HAS-BEEN ROCKS: METAMORPHIC HISTORY REVEALED BY PSEUDOMORPH TEXTURES, SYROS, GREECE

When metamorphic rocks preserve a part of their history in the form of pseudomorphs after preexisting minerals, petrologists are challenged to interpret these intriguing features. Multiple interpretations may be possible, depending on the evidence available. Charlie Guidotti was one petrologist who succeeded in persuasively deciphering pseudomorph textures in several papers about rocks from western Maine. We have tried to emulate his good science in studying pseudomorphs after aragonite and pseudomorphs after lawsonite in Syros marbles and blueschists. Rod-shaped, polycrystalline calcite pseudomorphs after aragonite, with calcite c-axes parallel to the long axes of the rods, are common in the marbles of Syros. These pseudomorphs are especially interesting because they have a shape-preferred orientation in which the long axes of the rods are oriented at a high angle to the foliation as defined by phengite crystals. We believe that the rod-texture developed during static growth (coarsening) of aragonite following the deformation that oriented the phengite and that the pseudomorphing reaction occurred during exhumation. Pseudomorphs of various mineral assemblages after lawsonite are observed in a variety of mafic schists and graphitic schists on Syros. Pseudomorph assemblages may include an epidote group mineral, phengite, paragonite, titanite, chlorite, quartz, and calcite, depending on the bulk composition of the rock and the metamorphic history of the sample. We believe that the pseudomorphic replacement of lawsonite developed by prograde (dehydration) reactions during static heating of these rocks at peak pressures (1.5 GPa) and that some of the pseudomorphs were further modified (to produce chlorite and albite) during exhumation. The extensive occurrence of both types of pseudomorph on Syros is consistent with a two-stage tectonic history: (1) dynamic metamorphism (with deformation) during burial; (2) static heating (without deformation) leading to pseudomorph formation during exhumation. In the context of the subduction history of this blueschist terrane, our observations suggest that little penetrative deformation occurred after the rocks were detached from the subducting slab.