The Tighsi area of the Egere/Aleksod Terrane (Tuareg Shield) contains mafic eclogites interlayered within anatectic 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. Oscillatory 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 crystallization at 811 +/- 15 degrees 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 intra-grain variations (mean epsilon Hf-j = -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 (2 sigma), attributed to post-peak decompression and heating. HP eclogite facies conditions in the Egere 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 degrees C and a U-Pb age of 584 +/- 6 Ma (2 sigma) 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 Egere terrane, where anatectic elongated domes of basement gneisses alternate with HP metasedimentary synforms.

Osumilite is reported in Palaeoproterozoic Al-Mg-rich granulites from the Khanfous area (Tekhamalt, In Ouzzal, Hoggar, Algeria). The main peak assemblages are osumilite+sapphirine+biotite+orthopyroxene+sillimanite and osumilite+orthopyroxene+sillimanite+quartz +/- biotite (+/- K-feldspar) in silica-deficient and silica-saturated granulites respectively. Osumilite coexists with F-rich biotite (XF approximate to 0.6). The observed microstructures, the mass balance of metamorphic reactions and P-T pseudosections modelled for bulk-rock and reaction-microdomain compositions indicate a clockwise P-T metamorphic evolution at ultrahigh temperatures, without substantial post-peak deformation. The peak P-T conditions recorded by the osumilite-bearing assemblages are 8.5-9.0kbar and 930-980 degrees C. During retrogression, osumilite was partially or totally replaced by fine-grained pseudomorphs of cordierite+orthopyroxene+K-feldspar+quartz at similar to 7kbar and similar to 850 degrees C. This study confirms that osumilite can occur only in Mg-rich metamorphic rocks that experienced ultrahigh-temperature metamorphism under anhydrous conditions. In the presence of a hydrous fluid, it is replaced, even at high temperatures, by cordierite-bearing assemblages. This important feature explains the rarity of osumilite in granulite facies rocks and its common replacement by cordierite+orthopyroxene+K-feldspar+quartz pseudomorphs. The peak conditions suggest that a delamination of the lithospheric mantle underneath the In Ouzzal crust brought the asthenosphere close to the Mohorovii discontinuity.

Le traitement d'images LANDSAT 7 ETM+ combine a un travail de terrain a permis de realiser une cartographie des series precambriennes de la region d'Amessmessa (Sud de l'In Ouzzal, Hoggar occidental). Les traitements utilises sont les compositions colorees, la fusion d'images, l'analyse en composantes principales, le rapport des bandes et les filtres directionnels. Les resultats montrent clairement que le filtrage directionnel et l'ACP permettent une fidele cartographie des lineaments. En ce qui concerne la lithologie, dans les compositions colorees (753 ou 543), la distinction entre les bassins et les domes gneissiques est nette. Ainsi les bassins composes de formation basique, ultrabasique et de series paraderivees apparaissent bleu pâle a bleu fonce alors que les orthogneiss charnockitiques avec leur structure fermees sont de teinte rosâtre. Par contre, il est impossible de distinguer les leptynites et les syenites, des orthogneiss charnockitiques en raison de la similitude de leur reflectance.

Field and petrostructural investigations in the Central Mauritanides provide new precisions on the polyorogenic character of the nappe edifice of this belt. The upper structural unit exposed in the Gaouâ area includes anatectic gneisses and high-grade metasediments affected by medium temperature, high-pressure metamorphism and cut by granitoids. They represent a basement unconformably overlain by a monometamorphic cover, the Gaouâ Group, of assumed lower Paleozoic age. Late Paleozoic regional metamorphism of metapelites from this cover is characterized by pyrophyllite–kyanite–chloritoid assemblages equilibrated at T around 420°C and P=1GPa. In the lower structural unit exposed in the south around Boufkerine-Farkâkâ, metapelites of the Gadel Group display slightly retrogressed garnet–kyanite–staurolite–rutile assemblages that equilibrated at T around 600°C and P⩾1.2GPa before the intrusion of 639Ma old plutons . The Gadel Group locally overlies in unconformity polycyclic gneisses. This continental assemblage represents an outboard terrane inserted between two monocyclic greenschist facies terranes: remnants of oceanic lithosphere in the east and arc-derived metasediments and metavolcanics cut by 670Ma old calc-alkaline plutons in the west. All units have been involved in late Paleozoic nappes emplaced to the east above the West African craton and its late Neoproterozoic to Cambro-ordovician cover. Though an east-directed vergence is locally recorded in the Neoproterozoic units, most early E-W trending stretching and mineral lineations are synchronous with greenschist facies metamorphism developed during the Late Paleozoic and roughly coeval with the Applachian nappe system.

