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Early Paleozoic U—Pb age of zircons from metagranophyres of the Grand-Saint-Bernard Nappe (zona interna, Aosta Valley, Italy). The U—Pb age of a metagranophyre from the Val de Rhemes (Brianconnais basement of the Aosta valley) of 511 ± 9 Ma suggests that the micaschists which dominate the pre-alpine basement of the Grand-Saint-Bernard Nappe are, in part, of Early Paleozoic age. The Brianconnais domain is characterized by a Cambrian to Ordovician alkaline to subalkaline (monzonitic) magmatism, similar in age to orthogneisses known in other parts of the Variscan belt and generally interpreted as related to an extensional event.

U-Pb ages obtained from zircons of granitoids from the Proterozoic western Anti-Atlas confirm the existence of Palaeoproterozoic magmatism in the inliers of Igherm (Ait Makhlouf granite: 2 050±6 Ma) and Bas Drâa (Sidi Said granite: 1 987±20 Ma) and neoproterozoic magmatism in the inliers of Bas Drâa (Taourgha granite: 575±4 Ma) and Kerdous (Tarcouate granodiorite: 583±11 Ma, Tarcouate gabbro-diorite: 560±2 Ma). The emplacement of these granitoids is therefore polycyclic: Eburnian and Panafrican.

The zircon and monazite U-Pb minimum age of the Hercynian peraluminous two-mica composite granite pluton of Cabeceiras de Basto (Northern Portugal) is 311±1 Ma (2 σ). This age, well constrained by a concordant monazite of 311±4 Ma (2 σ), is in good agreement with the age suggested for the synkynematic, syn-D3, Iberian granites of the Hercynian orogeny (315–305; Pinto et al., 1987). The ( 87 Sr/ 86 sr), and ɛ Nd isotopic ratios, calculated for 311 Ma, suggest that the three granite units which constitute the massif may derive from an heterogeneous crustal source dated (upper intercept of the discordia) around 1 200 Ma.

Resume Un metagranite de Vanoise meridionale (domaine brianconnais) a ete date par la methode U-Pb sur zircon a 479 ± 4 Ma. Cet âge confirme que les micaschistes qui constituent l'essentiel de ce socle ante-alpin datent, au minimum, du Paleozoique inferieur. Les rares composants magmatiques de Vanoise, de tendance alcaline, sont donc a rapprocher des orthogneiss appartenant a l'episode d'extension cambro-ordovicienne, connu dans d'autres parties de la chaine varisque.

International audience; Located at the internal border of the Grand-Saint-Bernard Zone, the diorite and its aureole lie on top of intensively studied Alpine eclogitic units but this pluton, poorly studied yet, has kept locally almost undeformed. The pluton intruded, at ~ 360 Ma, country-rocks mostly composed of dark shales with Na2O > K2O and minor mafic intercalations of tholeiitic basalt affinity. This association is characteristic of the Vanoise (France) basement series, where available age determinations suggest an Early Paleozoic age. Parts of the pluton, and of its hornfels aureole that is evidenced here for the first time, in the Punta Bioula section of Valsavaranche valley (NW-Italy), have been well-preserved from the Alpine deformation. Syn-emplacement hardening, dehydration-induced, probably prevented strain-enhanced Alpine recrystallization. Magmatic rock-types range continuously from subordinate mafic types at SiO2 ~ 48%, of hornblendite with cumulative or appinite affinities, to the main body of quartz diorite to quartz monzonite (SiO2 up to 62%). P-T estimates for the pluton emplacement, based on the abundance of garnet in the hornfelses, using also zircon and apatite saturation thermometry and Al-in-hornblende barometry, suggest T ~ 800-950 °C and minimum P in the 0.2-0.5 GPa range, with records of higher pressure conditions (up to 1-2 GPa?) in hornblendite phlogopite-cored amphibole. The high-K, Na > K, calc-alkaline geochemistry is in line with a destructive plate-margin setting. Based on major element data and radiogenic isotope signature (εNd360 Ma from − 1.2 to + 0.9, 87Sr/86Sr360 Ma from 0.7054 to 0.7063), the parental magmas are interpreted in terms of deep-seated metabasaltic partial melts with limited contamination from shallower sources, the low radiogenic Nd-content excluding a major contribution from Vanoise tholeiites. There is no other preserved evidence for Variscan magmatism of similar age and composition in the Western Alps, but probable analogs are known in the western and northern parts of French Massif Central. Regarding the Alpine tectonics, not only the age of the pluton and its host-rocks (instead of the Permo-Carboniferous age previously believed), but also its upper mylonitic contact, suggest revisions of the Alpine nappe model. The Cogne diorite allegedly constituted the axial part of the E-verging "pli en retour [backfold] du Valsavaranche", a cornerstone of popular Alpine structural models: in fact, the alleged fold limbs, as attested here by field and geochemical data, do not belong to the same unit, and the backfold hypothesis is unfounded.

