Armorican Domain Research Articles

Neogene and Pleistocene geodynamics: the paleoseismic evolution of Armorica (Western France)

The evolution of the passive Armorican margin (Western France) during the Neogene and Quaternary was analyzed using field data. The morphology of the margin attests to a late Hercynian shaping, further deformation during the Mesozoic mid-Atlantic opening, during the Alpine Orogeny, and ultimately, a Late Cenozoic uplift, mostly related to an onshore isostatic accommodation in response to erosion and limited tectonic activity. A very limited strike–slip dynamic, with very low seismicity, accommodated the Neogene–Pleistocene N170 strains around the rigid Armorican terrane. The South Armorican domain and English Channel floor include shear zones that adjusted the Alpine convergence, facilitating its transpressive slip to the west. The Permo-Triassic N150 faults were reactivated during the inversion phases that began after the Bartonian under the distal control of the Alpine convergence and the decrease in the Atlantic spreading rate after 34 Ma. The Armorican marine platforms were stable after the late Eocene and slightly subsident, experiencing pulsed episodes of transient lithospheric doming during the Neogene and Quaternary. Co-seismic activity onshore without surface rupture was recorded around ∼5.3 Ma, ∼3.7 Ma, ∼2.4–1.2 Ma, and ∼400–250 ka, in tandem with an inland exhumation driven by isostatic adjustment due to an intensification of periglacial erosion at the onset of the early interstadials or by agriculture. Low-magnitude and ubiquitous shallow seismic activities seem to be related today to an isostatic uplifted old brittle–ductile transition due to the accumulation of shearing strain.

The Cambrian Atlas – Ossa–Morena – North Armorican Rift, West Gondwana: along- and off-axis stratigraphic and volcano-tectonic patterns

Abstract The Cambrian Atlas – Ossa–Morena – North Armorican Rift extended along West Gondwana from the end of the Pan-African and Cadomian orogenies until the diachronous beginning of drift conditions related to the opening of the Rheic Ocean. The along-axis rift cross-cut the western parts of the Anti-Atlas, High Atlas and Coastal Meseta, which were linked to the Ossa–Morena Zone and the North Armorican Domain, whereas several joint tectonic branches connected with off-axis rift transects of the Central Iberian, West Asturian–Leonese and Cantabrian zones (Iberian Massif), the Central and South Armorican domains, the Occitan Domain, the Pyrenees, and southern Sardinia. The pre-rift unconformity, post-dating the orogenic collapse, is characterized by initial (half-)graben development and subsequent infill, with slope-related breccias and conglomerates controlled by the denudation of surrounding uplands. Synrift pulses show regional extension and are distinctly identifiable on the top of rift shoulders, recording episodes of carbonate production due to their association with karst and hydrothermal processes. The break-up unconformity ranges from volcanic-free angular discordances and paraconformities to generalized uplift and denudation of subaerially exposed areas, associated with the onset of granite-dominant large igneous provinces (LIPs). The Furongian–Tremadocian (Toledanian) and Ordovician (Sardic) phases have been interpreted as due to: (i) Andean-type subduction magmatism reaching the crust in an arc–back-arc setting; (ii) post-collisional decompression melting without significant mantle involvement; and (iii) partial melting of the lower continental crust affected by the underplating of hot mafic magmas linked to superplumes.

Katian volcanism in Central Armorican Domain (France) and Central Iberian Zone (Portugal): a single monogenetic basaltic field?

The Crozon peninsula and Buçaco syncline belong respectively to the Central Armorican Domain and the Central Iberian Zone, located to either side of the Variscan ‘Ibero-Armorican Arc’ structure at present. The Ordovician sedimentary successions at both sites are known to display identical lithofacies. The paired Upper Katian formations also exhibit comparable volcanic successions of anorogenic affinity, including volcaniclastic beds and pillow lavas. The tephra deposits are inferred to have resulted from the activity of subaqueous monogenetic basaltic tuff cones above a <100 m deep calcareous soft substrate, in the context of a passive margin. The Surtseyan-style hydrovolcanic eruptions were followed by effusion of pillow lavas. Both sites also display a large range of similar peperite facies, sometimes related to magmatic segregation structures likely to derive from vapour differentiation. The deeper doleritic dykes and sills, comparable in terms of mineralogy and geochemistry, contain identical metasedimentary xenoliths, whose high-temperature phyllosilicates display a very restricted range of compositions. In the Late Ordovician, Crozon and Buçaco were probably nearby volcanic edifices belonging to a single monogenetic field inside the Armorica microplate. The sudden appearance of high-latitude Katian carbonates, associated with volcanism only at these two peri-Gondwanan sites, just preceded the Gondwanan Hirnantian glaciation. Supplementary material: Analytical data for minerals and whole-rock samples and an M + –4Si–R 2+ diagram are available at https://doi.org/10.6084/m9.figshare.c.6096737

