The ovoid anorthositic gabbro at Bernic Lake, Manitoba

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Bond, W. D.
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Sedimentary and volcanic rocks of the Bird River Area have been intruded by peridotite and gabbroic magmas of various compositions, including anorthosite. At several localities the anorthosite displays a unique texture, with distinct elliptical and/or spherical ovoids averaging 6 inches across uniformly distributed within a massive gabbroic groundmass. This texture is exposed in sills along the shores of Bernic Lake, Manitoba, and also as concordant layers near the base of the gabbroic portion of the Bird River Sill. Structural observations both in the field and in thin section indicate the ovoids are pretectonic. Compositional observations indicate the ovoids are igneous in origin and are poly-crystalline aggregates of plagioclase feldspar that originated by the accumulation of phenocrysts. These aggregates were intruded as a crystal-mush magma that became concentrated in narrow sills through a flow differentiation mechanism. The ovoid anorthositic matrix at Bernic Lake is similar in texture, mineralogy and chemical composition to a gabbroic body thought to be intrusive into the Bird River Sill and a genetic relation is suggested.