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The Pan-African/Brasiliano orogenic belts are a part of numerous Neoproterozoic orogenic belts that belong to the long-lived orgenic cycle that distancing phase started at the Tonian around 1.0 Ga. The Tonian magmatism fairly documented in the Neoproterozoic belts of Borborema Province (NE-Brazil), seemed so far inexistent in the Central African Orogenic Belt (CAOB) although these two belts were geologically correlated. Through the Lu-Hf geochronological analysis on zircon of tonalite, the present work, coupled with the previous data, suggests the existence of a Tonian age magmatism in the Central Cameroon Domain of the CAOB although the latter is much reworked. The Nguesseck tonalite outcrops in the northern part of the Mbe-Sassa-Mbersi region, in the northern edge of Central Cameroon Domain of the CAOB and in the Tchollire-Banyo shear zone (TBSZ). The Lu-Hf data obtain on the zircon grains of this tonalite reveal juvenile HfTDM age of ca. 1.0 Ga. This age, combined to the previous geochemical data suggests that the protholites of this tonalite would have extracted from the source during the distancing phase (rifting and oceanization) of the Pan-african/Brasiliano orogeny at the early Neoproterozoic.

The Lafaye orbicular body was emplaced in the Villatange tonalite-granodiorite unit of the Guéret magmatic complex (Massif Central, France). It consists of plagioclasic orbicules (4–35 cm diameter) embedded in homogeneous cordierite granodiorite. Orbicule cores consist mostly of residual metasedimentary xenoliths or autolithic plagioclasic cumulates. Rims (0.7–8 cm thickness) are single- or multi-layered; layers, mostly comb-textured, comprise alternating sheets dominated by cordierite (XFe = 0.32–0.37) or plagioclase (mostly An25–30). Additional mineral phases are minor biotite (XFe = 0.52; AlVI = 0.58–0.92 atoms per formula unit) and interstitial quartz. Plagioclase and cordierite morphologies (needle-like, skeletal, branching or fan-shaped) indicate growth under high initial supersaturation. However, the final polyhedral shapes and primary zoning of many individual plagioclase crystals, as well as evidence of partial recrystallization, imply significant textural maturation. Whole-rock major and trace element data (A/CNK = 1.12–1.46) and Sr and Nd isotopic compositions (εNd(355 Ma) = −8.6 to −7.4; 87Sr/86Sr(355 Ma) = 0.7110–0.7147) suggest that the parental magma of the orbicules resulted from bulk assimilation of aluminous metasediments by a Villatange-type granodioritic magma. Heterogeneous nucleation and growth of plagioclase and cordierite around xenoliths/autoliths are interpreted in terms of (1) adiabatic decompression of magma pulses ascending in dykes leading to superheating and resorption of early solids, and (2) volatile exsolution, inducing undercooling, supersaturation, and rim crystallization. The variability of layers (number, thickness, mineral distribution, and texture) is considered to result from oscillatory crystallization combined with variable plagioclase growth rates linked to changes in the degree of supersaturation as a function of the extent of melt degassing, itself linked to magma transfer dynamics.

International audience

The Imiter inlier (eastern part of the Moroccan Anti-Atlas) is located on the northwestern border of the West African Craton (WAC) and exhibits a range of Pan-African granitoids. Three massifs that crosscut the Imiter Saghro Group were targeted in this work: the Igoudrane granodiorite, Bou Teglimt granodiorite and Bou Fliou granite. We present here additional geochemical analyses (major and trace elements) and Sm-Nd isotopic data, which define two distinct groups: (i) the Igoudrane massif (677 Ma) and (ii) the Bou Teglimt granodiorite (576 Ma) and the Bou Fliou granite (550 Ma). Geochemical data confirm the calc-alkaline signature of the studied granitoids. Both groups of granitoids are slightly peraluminous and show strong negative anomalies in Nb, Ta and Ti in multi-element plots normalized to the primitive mantle. The granitoids have low 143 Nd/ 144 Nd initial ratios (0.5116–0.5117), with T DM model ages ranging from 1.73 to 1.52Ga. The eNd (t) values are negative, decreasing from the Igoudrane samples (−1.1 to −3.1) to the Bou Teglimt granodiorite (−3.0 to −3.3) and Bou Fliou (−4.2 to −4.8). All these data suggest a mixed magmatic origin involving a juvenile mantle source and an old, at least Paleoproterozoic crust. Given Mesoproterozoic rocks are lacking or very scarce in the Anti-Atlas, these results confirm the existence of an old cratonic basement beneath the eastern Anti-Atlas, and therefore suggest that the northern border of the West African Craton must be placed further to the north, as suggested by previous work in this region.

Quel est l’impact du fonctionnement d’une centrale nucléaire sur l’environnement, les terres et les populations avoisinantes ? Quelles sont les phases industrielles de la filière, de l’extraction du minerai à l’exploitation des centrales, qui comportent les effets sensibles les plus controversés ? Deux chercheurs répondent à nos questions en exprimant des points de vue opposés.

International audience; We describe a case of S-type alkali-feldspar granite (quartz/K-feldspar/plagioclase/ biotite/garnet ∼ 30/35/20/13/2 vol%) showing skeletal and dendritic textures. It consists of three nested units distinguishable by the size of biotite crystals: a first-emplaced unit characterized by small biotite flakes (0.4–9 mm); a second unit with cm-sized biotite platelets (0.3–28 mm) outlining a syn-magmatic foliation; and a third unit formed of large biotite plates (0.7–850 mm) showing dendritic and radial growth patterns.Quartz occurs mostly as rounded to euhedral grains, but also shows complex shapes (serrated outline, re-entrants, etc.). Cathodoluminescence imaging reveals that grains consist of both rounded and skeletal bluish cores (quartz-1) rimmed by reddish overgrowths (quartz-2) that lead locally to the coalescence of neighbouring grains and equilibrated texture. Biotite is poikilitic enclosing dominantly quartz grains but also in lesser amount plagioclase and garnet. In the third unit, biotite shows textures typical of dendritic growth (bifurcating, radiating or branching structures, skeletal core). Plagioclase (An≤25) occurs as small euhedral to subhedral grains scattered in the matrix or forming glomerophyric aggregates. K-feldspar (Or74–100) mostly forms an almost equilibrated mosaic of anhedral perthitic crystals; it also appears in association with albite as a secondary phase replacing plagioclase. The crystallization sequence started with garnet, then proceeded with quartz, plagioclase, biotite and K-feldspar, ended with cordierite, and was followed by subsolidus reequilibration.The variability in size and morphology of quartz and biotite is interpreted in terms of variable undercooling depending on temperature of the host-rocks and on more or less intense degassing of magma batches with variable melt water content. Here, rapid crystallization prevented significant textural reequilibration and allowed the initial crystallization textures of quartz to be preserved. Overall, this study shows that behind the common “coarse-grained” texture of quartz in granite, subsist primary disequilibrium crystallization textures.