Tl-rich pyrite and Sb-rich melnikovite in Montlouis pyrite veins (central armorican domain, Janze, Ille-et-Vilaine; France)

Lower Ordovician sandstones of the “Grès Armoricain” formation in the Montlouis quarry near Janzé (Ille-et-Vilaine, France) display rare Fe-sulfide veins filled with a low-T (<300°C) assemblage of pyrite, marcasite, “melnikovite”, with trace amounts of galena and sphalerite. Pyrite occur as contorted cm-scale veins of columnar pyrite invading sandstones and as larger idiomorphic crystals coating vugs. It is usually overgrown by fibroradiated marcasite outer layer. Concentrically-layered spheroids of “melnikovite” separate pyrite from marcasite. This mineral sequence is consistent with crystallization from near-neutral, moderately reducing, H 2 S-rich hydrothermal fluids evolving toward acidic conditions. Vug-hosted cm-sized pyrite cubes may be partly covered by a millimeter-thick fine-grained pyrite film. Unlike the other sulfide ores of the Central Armorican domain that carry Sb and Pb indices, trace element analyses identify As, Tl, Sb and Pb anomalies all distributed between Fe-sulfides, i.e. columnar vein pyrite (up to 1.0 wt% As), “melnikovite” (up to 5 wt% Sb), marcasite (up to 7600 ppm Pb) and fibrous pyrite overgrowths (up to 2060 ppm Tl). It is the first time that a Tl anomaly is reported in sulfide ores from the Armorican massif. A potential reservoir for Tl could be the altered dolerite body observed in the quarry.

Variscan lamprophyres of the South Armorican Domain and comparison with lamprophyres of the Western European Variscan belt

Variscan lamprophyres of the South Armorican Domain and comparison with lamprophyres of the Western European Variscan belt

Provenance of Ediacaran-Ordovician sediments of the Medio Armorican Domain, Brittany, West France: Constraints from U/Pb detrital zircon and Sm–Nd isotope data

The temporal evolution of the sedimentary source areas of the Armorican Massif, involving Ediacaran to Upper Ordovician strata, is investigated to gain insight into the palaeogeographic affinities and changes that occurred as a result of Cadomian orogenesis. Until now, palaeogeographic reconstructions based on geodynamic, stratigraphic and paleontological data have shown geological continuity between the Armorican Massif and the Iberian and Bohemian massifs and have allowed researchers to locate the Armorican Massif near the West African Craton and the Trans-Saharan Belt. This study goes beyond the interpretations based on lithostratigraphic correlation, which may be influenced by allocyclic factors (e.g., sea-level change) or fauna assemblages that have a wide provincial distribution, to provide a correct assessment of sediment flux. To determine the palaeogeographic location more accurately, the provenance of the siliciclastic sediments was examined in this study using U–Pb LA-MC-ICP–MS geochronology on detrital zircons coupled with whole-rock Sm–Nd and zircon Lu–Hf isotope analysis. This work was carried out on the sedimentary succession of the Medio Armorican Domain. The oldest studied sedimentary rocks were shown to belong to the Brioverian succession, which contains mainly 519–781 Ma old zircons, likely derived from sources that are still present in the Armorican basement. Successively, the lower Paleozoic succession was deposited in the rift stages of the Rheic Ocean, with contributions from a new source of 827–1120 Ma old zircons.A comparison of the zircon populations showed an increase in negative εNd(t) and εHf(t) values of the sedimentary supply in the post-Cadomian samples. Moreover, it revealed that the Medio and North Armorican domains had different locations during the Lower Ordovician, and that some areas of the Iberian Massif and the Medio Armorican Domain close to the Sahara Metacraton and Arabian-Nubian Shield were contiguous.

