| The petrogenetic grid which has been used to interpret the origin of granulite facies metamorphic rocks has been in place with little or no modification for nearly 30 years. The concept of parity mapping is introduced to demonstrate that there are several viable alternatives to the established model in the system FeO-MgO-Al2O3-SiO2 (FMAS) involving the phases sillimanite, hypersthene, cordierite, sapphirine, garnet, spinel, quartz, and corundum. These alternatives should be considered before applications to chemically complex systems are made.; Parity and stability are the two characteristic properties of invariant points. Parity describes the handedness or sequence of reactions around an invariant point which is defined by the relative slopes of the reactions. A parity map tracks these slopes through the range of geologically reasonable temperatures and pressures as if the invariant point were located at any of these conditions. In this way the parity can be constrained in most cases independently of the P-T coordinates of the invariant equilibria.; The parity mapping concept is tested on a well-known one-component system with three phases, and a two-component system with four phases. The concept is then applied to the four-component system FeO-MgO-Al2O 3-SiO2 (FMAS). The calculated reaction slopes were used to construct grids that have implications for the interpretation of mineral assemblages in both silica-undersaturated and silica-saturated granulites. In both systems, the grids generated are topologically different from the commonly used grids proposed in the literature. Most notably, the slope calculations consistently give rise to the opposite parity of the invariant point [Qtz] in the silica-saturated system. This invariant point corresponds to the invariant point [Cor] in the silica-undersaturated system. The effects of variable phase compositions are also considered by calculating reaction slopes using additional sets of compositions reported in the literature. |