International audience

We present structural, petrological, geochemical and geochronological data compiled in a new geological map on the high-grade metamorphic rocks exposed in the Mbe – Sassa-Mbersi region located along the Tchollire-Banyo Shear Zone, at the northern edge of the Central Cameroon domain. The region exposes a complex assemblage of metabasic (hornblendite, metagabbro, amphibolite) and metasedimentary (garnet paragneiss, calc-silicate gneiss) rocks alternating with ubiquitous migmatitic intermediate to felsic gneisses. U-Pb ages on detrital zircon in paragneiss point to the contribution of Archean, Paleoproterozoic and Neoproterozoic sources, and suggest a maximum deposition age of 725 ± 12 Ma for their sedimentary protolith. The protolith of some amphibolites was emplaced during the Neoproterozoic as indicated by U–Pb ages on magmatic zircon at 600 ± 4 and 599 ± 6 Ma. Plutonic rocks with I-type chemical signatures were emplaced discontinuously over a period of ca. 80 Ma. The magmatic evolution started with the intrusion of magnesian medium-K tonalites and trondhjemites at 651 ± 3 Ma, followed by magnesian high-K hornblende-biotite granites and ferroan shoshonitic biotite granites until 599 ± 2 Ma. Finally, pegmatite dykes were emplaced along dextral shear zones at 573 ± 2 Ma. The metamorphic rocks show a penetrative shallow-dipping foliation partially to totally transposed in a NE–SW trending steep-dipping foliation bearing a shallow-dipping lineation. These ductile syn-migmatitic fabrics, developed under high-grade metamorphic conditions in the presence of melt, between the metamorphic peak at ca. 1.3 GPa–770 °C, and the following isothermal decompression down to ca. 0.8 GPa. In situ U–Pb dating of metamorphic zircon rims from a garnet migmatitic paragneiss yields an age of 582 ± 4 Ma. Localization of high-grade deformation is indicated by the development of the Tchollire-Banyo Shear Zone, characterized by mutually crosscutting subvertical and shallow-dipping mylonitic to ultramylonitic shear zones associated with sinistral and top-to-the SW kinematic criteria, respectively. These data indicate that the Central Cameroon domain exposed in the Mbe – Sassa-Mbersi region, represents the exhumed mid-to lower part of the former orogenic root of the Pan-African Central Africa Orogenic Belt that has undergone partial melting, lateral flow and intrusion of mafic to felsic calc-alkaline magmas between 650 and 580 Ma. The waning stage of the Pan-African orogeny is marked by the emplacement of syntectonic pegmatitic granite at ca. 575 Ma in subvertical E-W trending retrogressive dextral shear zones.

This study presents new U–Pb zircon ages for three granitoid bodies in the Imiter inlier at the eastern part of the Moroccan Anti-Atlas chain, which is located in the northwestern edge of the West African Craton (WAC). Analyzes of zircon grains yield the following ages: 538 ± 6 and 575 ± 10 Ma for the Igoudrane massif , 567 ± 6 Ma for the nearby Bou Teglimt granodiorite , and 582 ± 6 Ma for the Bou Fliou granite . These new data reveal that the Igoudrane massif is younger than previously thought (677 Ma), and that all the granitoids in the Imiter area are Ediacaran in age and are related to the Ouarzazate Group. Sm-Nd isotopic data yield negative eNd(t) values (−4.5 to −0.2) and T DM model ages of 1.04–1.82 Ga. These data indicate the presence of an inherited Paleoproterozoic to Archaean component in some zircon grains and show that generation of the parent magmas involved a Neoproterozoic juvenile contribution and recycling of an older crustal component. These new data allow reinterpreting the geochemical and isotopic data of the granitoids, and revisiting the structural significance to the Igoudrane massif, which was assumed until now to be coeval with the major Pan-African event of the Central Anti-Atlas. They also bring significant constraints to discuss the context and age of deposition of the Saghro Group, which is still a matter of debate. All these data provide evidence of an emplacement of the Imiter granitoids in a subduction complex, which affected all of the Anti-Atlas chain during the Late Proterozoic time.

International audience; The palladium-mediated C–H radio-iodination of arenes using sodium iodide as the primary isotopic source is reported and performed without chemical know-how in 30 min and applied to the synthesis of complex radio-iodinated compounds of biological interest.

V. Ngako and E. Njonfang addressed in their comment an interesting and debated issue about the collision geodynamics of the Pan-African Central African Orogenic Belt (CAOB) in Cameroon. However, the reason for this comment is still not clear to us since it mainly deals with issues far away from the scope of our study, namely, the pre-collisional set up of the belt. The comment raises two main points – the reworking process during plate collision and the Pan-African strain pattern – not discussed in our paper for the simple reason that it was not our goal and because our data set does not even allow such discussion. Having said that, the comment gives us the opportunity to clarify our view on the nature of the Adamawa-Yade domain (Central Cameroon) and the early evolution of the CAOB. We will successively discuss two other points raised in the comment: (1) our Fig. 12 depicting a possible pre-collisional configuration at the northern and southern margin of the Congo craton in the Early Neoproterozoic, and (2) the nature and extent of the Adamawa-Yade domain.

The Marbat basement (Sultanate of Oman) belongs to the Neoproterozoic accretion domain of the Arabian-Nubian shield. We present new geochronological, petrological and geochemical data as an extension of our previous study (Denele et al., 2017) re-interpreting this basement as a metamorphic core complex (MCC). We showed that this MCC consists of a metamorphic unit (Juffa complex) separated by an extensional detachment from a plutonic unit (Sadh complex and Tonalite plutons). Geochemical data show that the Juffa metasediments correspond to volcanogenic graywackes, suggesting deposition in front of a juvenile magmatic arc. New in situ U-Pb zircon data show that the protolith of the Juffa paragneisses is Tonian in age (960-830 Ma). The Juffa complex recrystallized under amphibolite facies conditions (950 MPa, 630 degrees C) corresponding to thermal gradient of 17-18 degrees C/km, i.e. close to that observed in fore-arc environment subjected to tectonic underplating. It was then retrogressed (< 400 MPa, < 400 degrees C) during exhumation of the MCC. Phase assemblages of the Sadh complex and Tonalite plutons record magmatic temperatures estimated at 700-840 degrees C and 610-840 degrees C from hornblende and zircon-saturation thermometry, respectively; pressures are grossly estimated at ca. 250 MPa. These data and structural evidence led us to consider that the Sadh complex is a plutonic unit emplaced during exhumation of the MCC. Hence, we re-interpret the previously published zircon core U-Pb ages of the Banded gneisses (ca. 860-830 Ma) as inherited, and zircon rim ages (ca. 815 Ma) as the age of their intrusion. This suggests that the parent magma of all these granitoids could have interacted with volcanic material equivalent to the Juffa complex. Overall, the Marbat area developed during the Tonian and involved (i) deposition of volcanosediments (Juffa complex) in an island-arc environment (960-830 Ma); (ii) metamorphism under middle pressure conditions during tectonic underplating at ca. 820-815 Ma, followed by emplacement of plutonic rocks in the upper crust during initiation of the MCC at ca. 815-810 Ma; and (iii) late-tectonic intrusions associated with MCC amplification at ca. 800-790 Ma (Mahall intrusives and Tonalite plutons).

