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Home All issues Volume 196 (2025) BSGF - Earth Sci. Bull., 196 (2025) 18 Full HTML
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BSGF - Earth Sci. Bull.
Volume 196, 2025
Article Number 18
Number of page(s) 23
DOI https://doi.org/10.1051/bsgf/2025019
Published online 07 November 2025

© G.R. Noura et al., Published by EDP Sciences 2025

Licence Creative CommonsThis is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1 Introduction

The spatio-temporal extent of mafic magmatism in the West African Craton has been determined from studies of mafic and ultramafic rocks in the Reguibat and Léo-Man shields and in intra-cratonic sedimentary basins. Among the mafic rocks are dyke swarms and dolerite sills, which are most often parts of the distribution systems of Large Igneous Provinces (LIPs) (Bryan and Ferrari, 2013; Ernst and Buchan, 2002; Ernst et al., 2013). The first mafic magmatic events (LIPs) in the West African Craton were recognized in the Archean around 2733 Ma (Tait et al., 2013). Other events are noticed in the basement formations between 2688 Ma and 580 Ma (Hafid et al., 2001; Aifa et al., 2001; Ama Salah, 1991; Baratoux et al., 2019; Cournède, 2010; Ernst et al., 2021; Kouyaté et al., 2013; Lefort and Aifa, 2001; Potrel et al., 1998; Söderlund et al., 2013a; Tait et al., 2013; Tapsoba et al., 2018; Thomas et al., 2002; Walsh et al., 2002; Youbi et al., 2013; Youbi et al., 2020) and reached a paroxysm in sedimentary basins at 200 Ma with the Central Atlantic LIP (Baratoux et al., 2019; Bassot et al., 1986; Chabou et al., 2007; Chabou et al., 2010; Rooney et al., 2010; Verati et al., 2005). Research on global reconstructions (e.g., Baratoux et al., 2019; Jessell et al., 2015; Youbi et al., 2011) has demonstrated that the mafic dykes with ages between ca. 580 Ma and c. 200 Ma belong to three LIPs: 580-550 Ma CIMP − Central Iapetus Magmatic Province (Thomas et al., 2002; Youbi et al., 2020), 369–250 Ma EUNWA: European North West African Magmatic Province (Moutbir et al., 2024; Youbi et al., 2011) and 201 Ma CAMP: Central Atlantic Magmatic Province (Marzoli et al., 2018), while those with ages between c. 2733 Ma and 580 Ma are currently associated with other LIPs. In the Paleoproterozoic domains of the West African Craton, which represents the host rock of younger LIPs, in addition to this radiometric evidence, geochemical studies have indicated the existence of ca. 2040–2060 Ma tholeiitic magmatism potentially also representing a LIP occurring during the Eburnean orogeny (Ama Salah et al., 1996; Augustin and Gaboury, 2017; Baratoux et al., 2011; Béziat et al., 2000; Dampare et al., 2008; Kouyaté et al., 2013; Pawlig et al., 2006; Sakyi et al., 2020; Senyah et al., 2016; Soumaila et al., 2004; Vidal et al., 1996).

In the Liptako area in Niger, tholeiitic magmatism was studied in Paleoproterozoic metabasalts and metagabbros (Ama Salah, 1991; Ama Salah et al., 1996; Affaton et al., 2000; Garba Saley, 2022; Hallarou, 2021; Soumaila, 2000; Soumaila et al., 2004, 2008, 2016a, 2016b) and post Birimian doleritic dykes (Ama Salah, 1991; Baratoux et al., 2019; Machens, 1973; Noura et al., 2023a). Petrographical, geochemical and geochronological data from the literature on the Liptako area dolerites concerned dykes with trends N010°-N020°, N110°-N120° and N130°-N140°. Radiometric dating on the N010°-N020° swarm provided a robust U-Pb baddeleyite age of 1791 ± 3 Ma on a N010° dyke (Baratoux et al., 2019), and an K/Ar biotite age of c. 1010–1370 Ma on a N020° dykes (Ama Salah, 1991). For the N130°–N140° swarm a K/Ar on biotite age of ca. 896 Ma, was obtained for a N135° dyke (Ama Salah, 1991). Geochemistry study conducted on the N020°, N110°–N120° and N130°–N140° trending dykes indicate continental tholeiitic and enriched mid-ocean ridge basalts characteristics (Ama Salah, 1991; Noura et al., 2023a) However, the Liptako doleritic dykes are represented by other trends varying between NNE (N000°–N015°) and NNW (N170°) highlighted in areas that have not been studied to date. The N010° trending Libiri dyke (which belongs to the NNE trend) was previously dated at 1791 ± 3 Ma by U-Pb on baddeleyite (Baratoux et al., 2019) on a sample from the Libiri gold mine pit.

The aim of this publication is to provide new information on the genesis and magmatic evolution of the NNW-NNE trending Libiri doleritic dyke swarm of Niger Liptako through a mineralogical and geochemical study, to assess their geodynamic setting.

