Abstract
Cordierite- and gedrite-bearing lithologies crop out along the base of the Central Metasedimentary Belt boundary thrust zone (CMBbtz) in Ontario, the boundary between the Central Metasedimentary Belt and the Central Gneiss Belt. It has been proposed that these rocks represent metamorphosed and partially melted Al- and Mg-rich sedimentary rocks. Whole-rock oxygen isotope values (δ18O in the range 6–8‰) from these rocks are not consistent with closed-system metamorphism of either soils or pelitic sediments, and cannot be explained solely by extraction of partial melt from pelitic rocks. The whole-rock δ18O and chemical composition of cordierite–gedrite rocks in the CMBbtz indicate that these rocks represent volcanic rocks hydrothermally altered by seawater. After hydrothermal alteration, these rocks were metamorphosed to the upper amphibolite facies, stabilizing the cordierite + gedrite assemblages. Diffusion modeling of quartz–garnet fractionations suggests intermineral exchange of oxygen in a closed system at moderate fugacity of H2O during slow cooling after regional metamorphism. We believe that cordierite–gedrite lithologies in the CMBbtz mark the site of Mid-Proterozoic rifting at the margin of Laurentia.
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