The Metamorphism of the Blue River Ultramafic Body, Cassiar, British Columbia, Canada

Abstract

The Blue River ultramafic body is an ‘Alpine’-type peridotite tectonically emplaced within spilitic volcanic rocks in northern British Columbia. The intrusive margins were sheared and serpentinized to a lizardite-chrysotile plus brucite assemblage during emplacement, prior to thermal metamorphism in the aureole of a younger batholith. Relatively anhydrous peridotite and hydrous serpentinite were both affected by thermal metamorphism. The body has been subdivided into units defined by the mineral assemblages observed in meta-peridotite and meta-serpentinite above and below the isograd for the advent of the mineral talc. Isograds were also established for prograde metamorphic olivine, tremolite, and enstatite. The intrusive was subjected to two metamorphic processes, oxidation and dehydration. The nucleation of metamorphic olivine in weakly metamorphosed serpentinite was erratic, and turbid porphyroblast cores are enriched in Fe and Mn. The dehydration reaction is thought to have been metastable. Above the talc isograd, serpentine, in both peridotite and serpentinite, reacted with original spinel to form ferritchromit and chlorite. The chlorite becomes progressively more aluminous with increase in grade. The oxidation process inhibited dehydration in meta-peridotite as a stable chlorite was formed. The process also served to reduce the Fe content of the silicate system, as shown by the composition of the olivine generated from excess serpentine in high grade meta-serpentinite.