Abstract The Archaean to Palaeoproterozoic Khanfous area from the Archaean In Ouzzal granulite terrane (Western Hoggar, Algeria) preserves exceptional thermal-peak (1150<T < 1300 °C) mineral parageneses, consisting of orthopyroxene+spinel+quartz, sapphirine+spinel+quartz and sapphirine+orthopyroxene+quartz, in quartz-rich Al–Mg granulites. Reaction textures coupled with P–T FMASH pseudosections indicate that rocks experienced complex multi-stage evolution. Our results suggest that the Khanfous area, as well as the entire northern In Ouzzal metacraton, experienced ultrahigh-temperature crustal metamorphism attributed to a 2 Ga Palaeoproterozoic event, followed by exhumation along a clockwise P–T path. The extreme temperatures attained suggest delamination of the lithosphere and ascent of the asthenosphere after crustal thickening.

In the Laouni terrane, which belongs to the polycyclic Central Hoggar domain, various areas contain outcrops of formations showing granulite-facies parageneses. This high-temperature metamorphism was accompanied by migmatization and the emplacement of two types of magmatic suite, one of continental affinity (garnet pyroxenites and granulites with orthoferrossilite-fayalitequartz), and the other of arc affinity (layered metanorites). Paragenetic, thermobarometric and fluid-inclusion studies of the migmatitic metapelites and metabasites make it possible to reconstruct the P-T-aH(2)O path undergone by these formations. This path is clockwise in the three studied areas, being characterized by a major decompression (Tamanrasset: 10.5kbar at 825degreesC to 6kbar at 700degreesC Tidjenouine: 7.5kbar at 875degreesC; to 3.5kbar at 700degreesC; Tin Begane: 13.5 kbar at 850degreesC; to 5 kbar at 720degreesC), followed by amphibolitization that corresponds to a fall of temperature (from 700 to 600degreesC) and an increase in water activity (from 0.2-0.4 to almost 1).The main observed features are in favour of petrogenesis and exhumation related to the Eburnean orogeny. However, the lacks of good-quality dating work and a comparison with juvenile Pan-African formations having undergone high-pressure metamorphism, in some cases reaching the eclogite facies, do not rule out the possibility that high-temperature parageneses are locally due to Pan-African events. (C) 2004 Elsevier Ltd. All rights reserved.

E030 THE AEROMAGNETIC MAP OF HOGGAR (TUAREG SHIELD ALGERIA). PROCESSING AND INTERPRETATION 1 N. BOURNAS 1 M. HAMOUDI 2 A. GALDEANO 3 K. OUZEGANE 2 AND J.R. KIENAST 4 1 University of Boumerdes Department of Geophysics Ave de l’independence Boumerdes 35000 Algeria 2 Univeristé de Bab Ezzouar Algeria 3 IPGP France 4 Université Paris VII France Abstract The Tuareg shield represents a part of the transsaharan Panafrican belt and is composed of Hoggar Aïr and the Iforas mountains. Historically the Hoggar is subdivided into three main structural blocks with different geology and separated by two mega shear zones (Bertrand and

