Abstract

An Archean conglomerate in the North Spirit Lake area of northwestern Ontario contains rare orthoquartzite pebbles. Detailed study of these pebbles shows that mineralogically they are very mature, consisting of as much as 99.8 percent quartz and a heavy mineral suite of zircon, tourmaline, and apatite. Textures are typically bimodal, characterized by rounded sand-sized quartz grains set in a 'matrix-cement' of thoroughly recrystallized finer quartz grains. These orthoquartzite pebbles provide the first definite evidence for local tectonic stability of the Canadian Shield before deposition of the immature sedimentary rocks that form part of an Archean (>2.6 Ga) greenstone belt of the Superior Province.

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