South Armorican Shear Zone Research Articles

3-D crustal VS model of western France and the surrounding regions using Monte-Carlo inversion of seismic noise cross-correlation dispersion diagrams

Summary Due to a too sparse permanent seismic coverage during the last decades, the crustal structure of western France and the surrounding regions is poorly known. In this study, we present a 3-D seismic tomographic model of this area obtained from the analysis of 2-year continuous data recorded from 55 broad-band seismometers. An unconventional approach is used to convert Rayleigh wave dispersion diagrams obtained from ambient noise cross-correlations into posterior distributions of 1-D VS models integrated along each station pair. It allows to avoid the group velocity map construction step (which means dispersion curve extraction) while providing meaningful VS posterior uncertainties. VS models are described by a self-adapting and parsimonious parameterization using cubic Bézier splines. 1268 separately inverted 1-D VS profiles are combined together using a regionalization scheme, to build the 3-D VS model with a lateral resolution of 75 km over western France. The shallower part of the model (horizontal cross-section at 4 km depth) correlates well with the known main geological features. The crystalline Variscan basement is clearly associated with positive VS perturbations while negative heterogeneities match the Mesocenozoic sedimentary basins. At greater depths, the Bay of Biscay exhibits positive VS perturbations,which eastern and southern boundaries can be interpreted as the ocean-continent transition. The overall crustal structure below the Armorican Massif appears to be heterogenous at the subregional scale, and tends to support that both the South-Armorican Shear Zone and the Paris Basin Magnetic Anomaly are major crustal discontinuities that separate distinct domains.

The La Bellière gold and antimony district (French Armorican Massif): A two-stage evolution model controlled by Variscan strike-slip tectonic

The La Bellière gold district, in the Ligerian domain of the Armorican Massif is the third one of France in term of production. It shows gold-bearing quartz veins neighboring with a complex network of Sb veins, hosted in the Neoproterozoic metasedimentary basement of the Mauges Nappe. New structural data, mineralogical, textural and geochemical assays have been carried out to clarify the gold and Sb veins relationships. Our results favor of two-step hydrothermal event with an early deep-seated gold event followed by a late shallower Sb event. The gold-bearing quartz veins are hosted by N60°E to N90°E sinistral strike-slip faults. These veins exhibit a four-stage paragenesis: 1) early barren quartz filling, 2) arsenopyrite-pyrite with minor scheelite assemblage, 3) gold and base metal sulfides crystallized after a ductile/brittle deformation stage, and 4) pyrite-carbonate coeval with a late brittle stage. These results support a continuous evolution model from a deep emplacement level (12 to 9 km) to a shallow one (7 to <5 km) with a brutal change in the P/T conditions that triggered the deposition of gold (stage 2) controlled by a seismic-valve mechanism. On the contrary, Sb veins correspond to open space vein types, hosted in a N20°W to N20°E conjugated strike-slip fault network arranged in a Riedel shear model, controlled by N130-140°E regional dextral strike-slip shear zone associated to the South Armorican Shear Zone system. A two-stage evolution is recognized, the first one consists of early pyrite-arsenopyrite with microcrystalline quartz, followed by a late stibnite stage formed after geodic quartz deposition. Quartz texture argues for a shallow emplacement (<5 km) of veins controlled by a suction-pump mechanism. Chronologically, the gold vein opening complies with a NE-SW maximum shortening direction different from the late-Variscan stress field responsible for the dextral Armorican shear zone during late Carboniferous, and constrains an indirect dating. Thus, a Visean age is proposed for the formation of gold veins. The overall features of gold deposits are close to those of mesozonal orogenic gold deposits in Europe. Conversely, Sb-bearing veins are generated by a N-S shortening during the late Carboniferous. The Sb event, distinct in time from the earlier gold one, might correspond to a different expression of the so-called “epizonal” type in the orogenic gold model. It is proposed that the Late Carboniferous “Or 300” event is not the unique metallogenic period for gold deposition in the French Variscan domain. Our study highlights the role of the sinistral strike-slip shearing of possible Visean age, in the gold metallogenesis of the Ligerian domain.

