Introduction

Mafic-ultramafic rocks can provide valuable information for unraveling the geological history of orogenic belts. For instance, those associated to ophiolitic complexes represent the remnants of fossil oceanic lithosphere and can indicate the suture zones (Dilek 2003; Şengör and Natal’in 2004; Robinson and Zhou 2008; Pearce and Robinson 2010). Many phanerozoic plutonic complexes have been identified in former oceanic arc setting. Less studied are the Proterozoic intrusions formed in island arc settings and mainly those formed in continental arcs. Such intrusions are often small in size and are elliptical- to ring-shaped complexes with distinct zoned rock units, the peridotites in the core, whereas pyroxenites and gabbros occur at the outer parts of the intrusion. This type of complexes occurs generally at convergent plate margins (Irvine 1974; Himmelberg and Loney 1995; Chen et al. 2009; Yang and Zhou 2009), but it can occur in almost any geodynamic context, including during post-collision extension periods (Tistl 1994; Mues-Schumacher et al. 1996; Chai et al. 2008; Zhang et al. 2008).

The In Allarene mafic-ultramafic complex, located in the southern part of the In Ouzzal terrane in the Hoggar (south of Algeria) (Fig. 1), is the only known mafic-ultramafic intrusion that is believed to be emplaced in this terrane during the Pan-African orogeny (Caby 2003; Talbi et al. 2007). This igneous complex, which covers an area of about 20 km2, shows ring structure with a peridotitic core surrounded by gabbroic lithologies. The complex was later intruded by granitic lithologies (Fig. 2). The overall ring structure, together with the dip of different lithologies, argues for a rather laminated lopolith that was emplaced as a result of the collision between a continental block of which the In Ouzzal was part and the surrounding terranes. This was in turn contemporaneous with the Touareg shield-West African craton collision at about c. 620 Ma (Berger et al. 2014; Bosch et al. 2016). In Ouzzal terrane is considered part of a continental block that includes in particular the Ahnet, Tirek, and Iforas terranes and has functioned as an active margin bordered by subduction zones (Caby 2003; Caby and Monié 2003). The In Allarene pluton would have been emplaced during the early stages of these subduction events. In this study, we present the petrological, mineralogical, and geochemical features of the In Allarene mafic-ultramafic complex, the largest unmetamorphosed igneous intrusion in the In Ouzzal terrane. Detailed petrological, mineralogical, and geochemical data are used to elucidate the magma sources and the geodynamic context of its emplacement.

Fig. 1
figure 1

a Sketch map of the Tuareg shield showing the main lithotectonic terranes, after Black et al. (1994) and Liégeois (2019). b Simplified geological map of the study area (unpublished ORGM rapport). Ed, Edembo; Ao, Aouzegeur; Se, Serouenout; Tz, Tazat; As, Assodé-Issalane; GO, Gour-Oumelalene; LATEA, (La, Laouni; Az, Azrou-n-Fad; Te, Tefedest; Eg-Al, Egéré-Aleksod; Ao, Aouilene); Is, Iskel; It, In-Tedeini; Tz, Tin-Zaouatene; Ah, Ahnet; TAS, Tassendjanet; IOGU, In Ouzzal granulitic unit; Ki, Tirek, Kidal; Ti, Tilemsi

Full size image
Fig. 2
figure 2

Geological map of mafic-ultramafic In Allarene complex and NW-SE cross-section showing structural relationships of the different units in the In Allarene complex

Full size image

Geological setting

The In Ouzzal granulitic block, one of the 25 terranes defined by Black et al. (1994) and Liégeois (2019), displays a north-south direction and more than 400 km long and about 80 km wide in its northern part. This terrane becomes thinner until it disappears towards the south at the expense of the Adrar des Iforas outcrop in Mali (Fig. 1a).

Petrological and geochronological studies carried out in the northern part of the In Ouzzal terrane show that it is an Archean crust segment which is composed of two major units: an Archean charnockitic orthogneiss unit dated at 3.3 to 2.5 Ga (Peucat et al. 1996) and a metasedimentary unit commonly associated with ortho-derived materials composed of mafic lenses (metanorites, spinel-bearing pyroxenites, and pyrigarnites) and ultramafic rocks (lherzolites and harzburgites), with subordinate leptynites and anorthosites. The majority of both ortho- and para-derived lithologies that constitute the In Ouzzal terrane show chemical features of Archean rocks. Mineralogical associations and structural characteristics of these rocks were later obliterated during the Eburnean event by a granulitic metamorphism of very high temperature that exceeded 1000 °C (Ouzegane et al. 2003; Benbatta et al. 2017).

