The Armorican Massif (western France) provides an excellent record of the Palaeozoic history of the Variscan belt. Following the Late Neoproterozoic Cadomian orogeny, the Cambro-Ordovician rifting was associated with oceanic spreading. The Central-and North-Amorican domains (which together constitute the core of the Armorica microplate) are bounded by two composite suture zones. To the north, the Léon domain (correlated with the ''Normannian High'' and the ''Mid-German Crystalline Rise'' in the Saxo-Thuringian Zone) records the development of a nappe stack along the northern suture zone, and was backthrusted over the central-Armorican domain during the Carboniferous. To the south, an intermediate block (''Upper Allochthon'') records a complex, polyorogenic history, with an early high-temperature event followed by the first generation of eclogites (Essarts). This intermediate block overthrusts to the north the Armorica microplate (Saint-Georges-sur-Loire), to the south: (i) relics of an oceanic domain; and (ii) the Gondwana palaeomargin. The collision occurred during a Late Devonian event, associated with a second generation of eclogites (Cellier). To cite this article: M.

27 p.International audience[1] The contribution of lateral forces, vertical load, gravity redistribution and erosion to the origin of mantled gneiss domes in internal zones of orogens remains debated. In the Orlica-Snieznik dome (Moldanubian zone, European Variscan belt), the polyphase tectono-metamorphic history is initially characterized by the development of subhorizontal fabrics associated with medium- to high-grade metamorphic conditions in different levels of the crust. It reflects the eastward influx of a Saxothuringian-type passive margin sequence below a Teplá-Barrandian upper plate. The ongoing influx of continental crust creates a thick felsic orogenic root with HP rocks and migmatitic orthogneiss. The orogenic wedge is subsequently indented by the eastern Brunia microcontinent producing a multiscale folding of the orogenic infrastructure. The resulting kilometre-scale folding is associated with the variable burial of the middle crust in synforms and the exhumation of the lower crust in antiforms. These localized vertical exchanges of material and heat are coeval with a larger crustal-scale folding of the whole infrastructure generating a general uplift of the dome. It is exemplified by increasing metamorphic conditions and younging of 40Ar/39Ar cooling ages toward the extruded migmatitic subdomes cored by HP rocks. The vertical growth of the dome induces exhumation by pure shear-dominated ductile thinning laterally evolving to non-coaxial detachment faulting, while erosion feeds the surrounding sedimentary basins. Modeling of the Bouguer anomaly grid is compatible with crustal-scale mass transfers between a dense superstructure and a lighter infrastructure. The model implies that the Moldanubian Orlica-Snieznik mantled gneiss dome derives from polyphase recycling of Saxothuringian material

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A correlation between allochthonous units exposed in the NW Iberian Massif and the southern Armorican Massif is carried out based on lithological associations, structural position, age and geochemistry of protoliths and tectonometamorphic evolution. The units on both sides of the Bay of Biscay are grouped into Upper, Middle and Lower allochthons, whereas an underlying allochthonous thrust sheet identified in both massifs is referred to as the Parautochthon. The Lower Allochthon represents a fragment of the outermost edge of Gondwana that underwent continental subduction shortly after the closure of a Palaeozoic ocean which, in turn, is represented by the Middle Allochthon. The latter consists of supra-subduction ophiolites and metasedimentary sequences alternating with basic, mid-ocean ridge basalt (MORB)-type volcanics, with inheritances suggesting the proximity of a continental domain. Seafloor spreading began at the Cambro-Ordovician boundary and oceanic crust was still formed during the Late Devonian, covering the lifetime of the Rheic Ocean, which is possibly represented by the Middle Allochthon. The opening of the oceanic domain was related to pulling apart the peri-Gondwanan continental magmatic arc, which is represented by the Upper Allochthon.

International audienceGarnet-chloritoid-bearing micaschists from the Gran Paradiso massif (Western Alps) contain evidence of a polymetamorphic evolution. Detailed textural observations reveal that two stages of garnet growth are present in the micaschists, interpreted as: (i) relics of an early metamorphism of pre-Alpine age and (ii) newly grown Alpine garnet, respectively. Both generations of garnet preserve growth zoning. Fromthermocalc-based numerical modelling of mineral assemblages in pressure-temperature (P-T) pseudosections, we infer that garnet 1 grew at increasing temperature and slightly increasing pressure, whereas garnet 2 grew at decreasing pressure and slightly increasing temperature. Estimated P-T conditions are ∼620 °C, 6 kbar for the peak of the pre-Alpine event, and of 490 °C, 18-20 kbar for the pressure peak of the Alpine event. Modelling of the modal proportion and chemical composition of garnet (i) shows that the subsequent decompression (to 14-15 kbar at 550 °C) must have been accompanied by moderate heating and (ii) does not support a stage of final temperature increase following decompressional cooling. This argues against a late thermal pulse associated with mantle delamination. Preservation of growth zoning in both generations of garnet and the limited amount of diffusive re-equilibration at the boundary between the two garnets suggests that the rocks were subjected to fast burial and exhumation rates, consistent with data obtained from other internal Alpine units