International audience

The West European Variscan chain is a remarkable illustration of how partial melting marks out the geodynamic evolution of mountain belt through time. Here, we focus on the Late Carboniferous melting events reported in the southeastern French Massif Central (Velay dome), with emphasis on the modes of partial melting, relationships between partial melting and magma emplacement, transition between the melting episodes and related P-T-t path. Following nappe stacking events under medium pressure/temperature conditions (M1 and M2 events), three melting events are identified in the southern envelope of the Velay dome. A first melting episode (M3 event) occurred within the biotite stability field at 325–315 Ma (T ≈ 720°C and P = 0.5–0.6 GPa). It led to the complete disappearance of muscovite and to the formation of migmatites consisting of biotite ± sillimanite melanosome and of granitic/tonalitic leucosomes depending on protolith composition. It is interpreted as the result of internal heating mainly linked to decay of heat producing elements accumulated in a thickened crust. It resulted in the formation of a partially molten middle crust with decoupling between the lower and upper crust, late-collisional extension and crustal thinning.The second episode of melting (M4 event) occurred at ca. 304 Ma (T 800°C and P 0.4 GPa), synchronously with emplacement of the Velay granites and growth of the dome. It led to the breakdown of biotite and growth of cordierite (locally garnet or tourmaline), with formation of diatexites and heterogeneous granites. This high-T event synchronous with crustal extension is considered to result from intrusion of hot mantle-derived and lower crustal magmas triggering catastrophic melting in the middle crust. This event ends with local retrograde hydrous melting within the stability field of biotite close to the solidus in response to local input of water during temperature drop in the late stage of emplacement of the Velay dome.The last evidence of melting in this area (M5 event) corresponds to emplacement of late granites generated under conditions estimated at ≈850°C and 0.4–0.6 GPa. They may have been generated from melting of specific lithologies triggered by injection of mafic magmas. These granites emplaced in a partly cooled crust (medium-grade conditions). The emplacement age of these granites is not well constrained (305–295 Ma) though they clearly post-date the Velay granites.The melting episodes in the Velay area and generation of granites appear to correspond to the conjunction between (i) the effects of collision-related crust thickening and (ii) those related to slab break off and asthenospheric mantle decompression melting. The driving process is mainly the internal radiogenic heat in a first stage, relayed by the propagation of a thermal anomaly initially located in the lower crust (M3 event), but which subsequently rose to the middle and upper crustal levels through magma transfer (M4 event). Overall, the Velay example is a remarkable illustration of the progressive dehydration and sterilisation of a thickened crustal segment. It documents how large amounts of granitic magmas can be produced at shallow crustal levels in relation to the injection of mantle-derived magmas.

New field observations, ages and geochemical data are presented for the Adamawa-Yade domain, forming part of Central African Orogenic Belt in Cameroon. This belt delineates the northern margin of the Congo craton and is part of the Braziliano/Pan-African orogenic system, which was formed during amalgamation of the West Gondwana continent at the end of the Neoproterozoic. U–Pb dating indicates that the protoliths of some orthogneisses from the Adamawa-Yade domain were emplaced during three distinct periods. Tonalite-Trondhjemite-Granodiorite (TTG) suites in the Makenene area intruded at ca. 3.0–2.5 Ga, and were affected by partial melting at ca. 2.08–2.07 Ga, during the Eburnean orogeny, and by a magmatic event with a metamorphic overprint at 0.64–0.61 Ga, during the Pan-African orogeny. A dioritic orthogneiss from the Maham area, north of Makenene, was emplaced at 2.07 Ga and also reworked at 0.62–0.61 Ga. The youngest granitic orthogneisses, sampled in the vicinity of Bafia, Bep and Ngaa-Bape, yield intrusion ages of 0.64–0.63 Ga. Some of these Neoproterozoic plutonic rocks show shoshonitic affinity and reveal eNd (630 Ma) values ranging from −1.2 to −6.9 (1.7 > T DM > 1.1 Ga), and 87 Sr/ 86 Sr initial ratios of 0.706 and 0.704, providing evidence for crustal reworking during the Pan-African collision. The isotopic geochemical characteristics provide evidence that potassic magmas result from the interaction between the Archaean TTG continental crust and magmas derived from a mantle enriched in silica and incompatible elements during subduction underneath a continental margin. The Bafia group appears as a complex geological entity comprised of an Archaean/Palaeoproterozoic basement and its Neoproterozoic sedimentary cover, both deformed and recrystallized at ca. 0.61 Ga. The crustal evolution of the Adamawa-Yade domain is very similar to that inferred for the northern margin of the Congo craton (the Ntem complex and Nyong series). This and the sedimentary record suggest that the Adamawa-Yade domain represents an Archaean/Palaeoproterozoic microcontinent, which was detached from the northern margin of the Congo craton in the early Neoproterozoic, but became re-accreted together with the Mayo Kebbi (magmatic) arc during the Pan-African orogeny.

We investigate the development of a Neoproterozoic metamorphic core complex (MCC) in the Marbat area (southern Arabian Peninsula). The MCC is a gneiss dome emplaced in a releasing bend along a dextral strike-slip fault system. Dome emplacement-related extension was at a high angle to the long axis of the dome and 65 degrees from the strike-slip fault system. Shortening is recorded in the upper plate of the MCC in a direction normal to the extension direction. Extensional emplacement of the gneiss dome accommodated lower crust exhumation and contributed to the formation of a ductile strike-slip releasing bend from an initial fault step. The Marbat example allows defining transversal stretching-dominated strike-slip MCCs, which would represent the early development stages of longitudinal stretching-dominated strike-slip MCCs. Conversely, longitudinal stretching-dominated strike-slip MCCs produced at low finite extension would result from strike-slip-dominated transtension. This study bears important implications for lower crust exhumation under pulla-part basin systems in obliquely convergent or divergent settings.

The Tchollire massif, in central north Cameroon, consists of elongated granite plutons that crop out along the Pan-African Tchollire-Banyo shear zone (TBSZ), a potential suture zone within the Central Africa Orogenic Belt. New structural and geochronological data on these granites constrain the tectonic regime and timing of the TBSZ. The plutons consist of syntectonic granites and granodiorite containing dioritic mafic enclaves. They show an S2 sub-vertical foliation, that trends NE-SW to ENE-WSW. The related L2 lineation is subhorizontal to shallowly plunging to the SW or NE. Kinematic indicators such as asymmetric folds, sigmoidal-shape boudins, shear bands, imbricated feldspar phenocrysts along antithetic fractures point to a sinistral sense of shear. Microstructural analysis shows that structures are acquired from the submagmatic to the low temperature solid state suggesting progressive deformation of the magma during its emplacement, crystallization and cooling. U-Pb zircon dating on this massif yields emplacement ages of 719 ± 12 Ma for the biotite-amphibole granite and muscovite granite, 652.2 ± 5.4 Ma for the biotite-granite and 632 ± 13 Ma for the leucogranite. These geochronological data show in addition, Palaeoproterozoic inherited ages of 1631 ± 30 Ma on the leucogranites of this massif, and point to a Palaeoproterozoic contribution in their genesis. The range of ages (ca. 87 Ma) points to the timing of syntectonic emplacement of felsic magmas coeval with sinistral transpression along the TBSZ during the Pan-African orogeny. These results show that the TBSZ has recorded prolonged deformation associated with crustal magmatism between the Palaeoproterozoic Adamawa-Yade domain to the southeast and the Sinassi-Mayo Kebbi Neoproterozoic magmatic arc to the Northwest.

The Variscan segment of the Pyrenees is well suited to study the timing of crustal-scale deformations as crustal flow and gneiss dome formation. This has been constrained from a synthesis of available structural and geochronological data of intrusive rocks, as well as new zircon U–Pb age determinations via laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). After a stage of moderate thickening by fold–thrust belt development in the upper crust between 323 and 308 Ma, the Variscan segment of the Pyrenees experienced crustal flow at c. 306 Ma and then gneiss dome formation at c. 304 Ma. Localization of the deformation along reverse-dextral shear zones occurred at c. 300 Ma. The Variscan segment of the Pyrenees recorded a high-temperature regime, which allowed crustal flow of the middle crust, but with limited amounts of heat which induced rapid cooling. The development of this enigmatic orogenic segment of the Variscan belt is closely contemporaneous with the formation of the Cantabrian Orocline and could correspond to a lithospheric-scale shear zone that accommodated buckling of the orocline. Late Variscan lithospheric delamination and asthenospheric upwelling associated with buckling in the core of the Cantabrian Orocline could explain the short-period high-temperature regime in the Variscan segment of the Pyrenees.