Wrench‐Related Dome Formation and Subsequent Orogenic Syntax Bending in a Hot Orogen (Variscan Ibero‐Armorican Arc, the Ouessant Island, France)

AbstractDuring the Carboniferous collision stage, the West European Variscan orogen was affected by oblique convergence, wrenching, plate‐scale oroclinal bending, and widespread exhumation of the deep crust. One of these exhumed units forms the Léon dome located on the northern flank of the Ibero‐Armorican Arc in the western part of the Armorican massif. Structural field data from the Ouessant Island reveal kinematic changes between 330 and 300 Ma that affected the northwestern margin of the Léon dome. This margin underwent two ductile deformation phases with opposite strike‐slip shear senses. Dextral strike‐slip wrenching combined with orogen‐normal shortening, orogen‐parallel stretching, and local top‐to‐the‐NE shearing occurred during the Léon dome formation and exhumation of its gneissic core at 330–310 Ma. During tightening of the Ibero‐Armorican Arc at ~300 Ma, more localized sinistral wrenching reactivated the northwestern boundary of the Léon dome. The resulting N70°E trending sinistral Porspoder‐Ouessant shear zone together with its conjugate dextral South Armorican shear zone controlled the lateral escape of the Léon‐North‐Central Armorican rigid tectonic wedge triggered by indentation of the Cantabrian orocline at the core of the Ibero‐Armorican Arc.

Meteoric fluid‐rock interaction in Variscan shear zones

AbstractVariscan shear zones in the Armorican Massif represent sites of strong fluid‐rock interaction. The hydrogen isotope composition of muscovite (δDMs) from syntectonic leucogranite allows to determine the source of fluids that infiltrated the footwall of three detachment zones and the South Armorican Shear Zone. Using temperatures of hydrogen isotope exchange estimated from microstructural data, we calculate the hydrogen isotope ratios of water (δDwater) present within the shear zones during high‐temperature deformation. A ~40‰ difference in δDwater values from deep to shallow crustal level reveals a mixing relationship between deep crustal fluids with higher δD values that range from −34 to −33‰, and meteoric fluids with δD values as low as −74‰ in the upper part of detachment footwalls.

Large-scale strain localization induced by phase nucleation in mid-crustal granitoids of the south Armorican massif

The reduction of grain size is believed to play a critical role in strain localization to form shear zones. Although many mechanisms have been proposed, the source of grain size reduction remains debated. The South Armorican Shear Zone (SASZ) is a crustal-scale strike-slip fault that deforms granitoids at mid-crustal conditions. The SASZ records the transition from protolith to ultramylonite, representative of increasing ductile shear strain. To investigate the evolution of strain localization, the different states of deformation were studied using a combination of detailed microstructural, chemical and electron backscatter diffraction analyzes. Increasing strain from protolith to ultramylonite resulted in (1) grain size reduction, (2) the development of interconnected monophase layers of mica and incipient mixed-phase zones composed of phengite-quartz ± K-feldspar in protomylonite and low-strain mylonite, and (3) the formation of fine-grained mixed-phase zones composed of K-feldspar-quartz ± phengite in high-strain mylonite and ultramylonite. We propose that the causes for interconnection of mica are the formation of cracks in the protolith combined with fluid-assisted nucleation. The latter process also plays a major role in phase mixing, as attested by the precipitation of K-feldspar at triple junctions in quartz-rich layers and in fine-grained tails of inherited K-feldspar porphyroclasts in ultramylonite. The transition from quartz-rich layers and mixed-phase zones is accompanied by a strong dispersion of the quartz lattice preferred orientation. These microstructural and textural evidences suggest that phase nucleation is the major process behind phase mixing, possibly accompanied by the action of grain-boundary sliding to open cavities during deformation. Instead of a single process, we therefore highlight a succession of weakening processes in the evolution of the SASZ, starting with the crystallization of mica as a first weakening material, and then evolving with the formation of very fine-grained mixed-phase zones made principally of feldspar, quartz and phengite.

