The first appearance of actinolite in the prehnite-pumpellyite facies, Sierra Nevada, California

Jurassic metavolcanic rocks in the western Sierra Nevada, California, display widespread assemblages typical of the transition from the prehnite–pumpellyite to the greenschist facies. Actinolite first appears in the assemblage quartz + albite + chlorite + epidote + pumpellyite ∠prehnite as an overgrowth on unaltered, relict grains of calcic clinopyroxene and as small acicular grains in chlorite. Chemographic and algebraic analysis of the mineral assemblages suggests that the appearance of actinolite is controlled by high-variance continuous reactions that produce actinolite + epidote at the expense of pumpellyite-(Mg) + minor chlorite. The data also suggest that the effective bulk composition of the metamorphic assemblage becomes more magnesian with increasing progress of the reactions. We suggest that this phenomenon is best explained by the irreversible consumption of relatively magnesian, relict igneous phases or altered groundmass as the reactions proceeded. Thus, both equilibrium and non-equilibrium phenomena play an important role in explaining the systematic evolution of mineral compositions in these low-grade rocks.