2 Geological setting and sampling

The West African Craton, stabilized at the end of the Paleoproterozoic (c. 1.9 Ga), forms a vast complex of Archean (c. 3.6–2.6 Ga) and Paleoproterozoic (c. 2.3–2.0 Ga) rocks that form the present-day configuration of the Léo-Man and Reguibat shields (Baratoux et al., 2011; Béziat et al., 2000; Egal et al., 2002; Gasquet et al., 2003; Hirdes and Davis, 2002; Koffi et al., 2022; Kouamelan et al., 2018; Liégeois et al., 1991; Lompo, 2009; Naba, 2007; Parra-Avila, 2017; Sakyi et al., 2020; Senyah et al., 2016; Thiéblemont et al., 2004; Vegas et al., 2008). These two shields are separated by the Taoudeni sedimentary basin. The Precambrian domains and their younger sedimentary cover were injected by mafic intrusions, notably doleritic dykes and sills, whose spatial distribution and grouping into discrete swarms based on trend was interpreted from the regional aeromagnetic map (Baratoux et al., 2019; Jessell et al., 2015) (Fig. 1). This map in Jessell et al. (2015) contains more than 3,000 dykes classified into 26 distinct orientation swarm types and emplaced during around 10 mafic magmatic events between c. 2733 Ma and c. 200 Ma (Baratoux et al., 2019; Jessell et al., 2015; Noura et al., 2023a).

The Liptako Paleoproterozoic basement is part of the Boulé Mossi domain of the Léo-Man shield. It is located at the NE margin of this domain and consists of two main types of units, greenstone belts and granitoid plutons. Infracambrian and Continental terminal 3 (Oligocene) sedimentary formations mark a major unconformity on the basement (Fig. 2). These greenstone belts and the granitoid plutons follow an overall NE-SW orientation due to Eburnean deformation, and are labelled from south-east to north-west, the Makalondi belt, Torodi pluton, Sirba belt, Dargol-Gothèye pluton, Diagorou-Darbani belt, Tera-Ayorou pluton and Gorouol belt. The greenstone belts consist of metabasites (metabasalts, metadolerites and metagabbros), meta-volcano-clastites (metavolcanic breccia and metatuff) and metasediments (schists, metaconglomerates, metagreywackes) (Ama Salah et al., 1996; Attourabi et al., 2024b; Dupuis et al., 1991; Garba Saley et al., 2024; Hallarou et al., 2020b; Ahmed et al., 2022; Machens, 1973; Noura et al., 2023a; Soumaila et al., 2008; Soumaila et al., 2016a,b). The emplacement ages of the greenstone belts have been determined from micaschists and metabasalts and range from 2400 ± 42 Ma to 2050 ± 12 Ma (Soumaila et al., 2008). The granitic massifs consist of tonalities, trondhjemites, and granodiorite series rocks intruded by plutonic bodies of various suites of granites, diorites and syenites (Abdou et al., 1998; Ahmed et al., 2022; Ama Salah et al., 1996; Attourabi et al., 2024b; Cheilletz et al., 1994; Dupuis et al., 1991; Lama, 1993; Pons et al., 1995; Soumaila, 2000). U/Pb, Sm-Nd and K/Ar ages obtained from Liptako granitoids range from 2000 ± 60 Ma to 2188 ± 12 Ma (Abdou et al., 1998; Ama Salah et al., 1996; Cheilletz et al., 1994; Dupuis et al., 1991; Lama, 1993; Pons et al., 1995; Soumaila et al., 2008). These greenstone belts and granitoids plutons are crosscut by quartzite, quartzo-feldspathic and pegmatitic veins (Ahmed et al., 2022; Attourabi et al., 2024b; Pons et al., 1995; Soumaila, 2000). On the geological map of Machens (1973) (Fig. 2), doleritic dyke trend in four major orientations: NNW-NNE, NW-SE, WNW-ESE, E-W. The NNW-NNE swarm, which is the subject of this publication, is characterized by a range in trends between NNE (N000°–N015°) and NNW (N170°). The dykes of this swarm are observed in outcrop in greenstone belt rocks and in granitoid plutons, in the form of irregular lines of blocks covered with a brownish alteration patina (Figs. 3a and 3b). The doleritic dykes were sampled in the areas of the Tera-Ayorou and Dargol-Gotheye plutons and the Diagorou-Darbani and Sirba greenstone belts (Fig. 2). The specific trends measured at the sampling localities and dyke thicknesses are listed in Table 1. We collected at least 2 samples from each dyke of N000°–N015° and N170° trends. The SD sample in Table 1 was collected in the Libiri gold mine and corresponds to the N010° Libiri swarm dated with a precise U-Pb age of 1791 ± 3 Ma (Baratoux et al., 2019). N020° and N140° trending dykes given less precise ages obtained by the K-Ar method ranging from 1378 ± 36 Ma to 1011 ± 46 Ma and 896 ± 25 Ma (Ama Salah, 1991), respectively.

thumbnail Fig. 1

Map of mafic sills and dykes in the West African Craton (Jessell et al., 2015), with modifications in red based on mapping from aeromagnetic overflights of the Liptako area dykes (ACDI, 1973).
thumbnail Fig. 2

