The Saghro Group consists of a thick volcanic-sedimentary sequence with intercalated basaltic lavas, the first magmatic event in eastern Saghro area. Nd isotopes of basaltic pillow lavas show T-DM model ages ranging from 640 to 580 Ma, which represent a maximum age for basalt eruption.Granitoids within the Saghro Group consist of a charnockitic suite, tonalites, granodiorites and monzogranites. They are high-K calc-alkaline (HKCA) with a post-collisional character, and were emplaced at high-levels in the crust. Their ages of emplacement are within the 580-560 Ma bracket, implying that the entire Saghro Group is slightly older than or partly coeval to granitoid emplacement and implying a common geodynamical setting. Sr-Nd isotopic compositions and Nd T-DM model ages point to a mixed origin, combining a juvenile mantle source and an Eburnean crustal component, which could be the West African Craton (WAC). The juvenile component in the Saghro granitoids could be the depleted upper mantle that has sourced the earlier basalts.Field observations, geochemical and geochronological data together support that, during the Pan-African orogeny, the Anti-Atlas was subjected to a regional transpressional to transtensional event. This event would have been responsible for the dissection of the northern margin of the WAC into several blocks, the development of deep sedimentary basins and the emplacement of HKCA magmas. (C) 2009 Elsevier Ltd. All rights reserved.

La boutonniére de Zenaga montre un enregistrement complet des orogenéses éburnéenne et panafricaine. L'Eburnéen est caractérisé par un métamorphisme général de haute température et un plutonisme complexe marqué par la granodiorite porphyroïde localement migmatitisée d'Azguemerzi qui comporte une signature isotopique d'origine mantellique. Elle s'est mise en place diapiriquement sous forme de dôme au cours de l'orogenése éburnéenne en développant un métamorphisme thermique local. La foliation de cette granodiorite résulterait de l'interférence entre sa mise en place et la déformation régionale dans les conditions du faciés amphibolite. Au Nord de ce secteur, sont injectés les granites clairs de Tazenakht plus potassiques, plus ou moins orthogneissifiés. Cette orthogneissification et les zones de mylonites et de phyllonites associées correspondent á des zones de cisaillement ductile synmétamorphes dans les conditions du faciés schistes verts durant l'orogenése panafricaine. Cette déformation résulterait d'un mouvement transpressif sénestre d'importance régionale. L'étude de cette région qui constitue la bordure Nord du craton ouest-africain a permis de révéler la superposition des orogenése panafricaine et éburnéenne et d'apporter une dimension régionale á l'interprétation de l'accident majeur de l'Anti-Atlas.

Les terrains volcaniques et volcanodétritiques du Néoprotérozoïque terminal (Vendien) affleurent largement dans le Haut Atlas occidental. IIs sont située au Nord de l'accident majeur de Tizi n'Test qui sépare le craton ouest africain du craton septentrional. Ces volcanites reposent en concordance sur une série grésopélitique pouvant être l'équivalent des séries de Tidilline et d'Anzi dans l'Anti-Atlas. Ce volcanisme par ses caractéristiques géochimiques calco-alcalin de marge active est affilié á l'histoire post panafricaine développée aussi dans l'Anti-Atlas. II différe de ce dernier par ses teneurs en K2O. Ce volcanisme s'est mis en place dans un contexte distensif. ll est généré par la fusion de la croûte subductée sous un craton septentrional. Au dessus de cet ensemble magmatique repose en concordance ou en légére discordance une puissante série calcschisteuse dans laquelle sont interstratifiées des dacites calco-alcalines montrant la continuité du volcanisme orogénique au moins jusqu'au Cambrien inférieur.

Little is known about the cleavage–fold relationships in the Lower Palaeozoic Brabant Massif, primarily because of the scarcity of suitable exposures. Moreover, to date, folds have only been described in the Ordovician and Silurian sequences along the southern extremity of the basement. However, excavation works for the construction of the TGV(railway)-track south of Brussels created an opportunity to study cleavage–fold relationships in the Lower Cambrian terrigenous series (Tubize Group) in a more central part of the Brabant Massif. The structural features observed seemed inconsistent with the suspected regional trends. Primarily, a divergent cleavage fan was observed in a region thought to have a regular cleavage attitude. The symmetrical but divergent disposition of the cleavage with regard to the fold hinges is explained by flexural folding of a pre-existing bedding-parallel compaction fabric. Cleavage development and folding are considered synchronous. The divergent cleavage fan reflects local strain variations. Also the steeply plunging hinge lines of the open, subangular folds are seemingly inconsistent with the regional trend characterized by subhorizontal fold hinges. Taking into account the structural position of the fold assemblage in the subvertical limb of a large-scale, upright, isoclinal fold structure, the fold assemblage is interpreted as an incongruous parasitic feature. The steeply plunging hinge lines are considered to be caused by fold hinge rotation during progressive coaxial deformation. Although this incongruous fold assemblage with its divergent cleavage fan is localized, it provides important information on both local and regional deformation circumstances in this part of the Anglo-Brabant Fold Belt.