Uranium metallogenesis of the peraluminous leucogranite from the Pontivy-Rostrenen magmatic complex (French Armorican Variscan belt): the result of long-term oxidized hydrothermal alteration during strike-slip deformation

In the French Armorican Variscan belt, most of the economically significant hydrothermal U deposits are spatially associated with peraluminous leucogranites emplaced along the south Armorican shear zone (SASZ), a dextral lithospheric scale wrench fault that recorded ductile deformation from ca. 315 to 300 Ma. In the Pontivy-Rostrenen complex, a composite intrusion, the U mineralization is spatially associated with brittle structures related to deformation along the SASZ. In contrast to monzogranite and quartz monzodiorite (3 3), the leucogranite samples are characterized by highly variable U contents (~ 3 to 27 ppm) and Th/U ratios (~ 0.1 to 5) suggesting that the crystallization of magmatic uranium oxide in the more evolved facies was followed by uranium oxide leaching during hydrothermal alteration and/or surface weathering. U-Pb dating of uranium oxides from the deposits reveals that they mostly formed between ca. 300 and 270 Ma. In monzogranite and quartz monzodiorite, apatite grains display magmatic textures and provide U-Pb ages of ca. 315 Ma reflecting the time of emplacement of the intrusions. In contrast, apatite grains from the leucogranite display textural, geochemical, and geochronological evidences for interaction with U-rich oxidized hydrothermal fluids contemporaneously with U mineralizing events. From 300 to 270 Ma, infiltration of surface-derived oxidized fluids leached magmatic uranium oxide from fertile leucogranite and formed U deposits. This phenomenon was sustained by brittle deformation and by the persistence of thermal anomalies associated with U-rich granitic bodies.

Crustal recycling and juvenile addition during lithospheric wrenching: The Pontivy-Rostrenen magmatic complex, Armorican Massif (France), Variscan belt

The South Armorican Shear Zone (SASZ), in the French Armorican Variscan belt, is a lithospheric wrench fault that acted during the Late Carboniferous as a transition zone between two distinct domains: a thickened domain to the south affected by extension and crustal magmatism, and a weakly thickened domain to the north subjected to dextral wrenching and crust- and mantle-derived magmatism. The Pontivy-Rostrenen complex is a composite intrusion emplaced along the SASZ. To the south, the complex is made of leucogranites whereas, to the north, monzogranites outcrop together with small intrusions of quartz monzodiorite. U-Pb dating of magmatic zircon by LA-ICP-MS reveal that most magmatic rocks were emplaced at ca. 315Ma (between 316.7±2.5Ma and 310.3±4.7Ma), excepted a late leucogranitic intrusion that was emplaced at 304.7±2.7Ma. The leucogranites (−4.8<εNd (T)<2.1; presence of Archean to Paleozoic inherited zircon) are strongly peraluminous (A/CNK>1.1) and formed by partial melting of metasediments and peraluminous orthogneisses. The monzogranite (−4.0<εNd (T)<−3.2; scarce Paleozoic inherited zircon) is moderately peraluminous (1<A/CNK<1.3) and formed by partial melting of an orthogneiss with a probable metaluminous composition whereas the quartz monzodiorite (−3.2<εNd (T)<−2.2; no inherited zircon) is metaluminous (0.7<A/CNK<1.1) and formed by partial melting of a metasomatized lithospheric mantle. The evolution of the magmas was controlled by fractional crystallization, magma mixing and/or peritectic mineral entrainment. At the scale of the Armorican Variscan belt, crustal partial melting, to the south of the SASZ, was triggered by lithospheric thinning and adiabatic decompression during extension. Conversely, to the north, asthenosphere upwelling during strike-slip deformation and subsequent slab tearing, as suggested by tomographic data, induced the melting of both the crust and the mantle fertilized during previous subduction events. This process is likely not exclusive to the Armorican Massif and may be applied to other regions in the Variscan belt, such as Iberia.