Geological map of the Liptako area in Niger (Machens, 1973). The different trends are color coded and refer to different swarms. The dated N010° Libiri swarm at 1791 Ma (Baratoux et al., 2019) belong to Samira-Libiri sector.
thumbnail Fig. 3

Field and petrographic photos of the NNW-NNE Libiri swarm: (a and b) outcrop mode of doleritic dykes from this swarm and (c, d, e and f) microphotographs showing typical textures and representative minerals in both petrographic types. Pl: plagioclase, Opx: orthopyroxene, Cpx: clinopyroxene, Amph: amphibole, Chl: chlorite, Ap: apatite, Ox: Fe-Ti oxides, Ep: epidote.
Table 1

Characteristics of sampled dykes of the NNW-NNE Libiri swarm.

3 Methodology

3.1 Thin section chemical mapping and electron microprobe analyses

Thin section chemical mapping and electron microprobe analyses were done at the GET Laboratory and Raimond Castaing Microanalytical Center of the University of Toulouse (France), respectively. One thin section was prepared from each collected sample. The instrument used for mapping and electron microprobe analyses was a Tescan Vega 4 LMU SEM (partial vacuum), coupled with Bruker Quantax SDD-type EDX 30 mm2 detector. The current used was 15 kV, the working distance varied from 10 to 16 mm, the pixel size was between 50 and 70 μm and magnification between 5 and 1000. The chemical mapping was used to characterize the zonations and spatial distribution of Ca, Mg, Al, Fe, Si, Ti, Na and K in pyroxene, Fe-Ti oxides, amphibole and feldspar crystals. Mineral chemistry was determined using the CAMECA SXFive electron microprobe (EPMA). The beam had 2 μm and was characterized by a voltage of 15 kV and a current of 10 nA. Standard minerals analyzed included albite (Na), baryte (Ba), Cr2O3 (Cr), corundum (Al), hematite (Fe), periclase (Mg), wollastonite (Si, Ca), sanidine (K), MnTiO3 (Ti, Mn), topaz (F), silicon aluminate glass with RB2O and Cs2O (Rb, Cs), and graftonite (P). Chemical maps and microprobe analyses on feldspars, pyroxene, amphibole and biotite were carried out on the SD, SLB, TD1 and TD2 samples and over twenty selected points were analyzed from each mineral to average out any chemical variability.

3.2 Geochemical analyses

Fifteen samples were analyzed at the ALS Geochemistry laboratory (Canada) by using ICP-MS (Inductively Coupled Plasma-Mass Spectrometry) coupled with ICP-AES (Atomic Emission Spectrometry) for whole rock major oxides and trace elements.

4 Results

4.1 Petrography

The doleritic textures, mineralogical compositions and degree of mineral alterations of the Libiri NNW-NNE dykes reveal two main petrographic types. The first type is characterized by dolerites with a well-preserved intergranular texture and the constituent minerals are plagioclase, K-feldspar, clinopyroxene, orthopyroxene, apatite, amphibole, biotite, sericite, chlorite, Fe-Ti oxides, and K-feldspar intergrowns with quartz showing the granophyric texture (Figs. 3c and 3d). This petrographic type comprises the SD, SLB and TDR dykes with N000° and N010° trends, intrusive into the Sirba greenstone belt. The second type consists of dolerites with a partially preserved ophitic texture. In this type, the same minerals are found as in the first one, with the exception of orthopyroxene, which is absent in the second type, while it contains epidote (Figs. 3e and 3f).This type is found in the TOD and TD dykes with N000° to N170° trends, cross-cutting the Tera-Ayorou and Dargol-Gotheye plutons and in the Diagorou-Darbani greenstone belt. Based on the bulk rock geochemical analyses (Sect. 4.3), the dykes were divided into two chemical groups (Tab. 1), which partially correspond to the petrographic types. In this case, the sample TD1 belongs petrographically to the first type but chemically to the second group. We will use these two geochemical groups when describing the chemical maps (Sect. 4.2), electron microprobe analysis results (Sect. 4.2) and geochemical composition of whole-rock (Sect. 4.3).

4.2 Mineralogy

4.2.1 Feldspars

Feldspars are represented in both geochemical groups by plagioclase which occurs in Group I dykes as divergent laths showing albite-type polysynthetic twins (Figs. 3c and 3d) while it correspond to the relict laths in Group II dykes. The laths are affected by sectorial sericite development in Group I dykes and more albitized in Group II dykes. The proportion of plagioclase crystals varies depending on the texture of the dykes. It is estimated in Group II dykes at around 40–45% whereas it accounts around 50–60% in Group I dykes. The altered crystals are transparent in natural polarized light or show a gray-black haze due to sericite alteration (Figs. 3c and 3d) and are surrounded by transparent apatite and/or brownish clinopyroxene crystals (Figs. 3e and 3f).

