Lithospheric Architecture and Precambrian Geology of the Hoggar and Adjacent Areas

The Hoggar, which forms the Algerian part of the Tuareg shield, is an ideal site for geoscientific studies. As the site of successive orogenies, it offers a single example of the problems that arise on a continental scale, whether in terms of magmatism, metamorphism, structure, metallogeny or geophysics. In this chapter, after a brief overview of the history of the work carried out in the Hoggar, we present the other ten chapters of this book.

The In Ouzzal Granulitic Unit (IOGU, southern Algeria) is an Archaean (3.2–2.5 Ga) and Paleoproterozoic (2.0 Ga) terrane of the Western Hoggar massif which has been affected by a very high temperature granulitic metamorphism. Its deep structure subject of controversy remains poorly known because of the lack of geophysical studies devoted to it. For this purpose, a magnetotelluric (MT) survey was conducted in October 2009 in the In Allarene area located south of IOGU, near Tirek and Amesmessa gold deposits. Thus, nine broadband soundings were collected along an East–West profile of 50 km long spreading on Kidal, Tassendjanet, In Ouzzal and Tirek terranes. MT data reveal three-dimensional In Allarene underlying structures. The crust of IOGU being reduced to its lower crust, is formed of two portions: an upper part of 7–8 km thick resting on a more conductive lower part. The Proterozoic crust of the adjacent terranes looks normal. The upper crust reaches about 15 km under Tirek and 15–20 km beneath Tassendjanet and Kidal. Unlike the Tekhamalt area, the two shear zones limiting IOGU do not have a clear electrical signature in the In Allarene area. On the other hand, the shear zone limiting Kidal from Tassendjanet, whose electrical signature is obvious, is rooted vertically in the crust and has a thickness of 4–5 km. According to the 3-D model, the most conductive feature is illustrated by In Allarene shear zone. The latter exhibit a high conductivity (~5 Ω m) at 5km depth and might be a key region to understand the extent of Neoproterozoic remobilization of this part of the In Ouzzal. This is all the more important as our results suggest that the In Allarene accident is a privileged place for the circulation of fluids responsible for gold mineralization known in the region, but exploited mainly a little further east (Tirek mine) and south (Amesmessa mine). These new data, which should be refined by one or more tighter profiles, already show that the In Allarene fault should be the subject of more extensive gold prospecting.

There has been considerable debate on the age of high-grade metamorphism in the Central Hoggar (Algeria). According to several authors, granulite facies metamorphism is of Paleoproterozoic age, while the Pan-African orogeny generated only greenschist and/or amphibolite-facies metamorphism. For others the high-grade metamorphism is Neoproterozoic. In an attempt to bracket the timing of deposition and metamorphism of the metasedimentary unit characterizing the Tamanrasset block (Central, Hoggar, Algeria), more than 200 zircon grains from metapelites have been investigated. Two varieties of zircons have been recognized according to the internal structure of the grains; the first population comprises cores of zoned zircon, which are considered as inherited. They display ages ranging from c. 2700 to c. 700 Ma. This wide range is related to multiple source provenances of the detrital grains. Furthermore, these cores reveal a high Th/U ratio. The second zircon population corresponds to metamorphic overgrowths that enclose the previous inherited cores besides new crystallized metamorphic zircon grains. The obtained results reveal that sediment deposition occurred after c. 700 Ma, and before the high-grade metamorphism which is well constrained to be at c. 660 (syn-collisional event) and c. 632 Ma (post-collisional event). The succession of mineral assemblages based on the microtexture observations along with the P-T pseudosections modeling suggest a clockwise P-T path with nearly isothermal decompression of the metapelites and garnet pyroxenites from the Tamanrasset block from pre-peak stage (M1) at 11.50 ± 1.0 kbar and 735 ± 50 °C to peak metamorphic stage (M2) at 10.25 ± 1.5 kbar and 820 ± 50 °C, and finally post-peak stage (M3) at 6.75 ± 1.0 kbar and 770 ± 50 °C conditions.