The Early Proterozoic Torngat Orogen resulted from the oblique collision of the Archean Nain and southeastern Rae provinces and evolved in four stages: (0) deposition of platformal supracrustal assemblages followed by subduction-related arc magmatism in the margin of the Rae Province; (I) crustal thickening and nappe tectonics; (II) sinistral transpression and formation of the Abloviak shear zone; (III) uplift on steeply dipping, east-verging mylonites along the eastern orogenic front.U–Pb geochronology on zircon and monazite from major rock units and syntectonic intrusions indicates that arc magmatism at ca. 1880 Ma was followed by 40 Ma. of deformation and high-grade metamorphism from ca. 1860 to 1820. Subsequent uplift and final cooling occurred ca. 1795 – 1770 Ma. Several ages of mineral growth that correspond to distinct structural and metamorphic events have been recognized: (1) 1858 – 1853 Ma zircon and monazite dates are interpreted as the minimum age of stage I and peak metamorphic conditions; (2) 1844 Ma zircons from anatectic granitoids in the Tasiuyak gneiss complex (TGC), syntectonic with stage II deformation, are interpreted to date the formation of the Abloviak shear zone; (3) 1837 Ma magmatic zircons from an intrusive granite vein deformed along the western contact of the TGC represent a discrete intrusive event; (4) 1825 – 1822 Ma metamorphic overgrowths and newly grown zircons in granitic veins from the western portion of the orogen (Lac Lomier complex) represent a period of renewed transpressional deformation; (5) 1806 Ma magmatic zircons from a post-stage II granite emplaced along the eastern edge of the Abloviak shear zone defines the transition between stage II and stage III events; (6) 1794 – 1773 Ma zircons from leucogranites and pegmatites that are associated with uplift of the orogen (stage III). 1780 – 1740 Ma dates for monazite and a 40Ar/39Ar hornblende age correspond to the latest stages of uplift and cooling of the orogen.

U-Pb and Th-Pb dating of monazite from hydrothermal quartz veins (“Alpine veins”) from the Lauzière massif (North Belledonne) together with Ar/Ar ages of adularias from the same veins constrain the age of the last tectono-metamorphic events that affected the External Crystalline Massifs (ECM). Ages obtained are surprisingly young. The study of the structural context of the veins combined with our chronological data, allow us to propose a tectonic scenario of the northern ECM for the 15-5 Ma period, which was poorly documented so far. The quartz veins are of two types: (i) the oldest are poorly mineralized (chlorite and epidote), flat-lying veins. The quartz fibres (= extension direction) are near vertical and seem to be associated with a subvertical dissolution schistosity superimposed upon an early Alpine deformation underlined by “mini-biotite”. They bear a sub-horizontal stretching lineation; (ii) the youngest veins are very rich in various minerals (anatase, rutile, phénacite, meneghinite, beryl, synchysite, ….). They are almost vertical. Their “en echelon” geometry as well as the horizontal attitude of their quartz fibres show a dextral strike-slip regime. Two groups of Th-Pb ages have been obtained: 11 to 10 Ma and 7 to 5 Ma. They were obtained from the most recent veins (vertical veins) sampled in different areas of the massif. The ca. 10 Ma ages are related to veins in the Lauzière granite and its metamorphic country-rocks at about 2 km from the eastern contact of the massif, while the ages of ca. 5 Ma correspond to veins occurring in mylonites along this contact. Adularias provided Ar/Ar ages at ca. 7 Ma. By contrast, a monazite from a vein of the Pelvoux massif (Plan du Lac) yielded a Th-Pb age of 17.6 Ma but in a different structural setting. Except fission track ages, there are very little ages of this range published in the recent literature on the Alps. The latter concern always gold mineralized veins (NE Mont Blanc and SW Lepontine dome). The last compressive tectonic regime dated between 15 and 12 Ma is coeval with (i) the late “Roselend thrust” event, which is recorded in the Mont Blanc by shear-zones with vertical lineation, (ii) the last movements in the basal mylonites of the Swiss Nappes, (iii) the horizontal Alpine veins from the Mont Blanc and Belledonne massifs (with vertical quartz fibres), which are similar to the early veins of the Lauzière. On the contrary, the vertical veins of the Lauzière, dated between 11 and 5 Ma, correspond to a dextral strike slip regime. This suggests that most of the strike-slip tectonics along the ECM took place during two stages (ca. 10 Ma and ca. 7-5 Ma) and not only at 18 Ma as had been proposed previously. Our ages are consistent with the late Miocene-Pliocene overlap of the Digne thrust to the South and to part of the normal movement along the Simplon fault to the North. Thus, all the external crystalline massifs were tectonically active during the late Miocene. This suggests that tectonic events in the external alpine belt may have contributed to some extent to the geodynamical causes of the Messinian crisis.

