Textural Evolution of Plagioclase Feldspar across a Shear Zone: Implications for Deformation Mechanism and Rock Strength

Caledonian amphibolite-facies shear zones developed in granulite-facies anorthosites and anorthositic gabbros of the Bergen Arcs, west-ern Norway, allow a detailed study of the relationships between fluid-infiltration, mineral reactions, the evolution of microstructure and deformation mechanisms. A sequence of rocks from the relatively pristine granulites into a shear zone has been studied by optical mi-croscopy, electron microprobe microanalysis (EMPA), scanning elec-tron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM), focusing on the pro-gressive development of microstructure in the plagioclase feldspars, leading up to their deformation in the shear zone. At the outcrop scale, fluid infiltration into the granulites is marked by a distinct colour change in the plagioclase from lilac^brown to white.This is associated with the breakdown of the intermediate composition plagioclase (An50) in the granulite to a complex intergrowth of Na-rich and Ca-rich domains. EBSD analysis shows that this inter-growth retains the crystallographic orientation of the parent feldspar, but that the Ca-rich domains contain low-angle grain boundaries. Within the shear zone, this complex intergrowth coarsens by grain boundary migration, annihilating these grain boundaries but retain-ing the Na-rich and Ca-rich zoning pattern. Analysis of nearest-neighbour misorientations of feldspar grains in the shear zone dem-onstrates that local crystallographic preferred orientation (CPO) is inherited from the parent granulite grain orientations. Random pair misorientation angle distributions show that there is no CPO in the shear zone as a whole, nor is there significant shape preferred orienta-tion (SPO) in single grains. These observations are interpreted in terms of fluid-induced weakening and deformation by dissolution^ precipitation (pressure solution) creep. KEY WORDS: deformation; EBSD; microtexture; plagioclase; shear zon