The Zabili granitic pluton (SW Chad) exposed in the Mayo Kebbi massif is dominated by a coarse-grained hornblende biotite granite grading into a fine-grained biotite granite along its southern margin. Petrologic (micrographic intergrowth of quartz and alkali feldspars, granophyric microstructures, the presence of fluorite and bastnaesite as accessory minerals) and geochemical data (high silica, alkalis and Fe/Mg, depletions in CaO, MgO, TiO 2 ; high Ga, Nb, Zr, Ga/Al, REE, depletions in Ba, Sr, Eu and compatible elements) indicate that this pluton consists of A-type granites crystallized from hot (apatite and zircon saturation temperatures ranging from 744 °C to 923 °C), extremely differentiated magmas. U–Pb zircon geochronology indicates that the magmas crystallized at 567 ± 10 Ma and reveals the presence of older Neoproterozoic xenocrystic zircons at 668 ± 5 Ma in both facies. Within the fine-grained biotite granite, discordant zircons with U–Pb and Pb–Pb ages ranging from Neoproterozoic to Archaean are also reported. The 668 ± 5 Ma old zircons are considered to derive from country-rocks while discordant zircons, characterized by angular shapes, internal fractures and inherited cores, are likely to represent multi-sources detrital crystals that have recorded at least one metamorphic event. Old pre-Neoproterozoic zircons are reported for the first time for rocks of the Mayo Kebbi massif and they attest to the contribution of an old basement (likely to be the Eastern Nigeria basement and/or the Congo craton) involved in a collisional event with a juvenile Neoproterozoic crust prior to the emplacement of the Zabili granitic pluton. Initial eNd values calculated for the Zabili pluton range from + 2.6 to + 7.0, the highest value recorded by one sample from the coarse-grained hornblende-biotite granite being close to the one of the depleted mantle at 570 Ma (eNd = + 7.4). Combining geochronology, Nd isotopes composition and geochemical modeling, leads us to suggest the following model for the origin of the Zabili granitic pluton: (i) contribution of juvenile magmas or partial melting of a juvenile basaltic protolith characterized by a short crustal residence time; (ii) interaction of granitic magmas with older continental materials as suggested by the presence of pre-Neoproterozoic zircons and lower initial eNd values of the fine-grained biotite granite; and (iii) fractional crystallization of feldspars and ferromagnesians to produce the observed geochemical features of sample GAB-B, from which an initial eNd value of + 7.0 has been calculated.

Publication of this special volume of the Bulletin de la Societe geologique de France, dedicated to the evolution of the southern segment of the European Variscan chain, is an opportunity for the French community of Geosciences to pay a friendly honoring tribute to Philippe Rossi, a tireless promoter of geology within the important institution that is the Bureau de Recherches Geologiques et Minieres (BRGM). After his secondary studies at the Lycee J.B. Corot in Savigny-sur-Orge, Philippe Rossi discovered geology at the Universite Paris-Sud (Orsay), where he defended in 1975 a Doctorat thesis entitled: “ Geologie et gitologie du Cerro San Cristobal Logrosan, Caceres, Espagne”. In the wake of his thesis, Philippe joined the BRGM where he went, from the 1980s, to play a prominent role as both a geologist specialist of the Variscan chain in Europe, and a key player of the evolution and accomplishment of the French mapping program (Programme de la Carte Geologique de la France). First of all, Philippe is recognized in our community as a researcher enthusiast about the petrologic and geodynamic evolution of the European Variscan chain. In 1986, he defended a Doctorat d’Etat thesis at the Universite P. Sabatier (Toulouse) entitled: “ Organisation et genese d’un grand batholite orogenique : le batholite calco-alcalin de la Corse ”. In parallel, Philippe has led the cartographic synthesis of Corsica at 1/250 000 published in 1980, and was involved in the survey of 7 geological maps at 1/50 000 in Corsica, to be published between 1984 and 2012. In 2009, he and his colleagues published a remarkable synthesis of the Corso-Sardinian segment of the Variscan chain, updated in 2014 with regard to the place of Corsica in the Variscan “puzzle”. This work conducted in collaboration with A. Cocherie within the BRGM, but also with many French and …

We show that the Permian volcanism in the Pyrenees was accompanied with emplacement of granite plutons with transitional chemistry. Reassessment of age, structure and petrological characteristics of the Aya pluton, intrusive in the Cinco Villas massif (Basque Country), indicates that it was emplaced at 267.1 ± 1.1 Ma in a dextral transtensive context and that it marks a transition between the high-K calcalkaline and the alkaline emissions of the first and second Permian volcanic cycles, respectively. This age is close to that of c. 267 Ma reported for lamprophyres dykes of the Pyrenean Axial Zone. These data characterize a period of transition between the Variscan orogeny and the subsequent Alpine cycle in the Pyrenees, with an earlier extensive regime that became widespread in the Pyrenees during the Mesozoic times.

International audience; The Archean geological evolution of the Carajás metallogenic province is marked by A-type granite emplacement coevally with a regional shortening episode at 2.7 Ga. These A-type granites intruded metavolcano-sedimentary series of the Itacaiúnas Supergroup. We studied three massifs: Igarapé Gelado, Estrela and Serra do Rabo. The granites present low Al2O3 values, high FeOt/(FeOt + MgO) and (K2O + Na2O)/CaO values, along with high contents of Zr, Y, Nb and rare-earth elements. The Sm–Nd model ages (2.97 Ga to 3.2 Ga) and the slightly negative {varepsilon}Nd(t) values (–0.38 to –2.06), obtained in the Estrela granite complex suggest that the magmas were most probably produced by partial melting of ca. 3.0 Ga rocks. Coexistence of ilmenite, ferropargasite, hastingsite and hedenbergite suggest crystallization of high-temperature melts under low fugacity of oxygen. The granites induced high-temperature ductile deformation in the contact aureole. The structural patterns of the granites suggest that their emplacement and deformation result from a continuous evolution under decreasing temperature due to the interference between inflation and horizontal shortening. The A-type granites, when first discriminated, were considered to be restricted to anorogenic or rift-related environments. Our data corroborate the fact that A-type granites can abridge a wide family of rocks formed in several tectonic regimes. The characteristics of the Carajás A-type granites point to an orogenic history considered to be related to an environment of magmatic arc.

We present a structural, AMS, microstructural and kinematic study of the Aston gneiss dome (French Pyrenees), which consists of a core made up of orthogneiss and paragneiss intruded by numerous sills of Carboniferous peraluminous granite. The orthogneiss corresponds to a former Ordovician granitic laccolith. Four Variscan events have been evidenced in this gneiss dome: (i) D1 deformation observed only as relics in the orthogneisses and their country-rocks located above the sillimanite isograd, and characterized by a NS to NE–SW non coaxial stretch associated to top to the south motions (NS convergence); (ii) D2-a deformation observed in the orthogneisses and their country-rocks, mainly migmatitic paragneisses, located below the sillimanite isograd and in the peraluminous granites whatever their structural level, and characterized by an EW to N120°E stretch associated to a top to the east flat shearing (lateral flow in the hot middle crust in a transpressive regime); (iii) D2-b deformation characterized by EW-trending megafolds corresponding to the domes in the middle crust and by EW-trending tight folds with subvertical axial planes in the metasedimentary upper crust; (iv) subvertical medium-temperature mylonitic bands developed by the end of the transpression. The Aston massif is a good example of decoupling between a cold upper crust and a hotter middle crust overheated by a thermal event originated in the upper mantle. This decoupling allowed the lateral flow of the migmatitic middle crust along a direction at high angle with respect to the more or less NS-trending direction of convergence. We suggest that the HT-LP metamorphism developed before the formation of the domes during D2-a, coevally with the emplacement of numerous sills of peraluminous granite, whereas the emplacement of the large calc-alkaline plutons in the upper crust occurred by the end of D2-b. Our data invalidate the previous geodynamical models based on either early or late extensional regime to explain the development of the HT-LP metamorphism. This new interpretation of the dynamics of the Variscan crust of the Pyrenees is consistent with recent studies conducted in Archaean and Palaeoproterozoic hot continental crusts having undergone oblique convergence, and characterized by a competition between vertical thickening and lateral flow induced by the important rheological contrast between two thermally different levels.