The reconstruction of Early Proterozoic crustal evolution and geodynamic environments, in Africa and South America, is incomplete if cratonic areas alone are studied. If the presence of high-grade gneisses is considered as a first clue to past collisional behaviour, 2 Ga high-grade gneisses are more abundant within the Pan-African–Brasiliano mobile belts than in the intervening pre-Late Proterozoic cratons. The West African craton and the Guiana–Amazonia craton consist of relatively small Archaean nuclei and widespread low- to medium-grade volcanic and volcanoclastic formations intruded by Early Proterozoic granites. By contrast, 2 Ga granulitic assemblages and (or) nappes and syntectonic granites are known in several areas within the Pan-African–Brasiliano belts of Hoggar–Iforas–Air, Nigeria, Cameroon, and northeast Brazil. Nappe tectonics have been also described in the Congo–Chaillu craton, and Early Proterozoic reworking of older granulites may have occurred in the São Francisco craton. The location of the Pan-African–Brasiliano orogenic belts is probably controlled by preexisting major structures inherited from the Early Proterozoic. High-grade, lower crustal assemblages 2 Ga old have been uplifted or overthrust and now form polycyclic domains in these younger orogenic belts, though rarely in the cratons themselves. The Congo–Chaillu and perhaps the São Francisco craton are exceptional in showing controversial evidence of collisional Eburnian–Transamazonian assemblages undisturbed during Late Proterozoic time.

The basement domes of the central part of western Alps may result either from a multistage tectonic evolution with a dominant horizontal shortening component, an extensional behaviour, or both. The Ambin massif belongs to the “Briançonnais” domain and is located within the HP metamorphic zone. It was chosen for a reappraisal of the tectonic evolution of the Internal Alps in its western segment. Structural investigations have shown that Alpine HP rocks were exhumed in three successive stages. The D1 stage was roughly coeval with the observed peak metamorphic conditions and corresponds to a non-coaxial regime with dominant horizontal shortening and north movement direction. Petrological observations and P-T estimates show that the exhumation process was initiated during D1, the corresponding mechanism being still poorly understood. The D2 stage took place under low-blueschist facies conditions and culminated under greenschist facies conditions. It developed a retrogressive foliation and pervasive shear-zones at all scales that locally define major tectonic contacts. D2 shear zones show a top-to-east movement direction and correspond actually to large-scale detachment faults responsible for the juxtaposition of less metamorphic units above the Ambin basement and thus to a large part of the exhumation of HP rocks toward the surface. D2 shear zones were subsequently deformed by D3 open folds, large antiforms (e.g. the Ambin dome) and associated brittle-ductile D3 shear-bands. The D1 to D3 P-T conditions and P-T path of the blueschists occurring in the deepest part of the Ambin dome, was estimated by using the multi-equilibrium thermobarometric method of the Tweeq and Thermocalc softwares. Peak pressure conditions, estimated at about 14–16 Kb, 500oC, are followed by a nearly-isothermal decompression that occurred concurrently with the major D1–D2 change in the ductile deformation regime. Eastwards, the Schistes Lustrés units exhibit a similar geometry on top of the Gran Paradiso dome but exhibit opposite D2 movement direction. Lower-grade units are lying above higher-grade units, the shear zones occurring in between being similar to Ambin’s D2 detachments. Thus at regional scale, the D2 detachments seem to form together with the Ambin shear-zones, a network of conjugate detachments. Such a pattern suggests that the exhumation history is mostly controlled by a D2+D3 crustal-scale vertical shortening resulting in the thinning of the previous tectonic pile formed during D1. The slab-break off hypothesis may explain such an extensional behaviour within the western Pennine domain. It is suggested that the thermo-mechanical rebound of the residual European slab initiated between 35 and 32 Ma the fast exhumation of the previously thickened orogenic wedge (stack of D1 HP slices). It was immediately followed by a collapse of the wedge that may correspond to the E-W Oligocene extensional event responsible for the opening of rifts in the West European platform.