The Azrou N'Fad terrane is remarkable by the presence of eclogites that are particularly rich in magnesium, as reflected in the compositions of the primary phases such as garnet (44.33% pyrope). These eclogites are only preserved in the cores of large lenses that mark out shear zones, and their margins are indeed completely transformed into amphibolite. Variations in pressure are indicated by the breakdown of primary omphacitic clinopyroxene, leading to a progressive depletion in jadeite component. This is linked to the successive formation of different types of plagioclase, with the most sodic appearing first. At the same time, the garnet becomes more and more enriched in iron, eventually breaking down into symplectites with hornblende + plagioclase or clinopyroxene + plagioclase + hornblende in contact with primary clinopyroxene and quartz. This last stage corresponds to the exhumation of the eclogite, with pressures falling from 15 ± 0.5 kbar to 11.1 ± 1.6 kbar at a temperature between 700 and 800 °C.

The mineralogy and phase relationships of corundum-bearing granulites from In Ouzzal (Hoggar, Algeria) are analysed within the context of the present knowledge of In Ouzzal metamorphism, as independently deduced from quartz-bearing rocks: a clockwise evolution characterized by peak temperatures ranging from 850degreesC to more than 1100degreesC at about 10 kbar, followed by decompression and cooling to about 5 kbar and 750 C. The corundum-bearing rocks have been divided into three types (A-B, C and D) according to their mineralogy. Type A-B is characterized by the occurrence of sapphirine-sillimanite-orthopyroxene phlogopite with or without garnet, Type C by the occurrence of spinel-sapphirine garnet, and Type D by the occurrence of garnet-spinel-sillimanite. Thermodynamic data for sapphirine have been adjusted from the current THERMOCALC dataset to fit in with available experimental data and current theoretical phase diagrams. MAS, K MASH, FMAS and KFMASH petrogenetic grids that connect quartz-present and corundum present grids are presented. Pseudosections derived from these grids account well for the observed textures. The three types of rock agree with the decompression path experienced by the quartz-bearing rocks. The occurrence of sapphirine corresponds to peak and decompression conditions, and trends of sapphirine and orthopyroxene compositions are consistent with this evolution.

The In Ouzzal terrane (Western Hoggar) is an example of Archaean crust remobilized during a very-high-temperature metamorphism related to the Paleoproterozoic orogeny (2 Ga). Pan-African events (≈0.6 Ga) are localized and generally of low intensity. The In Ouzzal terrane is composed of two Archaean units, a lower crustal unit made up essentially of enderbites and charnockites, and a supracrustal unit of quartzites, banded iron formations, marbles, Al–Mg and Al–Fe granulites commonly associated with mafic (metanorites and garnet pyroxenites) and ultramafic (pyroxenites, lherzolites and harzburgites) lenses. Cordierite-bearing monzogranitic gneisses and anorthosites occur also in this unit. The continental crust represented by the granulitic unit of In Ouzzal was formed during various orogenic reworking events spread between 3200 and 2000 Ma. The formation of a continental crust made up of tonalites and trondhjemites took place between ⩾3200 and 2700 Ma. Towards 2650 Ma, extension-related alkali-granites were emplaced. The deposition of the metasedimentary protoliths between 2700 and 2650 Ma, was coeval with rifting. The metasedimentary rocks such as quartzites and Al–Mg pelites anomalously rich in Cr and Ni, are interpreted as a mixture between an immature component resulting from the erosion and hydrothermal alteration of mafic to ultramafic materials, and a granitic mature component. The youngest Archaean igneous event at 2500 Ma includes calc-alkaline granites resulting from partial melting of a predominantly tonalitic continental crust. These granites were subsequently converted into charnockitic orthogneisses. This indicates crustal thickening or heating, and probably late Archaean high-grade metamorphism coeval with the development of domes and basins. The Paleoproterozoic deformation consists essentially of a re-activation of the pre-existing Archaean structures. The structural features observed at the base of the crust argue in favour of deformation under granulite-facies. These features are compatible with homogeneous horizontal shortening of overall NW–SE trend that accentuated the vertical stretching and flattening of old structures in the form of basins and domes. This shortening was accommodated by horizontal displacements along transpressive shear corridors. Reactional textures and the development of parageneses during the Paleoproterozoic suggest a clockwise P–T path characterized by prograde evolution at high pressures (800–1050 °C at 10–11 kbar), leading to the appearance of exceptional parageneses with corundum–quartz, sapphirine–quartz and sapphirine–spinel–quartz. This was followed by an isothermal decompression (9–5 kbar). Despite the high temperatures attained, the dehydrated continental crust did not undergo any significant partial melting. The P–T path followed by the granulites is compatible with a continental collision, followed by delamination of the lithosphere and uprise of the asthenosphere. During exhumation of this chain, the shear zones controlled the emplacement of carbonatites associated with fenites.

