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Volcanic rocks are found in most of the major rock assemblages in the western Cordillera. In-situ models of their origin (Souther, 1977) have been superseded by tectonic models based on evidence that the Cordillera is a collage of separate, once distant terranes that have been accreted to the continental margin. This concept is the basis for the three-fold subdivision of volcanic regimes in this chapter: (1) volcanic rocks of the miogeocline, (2) volcanism in the accreted terranes, (3) post-accretionary volcanism.

Following the separation of ancestral North America from a Precambrian continental mass in mid-Proterozoic time the deposition of a prograded terrace wedge along the western continental margin was accompanied by episodes of crustal extension and minor igneous activity. Tectonism about 1.2 Ga produced a widespread unconformity in the northern Cordillera between the Wernecke and Mackenzie Mountains supergroups and was accompanied by local effusions of alkali-tholeiite lavas and the emplacement of diabase dyke swarms and sills throughout the entire western half of North America. A second tectonic event about 780 Ma initiated deposition of the Windermere Supergroup in a relatively narrow rift depression that truncated the older terrace wedge deposits. Rifting was accompanied by widespread eruption of mafic volcanic rocks and, at least locally, by the explosive eruption of intermediate to salic magma. Deposition of the Windermere Supergroup was followed by a relatively stable regime that lasted through the early Paleozoic. Deposits of quartzite and carbonate reefs on the continental shelf and thick deep-water shale deposits on the outer slope

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