International audience; The metamorphic belt of the Western Alps was subjected to widespread extensional tectonism at the end of the Eocene (ca. 45–35 Ma). Extension was accommodated by hinterland-directed movements along gently inclined extensional shear zones, which facilitated rapid exhumation of high-pressure and ultra-high-pressure rocks. This deformation resulted in a normal metamorphic sequence. Extension in the inner parts of the Western Alps was coeval with shortening at the front of the belt (foreland-directed thrusts), which took place during decompression, and emplaced higher grade metamorphic units over lower grade metamorphic rocks, thus forming an inverse metamorphic sequence. Two mechanisms for this extensional episode are discussed: (1) collapse of an overthickened lithosphere, and (2) internal readjustments within the orogenic wedge due to subduction channel dynamics. We favour the latter mechanism because it can account for the development of the observed inverse and normal metamorphic sequences along foreland-directed thrusts and hinterland-directed detachments, respectively. This hypothesis is supported by published structural, metamorphic and geochronological data from four geological transects through the Western Alps. This study also emphasizes the importance of post-shearing deformation (e.g. horizontal buckling versus vertical flattening), which can modify the distribution of hinterland- and foreland-directed shear zones in orogenic belts.

International audience; The Ambin and South Vanoise Briançonnais basement domes, located within the HP metamorphic zone, were chosen for a reappraisal of the early tectonic evolution of the Internal Alps in their western segment. Finite strain analyses have shown that HP rocks were exhumed in two main successive stages, more or less expressed according to their structural position in the massifs. A partitioning of deformation, from core to rim of the domes, is thus evidenced. The D1 stage was roughly coeval with the observed peak metamorphic conditions and corresponds in the core of the Ambin dome, to a non-coaxial regime with dominant horizontal shortening and N to NW movement direction. The D2 stage, well-expressed in the upper part of the domes, took place under low-blueschist facies conditions and culminated under greenschist facies conditions. It developed a retrogressive foliation and pervasive E-verging shear-zones at all scales that locally define major Φ2 tectonic contacts. The D2 shearing is responsible for the development of spectacular F1–F2 fold interference structures recognised in the western part of the South Vanoise dome. In this area, we interpret the steeply-dipping HP-fabric (S1) defining the axial planes of the first generation of now-upright folds (F1) as linked to an early flat-lying nappe edifice (Φ1) subsequently steepened by the D2 shearing. Geometry and kinematic characteristics for the early D1 nappe-forming event are strongly consistent with most evolutionary models assuming that oceanic to European and Apulian crust-derived nappes were initially exhumed during N to NW-verging overthrusting events, synthetic to the subduction. They tend also to suggest that the Penninic HP-units may have suffered an early exhumation by low-angle thrusting, rather than by vertical extrusion.

Petrological and structural observations from the Ambin pre-alpine basement dome and from its Brianconnais and Piedmont covers show an early D1 nappe-forming event overprinted by a major D2 (+ D3) ductile shearing deformation. The D1 event is characterised by garnet-blueschist facies metamorphic assemblages retrogressed to greenschist facies conditions during D2 then D3 stages near the top of the dome. North-verging D1 structures preserved in the core of the dome are consistent with alpine evolutionary models, in which exhumation of HP–LT metamorphic alpine rocks occurs initially in a north–south direction. To cite this article: J. Ganne et al., C. R. Geoscience 336 (2004).

Concepts developed in the recently published model of the Air region (eastern Tuareg shield; Niger, Africa) as a college of three displaced terranes integrated in a single geodynamic model lead us to propose a terrane map of the Tuareg shield (500000 km 2 ). The 23 terranes recognized have their own lithological, metamorphic, magmatic, and tectonic characteristics and are separated by subvertical strike-slip megashear zones that can be traced for hundreds of kilometres, or by major thrust fronts. Some of these boundaries have ophiolitic assemblages or molassic deposits. The Tuareg shield was shaped and partly accreted during the Pan-African orogeny (750-550 Ma), but not as a homogeneous body.