The In Ouzzal terrane has long been considered to have been a rigid cratonic area during the Pan-African orogeny. It is recognized that this orogeny only manifested along the shear zones bordering this terrane, with limited extension greenschist metamorphism (Ouzegane et al. 2003). However, there is an increasing evidence that the paleo-continent, to which this terrane belongs, corresponds to an active margin at its two boundaries during the Pan-African orogeny (Fig. 3). Thus, Caby (2003), Caby and Monié (2003), and Berger et al. (2014) interpreted the existence of eclogites at the boundary between the In Ouzzal and the Tassendjanet terrane (Fig. 3) as the result of eastward subduction under the In Ouzzal. Caby (2003) considered that the mafic-ultramafic lithologies of the Zora camp complex (in the northern part of this block) that were metamorphosed in the granulite and blue-schist facies correspond to a lower crust of an island arc obducted on the Ahnet terrane after a westward vergence subduction under the In Ouzzal and Ahnet terranes. Bosch et al. (2016) and Bendaoud et al. (2017) show the presence of granitic and dioritic plutons throughout the Tirek terrane, dated between 660 and 630 Ma (Fig. 3). This is characteristic of arc magmatism that indicates a westward vergence subduction zone in this region. This is supported by several geophysical studies. For instance, Brahimi et al. (2018) showed that the western boundary of the block In Ouzzal–Ahnet–Tirek corresponds to a major accident separating two distinct continental crusts which had different rheological, gravimetric, and magnetic characteristics (Fig. 3). Moreover, Takherist (1990) demonstrated, using gravimetric data, that the Adrar Fault limiting the eastern side of this block is one of the most important accidents in the Hoggar (Fig. 3). Deramchi et al. (2020), using magnetotelluric data, showed that the two shear zones (the West Ouzzalian shear zone and the Adrar fault) are lithospheric scale and interpret them as witnesses of former subduction zones having convergent dips under the In Ouzzal (Fig. 3). These authors also showed that the mantle and a significant part of the In Ouzzal lower crust is conductive and were therefore rich in fluid and/or mineralization, which is characteristic of a subduction paleo-zone. It should be noted here that Fettous et al. (2019) dated the granulitic pseudotachylites within the In Ouzzal at 820 Ma (40Ar/39Ar), indicating remobilization of intra-ouzzalian shear zones in the Neoproterozoic.

Fig. 3
figure 3

Sketch map of the western part of the Tuareg shield (Black et al. 1994) showing (A) large shear zones interpreted as suture zones; (B) the geodynamic evolution of western Hoggar between 680 and 610 Ma, proposed by Caby (2003), with one subduction zone between the West African craton and the Tuareg shield and two others around the In Ouzzal; (C) vertical cross-section extracted from the obtained 3-D model by magnetotelluric data inversion (Deramchi et al. 2020) showing that the mantle under in ouzzal terrane and much of the lower crust are conductive and therefore rich in fluid and/or mineralization, which is characteristic of a subduction paleo-zone; and (D) the geodynamic evolution of the southwestern part of the Tuareg shield between 660 and 630 Ma, proposed by Bosch et al. (2016) and Bendaoud et al. (2017) showing a westward vergence subduction zone in this region

Full size image

The In Allarene mafic-ultramafic complex is the only pluton in the In Ouzzal terrane that is considered to be related to the Pan-African orogeny (Caby 2003; Talbi et al. 2007). Caby (1970, 1996) compared this pluton to lopoliths and sills that intruded the stromatolite-bearing series of the Tassendjanet region and emplaced before 680 Ma (Dostal et al. 1996; Caby and Monié 2003). It should also be noted here that Benmoussa et al. (1996) suggested that the In Allarene complex most likely have an Archean or Lower Proterozoic age. However, the intrusive and non-metamorphic nature of the massif that was intruded the Archean gneissic lithologies overlap the Eburnean foliation (~ 2000 Ma, Haddoum; unpublished map), and hence, both the Archean and the lower Proterozoic ages are discarded. However, the Pan-African is the most probable age of the In Allarene intrusion, as suggested by Caby (1996) and Talbi et al. (2007).

The intrusion shows an oval shape of about 20 km2 with a ring structure (Fig. 2a, b). Partially serpentinized peridotites form the core of the pluton and are surrounded by pyroxene-rich ultramafic rocks, then bordered by gabbroic lithologies. In some places, the gabbros exhibit magmatic stratification and show decimeter- to meter-thick segregation of anorthosites. The presence of fine-grained facies, particularly in the southern part of the massif, suggests shallow emplacement context (Caby 1996). The outer parts of the intrusion are intruded by later Pan-African granitoids (600 Ma; Fezaa et al. 2019) and a numerous of E-W- to NW-SE-oriented dolerite dykes (Fig. 2). Enclaves of few meters diameter are observed; they consist of fenites/syenites and tonalities, which have been considered by Benmoussa et al. (1996) as cogenetic with the intrusion. However, field observations show that they are rather xenoliths that belong to the Archean and lower Proterozoic country rocks. Indeed, the contact between the intrusion and the Archean country rocks are often intrusive, except the northwestern part of the complex where the contact with the NW dipping “Série Verte” sedimentary rocks (860–620 Ma, Caby 1996, Talbi et al. 2007) is marked by a tectonic fault (Fig. 2). The ring structure and the dip of the different lithologies of the pluton remind those features of dish-shaped stratified lopoliths.

Sampling and analytical techniques

Sampling was carried out over all representative lithologies of the mafic-ultramafic complex. Samples were systematically collected from the peridotitic core to the outer parts of the intrusion. Particular care was taken to collect the least altered rocks, where olivine and pyroxene minerals were well preserved.

Mineral chemistry was determined on polished thin sections using a CAMECA SX-100 five-spectrometer electron microprobe at the Université Pierre et Marie Curie Paris VI, France. Compositions of olivine, orthopyroxene, clinopyroxene, plagioclase, K-feldspar, amphibole, and spinel were measured with a 15-kV accelerating voltage, 20 nA beam current, and a counting time of 20 s on peaks and 10 s on background. Natural silicates and synthetic oxides were used as standards for all elements, except for fluorine, which has been calibrated on fluorite. Chemical compositions of minerals of the different studied rocks are provided in Tables 1, 2, 3, 4, 5, and 6 (Talbi et al. 2007).