Major softening at brittle‐ductile transition due to interplay between chemical and deformation processes: An insight from evolution of shear bands in the South Armorican Shear Zone

AbstractThe formation of S‐C/C′ fabrics in the South Armorican Shear Zone has been evaluated by detailed microstructural study where the focus was given to initiation and early evolution of the C/C′ fabric shear bands. Our observations suggest that the S‐C/C′ fabrics formed at distinct temperature conditions indicating &gt;550°C for the S fabric and 300–350°C at 100–400 MPa for the C/C′ fabric shear bands. The evolving microstructure within shear bands documents switches in deformation mechanisms related to positive feedbacks between deformation and chemical processes and imposes mechanical constraints on the evolution of the brittle‐ductile transition in the continental transform fault domains. Three stages of shear band evolution have been identified. Stage I corresponds to initiation of shear bands via formation of microcracks with possible yielding differential stress of up to 250 MPa. Stage II is associated with subgrain rotation recrystallization and dislocation creep of quartz and coeval dissolution‐precipitation creep of microcline. Recrystallized quartz grains show continual increase in size and decrease in stress and strain rates from 94 MPa to 17–26 MPa and 3.8 × 10−12 s−1–8 × 10−14 s−1 associated with deformation partitioning into weaker microcline layer and shear band widening. The quartz mechanical data allowed us to set some constrains for coeval dissolution‐precipitation of microcline which at our estimated pressure‐temperature conditions suggests creep at 17–26 MPa differential stress and 3.8 × 10−13 s−1 strain rate. Stage III is characterized by localized slip along white mica bands accommodated by dislocation creep at strain rate 3.8 × 10−12 s−1 and stress 9.36 MPa. Our mechanical data point to dynamic evolution of the studied brittle‐ductile transition characterized by major weakening to strengths ~10 MPa. Such nonsteady state evolution may be common in crustal shear zones especially when phase transformations are involved.

Pressure solution in rocks: focused ion beam/transmission electron microscopy study on orthogneiss from South Armorican Shear Zone, France

In order to characterize the µm-to-nm structures related to operation of pressure solution on phase boundaries in naturally deformed rocks, we have performed a detailed focused ion beam/transmission electron microscopy study in ultramylonite samples from South Armorican Shear Zone (France) that focused on grain boundary scale. We have studied phase boundaries between quartz, K-feldspar and white mica in both 2D and 3D and compare our evidences with theoretical dissolution precipitation models in the current literature. The dissolution (re)precipitation processes lead to the development of different features at different phase boundaries. In both quartz–white mica and quartz–K-feldspar phase boundaries, voids were ubiquitously observed. These voids have different shapes, and the development of some of them is crystallographically controlled. In addition, part of these voids might be filled with vermiculite. Amorphous leached layers with kaolinite composition were observed at the boundaries of K-feldspar–quartz and K-feldspar–white mica. The development of different features along the phase boundaries is mainly controlled by the crystallography of the phases sharing a common interface, together with the presence of fluids that either leaches or directly dissolve the mineral phases. In addition, the local dislocation density in quartz may play an important role during pressure solution. We suggest that the nanoscale observations of the quartz–white mica phase boundaries show direct evidence for operation of island-and-channel model as described in Wassmann and Stockhert (Tectonophysics 608:1–29, 2013), while K-feldspar–quartz phase boundaries represents amorphous layers formed via interface-coupled dissolution reprecipitation as described by Hellmann et al. (Chem Geol 294–295:203–216, 2012).

Extensional tectonics in the Hercynian Armorican belt (France). An overview

Abstract A synthesis of existing geological, structural and geophysical data shows that the south Armorican Hercynian belt was marked by syn-convergence crustal thinning and dextral wrenching that were in part coeval in late Carboniferous times. Our kinematic model is further supported by new structural data and 40Ar/39Ar ages on synkinematic leucogranites. Extension and strike-slip followed earlier crustal thickening and exhumation of high-pressure metamorphic units in late Devonian-early Carboniferous times. Crustal extension led to the development of core complexes cored by migmatites and crust-derived granite laccoliths. At this time, the South Armorican shear zone acted as a transfer zone separating the extending domain of South Brittany from the non-extending domain of Central Brittany submitted to dextral wrenching. The overall structural pattern and attached kinematics are compared with recent numerical models and illustrated by a 3D interpretative model that integrates geological and deep seismic reflection data (ARMOR 2 profile).

