Éditeur(s) : HAL CCSD Elsevier
Résumé : Kyanite and zoisite eclogites from the Limousin area in the Variscan French Massif Central (FMC) have been formed by deep subduction to 100 km depth at peak pressure-temperature conditions around 660 degrees C and 2.9 GPa. They belong to a thin tectonic unit comprising ophiolitic bodies devoid of HP-UHP markers and forming a transported suture zone now separating the two main lithotectonic units of the FMC with contrasted tectono-metamorphic features (namely the lower and upper allochthons). Composition of kyanite-eclogites is similar to supra-subduction-zone basalts found in back-arc basins or island arcs (LREE depleted, negative Nb-Ta anomalies, epsilon Nd: +6 to + 9). Zoisite eclogites have the major-element fingerprint of ultrabasic iron-rich plagioclase cumulates common in continental intrusions or forming Fe-Ti oceanic gabbros. The distribution of immobile trace-elements is however more comparable to alkaline Si-undersaturated rocks, but the Sr-Nd isotopic composition (Sr-87/Sr-86: 0.706; epsilon Nd: -3 to -5) suggests the intervention of an old radiogenic crustal component during their petrogenesis. The chemical composition of hydrous zoisite eclogites does not correspond closely to a common magmatic precursor. It is thus suggested that, by analogy with well exposed worldwide HP-UHP terranes, at least a part of the trace-element signature and the isotopic compositions are controlled by channelled fluid-rock interaction at UHP conditions, leading to localised segregation of zoisite that hosts nearly all Sr and LREE budgets of the bulk eclogite. Sr-Nd isotopic composition of these samples is thus controlled by zoisite and probably represents the isotopic composition of the fluid phase present during UHP recrystallisation. Zircons extracted from a zoisite eclogite were dated in-situ by LA-ICP-MS. Crystallisation of the magmatic precursor is bracketed between 489 and 475 Ma and the UHP event is dated at 412 +/- 10 Ma, in agreement with published ages for the Eo-Variscan HP stage in the FMC. A late resetting of the U-Pb isotopic system at 382 +/- 7 Ma is related to an anatectic high pressure-medium pressure event, well known in the Limousin area. The oceanic eclogites dated in this study are tentatively linked to the Galicia-South Brittany Lower Paleozoic Ocean; they show evidence of the existence of an intra-oceanic subduction zone at the Lower Ordovician and of partial closure of this oceanic domain by ocean-continent subduction during Early Devonian.
ISSN: 0024-4937

hal-00512835
https://hal.archives-ouvertes.fr/hal-00512835
DOI : 10.1016/j.lithos.2010.05.013

Éditeur(s) : HAL CCSD American Geophysical Union (AGU)
Résumé : New structural, microstructural, and 40Ar/39Ar data from the NW Massif Central (France) provide additional constraints on the timing and tectonic setting of late Variscan granite magmatism. Previous studies had emphasized the role of late orogenic extension in the emplacement of granite plutons in the Limousin region. In contrast, the new data set is consistent with syntectonic emplacement of magma in a dextral simple shear active from 350 to 300 Ma in a transpressional regime. As an alternative hypothesis to late orogenic extension, we propose that magmas migrated into tensional bridges between active P shears associated with a lithospheric shear zone comparable to a pop-up structure. The Galician region, in the western end of the Ibero-Armorican tectonic arc, exhibits major left-lateral ductile shear zones which can be interpreted as conjugate structures to the Limousin and Armorican shear zones. Citation: Ge´belin, A., M. Brunel, P. Monie´, M. Faure, and N. Arnaud (2007), Transpressional tectonics and Carboniferous magmatism in the Limousin, Massif Central, France: Structural and 40Ar/39Ar investigations
ISSN: 0278-7407

hal-00104212
https://hal-insu.archives-ouvertes.fr/hal-00104212
https://hal-insu.archives-ouvertes.fr/hal-00104212/document
https://hal-insu.archives-ouvertes.fr/hal-00104212/file/2005TC001822.pdf
DOI : 10.1029/2005TC001822

