Deep crustal shear zones, high-grade tectonites, and associated metasomatic alteration in the Limpopo belt, South Africa: Implications for deep crustal processes

Large thrust sheets of complexly folded (D-1) migmatitic granulites bounded by a regional system of anastomosing ductile shear zones (D-2\ characterize the Southern Marginal Zone of the Limpopo Belt. Tectonites within these shear zones are subdivided into ''primary straight gneisses'' with preserved textures that can unambiguously be referred to the D, exhumation stage, ''annealed straight gneisses'' that recrystallized during post D-2 cooling, and classic mylonites that post-date the Limpopo orogeny. Primary straight gneisses within these shear zones contain textural and mineralogical features that suggest they developed by a shear-related process of grain size reduction and fabric formation due to decreasing pressure and temperature under granulite facies conditions. This process is evident from the synkinematic replacement of garnet by cordierite and hypersthene symplectites. The resultant asymmetric sigmoidal corona textures enhance the shear fabric. In contrast, pelitic granulites from unsheared thrust sheets are characterized by symplectitic corona textures that clearly developed under static conditions. The deep-seated shear zones preserve evidence for high-temperature metasomatic alteration related to regional retrogression, demonstrating the important role of fluid-rock interaction during exhumation of the granulites. O-isotope fractionation patterns support petrologic data that alteration occurred under granulite facies conditions \>800 degrees C) as the result of the influx of externally derived fluids of low water activity. The fluids are represented by CO2 and saline fluid inclusions in minerals. These observations suggest that tectonites analogous to our primary straight gneisses may be characteristic features of high-grade gneiss terranes in general and that the role of fluids in deep crustal processes may be more important than previously assumed.