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The trondhjemitic Northern Light Gneiss, Saganaga Tonalite, and syenodioritic to granodioritic Icarus pluton are part of a typical Archean igneous sequence that began with the extrusion of large volumes of basic volcanics, which make up large parts of present-day greenstone belts, and ended with a small post-kine-matic alkalic stock. Radiometric dating indicates that these events encompassed approximately 50 m.y., from 2,700 to 2,750 m.y. ago.

K/Rb, Rb/Sr, Sr/Ba, and initial Sr87/Sr86 ratios for the Saganaga Tonalite and Northern Light Gneiss are similar to those for Archean basalts. A rare earth analysis for the tonalite is strongly depleted in the heavy rare earths with an abundance similar to chondrites and a light rare earth abundance similar to ocean ridge basalts. The depletion in the heavy rare earths is strongly suggestive of separation of a melt from a garnet-rich residue. Quartz “eyes” in the tonalite are probably partially resorbed quartz phenocrysts. It is proposed that both the tonalite and the trondhjemitic gneiss are derived by partial melting of either quartz eclogite or garnet-rich amphibolite at mantle depths. For both rocks the parent is assumed to be Archean mafic volcanics.

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