Éditeur(s) : HAL CCSD American Geophysical Union (AGU)
Résumé : International audience
Deciphering Variscan versus Alpine history in the internal Rif system is a key to constraining the tectonic evolution of the Alboran domain and hence the geodynamics of the western Mediterranean system during the Cenozoic. This study focuses on the evolution of the metamorphic envelope of the Beni Bousera massif and its relation to the underlying peridotites. Combining structural geology, metamorphic petrology, and LA-ICP-MS U-Th-Pb dating of monazite, this study contributes to the understanding of the tectonic history of the western internal Rif. The regional foliation (S2) is characterized by LP/HT mineral assemblages and obliterates a former foliation (S1) developed along a barrovian (MP/MT) metamorphic gradient. The dating of some metamorphic monazite grains from a micaschist and a migmatitic gneiss demonstrates that the crustal envelope of the peridotite recorded two distinct tectonometamorphic episodes. Data from monazite inclusions in S1 garnet suggest that the first event, D1, is older than 250-170 Ma and likely related to the Variscan collision, in agreement with the barrovian type of the metamorphic gradient. The second event, D2, is Alpine in age (at circa 21 Ma) and corresponds to a strong lithosphere thinning allowing subsequent subcontinental mantle exhumation. Such a tectonic context provides an explanation for the LP/HT metamorphic gradient that is recorded in the regional foliation of the western Betic-Rif system. This extension is probably related to a subduction slab roll-back in the western end of the Mediterranean realm during the Oligo-Miocene times. No evidences for a Tertiary HP/LT metamorphism have been identified in the studied area
ISSN: 0278-7407

Droits : info:eu-repo/semantics/OpenAccess
insu-01152517
https://hal-insu.archives-ouvertes.fr/insu-01152517
https://hal-insu.archives-ouvertes.fr/insu-01152517/document
https://hal-insu.archives-ouvertes.fr/insu-01152517/file/gueydan-2015.pdf
DOI : 10.1002/2014TC003769

Éditeur(s) : HAL CCSD Société géologique de France
Résumé : International audience
New geodynamical data from West Africa bring consistent informations on the pre-Mesozoic reconstruction within a large area running from the western Sahara to the Colombian cordillera. These new data support a Neoproterozoic Ocean (WANO) between the Amazonian (AMC) and West African (WAC) cratons previously to the Iapetus and Rheic oceans. We delineate 31 blocs detached from the surrounding three continents: NAC (North American Craton), AMC and WAC. 7 came from the WAC margin, 7 from the NAC, 6 from the AMC and 11 from an intermediate volcano sedimentary domain (COB) built on a 1200-1000 Ma oceanic crust. These imbricated blocks formed a tight mosaic by the Hercynian/Alleghanian tectonic event which gave way to the Pangea super-continent. But, during the Atlantic Ocean opening these blocks began to move. They were separated by new oceanic basins. However, previously to the Pangea, blocks from the COB domain formed two sets of garlands located on the northwestern Gondwana margin. The northern one moved to the North until the Silurian to collide the NAC (Taconic tectonic event) meanwhile the southern one remains on the Gondwana margin. All together were gathered by the Carboniferous/Permian time. Then, the framework for the opening of the Atlantic Ocean was not totally disconnected from the "Variscan" collage and many variscan weakness zones were re-used as initial breaking zones. Beyond this tectonic impact, the pre-mesozoic assemblage allows us to compare this "Caribbean" island arc with another one: the Indonesian "Banda" arc. Thus, West Africa is a geological key area for correlations between the Caribbean, the Appalachian, the Brazilian "Nordeste" and the West European domains and for the understanding of the Atlantic Ocean opening process.
EISSN: 1777-5817

hal-00842747
https://hal.archives-ouvertes.fr/hal-00842747
DOI : 10.2113/gssgfbull.184.1-2.5

Éditeur(s) : HAL CCSD
Résumé : International audience
Tin mineralization in Achmmach deposit (NE of Moroccan Central Massif) is associated with tourmaline-rich alteration halos and faults, within N70 shear zones, hosted in sandstones and metapelites of the Upper Visean-Namurian. Such deposits were reported as late-Variscan in age and related to the emplacement of late orogenic granite not outcropping in the studied area.From field constraints, it appears that the late Variscan phase is marked by a transition from transpression to extension with conditions of deformation evolving from ductile to brittle environments. During the transpressive phase (horizontal shortening direction roughly trending E-W), pervasive tourmaline is deposited under the form of metasomatic halos around fractures in the upper levels relative to the deep batholith. This hydrothermal activity is coeval with ductile shear zones within a large corridors trending N70. Tourmaline-rich alteration appears as a hardening stage contributing to the modification of the tectonic style of rocks. Thereafter, a strong brecciation occurs in response of re-using and re-opening of the strike-slip structure. The result is (1) the formation of dextral (N070, N020) and sinistral (N120) shears, and (2) the circulation of fluids within early tourmalinites and formation of tourmaline-rich breccia. Subsequently, during the late orogenic extensional phase, tourmalinite breccias reopened as normal faults acting as overpressure fluid pathways for developing other breccias and veins. Such event is marked by deposition of tin mineralization i.e., cassiterite followed by sulphides, and then fluorite and carbonates at the end.From analysis of the shear-zone structure and superimposition of tourmalines and tin mineralization in the same structures, it is possible to suggest a structural control continuum. Such control is well expressed during metasomatism and deposition of hydrothermal tourmaline under the transpressive phase. Thereafter, under the extensive phase allowing open-space, tin-mineralization were trapped.This study has been supported by AI MA/09/210
24ème RST
Pau, France

