Abstract

The 150 Ma Averill alkaline plutonic complex is situated in southern British Columbia within the Intermontane Belt of the Canadian Cordillera. It comprises concentrically arranged phases of pyroxenite, monzogabbro, monzodiorite, monzonite, and syenite. Gradational changes in the modal composition of the phases contrast with an abrupt change in crystallinity from euhedral mafic minerals in the ultramafic–mafic phases to an anhedral, interstitial habit for mafic minerals in the syenite. Whole-rock compositions have clear alkaline affinities (e.g., feldspathoid normative) and indicate a chemical discontinuity between the ultramafic–mafic phases and the late syenite phases. Melanite garnet is an important accessory mineral of the syenite and is characteristic of silica-undersaturated alkaline intrusions. Clinopyroxene, feldspar, amphibole, biotite, melanite, and titanite compositions compare closely with those of other alkaline complexes in the western Cordillera.

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