In mafic and ultramafic rocks that appear to have equilibrated under largely anhydrous conditions at temperatures and pressures appropriate for the lower crust and upper mantle, evidence for the reactions (GAR) olivine + plagioclase = spinel + orthopyroxene + clinopyroxene, (OL) spinel + orthopyroxene + plagioclase = garnet ± clinopyroxene, and (PLAG) orthopyroxene + spinel ± clinopyroxene = garnet + olivine is commonly recorded in reaction rims or corona structures (see Fig. 1). Even before the celebrated work of Sederholm (1916), these features fascinated petrologists because of their common occurrence in such rocks, but it was not until the advent of high-pressure experimental petrology (Birch and LeComte, 1960; Boyd and England, 1960) that their petrogenetic significance could be addressed with any certainty. Recently, experimental investigations (for example, Evans, 1977) have focused on the univariant analogs of these reactions in model end-members systems CaO-MgO-Al2O3-SiO2 (CMAS) and MgO-Al2O3-SiO2(MAS). However, because of their sluggishness, reversal of these reactions at temperatures of principal geological interest (500 to 800 °C) for lower crustal metamorphism has yet to be demonstrated. Application of experimental results to delineate phase relations in natural systems is thus limited by uncertainties introduced by extrapolating model end-member univariant reactions to lower temperatures as well as those involved in accounting for displacements of these reactions due to the extra components found in natural systems. The recent application of differential scanning calorimetry and high-temperature solution calorimetry (for example, Perkins and others, 1977; Newton and other, 1977; Newton and others, 1977; Navrotsky and Coons, 1976) has reduced uncertainties involved in the low-temperature extrapolation of the experimental data. However, residual uncertainties in end-member thermodynamic data and, more particularly, in the thermodynamic mixing properties of the mineral phases are still too large to permit accurate prediction of phase relations at these temperatures. In this work, the phase relations of rocks composed of these minerals are examined for conditions of equilibration characteristic of the clinopyroxene granulite facies for metamorphism (DeWaard, 1965); the development of these relations is based on analysis of mineral assemblages defined by mineral contacts in the core of a layered metagabbro body from the eastern edge of the Precambrian rocks of the Adirondack Highlands near Westport, New York (for example, Kemp, 1894; Buddington and Whitcomb, 1941).