Introduction

Sahara is the largest arid desert in the world. It is mostly composed of either flat, or mountainous areas, where geological formations and their mutual contacts are fairly well-exposed. All types of rocks occur, and their ages range from Archean to recent times. Within high-elevation zones, the Hoggar is the most prominent. It is made up of a large swell of Precambrian formations, decorated in the highs by impressive Neogene volcanic domes and peaks culminating at c. 3000 m above sea level. Precambrian formations at altitudes ranging from about 400 m up to 2500 m constitute the central part of the Tuareg Shield. Their boundaries are defined by unconformably overlying Lower Paleozoic Tassili formations.

The Archean–Proterozoic Tuareg shield is characterized by north-south trending major shear zones, which separate crustal blocks with contrasting geology. It is interpreted as an amalgamation of terranes (Black et al. 1994) that were welded between the West African Craton and the East Saharan Metacraton (Abdelsalam et al. 2002; Liégeois et al. 2013) during the 850–630 Ma Pan-African orogeny (e.g.Caby 2003; Liégeois et al. 2003; and references therein). This major event was achieved through episodes of dockings, or collisions, affecting continental blocks and of accretions of oceanic island arcs. It resulted into the welding of the north-eastern part of West Gondwana. Subsequent strike-slip movements along mega-shear zones built the current shape of the shield. Eocene initiation of the Hoggar Swell (Rougier et al. 2013) and Cenozoic intraplate volcanic episodes (Liégeois et al. 2005) constitute the last geological events that have affected the Tuareg Shield.

In the central Sahara, the Hoggar or Ahaggar massif in southern Algeria, together with the Adrar of Iforas in northern Mali and Aïr in northern Niger, form the Tuareg Shield, displays variably deformed and metamorphosed sedimentary, volcanic and plutonic rocks that span from the Paleoarchean to the Latest Cenozoic (~3400 to ~1.51 Ma, Fig. 1). Precambrian rocks are currently organized in 16 terranes. Their boundaries correspond to lithospheric mega-shear zones, which favoured emplacement of postorogenic plutons of the “Taourirt” suite (Azzouni et al. 2003). Suture zones are often highlighted by the following: (a) mafic and ultramafic complexes interpreted as ophiolites; (b) eclogite (e.g., Zetoutou et al. 2004; Doukkari et al. 2014, 2015; Berger et al. 2014) and whiteschist slices (e.g., Adjerid et al. 2015); (c) TTG-type calc-alkaline igneous suites (Bechiri-Benmerzoug 2009); (d) ultrahigh-temperature granulite metamorphism (e.g., Ouzegane et al. 2003); (e) gravity and magnetic anomalies related to spatial variations of lithosphere characteristics (Ayadi et al. 2000).

Fig. 1
figure 1

a Schematic map of Africa showing location of Tuareg shield and Algeria. b Terrane map of Tuareg shield (Black et al. 1994, modified)

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According to Black et al.’s (1994), Fig. 1) nomenclature, the Hoggar terranes, numbered from West to East, comprise the following: (a) in Western Hoggar, 1. Tassendjanet, 2. In-Ouzzal, and the Pharusian belt composed of 3. Tin Zaouatene, 4. In-Tedeini, and 5. Silet (formerly Iskel); (b) in the polycyclic Central Hoggar located in between North-South trending 4°50′E and 8°30′E shear zones, the LATEA metacraton (acronym for 6. Laouni, 7. Azrou N’Fad, 8. Tefedest, 9. Egere-Aleksod), 10. Serouenout, and 11. Assode-Issalane; (c) in Eastern Hoggar, 12. Aouzegueur, 13. Djanet, 14. Edembo; and (d) 15. Tassili sedimentary cover.

  1. 1.

    The Tassendjanet terrane is formed by alkaline to subalkaline granites and rhyolites of Paleoproterozoic age (Caby and Andreopoulos-Renaud 1983) affected by a high-pressure pan-African metamorphism under amphibolite facies (\Caby 2003), a carbonate cover with stromatolite-bearing horizons is assumed to have a Mesoproterozoic age (Caby and Monié 2003). A complex Neoproterozoic arc terrane (the Ougda complex, Dostal et al. 1996) rooted by gabbro-dioritic arc plutons that intruded in part the carbonate cover, which is unconformably overlain by ≥ 6000 m andesite flows and volcanic greywackes (Tassendjanet/Akofou complex) (Berger et al. 2014).

  2. 2.

    The In Ouzzal terrane consists in Archean crustal units, composed of orthogneissic domes and green stone belts, strongly remobilized during the Paleoproterozoic orogeny (2000 Ma, Peucat et al. 1996). Ouzegane et al. (2003) summarize this UHT metamorphic history as two granulitic stages of high temperature: a prograde evolution with peak conditions around 9–11 kbar and 950–1050 °C, leading to the appearance of exceptional parageneses with corundum-quartz, sapphirine-quartz and sapphirine-spinel-quartz in Al-Mg granulites, Al-Fe granulites and quartzites; followed by retrograde event characterized by a pressure drop to 5–7 kbar. This retrograde event is marked by intrusive carbonatite bodies and the occurrence of leptynite veins. The major effects of the Pan-African orogeny inside the In Ouzzal terrane comprise brittle faults and high-level subcircular intrusions, mostly granitic in composition, with sharp contacts with the country rocks. During the Pan-African orogeny, the In Ouzzal terrane preserved its Archean and Eburnean characteristics, rheological, geochemical and geochronological, which corresponds to a metacratonic behaviour (Haddoum et al., 2013).

