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The Egéré terrane (Central Hoggar, South Algeria) includes mafic eclogite lenses boudinaged in metapelitic rocks with high-pressure relicts. These metapelites show textural records of partial melting, mainly primary melt inclusions enclosed in garnet crystals and later crystallized as "nanogranitoids."Garnet porphyroblasts also contain inclusions of quartz, kyanite, phengite, biotite, staurolite, and rutile and show a smoothed prograde zoning with a Mn bell-shaped profile. The peak high-pressure metamorphic assemblage consists of garnet, kyanite, phengite (Si up to 6.36), quartz, rutile, ±ilmenite, ±feldspars, and melt. Phengite has partially transformed into fine-grained aggregates of biotite, plagioclase, and K-feldspar, a microstructure interpreted as resulting from a dehydration melting during exhumation. Phengite breakdown, along with other retrograde reactions, produced a late paragenesis with biotite, plagioclase, K-feldspar, quartz, almandine-rich garnet, ±sillimanite, ±staurolite, ±muscovite, and ilmenite. The thermodynamic modeling of P-T pseudosections allows us to constrain various steps of the metamorphic history: beginning of the garnet growth at 4.0 kbar and ~600 °C during prograde metamorphism; pressure peak at 14-20 kbar; temperature peak at 800-820 °C; formation of the last assemblage at 6.0-5.5 kbar and 725-685 °C. Partial melting likely started during the prograde path when crossing the H2O-saturated solidus, at T ≥ 650-670 °C and P ≥ 10 kbar, continued upon heating, up to the peak conditions, as well as during decompression. This evolution is interpreted in terms of subduction of the continental crust to mantle depths, followed by an exhumation through a clockwise P-T path during the Pan-African orogeny. The Egéré metapelites are relatively well-preserved eclogite-facies rocks, contain inclusions of "nanogranitoids"hitherto very little known in eclogite-facies metamorphic rocks, and represent an unusual trace of subduction within a Neoproterozoic orogen., This work is a contribution to the project PHC TASSILLI 15MDU943 and the FP7-IRSES-612572 project “MEDYNA” funded under REA Grant Agreement PIRSES-GA-2013-612572. Grateful thanks are due to the Direction Générale de la Recherche Scientifique et du Développement Technologique of Algeria for funding this work that forms part of the PRFU (E04N01UN160420180001) and PNR ATRST research projects in Algeria. We are extremely grateful to Office du Parc National de l’Ahaggar (Tamanrasset, Algeria) for logistic support during fieldwork, to Chunjing Wei and Abderrahmane Bendaoud for constructive reviews and to Fang-Zhen Teng for the editorial work.

International audience; The Tighsi area of the Egéré/Aleksod Terrane (Tuareg Shield) contains mafic eclogites interlayered within ana-tectic metapelites corresponding to metabasalts coeval with a shallow water marine sedimentation. In this study we present in-situ geochronological (U-Pb) and geochemical (major and trace elements, Hf and O isotopes) analyses from a 2.6 mm zircon megacryst found in a high-pressure kyanite pegmatite enclosed in the metapelites.Os-cillatory zoning, HREE-enriched patterns, positive Ce anomalies and the absence of negative Eu anomalies are consistent with crystallization of the megacryst from the anatectic melt. Ti-in-zircon temperatures indicate crys-tallization at 811 ± 15°C in an allanite/monazite buffered anatectic melt as evidenced by the low Th/U ratios. Oxygen isotopes yield large intra-grain variations (7.1-12.3‰) with a gradual lowering towards the edge of the grain in contact with secondary feldspathic veinlets. These results indicate fluid-assisted oxygen isotope disturbances , consistent with the low retentivity of O in zircon under wet conditions. Hf isotopes do not display in-tra-grain variations (mean εHf i = −20.7 ± 1.0) and support production of the leucosome by melting of crustal material. U-Pb analyses of the center of the crystal provide an age of 654 ± 5 Ma (2σ), attributed to post-peak decompression and heating. HP eclogite facies conditions in the Egéré terrane are thus significantly older than HP metamorphism in the western part of the shield (610-625 Ma) in agreement with multiple subduction events along the margins of the Tuareg Shield.The rim of the megacryst, close to feldspathic veinlets, is characterized by a significant decrease of trace elements (U, Y, HFSE), but preserved identical Hf isotope ratios, which is consistent with recrystallization processes. The rim displays a Ti-in-zircon temperature of 717 ± 28°C and a U-Pb age of 584 ± 6 Ma (2σ) coeval with the climax of batholith intrusion in Central Hoggar. Reheating and softening of the lower/middle crust at that time may have assisted and favored upward viscous flow of basement domes and escape tectonics along lithospheric shear zones. We propose that the final push of the Saharan metacraton in the east was responsible for the observed architecture of the Egéré terrane, where anatectic elongated domes of basement gneisses alternate with HP metasedimentary synforms.