International audience; Orogenic belts involving initially hot lithospheres, as exemplified by accretionary-type orogens, commonly show distributed deformation and retrograde PT paths with a concomitant decrease in pressure and temperature. Paths may track geotherms, indicating thermal equilibrium, consistent with slow strain and exhumation rates, limited strain localization, and consequently limited topographical gradients and distributed erosion. Such patterns are more common in Precambrian times than in younger periods of the Earth history. In contrast, orogens involving initially stiff lithospheres show exhumation PT paths that track isothermal decompression reflecting high strain rates along major shear zones, a feature typical of Phanerozoic collision belts. Field evidence, analogue and numerical models emphasize that strain localization has first-order consequences for the tectonic evolution of orogenic zones including structure, metamorphism, exhumation processes, topography, erosion and sedimentation modes.

Two identical zircon U-Pb ages have been obtained from the Riète orthogneisses at 470 ± 6 and 472 ± 2 Ma in the Aston and Hospitalet domes (Ariège, Pyrenees), respectively. New mapping data show that the protolith of these orthogneisses corresponds to Ordovician granitic laccoliths. Combined study of thin-sections and magnetic susceptibility on these rocks show that the laccoliths correspond to a suite consisting of granodiorites to leucogranites. U-Pb ages of the Aston and Hospitalet orthogneisses, very similar to the ages recently obtained from the Canigou (473 ± 4 Ma) and the Montagne Noire (southern French Massif Central) 469 ± 4 Ma orthogneisses, point to a major Early to Middle Ordovician event of granitic laccolith emplacement in the southwestern part of France, and more generally in western Europe. We underline that these laccoliths influenced the mechanical and thermal behaviour of the Variscan crust of the Pyrenees. Indeed, they have induced a rheological heterogeneity in the Variscan middle crust, which is at the origin of a structural contrast between the middle and upper crust. Moreover, these laccoliths played the role of screens, which have controlled transfer of magmas from the lower to the upper crust.

U–Pb spot zircon ages from metagabbros and metasediments of the Variscan lower crust in the Serre massif (southern Calabria) have been determined using LA-ICPMS and SIMS. The results, together with the literature data deriving from conventional and spot zircon ages and from Rb–Sr whole-rock isochrons, allow the geological history of the crustal block to be more tightly constrained from the Late Neoproterozoic to the Triassic, with particular emphasis on Variscan evolution. In a peri-Gondwana Pan-African/Cadomian basement, preserving memory of recycled older terrains with West African Craton affinities, mafic and felsic magmas emplaced at ca. 584 Ma (protoliths of metagabbros) and in the range 575–537 Ma (protoliths of felsic orthogneisses). Partially or completely recrystallized domains of zircons from metaigneous rocks yield ages ranging from 500 to 440 Ma. These ages (consistent with those of recrystallized cores of magmatic zircon from felsic orthogneisses and with Rb–Sr isochron data for the lower crust metasediments) suggest tectonothermal Ordovician activity, which is likely to be connected to the opening of Palaeozoic basin(s) separating the future Variscan blocks from Gondwana. Most zircon ages in the metasediments cluster in the 340–260 Ma range, which probably corresponds to two major events: (i) crustal thickening between 340 and 300 Ma (Barrowian metamorphic event) and (ii) lower-P/high-T metamorphism, Bt-dehydration melting, crustal extension between 300 and 260 Ma. Triassic ages ranging from 249 ± 4 Ma to 231 ± 5 Ma, together with whole-rock Rb–Sr isochron age at 234 Ma for the lowermost metapelites of the section, suggest tectonothermal activity, which prepared the opening of the Jurassic Neotethys. Similarities and differences relative to some circum-Mediterranean Variscan lower crust sections are also discussed, which contribute to the knowledge of south European Variscides evolution.

This study reassesses the development of compositional layering during the growth of granitic plutons, with emphasis on fractional crystallization and its interaction with both injection and inflation-related deformation. The Dolbel batholith (SW Niger) consists of 14, kilometre-sized plutons emplaced by pulsed magma inputs. Each pluton has a coarse-grained core and a peripheral layered series. Rocks consist of albite (An(

The late Indosinian Darongshan granite complex (Guangxi Province, South China) consists mainly of three plutons (Taima, Jiuzhou and Darongshan), with coarse-grained to subvolcanic rock types. There is a rough mineral evolution from the western to the eastern part of the complex, with ferromagnesian magmatic silicates sequentially distributed : Opx+Crd±Bt (Taima), Opx+Grt+Bt+Crd (Jiuzhou) and Bt+Crd (Darongshan). Restitic, cumulative or xenocrystic minerals (mainly Crd with fibrolite+spinel inclusions, Grt and probably Opx in some cases) are also encountered. Mineralogical, chemical and isotopic compositions of the granites suggest that the three plutons derive from a source dominated by a reduced, immature greywacke-psammite series, which has experienced high-amphibolite to granulite-facies metamorphism. This S-type granite complex is considered to result from various degrees of melting at different crustal levels by biotite dehydration-melting in a high T (800° to 950°C), low P (400–600 MPa) metamorphic environment. Abundant granulite-facies metasedimentary enclaves with refractory compositions are considered as xenoliths entrapped during magma ascent. The presence of scarce mafic microgranular enclaves and the high temperatures needed for melting could suggest that heat may have been partly contributed by mantle-derived magmas.

Two-phase quartz intergrowths with garnet, cordierite and tourmaline occur commonly in prograde high-temperature migmatites, granulites, as well as in the last crystallization stages of biotite granites. Structural, microtextural and mineralogical data show that they result from the breakdown of biotite in the presence of a melt phase associated with incongruent dissolution of feldspars into the melt and silica release (giving quartz in silica saturated rocks). Biotite breakdown and growth of Al-rich ferromagnesian minerals, occurring at the solid–liquid transition in the crust (early melting or final crystallization), is kinetically controlled by Fe and Mg mass transport, the network-forming cations Si and Al being locally compensated for by feldspar dissolution/crystallization. This process leads to significant changes with respect to equilibrium dehydration-melting reactions wherein quartz is a reactant and K-feldspar a reaction product. Therefore, quartz inclusions commonly occurring in garnets from granulite-facies metapelites and metagraywackes are not simply grains passively included during garnet growth. They may also correspond to newly crystallized phases. Resorption of feldspar may lead to more alkaline melt and to crystalline residue richer in Al than expected under equilibrium conditions. Hence, excess alumina in granulite-facies rocks is not necessarily related to initial alumina-rich whole-rock compositions (as currently considered), but may be due, at least partly, to kinetics of melting.

International audience; This paper presents a reappraisal of the main structural and metamorphic features of the Southern Svecofennian Arc Complex (southern Finland) and provides new data that allow us to reconsider its tectonic evolution. Results emphasize that (1) regional shortening was accommodated by distributed thickening with no clear evidence for large-scale thrusting, (2) syn-convergence lateral flow of ductile crust most probably competed with crustal thickening since early stages of deformation, (3) no evidence exists for thickening-driven extensional collapse associated with normal shear zones, the latest structures developed being compressive, (4) exhumation appears basically driven by distributed erosion that competed with crustal thickening. Observations are interpreted in terms of a peculiar mechanical behaviour of a hot and weak juvenile lithosphere undergoing compression.