  1. 3.

    Tin Zaouatene high-T–low-P amphibolite facies gneiss, graphitic micaschist, migmatite and anatectic granite, high-K calc-alkalic granitoids, and greenschist facies molasse.

  1. 4.

    The In Tedeini terrane is considered juvenile with oceanic affinities (Black et al. 1994) constituted by a Neoproterozoic greenschist-facies intruded by poorly known batholiths and plutons in which emplacement was strongly linked to movements along the major shear zones (Boissonnas 2008).

  1. 5.

    The Silet (ex Iskel) terrane is a narrow, c. 700 km-long, c. 60 km-wide, north-south trending strip stretching along the 4°25′ meridian within Hoggar. It is inserted between the In-Teidini Neoproterozoic juvenile terrane to the west (Black et al. 1994) and the LATEA metacratonic microcontinent to the east. It is occupied by two Neoproterozoic volcano-sedimentary series, namely the Pharusian I and the Pharusian II, which experienced contrasting tectonic episodes that are separated by the intra-Pharusian unconformity (Bertrand et al. 1966). Intercalated mafic–ultramafic cumulate bodies have been interpreted as representing remnants of ophiolitic units (Black et al. 1994, and references therein). An extensive TTG’s plutonic ensembles are exposed on central part of the terrane (Bechiri-Benmerzoug 2009) (Table 4). The igneous activity is older than the major Pan-African collisional stage, during which no igneous events occurred, in contrast with other parts of the Tuareg shield. Later on, Cambrian alkali-calcic granites of the “Taourirt” province were emplaced along Silet terrane boundaries (Azzouni-Sekkal et al. 2003). Most TTG’s rocks are juvenile and show an oceanic arc affinity (Bechiri-Benmerzoug 2011).

LATEA metacraton: four terranes (Laouni, Azrou-n-Fad, Tefedest and Egere-Aleksod) are composed by Archean and Paleoproterozoic amphibolite to granulite-facies metamorphic and magmatic rocks (Peucat et al. 2003; Bendaoud et al. 2008 and references therein) that defined the metacraton LATEA (Liégeois et al. 2003, 2013). During Mesoproterozoic and Early and Middle Neoproterozoic, ocean terranes (such as the juvenile terrane of Silet—ex Iskel—and the Tin Begane eclogite-bearing nappes) were accreted along its margins during the Cryogenic and Ediacaran periods (Caby et al. 1982; Liégeois et al. 2003; Bechiri-Benmerzoug et al. 2009). Around 630 Ma begins the collision between the Tuareg/West African craton, during which LATEA craton has become a metacraton dissected into several terranes marked by emplacement of high-K calc-alkaline batholith derived partly from Paleoproterozoic/Archean crustal sources (Acef et al. 2003; Liégeois et al. 2003; Abdallah et al. 2007). The end of the process of metacratonization is marked by intrusion of shallow circular plutons, such as the Temaguessine pluton (cf. 580 Ma, Abdallah et al. 2007).

  1. 6.

    The Laouni terrane is composed of Archean-Paleoproterozoic granulite- to amphibolite-facies basement overthrust onto Pan-African lithologies, such as Tessalit ophiolitic remnant in the south and eclogite lenses and associated oceanic material in the Tin Begane area (Liégeois et al. 2003).

  2. 7.

    The Azrou N’Fad terrane is defined as a NW-SE trending slice of basement located in between Laouni and Egere-Aleksod terranes. The transgressive Early Paleozoic Tassili sandstones mark its southern tip. Archean-Paleoproterozoic granulitic gneisses and supracrustal formations were remobilized during the Pan-African orogeny and intruded by calc-alkaline batholiths (Ben El Khaznadji et al. 2017).

  3. 8.

    The Tefedest terrane is composed by basement orthogneiss with lenses of amphibolites and eclogites retromorphosed in the amphibolitic facies. The metasedimentary cover is formed by metapelites of ferruginous quartzites and marble (Briedj 1993). Pan-Africain magmatism is abundant in this area, with the calc–alkaline batholith of Azrou N’Fad, crosscut by the Temaguessine subcircular pluton dated at 582 ± 5 Ma (U–Pb/zircon; Abdallah et al. 2007).

  4. 9.

    The Egéré terrane: The Precambrian basement displays two metamorphic series. The Arechchoum orthogneissic migmatitic series and garnet amphibolite lenses, referred to as Egere series, are characterized by strongly flattened folds. Eclogites are associated with metapelites and marbles in metasediments (Arab et al. 2014), whereas they are missing in Arechchoum orthogneiss, which led the authors to interpret the contact between the two series as tectonic in nature (Doukhari et al. 2015).

  5. 10.

    The Sérouènout terrane (Fig. 1b) consists mainly of metasediments considered to have formed in an old oceanic domain involved in Neoproterozoic convergence and subsequent continental collision (e.g., Bertrand and Caby 1978; Caby 2003; Liégeois et al. 2003). However, ophiolitic markers of oceanic lithosphere are scarce in this region. Peridotitic and gabbroic rocks, exposed in the south of the terrane, have been considered remnants of oceanic lithosphere (e.g., Bertrand and Caby 1978; Caby 2003), but they do not yield the high-P metamorphism typical of subduction zones. The only high-P rocks reported so far in the Sérouènout Terrane have been observed in the Ti-N-Eggoleh area (Adjerid et al. 2015 and references therein).

