Journal of African Earth Sciences 37 (2003) 127–131 www.elsevier.com/locate/jafrearsci Preface The Precambrian of Hoggar, Tuareg shield: history and perspective

The Hoggar is located in southern Algeria and con- the Precambrian rocks of the Tuareg shield. Bourcart stitutes the major part of the Tuareg shield, which also described the Atakor volcanic massif, later to be comprises also the Adrar des Iforas in Mali to the SW investigated in more detail by Bordet (1954). All these and the A€ıır in Niger to the SE. pioneering works were synthesized in 1937 in the sheet The first geological observations were made during no. 1 of the international geological map at the scale of the great geographical explorations of the 19th century, 1/5,000,000, whose explanation booklet was mainly especially after 1850: Barth (1863) in Eastern Hoggar, written by Kilian (1937). Duveyrier (1864) on the Atakor volcanic massif, con- In 1938, Lelubre began his research work in Hoggar, tributed important information. The Foureau–Lamy which lasted until the presentation of his thesis in 1952. expedition of 1898–1900 recognised the presence of a Lelubre built a synthesis in two volumes of the Western crystalline basement under sandstone formations (Fou- and Central Hoggar, which constituted the basis of the reau, 1898; the samples were studied by Gentil (1909)). following geological studies in the region. Lelubre can From about 1900 until the First World War, the geo- be regarded as having initiated the modern geological logical explorations followed the axes of the French studies in Hoggar. He considered that the Suggarian and military progression. In 1909, Chudeau published his the Pharusian ‘‘series’’ belonged to two distinct orogenic memoir on the ‘‘Sahara soudanais’’ (Sudanese Sahara), cycles. He distinguished moreover two groups within the in which he considered the ‘‘highly recrystallized’’ Suggarian: an old basement (Arechchoum gneisses) ( ¼ highly metamorphic) gneisses as Archaean but the being covered by a mainly metasedimentary series micaschists as Silurian. In 1911, Flamand, in his publi- (Egeere ‘‘series’’). At the same time in the Adrar des If- cation entitled ‘‘Recherches geeologiques et geeographi- oras in Mali, Roman Karpoff applied the same concepts ques sur le haut pays de l’Oranie et du Sahara’’ grouped and defined in addition the Nigritian ‘‘Series’’, which he all the metamorphic rocks in one basement, estimating believed to represent a distinct geological cycle, but that the youngest could be Cambrian. which is actually the youngest Precambrian series made After the First World War, the pace of geological of molasse sedimentary and volcanic rocks, the whole studies in Hoggar quickened. Buutler’s€ memoir (1922, being described in his thesis (Karpoff, 1958). The work 1924) is the first really precise description of Hoggar of Lelubre was remarkably highlighted during the geology. Between 1921 and 1932, Kilian introduced International Geological Congress in Algiers in 1952. fundamental clarifications in the understanding of the During the fifties, the great period of geological central Saharan structure and thus opened the way for mapping (made at the scale of 1/200,000 and published systematic research (e.g. Kilian, 1931). He defined the at the 1/500,000 scale) and of mining exploration began Tassilian unconformity separating the basement from within the framework of the BRMA (Bureau de Re- the unmetamorphosed Phanerozoic sedimentary rocks, cherches Minieeres de l’Algeerie). Nearly all the Hoggar and he subsequently proposed a major structural sub- was mapped and numerous unpublished reports were division of the Hoggar by dividing the basement into written. This is the period of geologists such as Areene, two terms: the younger slightly metamorphic Pharusian Blaise, Byramjee, Gueerangee, or Vialon. These root ‘‘series’’ and the older strongly metamorphic Suggarian works, although mainly unpublished, constituted the series (Kilian, 1932). Bourcart, alone in 1923 and with basis of further works. Monod in 1929–1930, extended the concept of a Tass- In 1962 Algeria become independent and the geo- ilian unconformity to the western Hoggar. They pub- logical studies, including mapping and mining, were lished jointly in 1932 a description of the Adrar Ahnet organised following three axes: (1) the university coop- area. Denaeyer (1923a,b, 1925), a Belgian geologist, eration allowed the nomination of cooperant professors provided the first detailed petrographical descriptions of in the newly created universities; Fabre and Fabriees

