QUANTIFYING LOWER CRUSTAL DYNAMICS IN THE WESTERN GRENVILLE PROVINCE THROUGH INTEGRATED MULTI-SYSTEM GEOCHRONOLOGY

Lower crustal rocks exposed within ancient orogenic belts are often the only source of information on the kinematics, thermal structure, and rheology within subduction-collision zones. In the western Central Gneiss Belt (CGB) of the Canadian Grenville Province, eclogite and associated high pressure (HP) rocks are commonly found within high strain zones between the structurally lowest parautochthon (reworked Laurentian margin) and overlying mid-level allochthonous domains. The timing of HP metamorphism and exhumation to mid-crustal levels remains uncertain, and thus an integrated petro-chronological investigation utilizing in-situ garnet and zircon REE and zircon U-Pb analysis, as well as garnet-whole-rock Lu-Hf analysis, is employed here.

Zircon U-Pb analyses from a meter-scale eclogite pod within the upper/lower Go Home domain boundary zone yield ²⁰⁷Pb/²⁰⁶Pb spot age probability maxima at ca. 1130 Ma and anchored discordia intercept at 1142±11 Ma. The REE composition of zircon from the pod core indicates that zircon grew in a garnet-bearing, plagioclase-free assemblage characteristic of eclogite-facies conditions. Mylonitic gneiss surrounding the eclogite pod has a probability maxima at ca. 1090 Ma and discordia intercept at 1106±12 Ma. Preserved growth zoning in garnet and zircon REE compositions suggest the bounding gneisses have a different metamorphic history than the adjacent eclogite pod.

Line traverses across garnet interiors from five eclogite samples yield a range of intracrystalline REE zonation patterns, with MREE and HREE commonly exhibiting significantly different to even inverse behavior. In some samples, HREE exhibit relaxed Rayleigh-type zoning, characteristic of diffusionally modified original growth zoning. Other samples show irregular or essentially flat HREE zoning patterns, commonly with resorption-related back diffusion along grain margins, suggesting compositional modification during or after peak thermal conditions. Forthcoming garnet-whole rock Lu-Hf isochron ages determined for each of these types of samples should yield the timing of garnet growth (for Rayleigh-type zoned garnets) and peak/post-peak thermal conditions (for garnets with flat profiles and resorbed edges), further constraining the metamorphic evolution of HP rocks in the western CGB.