  6. 11.

    The Assodé–Issalane terrane extends on 800 km from north to south (Fig. 1) and is characterized by high-temperature amphibolite facies metamorphism accompanied by regional K-rich leucogranite and by numerous high-K calc-alkaline batholiths and plutons dated between 620 and 570 Ma (Guérangé and Lasserre 1971; Bertrand et al. 1978; Liégeois et al. 1994). The metamorphic basement is a high-grade assemblage of banded and veined granitic to granodioritic gneisses and of a metasedimentary formation made up of fuschsite-bearing quartzites, calc-silicate gneisses and marbles. The whole was highly deformed under ductile conditions (Henry et al. 2009).

  7. 12.

    The Aouzegueur terrane, east of the Raghane shear zone, comprises a c. 730-Ma assemblage reminiscent of an oceanic environment (Caby and Andreopoulos-Renaud 1987) and a detrital sedimentary sequence (the Tiririne Group) separated from the former by an angular unconformity and intruded by a series of granitoid plutons and batholiths. The Tiririne Group becomes more metamorphic and more deformed northward: tight folds with N–S axial plane close to the 8°30 shear zone characterize the northern half of the area, while moderate folding affected the southern half. Greenschist-facies conditions are locally reached in the south, while they are more developed in the north (Henry et al. 2009).

  8. 13.

    The Djanet terrane is composed of detrital sedimentary series (Djanet Group), which was affected by greenschist-facies metamorphism. The Group is crosscut by magmatic intrusions between 571 and 558 Ma, related to the Late Ediacaran Murzukian orogenic episode, which has affected the Eastern Hoggar between 575 and 555 Ma (Fezaa et al. 2010).

  9. 14.

    The Edembo terrane is NW–SE elongated and bounded by shear zones adjacent to the Djanet terrane (Fig. 1b). It is characterized by amphibolite-facies metamorphism with abundant migmatites (e.g., Ouhot Complex, Table 5) and strong ductile deformation. Lithologies include biotite micaschists, metagreywackes with pebbles, phlogopite marbles, hornblende metabasalts, and migmatitic gneisses (Fezaa et al. 2010).

  10. 15.

    The Tin Serririne–Tin Mersoi basin, southeast of the Hoggar shield in Algeria and Niger, is constituted by Paleozoic series overlying the Hoggar basement. Near In Guezzam, the lower part of the series is composed by slightly metamorphosed magmatic and sedimentary complexes of Cambrian age. Other Paleozoic sedimentary formations include Ordovician to Carboniferous series in Algeria and to Permian in Niger (Djellit et al. 2006).

The aim of this paper is to review all radiometric dates concerning the Hoggar and published from 1963 up to as of 2017 (Tables 1, 2, 3, 4, 5, 6, 7). The data will be discussed separately afterwards.

Table 1 Isotopic ages published before 1980. Data arranged by terrane numbers
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Table 2 Archean isotopic ages published after 1980. Data arranged by terrane numbers
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Table 3 Paleoproterozoic isotopic ages published after 1980. Data arranged by terrane numbers
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Table 4 Neoproterozoic isotopic ages published after 1980. Data arranged by terrane numbers
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Table 5 Paleozoic isotopic ages and low-temperature thermochronology published after 1980. Data arranged by terrane numbers
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Table 6 Mesozoic low-temperature thermochronology published after 1980. Data arranged by terrane numbers
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Table 7 Cenozoic isotopic ages and low-temperature thermochronology published after 1980. Data arranged by terrane numbers
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Historical foundations of Hoggar geology

The knowledge of the historical geology of Hoggar was acquired quite recently. Field works and map making started in the nineteenth century and developed since the early twentieth century (e.g., Gautier 1908, 1928). Major breakthroughs are owed to Conrad Kilian (1898–1950) and, later on, Maurice Lelubre (1916–2005).

Conrad Kilian established definitely the following points:

  1. (a)

    Crystalline schists, reported previously as deformed Silurian formations, and granites are actually Precambrian in age. They are overlain unconformably by Tassili lower sandstones (“Grès inférieurs des Tassilis”) that are, in turn, conformably overlain by Silurian fossiliferous shales and upper sandstones (“Grès supérieurs”) (Killian 1924).

  2. (b)

    The Precambrian stratigraphy comprises two members. The older, highly metamorphosed “Suggarian” is separated from the younger, weakly metamorphosed “Pharusian” by a major unconformity decorated with metamorphic conglomerates (Killian 1932).

  3. (c)

    Another unconformity, marked again by conglomerates, was described by Karpoff (1946) in the Adrar des Iforas, the South- Western prolongation of Hoggar. It was recognized later on within the Pharusian. The Pharusian was then subdivided into an older “Relaidinian” and a younger “Nigritian” (Kilian 1947), two terms that never gained wide acceptance (Fig. 1).

In his Thesis Memoir Lelubre (1952), Maurice Lelubre applied successfully these subdivisions in a huge territory in Western and Central Hoggar. His interpretation of the “Pharusian” and the “Suggarian” as representing two successive orogenic cycles constituted a historical milestone in Hoggar geology.