0899-5362/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.jafrearsci.2003.10.002 128 Preface / Journal of African Earth Sciences 37 (2003) 127–131 played an important role; (2) in France, the CRZA framework of the 1977 geological map. During this (Centre de Recherches des Zones Arides) is largely period, independent efforts were oriented towards: (1) developed and is at the origin of the first multidisci- the Adrar des Iforas in Mali where a geological map was plinary research (including geology) in Hoggar. This is established by the CGG (Centre Geeologique et Geeo- the period of the major theses of the ‘‘Saharan’’ French physique, evolution of the CRZA) of Montpellier in geologists, Gravelle (1969), Boissonnas (1973), these two 1981 and where Late Precambrian plate tectonics was were actually being initiated in the days of BRMA, demonstrated (Black et al., 1979); (2) the A€ıır, in Niger, Caby (1970), Bertrand (1974), Latouche (1978) and Vitel initially mapped by Black et al. (1967) and where terrane (1979), all of them combined extensive field work geodynamics were introduced by Lieegeois et al. (1994). (leading to a geological map) and laboratory work. With This led to a strong rupture in 1994 in the global the exception of eastern Hoggar, these theses covered interpretation of the Hoggar geology. Based on the A€ıır nearly all the Precambrian of Hoggar and resulted many work and on the whole Hoggar knowledge, including international publications; (3) in Algeria, the SONA- BRMA reports, Black et al. (1994) suggested that the REM (Socieete Nationale de la Recherche Minieere) was Tuareg shield (550,000 km2) corresponds to the amal- created. It developed, in cooperation with the universi- gamation of more than 20 terranes during the Pan- ties, the CRZA and USSR, its own programs turned African orogeny. These terranes were shown to be towards mining exploration. contrasted, comprising juvenile oceanic Neoproterozoic All these works converged in 1977 to a fundamental terranes, remobilized or well-preserved Archaean and synthesis, the geological map of the Hoggar at the Palaeoproterozoic terranes. The Mesoproterozoic (1600– 1/1,000,000 scale, which is still in use. The main con- 1000 Ma) is remarkably absent. These terranes are N–S tributors to this map were Caby, Bertrand and the elongated, their main relative movements having re- SONAREM. This map gave for the first time a homo- sulted from a northern transpressional tectonic escape geneous view of the whole Hoggar with a subdivision due to their squeezing between the West African craton based on time, distinguishing Archaean and four Pro- to the west and the East Saharan craton (now Saharan terozoic units (Pr1, Pr2, Pr3 and Pr4). It was accom- metacraton) to the east. This model explained a series of panied by a remarkable description of the main major Hoggar features such as the difficulties of E–W lithological units identified. As in many areas, the initial correlations, but gave rise to new major questions subdivision in Suggarian and Pharusian based on the including the way along which the Hoggar terrane grade of metamorphism was questioned by the first amalgamation occurred and the origin of the various geochronological measurements (U-Pb on zircon) per- Hoggar terranes. formed on Hoggar rocks by Picciotto et al. (1965): for For the last 10 years, a small group of Algerian instance, the ‘‘Pharusian’’ Ouallen granite gave an age geologists who did their PhD thesis in Hoggar during of approximately 1900 Ma and the ‘‘Suggarian’’ granites the early nineties have been educating young geologists in the Tamanrasset area gave ages in the range 650–600 in this region with the collaboration of European labo- Ma, thus prompting the idea that