International audience; The Hercynian Bordères-Louron pluton (20 km2) in the Central Pyrenees intrudes Devonian and Carboniferous metasediments. It shows a concentric zoning and consists of a significant proportion of (quartz) gabbros in its periphery, and of granodiorites, biotite monzogranites and biotite-muscovite monzogranites in its core. AMS study shows that the pluton corresponds to an elongated dome with a N100°E-trending axis. The anisotropy intensity Ppara% is high in the south and in the core of the pluton, whereas it is low in the north. The shape parameter T indicates that the fabric is strongly planar in a large central band oriented NW-SE, whereas it is strongly linear in the western and eastern tips of the pluton. These characteristics suggest that the Bordères-Louron pluton emplaced in two episodes: (1) intrusion of mafic magmas along a N100°E sill parallel to the regional foliation of the host metasediments; and (2) injection of three successive silicic batches (granodiorite, biotite monzogranite, two-micas monzogranite) which pushed aside the early mafic injections. In situ U-Pb dating of zircon grains indicates that the emplacement age of the biotite monzogranite is 309 ± 4 Ma, synchronous with the D 2 dextral transpressive event and close to the ages of the eastern Pyrenean plutons, may be slightly older.

Resume L'evolution fini-hercynienne du Massif central francais correspond au passage entre un metamorphisme BP–HT (phase M3 a 314 ± 5 Ma) et un metamorphisme de plus haute temperature accompagne de la mise en place du dome granitique du Velay (phase M4 a 301 ± 5 Ma). Nous montrons que cette transition est marquee par le developpement de feuillets a sillimanite, qui representent des plans preferentiels de dissolution acide accompagnant une deformation ductile. Cela implique un chemin P – T – t retrograde antihoraire, en conditions subsolidus entre M3 et M4. Pour citer cet article : P. Barbey et al., C. R. Geoscience 337 (2005).

The Were Ilu ignimbrites are unlike other Oligocene rhyolites from the Ethiopian continental flood basalt province, in that they consist of plagioclase (An19–54), augite, pigeonite and Ti-magnetite, instead of anorthoclase, sodic sanidine, aegirine-augite and ilmenite. The minerals occur as (micro-)phenocrysts isolated within a glassy matrix or forming gabbroic and dioritic cumulophyric clots. Plagioclase is partially re-melted (sieve-textures with infilling glass). It is zoned with sudden changes in composition. However, the bulk zoning is normal with An-rich core (An45–54) and more sodic rim (An19–28). Ba and Sr concentration profiles of two plagioclase phenocrysts show a bulk rimward increase with compositions ranging from 250 ppm to 1,060 ppm and from 400 ppm to 1,590 ppm, respectively. The matrix glass has low CaO content (0.1–0.5 wt.%), a peralkalinity index of 0.79–1.04 and average Sr and Ba contents of 48±22 and 525±129 ppm, respectively. Geochemical modelling of Ba and Sr zoning profiles of plagioclase, based on experimental constraints, suggests that the cumulophyric clots can be derived from fractional crystallisation associated with limited assimilation (8 wt.%) from melts slightly less evolved than their rhyolitic matrix glass. These clots are not witnesses of intermediate magmas allowing the Daly Gap to be filled, but are cumulates differentiated from rhyodacitic melt. This indicates that parental magmas were stored in crustal magma chambers where they differentiated before being erupted at the surface.

The deepest Hercynian metamorphic terrains in the Pyrenees and in the nearby Montagne Noire are made up of medium-grade orthogneisses and micaschists, and of high-grade, often granulitic, paragneisses. The existence of a granitic-metamorphic Cadomian basement and of its sedimentary Lower Paleozoic cover was advocated from the following main arguments: (i) a supposed unconformity of the Lower Cambrian Canaveilles Group (the lower part of the Paleozoic series) upon both granitic and metamorphic complexes; (ii) a ca. 580 Ma U-Pb age for the metagranitic Canigou gneisses. A SE to NW transgression of the Cambrian cover and huge Variscan recumbent (“penninic”) folds completed this classical model. However, recent U-Pb dating provided a ca. 474 Ma, early Ordovician (Arenigian) age for the me-tagranites, whereas the Vendian age (581 ± 10 Ma) of the base of the Canaveilles Group was confirmed [Cocherie et al., 2005]. In fact, these granites are laccoliths intruded at different levels of the Vendian-Lower Cambrian series. So the Cadomian granitic basement model must be discarded. In a new model, developed in the Pyrenees and which applies to the Montagne Noire where the orthogneisses appear to be Lower Ordovician intrusives too, there are neither transgression of the Paleozoic nor very large Hercynian recumbent folds. The pre-Variscan (pre-Upper Ordovician) series must be divided in two groups: (i) at the top, the Jujols Group, mainly early to late Cambrian, that belongs to a Cambrian-Ordovician sedimentary and magmatic cycle ; the early Ordovician granites pertain to this cycle; (ii) at the base, the Canaveilles Group of the Pyrenees and the la Salvetat-St-Pons series of the Montagne Noire, Vendian (to earliest Cambrian?), are similar to the Upper Alcudian series of Central Iberia. The Canaveilles Group is a shale-greywacke series with rhyodacitic volcanics, thick carbonates, black shales, etc. The newly defined olistostromic and carbonated, up to 150 m thick Tregurà Formation forms the base of the Jujols Group, which rests more or less conformably on the Canaveilles Group. The high-grade paragneisses which in some massifs underlie the Canaveilles and Jujols low- to medium grade metasediments are now considered to be an equivalent of the Canaveilles Group with a higher Variscan metamorphic grade; they are not derived from metamorphic Precambrian rocks. So, there is no visible Cadomian metamorphic (or even sedimentary) basement in the Pyrenees. However, because of its age, the Canaveilles Group belongs to the end of the Cadomian cycle and was deposited in a subsident basin, probably a back-arc basin which developed in the Cadomian, active-transform N-Gondwanian margin of this time. The presence of Cadomian-Panafrican (ca. 600 Ma) zircon cores in early Ordovician granites and Vendian volcanics implies the anatexis of a thick (> 15 km?) syn-Cadomian series, to be compared to the very thick Lower Alcudian series of Central Iberia, which underlies the Upper Alcudian series. Nd isotopic compositions of Neoproterozoic and Cambrian-Ordovician sediments and magmatites, as elsewhere in Europe, yield Paleoproterozoic (ca. 2 Ga) model-ages. From the very rare occurrences of rocks of this age in W-Europe, it can be envisionned that the thick Pyrenean Cadomian series lies on a Paleoproterozoic metamorphic basement. But, if such a basement does exist, it must be “hidden”, as well as the lower part of the Neoproterozoic series, in the Variscan restitic granulites of the present (Variscan) lower crust. So a large part of the pre-Variscan crust was made of volcano-sedimentary Cadomian series, explaining the “fertile” characteristics of this crust which has been able to produce the voluminous Lower Ordovician and, later, Upper Carboniferous granitoids.

Zircon U–Pb data on post-tectonic granites that have intruded the Precambrian formations of the Kerdous inlier (western Anti-Atlas, Morocco) are Pan-African. Zircon grains from a syntectonic monzodiorite of the Tahala pluton yield a precise discordia line with an upper intercept at 2043.9 ± 1.8 Ma (MSWD = 0.67) interpreted as the emplacement age. This age is consistent with zircon U–Pb ages of granites from other inliers and confirms the existence of a widespread magmatic event at ≈2045 Ma in the Anti-Atlas. The Pan-African event left no resolvable imprint on the isotopic systems of the analysed zircons. The Kerdous Schists are, therefore, older than 2045 Ma. Rb–Sr and Sm–Nd data on the schists and the Tahala pluton suggest that both juvenile and Archaean material contributed to the generation of the Palaeoproterozoic formations in the Anti-Atlas.

