Contrib Mineral Petrol (1982) 81:240~251 Contributions to Mineralogy and Petrology 9 Springer-Verlag 1982 The Formation of Garnet in Olivine-Bearing Metagabbros from the Adirondacks Craig A. Johnson 1 and Eric J. Essene Department of Geological Sciences, The University of Michigan, Ann Arbor, Michigan 48109, USA Abstract. A regional study of olivine-bearing metagabbros in the Adirondacks has permitted testing of the P(pres- sure)- T (temperature)- X (composition) dependence of garnet-forming reactions as well as providing additional re- gional metamorphic pressure data. Six phases, olivine, or- thopyroxene, clinopyroxene, garnet, plagioclase and spinel, which can be related by the reactions: orthopyroxene + clinopyroxene + spinel + anorthite = garnet, and forsterite + anorthite = garnet occur together both in coronal and in equant textures indi- cative of equilibrium. Compositions of the respective miner- als are typically F025_72 , En44~75, En3044Fs9-23W047-49, PP~3 42Aim39 63Gq6-20, An2~49 and Sp16 ss- When they occur in the same rock, equant and coronal garnets are homoge- neous and compositionally identical suggesting that chemi- cal equilibrium may have been attained despite coronal tex- tures. Extrapolating reactions in the simple CMAS z system to granulite temperatures and making thermodynamic cor- rections for solid solutions gives equilibration pressures (using the thermometry of Bohlen et al. 1980b) ranging from about 6.5 kb in the Lowlands and southern Adiron- dacks to 7.0-8.0 kb in the Highlands for the assemblage olivine-plagioclase-garnet. These results are consistent with inferred peak metamorphic conditions in the Adirondacks (Valley and Bohlen 1979; Bohlen and Boettcher 1981). Thus the isobaric retrograde path suggested by Whitney and McLelland (1973) and Whitney (1978) for the formation of coronal garnet in olivine metagabbros may not be re- quired. Application of the same equilibria gives > 8.7 kb for South Harris, Scotland and 0.9 kb for the Nain Com- plex. Disagreement of the latter value with orthopyroxene- olivine-quartz barometry (Bohlen and Boettcher 1981) sug- gests that the use of iron-rich rocks (olivines > Fas0) results in errors in calculated pressures. Introduction Garnet has classically been recognized as a high P phase in metamorphosed igneous rocks. Its formation can be de- scribed by the reaction: Contribution No. 385 from the Mineralogical Laboratory, Depart- ment of Geological Sciences, The University of Michigan, Ann Arbor, Michigan 48109, USA 1 Present address: Climax Molybdenum Co., P.O. Box 961, Battle Mountain, Nevada 89820, USA 2 CMAS = CaO -- MgO - Al/O~ -- SiO z. Reprint requests to." C.A. Johnson or thopyroxene + plagioclase = garnet + quartz (1) in oversaturated rocks and in undersaturated rocks as: olivine + plagioclase = garnet (2) (Ringwood and Green 1964, 1966). In principle, assembla- ges such as orthopyroxene-plagioclase-garnet-quartz and olivine-plagioclase-garnet will fix P at a constant T for given mineral compositions (Wood and Banno 1973; Wood 1975; Schmid and Wood 1976). Garnet barometry is poten- tially widely applicable to granulitic terranes if it can be adequately calibrated. Reaction (2) is easier to adjust for variable solid solutions because olivines are well represented as binary solutions while complete description of natural orthopyroxenes requires consideration of variable MgSiO3, FeSiO3, CaSiO3 and MgA12SiO6 (Herzberg 1978 a). Two different kinds of experiments have been applied to garnet reactions: those on natural rock compositions and those on simple end-member reactions. Ringwood and Green (1964, 1966), Green and Ringwood (1967) and Ito and Kennedy (1971) have investigated garnet reactions in basaltic rocks but they concentrated mainly on the plagio- clase-out reaction forming eclogites. Application of these experiments below 900 ~ C is difficult in any case because of uncertainties in extrapolation: reversals are difficult to interpret in these multicomponent systems and the garnet-in boundary is unlikely to be a straight line (pace Green, Ito, Kennedy and Ringwood). Until experiments on multicom- ponent rock systems are completely and tightly reversed over a wide range of T, they will be difficult to apply to metamorphic rocks. Garnet-forming reactions have also been investigated in the simple system CMAS. Several experimenters (Kushiro and Yoder 1966; O'Hara et al. 1971; Herzberg 1972, 1976; Herzberg and Chapman 1976; Presnall 1976; Jenkins and Newton 1979; O'Neill 1981) have investigated the reactions: forsterite + anorthite = orthopyroxene + clinopyroxene + spinel (3) orthopyroxene + clinopyroxene + spinel + anorthite = garnet (4) orthopyroxene + clinopyroxene + spinel = garnet + forsterite. (5) Reaction (5) has been reversed between 800 and 1,100 ~ C (Jenkins and Newton 1979; O'Neill 1981) while reactions (3) and (4) are constrained only by half-reversals above 0010-7999/82/0081/0240/$02.40