Transgressive systems tract of a ria-type estuary: The Late Holocene Vilaine River drowned valley (France)

The understanding of control parameters of estuarine-infilling at different scales in time and space is important for near-future projections and better management of these areas facing sea-level change. During the last 20 years, the stratigraphy of late Quaternary estuaries has been extensively investigated leading to the development of depositional models which describe the sedimentary facies distribution according to changes in sea level, tidal range and wave activity but few of them describe in detail the influence of the estuarine geomorphology. On the Atlantic coast, recent studies provide a detailed understanding of the sedimentary infill succession of incised valleys during the late Holocene transgression with recent emphasis on the macrotidal bay of the Vilaine River in South Brittany. The latter has shown the importance of the inherited morphology in controlling the preservation of the sedimentary deposits. However, until now, there has been little information available on the geometry of the underlying bedrock of the estuary and its immediate approaches including the Holocene history of the most proximal part of the Vilaine River estuary. The recent acquisition of very-high resolution seismic reflection (Chirp) and sediment core data in the Vilaine Estuary provides the opportunity to address several of these issues leading to the identification of the controlling factors that played a role in the sedimentary filling of the estuary. The estuary of the Vilaine River is one of the most sheltered estuaries of the Atlantic coast and thus shows an excellent record of the successive transgressive phases since the last glacial period. The valley of this ria-type estuary is incised in the Hercynian magmatic and metamorphic rocks of the Armorican Massif along the striking branches of the South Armorican Shear Zone and filled with well-stratified sediments. The study of the morphology of the substratum in the estuary reveals narrow paleovalleys (1 km wide) with a maximum depth of 20 m. The shapes and dimensions of these paleovalleys are comparable with those observed upstream in the present-day Vilaine River valley. Facies analysis of seismic and sedimentary data, complemented by 21 AMS radiocarbon age dating, enables the reconstruction of the infilling of the estuarine valley. The valley is filled by Pleistocene to early Holocene alluvial, coarse-grained, pebbly sands overlain by mid- to late Holocene marine sands, silts and muds. From 10,000 to 5000 cal. BP, the marine transgression shows a rapid initial phase of transgression followed by a progressive slowing down which can be recognised over more than 90 km in an upstream direction. The rapid transgression developed homogeneously over the entire length of the estuary with a widespread expression of tidal currents (tide-dominated estuary). The slow phase of the transgression gives rise to a differentiated expression of the hydrodynamic factors according to the geomorphological setting. The sedimentary deposits of the innermost and narrow part of the estuary reflect a strong control of tidal and river currents when the influence of tidal currents increases at the expense of fluvial control in a downstream direction. The sedimentary filling of the outer reaches of estuary shows a strong influence of tidal currents modulated by changes in sea level where, from 5700 to 3000 cal. BP, the progressive decrease in accommodation space promotes the expression of wave action (wave-dominated estuary). Finally, from 5000 years BP to present day, the stabilisation of sea level gives birth to scour and lag sedimentary morphologies (scour and lag estuary). The sedimentary record appears thus strongly marked by the Holocene transgression and the geomorphological setting where hydrodynamic parameters superimposed on the geomorphological and eustatic contexts expressed in different ways in time and space govern the evolution of the estuarine system.

Giant quartz vein formation and high-elevation meteoric fluid infiltration into the South Armorican Shear Zone: geological, fluid inclusion and stable isotope evidence

Giant quartz veins associated with the South Armorican Shear Zone record important fluid circulation during the Hercynian period. Regional-scale mapping of veins allows two groups of veins to be identified, on the basis of their geometric relationship with the South Armorican Shear Zone. Veins in the first group are parallel to the shear zone, whereas those in the second group developed in a direction oblique to it. The former probably record infiltration of fluids along permeable pathways in highly deformed zones; the latter may represent crustal-scale tension gashes in the regional context. Most quartz veins have δ 18 O values between 10 and 16‰ indicating a mid-crustal origin for the fluids. Microthermometry on fluid inclusions from euhedral quartz indicates that late fluids were mostly aqueous with very low salinity (0–1.7 wt% eq.) and with homogenization temperatures ranging between 150 and 270 °C. Together with very low δ 18 O values of some euhedral quartz, down to −2‰, these features argue for a surface origin. Corresponding δ 18 O fluid values estimated near −11‰ are probably related to the high palaeo-elevation of meteoric precipitation. Scarce, but significant, H 2 O–CO 2 fluid inclusions in euhedral quartz indicate a metamorphic contribution. These were probably sourced from the exhumed metamorphic basement in the southern part of the region.