Chemical maps show variable chemical distributions of Ca, Na and K between the cores and margins of feldspar crystals (Fig. 4). In dykes belonging to Group I, plagioclase is characterized by a high concentration and homogeneous distribution of Ca between the cores and margins of the laths, whereas in dykes belonging to Group II, the laths are less Ca-rich and show a heterogeneous distribution of Na. Na-rich laths are often associated with K-rich or Si-rich grains and correspond to alkali feldspars (Kfs) or quartz and K-feldspar grains with granophyric texture (Gr), respectively (Figs. 4b, 4c, and 4e).

The electron microprobe analyses results of feldspars of both groups are shown in Table 2. Projection of these results in the Ab − Or − An diagram (Fig. 5a), shows a continuum of compositions from Ab25 to Ab96. Group I dykes are characterized by rare K-feldspars (orthoclase and sanidine) and calcic plagioclase with labradorite (Ab40–43-An55–58) and bytownite (Ab25–29-An70–74). Group II dykes contain less calcic plagioclase, ranging from albite (Ab96-An4), to labradorite (Ab42–48-An51–57). These An# values indicate that the Group I dykes are more primitive than Group II dykes. The presence of less calcic plagioclase in Group II dykes can be explained by fractional crystallization or albitization of early formed plagioclase crystals during deuteric alteration in a magma chamber.

thumbnail Fig. 4

Chemical maps of SD, SLB, TD1 and TD2 dykes belonging to both groups. Pl: plagioclase, Aug: augite, Pig: pigeonite, Hyp: hypersthene, Amph: amphibole, Kfs: K-feldspars, Ap: apatite, Gr: grain with granophyric texture, Tnt-Mg-Il: titanomagnetite-ilmenite.
Table 2

Selected points are representative electron microprobe analysis results of feldspars.

thumbnail Fig. 5

Compositional diagrams of (a) feldspar, (b) pyroxene (Morimoto et al., 1988), (c) amphibole (Leake et al., 1997) and (d) biotite (Speer, 1984 and Deer et al., 1986) of boths groups. Ab: albite, Ol: oligoclase, And: andesine, La: labradorite, By: bytownite, An: anorthite. Symbols with blue circles belonging to Group I and green triangles belonging to Group II.

4.2.2 Pyroxene, amphibole, chlorite

Pyroxene is represented by orthopyroxene and clinopyroxene in Group I (Figs. 3c and 3d), while in Group II, only clinopyroxene was found (Figs. 3e and 3f). In both groups, large clinopyroxene crystals locally show growth twins, which are overgrown by brown amphibole (Tab. 3, Figs. 3c, 3d, 3e, and 3f, Fig. 5c) or greenish-gray chlorite fibers (Figs. 3c, 3d, 3e, and 3f). Some large amphibole crystals are locally transformed into epidote in Group II dykes (Fig. 3e). These characteristics indicate a pseudomorphosis after uralitization of clinopyroxene crystals during interaction with late magmatic fluids or deuteric alteration.

The distribution of chemical elements within pyroxene crystals in the chemical maps (Figs. 4 and 5) shows zonation between Ca, Mg and Fe corresponding to augite, pigeonite and hypersthene compositions in Group I dykes whereas it indicates augite compositions in Group II dykes. Representative electron microprobe analysis results of selected points of pyroxenes in both groups are provided in Table 4. In the Wo − En − Fs diagram (Morimoto et al., 1988, Fig. 5a), the chemical compositions of pyroxenes in dykes belonging to Group I vary between those of augites (Wo30–38-En42–53-Fs13–20), pigeonites (Wo5–14-En46–67-Fs25–43) and clinoenstatite (Wo4–5-En59–78-Fs18–37). On the other hand, for dykes belonging to Group II, compositions fall between the augite range (Wo31–41-En33–47-Fs14–29) and those of diopside (Wo45–46-En27–29-Fs26–27). Pyroxenes in dykes belonging to Group I are relatively richer in Cr2O3 (0.11–0.67 wt%) and poorer in TiO2 (0.11–0.67 wt%) than those in dykes belonging to Group II (Cr2O3 = 0.00–0.24 wt%, TiO2 = 0.18–0.83 wt%). As indicated in Table 4, pyroxenes in both groups are characterized by an AlIV/AlVI ratio >1, typical of pyroxenes of mantle melt origin (Marcelot et al., 1983; Porras, 2010). In Group I, significant chemical zonation is observed in chemical maps within augite crystals, with Fe-Ca-rich cores and slightly Ca-poor margins, while within pigeonite and hypersthene crystals, Mg-rich cores and Mg-poor margins are observed (Figs. 4 and 5). In Group II dykes, augites are characterized by Ca-rich crystal interiors and Fe-Mg-Al-rich crystal margins, which correspond to amphibole or chlorite. Projection of the analytical results for amphibole (Tab. 3) for both groups into the Leake et al. (1997) classification diagram, shows that it corresponds to ferro-edenite (Fig. 6c).

Table 3

Selected points are representative electron microprobe analysis results of amphibole. Numbers of ions is based on 23 oxygens.

Table 4

Selected points are representative electron microprobe analysis results of pyroxene.

thumbnail Fig. 6

Diagram of variations in oxides and trace elements as a function of the Mg# parameter. Symbols with blue circles belonging to Group I and green triangles belonging to Group II.