Table 1 Chemical composition of olivine of the In Allarene complex
Full size table
Table 2 Chemical composition of orthopyroxene of the In Allarene complex
Full size table
Table 3 Chemical composition of clinopyroxene of the In Allarene complex
Full size table
Table 4 Chemical composition of amphibole of the In Allarene complex
Full size table
Table 5 Chemical composition of plagioclase of the In Allarene complex
Full size table
Table 6 Chemical composition of spinel of the In Allarene complex
Full size table

The bulk-rock compositions (major and trace elements) were carried out at the department of earth resources engineering, Kyushu University, using an X-ray fluorescence (Rigaku RIX 3100) method. Loss of ignition was measured based on the weight difference after ignition at 105 °C for 1 h and 1000 °C for 2 h. Rare earth element analyses were carried out on selected samples using inductively coupled plasma mass spectrometry (ICP-MS). Twenty-four samples with standard solutions were prepared using acid digestion methods and analyzed in an Agilent 7500 Series Quadrupole Reaction System. A calibration curve was constructed based on the measurements of the standard solutions. The analytical plasma condition included a radio frequency (RF) power of 1500 W, an RF matching of 1.64 V, and Ar carrier gas with flux of 1.2 L/min. Whole-rock chemical analyses are reported in Table 7 (INA and IA samples from Benmoussa et al. 1996).

Table 7 Major and trace elements data for the mafic and ultramafic rocks of the In Allarene complex. (INA and IA samples are from Benmoussa et al. 1996)
Full size table

Results

Petrography

Harzburgites and dunites

The harzburgites and dunites are recognized in the central part of In Allarene massif (Fig. 2). These rocks often show small spots or patches at their surface due to alteration effects; some outcrops are completely altered and transformed into serpentinite.

Polygonal olivine grains form cumulative texture under fresh harzburgite units (Fig. 4a). Most of the sub-rounded olivine grains are partially or totally serpentinized and form a mesh-like texture of lizardite and chrysotile. Pyroxenes represent 20–40 vol.% of the harzburgitic rock and are partly altered into serpentine or occasionally replaced by biotite flakes. They consist mainly of orthopyroxene and rare clinopyroxene (< 5 vol.%). Subhedral orthopyroxene and olivine aggregates represent the cumulus phases under the harzburgite variety. They are associated with magnetite, which seems to be a secondary mineral, related to the serpentinization process (Fig. 4a). Millimeter-sized euhedral chromian-spinel (Cr-spinel) are frequent either as disseminated grains within rocks or as inclusions within pyroxene minerals. Occasionally, Cr-spinels show fractures filled of serpentine minerals.

Fig. 4
figure 4

Photomicrographs in cross-polarized light of the mafic-ultramafic rocks from In Allarene complex. a Harzburgite, b lherzolite, c wherlite, d olivine-rich gabbro, e olivine-poor gabbro, f dolerite. Ol, olivine; Opx, orthopyroxene; Cpx, clinopyroxene; Mt, magnetite; Chr, chromite; Pl, plagioclase; Am, amphibole

Full size image

Lherzolites and wehrlites

These lithologies that also emerge in the central zone of the intrusion are mainly massive, dark, and slightly altered. They usually show heteroadcumulate textures composed by ubiquitous olivine, diopside, and augite grains. Within the matrix, these minerals correspond to the cumulus phases. Wehrlite is characterized by the predominance of clinopyroxene compared with orthopyroxene (Fig. 4b), while orthopyroxene is relatively enriched (however, lower than 8% vol) in the lherzolites.

Clinopyroxene is often twinned and occasionally transformed into amphibole (uralite) (Fig. 4c). Cr-spinel is the main accessory mineral in these rocks and is usually included in pyroxene phases. Serpentine minerals formed as a secondary mineral after intercumulus-phase alteration. A few brown amphiboles (0.1–0.3 mm across) exist as prismatic crystals and are considered primary origin.

Gabbros

Two types of gabbros were identified according to their grain sizes, olivine contents, and spatial distribution within the intrusion.

  1. 1.

    Dark-colored, olivine-rich gabbros—they occur in the contact with lherzolite-wehrlite facies and show granular, heterogranular to gabbroic textures (Fig. 4d). Olivine is abundant and shows more or less rounded form, rarely altered and occasionally observed as inclusions in clinopyroxene. Clinopyroxene is more abundant than orthopyroxene and often exhibits large subhedral twinned crystals. Laths of plagioclase are present and often altered. Magnetite and Cr-spinel occur as inclusions within pyroxene and olivine crystals.

  2. 2.

    Light-colored, olivine-poor gabbros—they occur at the outer part of the olivine-rich gabbros. They also present heterogranular and gabbroic texture but are fine-grained rocks (Fig. 4e). This type of gabbros often encloses decimeter- to meter-sized anorthosite pods and nodules and shows a fairly clear magmatic layering in some areas. Clinopyroxene is present as large subhedral twinned crystals and orthopyroxene is usually replaced by amphibole. Plagioclase exhibits tabular prismatic shape, rarely altered. Scarce olivine crystals are often totally serpentinized, but relics are preserved allowing their recognition.

In both types of gabbros, plagioclase is abundant and occasionally shows preferred orientation that gives the rock a marked magmatic layering. Plagioclase exsolutions into clinopyroxene grains also mark the subsolidus phenomena related to the cooling of magma.

