Petrology of the Flora Lake Stock, Lake of the Woods Region, Ontario, Canada

Abstract

The Flora Lake stock is a concordant plug of granitic rocks that was emplaced in a steeply dipping sequence of typical Precambrian “greenstones.” It occupies about 51/2 square miles and is roughly elliptical. The long axis of the ellipse strikes northeast-southwest parallel to the regional strike. Near the stock, the country rocks are mostly pillow and massive mafic volcanic rocks which have been regionally metamorphosed (almandine-amphibolite facies). In order of decreasing age, the stock consists of an outer ring of monzodiorite, an incomplete inner ring of monzonite, and a core of granite. Small bodies of metapyroxenire occur within the monzodiorite at the northeast end of the mass. The three major rock zones are characterized by primary flow structures (planar parallelism of minerals) which form strike patterns integral to each zone. Such structures are particularly pronounced in the monzodiorite and monzonite. In all cases they dip steeply, commonly outward. Chemically and mineralogically, each major rock unit is moderately uniform. Spectrographic, X-ray, and petrographic studies of plagioclase feldspar, biotite, pyroxene, and amphibole indicate that the chemistry of the major minerals in the intrusive sequence varies systematically. The Flora Lake stock may have evolved by anatexis and mobilization of basement rock at deeper levels in the crust. The profound distortion of the regional structural trend in adjacent “greenstones” suggests the stock was forcefully emplaced. The general absence of contact (thermal) metamorphic effects indicates emplacement under pressure-temperature conditions compatible with those of the almandine-amphibolite facies of the regional metamorphic terrain. The monzodiorite and monzonite were emplaced as viscous, crystal-liquid mushes. Well-developed flow structure, absence of chilled contacts, paucity of monzodiorite and monzonite dikes, and numerous evidences of strain in mineral grains in these rocks are compatible with the idea that the intruding material consisted of a high proportion of crystals to liquid. Late in the sequence of intrusion, granitic material must have become mobile, essentially completely magmatic, or granite magma was sweated out of more mafic material at depth. It moved upward and pushed its way into the monzodiorite and monzonite, completing the main intrusive activity in the stock.