Volume loss, fluid flow, and coaxial versus noncoaxial deformation in retrograde, amphibolite facies shear zones, northern Malawi, east-central Africa

Abstract

Two retrograde, amphibolite facies shear zones were studied to explore the relationship between retrograde mineral reactions, volume strain, fluid flow, mylonitization, and coaxial versus noncoaxial deformation. The two shear zones are the contractional Mafwewu Hills shear zone and the transcurrently displacing Mkamasa River shear zone of northern Malawi. In general, shear-zone formation is characterized by the breakdown of feldspar and biotite and the formation of sillimanite, quartz, and water. Silica, alkali, and alkali earth elements were mobile. Mass-balance calculations, based on major- and trace-element geochemistry, indicate as much as 50%–60% volume loss in mylonite. Fluid to rock ratios estimated from the calculated depletions of silica are as much as 200–400, indicating that the initiation and activity of the shear zones were accompanied by large amounts of fluids that infiltrated the shear zones. These fluids caused an almost complete loss of alkalies within the shear zone, leading to the development of almost “dry” and sheet-silicate–free mylonite. It is proposed that the associated destabilization of micas destroyed the pathways for fluid circulation and ultimately caused the cessation of shearing. In the Mafwewu Hills shear zone, shear-zone–related stretching lineations are oriented parallel to tectonic transport. In the Mkamasa River shear zone, the stretching lineations curve from a subhorizontal attitude in the wall rock into a subvertical attitude in mylonite characterized by large volume loss. Shear bands in the Mkamasa River shear zone indicate a sinistral sense of shear. A synoptic analysis of mesoscale S and Z asymmetric folds indicates that the orientation of the stretching lineations in mylonite characterized by large volume loss is subperpendicular to the regional tectonic transport direction. Aspect ratios of feldspar and quartz in XZ sections (X ≥ Y ≥ Z, principal axes of finite-strain ellipsoid) show oblate grain shapes and show a positive correlation with volume strain. Mean kinematic vorticity numbers are 0.26–0.44 in the wall rock and 0.57–0.79 in mylonite. In mylonite, small kinematic vorticity numbers correlate with large volume loss. The transport-perpendicular orientation of the stretching lineations in strongly volume-deficient mylonite is thought to be the result of a pronounced component of shortening normal to the shear zone; the resulting higher degree of coaxial deformation controlled the final orientation of the stretching lineations.