The Zippa Mountain intrusion is of Late Triassic age and is situated in the Iskut River area of northwest British Columbia. The pluton is elliptical in shape and 3.5 by 5 km in diameter. The pluton intrudes Palaeozoic and Triassic rocks within Stikinia and is compositionally zoned from clinopyroxenite at the pluton margins to a core of syenite. The Zippa Mountain pluton comprises aegirine-augite, potassium feldspar, and minor biotite, hornblende, nepheline, vishnevite, titanian andradite, titanite, and apatite. Based on new field, petrographic, and chemical data this intrusion is shown to be silica-undersaturated, strongly alkaline, and ultrapotassic. We interpret the pluton as a single pulse of magma, which entered a shallow-level crustal magma chamber. The potassic nature is a characteristic of the parental magma, but is enhanced by fractional crystallization and crystal sorting processes. The parental magma has affinities with arc-type magmas related to subduction (shoshonitic magma series), as is evidenced by high LILE/LREE ratio, and select depletion of HFSE. Upon emplacement, crystallization of clinopyroxene and then K-feldspar, and efficient physical sorting within the magma chamber, resulted in sidewall, marginal pyroxenite and roof-zone syenites, respectively. Continued fractionation in the core of the intrusion increased volatile contents and led to the crystallization of feldspathoids. Potentially, a mass of residual melt, and crystals of K-feldspar and feldspathoid, was buoyant relative to the surrounding pyroxenite, which allowed it it to rise and partly intrude the syenites.

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