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

Abstract

-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 M 1 is characterized by static growth of staurolite and garnet following thermal relaxation of the previously thickened crust. Strong non-coaxial deformation recording uniform topto-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 (M 2 ) was imprinted on the regionally metamorphosed rocks following the intrusion the Bossòst granite during D 2b . More coaxial deformation prevailed during synkinematic growth of M 2 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 M 1 staurolite-garnet assemblage by andalusite and cordierite of M 2 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 M 2 were approximately 3 kbar and 600• C. Pervasive crenulations and mesoscopic to regional southerly verging folds are the result of D 3 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 midWestphalian and granitic intrusions, which confine it to Late Carboniferous time (c. 305 Ma). Crustalscale 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.

Vertical strain partitioning in hot Variscan crust: Syn-convergence escape of the Pyrenees in the Iberian-Armorican syntax

“…Clockwise PTt paths in the infrastructure have been documented from peak pressures of ca. 4.5 kb with significant heating during decompression and later cooling along a steep geotherm (Mezger and Passchier, 2003;Mezger et al, 2004;Mezger, 2005;Vilà et al, 2007). Recently, Aguilar et al (2015) reported metamorphic relicts suggestive of an early hightemperature peak pressure of 7.5 Kbar in the Roc the France dome (Fig.…”

“…The northern sub-dome exposes a deep portion of the northern flank of the original dome, whereas the southern sub-dome exposes mostly the E-W elongate core and the southern flank of the original dome. Peraluminous granite sills are intercalated in the dome, particularly in its northern flank, which display mineralstretching lineations (Li) of two main trends (see also Mezger and Passchier, 2003). The dominant lineation is longitudinal, trending parallel or at a low angle to the map axis of the dome and a few transverse lineations are documented in the vicinity of the northern margin of the dome, sometime on the same outcrops than the longitudinal Li (see also Mezger and Passchier, 2003).…”

“…Attenuation of the isograds and associated high thermal gradient point to an extensional component of deformation at the InfrastructureSuperstructure transition Oxburgh, 1985, 1986;Van den Eeckhout and Zwart, 1988;de Saint-Blanquat, 1993). In the Axial Zone, extension was interpreted to have taken place before (Soula et al, 1986a), during (Mezger and Passchier, 2003) or after (Van den Eeckhout and Zwart, 1988;Vissers, 1992) shortening of the superstructure. In the eastern Pyrenees, HT deformation and magmatism have been shown to be coeval with regional-scale dextral-reverse shear zones (Carreras and Capella, 1994), in agreement with synkinematic emplacement of the plutons in the superstructure in an dextral transpressional context (Bouchez and Gleizes, 1995;Gleizes et al, 1998aGleizes et al, , 2001Gleizes et al, , 2006Aurejac et al, 2004;Carreras et al, 2004;Román-Berdiel et al, 2004.…”

“…Recent structural analysis of the Aston, Hospitalet and Bossost domes (Fig. 2) led to models of lateral flow of the infrastructure and dome formation during regional transpression (Mezger and Passchier, 2003;Denèle et al, 2007Denèle et al, , 2009Mezger, 2009). 3 Methodology and systematics of the structural markers…”

“…Peraluminous granite sills are intercalated in the dome, particularly in its northern flank, which display mineralstretching lineations (Li) of two main trends (see also Mezger and Passchier, 2003). The dominant lineation is longitudinal, trending parallel or at a low angle to the map axis of the dome and a few transverse lineations are documented in the vicinity of the northern margin of the dome, sometime on the same outcrops than the longitudinal Li (see also Mezger and Passchier, 2003). The fact that both longitudinal and transverse lineations are underlined by the same HT minerals suggests flattening strain in this part of the dome.…”