Structural and petrofabric analysis of an “Alpine-type” peridotite: the lherzolite of the French Pyrenees

A narrow E-W-striking “graben” containing Mesozoic sediments is separated from the Hercynian mountain chain of the Pyrenees in the south by the North-Pyrenean fault and in the north by a detachment plane. In the region around Vicdessos and the Etang de Lers (Ariège, France), three Alpine phases of deformation are recognized: the first and strongest has an E-W-trending fold axis and a subvertical axial-plane cleavage, the second and weakest also has an E-W-trending axis but a northward dipping axial-plane cleavage, and the third a subvertical axis and subvertical N-S-striking axial-plane cleavage. The northernmost zone has ultramafic bodies consisting principally of lherzolite. This zone is also characterized by the occurrence of metamorphism and brecciation. The metamorphism is of a dynamo-thermal type, resembling the Abukumatype of metamorphism, with a climax during the first Alpine deformational phase. Petrofabric analyses of some metamorphic minerals (calcite, dolomite, phlogopite, scapolite, and tremolite) indicate that this was a flattening phase. Although the lherzolites did not suffer from the Alpine metamorphism, they show fracture cleavages of the first and the third Alpine phases. However, the lherzolites contain many more older structures, such as a layering of spinel pyroxenites, an isoclinal folding with an axial-plane cleavage, and a weak, more open folding of these structures. Petrofabric analyses of olivine, enstatite, and diopside indicate that the fabric of the lherzolites is determined by the axial-plane cleavage of the isoclinal folds (\u03b1-olivine, \u03b1-enstatite, and \u03b1-diopside lying perpendicular to the axial-plane cleavage, and \u03b3-olivine, \u03b3-enstatite, and \u03b3-diopside lying parallel to the fold axis) which arose from a pre-Alpine syntectonic recrystallization. In general, the olivine grains show a fabric habit, i.e. they are referable to a triaxial ellipsoid, the short axis perpendicular to the axial-plane cleavage, the long axis parallel to the fold axis, but these axes do not always coincide with the optic elasticity axes. All these phenomena form indications that the lherzolites intruded as solid blocks. There is no indication of a tectonic intrusion; faults in the country rock and shear zones and slickenside structures in the lherzolites are absent. The brecciation of the lherzolites, which occurred principally along their margins, and of the country rock just in the zone containing lherzolites, is definitely linked to the emplacement of the lherzolites. Indications that the breccias originated by explosions are provided by such features as the funnel-shaped breccia bodies and the discordant nature of the breccias, which cut across the bedding planes. This situation suggests that the solid intrusion of the lherzolites could have been caused by degassing of the upper mantle followed by gaseous explosions, just before the Alpine orogeny. It seems highly probable that the lherzolites originated in the upper mantle, where their pre-Alpine structures and fabric would also have been formed. The gabbroic intrusions (ophites) are truly magmatic, and are probably genetically related to the lherzolites, but this relationship would also date back to the upper mantle.