The small elliptic Rechla granite pegmatite complex belongs to the c.525 Ma Rare Metal Granite (RMG) province of the Laouni terrane of the Pan-African Tuareg Shield (Hoggar). It intrudes a porphyritic biotite-granite and is particularized by a rim of Quartz, K-feldspar and Zinnwaldite pegmatite. The centre is occupied by a medium-grained granite, with quartz, albite (An01), rare microcline, topaz, lepidolite (≤ 8% MnO), wolframowodginite and Hf-zircon. The pegmatite rim comprises, toward the intrusion (i) thick K-feldspar lenses (palissadic crystals ≥ 50 cm), (ii) a laminated quartz-zinnwaldite-(beryl) sequence, described as a unidirectional solidification texture (UST), and (iii) a discontinuous band of fine-grained granite, with quartz, albite, topaz, lepidolite, titanowodginite and beryl. The laminated sequence overprints the K-feldspar lenses. It comprises thick (≤ 20 m) quartz lenses cross-cut by 10 cm-sized alternating bands of euhedral quartz and Mn-zinnwaldite (≤ 6.5% MnO). At the boundary with the fine-grained internal band, euhedral quartz crystals are projecting toward the inner wall. The chemical composition of the medium-grained granite is typical of a low-P peraluminous RMG deriving from highly potassic calcalkaline suites (A2 type) enriched in Ta (165 ppm, Ta/Nb between 2.4 and 2.6), expressedas columbo-tantalite and Mn-wodginite, with low P2O5 (0.05%) and ∑ REE (23 ppm) contents, with a pronounced tetrad effect and <0 Eu anomaly in the REE pattern. The fine-grained granite is equally fractionated with Ta 240 ppm (Ta/Nb = 2.4) and Be 500 ppm. The surrounding porphyritic biotite-granite is representative of the evolved magmas of the A2-type Taourirt suite in the nearby terranes. Geochemical modeling, using the filter press model, shows that the main Rechla magma is likely the fractionated product of this already differentiated magma, mainly involving quartz and Kfs. The pegmatite rim is interpreted as the result of the sequential crystallization of a Rechla-type melt, with late individualization of a Fe-rich magmatic-hydrothermal phase responsible for the quartz-zinnwaldite assemblage, leaving a strongly Be-enriched residual liquid (the fine-grained granite). As demonstrated by the Rechla occurrence, Ta concentration at levels similar to those in Beauvoir-type high-P peraluminous granites may be reached in the low-P low-Ta A2 suites, provided that extreme fractionation processes are established.

The purpose of this study is to discuss the data quality issues of an airborne geophysical survey (magnetic and radiometric) carried out in the 1970s in Algeria, and propose a convincing (compelling) solution to a complex noise problem that mainly affects the radiometric data. The quality of the collected radiometric data is moderate due to technological limitations, the use of uncalibrated gamma-ray spectrometer, and no background noise estimation procedure was implemented during the survey. The measurements present a serious signal-to-noise problem that alters the radiometric signatures of the soil and prevents better recovery of some geological signal. We motivate our study using a case example integrating gamma-ray spectrometric and magnetic data of the Tahifet region in the Central Hoggar area (southern Algeria). The datasets were corrected, gridded and filtered to visualize the radiometric and magnetic features and to define geological boundaries based on gamma-ray spectrometric responses. The obtained geophysical images were combined with published 1:200,000 geological maps to improve the geological knowledge of the region and to provide valuable information for the study of some poorly mapped structural and geological features.

The utilization of optical satellite images has become instrumental in lithological and alteration assessments, enabling the differentiation of diverse circular structures. This study specifically focuses on mapping volcanic massifs in the Hoggar region using remote sensing techniques, with a strong emphasis on evaluating alteration degrees based on VNIR and SWIR spectra. The application of the Crosta technique, involving the assignment of F (iron oxide image), F+H, and H (hydroxyl image) images to RGB primary colors, proves effective in delineating hydroxyl and iron oxide alterations within volcanic massifs. The introduction of PC1, showcasing maximum variance among terrestrial materials, further enhances the technique’s proficiency in mapping volcanic lava flows and deciphering alteration patterns. Through comparative analyses of the proposed technique’s application on various circular structures, including impact structures (Tin Bider and Ouarkziz) and volcanic structures (Achkal and In-Roundoum), the results reveal distinctive features. Volcanic structures exhibit clear contrasts with their host rocks, while impact structures are practically indistinguishable from them. This study concludes by underscoring the practical utility of remote sensing techniques in the study of circular structures and emphasizes the critical importance of understanding their origins.

The Hoggar region is known to be a zone where the upper mantle exhibits a negative gravity and thermal anomalies. The area is part of a system of large swells over the African continent such as those of: Aïr, Tibesti, Eghei, Darfur and Cameroon mounts. The Hoggar uplift is a broad structure elongated over a 1000 km span in the NE-SW direction. The elevation of the Hoggar is between 500 and 1000 m as a mean with areas where the elevation goes over 2000 m as in Tahat (Atakor, 2150 km²), and Adrar N’ajjer (2500 km²) corresponding to Cenozoic volcanic districts. Several geophysical investigations were conducted to image the lithospheric structure under the Hoggar such as; gravity, magnetic, magnetotelluric, heat flow and seismic tomography. The results obtained over 30 years of investigations, especially the seismic tomography in the region, show traces of a hot spot in the crust and the upper mantle under the Hoggar domal uplift characterized by low P-waves velocity. The seismic tomography revealed that the 4.5° mega-fault is a geological structure that has a deep extent in the lithosphere up to 70 km and also for the Adrar Fault (east of In Ouzzal domain) which is present down to upper mantle. To the north, the Sahara basins (In Salah) exhibit a low-velocity structure that could be associated with a thermal process. There is a good correlation between the zones with low velocity and the volcanic districts in the Hoggar and under the Sahara basin. Regarding the geodynamics of the African plate, it seems that the hot spot responsible for the uplift of the Hoggar could be located nowadays probably 400 km SW of the Tamanrasset.