insu-01546927
https://hal-insu.archives-ouvertes.fr/insu-01546927

Éditeur(s) : HAL CCSD Springer Verlag
Résumé : International audience
For the first time, an albite orthogneiss has been recognised and dated within the HP-LT blueschist facies metabasites and metapelites of the Ile de Groix. It is characterised by a peraluminous composition, high LILE, Th and U contents, MORB-like HREE abundances and moderate Nb and Y values. A U-Pb age of 480.8 ± 4.8 Ma was obtained by LA-ICP-MS dating of zircon and titanite. It is interpreted as the age of the magmatic emplacement during the Early Ordovician. Morphologically different zircon grains yield late Neoproterozoic ages of 546.6-647.4 Ma. Zircon and titanite U-Pb ages indicate that the felsic magmatism from the Ile de Groix is contemporaneous with the acid, pre-orogenic magmatism widely recognised in the internal zones of the Variscan belt, related to the Cambro-Ordovician continental rifting. The magmatic protolith probably inherited a specific chemical composition from a combination of orogenic, back-arc and anorogenic signatures because of partial melting of the Cadomian basement during magma emplacement. Besides, the late Devonian U-Pb age of 366 ± 33 Ma obtained for titanite from a blueschist facies metapelite corresponds to the age of the HP-LT peak metamorphism.
ISSN: 1437-3254

insu-00714570
https://hal-insu.archives-ouvertes.fr/insu-00714570
DOI : 10.1007/s00531-011-0732-5

Éditeur(s) : HAL CCSD
Résumé : International audience
The late stages of the Variscan orogeny in western and central Europe are characterized by the emplacement of numerous Carboniferous granitic intrusions. In the southern part of the Armorican massif, volcanic and plutonic rocks occur within the Carboniferous series of the Ancenis basin. Chemical analyses indicate that they belong to the same magmatic high-K calc-alkaline association of peraluminous composition, which likely derives from crustal melting of an igneous source. A LA-ICPMS U-Pb/zircon age of 319.3 ± 3.1 Ma dates, for the first time, this magmatic event and, by there, constrains the maximum age of both the end of the sedimentation and the northward tilting of the sediments in the Ancenis basin. This magmatic event is coeval with the sedimentation of the coal-bearing Namurian basins located along the Nort-sur-Erdre fault. These intrusions are interpreted as the feeding conduits of the volcano-sedimentary layers encountered in these basins. This would imply a minimal dextral displacement of ca. 20-25 km along the Nort-sur-Erdre fault.
Bulletin de la Socciété Géologique de France

insu-00656606
https://hal-insu.archives-ouvertes.fr/insu-00656606

Éditeur(s) : HAL CCSD Geological Society
Résumé : International audience
Deformation mechanisms, long-term kinematics and evolution of fold and thrust belts subjected to erosion are studied through 2D analogue experiments involving large convergence. First-order parameters tested include (1) décollements and/or plastic layers interbedded at different locations within analogue materials and (2) synconvergence surface erosion. Weak layers, depending on their location in the model, favour deformation partitioning characterized by the simultaneous development of underplating domains in the inner part of the wedge (basal accretion) and frontal accretion where the wedge grows forward. Interaction between tectonics and surface processes influences this behaviour. Development of antiformal thrust stacks controlled by underplating shows small- and large-scale cyclicity. Thin plastic layers induce folding processes, which are studied at wedge scale. Recumbent and overturned folds, with large inverted limbs, develop in a shear-induced asymmetric deformation regime via progressive unrolling of synclinal hinges. Surface erosion and underplating at depth induce further rotation (passive tilting) and horizontalization of fold limbs. Model results give insights to discuss the mechanisms responsible for the large-scale structures (i.e. antiformal nappe stacks, klippen and kilometre-scale recumbent fold-nappes) encountered in several mountain belts such as the Montagne Noire (French Massif Central), the Galicia Variscan belt (Spain) and the northern Apennines (Italy).
ISSN: 0016-7649

hal-00937644
https://hal.archives-ouvertes.fr/hal-00937644
DOI : 10.1144/jgs2013-012