Dolerite

Dolerites rocks outcrop at the external area of the intrusion following by the overlapping of doleritic dykes in many areas of In Allarene pluton. The dolerites show typical intergranular sometimes intersertal doleritic textures consisting of prismatic, occasionally oriented plagioclase crystals. Relatively large hornblende crystals often contain plagioclase inclusions, which confer an ophitic texture to the rock (Fig. 4f). Rare pyroxenes are usually observed as relics in hornblende crystals (uralite) or as inclusions within plagioclase laths. Skeletal biotite-phlogopite minerals can also be observed and are often partly chloritized. Altered K-feldspar crystals may occasionally occur in the rock and accessory minerals are mainly magnetite and ilmenite.

Mineral chemistry

Olivine

Olivine was analyzed in both peridotites and gabbros, where the mineral occurs as a cumulus phase. High fosterite contents (Fo ≈ 0.88) are observed in olivine of the central part of the intrusion, which is composed harzburgites, whereas olivine of the lherzolites (Fo ≈ 0.78) and gabbros (Fo = 0.70–0.75) shows lower fosterite contents. The Cr2O3 contents in olivine decrease from 0.35 wt.% in peridotites to 0.02 wt.% in the magnesium-poor gabbros (Table 1).

Pyroxene

The orthopyroxene of all mafic and ultramafic rocks is mainly enstatite, with XMg (100 × Mg/Mg + Fe2+) ranging between 65 and 91. The composition of orthopyroxene is Wo01–05, En89–63, and Fs09–03. The most magnesian orthopyroxene is that of harzburgites and the most ferrous is that of gabbros and dolerites poor in olivine. NiO and Cr2O3 contents are rather low, ranging between 0 and 0.13 wt.% and between 0 and 0.35 wt.% respectively. Al2O3 contents of orthopyroxene decrease from the dunites-harzburgites to the gabbros and dolerites (from 2.32 to 0.79 wt.%) (Table 2).

Diopside and augite are the principal clinopyroxenes in the majority of the studied rocks. Their XMg contents vary from 96 in the harzburgites-dunites to 76 in the dolerites. Their Al2O3 contents are between 0.67 and 2.78 wt.%, whereas their CaO exhibits high contents (between 20.56 and 24.32 wt.%) (Table 3).

Amphibole

Amphibole is mainly Ca-amphiboles; according to the classification of Leake et al. (1997), these amphiboles are mainly magnesio-hastingite or pargasite depending on their AlVI and Fe3+ compositions. Edenitic compositions are only observed in the amphiboles from the dolerite samples. Amphiboles from ultramafic rocks are enriched in Cr2O3 (1.0 to 1.22 wt.%) relative to those from olivine-poor gabbros and dolerites (0–0.12 wt.%). A wide range of MgO concentrations is observed (from 12.65 to 16.25 wt.%) where the highest values are reported in ultramafic rocks (Table 4).

Feldspar

Plagioclase is the dominant feldspar phase in the studied rocks. It occurs in gabbros and dolerites, where it often shows bytownite compositions ranging from An72 to An82 in the gabbros and An76 and An80 in dolerites (Table 5).

Chromian-spinel

Cr-spinels from both peridotites and olivine-rich gabbros show tight Cr2O3 contents (between 25.5 and 27.45 wt.%). Al2O3 contents vary between 26.48 and 30.90 wt.%, whereas the MgO concentrations range between 5.4 and 10.48 wt.%. These Cr-spinels exhibit also a range of FeOT from 28 to 33.59 wt.%. The TiO2 contents, ranging between 0.15 and 0.63 wt.%, are clearly higher than those of Cr-spinels from the mid oceanic ridge tholeiites (0.02 to 0.14; Singh 2013). XMg of the Cr-spinels varies between 18 and 38, Fe3+ exhibits low contents (1.78 to 2.35) and Cr# (100xCr/(Al + Cr)) ratios show intermediate to high values, ranging from 35 to 55 (Table 6).

Geochemistry

Major elements

The peridotites show relatively high L.O.I contents, varying between 6.29 and 10.96 wt.% (Table 7), reflecting a moderate to high alteration phenomena (serpentinization), already observed at thin-section scale (Fig. 4a). On the other hand, fresh gabbros and dolerites show lower L.O.I contents (0.15–5.38 wt.% and 0.91–2.75 wt.% respectively) indicating lower alteration effects. Because of some rock and mineral alteration, especially in ultramafic lithologies, all major oxide contents were recalculated in this study to 100% on anhydrous basis.

The peridotites are characterized by low SiO2 (42.17–45.50 wt.% in harzburgites and 44.31–47.40 wt.% in lherzolites) and high MgO (41.13–45.45 wt.% in harzburgites and 32.97–38.05 wt.% in lherzolites) contents. They also exhibit low Al2O3 (0.01–0.45 wt.% in harzburgites and 0.72–1.26 wt.% in lherzolites) and TiO2 (0.01–0.05 wt% in harzburgites and 0.05–0.15 wt.% in lherzolites) and variable CaO (0.07–0.31 wt.% in harzburgites and 5.16–8.33 wt.% in lherzolites) contents.