The geological structure, mineralogical and geochemical features of the Amesmessa gold quartz-vein type ore deposit located in In-Uzzal region of Hoggar shield (the Algerian Sahara) are described. The ore-bearing object has Proterozoic age and belongs to the gold - quartz - low-sulfide formation. The basis of its structure and a typical geological setting of the ore veins location are the fault paragenesis that had arisen in a process of the development of a big fault zone during the Ebourneisky tectonic and magmatic cycle. This object belongs to a formation typical for the metallogenic zones of the green belts of the ancient shields and has good prospects of accumulation of the ore resource potential.

International audience; The In Ouzzal terrane in western Hoggar (Southern Algeria) preserves evidence of ultrahigh temperature (UHT) crustal metamorphism. It consists in Archean crustal units, composed of orthogneissic domes and greenstone belts, strongly remobilized during the Paleoproterozoic orogeny which was recognized as an UHT event (peak T > 1000 °C and P ≈ 9–12 kbar). This metamorphism was essentially defined locally in Al-Mg granulites, Al-Fe granulites and quartzites outcropping in the Northern part of the In Ouzzal terrane (IOT).In order to test and verify the regional spread of the UHT metamorphism in this terrane, ternary feldspar thermometry on varied rock types (Metanorite, Granulite Al-Mg and Orthogneiss) and samples that crop out in different zones of the In Ouzzal terrane. These rocks contain either perthitic, antiperthitic or mesoperthitic parageneses. Ternary feldspars used in this study have clearly a metamorphic origin.The obtained results combined with previous works show that this UHT metamorphism (>900 °C) affected the whole In Ouzzal crustal block. This is of major importance as for future discussion on the geodynamic context responsible for this regional UHT metamorphism.

The analysis of gamma ray measurements provides valuable information about the basement's lithospherical model and the radioactive elements contained in the subsurface. To be exploited, the raw data need pre-processing operations which consist of corrections of the Compton effect, altimetry and the aircraft background, and 2D interpolations. These transformations may modify the data's stochastic component which reflects heterogeneities of the investigated medium. In this study, a fractal analysis is carried out on airborne gamma ray data measured in the Hoggar's area (Algeria). First, it is shown that the analyzed data exhibit fractal properties. Due to the dependence of the latter on the spatial location of the measures, the data can be considered as paths of multifractional Brownian motions. Another result worth noting is that the fractal nature of the raw data is not modified by the pre-processing operations. The fractal analysis can be then performed on the raw spectrometric measurements without submitting them to any pre-processing; thus, the stochastic component of the measurements is kept intact.

The mantle xenoliths included in Quaternary alkaline volcanics from the Manzaz-district (Central Hoggar) are proto-granular, anhydrous spinel lherzolites. Major and trace element analyses on bulk rocks and constituent mineral phases show that the primary compositions are widely overprinted by metasomatic processes. Trace element modelling of the metasomatised clinopyroxenes allows the inference that the metasomatic agents that enriched the lithospheric mantle were highly alkaline carbonate-rich melts such as nephelinites/melilitites (or as extreme silico-carbonatites). These metasomatic agents were characterized by a clear HIMU Sr–Nd–Pb isotopic signature, whereas there is no evidence of EM1 components recorded by the Hoggar Oligocene tholeiitic basalts. This can be interpreted as being due to replacement of the older cratonic lithospheric mantle, from which tholeiites generated, by asthenospheric upwelling dominated by the presence of an HIMU signature. Accordingly, this rejuvenated lithosphere (accreted asthenosphere without any EM influence), may represent an appropriate mantle section from which deep alkaline basic melts could have been generated and shallower mantle xenoliths sampled, respectively. The available data on lherzolite xenoliths and alkaline lavas (including He isotopes, Ra

International audience; The In-Ezzane volcanic district (EZD), located at the triple junction of Algeria, Niger and Libya belongs to the Eastern Hoggar, covers 350 km2 and includes 9 volcanic edifices that are probably aligned along NW-SE faults, parallel to the Tafassasset valley. The low volume (0.7 and 1 km3) of emitted lavas, the similar morphology of the monogenic cones and the lack of differentiated rocks indicate that the volcanic activity of the EZD was restricted in time and volume. The new K-Ar age (i.e. 2.86 ± 0.07 Ma) indicates that the EZD is contemporaneous with the last alkali volcanism paroxysm in Hoggar and with the nearby Libyan volcanics. The EZD alkali basalts (mainly basanite) show a remarkable homogeneous compositions both in major elements (44.8 ⩽ SiO2 ⩽ 45.8 wt.%; 5.2 ⩽ (Na2O + K2O) ⩽ 6.2 wt.%), trace elements (4.3 ⩽ Th ⩽ 5.5 ppm; 34.7 ⩽ La ⩽ 44.7 ppm; 16.1 < La/Yb(N) < 21.6) and radiogenic isotopes (0.70285 < 87Sr/86Sr < 0.70303; 0.51298 < 143Nd/144Nd < 0.51301; (19.212 < 206Pb/204Pb < 19.340, 15.589 < 207Pb/204Pb < 15.602 and 38.834 < 208Pb/204Pb < 38.903). Relative to the Hoggar alkaly basalts the EZD basalts appear systematically impoverished in incompatible elements and show a depleted signature both in Sr and Nd isotopes (almost MORB-like). The Pb isotopes are relatively enriched and intermediate between tholeiites and alkali basalts of the Hoggar. This unusual geochemical signature, is uneasy to reconcile with the known characteristics of the Hoggar swell, and would rather fingerprint a circum cratonic mantle lying beneath the west border of the Murzuq craton (Libya).