4.2.3 Biotite

Biotite is identified on the microphotographs and chemical maps as crystals size below to 70 μm. Microprobe results (Tab. 5) show that it contains traces of MnO (0.01–0.25 wt%) and Na2O (0–0.32 wt%) and relatively high content of TiO2 (0.05–4.71 wt%). In the mica classification diagram of Speer (1984) and Deer et al. (1986), the small grains plot within the biotite field with XFe = 0.5–0.65 and AlIV = 2.1–2.5 (Fig. 5d).

Table 5

Selected points are representative electron microprobe analysis results of biotite. Numbers of ions is based on 22 oxygens.

4.2.4 Apatite

Apatite is found as grains size below to 60 μm in plagioclase laths. On the chemical maps, it is mainly distinguished as Ca-rich grains or filaments in both groups (Fig. 4g).

4.2.5 Fe-Ti oxides

Fe-Ti oxides occur as subautomorphic or xenomorphic crystals in the coarse-grained portions of dykes, isolated or arranged around amphibole and chlorite crystals (Figs. 3c, 3d, 3e, and 3f). In some finer grained portions, Fe-Ti oxides are located in the interstices between plagioclase laths. On chemical maps, we distinguish subautomorphic crystals which are rich in Fe, Ti and Ca; corresponding to titano-magnetite-ilmenite (Fig. 4).

4.3 Geochemistry

4.3.1 Major and trace elements

Whole-rock analyses of major (wt%) and trace (ppm) elements of both groups are listed in Tables 6 and 7. The two petrological groups described above show significant bulk rock composition differences. Exceptions are the SiO2 contents which range from 51.0 to 54.2 wt% for Group I dykes and 54.7 to 56.6 wt% for Group II dykes. Na2O+ K2O contents are low in Group I (2.65–2.82 wt%) and relatively high in Group II (3.61–4.52 wt%). Group I dykes have TiO2 and Fe2O3 contents relatively low (TiO2 = 0.74–0.89 wt%, Fe2O3 = 11.15–11.9 wt%) compared to Group II dykes (TiO2 = 1.34–1.38 wt%, Fe2O3 = 13.60–15.35 wt%). However, MgO and CaO contents are high in Group I (MgO = 7.41–8.54 wt%, CaO = 8.73–9.42 wt%) and low in low in Group II (MgO = 2.90–3.47 wt%, CaO = 6.03–6.81 wt%). The difference in SiO2 and Na2O+ K2O contents presented by both groups is probably due to deuteric or hydrothermal alteration. For example, the effects of hydrothermal alteration are visible on microphotographs (Fig. 3) and chemical maps of Group II dykes (Figs. 4e, 4f, 4g, and 4h) and are exhibited by the destabilization of augite and plagioclase crystals. The low variation in MgO, TiO2 and CaO contents between the two groups can be associated with the magma evolution during high-Ca plagioclase and hypersthene-pigeonite-augite fractionation (Figs. 5a and 5b, Fig. 7b).

Mg# (100*((MgO/40.32)/(MgO/40.32+FeO(FeO = 0,9889*Fe2O3)/71.85)), (Grill, 2010) varies from 56 to 65 in Group I dykes and 30 to 35 in Group II dykes. This is indicated by the Mg# parameter in Figure 6 as two distinct geochemical groups. In total-alkali silica (TAS) diagram (Le Bas et al., 1986), (Fig. 8a), samples show two clusters of basaltic andesite to andesite composition with an evolved tholeiitic character. However, in Al-FeT+Ti-Mg (Fig. 8b), Group I dykes show characteristics similar to tholeiite basalt with high magnesium contents whereas Group II dykes have affinity close to tholeiite basalt with high iron contents.

Group I dykes show high Cr contents (344–505 ppm), while Group II dykes have lower contents (34–66 ppm). Zr and Nb contents are low in Group I (Zr: 88–116 ppm, Nb: 3.77–6.00 ppm) and relatively high in Group II (Zr: 152–175 ppm and Nb: 6.37–7.58 ppm). Ta and Yb contents are low in Group I (0.1–0.2 ppm and 1.65–2.26 ppm) and high in Group II (Ta: 0.4–0.5 ppm, Yb: 2.40–2.87 ppm

Extended primitive mantle normalized trace-elements, marked “N”, (Sun & McDonough, 1989) and chondrite normalized, marked as “n”, Boynton, 1983 plots for both groups (Fig. 7) are characterized by moderately sloping profiles. The mantle-normalized plot (Fig. 7a) displays positive peaks in Ba, U, Nd and K and negative peaks in Nb, Ta, P, and Ti. The ThN/LaN (1.00–1.29) and ThN/NbN (2.96–4.13) ratios are low in Group I and relatively higher in Group II (ThN/LaN: 1.31–1.42; ThN/NbN: 4.18–4.53). The chondrite-normalized REE plot (Fig. 7b) is characterized by LREE enrichment and a low Lan/Ybn (4.73–5.51) ratio for the dykes belonging to Group I and a higher one for the dykes belonging to Group II (Lan/Ybn = 5.14–6.21). The Cen/Ybn (3.49–4.60) ratio is higher in Group II than in Group I (Cen/Ybn = 2.21–2.44). All samples show negative Eu anomalies, with values decreasing slightly from Group I (Eu/Eu* = 0.79–0.93) to Group II (0.78–0.84), suggesting plagioclase fractionation.