Inversely, gabbros and dolerites exhibit relatively higher SiO2 and Al2O3 contents (44.98 to 60.57 wt.% and 8.37 to 21.30 wt.% respectively) and show lower MgO contents (varying between 4.52 and 20.28 wt.%) than those of the ultramafic rocks. The highest MgO contents characterize the olivine-rich gabbros, and the highest Al2O3 and CaO contents mark the most leucocratic, plagioclase-rich gabbros. The dolerites have compositions similar to those of olivine-poor gabbros (XMg varying between 0.45 and 0.53). TiO2 contents show a relatively low concentration (< 1.7 wt. %) in all analyzed rock suites.

Using MgO content variation as magmatic differentiation index, SiO2, Al2O3, CaO, TiO2, K2O, and Na2O contents show similar variation trends; they increase steeply with decreasing MgO contents (Fig. 5). This is consistent with the progressive accumulation of clinopyroxene and plagioclase in the rocks. However, total iron (Fe2O3t) and Cr increase with increasing MgO, consistent with accumulation of olivine and Cr-spinel in ultramafic rocks.

Fig. 5
figure 5

Major elements Harker diagrams of mafic-ultramafic rocks from In Allarene complex

Full size image

On the SiO2 vs CaO, SiO2 vs MgO, and MgO vs CaO diagrams (Fig. 6a–c), the composition of the constituent minerals of the ultramafic rocks show that the transition from the lherzolite to wehrlite is marked by clinopyroxene enrichement in the wehrlite compared with lherzolite

Fig. 6
figure 6

Whole-rock compared to mineral chemical compositions of the mafic-ultramafic In Allarene rocks: a SiO2 vs. CaO; b SiO2 vs. MgO (AM, primitive magmas of arc volcanism; BC, continental basalts from Wang et al. (2016)); and c MgO vs. CaO diagrams

Full size image

Trace elements

The peridotites show high concentrations of compatible elements, such as Cr (2410–4480 ppm) and Ni (664–816 ppm), and relatively low Co (50–103 ppm) and V (25–102 ppm) contents. Cr, Co, and Ni concentrations correlate positively with the high proportion of olivine and spinel minerals that host these elements in the peridotites. On the other hand, the lowest V concentrations are recorded in the peridotites, whereas the highest contents are observed in gabbros (68–215 ppm) and dolerites (165–299 ppm) due to the presence of clinopyroxenes that accommodate V in their crystal structure.

Incompatibles elements, such as Sr, Ba Th, Nb, Ta, Hf, and Zr are depleted in the peridotite samples. However, Sr and Ba enrichment together with positive Eu anomalies (normalized to chondritic values) are observed in some low-magnesian gabbro (olivine-poor gabbros) and dolerite samples. This may indicate plagioclase fractionation in these rocks. MgO contents positively correlate with compatible trace elements, such as Cr, Ni, and Co and show negative correlations with incompatible elements (high-field strength elements (HFSE) and large ion lithophile elements (LILE), such as Zr, Sr, and Y) in doleritic and gabbroic rocks (Fig. 7).

Fig. 7
figure 7

Harker diagrams of selected trace elements of the mafic-ultramafic In Allarene complex. Symbols are as in Fig. 5

Full size image

All the investigated igneous units display variable rare earth elements (REE) contents, with a slight enrichment in light REE (LREE) compared with heavy REE (HRRE). However, the sum of REE (ΣREE) abundances displays an increasing trend from ultramafic to mafic rocks. The dunite-harzburgites and wehrlites have the lowest ΣREE contents (3.77–4.53 ppm and 17.79–20 ppm respectively), whereas the olivine-poor gabbros and dolerites show the highest ΣREE contents (20.78–210.72 ppm).

The chondrite-normalized REE diagram of the In Allarene igneous rocks (Fig. 8a) shows two groups of rather sub-parallel and coherent patterns. The first group represent the peridotites and Mg-rich gabbros, which display flat patterns with slightly LREE enrichment compared to HREE ((La/Yb)CN 1.38 to 4.72). The second group correspond to the low-Mg gabbro and dolerite samples, which show a negative slope with more pronounced LREE enrichment compared to HREE ((La/Yb)CN 5.63 to 15.87). Both groups display weak negative and positive Eu anomalies, which may indicate respectively plagioclase enrichment or depletion in their mineralogical compositions.

Fig. 8
figure 8

Chondrite-normalized (Boynton 1984) REE patterns (a) and N-MORB-normalized (Sun and McDonough 1989) trace element patterns (b) of the mafic-ultramafic rocks from In Allarene complex

Full size image

MORB-normalized bulk-rock multi-element also distinguishes two groups of the analyzed samples: low-Mg gabbros and dolerites are relatively more enriched in LILE and HFSE than peridotites. However, both groups exhibit non-smooth MORB patterns with strong positive Ba and Ce anomalies, negative Nb-Ta and TiO2 anomalies, and negative to positive Sr anomalies (Fig. 8b).

Discussion

Petrogenesis

Most of the analyzed ultramafic rock samples from the In Allarene massif show low Al2O3 contents (0.01–1.26 wt.%), low Al/Si ratios (0.0015–0.02), and variable CaO contents (0.07–8.33 wt.%). These observations point to the crystallization of olivine, spinel, and orthopyroxene minerals first, then to the fractionation of clinopyroxene, since the residual liquid becomes richer in CaO. A small proportion (5% by volume) of clinopyroxene compositions fractionate Ca in peridotites. CaO and Al2O3 display a well-defined inverse correlation with MgO (Fig. 5), probably reflecting variable proportions of olivine and pyroxene crystallization and fractionation of plagioclase in gabbro and dolerite border units during the late magmatic stage. It is clear from Fig. 6a–c that the peridotite major element (MgO, CaO, and SiO2) compositions are controlled by the modal proportions of olivine, orthopyroxene, and clinopyroxene minerals. Harzburgites lie between olivine and orthopyroxene, while lherzolites shift towards clinopyroxene. These minerals, as well as plagioclase, also indicate their control on Mg-rich gabbro and dolerite compositions. Mg-poor gabbros and dolerites, which appear to correspond to liquids, are amphibole-rich and show alkali feldspar in their matrix.

