Polymetamorphism and ductile deformation of staurolite-cordierite schist of the Bossost dome: indication for Variscan extension in the Axial Zone of the central Pyrenees

摘要

The Bossòst dome is an E–W-trending elongated structural and metamorphic dome developed in Cambro-Ordovician metasedimentary rocks in the Variscan Axial Zone of the central Pyrenees. A steep fault separates a northern half-dome, cored by massif granite, from an E–W-trending doubly plunging antiform with granitic sills and dykes in the core to the south. The main foliation is a flat-lying S_1/2 schistosity that grades into a steeper-dipping slaty cleavage at the dome margins. Three major deformational and two metamorphic phases can be differentiated. S_1/2 schistosity is an axial planar cleavage to W-vergent recumbent folding that probably occurred in mid-Westphalian time. Peak regional metamorphism M₁ is characterized by static growth of staurolite and garnet following thermal relaxation of the previously thickened crust. Strong non-coaxial deformation recording uniform top-to-the-SE extension during D_2a is preserved in staurolite–garnet schists in a 1.5 km thick, shallowly SE-dipping zone in the southeastern dome. A 500 m thick contact aureole (M₂) was imprinted on the regionally metamorphosed rocks following the intrusion the Bossòst granite during D_2b. More coaxial deformation prevailed during synkinematic growth of M₂ phases in the inner part of the contact aureole around the northern part of the dome, where it obliterated D_2a fabrics. Progressive non-coaxial deformation continued in the southeastern antiform and is recorded by late-synkinematic growth of cordierite. Successive overprinting of the M₁ staurolite–garnet assemblage by andalusite and cordierite of M₂ is preserved in the southern part. The assemblage muscovite + cordierite + staurolite + biotite is considered metastable, given the low Mn and Zn contents of staurolite and cordierite, and interpreted as the result of prograde metamorphism during decompression. P–T conditions during M₂ were approximately 3 kbar and 600 °C. Pervasive crenulations and mesoscopic to regional southerly verging folds are the result of D₃ NNE–SSW compression post-dating ductile deformation and contact metamorphism. Polymetamorphic assemblages of the Bossòst dome preserve a regionally confined zone of ESE-directed extensional shearing within an overall N–S compressional setting. Exact timing of extensional shearing is not known, but can be constrained by recumbent folding during the mid-Westphalian and granitic intrusions, which confine it to Late Carboniferous time (c. 305 Ma). Crustal-scale flat-lying extensional shear zones with similar orientation and time frame are observed in the Hospitalêt massif of the eastern Axial Zone. This suggests that crustal extension, though probably restricted by regional strain partitioning over orthogneiss or intruding granitic bodies within an overall compressive setting, was not uncommon in Late Carboniferous time in the Axial Zone of the Pyrenees.