188.8.131.52 The Staurolite Zone
The staurolite zone can easily be overlooked because it forms a very narrow zone and because staurolite only grows within the most pelitic horizons of the Phe formation which are rare. The rocks from this zone have a lustrous silvery aspect and are coarser grained than the rocks from the garnet zone. Staurolite when present forms honey-brown prismatic blasts which quite frequently reach centimetric size. The diagnostic mineral assemblage for this zone is:
staurolite + garnet + biotite + muscovite + chlorite + plagioclase + quartz
In the less pelitic horizons of this zone, staurolite is however absent and the mineral assemblage remains the same as for the garnet zone. The very limited thickness of the staurolite zone is partly explained by the fact that, for a Barrovian type metamorphism, the P-T conditions, where the assemblage St ± Grt + Bt ± Chl + Ms + Qtz + H2O is stable, are limited by the reactions Grt + Chl = St + Bt + H2O and St + Chl = Bt + As + H2O which define a very restricted domain of temperature (30°C) in the MnKFMASH system (for Xsps = 0.1, the maximum content in the analysed garnets).
As for the Garnet zone, two main phases of deformation are preserved within these rocks. The main penetrative schistosity is defined by the phyllosilicates and moulds the staurolite and garnet porphyroblasts. Some elongated staurolite grains have been reoriented NE-SW by this deformation and shear sense criteria (mantled porphyroblasts, C/S and C'-type shear bands) indicate a top to the NE sense of shear. Again, this foliation is related to ductile extensional movements along the ZSZ (D4). An earlier foliation is preserved as inclusions within the staurolite blasts, but these inclusions give no information neither on the shear sense of this deformation nor on the crystallisation history of the staurolite with respect to this early tectonic event. It is however more than likely that this early event is the same D3 event as in the garnet zone. Staurolite growth is thus either syn- or post-tectonic with respect to the burial of the HHCS below the TH. Garnet is frequently found as inclusions within the staurolite poikiloblasts which indicates that the rock from the staurolite zone first underwent garnet zone metamorphic conditions (Fig 5.14).