measured ages re- ratories, mainly in France and in Belgium. In turn this flected extensive rejuvenation at the end of Precambrian shed new light on Hoggar geology. This Special Issue is times. The concept that developed later was to consider a consequence of this new approach. It is also the final the Suggarian as the equivalent of the Eburnian (2Ga contribution of the co-operative program between the orogeny) or of the middle Palaeoproterozoic (2.2–1.8 French university of Paris VII and the university of Ga) and the Pharusian as the equivalent of the Pan- Algiers (USTHB) named ‘‘Heeritage eeburneeen et struc- African (0.6 Ga orogeny) i.e. related to the Neopro- turation pan-africaine du Hoggar: eetude geeologique et terozoic. geeophysique’’. The 1977 geological map (Sonarem et al., 1977) dis- This Special Issue entitled ‘‘The Precambrian of plays remarkably the existence of N–Soriented mega- Hoggar, Tuareg shield’’, contains 11 papers dealing with shear zones. The two most important extend respectively the petrology, geochemistry, geochronology, tectonics, along the 4°500 and 8°300 meridians, that gave them their geophysics and geodynamics of Hoggar. As it can be name. They were considered as separating the Western understood from the historical description above, the Hoggar (mainly ‘‘Pharusian’’), the Central Hoggar Hoggar is the result from the amalgamation of terranes (mainly ‘‘Suggarian’’) and the Eastern Hoggar. This during the Pan-African orogeny. However, it contains subdivision constituted the basis of further studies at terranes that have well preserved Archaean and Palaeo- least for 15 years. proterozoic lithologies, including metamorphic parage- During the eighties, the cooperation between Algeria neses. Also, in some areas, pre-amalgamation rocks such and France became more formal through the initiation as island arc lithologies including high-pressure rocks of ‘‘Accord-Programmes’’ between universities and such as eclogites have been preserved from pervasive ORGM (formerly SONAREM). The Hoggar became collisional and subsequent post-collisional events. The better known but always within the homogeneous successive collisions were responsible for the exhuma- Preface / Journal of African Earth Sciences 37 (2003) 127–131 129 tion of these rocks while the final protracted post-colli- tion to predominant Archaean rocks in the age range sional period is characterized transpressive mega-shear 2.7–2.6, 2 Ga ultra high-temperature (more than 1000 zones, which are the result of the amalgamation. In turn, °C) granulitic metamorphism (e.g. corundum-quartz, they were the cause of the dissection of the pre-existing sapphirine-quartz, sapphirine-quartz-spinel) and 2 Ga microcratons (metacratonization) and of the genesis carbonatites. A series of exceptional parageneses are and emplacement of widge range of igneous rocks. Just reviewed and a geodynamical model is proposed where before its rapid peneplanation and covering by a sand- delamination and shear zones play a major role to ob- stone cap, the Hoggar was subjected to transtension, tain the high heat flow required. generating small rift basins filled by molassic sediments In the fifth paper, Peucat et al. present TIMSand and shallow-depth alkali-calcic and alkaline plutons. All SIMS U–Pb zircon ages and Nd TDM model ages on the these points are tackled in this Hoggar Special Issue. Gour Oumelalen area (NE LATEA), marked by well- This volume includes three parts: (1) three papers con- preserved Archaean lithologies. They reveal the presence cerning the general structure of Hoggar, from west to of Archaean protoliths at 2.7 Ga with Nd TDM at 3.0–3.2 east; (2) four papers dealing with the preserved Ar- Ga, of metasediments at 2.2 Ga and an age of 1.85 Ga chaean–Palaeoproterozoic lithologies; (3) four papers for the charnockitic metamorphism (10 kbar, 850 ± 20 focused on the Pan-African orogeny. °C). The old Pb–Pb age of 3.5 Ga published in the seventies is not confirmed. The LATEA metacraton comprises a series of 2.0–1.9 1. General structure Ga granulitic lithologies. In the sixth paper, Bendaoud et al. describe ferrous granulites in the Tidjenouine In the