Éditeur(s) : HAL CCSD
Résumé : International audience
In the external Western Alps, the collision started with the shortening of the proximal European (Dauphinois) margin. One peculiar feature is the crustal thickening that finally led to the exhumation of the basement External Crystalline Massifs (ECM) during the Neogene. In these massifs, various age of shortening, structural style and sequence of deformation have been proposed leading to different kinematics along the Alpine arc that in turn do not imply the same crustal/lithospheric strength. In this contribution, we present new structural and microstructural data along with Ar/Ar ages from field studies in the Oisans-Grandes Rousses massifs, between Grenoble and Briançon). From balanced cross-sections, we show that about half of the external zone crustal shortening is accommodated within the ECM with similar Ar/Ar ages in all massifs. Furthermore, the shear zones present very large thicknesses (several hundred meters) and do not reactivated either the inherited Liassic normal faults or the Variscan foliation. Finally, the main shortening structures are localized beneath inherited Liassic syn-rift basins. These results suggest that the crust was weak during its shortening. Such behavior is most likely due to the combination of several factors: the crust burial (below the overthrusting internal units) down to about 10km, the presence of weak inherited syn-rift basins, and the strain softening in shear zones that evolve in mica-rich phyllonites. These results are consistent with the studies in the Aar massif and should most likely apply to the Mont Blanc massif, whose relationships with its cover are still debated.
EGU General Assembly
Vienne, Austria

insu-00839426
https://hal-insu.archives-ouvertes.fr/insu-00839426
BIBCODE : 2013EGUGA..15.8837B

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Structural controls are critical during magmatic-to-hydrothermal transition in the formation of intrusion-related gold deposits. They may explain why only some parts of intrusions are mineralized and why only very few intrusions host large deposits. Moreover, most of these gold deposits postdate peak regional metamorphism and were formed in zones of relatively low strain. Indeed, the efficiency of structural gold traps is highest along favourably oriented intrusion/host-rock contacts where mechanical instability maintains high permeability in the cracked thermal aureole. This may be reinforced by melt injections from an underlying root zone. We present a structural analysis of the deformation features of a granite-pluton roof; large-scale dykes and a network of gold veins are intensively developed in this roof, recording a succession of mechanical instabilities. Our gravity survey underlines the presence of a pluton feeder zone located just beneath the mineralized network. It is argued that interferences between regional stress and melt injection in the feeder zone favoured the development of the network by strain located close to the granite roof. This stresses the role of mechanical instabilities triggered by the combined effects of regional stress and melt dynamics in determining the location and size of this type of gold deposit.
ISSN: 0191-8141

insu-00914780
https://hal-insu.archives-ouvertes.fr/insu-00914780
https://hal-insu.archives-ouvertes.fr/insu-00914780/document
https://hal-insu.archives-ouvertes.fr/insu-00914780/file/manuscript.pdf
DOI : 10.1016/j.jsg.2013.11.006

Éditeur(s) : HAL CCSD Springer Verlag
Résumé : International audience
Unravelling the detailed pressure–temperature–time-deformation (P–T–t-D) evolution of magmatic and metamorphic rocks provides essential insights into the timing and duration of partial melting and related plutonism during crustal flow and migmatitic dome formation. The Montagne Noire Axial Zone (MNAZ) is a migmatitic dome located within the Variscan orogen in the southern French Massif Central. The timing of the main thermal event that was responsible for intense partial melting is still highly debated. In this study we present new laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) age data on micaschists, migmatites and granites that clarify the P–T–t-D evolution of the MNAZ. Structurally controlled samples were collected in order to constrain the timing of metamorphism, migmatization and plutonism regarding the main structural pattern D1, D2 and D3. D1 and D2 correspond to nappe stacking and dextral transpression, respectively. D3 is related to vertical shortening and coaxial thinning with a preferential NE–SW- to E–W-directed stretching. LA-ICP-MS analyses on the syntectonic Anglès, Soulié and Martys granites yielded U–Th/Pb monazite ages of 305 ± 1.5, 306 ± 1.9 and 314 ± 2 Ma, respectively. Five migmatitic rocks sampled in the eastern and central Espinouse area yielded in situ ages ranging between 312 ± 2 and 301 ± 2 Ma. Along the dome envelope, two garnet–staurolite-bearing micaschists near Saint-Pons-de-Thomières village gave in situ U–Th–Pb ages of 312.1 ± 2.1 and 309.0 ± 3.1 Ma. A fine-grained gneiss with a D3 fabrics in the eastern dome envelope yield a 208Pb/232Th mean age at 305.7 ± 3.9 Ma. All ages obtained in this study for the micaschists, migmatites and granites range between 315 and 301 Ma. We interpret this time span as the record of the high thermal event responsible for intense crustal partial melting within the lower and middle crust. The onset of partial melting occurred at ca. 315 Ma that marked the beginning of transpressional deformation D2. Based on structural and petrological studies, our new U–Th–Pb results suggest that (1) partial melting may have started at ca. 315 Ma and lasted 15–10 Myr and (2) D2 et D3 developed between 315 and 300 Ma and were synchronous. D1 deformation ended at 315 Ma. The onset and duration of D1 related to nappe stacking and crustal thickening is still uncertain.
ISSN: 1437-3254

