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The Precambrian rocks, older than 2,600 m.y., in the Saganaga Lake-Northern Light Lake area on the Minnesota-Ontario boundary include a thick succession of metavolcanic rocks, chiefly greenstone but with some intermediate to silicic types, and minor metasediments; the Northern Light Gneiss; the Saganaga Tonalite; alkalic syenodiorite and granodiorite; and the Knife Lake Group. A variety of dikes and sills in the older units are of different ages; the youngest are related to the Keweenawan igneous activity, approximately 1,100 m.y. ago.

The Northern Light Gneiss consists principally of fine-grained biotite leucotonalite, or trondhjemite, that is interlayed with amphibolite and lesser amounts of metarhyodacite and metarhyolite. The gneiss is part of the volcanic pile of greenstone and related rocks formerly referred to the Keewatin.

Both the greenstone and the Northern Light Gneiss were intruded by the Saganaga Tonalite. This rock was formerly called the Saganaga Granite, but the “granite” is a sheared and altered phase in which microcline was introduced, and small amounts of epidote, chlorite, sericite, and carbonate were formed at the expense of the primary hornblende, biotite, and oligoclase.

The Saganaga Tonalite and the Northern Light Gneiss were intruded by a small pluton near Icarus Lake in the eastern part of the area. The pluton consists of a western phase of syenodiorite and an eastern phase of somewhat younger granodiorite. Both rocks are alkalic and are characterized by soda-rich pyroxene and amphibole.

Two periods of folding in the region were previously recognized as the Lauren-tian and Algoman orogenies. The Saganaga Tonalite is a late-kinematic intrusion in the folded greenstone and related rocks. Uplift and vigorous erosion uncovered the tonalite which supplied large boulders and finer detritus to the sediments of the Knife Lake Group deposited on the western margin of the batholith. At Cache Bay the conglomerate resting on the tonalite is composed almost wholly of clasts of tonalite, but north of Cache Bay the conglomerate on the greenstone is composed largely of pieces derived from the greenstone and related rocks. The tonalite also supplied a large component of the graywacke-argillite sequence that is intercalated with the conglomerate at Cache Bay.

The Knife Lake conglomerate and graywacke-argillite sequences were folded during the Algoman orogeny, and at Cache Bay the beds dip approximately 70° W. Although these events were deciphered from the geologic record, efforts to resolve the two periods of folding by radiometric age determinations have not been completely successful. The Saganaga Tonalite is dated by a whole-rock Rb-Sr isochron and by U-Pb ages on sphene and zircon at 2,730 m.y. The older Northern Light Gneiss probably was formed not more than 2,750 m.y. ago, and the younger Icarus pluton was emplaced approximately 2,700 m.y. ago; hence a complicated geologic history is encompassed in a span of 50 m.y. or less.

The Saganaga Lake-Northern Light Lake area is part of a mobile belt in which folding and igneous activity were essentially continuous from the time of eruption of the greenstone and related rocks to the time of folding of the Knife Lake Group.

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