International audience

Granites were shown to be excellent geochronological, structural and geodynamic markers. Among several generations of granites described in the Neoproterozoic of Ethiopia, we studied the post-tectonic Konso pluton to characterise the post-Pan-African evolution of the Mozambique Belt (MB) of southern Ethiopia. The Konso pluton is a composite intrusion of slightly peraluminous and ferro-potassic, bt (biotite)–leucogranites, bt–hbl (hornblende)–granites and subordinate coeval metaluminous monzodiorites, intruded into high-grade gneiss–migmatite associations of the MB. The whole suite displays chemical features of A-type granites. It is LIL- and HFS-elements enriched with Y/Nb and Yb/Ta≥1.2. The granites and leucogranites show non-fractionated to fractionated REE patterns [(La/Yb)N=0.3–9.4] with strong negative Eu anomalies. The monzodiorites show fractionated REE patterns [(La/Yb)N=5.5–7.4] with negligible negative Eu anomaly. The low initial (87Sr/86Sr)450 ratios (0.70113–0.70441) and positive eNd(450) values (+1.8 to +3.3) suggest an isotopically primitive source. The Konso granites are likely to be derived from a basaltic parent, with minor contamination by crustal material with high Y/Nb and low Sr initial isotopic ratios. Age of pluton emplacement is constrained by a Rb–Sr isochron and zircon U–Pb data at 449±2 Ma. The Konso pluton is, therefore, the witness of an Ordovician A-type magmatic event, which marks a change from convergence, related to the Pan-African collision, to extension in the Mozambique Belt of southern Ethiopia.

The Negash pluton (50 km 2 ) consists of late Pan-African, high-K, calc-alkaline granitoids intruded into low-grade metavolcanics–metasediments. This almost circular massif consists of monzogranites, granodiorites, diorites–gabbrodiorites, and hybrid diorites. The anisotropy of magnetic susceptibility (AMS) method was used to determine internal structures of the pluton. The foliation trajectories are concentric and inward dipping. The lineation pattern displays an external zone characterised by horizontal concentrically oriented lineations and an internal zone with NW–SE oriented lineations. These petro-structural data clearly locate the feeder zone at the north-western tip of the pluton and indicate the subsequent expansion of the magmas towards the SE. The pluton is a result of in-situ assembly of four magma batches, which were forcefully injected into pre-existing foliated country rocks in relation to transpressional tectonic regime. Two types of mafic–felsic magma interactions are recognised: homogeneous and heterogeneous hybrid diorites at the north-western part, and mingled interfaces at the diorite–granodiorite contact zones mainly visible in the eastern and south-eastern parts. The in-situ mingling along diorite–granodiorite contacts was achieved at the level of emplacement during the injection of dioritic magma into the felsic magmas while the hybrid diorites are assumed to result from two-way conduit mixing and mingling during simultaneous rising of mafic and felsic magmas.

This work is a review of Boris Choubert’s paper (1935), which was published in French under the rather devalorizing title: “Research on the Genesis of Palaeozoic and Precambrian Belts.” Despite its innovative content, this article had no impact either at the time of its publication or even later. It begins with the construction of a remarkable fit of the circum-Atlantic continents. This was based on the −1.000 meters isobath instead of the shoreline. Thirty years before Bullard et al. (1965), it demonstrated in an indisputable way the reality of the continents motion on the surface of the Earth. Therefore, Choubert designated Wegener’s “continental drift” as the main cause of tectonics. Even going beyond Wegener’s theory, he argued that this mechanism was efficient well before the formation of the Triassic Pangæa, during the whole Palaeozoic to result in the building of the Caledonian and Hercynian mountains. Although he was still encumbered by the vocabulary of the time regarding geosynclines, Boris Choubert described tectonics based on the horizontal mobility of the Precambrian continental blocks. Oddly enough, he did not apply this model to the Precambrian structures, which he attributed to the effects of the Earth’s rotation on the continental crust during its solidification. At the time of its publication, this paper was a very important step towards understanding global tectonics. Unfortunately, Choubert’s contemporaries did not generally recognize its significance. Ce travail est une analyse de l’article de Boris Choubert (1935) paru en français sous le titre un peu réducteur: « Recherches sur la genèse des chaînes paléozoïques et antécambriennes. » Malgré son contenu novateur, cet article n’a eu aucun retentissement à l’époque de sa publication, ni même plus tard. Il débute par la construction d’un remarquable assemblage des continents péri-atlantiques fondé sur l’isobathe −1.000 mètres et non pas sur la ligne de rivage. Trente ans avant Bullard et al. (1965), il démontre de façon indiscutable la réalité du déplacement des masses continentales à la surface de la Terre. Dès lors, Choubert privilégie totalement la « dérive des continents » comme moteur de la tectonique. Allant au-delà même de la vision de Wegener, il fait jouer ce mécanisme bien avant la formation de la Pangée triasique, pendant toute la durée de l’ère primaire, au cours de l’érection des chaînes calédonienne et hercynienne. Bien que restant prisonnier du vocabulaire de l’époque sur les géosynclinaux, Boris Choubert décrit une tectonique fondée sur la mobilité des blocs continentaux précambriens. Curieusement, il n’applique pas ce modèle aux déformations précambriennes qu’il attribue aux effets de la rotation de la Terre sur une croûte continentale en voie de solidification. Lors de sa parution, cet article représentait un pas très important vers la compréhension de la tectonique globale et il est regrettable qu’il n’ait pas été mieux reçu par ses contemporains. ISSN:0037-9409 ISSN:1777-5817

In this article, we discussed about the petrography and geochemistry of magmatic rocks of the Mbip massif located SW of Tchollire, in Central North Cameroon (Central African Fold Belt). Petrographic study shows that this massif is made of granodiorite, amphibole-biotite granite, and biotite granite which often contain enclave of mafic rocks (gabbro). Granodiorites and granites show porphyritic texture and consist dominantly of plagioclase and alkaline feldspar phenocrysts, quartz, biotite and often few amphibole. The gabbro enclaves are characterized by a granular porphyroid texture constituted of amphibole, plagioclase and olivine phenocrysts, all in a matrix of small crystals of plagioclase, olivine, amphibole, pyroxene and opaque minerals. All the analyzed rocks provide geochemical features of sublkaline serie and have nothing to do with the alkaline nature previously signaled in the former works. Granodiorites and granites are shoshonitic to calc-alkaline strongly potassic affinity consistent with emplacement in the continental collision setting. They present positive anomalies in Rb, Th, U; negative anomalies in Sr, P, Ti, Nb, Ta and some facies contain hydrated minerals such as amphibole. These characters are consistent with crustal and mantle contribution in their genesis. The mafic rocks (gabbro) have geochemical characteristics conferring a mantle origin, as confirmed by the high Mg# value (49.9). On the whole, the fractional crystallization of the magma formed by melting of crustal material and the magmatic mixing are the main petrogenetic process in the Mbip massif. Petrographic studies and geochemical data shows that the Mbip massif was emplaced in an active continental margin, into transitional regime from the end of maximum compression to the beginning of relaxation.