hal-01526942
https://hal.archives-ouvertes.fr/hal-01526942
DOI : 10.1007/s00531-016-1417-x

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
This article summarizes the geodynamic evolution of the Variscan to Mesozoic Tethyan subduction history, based on a review of geochronological data from Eastern Anatolia and the Lesser Caucasus, and new isotopic ages for the Georgian crystalline basements. The geological history of the basements of Georgia (Transcaucasus) and NE Turkey (eastern Pontides) appears to be similar and provides evidence for a continuously active continental margin above a north-dipping subduction since at least the Lower Jurassic. New La-ICPMS U-Pb ages from the Georgian basement provide further evidence for the derivation of the Transcaucasus and its western continuation (the eastern Pontides) from Gondwana. A migmatized granodiorite provides preserved magmatic zircon cores with an age of 474 ± 3 Ma, while the age of migmatization is constrained by its 343 ± 2 Ma metamorphic rims. Metamorphism is synchronous with widespread I-type granites that were emplaced at 335 ± 8 Ma in the neighbouring Dzirula massif, and in the eastern Pontides. These U-Pb ages are in close agreement with recently obtained Ar/Ar ages from biotites and muscovites from metamorphic schists and U-Pb ages ranging from 340 to 330 Ma in the Georgian basement. The narrow range of ages suggests that the Variscan LP-HT metamorphic event in the eastern Pontides and Georgia was of short duration and likely related to mantle-derived intrusives. Furthermore, we suggest that (1) rifting of the Pontides-Transcaucasus block (PTB) from Gondwana at 450–350 Ma could have been driven by roll-back of the south-dipping Rheic slab, (2) that the main metamorphic and coeval magmatic events are related to the accretion of the PTB to the Eurasian margin at c. 350 Ma, while the source of magmatism is ascribed to slab detachment of the south-dipping slab at 340 Ma and that (3) three subduction zones may have been contemporaneously active in the Tethyan domain during the Jurassic: (i) the Lesser Caucasus South Armenian Block (SAB) shares a similar Gondwana affinity, but bears younger ages for its MP-MT metamorphic evolution and calc-alkaline magmatism bracketed from 160 to 123 Ma; (ii) north-dipping subduction below the PTB from c. 210 Ma to 150 Ma; (iii) northward intra-oceanic subduction bracketed from 180 to 90 Ma between the SAB and the PTB.
ISSN: 0264-3707

hal-01347761
https://hal.archives-ouvertes.fr/hal-01347761
DOI : 10.1016/j.jog.2016.03.003

