Tectonic implications from sapphirine‐bearing lithologies, south‐west Grenville Province, Canada

Abstract

Sapphirine has been found in two types of magnesian, metabasic lenses from tectonite zones within the Central Gneiss Belt of the south‐west Grenville Province, Canada. The first type (association I) comes from a lenticular mafic lens within highly tectonized anorthosite, the second type (association II) comes from meta‐eclogitic pods with foliated amphibolite rims. In each case the sapphirine‐bearing assemblages record a wealth of reaction textures. The primary mineralogy in association II is represented by high alumina clinopyroxene, garnet and kyanite ± plagioclase and records pressures of around 14‐16 kbar; in association I the primary mineralogy is represented by plagioclase, two pyroxenes and possibly olivine but here the equilibrium pressure is unknown. The host gneisses equilibrated at approximately 8 to 10 kbar and 700‐750°C by continuous cation exchange reactions during and after the culmination of the Grenvillian orogeny at 1.16‐1.0 Ga. It is unlikely that the higher pressures recorded in the meta‐eclogitic pods represent an earlier high‐pressure metamorphism as the pods are restricted to shear zones. A tectonic mode of emplacement into a crust undergoing granulite facies metamorphism is more likely. Sapphirine formed by discontinuous decompression reactions; in association II this involved a reaction between garnet and kyanite and resulted in the formation of magnesian granulite facies assemblages. At the same time primary clinopyroxene became much less aluminous by evolving plagioclase. Pressures and temperatures from coexisting phases, that are believed to have equilibrated at the same time as sapphirine formation, are estimated as 11 to 12 kbar and 750°C. These probably represent the peak conditions for granulite facies metamorphism in the south‐west Grenville Province.