Late orogenic exhumation of the Variscan high‐grade units (South Armorican Domain, western France), combined structural and40Ar/39Ar constraints

The current structure of the South Armorican Domain in the French Variscan Belt, provides an excellent record of the late orogenic evolution, as documented by the complex interactions between extensional structures and partial melting. In the Golfe du Morbihan, the South Brittany Migmatite Belt (SBMB), tectonically overlain by non‐melted metamorphic units, was studied by combined structural and40Ar/39Ar methods. The recognition of successive deformation stages points to a strain continuum initiated at depth at the end of a partial melting event, and continued to near‐surface brittle faulting, encompassing ductile strain localization over a major top‐to‐the‐ESE extensional shear zone (SSZ) consistent with an overall WNW‐ESE orogen‐parallel stretching. This multidisciplinary study making the link between structure, texture, microchemistry and40Ar/39Ar geochronology allow distinguishing between cooling, recrystallization and syn‐tectonic crystallization ages. The first extensional insights, witnessed by dyke intrusion were dated at ∼319 ± 6 Ma immediately after the SBMB partial melting. Strain localization along the SSZ, occurred until ∼302–298 Ma when SBMB crossed the ductile‐brittle transition. Moreover, in situ40Ar/39Ar analysis, of syn‐tectonic white micas sampled in the ductile and brittle structures yield similar ∼300 Ma. Formation or reactivation of the SSZ may have occurred during the rapid cooling of the SBMB settled at ∼305 Ma by conventional40Ar/39Ar analysis. Large‐scale late orogenic extensional event of the previously overthickened Variscan crust thus lasted over ∼20 ± 3 Myr in the same extensional tectonic regime guided by the South Armorican Shear Zone.

New U-Pb zircon and 40Ar/ 39Ar muscovite age constraints on the emplacement of the Lizio syn-tectonic granite (Armorican Massif, France)

LA-ICP-MS U-Pb analyses performed on zircon grains from the Lizio granite yielded an emplacement age of 316 ± 6 Ma. Typical S-C structures show that the Lizio granite was emplaced contemporaneously with dextral shearing along the northern branch of the South Armorican Shear Zone and that it was therefore active at that time. 40Ar/ 39Ar analyses performed on muscovite grains yielded plateau dates ranging between 311.5 and 308.2 Ma. Muscovite chemistry is typical of primary magmatic muscovite, which precludes a late fluids-induced resetting of the K-Ar isotopic system. 40Ar/ 39Ar dates thus likely correspond to the cooling ages below the argon closure temperature. Considering the uncertainties on the measured ages, we can propose that either the Lizio granite cooled down quickly in less than a million of years or that it remained in a hot environment for several millions of years after its emplacement. This latter scenario could have been sustained by shear heating during dextral shearing along the northern branch of the South Armorican Shear Zone.

Mylonites of the South Armorican Shear Zone: Insights for crustal-scale fluid flow and water–rock interaction processes

Mylonites display petrographical and geochemical characteristics that can be related to syn-deformation fluid circulation. The South Armorican Shear Zone, a major structural feature of the Armorican Massif (France), is outlined by the presence of mylonitic rocks cropping out mostly in open quarries. These mylonites were essentially formed at the expense of peraluminous granitic bodies. Deformation occurred from ductile conditions in the biotite stability field (>400°C) down to lower greenschist cataclasis and brecciation, where carbonation developed. U–Pb analyses on zircon and monazite define a minimum duration of 15Ma for the deformation and hydrothermal history, between 315Ma and 300Ma. Fluid circulations are well documented, by way of petrographic observation (chlorite and carbonate crystallization), mineralogical composition analysis (muscovite chemistry), erratic mobility behavior of some elements (As, Sn, U for instance), and stable isotope composition analysis of the infiltrated rocks. High temperature deformation is not accompanied by alteration of the O isotope system, which implies either low fluid/rock ratio and/or the involvement of δ18O crustal fluids with a composition similar to that of the rocks. On the other hand, some low temperature mylonites show a drastic decrease in the δ18O values, which has to be related to the influx of surface derived waters. The heat source necessary for this crustal scale downward infiltration of fluids followed by upward motion was likely provided by the exhumation of lower crustal units in the South Armorican domain.

