Prehnite-Pumpellyite to Greenschist Fades Transition in the Karmutsen Metabasites, Vancouver Island, B.C.

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Abstract
Mineral paragenescs in the prehnite-pumpellyite to greenschist fades transition of the Karmutsen metabasites are markedly different between amygdule and matrix, indicating that the size of equilibrium domain is very small. Characteristic amygdule assemblages (+ chlorite + quartz) vary from: (1) prehnite + pumpeUyite + epidote, prehnite + pumpellyite + calcite, and pumpellyite + epidote + calcite for the prehnite-pumpellyite facies; through (2) calcite + epidote + prehnite or pumpellyite for the transition zone; to (3) actinolite + epidote + calrite for the greenschist facies. Actinolite first appears in the matrix of the transition zone. Na-rich wairakites containing rare analcime inclusions coexist with epidote or Al-rich pumpellyite in one prehnite-pumpellyite facies sample. Phase relations and compositions of these wairakite-bearing assemblages further suggest that pumpellyite may have a compositional gap between 0.10 and 0.15 XFe°. Although the facies boundaries are gradational due to the multi-variance of the assemblages, several transition equilibria are established in the amygdule assemblages. At low Xco2, pumpellyite disappears prior to prehnite by a discontinuous-type reaction, pumpellyite + quartz + CO2 = prehnite + epidote + calcite + chlorite + H2O, whereas prehnite disappears by a continuous-type reaction, prehnite + CO2 = calcite + epidote + quartz-l-H2O. On the other hand, at higher XCO2 a prehnite-out reaction, prehnite + chlorite + H2O + CO2 = calcite + pumpellyite + quartz, precedes a pumpellyiteout reaction, pumpellyite + CO2 = calcite + epidote + chlorite + quartz + H2O. The first appearance of the greenschist facies assemblages is defined at both low and high XCOj by a reaction, calcite + chlorite + quartz = epidote + actinolite+ H2O + CO2. Thus, these transition equilibria are highly dependent on both XFe3+ + of Ca-Al silicates and XH20 of the fluid phase. Phase equilibria together with the compositional data of Ca-Al silicates indicate that the prehnite-pumpellyite to greenschist facies transition for the Karmutsen metabasites occurred at approximately 1.7 kb and 300°C, and at very low Xco2, probably far less than 0.1.

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