The Hoggar mantle xenoliths provide insights into the evolution of the subcontinental lithospheric mantle in response to the geodynamic events that shaped NW Africa since, at least, the Panafrican orogeny. The Pan-African heritage is preserved in deformed lherzolite xenoliths (porphyroclastic to equigranular) from peripheral Hoggar localities (Tahalgha and Eggéré districts, Kourim et al., Journal of Petrology 55:2249–2280, 2014; Kourim et al., Tectonophysics, Special Issue: Constraints on Composition, Structure and Evolution of the Lithosphere 650:18–33, 2015; this study). These samples are distinguished by only incipient annealing, LREE-depleted clinopyroxene compositions, well-preserved olivine preferential crystallographic orientations (axial-[010]) consistent with transpressional regime, and low equilibrium temperatures (750–900 °C) achieved after lithosphere thermal relaxation. They are considered to represent the sub-continental lithosphere after the rejuvenation processes that took place during the late stages of the Pan-African orogeny, likely associated with igneous refertilization. Extensive lithospheric rejuvenation occurred either regionally, as a result of lithospheric delamination or thermo-mechanical erosion after thickening, or more locally along meridian shear zones. The Cenozoic events are marked by partial to complete annealing of pre-existing deformation microstructures, increased equilibrium temperatures (up to 1150 °C), extensive olivine-, clinopyroxene- (±amphibole-) forming metasomatic reactions, and changes in olivine CPOs and seismic properties. These modifications are observed either at the scale of magma conduits and their wall rocks (Kourim et al., Journal of Petrology 55:2249–2280, 2014; Kourim et al., Tectonophysics, Special Issue: Constraints on Composition, Structure and Evolution of the Lithosphere 650:18–33, 2015) or at the whole Hoggar scale, as shown by increasing degree of textural annealing and metasomatism from Tahalgha and Eggéré to Manzaz (i.e. from outer to central Hoggar). Our data show little changes at intermediate scale, as might have been expected, in particular, near or across major shear zones such as the 4°35′. This finding favors relatively large-scale asthenospheric upwelling related to upper mantle instabilities or local convections, rather than a process involving merely the reactivation of pan-African lithospheric faults.

The water resources in the Ahaggar region are extremely limited, with the alluvial terraces intersected by wadis being the sole source of water. Despite considerable fracturing, the bedrock composed of eruptive and metamorphic rocks is unsuitable for the development of significant aquifers due to its unfavorable hydrodynamic characteristics. The hydrographic network and hydrogeology of the region are determined by major faults, some of which create impermeable boundaries due to their siliceous filling, while others accommodate wadis and facilitate the formation of alluvial aquifers. These alluvial aquifers are recharged by the wadi floods that occur during the summer “monsoon” in the Sahel region. The isotopic content and the levels of radioactive Carbon and Tritium in the water of the alluvial aquifer are similar to current precipitation, indicating a recently formed water table. In contrast, the isotopic, radioactive Carbon, and Tritium compositions of water from the bedrock reveal an older origin during a more humid and colder period.

In the region of Béchar and Ougarta, several Proterozoic formations outcrop, displaying various structures (unpublished EREM, 1978 to 1987; Bouima, 1986, 2002; Remichi, 1987; Ait-Kaci, 1990; Duée et al., 1992; Zerrouki, 1993, 2000). These formations present lithological features that suggest a correlation with those known in their Moroccan counterparts. On the surface, the Proterozoic formations of the Ougarta chain become progressively younger from east (Sebkha El Mellah) to west (Damrane). The highly folded Sebkhat El Mellah series is likely related to the base of the PII-2, while the sedimentary formation of Damrane would correspond to its top. These formations were established before the first Pan-African tectonic phase. The Damrane massif consists of three formations (sedimentary, andesitic-basaltic, and rhyolitic) separated by structural unconformities (Preidl et al., 1985). These characteristics, combined with a very low degree of alteration, make the rhyolitic formation equivalent to the P-III of the Anti-Atlas, on which Cambrian deposits are stratigraphically unconformable. North of Béchar, the lithological and structural characteristics of the dolomite formations and shales of ThéniaZerga (north of Boukais) can be attributed to the PII-2 of the Anti-Atlas. Above these, the “limestone” and “red sandstone” series of Boukais are found. The Boukais volcanic series (north of Béchar) is considered PIII. Its formation is characterized by transtensive events that generated “pull-apart” basins, similar to those in the Anti-Atlas. It differs from the basaltic andesite formation of Damrane in its composition and the nature of the transition from red sandstones to volcanic facies.