Differentiation of peraluminous leucogranites “en route” to the surface

Leucogranites display petrological and geochemical heterogeneities that can be related to either primary processes in the source of melts or to secondary processes such as fractional crystallization. In the Armorican Massif, a Hercynian domain in western France, syntectonic leucogranites were emplaced along major shear zones, the so-called South Armorican Shear Zone. The Lizio and Questembert massifs are derived from a similar metasedimentary source ( ε Nd(T) between − 4 and − 6) and have similar magmatic volumes. They display, however, distinct depths of emplacement, with Questembert being emplaced ca. 5–10 km shallower than Lizio. The shallower emplacement of Questembert is recorded by contact metamorphic assemblages and the levels of intrusion in correlated sedimentary sequences. The Questembert magmas are more differentiated than the Lizio ones, as demonstrated by a lower amount of modal biotite, a higher SiO 2 content (72.8–74.2 wt.% vs. 71.7–73.3 wt.%) and a more pronounced peraluminous character. The high-SiO 2 Questembert magmas can be derived from the low-SiO 2 Lizio ones by 15–20 wt.% fractionation of the assemblage alkali feldspar + plagioclase + biotite. Fractionation of zircon and monazite, hosted in biotite, is indicated by a systematic decrease in Zr and Th + LREE contents. The magmatic evolution is well recorded by the oxygen isotope evolution of whole rocks and mineral separates. The Questembert massif records evidence of a two stage fluid–rock interaction. A high-T stage corresponds to the magmatic fluid exsolution, and is recorded in the most evolved samples: appearance of tourmaline in the matrix, changes in the composition of muscovite, increase in whole rock Be, Sn, Rb and W contents, fractionation of K/Rb, Zr/Hf and Nb/Ta ratios, and fractionation of REE patterns. A second post-solidus alteration is indicated by oxygen isotope disequilibrium between quartz and feldspar in the Questembert massif and requires the circulation of low- δ 18O fluids, likely derived from the surface. We propose that the distinct petrological, mineralogical and geochemical evolutions of the Questembert and Lizio magmas are related to processes acting during the ascent of magmas through the continental crust. Questembert is more differentiated than Lizio because it covered a greater distance during its vertical migration, enhancing crystal fractionation from melts.

The top-to-the-southeast Sarzeau shear zone and its place in the late-orogenic extensional tectonics of southern Armorica

AbstractThis study presents new structural and monazite chemical U-Th/Pb geochronological constraints for the magmatic rocks of the Golfe du Morbihan area, in southern Brittany, south of the South Armorican shear zone (SASZ). A major extensional shear zone, defined here as the “Sarzeau shear zone” (SSZ), separates Carboniferous migmatites and the Ste-Anne d’Auray type anatectic granite from highly retrogressed micaschists in its footwall and hangingwall, respectively. Late Carboniferous leucogranite dykes, called the Sarzeau granite that intrude the Lower Unit are progressively sheared and mylonitised within the SSZ. The SSZ is characterised by a low to moderately SE-dipping foliation and a NW-SE trending stretching lineation. Kinematic criteria indicate a top-to-the-SE sense of shear. Below the SSZ, NNE-SSW-trending, leucogranitic dykes sometimes present a wall-parallel magmatic layering. These dykes that intrude into vertical NW-SE trending migmatites are interpreted here as emplaced as tension gashes, whose opening direction is consistent with the NW-SE regional stretching. The 316-321 Ma U-Th/Pb ages yielded by the monazite in the dykes comply with the interpretation of a synkinematic magmatism. In the Golfe du Morbihan, geometric relationships between the SSZ and the migmatitic host rocks do not support a previous interpretation as a metamorphic core complex. Regionally, the SSZ kinematics is consistent with the Late Carboniferous orogen-parallel extension, already recognised in other areas of southern Armorica, but does not support the 200 km-long flat detachment fault model.

