Depressurized cavities within high-strain shear zones: Their role in the segregation and flow of SiO 2-rich melt in feldspar-dominated rocks

Abstract

We observe void growth and coalescence into cavity-bearing shear bands during deformation of wet synthetic anorthite aggregates containing <3 vol. % silica-enriched melt. Samples were deformed in the Newtonian creep regime to high strain during torsion experiments at 1100°C and 400 MPa confining pressure. Localized cavity-bearing shear bands show an S-C'-geometry: the bands (C') are oriented at about 30° to the compression direction of the imposed simple shear and the internal foliation (S) of the bands is rotated towards the horizontal external shear plane. Cavity-bearing shear bands started to nucleate in the sample periphery above a shear strain threshold of ≈2. Quartz crystallized from the water-saturated SiO. 2-rich melt within large cavities inside these bands, which requires that the melt is decompressed by >200 MPa during their formation. The dynamically evolving cavities are sites of locally reduced pressure that collect the melt distributed in the adjacent matrix. Therefore, cavitation damage under ductile conditions may result in the development of an efficient melt channelling system controlling SiO. 2-rich melt flow in the lower crust. Electron backscatter diffraction analysis shows that the quartz inside the cavity bands has a crystallographic preferred orientation (CPO). The development of the CPO is explained by the preferred dissolution of crystals oriented with the rhombohedra and trigonal dipyramids orthogonal to the compression direction and by preferential growth of crystals aligned with their <0001> axis in the extension direction of the externally applied simple shear deformation. © The Author 2012. Published by Oxford University Press. All rights reserved.