Boullier, A.M., Liegeois, J.P., Black, R., Fabre, J., Sauvage, M. and Bertrand, J.M., 1986. Late Pan-African tectonics marking the transition from subduction-related talc-alkaline magmatism to within-plate alkaline granitoids (Adrar des Iforas, Mali). In: B. Johnson and A.W. Bally (Editors), Intraplate Deformation: Characteristics, Processes and Causes. Tectonophysics, 132: 233-246. The Pan-African Trans-Saharan belt, resulting from oceanic closure and oblique collision between the West-African Craton and the Tuareg-Nigerian shields around 600 Ma ago, displays in the Iforas a rapid switch from subduction and collision related talc-alkaline magmatism to typical A-type granitoids (560-540 Ma). The transition is clearly seen in associated spectacular acid dyke swarms, early talc-alkaline E-W trending sets being cut by alkaline N-S trending arrays. Isotopic evidence shows that the change in chemistry involves a different more primitive mantle source. Structural observations indicate that the transition from talc-alkaline to alkaline magmatism is related to reversals in the stress field during and after collision. In the model proposed, the first major reversal from D, with ei ca. 13S0N-llOON to Ds with ui ca. 50°N-60” N is thought to have been responsible for the slicing of the subducted plate thereby allowing the rise to shallow depth beneath the continental lithosphere, of asthenospheric mantle believed to be the source of the alkaline magmatism. The tapping of this new source occurred between Ds and D4 events contemporaneous with a rotation of the maximum compressive stress back from 60° N to 105“ N.

The Karakorum gneisses outcrop north of the complex suture separating the Indian-Pakistan plate from the Europe-Asia block; they grade to deformed earlier members of the Karakorum batholith ranging in age from Cretaceous to Miocene and are cross-cut by its later members. The main interest of the region lies in the fact that very young high-grade gneisses (Miocene), outline the southern edge of the Europe-Asia Plate. The tectonic and metamorpic evolution of the Braldu-Baltoro region is interpreted here as resulting from a poly phased history. The following structural sequence has been defined : - (1) A Dl isoclinal folding was accompanied by subparallel healed shear zones and by intense boudinage, and cross-cut by a dense net of post-Dl hetero-geneous leucogranitic veins and stocks; - (2) a major phase of EW trending recumbent folds (D2), is followed by (3) large open D3 folds generating EW trending domai structures (Dassu and Panmah domes); and (4) a late set of brittle to locally more ductile str...

A large composite calc-alkaline batholith, in the Iforas region, Mali, occurs close to the Pan-African suture between the 2000 Ma old West African craton and the Trans-Saharan mobile belt. Its location in an embayment of the West African craton is probably responsible for the important production of magma. The Iforas batholith intrudes the western border of an old continental segment affected by early nappe tectonics (D1 event) and is flanked to the west by the Tilemsi palaeo–island arc. The batholith comprises several successive stages. The cordillera (>620 Ma), probably post-dating the D1 event, is essentially composed of volcanosedimentary sequences. The collision (620–580 Ma) is marked by the production of abundant granitoids mostly emplaced by the end of the D2 EW compressional event. The post-collision tectonic stages (D3 and D4; 580–540 Ma) are characterized by strike-slip movements, reversals in the stress field, and a rapid switch from calc-alkaline to alkaline magmatism. Magmas corresponding to each step show distinctive geochemical trends but all share low 87Sr/86Sr initial ratios (0·7035–0·7061). The possible successive sources have been evaluated from different entities in the Inforas region: Eburnean granulites for lower crust, Tilemsi palaeo–island arc for depleted subduction source and the Tadhak undersaturated province for asthenospheric more primitive mantle. A geodynamic model is proposed where all the calc-alkaline groups originated from a classical subduction source (depleted upper mantle modified by hydrous fluids from the subducted oceanic plate) which, some fifty million years after the beginning of the collision, was taken over by an asthenospheric source producing the alkaline province.

Bertrand Jean-Michel, Burbage Frank, Chouchan Nathalie. Libéralisme et démocratie : éléments d'une réflexion critique. In: Raison présente, n°78, 2e trimestre 1986. Les impasses du libéralisme. pp. 35-56.

Turing's project is to prove, without subscibing first to a questionable definition of thought, that computers can think. Therefore, he uses a fiction story, a game in which a computer must answer instead of a person. Conclusions should come up from the performances of the computer. The article shows that : - The game has a limited value, - Turing endsup by doing what he claimed to avoid, which is introducing again in his text a conception of thought, - He then develops a mechanistic conception "tainted" with empiricism (the idea thar thought is all clearly definined by the acts of a functional and transparent language, that it is to be approached through its analysis, that it has an original state...)., Prouver que des machines peuvent penser, sans souscrire au préalable à une définition problématique - de la pensée, tel est le projet de Turing. Pour ce faire, il utilise le détour d'une fiction, d'un jeu au cours duquel un ordinateur doit prendre la place d'un sujet Les performances de la machine doivent permettre de conclure. L'article montre : - que le jeu a une valeur limitée, - que Turing finit par réintroduire dans son texte une conception de la pensée ; ce à quoi il prétendait initialement échapper, - qu'il développe alors une conception mécaniste et "entachée" d'empirisme (l'idée que la pensée est toute entière définie par des actes d'un langage fonctionnel et transparent, qu'elle s'appréhende par son analyse, qu'elle connaît un état d'origine...), Bertrand Jean-Michel. Test de Turing : jeu d'imitation ou test d'intelligence ?. In: Quaderni, n°1, Printemps 1987. Genèse de l'intelligence artificielle. pp. 35-45.