Precambrian basement in the Rheic suture zone of the Central European Variscides (Odenwald)

Detrital zircon age spectra of Ediacaran paragneiss from the Rheic suture between the Rhenohercynian Zone and Saxothuringian Zone suggest that they originated from different parts of peri-Gondwana. The paragneiss from the Northern Phyllite Belt displays an age spectrum of detrital zircons with a high amount of Neoproterozoic (82%) and Mesoproterozoic zircons (11%) typical for Amazonian provenance, whereas the spectrum from the metagreywacke of the Odenwald (Mid-German Crystalline Zone) shows a Mesoproterozoic age gap which is correlated with the West African Craton. The metagreywacke of the Odenwald contains 20% Paleoproterozoic and 32% Archean zircons, whereas the paragneiss of the Northern Phyllite Belt (Wartenstein Crystalline) contains only 6% Paleoproterozoic and no Archean zircons. The paleoposition of the basement of Northern Phyllite Belt was proximal to the Avalonian magmatic arc of the London–Brabant high. The Armorican metagreywacke of the Odenwald occupied a distal position to a Neoproterozoic magmatic arc, probably in a back-arc basin related to the West African Craton. Such a U–Pb age spectrum of detrital zircons together with a Mesoproterozoic age gap is typical for sediments of Armorica in Europe during the Ediacaran to Carboniferous. Neoproterozoic igneous rocks extruded at 566 ± 2 Ma forming a volcano-clastic sequence of the Cadomian magmatic arc which is the wall rock of the Silurian to Carboniferous plutons of the entire West Odenwald. This is the first occurrence of an extensive Cadomian crystalline basement in the Mid-German Crystalline Zone. Metagranite dykes crosscut the foliation of the Cadomian para- and orthogneiss at 542 ± 3 Ma. The deformation and migmatization of the Cadomian basement are bracketed between 566 and 542 Ma. A similar late Cadomian event is known from the Bohemian Massif and the Armorican Massif. Large Cadomian plutons with an age around 540 Ma, like that of the northern Odenwald, are common for Armorica. Silurian to Devonian granitoids (434 ± 4 Ma, 411 ± 5 Ma) are witness to an active margin along the northern boundary of Armorica. The Cadomian basement of the Odenwald together with the Palaeozoic granitoids is overprinted by a high-grade metamorphism at 384 ± 4 Ma (U–Pb on zircon) and cooled down below ca. 500 °C at 370 Ma (K–Ar ages of amphibole; U–Pb on titanite). Such a combination of late Cadomian and early Variscan ages could be correlated with the Munchberger Nappe, the Tepla–Barrandian Unit, Central Armorican Domain and the Massif Central.

Pre-Variscan tectonic setting of the south margin of Armorica: Insights from detrital zircon ages distribution and Hf isotopic composition of the St-Georges-sur-Loire Unit (S. Armorican Massif, France)

The Armorican Massif experienced a poly-orogenic evolution, represented by the Neoproterozoic Cadomian and the Devonian-Carboniferous Variscan tectonothermal events. In the southernmost part of the Armorican Domain (or Armorica microcontinent), the St-Georges-sur-Loire Unit consists of a block-in-matrix subunit, and a turbiditic subunit in the South and North, respectively. The St-Georges-sur Loire unit is interpreted as a remnant back-arc basin formed in Silurian-Early Devonian (ca. 440–400 Ma), and was deformed during the Variscan orogeny. Most of the sedimentary and magmatic blocks enclosed in clastic matrix are olistoliths derived from an assumed southern area called the “missing domain”, presently exposed as the Variscan Mauges Nappe composed of the Cadomian rocks and Early Paleozoic sedimentary cover. The “missing domain” was subducted beneath the Armorican massif during the Variscan collision. Detrital zircon age spectra from sandstones and siltstones from the olistostrome matrix reveal Neoproterozoic and Early Paleozoic age clusters, and less abundant Paleoproterozoic and Archean ages. Hf isotopic compositions of detrital zircons indicate dominantly negative εHf (t) values which are in agreement with a recycled continental source. Minor positive values correspond to a juvenile component that was formed during the Neoproterozoic Cadomian orogeny or Archean-Paleoproterozoic events. The Cambrian and Early Ordovician ages can be related to the pre-Variscan rifting of the Pannotia megacontinent that separated the Armorica microcontinent from Gondwana. Neoproterozoic ages imply a source of the magmatic rocks exposed in the Cadomian belt of the northern part of the Armorica microcontinent, and in the Mauges Nappe.