The term “Pharusian” is still in usage. It matches Neoproterozoic formations that are currently subdivided into “Pharusian I” and “Pharusian II” (Bertrand et al. 1966), two terms replacing the former “Relaidinian” and “Nigritian”, respectively. When isotopic ages became available, the “Suggarian” was shown to correspond to highly metamorphosed Archean and Paleoproterozoic formations, but also to some Neoproterozoic formations as well, so that the term became obsolete.

During the 50s of the last century, many field works were performed, either for academic theses, or for economic geology purposes, but no radiometric data were available. Then, geochronological laboratories developed dramatically worldwide and produced an increasing number of isotopic ages. As analytical apparatus and methods were considerably improved during the 80s of the last century, isotopic data will be presented in two parts: (1) data collected before 1980 and (2) data collected after 1980.

Analytical methods

Several analytical methods have been used in order to obtain meaningful ages in terms of geological events. Whether they succeeded, or not, is the subject of this paper. All methods are based on natural and artificial radioactivity and assume closed systems. Because dated formations in Hoggar either are Precambrian, or lack carbon as a major component, the 14C method was not applied. Current methods apply either to mineral, or to whole-rock systems. They differ by the parent–daughter couples measured and by their temperatures of closure.

The Rb-Sr isotopic system

The Rb-Sr isotopic system was widely used in granitoids and gneisses, because they are generally rich in Rb and variously depleted in Sr. Single mineral (e.g., muscovite, biotite) or whole-rock dates should use an assumed value of initial 87Sr/86Sr (generally, 0.712). For minerals rich in Rb and poor in Sr (e.g., mica), that does not matter too much.

Results are not equally reliable and depend on the mineral analysed and the temperature, under which the system is closed, e.g., 500–300 °C for muscovite, 350–300 °C for biotite. Dates represent nothing more than cooling ages, which vary according to closure temperatures of minerals. Any thermal event raising temperature above closure temperatures will reset the isotopic clock.

Using mineral(s)–whole-rock isochrons has the advantage to give reasonable initial 87Sr/86Sr values. Whole-rock isochrons are even better, as isotopic closure is considered to take place as temperatures close to the granite solidus. However, this method should be used only in case of cogenetic samples, i.e., identical initial 87Sr/86Sr value, without any later disturbance. These required conditions are seldom completed, so that dates are only indicative and yield frequently large uncertainties (see Cahen et al. 1984).

The Sm-Nd isotopic system

This isotopic system presents the advantage that, contrary to Rb-Sr that may be mobile in hydrothermal environments, Sm and Nd are considered as immobile. Isotopic ages may be obtained via mineral(s), e.g., garnet, pyroxene, and/or whole-rock isochrons, following the same procedure as Rb-Sr. Closure temperatures for garnet are pretty high, i.e., 700–600 °C, and Sm-Nd datings involving garnet are fairly robust. If no isochrons can be obtained, due to isotopic heterogeneity, two model ages can be calculated, based on CHUR (CHondritic Uniform Reservoir) and DM (Depleted Mantle) evolution with time. Model ages are calculated in order to indicate when the source of the rock analysed was separated from CHUR, or DM. However, they are seldom meaningful in terms of geological history.

The isotopic systems containing Ar

Two methods are used, the conventional K-Ar method and the 39Ar-40Ar, in which 39K is converted artificially in 39Ar. The first method is used either on minerals, or on whole-rocks, especially in K-bearing mafic rocks that are too poor in Rb. The second method is suitable for K-bearing minerals, e.g., amphibole, biotite, and muscovite, and its advantage is that the two isotopic analyses are made on the same mass spectrometer. Recently, the method evolved to miniaturization, with laser ablation techniques. However, limitations due to closure temperatures are similar, or more severe than for Rb-Sr isotopic system, e.g., 550–450 °C for amphibole, 350–300 °C for muscovite, 320–250 °C for biotite, and 350–150 °C for feldspar.

The U-Pb isotopic systems

The dating method relies on two discrete decay chains, i.e., 238U → 206Pb and 235U → 207Pb that have different half-lives. Radioactive minerals that incorporate readily U and Th, but reject Pb, are suitable for U-Pb geochronology. Only Pb radiogenic isotopes can be detected in these minerals, with no Pb initial isotopic composition. Supplementary advantages of the dating method are high-closure temperatures, e.g., 900–750 °C for zircon, 700–600 °C for titanite, 500–400 °C for rutile, and 450–380 °C for apatite and monazite(Ce), and to provide two chronometers that can be compared in the same sample. Best cases are identical ages plotting on the concordia curve in the 206Pb/238U–207Pb/235U diagram.

Analytical techniques have developed throughout the last decades of the twentieth century. In the early days, poorly precise apparatus required large populations of crystals, hiding the intrinsic complexity of each grain, especially in the case of zircon. Zircon is very chemically inert and resistant to mechanical weathering, so that zones or even whole crystals can survive melting of parent rock with their original uranium-lead age intact. Thus, crystals with prolonged and complex histories may contain zones of strikingly different ages (usually, with the oldest and youngest zones forming the core and rim, respectively, of the crystal).

