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

The Blue River ultramafic body is an ‘Alpine’-typeperidotite tectonically emplaced within spilitic volcanic rocksin northern British Columbia. The intrusive margins were shearedand serpentinized to a lizardite-chrysotile plus brucite assemblageduring emplacement, prior to thermal metamorphism in the aureoleof a younger batholith. Relatively anhydrous peridotite andhydrous serpentinite were both affected by thermal metamorphism.The body has been subdivided into units defined by the mineralassemblages observed in meta-peridotite and meta-serpentiniteabove 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, oxidationand dehydration. The nucleation of metamorphic olivine in weaklymetamorphosed serpentinite was erratic, and turbid porphyroblastcores are enriched in Fe and Mn. The dehydration reaction isthought 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 increasein grade. The oxidation process inhibited dehydration in meta-peridotiteas a stable chlorite was formed. The process also served toreduce the Fe content of the silicate system, as shown by thecomposition of the olivine generated from excess serpentinein high grade meta-serpentinite.