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
In many orogens, high-pressure (HP) metamorphic rocks such as eclogite occur as lenses in quartzofeldspathic gneiss that equilibrated at much lower pressures. The pressure–temperature–time (P–T–t) history of eclogite relative to host gneiss provides information about mechanisms and timescales of exhumation of orogenic crust. The Montagne Noire of the southern Massif Central, France, is an eclogite–bearing gneiss (migmatite) dome located at the orogen-foreland transition of the Variscan belt. Results of our study show that it contains the youngest eclogite in the orogen, similar in age to migmatite and granite that crystallized under low-pressure conditions. P–T conditions for an exceptionally unaltered eclogite from the central Montagne Noire were estimated using a pseudosection supplemented by garnet–clinopyroxene and Zr-in-rutile thermometry. Results indicate peak P∼1.4 GPaP∼1.4 GPa and View the MathML sourceT∼725°C for Mg-rich garnet rim (50 mol% pyrope) + omphacite (36 mol% jadeite) + rutile + quartz. U–Pb geochronology (LA-ICP-MS) of 16 zoned zircon grains yielded ∼360 Ma (4 cores) and ∼315 Ma (12 rims and cores). Rare earth element abundances determined by LA-ICP-MS for dated zircon are consistent with crystallization of ∼315 Ma zircon under garnet-stable, plagioclase-unstable conditions that we interpret to indicate high pressure; in contrast, the ∼360 Ma zircon core corresponds to crystallization under lower pressure plagioclase-stable conditions. Based on garnet zoning and inclusion suites, rutile textures and Zr zoning, P–T results, and zircon petrochronology, we interpret the ∼315 Ma date as the age of eclogite-facies metamorphism that only slightly preceded dome formation and crystallization at 315–300 Ma. This age relation indicates that eclogite formation at high pressure and migmatite dome emplacement at low pressure were closely spaced in time. We propose that collapse-driven material transfer from the hot orogen to the cool foreland resulted in thickening of the orogen edge, leading to eclogite facies metamorphism of the deep crust. Soon after, the low-viscosity partially molten crust flowed from the plateau toward the foreland, incorporating and exhuming eclogite. The P–T–t history of the Montagne Noire eclogite shows that some dome material ascended from >40 km depth to shallow crustal levels, likely in a single decompression event, and that migmatite domes are therefore very efficient at exhuming the deep crust.
ISSN: 0012-821X

hal-01243499
https://hal.archives-ouvertes.fr/hal-01243499
DOI : 10.1016/j.epsl.2015.08.026

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
A new portable sampling system was developed to extract Radium isotopes from large volumes (up to 300 L) of fresh surface- and ground-waters of low Ra-activities (<5 mBq/L). Ra is quantitatively adsorbed on a small amount (6.5 g) of MnO2-coated acrylic fibers, which are then dried and burned at 600 °C in the laboratory. The resulting Mn-oxide powder (about 2 cm3 when compacted) is then analyzed through gamma-ray spectrometry which allows measurement of the whole Ra quartet (226Ra, 228Ra, 224Ra and 223Ra) in a single counting of a few days. The usual relative standard combined uncertainties (1σ) are 2–3% for 226Ra, 228Ra and 224Ra; and less than 10% for 223Ra. This method was applied to the analysis of Ra in karstic waters of the Lez aquifer, and surface- and ground-waters of the upper and middle Vidourle watershed (South of France). The analyzed waters have relatively low 226Ra activities (1–4 mBq/L) in both cases, regardless of the contrasted geology (Mesozoic limestone vs crystalline Variscan basement), but clearly distinct (228Ra/226Ra) ratios in agreement with the differences in Th/U ratios of the two drained areas. Short-lived Ra isotopes (224Ra and 223Ra) appear to be mainly influenced by near-surface desorption/recoil processes for most of the sampling sites.
ISSN: 0969-8043

hal-01491281
https://hal.archives-ouvertes.fr/hal-01491281
DOI : 10.1016/j.apradiso.2016.12.010

Éditeur(s) : HAL CCSD Springer Verlag
Résumé : International audience
During the development of the Variscan orogeny, large amounts of granitic melt were produced, giving rise to the intrusion of granitoids at different structural levels. Despite numerous studies, ages available from previous work on the Cevennes granites remain largely imprecise. In order to better constrain the age and emplacement mode of these granites, we have combined U-Pb dating on monazites and zircons and Ar-40/Ar-39 dating on biotites with petrological observations, major element chemical analysis and SEM zircon imaging on five samples from the Aigoual-St Guiral-Liron and Mont Lozere granitic massifs. The results revealed that granitic intrusions and cooling in Southern Cevennes occurred in a short time span at similar to 306 Ma after the main episode of regional metamorphism. Petrological and chemical data suggest that they result from a mixing between mantle-derived basic magmas (lamprophyres) and lower crust acid magmas. At a regional scale the production of these melts occurred at the end of crustal thickening induced by nappe stacking, at the same time as the late anatectic events recorded further north in the Velay dome and the granulite facies metamorphism recorded in metasedimentary granulite enclaves brought up by Tertiary volcanoes of the Velay area (Bournac).
ISSN: 1437-3254

hal-00412183
https://hal.archives-ouvertes.fr/hal-00412183
DOI : 10.1007/s00531-007-0187-x