Until the 90s of the last century, data interpretations were based on analytical results aligned along a straight line, named discordia, which crosscuts the concordia curve at upper and lower intercepts. Due to improvement of analytical techniques, the required masses of crystal populations became smaller and smaller, from milligrammes down to microgrammes. Currently, instead of populations, single crystals are selected on the basis of various criteria, such as typology and zonation examined by SEM imagery, to unravel their complex evolution from concordant data. Carefully selected single grains are still analysed via the highly precise thermal ionization mass spectrometry (TIMS). In situ micro-beam analyses are routinely performed via secondary ion microprobe spectrometry (SIMS) or laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS).

Age precisions

According to techniques used, analytical errors on isotopic ratios are variable, ranging from 1 to 2% for old data down to less than 0.2% for recent ones. In published papers, age regressions were made using various softwares. Now, the most popular software package is ISOPLOT. Though the first purpose was to create a uranium-decay dating program (Ludwig 1991), the last updates, ISOPLOT 3.75 and ISOPLOT 4.15 (Ludwig 2012), can calculate and plot isochrons and concordia intercepts for a wide variety of isotopic systems.

Isotopic data published before 1980

To our knowledge, the very first published radiometric datings were made in 1960 using K-Ar method. They were followed in 1963 by two communications presented at the Academy of Sciences of Paris. The first was devoted to biotite ages via K-Ar and Rb-Sr methods (Eberhardt et al. 1963). The dated terranes were In Ouzzal (biotite pegmatite), Laouni (In Abalessa porphyritic granite) and In-Tedeini (Imezzarene margin granite), whereas the second dealt with K-Ar and Rb-Sr methods on phyllosilicates (biotite, muscovite, zinnwaldite) and whole-rocks, with preliminary data using U-Pb method on zircon (Lay and Ledent 1963). The dated terranes were Laouni (Anfeg batholith and Tin Begane biotite micaschist), Tassendjanet (Ouallen migmatitic granite), Tefedest (In-Ecker muscovite schist), Serouenout (micaschists) and Tin Zaouatene (In Rabir granite, Tin Touafa biotite muscovite granite, Tinnirt microcline granite and Ti-N-Missaou muscovite quartzite).

During the two decades 1960–1980, datings began to cover randomly the different Precambrian formations, with the aim to unravel the major events having built the shield (Fig. 2). Yet, geochronological studies were more focussed to the western terranes, owing to the discovery of Archean In Ouzzal high-grade granulitic formations. The available analytical apparatus was less improved than currently, and the quality of data was severely limited as in the following: (a) Rb-Sr, K-Ar and U-Pb results were largely discordant with 207Pb-206Pb > U-Pb > K-Ar ≥ Rb-Sr ages, (b) single mineral analyses precluded the use of isochrones or discordia curves, and (c) ages obtained on crystal populations were inherently flawed by isotopic heterogeneity.

Fig. 2
figure 2

Distribution of isotopic ages published before 1980, arranged by terranes from west to east

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In the early 1980s, a comprehensive framework for the Hoggar structure is identified mainly using the radiometric data (19 publications, 162 radiometric data; Table 1) and nine techniques: 1/− Rb-Sr minerals (56 analysis); 2/− Rb-Sr whole-rock isochron (30 analysis; Table 1); 3/− K-Ar minerals (amphibole, biotite and muscovite; 30 analysis); 4/− U-Pb zircon (28 analysis); 5/− K-Ar whole-rock (07 analysis); 6/− Rb-Sr whole-rock-minerals (06 analysis); 7/− U-Th-Pb zircon (02 analysis); 8/− Pb-Pb zircon (01 analysis); and 9/− U-Pb-Th apatite (01 analysis) (Table 1, Fig. 2).

Thus, the features evidenced by the data were approximate only. The major results, as of 1980, were as follows:

  1. (1)

    The so-called Suggarian formations yielded an unexpected large age range from Archean to Neoproterozoic, thus questioning its reliability as a geological group. On the contrary, the Pharusian formations displayed a more restricted range of Neoproterozoic dates.

  2. (2)

    Several orogenic episodes were substantiated: Archean events in cratonic fragments, Paleoproterozoic Eburnean orogeny, and Neoproterozoic Pan-African orogeny.

  3. (3)

    Early Paleozoic dates would correspond to resetting by hydrothermal episodes related to strike-slip shear zones, before Tassili sandstone deposition in the Late Cambrian.

  4. (4)

    Scarce Mesoproterozoic dates were interpreted as suggesting the possible role played by the Kibaran event defined in central-southern Africa (Table 1, Fig. 2).

  5. (5)

    K-Ar datings of Atakor felsic volcanic rocks evidence Neogene episodes.

Isotopic ages published after 1980

Together with remarkable improvements of the analytical apparatus, sampling strategy changed, with datings focussed on restricted areas carefully selected with specific purposes, mostly for doctoral theses. All field studies (Fig. 3) are currently accompanied by geochronological data. Though the shield is still unevenly covered, more precise analytical results allowed more accurate deciphering of the geological history. In addition, recent attention was paid to the detrital zircon component of (meta)sedimentary formations, which explains in part the flood of data collected since the beginning of the twenty-first century.