New data implying crustal activation of Eastern Avalonia along the Anglo-Brabant fold belt are presented. Late Ordovician subduction-related magmatism in East Anglia and the Brabant Massif, coupled with accelerated subsidence in the Anglia Basin and in the Brabant Massif during Silurian time, indicate a foreland basin development. Final collision resulted in folding, cleavage development and thrusting during the mid-Lochkovian to mid-Eifelian. In the southeast of the Anglo-Brabant fold belt, Acadian deformation produced basin inversion and the regional antiformal structure of the Brabant Massif. The uplift, inferred from the sedimentology, petrography and reworked palynomorphs in the Lower Devonian of the Dinant Synclinorium is confirmed by illite crystallinity studies. The tectonic model discussed implies the presence of two subduction zones in the eastern part of Eastern Avalonia, one along the Anglo-Brabant fold belt and another under the North Sea in the prolongation of the North German–Polish Caledonides.

Geologically, Upper-Volta belongs to the West African Shield, which is composed of metamorphic and crystalline rocks of the Liberian and Birrimian orogenic cycles (geochronologic limits: 2,700-1,600 m.y.).The Oursi area comprises a dome of syntectonic granite partly encircled by a complex syncline of porphyrites and related rocks, ophiolitelike greenstones, and volcano-sedimentary rocks. Within these are intrusions of magnetite-bearing gabbros which are subconcordant with the folded structures and may be Birrimian tarditectonic intrusions. In all the masses, the gabbros are layered with the magnetite concentrations forming an integral part of them.Ten significant magnetite deposits have been determined. They are characterized by bundles of magnetite veins with thicknesses ranging from 0.5 to 40 m and lengths of a few hundred meters. At the surface the magnetite concentrations are always martitized. In some places, maghemite may also occur. The final stage of alteration consists of a general transformation into goethite.The magnetite veins are associated with different types of gabbro: augite-gabbros, augite- and hypersthene-gabbros (gabbronorites), and hypersthene-gabbros (norites). Quartz is generally absent but may appear in large quantities in some rocks, e.g., quartz-bearing gabbronorites. Most of the gabbros are igneous cumulates. The cumulus crystals comprise plagioclase, hypersthene, and augite, whereas the intercumulus material is hypersthene, augite, and ilmenite. The veins are essentially an association of magnetite and ilmenite grains with accessory grains of sulfides, such as pyrrhotite, pentlandite, and arsenopyrite. The magnetite is characterized by the development of two types of microtexture: cloth microtexture, determined by the superposition of two series of exsolution lamellae in the (100) plane of the magnetite, and trellis microtexture, determined by the development of ilmenite lamellae in the (111) planes of homogeneous magnetite.Trellis cloth zonation occurs where the two microtextural types are found together in the magnetite. Primary ilmenite is associated with the magnetite grains and reaction rim zonation occurs at the contact between magnetite and ilmenite.The results for some magnetite samples, analyzed by conventional methods and by electron and laser microprobes, show that the V 2 O 5 concentration never rises above 1.4 percent. Goethite samples contain less than 100 ppm vanadium; magnetite, maghemite, and hematite ore have the same vanadium content. The V 2 O 5 concentration and the relatively low Cr 2 O 3 content (<0.11 wt %) of the Oursi ore are similar to those found in Bushveld ores formed during the last magmatic differentiation phases.Microprobe scanning shows that the titanium content decreases along the border of the magnetite grains and that the vanadium content increases at the same time. These results confirm, on the mineral scale, the same negative correlation between the titanium and vanadium content as was observed in similar ores in the Bushveld, Skaergaard, and Bjerkrem-Sogndal masses.

Structures stained by ZIO technique contain both Zn and Os. The distribution of both elements is similar and the ratio Zn/Os is constant.