Éditeur(s) : HAL CCSD American Geophysical Union (AGU)
Résumé : International audience
We decipher late-orogenic crustal flow characterized by feedback relations between partialmelting and deformation in the Variscan Montagne Noire gneiss dome. The dome shape and finite strainpattern of the Montagne Noire Axial Zone (MNAZ) result from the superimposition of three deformations(D1, D2 and D3). The early flat-lying S1 foliation is folded by D2 upright ENE-WSW folds and transposed in thecentral and southern part of the MNAZ into steep D2 high-strain zones consistent with D2 NW-SE horizontalshortening, in bulk contractional coaxial deformation regime that progressively evolved to noncoaxial dextraltranspression. The D2 event occurred under metamorphic conditions that culminated at 0.65 ± 0.05 GPa and 720± 20°C. Along the anatectic front S1 and S2 foliations are transposed into a flat-lying S3 foliation with top-to-NEand top-to-SW shearing in the NE and SW dome terminations, respectively. These structures define a D3 transitionzone related to vertical shortening during coaxial thinning with a preferential NE-SW to E-W directedstretching. Depending on structural level, the metamorphic conditions associated with D3 deformation rangefrom partial melting conditions in the dome core to subsolidus conditions above the D3 transition zone. Wesuggest that D2 and D3 deformation events were active at the same time and resulted from strain partitioningon both sides of the anatectic front that may correspond to a major rheological boundary within the crust.
ISSN: 0278-7407

hal-01208098
https://hal.archives-ouvertes.fr/hal-01208098

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The detailed morphology and internal structure of the Calabrian accretionary wedge and adjacent Eastern Sicily margin are imaged in unprecedented detail by a combined dataset of multi-beam bathymetry and high-resolution seismic profiles. The bathymetric data represent the results of 6 recent marine geophysical surveys since 2010 as well as a compilation of earlier surveys presented as a 2 arc-sec (60 m) grid. Several distinct morpho-tectonic provinces are identified including: the deeply incised Malta–Hyblean Escarpment, numerous submarine canyons, broad regions of relatively flat seafloor dominated by fields of sediment waves, the gently undulating anticlinal fold-and-thrust belts of the external Calabrian accretionary wedge and the adjacent portion of the Western Mediterranean Ridge. The Calabrian arc can be divided into 4 domains (from SE to NW): 1) the undeformed Ionian abyssal plain, 2) the external evaporitic wedge, 3) the internal clastic wedge, 4) the Calabrian backstop (Variscan crystalline basement). The Calabrian accretionary wedge can also be divided laterally into two major lobes, the NE- and the SW lobes, and two minor lobes. The kinematics of the limit between the two major lobes is investigated and shown to be sinistral in the external (evaporitic) wedge. A network of radial slip lines within the southernmost external wedge unequivocally demonstrate ongoing dextral displacement of a rigid indenter (representing the corner of the clastic wedge) into the evaporitic wedge thereby confirming the geodynamic model of an active lateral slab tear fault here off eastern Sicily. The slab tear produces a series of major sub-parallel dextral strike-slip faults offshore Mt. Etna and south of the Straits of Messina consistent with the relative motions between Calabria and the Peloritan domain (NE Sicily). Abundant strike-slip faulting, and wide-spread folding and thrusting observed throughout the entire accretionary wedge, indicate regional shortening between the Ionian abyssal plain (foreland) and the Calabrian–Peloritan backstop caused by active subduction.
ISSN: 0012-821X

hal-01523990
https://hal.archives-ouvertes.fr/hal-01523990
DOI : 10.1016/j.epsl.2016.12.020

Éditeur(s) : HAL CCSD Springer Verlag
Résumé : International audience
Geochronological, mineralogical, and geochemical analyses have been focussed on the Mont-Loze're- Borne plutonic complex and surrounding rocks (Ce'vennes, French Massif Central) in which B-W-Sn and As-Au-Sb mineralization is encountered. Two main results are highlighted: (1) the existence of a 301-306 Ma magmatohydrothermal event unrelated to the emplacement of the Pont-de-Montvert-Borne plutonic body at 316 Ma; (2) the magmatic and hydrothermal features are strongly associated, both in time and in space, thus demonstrating an intimate connection between mineralizing processes and magmatism in this part of the French Massif Central. We also show that mineralization and associated hydrothermal occurrences do not correspond to a simple and single geochemical signature and that a contamination model must be invoked in order to account for the complexity of isotopic results. This study demonstrates that the application of the O and H isotopic signatures as tracers of the source and nature of fluids in an orogenic context requires some specific care. Finally, a model of the tectonic-magmatic- hydrothermal evolution of the study area is suggested in which we discuss two alternative scenarios. The first one implies the existence of two different hydrothermal/ mineralizing events (Bo-W-Sn and As-Au-Sb ones). The second one suggests the same source for all hydrothermal and mineralized structures.
ISSN: 1437-3254

insu-00576546
https://hal-insu.archives-ouvertes.fr/insu-00576546
https://hal-insu.archives-ouvertes.fr/insu-00576546/document
https://hal-insu.archives-ouvertes.fr/insu-00576546/file/Chauvet-InternationalJournalEarthSciences-2012.pdf
DOI : 10.1007/s00531-011-0639-1

