Geology and geochemistry of the Rainy Lake Area

Abstract

Abstract Major stratigraphic units in the Rainy Lake area include graywackes (Coutchiching), mafic and felsic volcanic rocks (Keewatin), and conglomerates and associated finer-grained clastic rocks (Seine), Tonalites (Laurentian) intruded the Coutchiching and Keewatin, and, following a period of folding, contributed clasts to the younger Seine sediments. Granitic plutosn (Algoman) were emplaced during a younger igneous and metamorphic event. Radiometric dating places all of the major rock-forming events between 2700 and 2600 Ma ago. Uplift and stabilization 26000 to 2400 Ma ago resulted in widespread retrograde metamorphism and local cataclasis. Compositional differences between the Coutchiching gneisses in Rice Bay and the lower grade graywackes in the Bears Passage belt reflect different source terranes. Major rock types in the Keewatin are massive and pillow basalts of tholeiitic composition, and felsic rhyodacites with quartz phenocrysts; intermediate rocks are uncommon. Gabbroic sills were emplaced in the Keewatin as the volcanic pile thickened, and anorthosite cumulates reflect differentiation of larger bodies of magma. Sills of leucotonalite and leucogranodiorite are abundant at Rice Bay as slightly discordant bodies in the Coutchiching gneisses and in the overlying Keewatin volcanic rocks. Laurentian intrusions are highly sheared and altered tonalites and granodiorites. Algoman intrusions are less deformed, elliptical-shaped plutons of granodiorite and quartz monzonite. The Ottertail Lake Pluton, the largest body, is zoned from biotite-hornblende granodiorite at the margin to leucocratic quartz monzonite at the center. Angular inclusions of Keewatin rocks are abundant at the margin. These decrease in size and angularity inward where they are last recognized as small mafic clots and schlieren in quartz monzonite. The diversity and general bimodal character of the major igneous rocks units have resulted from different source materials, degrees of melting, and the crustal history of the magmas — fractional crystallization and contamination. Keewatin volcanic rocks evolved through differentiation of tholeiitic magmas, and similarities between the rhyodacites and the Laurentian tonalites suggest that these may be related. Leucocratic sills abundant in Rice Bay are not genetically related to the Keewatin rocks but have some similarities to the Algoman rocks. Marked variations in the Ottertail Lake stock probably resulted from both contamination and differentiation. The origin of these magma is not clear but deviation from the same source as the Keewatin rocks is unlikely. With the development and thickening of the crust, a greater diversity of material became available for magmatic recycling.