Syntectonic crustal melting and high-grade metamorphism in a transpressional regime, Variscan Massif Central, France

Hot collisional orogens are characterized by abundant syn-kinematic granitic magmatism that profoundly affects their tectono-thermal evolutions. Voluminous granitic magmas, emplaced between 360 and 270 Ma, played a visibly important role in the evolution of the Variscan Orogen. In the Limousin region (western Massif Central, France), syntectonic granite plutons are spatially associated with major strike–slip shear zones that merge to the northwest with the South Armorican Shear Zone. This region allowed us to assess the role of magmatism in a hot transpressional orogen. Microstructural data and U/Pb zircon and monazite ages from a mylonitic leucogranite indicate synkinematic emplacement in a dextral transpressional shear zone at 313 ± 4 Ma. Leucogranites are coeval with cordierite-bearing migmatitic gneisses and vertical lenses of leucosome in strike–slip shear zones. We interpret U/Pb monazite ages of 315 ± 4 Ma for the gneisses and 316 ± 2 Ma for the leucosomes as the minimum age of high-grade metamorphism and migmatization respectively. These data suggest a spatial and temporal relationship between transpression, crustal melting, rapid exhumation and magma ascent, and cooling of high-grade metamorphic rocks. Some granites emplaced in the strike–slip shear zone are bounded at their roof by low dip normal faults that strike N–S, perpendicular to the E–W trend of the belt. The abundant crustal magmatism provided a low-viscosity zone that enhanced Variscan orogenic collapse during continued transpression, inducing the development of normal faults in the transpression zone and thrust faults at the front of the collapsed orogen.

Progressive shortening axis rotation recorded by Variscan synkinematic granites : example of the South Armorican shear zone in the Vendée (France)

The Variscan continental collision is expressed by large shear zones in Western Europe. The synthesis of granite ages, related to different deformation fields in the Vendée area, suggests a geodynamic model for the tectonic evolution of this part of the Variscan belt between 370 Ma – 320 Ma. After the first step of the continental collision, leading to high temperature and anatexis at 375-360 Ma, the south-eastern part of the Armorican massif underwent large dextral shearing along N110-N125 trending shear zones, related to a bulk NNW-SSE shortening direction, up to early Visean time. Large-scale displacements progressively decreased at around 345-340Ma. During middle Visean time, the shortening axis direction rotated towards a NNE-SSW position implying changes in the regional deformation field. The occurrence of N70-N100 sinistral and N110-N130 dextral conjugate shear zones within leucogranites are related to that epoch. Finally, a new dextral shear zone system, trending N130-N150 along the Parthenay shear zone, occurs during late Visean time. This progressive middle Visean change of shortening direction probably corresponds to a major change in the Iberian plate motion and indentation during the Mississipian collision.

Fammenian–Tournaisian dextral ductile shear in the French Variscan belt

The South Armorican Shear Zone consists of a set of faults that runs across the southern Armorican Massif and extends eastwards to the Massif Central. One of its branches, the Cholet Shear Zone of South Brittany, can be correlated with the North-Millevaches–La Courtine Shear Zone in the Massif Central. It was active immediately after the regional Frasnian anatexis (372–368 Ma) as a right-lateral strike-slip fault. The horizontal offset, which can be estimated between 110 and 170 km, was achieved before the emplacement of non-deformed Late Tournaisian calc-alkaline and peraluminous granites (355–350 Ma). This newly established age of activity (Fammenian–Tournaisian) of the Cholet–La Courtine Shear Zone (CCSZ) has to be taken into account in geodynamical reconstructions of the Variscan belt of western Europe. To cite this article: C. Cartannaz et al., C. R. Geoscience 338 (2006).