Moreover, the relationships between MgO contents and major oxides (Fig. 5), as well as compatible trace elements, such as Cr, Co, and Ni (Fig. 7), indicate a control of the chemical composition of the rocks by crystal fractionation/accumulation of olivine, pyroxene, and plagioclase. Negative correlations between MgO and SiO2, CaO, Al2O3, K2O, and Na2O (Fig. 5) are indicative of clinopyroxene and plagioclase fractionation/accumulation in the mafic units, whereas positive correlations between MgO and Fe2O3t and compatible trace elements (e.g., Cr, Co, and Ni) (Figs. 5 and 7) are consistent with olivine, spinel, and orthopyroxene fractionation/accumulation in the ultramafic units (peridotites). Therefore, it is most likely that the cumulate texture of peridotites suggests that olivine accumulation was a dominant process, which is in good agreement with the high MgO contents in bulk-rock compositions and the decreasing of MgO values with increasing SiO2 and Al2O3 contents. Increasing in TiO2 with decreasing in Cr from peridotites to gabbros suggests a fractionation of Cr-spinel, which commonly occurs in the peridotites and olivine gabbros (Cai et al. 2012). On the other hand, the observed negative correlation between TiO2 and MgO values suggest that Ti was incompatible in the crystallization process and that Fe-Ti oxide did not reach the solidus in the system (Yang and Zhou 2009). In addition, the CaO and Al2O3 enrichment with MgO depletion (Fig. 5) may suggest that plagioclase was not a major fractional phase in the peridotites, but rather became a dominant mineral in the gabbroic and doleritic units. This suggestion is consistent with the variation of mineral compositions in the different rock units. Cumulus texture defined by plagioclase in the olivine gabbros is consistent with the strong Sr enrichment in gabbros (Fig. 7).

Geodynamic setting

Several mafic-ultramafic complexes affiliated with the Neoproterozoic are known in Hoggar (Abed 1983; Dupont 1987), but very few have been studied. These include the Tin Zebane complex in the west of Hoggar (Aït-Djafer et al. 2003) and the Laouni complex (Cottin et al. 1998). Both were considered late orogenic complex. However, recent geochronological work on the Laouni complex (Bowden et al. 2014) shows that it was established around 640 Ma and is therefore pre- to syn-collisional. In what follows, we present the characteristics of the In Allarene complex that make it possible to determine the geodynamic context of its emplacement.

The In Allarene mafic-ultramafic complex displays similarities and differences with Alaskan-type complexes. For morphological and mineralogical similarities, we can note the following: (1) The ring structure of the In Allarene intrusion composed by a peridotitic core and bordered by gabbroic rocks correspond to the structural and petrological features of worldwide Alaskan-type mafic-ultramafic complexes. (2) This intrusion is located along a major fracture zone like most of Alaskan-type intrusions in the worldwide. (3) The lack of metamorphic texture. (4) Several mineralogical characteristics such as (a) the high fosterite contents (Fo ≈ 88) of the peridotites and the An-rich plagioclase of the gabbros and pyroxenites (e.g., Irvine 1974; Helmy and Mahallawi 2003; Abdallah et al. 2019) and (b) the CaO contents of orthopyroxenes of the gabbros and dolerites varying between 0.41 and 1.43 wt.% (e.g., Eyuboglu et al. 2010).

The main differences are the relative abundance of orthopyroxene and the rarity of amphibole in the In Allarene peridotites compared with Alaskan-type intrusions (Himmelberg and Loney 1995; Helmy and El Mahallawi 2003). The features will be discussed in more detail at the end of this section.

The geodynamic context of In Allarene complex emplacement can also be investigated thanks to mineral compositions and geodynamic discrimination diagrams using both mineral and whole-rock chemical compositions of the studied rocks. As intracontinental tholeiites have a very close chemical signature with those produced in a subduction context (Wang et al. 2016), particular attention was given to the affinity of the studied samples regarding these two types of magmatism. Talbi et al. (2007) used the geodynamic discrimination diagrams of Leterrier et al. (1982), based on the composition of clinopyroxene. The results show that on the Ca vs. Cr + Ti diagram (Fig. 9a), the majority of clinopyroxenes of the In Allarene intrusion fall within the orogenic domain; on the Ti vs Ca + Na diagram (Fig. 9b), the pyroxenes plot in the sub-alkaline magma domain; and on the AlT vs. Ti diagram, they fall within the tholeiitic arc domain (Fig. 9c). On the other hand, all clinopyroxene compositions plot in the Alaskan-type fields determined in the Alz (percentage of tetrahedral sites occupied by Al) vs. TiO2 diagram (Fig. 9d), where they follow the arc cumulate trend and not that of rift cumulate.