A Sb ± Au mineralizing peak at 360 Ma in the Variscan belt

40Ar/39Ar absolute dating on tobelite (an ammonium-rich white mica) has been performed in order to provide geochronological constraints on the Sb ± Au mineralization and hydrothermalism at the Saint-Aubin-des-Châteaux base metal-Sb ± Au occurrence (Variscan Central Armorican Domain, France). The results show that the Sb ± Au deposition occurred atca.360 Ma. Coupled with recent results obtained in neighboring areas, this occurrence seems to belong to a large-scale Early Carboniferous economic Sb ± Au mineralizing peak in the southeastern part of the Central Armorican Domain. The emplacement of a coeval widespread mafic magmatism in the region appears to represent a major trigger for this mineralizing system at shallow depths (less than 3 km). In the light of these new data, this Early Carboniferous mafic magmatic event must be considered for the overall understanding of the genesis of mineralizing systems at the scale of the whole Variscan belt. Finally, at Saint-Aubin-des-Châteaux, evidence of an Early Permian hydrothermal event is also reported through LA-ICP-MS U-Pb dating of fluorapatite, arguing for the re-use and re-opening of Early Carboniferous mineralizing plumbing system by late (i.e.Permian) fluid flow pulses although no metal mobilization was associated with this event.

Devonian operculate corals (Calceolidae, Cnidaria) from the Massif Armoricain, France

Wright, A.J., Plusquellec, Y. & Gourvennec, R., February 2016. Devonian operculate corals (Calceolidae, Cnidaria) from the Massif Armoricain, France. Alcheringa 40, xxx–xxx. ISSN 0311-5518The operculate coral Calceola gervillei Bayle, 1878 is described for the first time on the basis of the type material from the Cotentin region of Normandy (North Armorican Domain), from Early Devonian (likely upper Lochkovian to lower Pragian) strata, and is chosen as the type species of the monotypic new genus Gerviphyllum. The new genus is also present in the l’Armorique Formation (lower Pragian) of the Plougastel Peninsula (Central Armorican Domain) as Gerviphyllum sp. cf. G. gervillei. One locality in the upper Emsian (Polygnathus serotinus Conodont Zone) Le Fret Formation, on the northern coast of the Crozon Peninsula, has yielded operculate coral specimens described here as ?Chakeola sp., the first (tentative) record of the genus outside eastern Australia, south China and Vietnam. The operculate coral Calceola collini sp. nov. is described from six localities in the early Middle Devonian (Eifelian: Polygnathus costatus Conodont Zone) Saint-Fiacre Formation of the Plougastel and Crozon Peninsulas (Central Armorican Domain), despite the fact that knowledge of the internal characters, especially of the operculum, of the type species Calceola sandalina is very limited. From an extensive review of published references to Calceola from France, we conclude that only the record of Collin (1929) is valid.Anthony Wright (tony.wright@optusnet.com.au), GeoQuEST Research Centre, School of Earth & Environmental Sciences, University of Wollongong, Wollongong, NSW, 2522, Australia; Yves Plusquellec (yves.plusquellec@univ-brest.fr) and Rémy Gourvennec (remy.gourvennec@univ-brest.fr), Université de Bretagne Occidentale, CNRS-UMR 6538 ‘Domaines océaniques’, Laboratoire de Paléontologie, UFR Sciences et Techniques, 6 avenue Le Gorgeu, CS 98837, F-29283, Brest, France. Received 1.10.2015; revised 8.12.2015; accepted 14.12.2015.

