Origins of the Zippa Mountain pluton: a Late Triassic, arc-derived, ultrapotassic magma from the Canadian Cordillera

摘要

The Zippa Mountain intrusion is of Late Triassicage and is situated in the Iskut River area of northwest BritishColumbia. The pluton is elliptical in shape and 3.5 by 5km indiameter. The pluton intrudes Palaeozoic and Triassic rockswithin Stikinia and is compositionally zoned fromclinopyroxenite at the pluton margines to a core of syenite. TheZippa Mountain pluton comprises aegirine-augite, potassiumfeldspar, and minor biotite, hornblede, nepheline, vishnevite,titanian andradite, titanite, and apatite. Based on new field,petrographic, and chemical data this intrusion is shown to besilica-undersaturated, strongly alkaline, and ultrapotassic. Weinterpret the pluton as a single pulse of magma, which entered ashallow-level crustal magma chamber. The potassic nature is acharacteristic of the parental magma, but is enhanced byfractional crystallization and crystal sorting processes. Theparental magma has affinities with arc-type magmas related tosubduction (shoshonitic magma series), as is evidenced by highLILE/LREE ratio, and select depletion of HFSE. Uponemplacement, crystallization of clinopyroxene and then K-feldspar, and efficient physical sorting withing the magmachamber, resulted in sidewall, marginal pyroxenite and roof-zonesyenites, respectively. Continued fractionation in the core of theintrusion increased volatile contents and led to the crystallizationof feldspathoids. Potentially, a mass of residual melt, and crystalsof K-feldspar and feldspathoid, was buoyant relative to thesurrounding pyroxenite, which allowed it it to rise and partlyintrude the syenites.