The Aleksod region is composed of metasedimentary rocks and large areas of biotite and hornblende-bearing migmatites. Anatexis associated with the main deformation stages, occurred under high pressure and temperature conditions estimated at 13±2 Kbar and 750±50° C. The bulk mineralogical composition of the Telohat migmatites shows that their protolith was granodioritic. Internal structures of zircons and U-Pb data suggest a polyphased evolution, with a 2131±12 Ma age for the protolith and a 609±17 Ma age for the Pan-African tectono-metamorphic evolution, thus precluding any Kibaran event in the Aleksod area. Leucosomes are richer in Sr and display lower Rb, Zr, Nb, Y, Th, U and REE contents than melanosomes wherein accessory phases are stored. Eu contents are also lower in the leucosomes but in lesser proportion than the other rare earth's, leading to a significant positive anomaly. Petrogenetic modelling accounting for accessory mineral phases clearly shows that the trace element contents of leucosomes and melanosomes follow a distribution law consistent neither with equilibrium nor fractional melting. Their trace element patterns are best explained by the model of disequilibrium melting, with mixing of a few residual phases. The present results and previous Sr isotopic data as well raise the question of disequilibrium melting in anatexis of crustal material

Time and space relationships between Pan-African intrusives in the Iforas belt (northeast Mali) are discussed. On the basis of structures, lithological composition and plutonism, three domains have been defined. These are, from west to east: (1) the Tilemsi accretion domain; (2) the central domain characterized by an early Pan-African tectonism with large-scale basement nappes; and (3) the eastern domain, a later Pan-African trough deposited on continental basement. Domains (1) and (2) are separated by a huge complex batholith. Geochronological results confirm the structural distinctions and the existence of two major events. As shown by Rb/Sr isochron ages, domains (2) and (3) were stabilized 645 Ma ago. While the multistage evolution of the batholith can be bracketed between 614 and 570 Ma, RbSr biotite whole rock pairs give very similar ages which indicates a short time lapse between emplacement and cooling. The initial ratios, in the range 0.705–0.708 are, as in many other batholiths, slightly higher than the mantle growth curve, indicating a complex origin where crust and mantle are involved. As granitoids occupy some 60% of the surface area in the Iforas belt, the Pan-African epoch corresponds to an important crustal addition in the Iforas segment.

Discovery of large-scale deep-seated thrusts in Central Hoggar, with a plurifacial evolution ranging from lower amphibolite facies to upper greenschist facies conditions and linked to a regional refoliation, has led us to reconsider the Pan-African tectonic and metamorphic history in that region. Two areas are described, and a review of other thrusts leads to an interpretative cross section of a large portion of reactivated continental crust. The age and kinematics of this structural reworking have been approached using U/Pb zircon dating in the Tamanrasset region. Despite the difficulty of estimating the age of the initiation of the assumed intracontinental A-type subduction, the results provide a time span of 30–40 m.y. between the climax of granitoid emplacement and a late retrogressive offset along the thrust planes. Some key ages were determined: (1) 2075 ± 30 Ma is the age given by granulite facies remnants which escaped from the refoliation, the corresponding lower intercept at 530 ± 70 Ma confirms the Pan-African imprint; (2) 615 ± 5 Ma reflecting the age of syntectonic to late-tectonic granitoids emplaced in reworked gneisses and in preserved granulites; (3) 580 Ma, the concordant age of sphenes and monazites from the same granitoids, which is interpreted as corresponding to the end of medium-grade conditions. No evidence has been found of a ∼1000 Ma age: a Kibaran event does not appear to exist in Central Hoggar. The age similarity between the observed deep intracontinental evolution of Central Hoggar and the collision-related tectonics of Western Hoggar and Iforas suggests a common origin for both phenomena.