A turning-point in the evolution of the Variscan orogen: the ca. 325 Ma regional partial-melting event of the coastal South Armorican domain (South Brittany and Vendée, France)

AbstractBy drastically reducing the bulk strength of crustal materials, partial-melting is one of the main parameter controlling the rheological behaviour of the continental crust. With more than ca. 50% of the outcropping surface characterised by migmatites and granites, the coastal South Armorican domain, offers an opportunity to study deep-orogenic processes and more particularly, to understand the role of partial-melting for the late-evolution of the Variscan belt. To date, time-constraints are scarce hindering the understanding of this crucial stage in the Variscan belt evolution. This paper provides 29 new U-Th/Pb chemical ages on monazite collected over five sampling areas consisting in migmatite domes and late regional classic plutons. Based on structural, textural and chemical criteria, three main U-Th/Pb age-groups are distinguished. The first group, settled at ca. 335–330 Ma concerns samples of restites and core-domains of the monazite crystals for most of the granite massifs. Its significance is ascribed to inherited crystallisation ages probably recording the crossing of prograde monazite forming reactions (i.e. metamorphic isograds) during increasing P-T conditions in an overall nappe-stacking context. The second group that clusters at ca. 325–320 Ma corresponds to newly formed monazite grains that crystallised from juvenile silicate melts. Ages of this group are interpreted as crystallisation ages of leucosomes after a major partial-melting event that affected the whole domain. The last ca. 320 Ma group corresponds to rim-domains of monazite crystals. It is interpreted as the emplacement age of most of the large-scale granite massifs and therefore fixes the end of the partial-melting event.The inception and drastic generalisation of partial-melting at peak-P conditions therefore coincides with a major change in the tectonic regime recorded at regional-scale. In the lights of these results, this implies that (1) either continuous stacking of continental crustal units, rich in radiogenic elements, led to an increase of temperature within the orogenic wedge provoking partial-melting, the resulting drop in the crustal strength inducing collapse and lateral expansion of the belt, or (2) a drastic change of the boundary conditions has induced hot asthenospheric upwelling which in turn led to coeval extension and partial-melting. At a more local scale, strain benefited of the low-strength of the magmatic bodies prior to complete crystallisation promoting intense strain localisation within the South Armorican domain large-scale laccoliths often referred to as synkinematic plutons.

Ediacaran to Cambrian oceanic rocks of the Gondwana margin and their tectonic interpretation

In tectonic maps of Variscan Europe, allochthonous pieces of Cadomian basement clearly stand out with their predominant metabasic to ultrabasic elements, the so-called exotic terranes with ophiolites. Most of these domains are observed in basements of the Central Iberian Allochthone, the South Armorican domain, the nappe structures of the French Massif Central, the Saxothuringian Zone and the Bohemian Massif. Similar relics can be recognized in many Alpine basement areas, and correlations with supposedly more autochthonous basements, such as the Ossa Morena Zone and the Central Iberian basement, can be envisaged. All of these relics are thought to represent the interrupted trace of a former continuous or discontinuous structure, characterized by the presence of ocean-derived proto-Rheic rock suites. These can be interpreted as pieces of former magmatic arcs of Ediacaran to Cambrian age accreted to the Gondwana margin, which later were scattered as allochthonous units during the Variscan plate-tectonic processes. The presence of similar rock suites of Ordovician age in the Alpine realm is explained by the accretion of exotic China-derived basements and their collision with the Gondwana margin during the opening of the Rheic Ocean.

Paramagnetic metamorphic mineral assemblages controlling AMS in low-grade deformed metasediments and the implications with respect to the use of AMS as a strain marker

