The emplacement and metamorphism of the blue river ultramafic body, cassiar district, British Columbia, Canada

Abstract

The blue river ultramafic body is one of many “alpine” peridotites intruded into the Sylvester volcanic series in Northern British Columbia. The primary duniteharzburgite assemblage consists of olivine (FO (_89) – FO (_95)) enstatite (En (_90) – En (_92), and spinel. The spinel chemistry is variable, dunite spinels give Crx100/ (Cr+Al) ratio of 58 and above, and peridotite spinels are invariably below this figure. The assemblage, which was evidently intruded hot as it has formed a “hornblende hornfels” facies amphibolite aureole, is inferred to have equilibrated with volcanic magma, present in the form of bodies gabbro. The body has been truncated by the Cassiar batholith. Metamorphism has affected both the primary assemblage, and also an early generation of marginal serpemtinite. Isograds have been established which mark the incoming of metamorphic olivine, tremolite, olivine with talc, and enstatite. Metamorphic olivine porphyroblasts in the outer aureole of the batholith are zoned. They have an inner turbid core enriched in Fe and Mn, and a clear outer margin enriched in Mg. Ni will enter olivine only in the absence of sulphur. A maximum core to margin range of FO (_85) – FO (_97) we found 4,000m from the contact. Above the olivine with talc isograd the assemblage is influenced by increased FO (_2). Primary spinels in both peridotite and serpentinite are oxidized. Al substitutes in the serpentine structure to form chlorite, and the spinel absorbs Fe, Mn and Ni. The al content in the chlorite increases towards the batholith contact. Metamorphic olivines are weakly or non zoned, and they reach a composition of Fo (_95). Metamorphic enstatite is similarly Fe depleted at En (_93). The body underwent partial alteration to antigorite during the waning stages of thermal metamorphism, and more pervasive serpentinization on cooling. Euhedral olivine porphyroblasts in a matrix of relict serpentine retain their original outlines, and they undergo volume for volume replacement. The lizardite-chrysotile assemblage contains abundant brucite, which envelopes serpentine pseudomorphs after metamorphic olivine. This is indicative of cation mobility during serperntinization.