GEON 9 RETROGRESSION IN THE CENTRAL METASEDIMENTARY BELT BOUNDARY ZONE (CMBBZ), GRENVILLE PROVINCE, ONTARIO, CANADA?

The Central Metasedimentary Belt boundary zone (CMBbz) is a 10 km thick, ~200 km long crustal-scale shear zone composed of stacked crystalline nappes and ductile shear zones; the CMBbz juxtaposes the Central Gneiss Belt and the Central Metasedimentary Belt in the Grenville Orogen in Ontario, Canada. Near the town of Killaloe, deformed gneisses at the base of the structurally lowest nappe represent mixed felsic and mafic igneous protoliths. Metamorphic assemblages and textural variations provide evidence for multiple deformation events in the shear zone here. Greasy greenish pods in relatively mafic orthogneiss contain cm-scale orthopyroxene grains, suggesting that peak metamorphism occurred at the granulite facies. Matrix textures and metamorphic assemblages in both felsic and mafic orthogneiss from the same outcrops are consistent with an overprinting, penetrative amphibolite-facies deformation and metamorphic event. Matrix monazite yields electron microprobe U-Th-Pb dates on a grain aligned with foliation. These dates are consistent with the well-established timing of early and peak Ottawan deformation and metamorphism in the CMBbz: ca. 1100 ± 15 Ma for the core and ca. 1060 ± 30 Ma for the rim of the monazite grain. Although we cannot constrain the timing of the granulite facies event, other than to suggest that it predated deformation and metamorphism at the amphibolite facies, we do see textural evidence in thin section that the orthopyroxene grains are extensively retrogressed from hypersthene to clots of fine-grained, low-grade minerals including sheet silicates, calcite, and quartz. These clots appear undeformed. U-Th-Pb dates from a monazite grain within a retrogressed clot are 900 ± 50 Ma. These dates are younger than the likely timing of deformation events associated with the Grenvillian orogeny. Interpreted in combination with other Geon 9 U-Th-Pb dates from similar unzoned monazite grains in Grenville gneisses, we suggest that Geon 9 monazite dates record fluid infiltration and static retrogression of high-grade metamorphic assemblages during the post-kinematic exhumation of the orogen. Interestingly, Geon 9 dates from the Grenville Orogen in Ontario are consistent with many argon cooling ages reported in the literature on the orogen.