The 4 degrees 50'Shear zone (SZ), one of the major structures of western Gondwana, formed in Hoggar during the PanAfrican orogeny is still poorly studied; especially ages and deformation characteristics are lacking. We performed microstructural analysis and EBSD measurements on nine selected mylonites from the 4 degrees 50'SZ, and zircon U-Pb dating on two granitoid samples. The protolith of these mylonites are granitoids. Most samples display segregation of dominant feldspar and minor quartz in separate layers that underline the N-S subvertical mylonitic foliation. Feldspar grains display evidence of intracrystalline deformation, supporting dextral shear-sense. Plagioclase and K-feldspar are frequently corroded with embayments infilled by secondary K-feldspar and plagioclase respectively. Interstitial quartz grains disseminated in feldspar aggregates may infill fractures in feldspar grains. Quartz ribbons, usually parallel to the foliation, locally crosscut feldspar layers. Quartz is partially recrystallized and elongate crystals form a secondary foliation at similar to 30 degrees to the main foliation, supporting a later sinistral shearing. In some samples, amphibole is associated with quartz and oriented parallel to the foliation. In all samples, quartz CPO suggests a dominant prism- slip-system, activated at medium/high temperature (similar to 500-700 degrees c), but stress-induced oriented crystallization may have contributed to this CPO. K-feldspar CPO supports the activation of the [100](010) slip system while the plagioclase CPO points to activation of [100](001) system. These CPO support dislocation creep under amphibolite facies conditions. These new data, in addition to U-Pb dating, suggest the following evolution: 1) successive intrusions of granitoids in the SZ (similar to 661-similar to 639 Ma) due to partial-melting of the lower-crust and, possibly also of the lithospheric-mantle, 2) the 4 degrees 50' SZ was rooted in the lower-crust or the upper-mantle, and 3) dextral shearing lasted over >= 20My. This was followed by successive magma batches that intruded these rocks during a late migmatitic episode (similar to 623-similar to 609 Ma). After similar to 609 Ma, a moderate sinistral reactivation of the 4 degrees 50' SZ occurred and was accommodated through dislocation creep mainly localized in quartz ribbons. During accretion of the Western Gondwana, the 4 degrees 50' SZ was active during subduction. During >= 20My, it was successively intruded by granitoids that cooled down slowly, and have been deformed in the submagmatic state then in the solid-state.

The Torak massif located to the west of the Atakor volcanic district is a granitic pluton NNE-SSW elongated and crosscut by several NNW-SSE lineaments. It is intrusive within the S-W Tefedest terrane composed of Eburnean (c. 2 Ga) gneisses and of Pan-African (c. 615 Ma) syntectonic granitic batholiths. New field mapping, petrographic, and mineralogical data (feldspars and mica chemistry) and geochemical data (major and trace elements, REE) reveal the following: the Torak granitic massif shows a fairly simple and homogenous mineralogy dominated by feldspars (orthoclase and microcline, albite, and oligoclase) and lithium micas (siderophyllite to protolithionite and phengite to Li-phengite). The Torak granite chemistry is remarkably homogeneous both in major elements (73.95 ≤ SiO2 ≤ 76.85 wt.%; 7.68 ≤ (Na2O + K2O) ≤ 9.55 wt.%) and trace elements (500 ≤ Rb ≤ 780 ppm; 9.774 ≤ La ≤ 45.267 ppm; 0.316 ≤ (La/Yb)N ≤ 2.418; 0.027 ≤ (Eu/Eu*) ≤ 0.064). This data indicates that Torak rocks are highly fractionated calk-alkaline (HFCA) and co-genetic granites having evolved by assimilation-fractional crystallization (AFC). Similar to the surrounding GIIb granites of the Taourirt province (Azzouni-Sekkal & Boissonnas in Bulletin Socièté Géologique France 164: 597–608, 1993; Azzouni-Sekkal et al. in J Afr Earth Sci 37: 337-350, 2003), a similar mixed deep source (asthenosphere + old lower crust) can be described here. The country rocks could correspond to the old Archaean–Paleoproterozoic LATEA metacraton. The emplacement of the Torak pluton could be linked to the Murzukian intracontinental orogenic phase that occurred in the eastern part of the Tuareg Shield (Fezaa et al. in Precambrian Res 180: 299-327, 2010).

The Taessa lavas are located in the Atakor volcanic domain, (Hoggar Algeria). The Atakor district has undergone a significant magmatic activity during the Mio-plio-quaternary, which lead to the outpouring of massive amounts of alkaline lava. This magmatic activity resulted in the reactivation of the Pan-african orogenesis accidents that occured during the collision between the african and european continents (Liégeois in Plates, Plumes and Paradigms: Geological society of America Special Paper, pp. 379–400, [10]). The petrographic and mineralogical studies relating to the Taessa’s alkali basalts highlighted the persistence of two basaltic groups: olivine-pyroxene basalt and olivine-green-pyroxene basalt. They are mainly represented by magnesian olivines, whose crystallization process ended by 693 °C, according to the Fabriès geothermometer (Contr Mineral Petrol 69:329–336, [6]) olivine-spinel, pyroxenes (diopside, augite and hedenbergite) which crystallized by 500–1300°C (phenocrysts), and by 1100–1300 °C (microlites), according to Lindsley geothermometer (Pyroxene thermometry. American Mineralogist 68:477–493, [11]), plagioclases (andesine –labrador) and sanidine, Oxides (titano-magnetites and spinel).

The main objective of this study was to model the lithospheric structure of the Manzaz volcanic district (Hoggar) using the magnetotelluric (MT) method. For this purpose, eleven MT stations forming a 70-km long NW-SE profile, intersecting the Manzaz, were modeled. The 2D resistivity model shows an anomalously conductive crust resting on a lithospheric mantle of normal resistivity. The anomalously high conductivity of the middle and lower crust results probably from magma ascent from the asthenosphere to the surface. It could correspond to the presence of partial melting, trapped fluids released by the magma and/or precipitation of mineralization. The conductive structures underlying the Manzaz could correspond to intracrustal magmatic chambers or to magmatic underplating zones at the Moho or at lower/upper crust discontinuities.