Fig. 9
figure 9

Clinopyroxene compositions of mafic rocks from In Allarene complex plotted on the a Ti vs. Ca + Na, b Ti + Cr vs. Ca, c Ti vs. Altotal diagrams of Leterrier et al. (1982) and d Alz (percentage of tetrahedral sites occupied by Al) vs. TiO2 diagram. Alaskan-type complexes fields of Quetico are from Pettigrew and Hattori (2006) and Tulameen field is from Rublee (1994). Alkaline and non-alkaline field, arc cumulate trend, and rift cumulate trend are from Le Bas (1962) and Loucks (1990)

Full size image

Furthermore, the Al2O3 vs. Cr2O3 diagram of Franz and Wirth (2000) (Fig. 10a) shows that the Cr-spinel of the studied peridotites fall within the field of island arc ultramafic cumulates. The same is true in the ternary Cr-Al-Fe3+ diagram (Fig. 10b), where these spinel minerals plot within the Alaskan-type spinel field (Conrad and Kay 1984; De Bari et al. 1987; De Bari and Coleman 1989; Helmy and El Mahallawi 2003; Ahmed et al. 2008; Abdel Halim et al. 2016). The projection of the In Allarene mafic rocks on the TiO2/10-MnO-P2O5 ternary diagram of Mullen (1983) (Fig. 11a) shows that almost all gabbros and dolerites plot below the calc-alkaline basalt domain of island arcs (and boninites). Similarly, the AFM (FeOT-MgO-(Na2O + K2O)) ternary diagram of Beard (1986) (Fig. 11b) shows, in addition to the tholeïtic feature, that the peridotite and Mg-rich gabbro samples plot in the field of arc-related ultramafic cumulates. However, the Mg-poor gabbro and dolerite samples plot within and between the arc-related mafic cumulates and arc-related gabbros fields. This suggests that (1) most of the rocks of In Allarene complex represent cumulative rocks, (2) a group of the gabbros and dolerites represents magmatic liquid, and (3) a second group of gabbros and dolerites corresponds to cumulates that have trapped a variable amount of magmatic liquid.

Fig. 10
figure 10

Spinel composition from the peridotites and olivine-rich gabbros from the In Allarene intrusion plotted on Cr2O3 vs. Al2O3 (a) and on the Al-Cr-Fe3+ (b) ternary diagrams. Fields of mantle array and arc cumulates are from Franz and Wirth (2000); compositional fields of mantle Cr-spinel, ferritchromite, and magnetite are from Barnes and Roeder (2001); the high-T altered Cr-spinel is from Arai et al. (2006) and the Alaskan-type spinel is from Helmy and El Mahallawi (2003), Ahmed et al. (2008), and Abdel Halim et al. (2016)

Full size image
Fig. 11
figure 11

Ternary MnO-(TiO2/10)-P2O5 (after Mullen 1983) (a) and AFM ((K2O + Na2O)-Fe2O3-MgO) (b) discrimination diagrams for gabbros and dolerites from In Allarene complex fields of cumulate and non-cumulate mafic-ultramafic rocks are from Beard (1986). Symbols are as in Fig. 5

Full size image

Comparison between the relative contents of Th and Ta is one of the most reliable ways of distinguishing tholeiites originated from active margins from those emplaced within intracontinental context. Thus, whole-rock composition of the least magnesian gabbros and dolerites plot within the active continental margins field on both Ta/Yb vs. Th/Yb (Fig. 12a) and Ta vs. Th diagrams (Fig. 12b).

Fig. 12
figure 12

Th/Yb vs. Ta/Yb diagram of Pearce (1983) modified by Gorton and Schandl (2000) (a), Ta vs. Th diagram of Schandl and Gorton (2002) (b), and ternary Zr/Sm-Ti/V-Sr-Nd of Wang et al. (2016) (c) showing the active continental margin affinity of the mafic rock samples from In Allarene complex

Full size image

Another diagram to distinguish arc magmatic rocks from intracontinental lithologies is the Zr/Sm-Ti/V-Sr-Nd ternary diagram of Wang et al. (2016). These authors show that intracontinental magmatism exhibits significantly lower Sr-Nd ratios than those of arc magmatic rocks. In the present In Allarene complex case study, all the rocks are distinct from the intracontinental basalt field; they plot within the arc magmatism field and follow the typical trend of the latter (Fig. 12c). Similarly, these rocks fall within the arc magmatism fields in both the Th-Hf/3-Ta (Wood 1980) and La/10-Y/15-Nb/8 (Cabanis and Lecolle 1989) ternary diagrams (Fig. 13a and b respectively).

Fig. 13
figure 13

Ternary Th-(Hf/3)-Ta plot after Wood (1980) (a) and (La/10)-(Y/15)-(Nb/8) diagram after Cabanis and Lecolle (1989) (b) for the mafic rocks from In Allarene complex (see text for explanation)

Full size image

The behavior of chondrite-normalized REE patterns of the In Allarene peridotites (Figs. 8 and 14) suggests a normal fractional crystallization (Chai et al. 2008) and contains variable amounts of interstitial liquid enriched in incompatible elements (Zhang et al. 2011). The presence of slight cerium anomalies may indicate serpentinization processes involving fluids of mainly seawater origin (Douville et al. 2002; Rouméjon et al. 2015). Variations in (La/Sm)N and (La/Yb)N ratios, as well as the presence or absence of Eu anomalies could indicate crustal contamination of some samples, but most likely the trapping of small amount of melt by these cumulates or the presence of magmatic rocks among the gabbros and dolerites poor in Mg.