International audience; The Eastern Hoggar is by far the least well known part of the Tuareg Shield. It is composed, from west to east, of the Aouzegueur, Edembo and Djanet Terranes. The Djanet Terrane is the easternmost Hoggar terrane and comprises a greenschist-facies clastic sedimentary sequence, the Djanet Group, intruded by granitoids. Laser ablation (LA-) ICP-MS U-Pb zircon ages of detrital zircons from these sediments yield a large range of ages similar to ages known in Central and Western Hoggar; the youngest is 590 +/- 10 Ma (2 sigma), which is the maximum age of deposition of the Djanet Croup. The Djanet Group is intruded by the Djanet Batholith (571 +/- 16 Ma), by high-level subcircular plutons such as the Tin Bedjane Pluton (568 +/- 5 Ma) and finally by the felsic Tin Amali Dyke Swarm (558 +/- 5 Ma), all ages being SHRIMP U-Pb zircon. The deposition and metamorphism of the Djanet Group thus occurred between 590 Ma (the age of the youngest detrital zircons analyzed in the Djanet Group) and c. 570 Ma (the age of intrusive granitoids). Nd T-DM two-stage model ages (1.30-2.03 Ga), initial Sr-87/Sr-86 ratios (0.7035-0.7095) and epsilon(Nd) (-3 to -11) of the three intrusive suites, all being high-K calc-alkaline in composition, indicate mainly an old, Rb-depleted continental source. Migmatization in the adjacent Edembo Terrane is dated at 568 +/- 4 Ma (U-Pb zircon SHRIMP age), contemporaneous with the intrusion of the Djanet plutons. Rocks and events in the Edembo and Djanet Terranes are all intracontinental. We propose that the Djanet Terrane lies on the boundary of a craton located to the north-east, that we defined here as the Murzuq craton. The existence of the Murzuq craton is supported by sedimentary and geophysical data. We suggest that the deformation of Eastern Hoggar occurred 575-555 Ma, due to the indentation of the Murzuq craton and has no link with the older convergence with the West African craton as was the case for the central and western parts of the Tuareg Shield. This determines a late Ediacaran intracontinental Murzukian event that probably also occurred to the east in the Tibesti area.

International audience; The N-S oriented Raghane shear zone (8 degrees 30') delineates the western boundary of the Saharan metacraton and is, with the 4 degrees 50' shear zone, the most important shear zone in the Tuareg shield. It can be followed on 1000 km in the basement from southern Air, Niger to NE Hoggar, Algeria. Large subhorizontal movements have occurred during the Pan-African orogeny and several groups of granitoids intruded during the Neoproterozoic. We report U-Pb zircon datings (laser ICP-MS) showing that three magmatic suites of granitoids emplaced close to the Raghane shear zone at c. 790 Ma, c. 590 and c. 550 Ma. A comprehensive and detailed (158 sites, more than 1000 cores) magnetic fabric study was performed on 8 plutons belonging to the three magmatic suites and distributed on 200 km along the Raghane shear zone. The main minerals in all the target plutons do not show visible preferential magmatic orientation except in narrow shear zones. The AMS study shows that all plutons have a magnetic lineation and foliation compatible with the deformed zones that are zones deformed lately in post-solidus conditions. These structures are related to the nearby mega-shear zones, the Raghane shear zone for most of them. The old c. 793 Ma Touffok granite preserved locally its original structures. The magnetic structures of the c. 593 Ma Ohergehem pluton, intruded in the Aouzegueur terrane, are related to thrust structures generated by the Raghane shear zone while it is not the case of the contemporaneous plutons in the Assode-Issalane terrane whose structures are only related to the subvertical shear zones. Finally, the c. 550 Ma granite group has magnetic structure related to the N-S oriented Raghane shear zone and its associated NNE-SSW structures when close to them, but NW-SE oriented when further. These NW-SE oriented structures appear to be characteristic of the late Neoproterozoic evolution of the Saharan metacraton and are in relation to the convergence with the Murzuq craton. This evolution reflects the rheological contrast existing along the Raghane shear zone marking the western boundary of the Saharan metacraton.

