Dynamic recrystallization and deformation mechanisms of naturally deformed Carrara Marble: a study on one- and two-phase carbonate rocks

Carrara marble is a greenschist facies metamorphic marble from the Alpi Apuane, Tuscany, Italy. It is widely known for its chemical purity, its homogenous microstructure and isotropic texture (crystallographic preferred orientation). During two deformation events (D1 and D2) the Liassic sedimentary precursor of Carrara marble was metamorphosed and deformed.
Besides statically recrystallized microstructures, two end members of dynamically recrystallized microstructures are preserved in Carrara marble (Molli et al. 1997, Molli & Heilbronner 1999, Molli et al. 2000). Both end members can be related either to sub grain rotation (SGR) recrystallization or to grain boundary migration (GBM) recrystallization, respectively. In order to characterize the different dynamically recrystallized microstructures and to relate them to mesoscopical structures of D1 and D2 a combined study of structural field geology and microstructural analysis was carried out. On the basis of a detailed structural mapping overprinting relationships between D1 and D2 were investigated and typical deformation structures were sampled. The microstructural analysis of theses shear zones and folds revealed that D1 structures are generally characterized by GBM microstructures and D2 structures by SGR microstructures.
Systematic determination of deformation temperatures of D1 and D2 shear zones along a profile parallel to the main tectonic transport direction by calcite-dolomite thermometry shows two individual temperature trends for D1 and D2. The D1 deformation temperatures decrease from West to East from 430°C to 370°C and those of D2 decrease from 370°C in the East to 295°C in the West. This continuous temperature gradient makes it possible to study dynamic recrystallization microstructures over a temperature range of approximately 150°C.
In order to investigate the influence of strain on the formation of microstructure and texture in naturally deformed Carrara marble a mm-scaled D2 shear zone was analyzed. The shear zone was active at a constant temperature of 325°C and cross-cut the S1 main foliation, which acted as a passive strain marker. Therefore, the observed variations of microstructure and texture are solely a function of the accommodated strain and the strain rate. The protolith is characterized by a core-mantle microstructure with coarse porphyroclasts, which are embedded in a matrix of small recrystallized grains. With increasing strain the microstructure becomes progressively recrystallized, so that the center of the shear zone is completely recrystallized and a fine-grained matrix of equally shaped recrystallized grains is developed. At the same time the texture changes from an e-twinning type (single c-axes point maximum parallel to the compression direction) in the protolith to a basal <a> texture (single c-axes maximum perpendicular to the shear plane) in the center of the shear zone. This texture transition is caused by SGR recrystallization, which forms continuously new grains with slightly rotated crystallographic orientations in the same sense as the overall shear deformation. Strain rate estimates on the basis of experimentally derived flow laws suggest that the shear zone is highly localized in space and in time.
Localization of deformation was probably caused by the presence of a pre-existing coarse-grained calcite vein. According to the Hall-Petch relationship dislocation glide is favoured by coarse-grained materials, which is why deformation may started in the vein calcite by twining. After a certain amount of strain twinning is exhausted and the deformation continued by dislocation creep associated by dynamic recrystallization.
Many calcite shear zones in the Alpi Apuane contain substantial amounts of dolomite, representing a second phase in ‘pure’ Carrara marble. Second phases are considered to influence the mechanical behaviour of a mono-mineralic rock, so that the strength of the phase mixture deviates from the one of the pure end members (Tullis et al. 1991, Dresen et al. 1998, Bruhn et al. 1999). In order to investigate the influence of dolomite on microstructure, texture and the dominant deformation mechanism of calcite a shear zone from the Eastern Alpi Apuane was analyzed. The shear zone is composed of alternating calcite and dolomite rich layers. Dolomite is presumably derived from dolomite veins, which become progressively fractured and sheared so that at the highest strains completely mixed layers of calcite and dolomite are developed. Pure calcite layers are characterized by a GBM microstructure and a strong basal <a> texture, indicating that they have deformed dominantly by dislocation creep. With increasing dolomite content the calcite grains become smaller and less lobate, while the dolomite grains maintain their initial size and shape. Also the texture of calcite becomes weaker and the initial orthorhombic symmetry (with respect to the shear plane) is almost randomized at a dolomite content of ~50 vol%. This transition suggests that the mixed layers deform dominantly by diffusion creep. Heterogeneous nucleation (Kruse & Stünitz 1999) of dolomite along calcite grain boundaries and triple junctions leads to a dispersion of calcite and dolomite, supporting the activity of diffusion creep and grain boundary sliding. The progressive addition of dolomite to calcite may lead to the transition from dislocation creep dominated deformation in the pure calcite layers to diffusion creep dominated deformation in the mixed layers. The rheological behaviour of each layer was estimated with the help of a deformation mechanism map. It turned out that the mixed layers deform either by higher strain rates and constant stresses or at lower stresses and a constant strain rate, both suggesting a localization of deformation in the mixed layers.