A regional analysis of the anisotropy of magnetic susceptibility (AMS) has been performed on low-grade metamorphic, deformed homogeneous siltstone beds (HSBs) of the Plougastel Formation in the Central Armorican Domain together with exhaustive compositional analyses of the studied specimens. Despite sampling a single horizon, different paramagnetic minerals are controlling the AMS in the Crozon peninsula sites (white mica and chlorite) and in the inland sites of the Central Armorican Domain (white mica, chloritoid and some chlorite). Both the Crozon peninsula and inland datasets show a hockey-stick shaped pattern on a plot of the shape parameter T versus the corrected degree of anisotropy P J , although the whole pattern is shifted to higher P J values for the inland dataset. High-field AMS indicates that the low-field P J and T values from two inland sites are slightly enhanced by a small ferromagnetic ( sensu lato ) contribution. Furthermore, variation in P J and T values within single sites can be attributed to an effect of the observed quartz/white mica ratio, as quartz grains disrupt the petrofabric intensity. Our findings clearly demonstrate that the AMS of the HSBs in the Central Armorican Domain is strongly influenced by compositional variations and does not merely indicate tectonic strain, even in very similar tectonostratigraphic settings. Supplementary material: Outcrop coordinates with information on the HSBs and samples, and the summarized low-field, room-temperature AMS data are available at www.geolsoc.org.uk/SUP18578 .

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.

Pluton-dyke relationships in a Variscan granitic complex from AMS and gravity modelling. Inception of the extensional tectonics in the South Armorican Domain (France)

The Carnac granitic Complex (South Armorican Domain, Western France) was emplaced during Late Carboniferous times in the deepest Variscan unit, roofed by two major extensional shear zones. Through the acquisition and interpretation of field data, Anisotropy of Magnetic Susceptibility and gravity data, emphasized by petrological and structural observations, we address the emplacement model and possible magmatic processes involved between dyking to massive plutonism in a synkinematic context. Gravity modelling highlights an overall eastward thinning of the pluton, and several deep zones in the western part of the complex, interpreted as the pluton feeder zones. The internal granitic fabric, developed in a sub-solidus state, shows marked planar-linear anisotropy, consistent with a vertical shortening in the WNW-ESE regional stretching regime and eastward magma spreading. This study documents the occurrence of numerous NNE-SSW trending dykes within the eastern part of the pluton, suggesting that this granitic Complex formed by the coalescence of dykes oriented perpendicular to the regional stretching direction, and thus interpreted as large-scale “tension gashes”. The synkinematic character of the Carnac Complex intrusion, recently dated at ca. 319 ± 6 Ma, thus times the inception of the late-orogenic extensional deformation experienced in the whole South Armorican Domain.

Geochronology and geochemistry of Ordovician felsic volcanism in the Southern Armorican Massif (Variscan belt, France): Implications for the breakup of Gondwana

Low-grade, porphyritic felsic metavolcanics make up a significant volume of the South Armorican domain (Variscan belt, France). Their age and geochemistry are the first focus of this paper. These rocks have long been considered to be Silurian in age because they are either thrust over Silurian sediments (the so-called “Porphyroid Nappe”) or inserted within the Silurian sediments, acting as a decollement level (Para-autochthon), and because of a previous zircon age. Five new U–Pb SHRIMP zircon dates for the felsic volcanics from the Porphyroid Nappe and its Para-autochthon define Ordovician ages between 473±5 and 494±4Ma. These Ordovician metavolcanics are mainly of calc-alkaline affinity, probably in response to the reworking of the Ediacaran sediments derived from the Cadomian belt (Brioverian), variably mixed with a Paleoproterozoic (Icartian) component in metatuffs.A second focus of this paper is the integration of this volcanism at the scale of the Variscan belt. Because of its Early Ordovician age and calk-alkaline chemistry, the felsic volcanism from the Porphyroid Nappe is similar to the well-known Ollo de Sapo Formation in the Iberian Massif. Hence, the volcanism allows a better understanding of (i) the Early Ordovician rifting along the northern Gondwana margin, and (ii) the large-scale structure of the Variscan belt from the Iberian Massif to the French Massif Central (i.e. along the Ibero-Armorican Arc).

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.

