Partial melting and localisation as functions of composition and strain rate in metapelites

Abstract

The understanding of deformation-induced partial melting and localisation in metapelitic rocks is of importance for the mechanical behaviour and chemical differentiation of the continental crust. Regions of intense deformation and shear localisation are the major sources of melt in the crust. The process of partial melting is directly related to shear localisation and migration pathways for the melt. Existing experimental studies have not fully established the dependence of several main factors that may have a large impact on localisation. In this study the focus will lie on the effect of (1) composition, (2) strain rate and (3) finite strain on melting and localisation in sheared metapelites. This creates a larger framework for and a better understanding of deformation-induced features in metapelites. These features have been studied by deforming samples in a high pressure and temperature tri-axial torsion apparatus. The quartz-content is respectively positively and twice negatively dependent on shear strength, localisation and melt production. Rate-strengthening viscous creep occurred: indicating a strong dependence of localisation on strain rate. Deformation accommodation behaviour was found to change radically with increased finite strain. Weakening occurred mostly because of the increase of available gliding planes with increased strain. A novel approach with the FIB-SEM analysis was used to establish the 3D structure of melt zones. This method has proven to be promising but also challenging. Results only allowed tentative interpretations of the 3D melt distribution. This study has provided a better understanding of localisation and partial melt in deformed metapelites and has also provided insights into the potential of a Focused Ion Beam–Scanning Electron Microscope (FIB-SEM) as a tool for geology.