The transition from feldspar amphibolite to eclogite is a very wide P - T field that extends from somewhere close to 5 kbar where the garnet-amphibole pair starts to appear, to 10-20 kbar at albite-out reaction, then up to 25-30 kbar where an hydrated phase such as amphibole can be stable with pyroxene and garnet. Thus the assemblage garnet (py) + amphibole (tr) + epidote (cz) +/- plagloclase (ab) +/- clinopyroxene (di) +/- quartz (qz) +/- fluid is commonly reported in a large number of metamorphic terrains. These mineral phases are complex solid-solutions which adapt to variations in evironmental conditions mainly by means of continuous reactions. The reaction space, introduced by Thompson in 1982a, provides a very elegant and powerful tool to approach these high-variance assemblages. The reactions: A) ab = ed + 4qz; B) 2ab + 2mc = py + 2qz + tk + 2pl; C) ab = di + tk + pl + qz; D) 2cz + 2mc = tr + 3tk + qz where italicised letters denote exchange vectors, permit the development of a condensed-reaction space in NCMASH for amphibole eclogites, for garnet amphibolites and for pyroxene amphibolites. The displacement of the faces corresponding to the elimination of additive components (phases) and to the loss of exchange-capacity of the phases are discussed as a function of whole-rock chemistry and (de-)hydration reactions. The relationship with the P - T plane shows the sensitivity of the reaction space to assess variations in P - T conditions and to examine critically the internal consistency of conventional thermobarometric determinations. The procedure formulated herein is applied to the evolution of the metabasites from Moso (Oetztal unit, Austroalpine domain, Eastern Alps). Four main stages are summarized: (1) omphacite + garnet + barroisite + albite/oligoclase + epidote + quartz + rutile +/- calcite; (2) Ca-clinopyroxene + garnet + subsilicic pargasite (sadanagaite) + albite/oligoclase + epidote + quartz + rutile-ilmenite; (3) pargasite + oligoclase-andesine + epidote + quartz +/- chlorite + ilmenite; (4) actinolite + albite + epidote + quartz +/- chlorite + sphene. The compositions of coexisting phases at each stage of evolution define a path in the reaction polytope which implies an increase in amphibole-, pyroxene- and quartz-abundance and conversely a decrease of garnet and epidote. When the intersection of garnet- and epidote-out faces is reached estimated pressure and temperature are in the order of 9-12 kbar, 700-degrees-C. The P - T path suggested is a simple loop characterized by a temperature-increase at roughly isobaric conditions, slightly above 10 kbar, followed by both pressure-, and temperature-decrease down to almost-equal-to 400-degrees-C, almost-equal-to 3-4 kbar. The high-pressure stage of evolution has been attributed to pre-Alpine ages.
