Abstract

Three types of subvertical leucogranites and pegmatites populate the Red Cross Lake pegmatite field, located in the Red Cross Lake greenstone belt of the Sachigo Subprovince, in the northwestern part of the Archean Superior Province of the Canadian Shield. The greenstone belt is situated in the northern margin of the Oxford–Stull domain of the North Caribou terrane, in tectonic contact with the Hudson Bay terrane. The tholeiitic greenstone belt, flanked by plutonic masses of tonalitic gneiss to the north and tonalite–granodiorite gneiss to the south, consists of a bimodal assemblage of metabasalt + subvolcanic metagabbro and felsic lapilli tuff of andesitic to rhyolitic compositions, mafic mylonite with a mylonitized granitoid, and volcaniclastic metagreywacke. Regional context and petrochemical features of the south-flanking tonalitic-granodioritic masses and of the greenstone-belt lithologies indicate a largely juvenile, deep-oceanic origin. To the north, the isotopic signatures of tonalitic-granodioritic masses indicate an older continental association. The leucogranites and pegmatites and related intrusions were subvertically emplaced into, or alongside, the predominantly mylonitized metavolcanic rocks of the WNW–ESE-trending North Kenyon Shear Zone. Some of the leucogranites are geochemically rather primitive, but they grade into more evolved pegmatite bodies and ultimately into extremely fractionated lepidolite-subtype pegmatites. Investigation of the origin of the rare-element enrichment in these pegmatites, within the limits of our reconnaissance sampling, did not show above-average enrichment of rare lithophile elements in any of the main regional map units.

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