Microstructural and textural development of calcite marbles during polyphase deformation of Penninic units within the Tauern Window (Eastern Alps)

The evolution of calcite microstructures and crystallographic preferred ori entations (CPOs) is well understood due to well constrained experimental st udies. However, the interpretation of naturally deformed calcite marbles is more difficult because of less constrained strain paths, a multiphase defo rmation history, and variable P-T conditions. The Penninic units within the Tauern Window (Eastern Alps) have been affected by several deformation eve nts and metamorphic overprint. Generally, three major deformational events can be distinguished. D-1 is related to underthrusting of Penninic units be neath the Austroalpine nappe complex, and top-to-the-N nappe stacking withi n the Penninic continental units. Deformation stage D-2 is interpreted as r eflecting the subsequent continent collision between the Penninic continent al units and the European foreland. D-3 is related to the formation of the dome structure of the Tauern Window. This polyphase deformation history can be partly reconstructed by the evolution of calcite microfabrics and CPOs. Three types of calcite-fabrics are distinguished within the Penninic units of the Tauern Window and the Lower Austroalpine unit. D-1-fabrics are chara cterized by equilibrated microstructures and LT-CPOs. The CPOs are generall y strong and symmetric, with one well developed cluster near the Z-axis of the finite strain ellipsoid. These fabrics have locally been overprinted by subsequent amphibolite to greenschist facies metamorphism. Generally, the occurrence of LT-fabrics coincides with the occurrence of amphibolite facie s metamorphic mineral assemblages in the central part of the Tauern Window, while HT-fabrics have been observed outside this area. Fabrics from the ce ntral part of the Tauern Window have likely been strengthened during subseq uent thermal equilibration, while the fabrics that have been observed at th e peripheral parts have been less affected by subsequent metamorphic overpr int. Therefore, D-1-fabrics do not reflect D-1 conditions, but subsequent t hermal equilibration. Similar observations have been made for the evolution of D-2-fabrics. LT-fabrics dominate inside the amphibolite facies isograde , HT-fabrics occur outside (greenschist facies metamorphic conditions). The fabrics are characterized by high finite strains near the margins of the T auern Window, the intensity of which decreases towards the central parts, w here microstructures are characterized by recrystallization and thermal equ ilibration due to amphibolite facies metamorphic overprint. The strong CPOs document this influence. The HT-fabrics and microstructures within periphe ral areas indicate that they have less been affected by this thermal event, and, therefore, are more indicative for the deformational conditions durin g D-2. D-3 is restricted to distinct shear zones along the tectonic boundar ies of the Tauern Window. From the central parts to the shear zone boundari es, a clear evolution of microfabrics can be observed. In internal parts, m icrostructures are characteristic for intracrystalline plasticity with a do minating activity of r(-)-glide. Twinning was less important during the fin al phases of deformation. On approaching the shear zone boundaries, the gra in size decreases due to dynamic recrystallization and secondary grain size reduction until ultramylonites are formed. Within these domains grain boun dary sliding seems to have been dominant. In conclusion, calcite CPOs from polyphase areas do not only include inform ation on the deformation conditions, but also bear information about the th ermal overprint subsequent to the main deformational event. (C) 2000 Elsevi er Science B.V. All rights reserved.