Fig. 3
figure 3

Field features. a Archean. “Série Rouge” grey gneiss cut by a granite dyke (Egere-Aleksod terrane). b Paleoproterozoic. Tidjenouine granulitic orthogneiss (Laouni terrane). c Neoproterozoic. Timgaouine Conophyton (Silet terrane). d Neoproterozoic. Anou-Eheli batholith (Silet terrane). e Precambrian-Cambrian boundary. Tioueine Taourirt complex (Silet-In Tedeini terrane boundary). f Neogene. Tizouiedj phonolite necks, Atakor volcanic district (Azrou N’Fad terrane). Photographic credits. a Nadia Bouregda, b–f. Faten Bechiri-Benmerzoug

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Since 1980, over 243 radiometric data were published (47 publications), the most used technique in the Hoggar and probably in the world is U-Pb zircon (102 analysis, LA-ICP-MS, SHRIMP, TIMS and SIMS). Followed by K-Ar whole-rock (35 analysis), U-Th-Pb apatite (22 analysis), Ar-Ar minerals (21 analysis), fission track apatite (16 analysis), Rb-Sr whole-rock (15 analysis), Sm-Nd whole-rock-minerals (13 analysis), Ar-Ar whole-rock (11 analysis), Rb-Sr whole-rock-minerals (07 analysis), and U-Pb titanite (1) (Fig. 3, Tables 2, 3, 4, 5, 6). Increasingly precise ages due to improved measuring devices contributed to understand better the geodynamic evolution of the Hoggar.

All radiometric data conducted after 1980 by terranes are represented in Fig. 4. Three distinct periods are emphasized: (a) Archean-Paleoproterozoic (Tables 2 and 3), (b) Neoproterozoic-Early Paleozoic (Tables 4 and 5), and (c) Cenozoic (Table 7). The absence of Mesoproterozoic and Mesozoic (Table 6) is a major feature in the geodynamic evolution of the Hoggar shield.

Fig. 4
figure 4

Diagram of all isotopic ages published after 1980, arranged by terranes from west to east

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Archean igneous episodes (Fig. 5, Table 2) were found exclusively in the In-Ouzzal, with e.g., 3473–2946 Ma charnockite (Ben Othmane et al., 1984; Sm-Nd, Rb-Sr WR model ages, U-Pb zircon), 3270 ± 11 to 2506 ± 15 Ma tonalitic gneisses (U-Pb zircon/SHRIMP and TIMS), 2772 ± 9 to 2572 ± 4 monzogranitic gneisses (U-Pb zircon/SIMS and TIMS) and 2731 ± 6 to 2650 ± 10 Ma granite gneiss (Peucat et al. 1996), and in the Egere-Aleksod terranes, with 2750 ± 100 to 2568 ± 120 Ma red gneiss complex (Sm-Nd WR and U-Pb/zircon TIMS and SIMS; Table 2) (Peucat et al. 2003) (Fig. 3a). For a more complete review, see Drareni et al. (2007).

Fig. 5
figure 5

Distribution of Archaean isotopic ages (with uncertainty) published after 1980, arranged by terranes from west to east

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Detrital zircon crystals found in quartzites (Ihaouhaoune, In-Ouzzal) yield cores at 2900 ± 100 Ma and rims at 2000 Ma (Hellal 1987; U-Pb zircon). In the Djanet terrane (Eastern Hoggar), xenocrystic zircon deposited in conglomerate yields also Archean ages (U-Pb zircon/LA-ICP-MS, 3232–2650 Ma, Fezaa et al. 2010). In the Edembo terrane, protolith of the Ouhot migmatite yields 2940 ± 17 Ma (U-Pb zircon (SHRIMP, Fezaa et al. 2010). The sources of Archean zircon crystals are likely the In Ouzzal and Egere-Aleksod terranes.

The Paleoproterozoic episodes identified by the authors before the 1980s (Fig. 2, Table 1) were confirmed by coeval magmatic and metamorphic events in the In-Ouzzal (around 2000 to 1700 Ma, Bernard-Griffiths et al. 1988; Maluski et al. 1990; Peucat et al. 1996) (Table 3) and the Egere-Aleksod terranes (around 2200–1900 Ma, Peucat et al. 2003) (Fig. 6). In the Tassendjanet terrane, a Paleoproterozoic igneous episode occurred at 1755 ± 10 Ma, whereas, in the Laouni terrane, the Paleoproterozoic is represented exclusively by metamorphic events (2075 ± 30 to 2062 ± 39 Ma) (Bertrand et al. 1986a, b; Bendaoud et al. 2008) (Fig. 3b). Detrital zircons from the Edembo and the Djanet terranes recorded Paleoproterozoic ages from 2438 to 1847 Ma. All Paleoproterozoic ages were recorded in metacratonic areas (Liégeois et al. 2013) and reflect the major role played by the Eburnean orogeny.

Fig. 6
figure 6

Distribution of Paleoproterozoic isotopic ages (with uncertainty) published after 1980, arranged by terranes from west to east

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The following Mesoproterozoic era corresponds to a long-lasting period of quiescence, referred to as the “boring billion” (Roberts 2013). Neither high-grade metamorphism nor igneous emplacement ages were recorded so far. Yet, as far as the Mesoproterozoic era is concerned, Caby (2003) reported unpublished Pb-Pb ages of 1145–1100 Ma obtained on galena from a stratabound lead occurrence within high-grade Tirek marbles.

