Recrystallization of high-pressure marble (Sivrihisar, Turkey)

In the Sivrihisar Massif, Turkey, the southern margin of a blueschist facies terrane was affected by heating and deformation associated with later regional (Barrovian) metamorphism, creating a recrystallization front that can be observed in calcite microstructures. Calcite marble records a progression from high-pressure (HP) textures in a northern domain to completely recrystallized (greenschist to amphibolite facies) textures in a southern domain, with intermediate stages of partial recrystallization of the HP textures. HP marble is characterized by rod-shaped calcite (aragonite pseudomorphs) arranged in series with the long axis of each grain oblique to compositional layering; obliquity varies from 50° to nearly parallel, but high angles are most common. In a narrow (~ 200 m thick) transition zone that contains partially retrogressed blueschist and eclogite, the HP texture is partially overprinted by chains of small, equant, recrystallized grains that preserve the crystallographic orientation of the pre-existing rod-shaped calcite, as revealed by electron backscatter diffraction analysis; rod-shaped crystals occur locally. In a ~ 100 m thick zone south of the transition zone, all calcite grains are equant but have a crystallographic preferred orientation. Further south in the Barrovian sequence, recrystallized marble contains equant calcite that is randomly oriented. The similarity in fabric elements (foliation, lineation) of the two domains further shows the effect of Barrovian-related processes on the HP domain, including the rotation of the calcite rods during shearing along foliation planes. White mica ⁴⁰Ar/³⁹Ar ages for phengite in HP marble are ~ 88 Ma, and for muscovite in Barrovian marble are ~ 59 Ma, indicating ~ 30 Ma time interval between initial exhumation and cooling of the HP rocks and final cooling of the rocks that were deformed and heated during the later metamorphic event.