogite samples from across the western CGB, will be used to better constrain the metamorphic evolution of eclogites and host gneisses and inform orogen-scale geodynamic models for the western Grenville Province and collisional orogenesis in general.