Fibrous Marble Doesn't Lie

The rheology of subducted crust strongly influences the geophysical and geochemical evolution of plate boundaries, including the interaction of metamorphism, fluid-rock interaction and deformation during subduction and exhumation. On a grain scale, deformation mechanisms at HP are not well known in part because of the lack of fabrics that can be definitively linked to HP conditions, and because of the lack of exposure of structurally coherent HP rocks. The Sivrihisar massif, west-central Turkey, contains pristine lawsonite eclogite, indicating preservation of HP (up to 25 kbar) assemblages and fabrics that are part of a Tethyan subduction complex. The marble is interlayered with eclogite and blueschist and is dominated by acicular calcite crystals (fibers). Fiber length is dependent on the thickness of individual compositional layers in marble (mm to cm-scale) and fiber orientation ranges from parallel to perpendicular relative to compositional layering; the typical obliquity is 35-50 degrees. In some samples, fibers are variously oriented from layer to layer. Fibers do not occur in a Barrovian sequence that overprints the HP terrane; in the Barrovian zones marble has equant crystals.

The predicted HP carbonate phase is aragonite, but aragonite is rarely preserved in HP terrains owing to the ease of polymorphic transformation to calcite during exhumation. Although the aragonite structure is not preserved, aragonite crystal shape may be preserved during exhumation. The fibrous calcite in the Sivrihisar massif may be pseudomorphed aragonite. EBSD analyses did not identify aragonite as a phase in the marble so far (only calcite is present), but has demonstrated that the long axis of the fibers is the c-axis, as would be predicted for aragonite. Continued studies of carbonate lithologies, which have been understudied in subduction complexes, will enable us to better understand the formation and exhumation of HP rocks.