Abstract: Gabbros and dolerites in the Western Gneiss Region of Norway have been subjected to a high P-T regime and preserve evidence of the existence of several stages in the consequent reaction sequence. Incomplete reaction stages are characterized by corona structures between relict igneous phases and by frequent pseudo-morphs after igneous olivine, plagioclase and augite. The increasing degree of reaction is recorded by successive increases of modal garnet, omphacite and phlogopite (±orthopyroxene) to produce eclogite, often with excellent pseudomorphic preservation of the igneous fabric.

Pseudomorphic replacement of olivine by orthopyroxene (in coronas and aggregates) and of plagioclase by local assemblages of garnet, spinel, and sodic plagioclase, is interpreted as a transient reaction stage with restricted and selective diffusion between the original mafic and felsic domains. Complete eclogitization is compatible with more extensive diffusion, especially of Na and Al (on a mm scale) leading to omphacite production by replacement of olivine, augite, and orthopyroxene. Concomitant reactions in the felsic domains lead to total replacement of the transient phases by garnet, with or without inclusions of minute omphacite grains.

Strong Ca zoning of garnet in the coronites is inter-preted as a relict growth zoning, attributed to local controls by diffusion and subreactions in the plagioclase host and to local garnet + plagioclase + spinel equilibria. In contrast, complete eclogitization is associated with diffusional homogenization of garnet by (Mg, Fe)Ca₋₁ and MgFe₋₁ exchange with omphacite ± phlogopite.