Migmatitic gneiss, two units locally, different compositions, anatexis with phase separation

Sheets of granite are confined to shear zones in the tectonite peridotite unit of the Thetford Mines ophiolite complex. Their orientation is parallel to the thrust plane at the base of the complex. The granite, which is calc-alkaline and rich in potassium, does not belong to the ophiolite sequence. We show that it was tectonically incorporated within the partially serpentinized peridotite during thrusting and emplacement of the complex. During this process, the granite was strongly deformed and recrystallized at low pressure (500 bar (50 MPa)) and low temperature (500 °C or less), which gave rise to the mineral assemblage orthoclase, albite, hydromuscovite, and hydrothermal biotite. This episode was followed by a rodingite alteration, characterized by the formation of grossular, which was associated with the development of economic chrysotile within the enclosing peridotite. The K–Ar ages of biotite, muscovite, and feldspar from the granite cluster around 450 Ma. These cooling ages suggest that the emplacement and alteration of the granite and peridotite had ended by Late Ordovician time. Des écailles de granite occupent des zones de cisaillement dans la péridotite tectonite du complexe ophiolitique de Thetford Mines. Elles sont orientées parallèlement au plan de chevauchement basal de ce complexe. Le granite, calco-alcalin et riche en potassium, est étranger à la séquence ophiolitique. On montre qu'il a été incorporé tectoniquement à la péridotite partiellement serpentinisée au cours du processus de charriage et de mise en place du complexe dans son cadre structural actuel. Au cours de ce processus, le granite a été fortement déformé et il a recristallisé à faible pression (500 bar) (50 MPa) et basse température (500 °C ou moins) avec néoformation d'orthoclase, d'albite, d'hydromuscovite et de biotite hydrothermale. Cet épisode a été suivi par une altération rodingitique, caractérisée principalement par la formation de grossulaire, en relation avec le développement du minerai d'amiante-chrysotile dans la péridotite encaissante. Les âges K–Ar des biotites, muscovites et feldspaths du granite sont de 450 Ma environ. Ils indiquent l'âge du refroidissement et suggèrent que la mise en place et l'épisode d'altération du granite et de la péridotite étaient terminés à l'Ordovicien supérieur.

Zircons from the Anfeg granite (Algeria) were investigated by electron and Raman microprobe techniques and by laser probe mass spectrometry. The latter method enables the analysis of 200-300 Å depth layers. The analytical results show that the composition of the zircon grains is heterogeneous. In addition, the element distributions vary greatly from rims to cores. The variation of Si and Zr contents in the layers suggests the existence of zones enriched in Si or Zr. The outer zones are generally depleted in Si, sometimes showing less than 4 atom % This phenomenon may result from the substitution of (OH)-4 for SiO4-4. Raman spectroscopy confirms the presence of (OH)-. The zircons are also rich in substituted cations. The latter are irregularly distributed, with high concentrations in narrow zones : e.g. Ge 3.6, Hf 2.2, Sm 2.3, Yb 1.1, Gd 0.64 atom %. It is shown that the U rich zones are not rich in radiogenic Pb. This suggests that migration of certain elements has occurred in the crystal cores. Such elements were intermediate in the U-Pb radioactive decay chain, possible near 222Rn., Des zircons du granite d'Anfeg (Algérie) ont été étudiés par microsonde électronique, par microsonde Raman et par spectrographie de masse à sonde laser (L.P.M.S.). Cette dernière technique permet d'effectuer des analyses par couches de 200 à 300 Å d'épaisseur. Les mesures mettent en évidence l'hétérogénéité de la composition chimique des zircons issus d'un même échantillon. De plus, la répartition des éléments en fonction de la profondeur, à partir de la surface, est fortement variable. La variation.de la teneur en Si et en Zr de couche en couche indique l'existence de zones avec excès de Si ou de Zr. Les zones externes sont généralement fort appauvries en Si dont parfois la teneur ne dépasse pas 4 % (% atomique). Ce phénomène peut être attribué au remplacement de SiO4-4 par les ions (OH)-4 dont la présence a été constatée par microsonde Raman. Les zircons sont riches en éléments de substitution répartis d'une façon irrégulière avec fortes concentrations localisées dans des zones étroites où leur teneur atteint des valeurs considérables notamment : Ge 3,6 %, Hf 2,2 %, Sm 2,3 %, Yb 1,1 %,Gd 0,64%. Les zones riches en Pb radiogénique sont décalées par rapport à celles riches en U. Ce fait indique la migration des éléments intermédiaires de la chaîne radioactive au sein des cristaux, notamment au niveau de 222Rn., Kosztolanyi Charles, Eloy Jean-François, Bertrand Jean-Michel. Etude de l'hétérogénéité des zircons du granite d'Anfeg (Algérie) à l'aide de méthodes microanalytiques. In: Bulletin de Minéralogie, volume 109, 3, 1986. pp. 265-274.