The Tin Zebane gabbro–anorthosite layered mafic intrusion represented by plagioclase-rich cumulates forms a set of small lenticular to round-shaped mainly undeformed bodies intruding the Pan-African high-pressure metamorphic rocks from western Hoggar (Tuareg shield, southwest Algeria). The coarse-grained anorthosites are mainly made of slightly zoned bytownite (An86–74) with the higher anorthite content at the cores. Anorthosites are interlayered with leucogabbros and gabbros that show preserved magmatic structures and with olivine gabbros characterised by coronitic textures. The primary assemblage in gabbros includes plagioclase (An93–70), olivine (Fo77–70), zoned clinopyroxene (En43–48Fs05–13Wo41–49 with Al2O3 up to 4.3 wt.%) and rare orthopyroxene (En73–78). Pyroxenes and olivine are commonly surrounded by Ca-amphibole. The olivine–plagioclase contact is usually marked by a fine orthopyroxene–Cr-spinel–amphibole symplectite. A magnesian pigeonite (En70–75Fs19–20Wo6–10) is also involved in corona. The coronitic minerals have equilibrated with the primary mineral rims at P–T–aH2O conditions of 797±42 °C for aH2O=0.5 and 808±44 °C for aH2O=0.6 at 6.2±1.4 kbar. The Tin Zebane gabbroic rocks are depleted in REE with a positive Eu anomaly, high Sr (>10∗chondrite) and Al2O3 concentrations (17–33%) that support plagioclase accumulation with the extreme case represented by the anorthosites. The REE patterns can be modelised using plagioclase, clinopyroxene and orthopyroxene REE signature, without any role played by accessory minerals. High MgO content points to olivine as a major cumulate phase. Anorthositic gabbros Sr and Nd isotopic initial ratios are typical of a depleted mantle source (Sri=0.70257–0.70278; εNd=+5.9 to +7.8). This isotopic signature is identical to that of the 10-km wide 592 Ma old dyke complex composed of alkaline to peralkaline granites and tholeiitic gabbros and one single bimodal complex can be inferred. The source of the Tin Zebane basic rocks corresponds to the prevalent mantle (PREMA). The Tin Zebane complex was emplaced along the mega-shear zone bounding to the west the Archaean In Ouzzal metacraton. The model proposed suggests a linear lithospheric delamination along this rigid and cold terrane due to post-collisional transtensional movements. This allowed the asthenosphere to rise rapidly and to melt by adiabatic pressure release. Transtension along a rigid body allowed these mantle melts to reach the surface rapidly without any crustal contamination.

Ferrous granulites in the area of Tidjénouine (Central Hoggar) exhibit a remarkable mineralogical composition characterized by the association orthoferrossilite–fayalite–quartz. These granulites are metamorphosed mafic igneous rocks showing the juxtaposition of different metamorphic parageneses. Peak paragenesis with garnet–clinopyroxene–amphibole–plagioclase–quartz reach to assemblage with orthopyroxene–plagioclase2. Secondary orthopyroxene reacted with garnet to produce symplectites with fayalite+plagioclase+quartz. The latest stage corresponds to an orthopyroxene–fayalite–quartz–plagioclase assemblage. The metamorphic history of the ferrous granulites is inferred by combining the study of phase relations with the construction of a petrogenetic grid and pseudosection in the CFMASH and CFAS systems using the Thermocalc program of [J. Metamorph. Geol. 6 (1988) 173]. The evolution of paragenetic minerals indicates a metamorphic P–T path through the following conditions: 7.1±1 kbar at 880 °C, 4.9±1.6 kbar at 750 °C and 3–4 kbar at 700 °C, which is consistent with a clockwise P–T path recorded throughout the area.