Deep structural control on distribution of raised beaches and Quaternary sedimentation in Brittany? A review

This paper is a review of the possible influences of deep structures on Quaternary sedimentation and the distribution of raised beaches, a problem which has never been raised before in any paper. The distribution of the Quaternary outcrops is very patchy in Brittany and seems to be organized in a random way, partly because the control by the short wavelength basement structures is not easy to recognize but also because the most efficient control is associated with long wavelength structures which are far beyond the scale of the usual geological maps and the immediate observations. Gravity data and terrain-levelling give far better results than the other geophysical techniques when the Moho undulations are compared with the surficial data. The two main domains involved in the long wavelength deformations are (a) the Alpine domain where all the Moho undulations are concentric with the Alps; and (b) the Armorican domain in which all the undulations are west–northwest–east–southeast oriented, parallel with the axis of the initial opening of the Bay of Biscay. Study of the topography and of the superficial structures associated with these undulations reveals that the undulations delineated in the Alpine Domain result from compression (buckling). These associated with the Armorican Domain result from extension (boudinage). Both phenomena show a typical wavelength of 200–250 km, which is in agreement with the results of the physical and numerical modeling currently available. These two types of deformations are crosscutting in Brittany. The general uplift of Western and Central Brittany is a well-controlled geological phenomenon. This is shown by the incision of the rivers which increases towards the west, suggesting larger uplift in the west than in the east. This uplift has been also demonstrated by two different terrain-levelling techniques. The distribution of the Pleistocene/Holocene sediments is compared with the deep structures. The westernmost Alpine undulation controlled the north–south deposition of the recent loess and swamps in eastern Brittany. The Biscay Bay related structure controlled the deposition of loess along the northern coast of Brittany, and also the location of the Pleistocene raised beaches. The gradient in beach age results from the preferential uplift of the westernmost part of Brittany.

The early land plants from the Armorican Massif: sedimentological and palynological considerations on age and environment

AbstractThe Châteaupanne Unit belongs to the South Armorican domain of the Armorican Massif (France), which is part of the Variscan belt. This unit includes two Lower Devonian plant levels and one of them corresponds to the Basal Member of the Chalonnes Formation. A sedimentological and palaeontological analysis of these fossiliferous deposits from the Châteaupanne quarry (Montjean/Loire, Maine et Loire, France) is presented here for the first time. The age determination based on palynology indicates that the locality records the earliest occurrence of plant megafossils in the Armorican Massif. Their presence suggests an emergence event that has never been described before. Our study highlights the promising potential of the Basal Member of the Chalonnes Formation to aid in understanding these occurrences, and provides new insights into the history of the Variscan belt.

Palaeozoic oolitic ironstone of the French Armorican Massif: a chemical and structural trap for orogenic base metal–As–Sb–Au mineralisation during Hercynian strike-slip deformation

In the Saint-Aubin-des-Châteaux quarry (Armorican Hercynian belt, western France), an epigenetic hydrothermal alteration affects an oolitic ironstone layer intercalated within the Lower Ordovician Gres armoricain Formation. The hydrothermal overprint produced pervasive and massive sulphidation with stratoid pyritised lenticular bodies within the oolitic ironstone layer. These sulphide lenses are spatially associated with strike-slip faults and extend laterally from them. After the massive sulphidation stage (Fe–As, stage 1), subsequent fracturing allowed the deposition of base metals (stage 2) and Pb–Sb–Au (stage 3) parageneses in veins. The dominant brittle structures are vertical extension veins, conjugate shear veins and strike-slip faults of various orders. All these structures are filled with the same paragenetic sequence. Deformation analysis allows the identification of structures that developed incrementally via right-lateral simple shear compatible with bulk strain affecting the Central Armorican Domain. Each increment corresponds to a fracture set filled with specific parageneses. Successive hydrothermal pulses reflect clockwise rotation of the horizontal shortening direction. Geothermometry on chlorite and arsenopyrite shows an input of hot hydrothermal fluids (maximum of 390–350°C) during the main sulphide stage 1. The subsequent stages present a marked temperature drop (300–275°C). Lead isotopes suggest that the lead source is similar for all hydrothermal stages and corresponds to the underlying Neo-Proterozoic basement. Lead isotope data, relative ages of deformation and comparison with neighbouring deposits suggest that large-scale fluid pulses occurred during the whole Hercynian orogeny rather than pulses restricted to the late Hercynian period. The vicinity of the Hercynian internal domain appears as a key control for deformation and fluid flow in the oolitic ironstones, which acted as a chemical and structural trap for the hydrothermal fluids. The epigenetic mineralisation of Saint-Aubin-des-Châteaux appears to be very similar to epigenetic sulphidation described in banded iron formation-hosted gold deposits.