Quelles sont les valeurs qui caracterisent sur le territoire francais les rayonnements ionisants et les radionucleides d'origine naturelle ? Comment distinguer dans le resultat d'une mesure la part qui resulte eventuellement d'une activite humaine ? Afin d'essayer de repondre a ces questions, un questionnaire a ete adresse aux differents organismes qui effectuent en France des mesures de radioactivite dans l'environnement. Les informations mentionnees dans cette etude se rapportent exclusivement a des observations faites dans l'environnement a l'exterieur des bâtiments et du perimetre des installations nucleaires de base (INB). En outre, elles ne concernent, sauf cas particuliers signales, que des mesures de rayonnements ou de radioelements d'origine naturelle dont les resultats n'ont pas ete perturbes par l'action de l'homme. L'enquete montre la difficulte actuelle de dresser en France un bilan exhaustif des connaissances acquises concernant les caracteristiques du bruit de fond radioactif d'origine naturelle dans l'environnement. Aujourd'hui en France, dans le domaine de la radioactivite naturelle, les efforts consentis manquent de coordination et d'orientation vers certaines priorites. A ce titre, les mesures des 238 U et 232 Th precurseurs des 226 Ra et 228 Ra, des descendants solides emetteurs alpha du radon (par la mesure des EAP du 222 Rn et 220 Rn), des emetteurs gamma post emanation (214 Pb, 214 Bi, 212 Pb), du 210 Pb, 210 Po et du 14 C doivent etre privilegiees.

The Archaean, syntectonic, A-type Estrela Granite Complex (Carajas Mineral Province, Brazil) consists of three plutons emplaced in a greenstone sequence under low-pressure conditions (180

U–Pb age determinations with an ion probe on zircons within orthogneisses from the Canigou Massif, eastern Pyrenees, give an emplacement age for their granitic protolith of 475±10 Ma (Arenigian). This invalidates the previous interpretation of basement-cover relationships between the orthogneisses and the overlying Lower Cambrian Canaveilles metasedimentary series, and corroborates the intrusive character of the orthogneisses, which is further supported by field evidence. The data also define two groups of inherited zircons (Paleo/Meso-Proterozoic and Cadomian) in addition to partial lead loss during the Hercynian Late Carboniferous high temperature–low pressure metamorphic event. These results are in agreement with ages reported from other orthogneissic bodies across the entire Variscan chain of western Europe, and indicate the existence of a widespread Cambro-Ordovician magmatic event.

From new field and single zircon geochronological data, we reinterpret the structure of the Canigou orthogneisses (Eastern Pyrenees) as an Early Ordovician rapakivi granite laccolith. This laccolith is surrounded by a contact metamorphic aureole predating the Hercynian tectono-metamorphic events. This aureole consists of migmatites at the contact with unmelted orthogneisses, of skarns in the carbonate beds and of retrogressed Al-rich porphyroblasts over some hundred metres beyond the contact. The metallogenic activity (Fe, F) observed around the massif might be related to the laccolith emplacement.

The granitoids of Marajoara in the Rio Maria terrain (Carajas Mineral Province, Brazil) consist of: (i) a broad unit of 2.96 Ga syntectonic tonalites (Arco Verde Tonalites) displaying a trondhjemitic differentiation trend; (ii) 2.93 Ga syntectonic monzogranites (Guaranta); and (iii) 2.87 Ga post-tectonic monzogranites (Mata Surrao) and granodiorites (Rio Maria), displaying a calc-alkaline differentiation trend. Deformation of the Arco Verde tonalites is heterogeneous with low strain domains (well preserved magmatic banding and textures) and orthogneissic domains displaying an E–W trending mainly subvertical foliation, associated with horizontal lineations, upright folds and subvertical shear zones. Microstructures and phase assemblages suggest that deformation occurred within a large temperature range (i.e. during magma emplacement and cooling), from high-T conditions (synmagmatic shear zones and subsolidus ductile deformation with intense quartz and feldspar recrystallization; Pl+Qtz+Hbl+Bt assemblages) to medium- and low-T conditions (ductile to brittle deformation with weakly recrystallized quartz and undulose extinction in feldspars; Qtz+Pl+Bt+Mu or Chl+Ep+Ab+Qtz assemblages). These data, finite strain analysis and structures reported from the surrounding greenstone belts suggest that deformation did not result from a post-emplacement prograde tectono-metamorphic event as considered previously, but that the Marajoara granitoids are synkinernatic intrusions which were deformed together with the supracrustal rocks during a regional NS horizontal shortening. Although the Rio Maria terrain presents similarities with Archaean domains controlled by diapiric processes (lithologies dominated by thick greenstone sequences and TTG plutons, and forming a dome-and-keel structure), its structural evolution is controlled dominantly by a transpressional event which shaped the granite–greenstone terrains. The Rio Maria area, probably as many Archaean ‘grey gneisses’ domains, represents an intermediate case between terrains controlled by Raleigh–Taylor instabilities in a thermally softened crust with insignificant external forces related to plate convergence (e.g. east Pilbara craton) and those controlled by thrust tectonics related to convergence of rigid plates (e.g. Superior Province). The closest analog to the Rio Maria terrain seems to be the Chilimanzi area in the Zimbabwe craton.

Resume Deux ensembles magmatiques contemporains, d'âge paleoproterozoique, peuvent etre distingues par leur mineralogie et leur composition chimique, dans les formations paleoproterozoiques des boutonnieres du Bas Drâa et de la Tagragra d'Akka (Anti-Atlas Occidental, Maroc). Le premier ensemble est constitue de diorites, de monzogabbrodiorites, de granodiorites et de granites (50.9 La nature et la signature geochimique de ces deux types de granitoides dont la mise en place est contemporaine, impliquent une interaction entre des magmas d'origine mantelique et une croute continentale. La nature silicoclastique des series sedimentaires paleoproterozoiques dans lesquelles se sont mis en place ces granitoides confirme la presence d'un domaine continental d'âge probablement archeen. L'absence de formations volcaniclastiques dans ces deux boutonnieres exclut l'existence d'un arc volcanique. Un contexte d'arriere-arc similaire a ce qui a ete decrit en Argentine pour le Paleozoique est propose. Une comparaison avec la Dorsale Reguibat et la Dorsale de Leo montre l'existence de deux grands domaines paleoproterozoiques dans le craton ouest-africain: (i) un domaine correspondant a une ancienne croute archeenne recyclee situee a l'Ouest et au Nord (blocks de Man, du Tiris et de l'Anti-Atlas); et (ii) un domaine juvenile paleoproterozoique situe au Sud-Est au Nord-Est (domaine Baoule-Mossi et et Dorsale Reguibat orientale).

The Ntui-Betamba area (southern Cameroon) is composed of high-grade migmatitic gneisses in which two lithological units are distinguished: (i) a metasedimentary unit (kyanite-biotite-garnet gneisses, biotite-muscovite-garnet gneisses, calc-silicate rocks and quartzites) interpreted as a continental margin sedimentary series; and (ii) meta-igneous rocks comprising alkaline ultramafic to mafic pyroxene gneisses and amphibolites and amphibole-bearing alkaline orthogneisses. These units recrystallised under HP-HT conditions ( T =750–800 °C, P ≥0.9–1.3 GPa) and were deformed in relation to a major tangential tectonic event with a north-northeast-south-southwest kinematic direction. This lithological association and its tectono-metamorphic evolution show striking similarities with the Yaounde gneisses, suggesting that the extensional depositional environment envisaged for this formation can be extended farther north, towards the Adamawa Shear Zone (Lorn series). The contrasted metamorphic evolution between areas located to the south of the Adamawa (high pressure: Yaounde, Ntui-Betamba), and those located to the north (low pressure: Banyo), along with widespread remains of a Palaeoproterozoic crust, suggest important crustal thickening during tangential tectonics in southern Cameroon. As a consequence, the Adamawa Shear Zone is not simply a late Pan-African transcurrent or transpressive shear zone but appears to have been formerly a major (possibly intracontinental) thrust zone.