first paper, Caby focuses on the Neoproterozoic area (Central LATEA) marked by the remarkable subduction- and collision-related stages in the Western association orthoferrosilite-fayalite-quartz. This com- Hoggar. After reassessing available field (with several plex metamorphic history occurred from 7 to 4.9 kbar synthetic maps), geochemical and geochronological data, then to 3–4 kbar from 880 to 700 °C during a phase of Caby proposes a new geodynamic scenario with several thrust tectonics. A thrust structure can explain differ- palaeosubductions zones implying an important dia- ences in P–T conditions observed in other nearby chronism of the Pan-African building of the Hoggar. He granulites. also provides some new information on the late molasses. In the seventh paper, Derridjet al. describe, in the Tin The second paper by Lieegeois et al. shows that four Begane area (southern LATEA), garnet pyroxenite from terranes from the Central Hoggar belonged to one small a high-pressure granulitic facies. The passage from a D1 Archaean/Palaeoproterozoic craton (LATEA) that was phase (13.5 kbar, 800 °C) to a D2 decompression phase partly destabilized and dissected during the post- (10.7–4.8 kbar, 800–700 °C in coronas and opx–– collisional stage of the Pan-African orogeny, forming a pl ± hb ± sp) suggests a crustal thickening followed by an metacraton. This status implies the absence of major uplift. Retrogression path to greenschist facies is linked subduction and collisional events but the presence of to Pan-African shear zones. preserved early Neoproterozoic oceanic thrust sheets and the intrusion of granitic batholiths along the mega-shear zones dissecting the metacraton. One oceanic thrust 3. The Pan-African orogeny sheets and one granitoid body are dated and their petrol- ogy, geochemistry and isotope geochemistry studied. In the eighth paper, in the light of structural, petro- The third paper by Bournas et al. provides an aero- logical and 39Ar/40Ar laser geochronological data, Caby magnetic map of Eastern Hoggar. The three methods and Monie re-examine the western terranes of Hoggar, used to locate major contacts and faults (Euler decon- which include a Palaeoproterozoic basement, an volution, analytical signal, local wave number) allow to approximately 680 Ma mafic arc, a subduction-related suggest a general structural scheme in which the eastern high pressure belt (550–600 °C, 14–18 kbar) exhumed boundary of the Assodee-Issalane terrane is the suture around 615–600 Ma and a low pressure granulitic belt zone between the Central Hoggar and the Eastern (750–800 °C, <4.5 kbar), which cooled at a rate of 15 °C/ Saharan craton ( ¼ Saharan metacraton). Ma between 600 and 580 Ma, subsequently to the emplacement of late granitic plutons. In the ninth paper, Acef et al. study the Anfeg 2. The Archaean and Palaeoproterozoic events granodioritic batholith intruded at approximately 608 Ma into LATEA during its metacratonization: its In the fourth paper, Ouzegane et al. review the Ar- emplacement occurred within the 2 Ga basement along chaean and the Palaeoproterozoic evolution of the In thrust planes related to the functioning of the Pan- Ouzzal granulitic terrane. This unique terrane in Hoggar African subvertical shear zones. Sr–Nd isotopes point to hosts early Archaean (3.3–3.2 Ga) lithologies, in addi- an old Rb-depleted granulitic source mixed with some 130 Preface / Journal of African Earth Sciences 37 (2003) 127–131 mantle material. This agrees with an asthenospheric We kindly thank A.B. Kampunzu for his thorough uprise provoked by a linear delamination along the editorial review and A. Bendaoud for his help finalizing mega-shear zones. this special issue. In the tenth paper, Ait-Djafer et al. study the Tin Zebane gabbro-anorthositic layered complex, emplaced along the western edge of the In Ouzzal metacraton Acknowledgements to reviewers together with an alkaline–peralkaline granitic dyke swarm dated at approximately 592 Ma. Late symplec- We would like to warmly acknowledge our colleagues tites from this layered complex crystallized at 6.2 kbar who accepted to review the articles submitted to this for 800 °C and