Table 6

Whole-rock major (wt%) and trace elements (ppm) compositions of Group I dykes. LOI = Loss On Ignition, N = primitive mantle, n = chondrite.

Table 7

Whole-rock major (wt%) and trace elements (ppm) compositions of Group II dykes. LOI = Loss On Ignition, N = primitive mantle, n = chondrite.

thumbnail Fig. 7

(a) Extended primitive mantle normalized trace-element plot and (b) chondrite normalized REE plot for both groups. Primitive mantle normalizing values are from Sun and McDonough (1989) and chondrite normalizing values are from Boynton (1984). Blue and green symbols correspond to groups I and II, respectively.
thumbnail Fig. 8

(a) Total-alkali silica (TAS) (Le Bas et al., 1986) and (b) Al-FeT+Ti-Mg (Jensen, 1976) classification diagrams showing tholeiitic nature of dykes in both groups. Symbols with blue circles belonging to Group I and green triangles belonging to Group II.

5 Discussion

5.1 Hydrothermal and deuteric alterations

As demonstrated above, the NNW-NNE Libiri swarm can be divided into two petrological types based on their textures and degree of alteration. The two types correspond partially, but not completely, to two groups determined using the bulk rock geochemistry. The mineral chemistry also shows systematic differences between these two groups. The Group I dykes are more primitive and less altered than Group II dykes. The presence of orthopyroxene and more calcic plagioclase in Group I dykes compared to Group II dykes can be associated to the magma evolved during minerals fractionation whereas the difference in dyke textures and altered crystals can be explained by deuteric or hydrothermal alteration. Petrographic characteristics of Group I dykes are similar to the results obtained by Ama Salah (1991) on the N020° oriented dykes intersecting the Sirba belt. This author observed an intergranular texture and minerals such as orthopyroxene, clinopyroxene, plagioclase, amphibole, biotite, sericite and Fe-Ti oxides. Group II dykes have the same characteristics as the petrographic results (large ophitic clinopyroxene and plagioclase, amphibole and minor Fe-Ti oxides) described in Baratoux et al. (2019) on the N010° Libiri dated dyke, sampled in the Samira gold mine. However, the petrographic characteristics of both group dykes differ from those of the WNW-ESE to NW-SE-trending Liptako area doleritic dykes (ophitic texture, augite, pigeonite, plagioclase, apatite, abundance of chlorite, biotite, Fe-Ti oxides) highlighted by Noura et al. (2023a).

The gray-black opacity of plagioclase laths observed in dykes belonging to Group II and the presence of alteration minerals (sericite, epidote and chlorite) on plagioclase, amphibole, augite and biotite crystals observed in dykes of both groups are comparable to results described by Cournède (2010); Sengupta et al. (2014); Noura et al. (2023a) and interpreted in the West African Craton in relation to alteration activities or a low grade post-magmatic metamorphic process, such as amphibole epidotization, plagioclase sericitization and biotite chloritization. Moreover, the late magmatic minerals (ferro-edenite and biotite) highlighted as isolated crystals or on the margins of augite large crystals in these dykes, are typical of deuteric alteration, but sericite, epidote and chlorite minerals are originating from hydrothermal alteration or low grade post-magmatic metamorphism.

Deuteric alteration can cause significant mineralogical and geochemical changes during the recrystallization of minerals already formed in the magma chamber (Mottl, 1983; Sengupta et al., 2014). Mineralogical change has been highlighted on the microphotographs in Figures 3 and 4 through uralitization of augite, biotite chloritization and ferro-edenite epidotization. In addition, the partially preserved ophitic textures of the Group II dykes and their higher proportions of late-magmatic (ferro-edenite, biotite) and secondary (chlorite and epidote) minerals also indicate the relative degree of hydrothermal or deuteric alteration in Group II dykes compared to Group I dykes. The presence of ferro-edenite and biotite at the margins of the large augite crystals is similar to deuteric alteration described in the Liptako area WNW-ESE to NW-SE trending doleritic dykes; suggesting the evolution of magma under variable fluid pressures, leading to interaction between crystals and hydrothermal aqueous fluids (Noura et al., 2023a). In addition, LILE (K, Sr, Rb, Ba and Cs) and Na contents can be used to test hydrothermal alteration. When we examined the relative variation of Ba (272–751 ppm), Sr (195–234 ppm) and Cs (0.63–1.09 ppm) contents in Group I dykes compare to Group II dykes (Ba: 411–616 ppm, Sr: 208–309 ppm, Cs: 0.4–1.42 ppm), it seems that Group II dykes are slightly more affected by hydrothermal alteration. According to Backman et al. (1988), in presence of hydrothermal alteration, geochemical changes can be reflected in the variability of Rb contents within the rock groups. Rb contents in Group I dykes (Rb: 22.9–30.1 ppm) and in Group II dykes (Rb: 22.2–52.9 ppm) indicate the same evolution. Although hydrothermal or deuteric alteration was more important in Group II dykes than in Group I dykes and represented one of the important criteria for identifying the two groups, REE patterns indicate that it did not significantly affect the geochemical signature of the dykes in either group.

