Cretaceous intrusions in the Commerce Mountain and adjacent areas of southeastern British Columbia and southwestern Alberta

Abstract

An older oversaturated monzonitic and a younger undersaturated syenitic suite of Cretaceous igneous rocks are present at Commerce Mountain and adjacent areas of the southern Canadian Rockies. The undersaturated suite consists of foid-bearing alkali feldspar syenite and trachyte, with lesser amounts of phonolite and foid syenite. Normative data indicate a higher degree of undersaturation attributable to the presence of garnet. Sr and Nd isotopic data are consistent with generation of the magma by partial melting of an enriched source in the upper mantle or the lower continental crust; Pb isotopic data are more consistent with a source in the lower continental crust. High Sr and Ba concentrations in analcime-bearing alkali feldspar megacrysts support early crystallization at high pressure. Replacement of the analcime by muscovite suggests emplacement and continued crystallization at shallow depth under low pressures, at which the analcime was unstable. Prior to emplacement in the upper crust, the Commerce Mountain suite evolved in composition from mafic analcime-bearing perthite syenite to felsic analcime-nepheline-bearing perthite syenite to nepheline (micro)monzosyenite and to analcime (micro)syenite compositionally similar to analcime phonolites found in adjacent areas. Extensive metamorphic-metasomatic alteration suggests that Commerce Mountain was a volcanic center, possibly associated with the eruption of the Crowsnest Formation volcanic rocks.