Mineral paragenesis of pumpellyite in low-grade mafic rocks
Our understanding of low-grade, mafic, metamorphic rocks is relatively primitive compared to higher grade equivalents, in large part because of the abundance of relict minerals, the difficulties of studying fine-grained rocks, and the difficulties of establishing the existence and scale of chemical equilibrium. However, work carried out during the past decade has identified the systematic correspondence between effective bulk compositions and mineral assemblage that is required by the metamorphic facies concept, and a large body of data defines the mineral pangeneses of coexisting pumpellyite (Pmp), prehnite (Prh), amphibole (Ab), and epidote (Ep). At pressures and temperatures typical of many regional metamorphic terrains, rocks containing Ep + Ab + quartz (Qtz) may have, in addition, chlorite (Chl) + actinolite (Act) in magnesian bulk compositions, Chl + Pmp in more ferroan compositions, and Prh in compositions relatively lower in aluminum and ferric iron. Act + Pmp ± Chl ± Prh may coexist in a narrow range of compositions with intermediate values of Mg/(Mg + Fe2+). Improvements in thermodynamics data bases now permit calculation of pressures and temperatures for low variance assemblages. The results of systematic examination of mineral assemblages and calculations of petrogenetic grids for epidote-bearing rocks suggest that the stability field of the Prh-Pmp facies is entirely contained within the overlapping stability fields of the Pmp-Act and Prh-Act facies. At pressures typical of the most common metamorphic field gradients, the stability fields of the three facies cannot be distinguished.
The predictability of mineral assemblages and the recent success in calculating meaningful pressure and temperature for low-grade rocks suggest that, under favorable circumstances, mineralogical approaches rooted in the assumptions of equilibrium thermodynamics are likely to prove as useful for determining the physical conditions of low-grade metamorphism as they have been for high-grade rocks.