Low-Grade Metamorphism of Tuffaceous Rocks in the Karmutsen Group, Vancouver Island, British Columbia,
The Triassic Karmutsen Group of central Vancouver Island consists of 18,000 ft of pillow lava and breccia, aquagene tuff, massive amygdaloidal volcanic flows, and thin inter-lava limestone beds. Low-grade metamorphism of the tuffaceous rocks has resulted in the development of laumontite, prehnite, pumpellyite, epidote, analcime, and albite. The glass alteration process in the tuffaceous rocks was one of hydration and solution. The large overlap of critical minerals such as laumontite, wairakite, prehnite, pumpellyite, and epidote is explained in terms of ionic equilibria. The observed mineralogical differences are explained in terms of minor variations in the activities of ionic species in the aqueous phase, rather than large changes in pressure and temperature. However, the gross regional pattern is explained in terms of the thermal stability of the hydrous calcium aluminosilicate minerals (that is, laumontite at the top of the section and prehnite at the base of the section).