Date of Award
Master of Science (MS)
A 10 km2 portion of a subsolvus, syntectonic or posttectonic grano dioritic intrusion located about 16 km east of Kenora, Ontario was extensively sampled for study by petrographic modal analysis and trend surface analysis. The Dogtooth pluton, the main intrusive body sampled, is part of a granitic batholith known as the Dryberry dome, which is interpreted to have been intruded into supracrustal volcanic and sedi mentary rocks about 2480 million years ago during the Kenoran Orogeny. The Longbow pluton, which was also sampled, occurs at the west margin of the Dryberry dome in contact with the supracrustal rocks to the west of the study area.
The rocks of the Dogtooth pluton are pink, fine- to medium-grained, and have either hypidiomorphic granular or allotriomorphic granular texture. The rocks are characterized over much of the study area by the presence of polycrystalline quartz aggregates up to 2 cm in diameter. The pluton is approximately 75 percent granodiorite or trondhjemite in composition at the present level of exposure, with the remainder of the samples being quartz monzonite, granite, monzonite, and syenite. The average composition of the rocks sampled is 28.4 volume percent quartz, 46.9 percent plagioclase, and 19.7 percent alkali feldspar, which falls very close to the ternary eutectic for the KA1Si 308 –Na ALSi 308 –SI02 system. The rocks analyzed are low in mafic mineral content, averaging only about 0.7 volume percent biotite and 0.2 percent opaque minerals as the only mafic minerals.
The country rocks which the Dogtooth pluton intrudes are_ gneissic tonalites, gneissic trondhjemites, and amphibolite. Trend surface analysis shows a strong inversely proportional relationship between pla gioclase and alkali feldspar and a moderate inversely proportional rela tionship between alkali feldspar and quartz. These trends may be caused by cooling temperatures and water vapor pressures in the first case and differentiation in the second case. The processes operative during the emplacement of the Dogtooth pluton were probably forcible intrusion, stoping, and, less important, assimilation. Some areas of fine-grained rocks within the pluton may represent conduits for residual fluids from the crystallization of the pluton.
The rocks of the Longbow pluton are gneissic trondhjemites and gneissic tonalites. They are medium-grained, whitish gray to dark gray in color and have hypidiomorphic granular texture. There are at least three different intrusive phases within the pluton, with two earlier phases present as xenoliths in a later phase. Amphibolite xenoliths are also common in all three phases. The mafic minerals present are mainly biotite (5-15 volume percent) with a trace of amphibole. The quartz content averages over 20 percent for the rocks analyzed. The Longbow pluton appears to have assimilated large amounts of amphibolite and converted the amphibole to biotite, possibly indicating high water vapor pressures during emplacement.
Pilatzke, Richard H., "Geology of the western portion of the Dogtooth pluton, Lake of the Woods region, southwestern Ontario" (1976). Theses and Dissertations. 232.