aH O ¼ 0.5 to 0.6. This complex is a 2 special issue and without whom it would not have been plagioclase-rich cumulate linked to a mantle source set up. (e ¼þ6; Sr ¼ 0.7024). This emplacement was possible Nd i Many thanks to because of the post-collisional transtensional move- ments that occurred along the rigid body of the In J.M. Bertrand, Chambeery, France; Ouzzal metacraton, inducing linear delamination and R. Black, Paris, France; rapid uprise of the mantle melts. S. Bogdanoff, Paris, France; The end of the Pan-African orogeny in Hoggar is J. Boissonnas, Bruxelles, Belgium; marked by the intrusion of subcircular shallow-depth F. Bussy, Lausanne, Suisse; plutons called ‘‘Taourirt’’ in Central Hoggar. Azzouni- R. Caby, Montpellier, France; Sekkal et al., in the last paper, redefine this expression B. Cesare, Padova, Italy; that covers different subgroups similar by their deep A. Cheilletz, Nancy, France; source (mantle + old granulitic basement) but acquiring L. Corriveau, Queebec, Canada; particularities through their interaction with their J.Y. Cottin, Saint Etienne, France; country-rocks, ranging from 2 Ga granulites to Neo- D. Demaiffe, Bruxelles, Belgium; proterozoic thrust island arcs assemblages. The Taourirt J. Dostal, Halifax, Canada; province is mainly alkali-calcic but can be alkaline and is J.C. Duchesne, Lieege, Belgium; linked to the late transtensive movements (525 Ma) O. Eklund, Turku, Finland; along the mega-shear zones that dissected LATEA. Not E. Ferre, Carbondale, USA; a long time after, at the beginning of the Ordovician, the A. Giret, St Etienne, France; Hoggar was completely eroded and covered by the M. Guiraud, Paris, France; Tassilis sandy sedimentation. P. Keating, Strasbourg, France; This Special Issue shows that the Tuareg shield con- G. Rebay, Pavia, Italy; stitutes a fantastic area for studying a Precambrian orogen R.J. Stern, Dallas, USA; on its whole width, from a craton to another and for all M.J. de Wit, Cape Town, RSA. disciplines. The current global model is assessed by current studies but it clearly needs refinements, which require a K. Ouzegane large panel approach from detailed geological mapping to J.-P. Liegeois high-tech studies. With such studies, the interpretations J.R. Kienast and conclusions obtained in Hoggar will likely be exported E-mail address: [email protected] to other areas in the world. Also, such studies require cooperation between African countries, namely Algeria, Mali and Niger, sharing together the Tuareg shield. This Special Issue is a contribution to the France– References Algeria project CMEP––MDU476 (‘‘Heeritage eeburneeen et structuration pan-africaine du Hoggar: eetude geeolog- Barth, H., 1863. Voyages et deecouvertes dans l’Afrique septentrionale ique et geeophysique’’; leaders: Kienast and Ouzegane), to et centrale pendant les anneees 1849 a 1855 (traduction par P. the IGCP 485 (‘‘Cratons, metacratons and mobile belts: Ithier). 8°, 4 volumes, Paris.  keys from the West African craton boundaries Eburnian Bertrand, J.M.L., 1974. EEvolution polycyclique des gneiss preecambri- ens de l’Aleksod (Hoggar central algeerien). Aspects structuraux, versus Pan-African signature, magmatic, tectonic and peetrologiques, geeochimiques et geeochronologiques. Theese d’EEtat, metallogenic implications’’; leaders: Ennih and Lieegeois), Montpellier, 350 p. to the NATO project EST/CLG 979766 (‘‘Geodynamical Black, R., Jaujou, M., Pellaton, C., 1967. Notice explicative de la carte evolution of the Hoggar Pan-African lithosphere’’; geeologique de l’A€ıır aal’ eechelle 1/500000ieeme. Direction des Mines et coordinators: Lieegeois and Ouzegane) and to the prep- de la Geeologie, Niamey, Niger. Black, R., Caby, R., Moussine-Pouchkine, A., Bayer, R., Bertrand, aration of the book entitled ‘‘The Geology of the African J.M.L., Boullier, A.M., Fabre, J., Lesquer, A., 1979. Evidence for plate’’ that will be edited by ICESA (International late Precambrian plate tectonics in West Africa. Nature 278, 223– Commission on Earth Sciences in Africa). 227. Preface / Journal of African Earth Sciences 37 (2003) 127–131 131

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