Eclogite, felsic orthogneiss and garnet-staurolite metapelite occur in a 5 km long profile in the area of Międzygo´rze in the Orlica-S´nie_ znik dome (Bohemian Massif). Petrographic observations and mineral equilibria modelling, in the context of detailed structural work, are used to document the close juxtaposition of high-pressure and medium-pressure rocks. The structural succession in all lithologies shows an early shallow-dipping fabric, S1, that is folded by upright folds and overprinted by a heterogeneously developed subvertical foliation, S2. Late recumbent folds associated with a weak shallowdipping axial-plane cleavage, S3, occur locally. The S1 fabric in the eclogite is defined by alternation of garnet-rich (grs = 22-29 mol.%) and omphacite-rich (jd = 33-36 mol.%) layers with oriented muscovite (Si = 3.26-3.31 p.f.u.) and accessory kyanite, zoisite, rutile and quartz, indicating conditions of 19-22 kbar and 700-750°C. The assemblage in the retrograde S2 fabric is formed by amphibole, plagioclase, biotite and relict rutile surrounded by ilmenite and sphene that is compatible with decompression and cooling from 9 kbar and 730°C to 5-6 kbar and 600-650°C. The S3 fabric contains in addition domains with albite, chlorite, K-feldspar and magnetite indicating cooling to greenschist facies conditions. The metapelites are composed of garnet, staurolite, muscovite, biotite, quartz, ilmenite and chlorite. Chemical zoning of garnet cores that contain straight ilmenite and staurolite inclusion trails oriented perpendicular to the external S2 fabric indicates prograde growth, from 5 kbar and 520°C to 7 kbar and 610°C, during the formation of the S1 fabric. Inclusion trails parallel with the S2 fabric at garnet and staurolite rims are interpreted to be a continuation of the prograde path to 7.5 and 630°C in the S2 fabric. Matrix chlorite parallel to the S2 foliation indicates that the subvertical fabric was still active below 550°C. The axial planar S2 fabrics developed during upright folding are associated with retrogression of the eclogite under amphibolite facies conditions, and with prograde evolution in the metapelites, associated with their juxtaposition. The shared part of the eclogite and metapelite P-T paths during the development of the subvertical fabric reflects their exhumation together.

The garnet blueschists from the Ile de Groix (Armorican Massif, France) contain millimetre-to centimetre-sized pseudomorphs consisting of an aggregate of chlorite, epidote and paragonite. The pseudomorphed phase developed at a late stage of the deformation history, because it overgrows a glaucophane-epidote-titanite foliation. Garnet growth occurred earlier than the beginning of the ductile deformation, and thus garnet is also included in the pseudomorphs. Microprobe analyses show that garnet is strongly zoned, with decreasing spessartine and increasing almandine and pyrope contents from core to rim. Grossular content is higher in garnet cores (about 35 mole%) compared to garnet rims (about 30 mole%). Blue amphibole has glaucophane compositions with a low Fe 3+ content and become more magnesian when inclusions in garnet (X Mg ¼ 0.62-0.65) are compared with matrix grains (X Mg ¼ 0.67-0.70). Matrix epidote has a pistacite content of about 50 mole%. On the basis of their shape and the nature of the breakdown products, the pseudomorphs are attributed to lawsonite. A numerical model (using THERMOCALC THERMOCALC) has been developed in order to understand the reactions controlling both the growth and the breakdown of lawsonite. Lawsonite growth could have taken place through the continuous hydration reaction Chl + Ep + Pg + Qtz + Vap ¼ Gln + Lws, followed by the fluid-absent reaction Chl + Ep + Pg ¼ Grt + Gln + Lws. Peak P-T conditions are estimated at about 18-20 kbar, 450°C. This indicates that lawsonite growth took place at increasing P and T, hence can be used as a geobarometer in the buffering assemblage garnet-glaucophane-epidote. The final part of the history is recorded by lawsonite breakdown, after cessation of the ductile deformation, and recording the earliest stages of the exhumation.

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