Contact Metamorphism of the Karmutsen Volcanics, Vancouver Island, British Columbia

The Upper Triassic Karmutsen Subgroup of northeastern VancouverIsland exhibits a clear-cut contact metamorphism of basalticflows, pillows, and pillow breccias in the aureoles of the CoastRange Intrusions. The thermal effect is superimposed upon prehnite-pumpellyitefacies regional metamorphic rocks. The metamorphism of flowsis nearly isochemical except for H₂O. Aquagene tuffs and pillowbreccias, which have undergone extensive chemical alterationthrough diagenesis and burial metamorphism, involve considerablemetasomatism (gain of Si, Na, and Ca and loss of Fe, Mg, andTi) and approach the original basaltic composition in the contactzone. Two prograde metamorphic zones are recognized: an actinolite-chlorite-epidote-albitezone, about 2600 m wide; and a hornblende-calcic plagioclase(An>20) zone, about 900 m wide. Microprobe analyses indicatethat (1) amphibole increases in Al₂O₃, TiO₂, and Na₂O and chloritein Al₂O₃ with progressive metamorphism; (2) the pistacite contentof epidote decreases from Ps = 30 in the prehnite-pumpellyitezone to Ps = 25 in the epidote-actinolite zone; and (3) theAn content of plagioclase jumps from An₀₀–₀₄ for the outerthermal zone to An ₂₀–₇₀ for the inner zone with a compositionalgap between An₀₄ and An₂₀. The following evidence suggests a low-pressure contact metamorphiccondition and therefore a shallow level of the intrusion: (1)the occurrence of prehnite-actinolite-chlorite and absence ofpumpellyite-actinolite-chlorite in the outer thermal zone; (2)wide compositional gaps in both amphibole and plagioclase acrossthe metamorphic zones; (3) absence of an albite-epidote-hornblendeassemblage in transition zone; and (4) the thickness of thesection (Quatsino-Bonanza) overlying the Karmutsen. Mineralparageneses and inferred phase relations suggest that the transitionfrom the prehnite-pumpellyite zone to the epidoteactinolitezone occurs at about 350 ?C and from the epidote-actinolitezone to the plagioclase-hornblende zone at about 470–500?C at relatively high fo₂ conditions.