Here, we suggest a new algorithm to study the local fractal properties of airborne spectrometric data considered as 2D-mBm paths. This algorithm is first validated on synthetic two-dimensional multifractional Brownian motion (2D-mBm) paths, and then implemented on natural Gamma rays measurements recorded in Hoggar (Algeria). The regularity (Holder exponent) maps obtained from the different channels (K, Th and U) show a strong correlation. Besides, these maps allow depicting the location of some faults affecting the studied area, especially the main 4°50’ fault. Our findings show that the Holder exponent can then be used as a tool for the geological characterization.

Pyroxenite, almost entirely composed of iron-rich orthopyroxene with very little fayalite and quartz, forms the extremity of a lens of pyroxenites and basic charnockites. This lens is one of a group of numerous bodies of basic and ultrabasic rocks belonging to the charnockitic suite of the Gour Oumelalen (Ahaggar, Algeria), which are surrounded by garnet-sillimanite gneisses and marbles. This orthopyroxenite gives place in few decimeters to a clinopyroxenite with orthoferrosilite and fayalite. Basic charnockite is a norite with brown amphibole and biotite. The interaction between a vein of hypersthene pegmatite and the clinopyroxenite has produced secondary minerals, amphibole (hastingsite) and garnet (almandine). Physical properties of the iron-rich OPX from the "orthoferrosilitite" are given. Microprobe analyses of ferromagnesian silicates in these rocks permit some estimates, based upon experimental data, of temperature and pressure. The composition of coexisting clinopyroxene-orthopyroxene suggests that temperatures ranged from 800 °C to 860 °C, The composition of orthoferrosilite (OFS 87-92) coexisting with fayalite (Fa 93 -95) applied to various experimental data indicates pressures of 8. 1-8.7 kbar at 860 °C (8-9 kbar at 800 °C). Temperatures estimated from garnet-biotite geothermometry in gamet-sillimanite gneisses are between 760 °C and 850 °C. The metamorphic pressure, measured from the garnet-plagioclase-sillimanite-quartz equilibrium, is close to 8.5 kbar at 790 °C. These diverse P-T estimates define the final metamorphic crystallisation conditions as 760 °C-860 °C and 8-9 kbar, which agree well with those recorded from other Proterozoic granulite facies series., Cette étude présente des données pétrographiques et minéralogiques sur une pyroxénite formée presque uniquement de pyroxène riche en fer (Fs90), d'un peu de fayalite et de quartz. Cette "orthoferrosilitite" constitue l'extrémité d'une lentille de pyroxénites et de charnockites basiques qui appartiennent à un vaste ensemble de roches basiques et ultramafiques faisant partie de la série charnockitique des Gour Oumelalen (N.E. Ahaggar, Algérie). La lentille comprenant ces roches à orthoferrosilite est environnée par des gneiss à grenat et sillimanite et des marbres. L'orthopyroxène passe, en quelques centimètres à une clinopyroxénite à orthoferrosilite et fayalite. L'interaction entre une veine de pegmatite à hypersthène et la clinopyroxénite a produit des minéraux secondaires, amphibole (hastingsite) et grenat (almandin). Les propriétés physiques du pyroxène de 1' "orthoferrosilitite" ont été étudiées. L'analyse à la microsonde des différents minéraux a permis une estimation des conditions physiques de formation de ces roches. La composition des pyroxènes coexistants suggèrent que les roches à orthoferrosilite ont achevé leur cristallisation entre 800 °C et 860 °C. La composition de l'orthoferrosilite (Fs87-92) en équilibre avec la fayalite (Fa93-95) indique, à partir de données expérimentales récentes, une pression de formation d'environ 8,2-8,7 kbar à 860 °C. Les températures, estimées à partir des couples grenat-biotite dans les métapélites à grenat et sillimanite (780 °C-850 °C), sont en bon accord avec les résultats précédents. L'utilisation du géobaromètre grenat-plagioclase à partir des calibrages de Newton et Haselton (1981), conduit à une pression de 8,5 kbar ± 1 kbar à 790 °C. Ces diverses estimations, obtenues par des méthodes indépendantes, permettent de considérer que les roches à orthoferrosilite ont achevé leur évolution dans les conditions du faciès granulite de moyenne pression. L'existence de reliques de textures magmatiques et de températures plus élevées obtenues à partir des charnockites basiques, indiquent que les conditions définies pour les équilibres subsolidus correspondent à un stade terminal de l'évolution des charnockites et des pyroxénites associées., Latouche Louis. L'orthoferrosilite et les roches associées de la région des Gour Oumelalen (N.E. Ahaggar, Algérie). In: Bulletin de Minéralogie, volume 106, 3, 1983. pp. 329-339.