Tapie, chef d'entreprise gère son image comme une marque commerciale qui condense et supporte des effets de croyance. A travers l'analyse détaillée du déroulement du magazine télévisé Ambition se révèlent des stratégies discursives : Tapie, héros multicodique, joue de tous les codes et métadiscours, de toutes les places, et instaure un contrat de parole visant à la reconnaissance de sa place d'énonciateur pour mieux utiliser les possiblités de la simulation télévisuelle., Bertrand Jean-Michel. Un héros médiatique : Bernard Tapie. Notes critiques sur l'émission "Ambition" du 11 avril 1986. In: Quaderni, n°4, Printemps 1988. Les mises en scène télévisuelles. pp. 77-88.

L'analyse quantitative de la déformation des roches apparaît indispensable si l'on veut pouvoir comprendre et discuter les mécanismes de la déformation à différentes échelles d'observation. Dans ce mémoire, l'étude de la déformation finie à l'échelle microscopique, a été réalisée à partir de la forme et de l'orientation de cristaux constituant les populations de marqueurs. A cette fin, nous avons développé un outil permettant d'avoir facilement accès, aux paramètres de forme et d'orientation, sur lesquels reposent les différentes méthodes d'estimation du taux de déformation (Lapique 1987, Lapique et al. 1988). Nous avons opté pour une analyse interactive, où le manipulateur choisit et extrait lui même de l'image vidéo, les objets qui l'intéressent. L'analyseur vidéographique interactif permet l'observation et le traitement à partir d'une image couleur, ce qui met le géologue dans les conditions normales d'observation, lui permet la reconnaissance des phases minérales et lui facilite la saisie de la forme des objets (digitalisation de leur contour au moyen d'une tablette à numériser). Différentes méthodes de quantification de la déformation finie ont été testées (Panozzo 1983, 1984, Shimamoto et Ikeda 1976, RflF de Dunnet 1969, Fry 1979, diamètres de Féret) à partir de fabriques naturelles déformées artificiellement. Il ressort que la méthode des diamètres de Féret (Lapique 1987) donne les résultats les plus fiables en rapport axial de l'ellipse de déformation finie et en orientation de son grand axe quelque soit l'état initial, surtout dans le cas de faibles intensités. L'application des ces méthodes d'estimation du taux de déformation à la zone de cisaillement d'Abeibara-Rahrous (Adrar des Iforas, Mali) a permis de souligner l'importance de l'échelle d'observation. La combinaison de ce type de méthodes et d'autres englobant les phénomènes de rotation des marqueurs (Fernandez 1984, 1987) permet de cerner d'une manière plus précise, même à l'échelle microscopique, l'intensité de la déformation subie par cette zone. En considérant les phénomènes de déformation ductile (étirement des marqueurs tels que les quartz ou les opaques), et la rotation de marqueurs que l'on peut considérer comme essentiellement "rigides" comme les feldspaths, nous obtenons des estimations de l'ordre de Y = 6 qui se rapprochent des estimations réalisées à des échelles différentes (Boullier 1982, 1986). La digitalisation de la forme des marqueurs donne accès également à l'étude de l'orientation des fabriques et des dissymétries traduisant le sens de cisaillement, ainsi qu'à la distribution des rapports axiaux, des tailles et des indices d'ellipticité des populations de marqueurs. Ces paramètres permettent de comparer le comportement de marqueurs de différente nature et de situer les phénomènes de granulation ou de recristallisation en fonction de la quantité de déformation enregistrée par le minéral (dès les valeurs de rapport axial de 3.5 pour le quartz et de 6 pour les opaques). Ils permettent également de tester les hypothèses sur les formes théoriques des marqueurs. Une deuxième application concerne la détermination du régime de la déformation subie par les orthogneiss du Grand Paradis (Alpes occidentales, Italie). Les techniques et méthodes d'estimation de la déformation finie ont été appliquées, parallèlement à une étude des orientations préférentielles de réseau des quartz. Ces études ont permis de conclure que lors de la phase éocène, le régime de déformation était en cisaillement simple. Cette déformation s'est traduite par le jeu ou le rejeu de zones de cisaillement très localisées, avec des mouvements vers l'Est ou vers l'Ouest, qui limitent des compartiments ,où l'aplatissement est dominant (fabriques symétriques dans le plan XZ)., pas de résumé