Coronite and eclogite formation in olivine gabbro (Western Norway): reaction paths and garnet zoning

Abstract

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 pseudomorphs 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 AI (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 interpreted 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_ I exchange with omphacite_+phlogopite.

Pre-Caledonian granulite and gabbro enclaves in the Western Gneiss Region, Norway: indications of incomplete transition at high pressure

“…All samples with ophitic relics exhibit corona textures; all these samples are undeformed. Similar corona textures have been observed in meta-dolerites and meta-gabbros elsewhere in southwest Norway (Griffin & Heier, 1973;Mørk, 1985Mørk, , 1986.…”

“…This age suggest that the gabbro is MidProterozoic in age so that it was emplaced prior to the Caledonian high-pressure event. The gabbro underwent very little reaction under these conditions and behaved even more metastably than the meta-gabbros described by Mørk (1985Mørk ( , 1986. Whether or not the gabbro expe- …”

“…To date, incomplete eclogitization of rocks in the Western Gneiss Region is described in mafic rocks in the north by Mørk (1985Mørk ( , 1986) and on Bårdsholmen in Sunnfjord, where small occurrences of felsic granulite occur that experienced partial eclogitization Engvik, Austrheim & Andersen, 2000). Further evidence of partial eclogitization may be found in the following places.…”

Eclogitized gabbros from the eastern Grenville Province: textures, metamorphic context, and implications

“…In such cases, under a given P-T regime, the type of mineralogical transformation is largely controlled by the rates of element diffusion between microdomains of contrasting compositions such as plagioclase and olivine (Mork 1985(Mork , 1986Rivers and Mengel 1988;Rubie 1990). For instance, the development of orthopyroxene with low Al2O3 content around olivine and the abundance of corundum and kyanite inclusions within plagioclase reflect limited diffusion of A1 outside the plagioclase domains during the HP transformations (see also Mongkoltip and Ashworth Can.…”

Arrested development – a comparative analysis of multilayer corona textures in high-grade metamorphic rocks

“…Relative diffusion fluxes and attendant chemical potential differences shift and evolve from one steadystate configuration to another under new P -T -X conditions. Sequential corona development with changing P and T has been demonstrated in prograde coronas found in mafic rocks between olivine and plagioclase by Griffin (1972) and Mork (1986). Griffin (1972) derived a sequential model for corona formation that involved cooling from temperatures in excess of the dry basalt solidus (> 1200 • C), between 0.8 and 1.1 GPa, and the crossing of univariant equilibria (Figs.…”