HT-fabrics in a garnet-bearing quartzite from Western Portugal: geodynamic implications for the Iberian Variscan Belt

Abstract

The study of a garnet-bearing quartzite from a major suture zone in Iberia reports an unusual high-T fabric. Quartz c-axis patterns were plotted using shaped garnet as reference axis for the finite stretch (X-axis). The pole figures are characterized by a dominant single maximum around X together with other point maxima along the XY plane (mylonitic foliation). These patterns suggest that dominant <c> prism slip and subordinated <a> prism slip operated during quartz plastic deformation in agreement with P–T conditions for syntectonic garnet growth (4–5 kbar and 700 ± 50 C) and, pre-dating the well-known (late) Variscan D1 event (<6 kb and 600 ± 30 C). The geotectonic framework suggests that the fabrics were formed along the western shear margin of the Ossa-Morena Zone during the early stages of the Variscan orogeny.