Abstract

Undeformed lenses of olivine–orthopyroxene–gabbro belonging to the 1429 Ma Shabogamo Intrusive Suite (Parautochthonous Belt of the eastern Grenville Province) host the first occurrence of eclogite mineral assemblages reported from the Grenville Province. These rocks preserve igneous textures overprinted by eclogite mineralogy and by textures related to subsequent unloading. High-pressure signatures include omphacite pseudomorphs after augite, plagioclase replacement by garnet + kyanite + corundum, and orthopyroxene–omphacite–garnet coronas between olivine and plagioclase. Transformations related to decompression include replacement of garnet + kyanite + corundum by spinel + plagioclase, growth of plagioclase collars between clinopyroxene and garnet, and de-jadeitization of omphacite. Clinopyroxenes with numerous plagioclase inclusions and garnets surrounded by clinopyroxene–plagioclase symplectites in granoblastic mafic rocks from the same area are also evidence of decompression from high pressures. These rocks record pressure–temperature (PT) conditions of 1600 MPa at 700–800 °C and steep subsequent PT paths. It is suggested that the entire parautochthon in this part of the Grenville Province experienced deep burial during the Grenvillian metamorphism. In contrast, the Grenvillian metamorphic signature in the overlying allochthonous sheet is known to be minor, thus precluding any relationship between its emplacement and very high Grenvillian pressures in the parautochthon.

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