Contrasting assemblages and petrogenetic evolution of corona and noncorona gabbros in the Grenville Province of western Labrador

Abstract

Contrasting coronas around olivines, pyroxenes, and ilmenites occur in metagabbros in two adjacent terranes in the Grenville Province of western Labrador. In the parautochthonous Gagnon terrane, coronas around olivines display evidence of evolution from double coronas of orthopyroxene- and fluorine-rich pargasitic amphibole, through triple coronas of orthopyroxene – garnet (clinopyroxene) – pargasitic amphibole, to double coronas of orthopyroxene – garnet (clinopyroxene) (parentheses enclose phase occurring as inclusions in previous phase). In the allochthonous Lac Joseph terrane, coronas around olivines consist of orthopyroxene – [clinopyroxene–spinel] or orthopyroxene–[pargasitic amphibole – spinel] (brackets enclose phases occurring in symplectic intergrowths). Possible reactions that explain the development of these assemblages are investigated, and it is concluded that the system was closed at the level of the corona. The silica required for the olivine–plagioclase reaction in Gagnon terrane was generated by the FMQ reaction during cooling and controlled the rate of reaction along olivine–plagioclase contacts. Alumina was relatively immobile, resulting in the formation of corundum and spinel inclusions in plagioclases in Gagnon and Lac Joseph terranes, respectively.Evolution of the corona gabbros in Gagnon terrane took place in two stages during isobaric cooling after emplacement, whereas formation of the symplectite coronas around olivines in Lac Joseph terrane is compatible with a single-stage reaction during isobaric cooling following emplacement. It is qualitatively estimated that the garnet-bearing coronas in Gagnon terrane formed at higher pressures than the spinel-bearing coronas in Lac Joseph terrane.Greenschist- and amphibolite-facies assemblages in noncorona gabbros in Gagnon terrane occur in locations corresponding to the regional greenschist- and amphibolite-facies metamorphic zones and are accordingly interpreted as being Grenvillian features. There is no apparent Grenvillian imprint in the gabbros in Lac Joseph terrane.