Corona microstructures between olivine and plagioclase of the Chenaux gabbro in the Grenville Province, Ontario.

Abstract

A corona is a small-scale structure developed around a mineral grain by reaction with its surroundings. It could indicate a change in chemical and/or physical factors during a prograde or retrograde metamorphic event, or associated with cooling from igneous temperatures (Kretz, 1994). The olivine-plagiocalse corona is one of distinct and most common examples of them in metamorphosed rocks.
In Ontario, the Grenville Orogen comprises the Central Gneiss Belt and the structurally overlying Central Metasedimentary Belt, divided into a northeastern Elzevir terrane and a southeastern Frontenac terrane. The Chenaux gabbro intrusion is one of plutons in the Raglan gabbro belt at the base of the Elzevir terrane hanging wall (Pehrsson et al., 1996) (Fig.1). It was emplaced during the late stage of the regional metamorphism and deformation, culminated at ca.1.1 Ga.
The Chenaux Gabbro is partly deformed and recrystallized on the amphibolite facies condition. Intrusive contacts between the gabbro and surrounding carbonate rocks are observed at various places (Fig.2 (1)). It has occasionally igneous layering (Fig.2 (2)). The principal constituents of the gabbro are plagiolclase, olivine, clinopyroxene, orthopyroxene and calcic amphiboles, with small amounts of biotite, spinel and Fe-Ti oxides.