Transition from the zeolite to prehnite-pumpellyite facies in the karmutsen metabasites, Vancouver Island, British Columbia

The upper Triassic Karmutsen metabasites from northeast Vancouver Island, B.C., are thermally metamorphosed by the intrusion of the Coast Range Batholith. The amygdaloidal metabasites developed in the outer portion of the contact aureole show a progressive metamorphism from zeolite to prehnite-pumpellyite facies. The size of an equilibrium domain is extremely small for these metabasites, and the individual amygdule assemblages are assumed to be in equilibrium. Two major calcite-free assemblages (+chlorite+quartz) are characteristic: (i) laumontite+pumpellyite+epidote in the zeolite facies and (ii) prehnite+pumpellyite+epidote in the prehnite-pumpellyite facies. The assemblages and compositions of Ca-Al silicates are chemographically and theoretically interpreted on the basis of the predicted P-T grid for the model basaltic system, CaO-MgO-A1₂O₃-Fe₂O₃-SiO ₂-H₂O. The results indicate: (1) local equilibrium has been approached in mineral assemblages and compositions; (2) the X_Fe3+ values in the coexisting Ca-Al silicates decrease from epidote, through pumpellyite to prehnite; (3) with increasing metamorphic grade, the Fe³⁺ contents of epidotes in reaction assemblages decrease in the zeolite facies, then increase in the prehnite-pumpellyite facies rocks. Such variations in the assemblages and mineral compositions are controlled by a sequence of continuous and discontinuous reactions, and allow delineation of T-X_Fe3+ relations at constant pressure. The transition from the zeolite to prehnite-pumpellyite facies of the Karmutsen metabasites is defined by a discontinuous reaction: 0·18 laumontite+pumpellyite+0·15 quartz = 1·31 prehnite+ 0·78 epidote+0·2 chlorite+ 1·72 H₂O, where the X_Fe3+ values of prehnite, pumpellyite and epidote are 0·03, 0·10 and 0·18, respectively. These values together with available thermodynamic data and our preliminary experimental data are used to calculate the P-T condition for the discontinuous reaction as P = 1·1 ±0·5 kb and T = 190±30°C. The effects of pressure on the upper stability of the zeolite facies assemblages are discussed utilizing T-X_Fe3+ diagrams. The stability of the laumontite-bearing assemblages for the zeolite facies metamorphism of basaltic rocks may be defined by either continuous or discontinuous reactions depending on the imposed metamorphic field gradient. Hence, the zeolite and prehnite-pumpellyite facies transition boundary is multivariant.