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The metamorphic and plutonic rocks upon which the miogeocline was developed are considered to be the basement rocks of the Canadian Cordillera. They are recognized by an unconformable relationship with Proterozoic or younger bedded rocks and/or on the basis of reliable geochronological data. According to these criteria, basement rocks in the Cordillera are exposed only in fold and thrust nappes of the Omineca Belt, and, at one locality, unconformably beneath the Windermere Supergroup in the Foreland Belt. Basement rocks sampled in diatremes and drill holes provide additional geochronological data.

drill holes provide additional geochronological data. The principal basement exposures comprise mainly granitic paragneiss and orthogneiss, with minoramphibolite and metasedimentary rocks. U-Pb zircon dating provides the most reliable age assignment. These age determinations suggest that most Precambrian crystalline rocks of the Cordillera fall into three groups, 1.85-2.1 Ga, 1.1-1.2 Ga, and 0.7-0.8 Ga; the first and third of these categories are probably represented throughout the full length of the Canadian Cordillera. Relationships with the structurally or stratigraphically associated Windermere Supergroup suggest but do not prove that all of these rocks are part of the North American craton. The earlier Proterozoic ages (1.85-2.1 Ga) probably represent those of one or more Proterozoic tectonic provinces of the western Canadian Shield. Granitic rocks from a diatreme in the Mackenzie Mountains (1.1-1.2 Ga) may be associated with a Middle Proterozoic orogenic event in the northern Cordillera. The latest Proterozoic rocks may be related to magmatism caused by pre-Windermere rifting, eventually resulting in the

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