Abstract

The lower Cretaceous Crowsnest Formation of the Crowsnest Pass area, Alberta, is predominantly trachytic, but some sodic, analcite-rich, rocks occur. It consists of bedded pyroclastic and epiclastic material with minor flows and dikes. Most of the volcanism appears to have been explosive in nature. Almost all the igneous material is porphyry tic; nearly all the phenocrysts are zoned, and evidence of resorption is common. Sanidine phenocrysts show evidence of repeated growth and solution with as many as 60 cycles per crystal. The primary, igneous, analcite phenocrysts of the analcite phonolites and blairmorites crystallized at depth, and were preserved by rapid vertical transport and quenching at the surface. In many cases, however, partial or complete decomposition of the crystals occurred, and thermal discoloration of the analcite can be reproduced in the laboratory.

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