5.2 Magma genesis

5.2.1 Magma source

The Th/Ta ratio (6.96–17.8) obtained from both dyke groups of the Libiri swarm are typical of orogenic basalts (Th/Ta > 5, Cabanis and Thieblemont, 1988). In the Th/Nb vs. TiO2/Yb (Pearce et al., 2021) diagrams, the samples fall in the subduction zone field (Fig. 9). It should be emphasised that, even if these rocks project into the orogenic domain, they may also correspond to continental tholeiites. This is supported by the Y/Nb (3.64–4.47) ratios of 13 samples, which are typical of continental tholeiites (Y/Nb < 5, Winchester et Floyd, 1975). The SZLM (subduction-modified lithospheric mantle) fields correspond, according to these authors, to magmatic sources originating in the lithospheric mantle modified by subduction components. The Libiri NNW-NNE doleritic dyke swarm either originated from subduction zone activity or inherited such a context from the source. The fact that the dykes cannot be emplaced in an orogenic context (Noura et al., 2023a) leads us to conclude that the source of these dykes is probably lithospheric mantle which had been metasomatized during earlier subduction zone associated with emplacement of Birimian rocks. This is evident when we consider the important negative Nb anomaly in the mantle-normalized trace-element spectra (Fig. 7a) and the slight slope in the Ti-Yb segment. The Lan/Ybn ratio (4.73–5.81) in Group I and Lan/Ybn (5.14–6.21) in Group II are essentially typical to those of garnet lherzolite sources (Lan/Ybn > 5, McDonough & Sun, 1995; Oinam et al., 2020). However, if melting of hydrated spinel lherzolite is also considered then Group I and Group II melts could have been sourced from partial melting of hydrated spinel or garnet lherzolite, at depths between 60 km to over 120 km in the mechanical lithosphere (Griffin et al., 2009).

thumbnail Fig. 9

Diagram Th/Nb vs. TiO2/Yb (Pearce et al., 2021) showing an inherited lithospheric modified mantle by subduction components origin of the dykes in both groups. SZLM = Subduction-modified lithospheric mantle, OIB = Oceanic Island basalts, OPB: Oceanic plateau basalts, MORB= Mid-ocean ridge basalts. Blue and green symbols correspond to groups I and II, respectively.

5.2.2 Magma evolution

The Mg# parameter and the DI (differentiation index) (Quartz + Alkali Feldspar + Albite + Nepheline + Leucite, Thornton et al., 1960) are good indicators to test the basaltic magma evolution during its ascent (Ahmed et al., 2018; Vicat et al., 2001). Variation in Mg# and DI reflect magma that has evolved slightly between the two groups (c.f. Cox, 1980 and Vicat et al., 1997) with Group I magma being more primitive (DI = 16.26–20.19, Mg# = 55–65) and Group II being more evolved (DI = 22.82–50.00, Mg# = 30–35). This interpretation is also support by SiO2 contents which are relatively low in Group I dykes (51.0–54.2 wt%) and higher in Group II dykes (54.7–56.6 wt%) and the presence of more calcic-plagioclase in Group I dykes relative to Group II dykes. This difference can be associated to fractional crystallization or magma contamination by felsic rocks for Group II dykes. The relatively high (1.34–1.38 wt%) and low (0.74–0.89 wt%) TiO2 contents in both Group are common to differentiated low-Ti basalts (TiO2 < 2.5, Hou et al., 2011) and continental LIPs described in Ernst et al. (2021), Pearce et al. (2021).

The Th/Ta and La/Yb ratios, which can be used to test crustal contamination (Condie, 1997), show higher values in Group I dykes (Th/Ta = 8.80–17.8, La/Yb = 7.02–8.17) relative to Group II dykes (Th/Ta = 6.96–9.26, La/Yb = 7.63–9.21). Similar ratio values, obtained by Tapsoba et al. (2018) on doleritic dykes in Burkina Faso (Th/Ta = 1.31–11.44 and La/Yb = 1.83–12.72), reflect magma contamination by the medium and upper continental crust. In the TaN/LaN vs. HfN/SmN (La Fleche et al., 1998) and Nb/U vs. (Th/Nb)N (Panda et al., 2023) diagrams, sample projections indicate metasomatism of the source by subduction-related processes (Fig. 10a) and magma contamination by the middle crust (Fig. 10b). According to these authors, the (Th/Nb)N, TaN/LaN and HfN/SmN ratios were calculated relative to the primitive mantle of McDonough & Sun (1995). Both processes are also distinguished on the primitive mantle-normalized diagram by K and U enrichment and Nb depletion, typical of continental crust contaminants (Dupuy and Dostal, 1984; Rudnick and Gao, 2003). Furthermore, samples belonging to both groups have Th/Nb (0.38–0.54) and Th/La (0.13–0.17) ratios below the values normalized to the primitive mantle (ThN/NbN = 3.25–4.58 and ThN/LaN = 1.07–1.43), Tables 6 and 7. This behavior reflects a depletion in Nb relative to Th, which is typical of variable crustal contamination that occurred during interaction between crystals and late-magmatic fluid in the deeper magma chamber (Noura et al., 2023a) or magma generated from a lithosphere that had been metasomatized during a prior subduction event (c.f. Pearce et al., 2021). The simple projections in these diagrams and the low variation of La/Nb rations values from 2.75–3.63 in Group I dykes to 2.88–3.21 in Group II dykes suggest that in situ contamination has played a very minor role in distinguishing the two groups.