Éditeur(s) : HAL CCSD Geological Society of London
Résumé : International audience
The sediments of the Mauges Unit located in the internal zone provide an opportunity of studying the evolution of relief during Palaeozoic time. U-Pb dating on zircon and 39Ar/40Ar on white mica are used to constrain the age and nature of the sources. The first relief identified is marked by an Early Devonian unconformity interpreted as the opening of a northern back-arc basin. Detrital minerals are first reworked from underlying layers indicating a local supply. Magmatic zircons at c. 400 Ma then record the emergence of a magmatic arc. During the Middle Devonian, the gap in the sedimentary record is attributed to an emersion followed by the disappearance of the relief during the Late Devonian. At the Devonian-Carboniferous boundary, the main collision is followed by the onset of a relief. The continental sedimentation in the Ancenis Basin (late Tournaisian-Viséan) is a coarsening-upwards megasequence indicating an increasing and/or approaching relief. The detrital minerals record the progressive exhumation of Variscan metamorphic (mica at c. 350 Ma) and magmatic rocks (zircons at c. 390-340 Ma). The Serpukhovian-Bashkirian sedimentation records the erosion of a proximal metamorphic source (Champtoceaux with micas at c. 350-340 Ma) showing a much shorter drainage system
The Variscan Orogeny: Extent, Timescale and the Formation of the European Crust

ISBN : 978-1-86239-658-6
insu-01016985
https://hal-insu.archives-ouvertes.fr/insu-01016985
DOI : 10.1144/SP405.6

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Intraplate deformation is most often linked to major stress applied on plate margins. When such intraplate events are accompanied by magmatism, the use of several dating methods integrated within a multidisciplinary approach can bring constraints on the age, nature and source mobilized for generating the magma and in turn on the nature of the intraplate deformation. This study focuses on the large gabbro Arrikine sill (35 km in extension) emplaced within the Silurian sediments of the western margin of the Murzuq cratonic basin in southeastern Algeria. Its emplacement is dated during the early Devonian (415-400 Ma) through the determination of a reliable paleomagnetic pole by comparison with the Gondwana Apparent Polar Wander Path (APWP). This age can be correlated with deep phreatic eruptions before Pragian time thought to be at the origin of sand injections and associated circular structures in Algeria and Libya. For the sill, the K–Ar age of 325.6 ± 7.7 Ma is related to a K-rich aplitic phase that has K-enriched by more than 20% the Devonian gabbro. Laser-ICP-MS U–Pb method dates only inherited zircons mostly at c. 2030 Ma with additional ages at c. 2700 Ma and younger ones in the 766-598 Ma age range. The Arrikine sill is a high-Ti alkaline gabbro having the geochemical composition of a hawaiite akin to several intraplate continental and oceanic provinces, including the contemporaneous Aïr ring complexes province in Niger, but also to the Mauna Loa volcano in Hawaii. This peculiar composition akin to that of the contemporaneous Aïr province is in agreement with a lower Devonian age for the Arrikine sill.The lower Devonian Arrikine sill emplacement is related to a “Caledonian” transtensive reactivation of the western metacratonic boundary of the Murzuq craton. This event also generated in the Saharan platform the so-called “Caledonian unconformity” of regional extension, the Aïr ring complexes and magmatic rocks that produced sand injections. It could be related to rifting of the Hun terranes that occurred at the plate margin to the north (Stampfli and Borel, 2002, Blackey, 2008 and references therein). The mid-Carboniferous (c. 326 Ma) reactivation corresponds to Variscan compression on NW Africa generating aplitic fluids, but also to the major “Hercynian unconformity” of regional extension. The generation of the Arrikine magma is attributed to partial melting through adiabatic pressure release of uprising asthenosphere along tectonically reactivated mega-shear zones, here bordering the relictual Murzuq craton enclosed in the Saharan metacraton.
ISSN: 0899-5362

hal-01306637
https://hal.archives-ouvertes.fr/hal-01306637
DOI : 10.1016/j.jafrearsci.2015.11.019