Garnet pyroxenite from high pressure granulite facies occurs with different mineral assemblages which involve garnet, clinopyroxene, orthopyroxene, plagioclase, amphibole and quartz with spinel developing as symplectite with orthopyroxene and plagioclase in large cracks. Three successive parageneses have been identified. The primary assemblage is characterised by the presence of quartz. The second assemblage involves orthopyroxene–plagioclase–hornblende symplectite, and the third assemblage is characterised by the development of spinel in symplectites with orthopyroxene and plagioclase. Using THERMOCALC (V2.7), a quantitative pseudosection in the system CaO–FeO–MgO–Al2O3–SiO2–H2O has been calculated. The assemblage involving quartz developed at high pressure, while the assemblage involving spinel developed at lower pressure. The peak of metamorphism in Tin Begane was calculated at 860 °C and 13.5 kb with aH2O=0.2. These conditions are followed by a decrease of pressure down to 4.8 kb.

Tamanrasset (Central Hoggar, Algeria) is part of the 2000 Ma Eburnean terrane in which spectacular examples of granulite-grade metapelites and metabasites occur. Reactional textures in metabasites, supplemented by geothermobarometry, fluid inclusions, and mineral equilibria indicate a complex metamorphic history, characterized by a strong decompression during postpeak uplift. Peak metamorphic pressure-temperature (PT) conditions of 800 degreesC and 10 kbar, respectively, were followed by rapidly decreasing pressure (to 700 degreesC and 6 kbar) at nearly constant temperature, during which time the rocks acquired their present mineral assemblages and textures (garnet pyroxenites). This isothermal decompression path was followed by nearly isobaric cooling. Throughout this history, water activity remained consistently low (a(H2O) = 0.1-0.5), comparable to values in neighboring migmatitic metapelites. Fluid inclusion studies suggest that the low water activities are due to CO2-rich fluids, identified as a synmetamorphic fluid (CO2 density: 1.05 g/cm(3)) generated by reactions during decompression. This fluid may have been generated during melting of the metapelite. The clockwise PT evolution path is consistent with crustal thickening during collisional tectonics.

Granulites from the In Ouzzal complex, Algeria, preserve excellent indicators of high-temperature conditions. They display a series of reaction textures involving sapphirine, sillimanite, spinel, garnet and cordierite in addition to orthopyroxene, quartz and rutile. The peak conditions are represented by five assemblages characterized by quartz+orthopyroxene+rutile with the possible addition of spinel or sillimanite+sapphirine. Geothermometry and geobarometry show that the peak metamorphism conditions were T = 1000 degrees C and P = 10-11 kbar. As most of the minerals contain a significant amount of Fe3+ and Cr, the parageneses have been studied within the FMASTOCr (FeO-MgO-Al2O3-SiO2-TiO2-O-2-Cr2O3) system. Neither of the two simple systems FMAS nor FMASCr account for the observed phase relationships. The assemblages occurring in the rocks are also inconsistent with previous FMASTO grids. However, the observed chemical relationships are different from those used by former authors. In particular, the collinearity of sillimanite, sapphirine and I orthopyroxene lead to a different compatibility diagram. Therefore, we have investigated a new set of petrogenetic grids based on our own observations and valid for the FMASTO system with sapphirine, spinel, sillimanite, garnet, cordierite, magnetite, ilmenite and rutile in presence of quartz and orthopyroxene. The grids account for the parageneses observed in the Khanfous area (North In Ouzzal) and for most of the recorded high-temperature granulites.