Metamorphic history of the Central Pyrenees Part II, Valle de Arán, Sheet 4

The structural geology and metamorphic petrology of the Bosost area in the Valle de Arán (Central Pyrenees) is discussed. The rocks exposed in this area consist of Cambro-Ordovician mica-schists with numerous granite and pegmatite bodies, phyllites and limestones; Silurian slates and schists and Devonian schists and limestones. The major structure dating from the Hercynian orogeny is the Garonne dome with essentially horizontal schistosity. Large steep folds in the Devonian accompanied by axial plane slaty cleavage are folded disharmonically with regard to the Cambro-Ordovician. Both kinds of structures date from the main phase. A second and later phase with N-S foldaxes was accompanied by laminar flow in E-W direction as shown by numerous rotated porphyroblasts. A third and fourth phase of deformation have folds with vertical axial planes in NW-SE and E-W direction. These last three phases are characterized by minor and microfolds only. The method of investigating microstructures mainly with regard to porphyroblasts is discussed first; then its application. This resulted in the establishment of four metamorphic zones: a biotite-zone; a staurolite-andalusite-cordierite-zone; an andalusite-cordierite-zone and a cordierite-sillimanite-zone; in this order with increasing grade. The higher zones have passed through each of the lower grade ones, so that the cordierite-sillimanite-zone has the most complex history. Staurolite is the first aluminium-silicate to crystallize with increasing temperature, than andalusite, cordierite and finally sillimanite. Before sillimanite started to form, staurolite was already unstable; at the beginning of sillimanite crystallization andalusite became unstable. Cale-silicate rocks in the Ordovician limestone in the cordierite-sillimanite-zone contain bytownite, grossularite, diopside and vesuvianite. Granite and pegmatite bodies and sills occur in all the aluminium-silicate bearing zones, but most abundant in the cordierite-sillimanite-zone. Their emplacement lasted from shortly after the first phase until after the fourth. The culminating point lies around the fourth phase. The granites are mainly composed of albite, quartz, and muscovite; the pegmatites carry smaller or larger amounts of microcline. Chemical analyses of phyllites and mica-schists have shown that the composition of both rock groups is essentially the same. With increasing silicium content, aluminium and potassium decrease. The granites have high alcali percentages with sodium predominating over potassium.