The Neoproterozoic era is dominant in the geological history of the Hoggar shield (Fig. 7, Table 4). This era is marked by continental crustal growth illustrated by emplacement of a large number of granitoids. In Central Hoggar (Laouni and Egere-Aleksod terranes, parts of the LATEA metacraton), the granitoids intrude garnet-bearing lithologies (eclogite, amphibolite, gneiss). The amphibolite-facies metamorphic events at around 680 Ma accompanied tangential shearing and eclogite obduction (Liégeois et al. 2003). They predated granitoid emplacement at 630 to 550 Ma. The first period between 630 and 600 Ma is characterized by emplacement of the syn-collisional calco-alkaline batholiths, such as Amsel dated at 630 ± 5 and 599 ± 3 Ma (LA-ICP-MS U-Pb zircon, Talmat-Bouzeguela et al. 2011), Anfeg (dated at 615 ± 5 Ma by Bertrand et al. 1986a, b and recalculated at 608 ± 7 Ma by Acef et al. 2003) and subcircular plutons with alkaline affinity as Ounane granodiorite dated at 629 ± 6 Ma (SHRIMP U-Pb zircon, Abdallah et al. 2008). During the second period from 580 to 550 Ma, the post-tectonic batholiths with an alkaline character are dominated as the In Tounin batholiths (LA-ICP-MS U-Pb zircon 552 ± 3 Ma, Abdallah et al. 2011), Tihoudaïne and the Tisselliline granite dated at 580 ± 6 Ma and 572 ± 6 Ma (SHRIMP U-Pb zircon), respectively (Abdallah et al. 2008).

Fig. 7
figure 7

Distribution of Neoproterozoic isotopic ages (with uncertainty) published after 1980, arranged by terranes from west to east

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In the In-Ouzzal terrane characterized by granulitic basement (Ouzegane et al. 2003), alkali-calcic granitoids were emplaced, like North Tihimatine sub-circular granitic pluton dated at 601 ± 4 Ma (SHRIMP U-Pb zircon) and 600 ± 5 Ma (LA-ICP-MS U-Pb zircon, Fezaa et al. 2011), whereas, in the nearby Tassendjanet terrane, metamorphic events were identified, i.e., 719 ± 7 Ma amphibolite episode within the Ougda volcanic arc and 611 ± 5 to 577 ± 6 Ma HP episodes within the Tidjeridjaouijne belt (40Ar-39Ar amphibole, phengite and biotite; Caby and Monié 2003) coeval with Tin Zebane dyke swarm emplacement (592.2 ± 5.8 Ma, Rb-Sr WR isochron; Hadj Kaddour et al. 1998). In Eastern Hoggar (Edembo terrane), the Ouhot migmatite was metamorphosed and partially melted at 568 ± 4 Ma, coevally to 571–558 Ma alkali-calcic granitic batholiths in the nearby Djanet terrane (Fezaa et al. 2010, Table 4).

Juvenile terranes of the metasediments Pharusian belt (Tin-Zaouatene, In-Tedeini and Silet terranes) are characterized by the occurrence of Neoproterozoic geological formations only. For example, in the Silet terrane (Bechiri-Benmerzoug 2009), numerous granitic batholiths (sodic low-HREE and potassic TTG) were emplaced during the Tonian (three episodes at 868 ± 8, 839 ± 4 and 742 ± 5 Ma, Table 4) and mostly the Cryogenian (tonalite, granodiorite and monzogranite rocks from 651 ± 6 to 638 ± 5 Ma) (Fig. 3c, d). They predate the final collision of the Silet terrane onto the LATEA metacraton (Laouni terrane) at 629 ± 5 Ma (Bertrand et al. 1986a, b). No Ediacaran batholiths were found so far. The Neoproterozoic volcanism which is represented by Irrelouchem volcanic series (basalt, rhyodacite and ignimbrites) exposed in the Tin-Dahar area (Silet terrane) is dated at 680 ± 36 Ma (isochron Rb-Sr WR, Dupont 1987; Table 4).

The Ediacaran period is marked by North-South trending strike-slip shearing episodes, accompanied by emplacement of the Taourirt igneous province. The Taourirt complexes (Azzouni-Sekkal et al. 2003) are composed of alkali-calcic and (per)alkaline A-type granitoids and associated gabbros and alaskites (highly evolved alkali feldspar granites). Their emplacement ages are not well-constrained by Cambrian Rb-Sr whole-rock isochrons (Azzouni-Sekkal et al. 2003) and 39Ar-40Ar biotite dates (Cheilletz et al. 1992) ranging from 540 to 505 Ma (Table 5), which may represent late thermal events fostered by strike-slip movements along north-south trending shear zones. For example, the In Tounine Taourirt complex yields 552 ± 3 Ma LA-ICP-MS U-Pb zircon age, interpreted as true emplacement age (Abdallah et al. 2011), 502 ± 42 Ma (Azzouni-Sekkal et al. 2003) or 521 ± 17 Ma (Cheilletz et al. 1992) Rb-Sr whole-rock isochrons, and 535 ± 3 Ma 40Ar-39Ar biotite age (Cheilletz et al. 1992), suggesting that the Taourirt igneous episodes may have occurred mostly during the Ediacaran and that Cambrian ages may reflect either a slow cooling process, or more likely a late-stage hydrothermal event. Yet, the 523 ± 1 Ma U-Pb zircon (TIMS) age on Tioueine alkali feldspar syenite (Paquette et al. 1998) is the only to correspond to late Early Cambrian emplacement (Fig. 3e).