Fig. 14
figure 14

Geodynamic discrimination REE and trace element patterns of mafic rocks from the In Allarene complex. Colored fields represent the compositions of gabbros and diorites from Pettigrew and Hattori (2006) and Abdallah et al. (2019)

Full size image

Positive Eu anomalies in the less magnesian gabbros and dolerites can also indicate plagioclase fractionation. The observed LILE enrichment of the In Allarene igneous rocks, along with Sr, Rb, and Ba positive anomalies and HFSE depletion with Nb, Ta, and Ti negative anomalies compared to N-MORB (Fig. 8b), suggests involvement of subduction-related components in a depleted mantle source. Once again, these features are similar to those observed in the Quetico intrusions of the upper western province (Pettigrew and Hattori 2006) and the eastern Egyptian desert intrusions (Abdallah et al. 2019), which are considered to have been emplaced in an arc environment (Fig. 14).

The In Allarene complex has clinopyroxene and spinel chemical compositions similar to those of subduction zone complexes. The same applies to the chemical composition of rocks, both in major and trace elements. However, the early presence of orthopyroxene in ultramafic rocks is not very common for mafic-ultramafic subduction zone complexes, although it does occur (e.g., Khedr et al. 2020).

On the other hand, the geochemistry of mafic-ultramafic complexes with geochemistry showing arc affinities but with some orthopyroxene abundance in ultramafic terms has been interpreted in different ways. However, all the geodynamic contexts invoked involve a mantle modified by a subduction: (1) an arc context (e.g., Khedr et al. 2020), (2) an emplacement in a back-arc basin context (e.g., Gu et al. 1995), and (3) a post-collisional setting; the mafic-ultramafic complexes are generated from a metasomatized mantle by an earlier subduction (Azer et al. 2017).

Another cause of the presence of orthopyroxene and the rarety of amphibole at the early stage of crystallization in the In Allarene intrusion could be the result of contamination by the Archean crust of the In Ouzzal and that crustal assimilation can occur before the magmas entered the highest magma chambers (Campbell 1985).

Indeed, it should be noted that Alaskan-type ultramafic-mafic intrusions occur in oceanic arcs near the end of the subduction during accretion-collision with the continent (Ramiz et al. 2018 and reference within), whereas the region studied corresponded to an active margin during the Pan-African orogeny.

The results of this study clearly show the existence of an arc-like metasomatized mantle under the In Ouzzal terrane during the Pan-African orogeny.

It is interesting to note that, in the region, the geographically closest Neoproterozoic mafic magmatism corresponds to Tirek gabbros, which are about 15 km far and have been dated at 630 Ma using a U-Pb method on zircon (Marignac et al. 1996), suggesting emplacement during the subduction period (Bosch et al. 2016).

Recently, Fezaa et al. (2019) showed that the alkali-calcic granites, dated at 600 Ma, comprise 20 to 40 vol.% of melts from the In Ouzzal archean crust. These authors invoke a metacratonization model during the post-collisional period. This leads to say that transpressive movements along large intra- and peri-In Ouzzal faults resulted in asthenosphere uplift and local In Ouzzal crust melting along the major faults, which generate granitic magmas and granitoid pluton emplacement. Inherited zircons in this massif, which was dated at 600 Ma, record an age around 650 Ma which corresponds to the paroxysm of subduction magmatism in the Hoggar. We, therefore, suggest that the alkali-calcic granites, as well as the gabbros and peralkaline granites of Tin Zebane (Aït-Djafer et al. 2003; Hadj Kaddour et al. 1998), the gabbros of Tileouine (Berger 2008), the quartz-diorites of Tirek (Caby et al. 1985), and the monzogranites of Kidal (Bosch et al. 2016) which were emplaced between 600 and 580 Ma could correspond to a post-collisional period of important delamination of the lithospheric mantle, possibly related to slab detachment. This is contemporaneous with the HT-BP metamorphism observed in both east and west of the In Ouzzal, Tassendjanet (Caby and Monié 2003), Tirek (Bendaoud et al. 2017), and the Iforas (Boullier and Barbey 1988) terranes.

Conclusion

The Hoggar, which includes lithologies ranging from Archean to Neoproterozoic, owes its current structure to the Pan-African orogenesis (750–550 Ma). It corresponds to the amalgamation of crustal blocks by the accretion of arcs and intercontinental collisions following the closure of oceanic domains.

The mafic-ultramafic complex of In Allarene, which is located in the southern part of archeo-paleoproterozoic In Ouzzal terrane, is considered as neoproterozoic and pre-tectonic intrusion (Caby 1996; Talbi et al. 2007). It consists of peridotites (harzburgites and lherzolites) in the core and gabbros and dolerites in the outer part of the intrusion.

Textural, mineralogy, and geochemistry results of these rocks demonstrate a cumulate emplacement for this complex. Some gabbros and dolerites, which correspond to the mafic rocks that are the poorest in magnesium and the richest in alkali, show characteristics of magmatic liquids or cumulate rocks that have trapped a variable amount of magmatic liquids.

Major element compositions together with LILE enrichment relative to HFSE, Nb-Ta, and Ti negative anomalies indicate island arc geochemical signature related to a magmatism in a subduction zone setting. Moreover, clinopyroxene and spinel chemical compositions suggest arc cumulate affinity. Likewise, petrological, mineralogical, and geochemical discriminative features indicate that the mafic-ultramafic In Allarene intrusion have a strong arc magmatism affinity. These results confirm the existence of an arc-type metasomatized mantle under the In Ouzzal terrane during Pan-African orogeny.