METAMORPHIC CONTROLS ON TEXTURAL EVOLUTION AND RHEOLOGICAL WEAKENING IN A LOWER-CRUSTAL, ALPINE SHEAR ZONE

During Alpine deformation/metamorphism, the Greiner shear zone (SW Tauern Window, Eastern Alps) experienced large variations in rheology due to textural evolution related to a complex interaction between both deformational and metamorphic processes. The "strongest" syn-deformational texture preserved in the Greiner zone is hornblende garbenschiefer (radiating, elongate, porphyroblastic hornblende that cross both lineation and foliation). The development of garbenschiefer appears to have strengthened these domains sufficiently that strain was partitioned into other areas within the shear zone, causing minimal subsequent deformation in garbenschiefer domains while hornblende remained in the stable assemblage.

At many localities within the Greiner zone, hornblende garbenschiefer has reacted by a devolitization reaction to pseudomorphs of biotite, chlorite, garnet, plagioclase ± epidote ± staurolite in the presence of white mica. In some areas, these pseudomorphs are deformed and the former garbenschiefer domains are incorporated into a mylonitic biotite schist. These observations are consistent with variations in timing of hornblende destruction relative to deformation controlled either internally by kinetics and/or effective fluid/solid composition or externally by regional isothermal decompression and/or variations in externally derived fluid composition. Preliminary work indicates that in interspersed graphitic and non-graphitic schist the dominant factor controlling reaction is a(H2O), which in turn is controlled locally by the presence or absence of graphite. In other areas subtle variations in solid composition have a strong effect on the duration of hornblende stability. In addition, hornblende may exist metastably in domains with slow diffusion (related to variations in deformation mechanism) in the plagioclase-rich matrix. Singular value decomposition, pseudosections, thermobarometric calculations, and P-X diagrams are used to quantify these results.

These observations highlight the connection between metamorphic reactions and deformational processes. They further show the importance of subtle variations in bulk and/or fluid composition in controlling rock strength at depth.