Polymetamorphism during the Grenvillian Orogeny in SE Ontario: Results from trace element mapping, in situ geochronology, and diffusion geospeedometry

Abstract The Flinton Group is a metasedimentary succession of the Grenville Province in SE Ontario, potentially allowing insight into tectono‐thermal evolution continental crust during Mesoproterozoic. At its Green Bay locality, metapelites staurolite zone contain abundant, post‐kinematic garnet porphyroblasts. Whereas larger crystals are typically impinged, smaller isolated from each other, occasionally exhibiting elongated shapes with apparently trigonal morphology. Central sections population representative sample reveal that composed different compositional and microstructural domains. In largest population, contains rectangular to rhombic domains, marked by sharp increases concentrations Nb, V, Ti, Cr. These domains associated irregularly shaped patches, characterized spatially heterogenous enrichments Ca LREE, depletions contents P, Y, MREE, HREE, accompanied increased densities comparatively coarse‐grained quartz inclusions hosting apatite. Microstructural relationships indicate these correspond portions pseudomorphed biotite, Cr outlining original biotite shapes. patterns formed Ca, REE apatite participated grain‐fluid interactions operated metasomatic replacement garnet. statistical analyses number size distributions confirm initially nucleated on controlled kinetics attachment detachment processes at garnet/biotite interface, resulting typical impingement habit. situ Lu–Hf geochronology applied did not pseudomorph hence enriched points first metamorphic event c. 1080 31 Ma. Subsequent pseudomorphism white mica Al 2 O 3 ‐ FeO‐mobile system resulted concomitant crystallization new generation, forming overgrowths generation nuclei fine‐grained matrix. Garnet this grew morphology, aligned direction perpendicular rock matrix foliation. open‐system behaviour limits use whole‐rock‐based geochronological thermobarometrical applications. However, previously published U–Pb ages monazite included rims took place 976 4 Ma, likely period hydrothermal activity late history Grenvillian Orogeny. Diffusion geospeedometry calculations growth lasted for less than 6 Myr.