During the Paleozoic era (Fig. 8, Table 5), two major periods were found. In the Early Cambrian, Rb-Sr whole-rock isochrons and 39Ar-40Ar biotite ages on Taourirt complexes, though interpreted formerly as igneous emplacement ages, may actually represent late thermal episodes. Later on, in the southern and eastern Tassili sedimentary cover, the 348 ± 16 Ma (Late Devonian to Early Carboniferous) Tin Serririne dolerite lava (Djellit et al. 2006) and the 326 ± 8 Ma (Serpukhovian) Arrikine gabbroic sill (Derder et al. 2016), measured by 40K-40Ar isotopes on whole-rocks, took part to the widespread Carboniferous igneous events occurring in the nearby North African sedimentary basins.

Fig. 8
figure 8

Distribution of Paleozoic isotopic ages (with uncertainty) published after 1980, arranged by terranes from west to east

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The Mesozoic era (Table 6) was a period of quiescence in terms of ductile deformation, metamorphic and igneous episodes. During the Cenozoic era (Fig. 9, Table 7), renewed volcanic activity began at ca. 44.0 ± 0.5 Ma, with the outpouring of Taharaq trachybasalt flow followed by a 34.5–24.4 ± 0.5 Ma sequence of tholeiitic flood basalt lava flows (Aït-Hamou et al. 2000). Then, the volcanic activity became more and markedly alkaline in several discrete episodes separated by periods of quiescence. It was active until recent times, with scarce Neolithic artefacts intercalated with lava flows (Benmessaoud 2014). For a more complete review, see Liégeois et al. (2005). The first Late Eocene and Oligocene episodes were confined in the Egere-Aleksod terrane, in the central part of the Hoggar Swell. During Neogene episodes, new districts were formed in between the 4°50′E and the 8°30′E north-south shear zones within the Hoggar Swell, with most of them occupying a diametrical SW-NE trending alignment referred to as the Oued Amded lineament (Aït-Hamou 2006). Discrete episodes are coeval to the successive phases of Africa–Europe convergence (Rougier et al. 2013). Current activity is marked by thermal and/or mineral springs in the south of the Atakor district (for a review of the Atakor volcanic district, see Azzouni-Sekkal et al. 2007) (Fig. 3f). Weak seismic activity, recorded since the 1950s by the Tamanrasset seismological station, occurs near the 4°50′E shear zone, with a recent crisis beginning on May 20th, 2010 (Bourouis et al. 2013; Babkar et al. 2014). It provides ample evidence that, though located within intraplate settings, the Hoggar Swell is not yet stable.

Fig. 9
figure 9

Distribution of Cenozoic isotopic ages (with uncertainty) published after 1980, arranged by terranes from west to east

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Two studies (Carpena et al. 1988; Rougier 2012) using thermochronological techniques (U-Th/He apatite and zircon, apatite fission tracks) were made to unravel exhumation mechanics of the Hoggar Swell (Fig. 10, Tables 5 to 7). Discrete low-temperature episodes affected Paleoproterozoic carbonatites in the In-Ouzzal terrane during the Pan-African orogeny and, afterwards, during the final stage of the Variscan orogeny, as emphasized by Ediacaran (628 Ma), Mid-Cambrian (500 Ma) and, later on, Mid-Permian (263 Ma) apatite fission track ages (Carpena et al. 1988). Elsewhere, apatite fission tracks and U-Th/He ages spread from the Early Permian (285 Ma) to the Pliocene (5 Ma) (Rougier et al. 2013). Cretaceous sedimentary remnants at high elevations suggest subsidence during the Mesozoic, with burial of more than 1 km after the Early Cretaceous. Thermal models reflecting large-scale vertical processes demonstrate widespread Eocene exhumation of the entire shield before volcanic activity began in the Late Eocene (Rougier et al. 2013).

Fig. 10
figure 10

Distribution of low-temperature thermochronology, arranged by terranes from west to east

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Summary and concluding remarks

Increasingly precise isotopic dating techniques illustrate the protracted geological history of the Hoggar Shield.

The first continental nuclei were formed in the Archean era. A second series of continental terranes were created during the Paleoproterozoic Eburnean orogeny. A long-lasting period of quiescence, referred to as the “boring billion”, corresponds to cratonization processes in the Mesoproterozoic.

Cratonic to metacratonic terranes were reworked and accompanied juvenile terranes during the Neoproterozoic–Lower Paleozoic Pan-African orogeny. At the final stages of the orogeny, the Hoggar Shield acquired its definitive shape defined by strike-slip movements along north-south-trending shear zones.

After scarce Carboniferous emplacement of mafic magmatic formations, the Mesozoic and the beginning of the Cenozoic are again a long period of quiescence, with neither high-grade metamorphism, nor igneous emplacements. Widespread Eocene exhumation predated the Late Eocene to recent volcanic activity.

Low-temperature thermochronological ages show Mesozoic subsidence, with burial up to 1 km after the Early Cretaceous, followed by Eocene exhumation giving to the Hoggar its current landscape.

Comparison of the different dating techniques indicates that emplacement ages of magmatic formations and/or protoliths of orthogneisses should be measured by U-Pb zircon ages, whereas the other techniques show only late-stage hydrothermal processes, except K-Ar techniques for Cenozoic volcanic formations. Thus, new U-Pb zircon ages are warranted in order to get more precisely dated geological episodes.