Aragonite-grossular in eclogite-facies marble, Alpine Corsica
Résumé
The occurrence and preservation of aragonite in eclogite-facies rocks of north-eastern Corsica is linked to an uncommon microtexture. Aragonite exclusively occurs as oriented fibres in garnet crystals of a graphitic, more or less siliceous marble that immediately overlies a serpentinite body of the meta-ophiolitic unit. The arrangement of the fibres is grossly radial, but more clearly sectoral in subhedral garnet, the fibres growing perpendicular to the garnet/matrix interface. Raman mapping reveals that the carbonate is calcite in the matrix and in poikiloblastic garnet cores, and that the fibres in the garnet mantle are aragonite alone in the case of a carbonate matrix, and both aragonite and quartz (in distinct fibres) in a quartz–carbonate matrix. These features are interpreted as prograde intergrowths, the result of garnet nucleation and growth in a calcite and then aragonite matrix (±quartz). Upon further heating and/or decompression, the aragonite matrix transformed back to calcite while the carbonates included in garnet retained their original structure, in spite of the relatively high temperature attained (ca. 500 °C). These aragonite relics are one more example of the preservation of a high-pressure polymorph through mechanical shielding of inclusions in a rigid host. The aragonite–garnet intergrowths are similar to quartz–garnet intergrowths described in amphibolite-facies graphitic schists. They are evidence that oriented inclusions in garnet are not necessarily precipitates (‘exsolutions'). Unlike precipitates, their orientation is controlled more by the shape of the garnet growth front than by symmetry constraints.