thumbnail Fig. 10

(a) TaN/LaN vs. HfN/SmN (La Fleche et al., 1998), (b) Nb/U vs. (Th/Nb)N (Panda et al., 2023) diagrams showing the source of contamination. A, B, C: mantle sources metasomatized by subduction-related processes. D, E: mantle sources with carbonatite metasomatism. N = primitive mantle. Blue and green symbols correspond to groups I and II, respectively.

5.3 Geodynamic setting

The geodynamic setting of mafic rocks can be interpreted by using high field strength elements which resist low-grade metamorphic processes (Th, Nb, Hf, Zr, Ti, Ta, Yb, Pearce and Cann, 1973; Wood, 1980). On the Th/Yb vs. Ta/Yb diagram (Pearce, 1982), sample projection suggests that both groups derive from an island arc setting (Fig. 11b). These geotectonic settings have been reported by geochemical studies carried out on the Birimian (2.2–2.1 Ga) metabasalts of the Sirba (Ama Salah et al., 1996) and Diagorou-Darbani belts (Soumaila et al., 2004; Soumaila et al., 2016a), the granitoids of the Gorouol belt (Hallarou, 2021) and the anorthosites of the Makalondi belt (Garba Saley, 2022). By extension, juvenile and mature volcanic oceanic shelf and island arc contexts have been described for other Birimian rocks elsewhere in the West African Craton (Asiedu et al., 2004; Augustin and Gaboury, 2017; Baratoux et al., 2011; Béziat et al., 2000; Dampare et al., 2008; De Kock et al., 2011; Grenholm et al., 2019; Lompo, 2009; Sakyi et al., 2020; Senyah et al., 2016). Thus, the basaltic andesite characteristics presented by these doleritic dykes can be explained by involvement of melts derived from a lithospheric mantle modified by older subduction of Birimian rocks.

Given the U-Pb age of 1791 ± 3 Ma (Baratoux et al., 2019) on the SD dyke of our NNW-NNE doleritic dykes, we can infer that Libiri swarm postdates the end of the Eburnean orogeny around 1800 Ma (Bonhomme, 1962) and would indicate that metasomatism of the lithospheric mantle could have been caused by this phenomena. Additional U-Pb dating of the NNW-NNE dykes will be important to confirm the 1791 Ma age for the NNW-NNE swarm as a whole.

Baratoux et al. (2019) offer a reconstruction which combines the 1791 Ma Libiri dyke with coeval dykes (1780–1790 Ma) in Sarmatia portion of Baltica (e.g., Bogdanova et al., 2013; Shumlyanskyy et al., 2016) and Amazonia (Norcross et al., 2000; Reis et al., 2022; Santos et al., 2002), and show an overall radiating swarm focused on the Libiri swarm region. The establishment of the Libiri swarm was linked according to Baratoux et al. (2019) to a major mantle plume.

thumbnail Fig. 11

Diagram Th/Yb vs. Ta/Yb (Pearce, 1982) showing the geodynamic setting of the NNW-NNE Libiri swarm. Th = Tholeiitic, CA = Calc-alkaline, SHO = Shoshonitic, MORB= Mid- Ocean ridge basalts. Blue and green symbols correspond to groups I and II, respectively.

6 Conclusion

The mineralogical and geochemical results demonstrate that the NNW-NNE doleritic dyke swarm in the Liptako region of Niger, consists of evolved tholeiitic basalts. Given the match of the SD sample, collected at the same site as the sample dated at 1791 ± 3 Ma from the Libiri pit of the Samira gold mine (Baratoux et al., 2019), we propose that these dykes collectively represent the 1791 Ma Libiri dyke swarm which was emplaced c. 100–150 myr after the end of the Eburnean orogeny and have inherited the geochemical signature of lithospheric mantle which had been metasomatized by Eburnean subduction. The upwelling of magma in a post-orogenic context is linked to the arrival of mantle plume and interpretation of the swarm as part of a broad 1790 Ma Large Igneous Province present in the West African craton, formerly adjacent Sarmatia (part of Baltica) and Amazonia. Two geochemical groups can be distinguished in the 1790 Ma Libiri swarm. These groups differ notably in terms of their SiO2 content, as well as the Mg# and An# of their plagioclase. However, their similar REE slopes suggest that Group II rocks are more evolved than Group I rocks. Group II rocks being characterised by greater alteration than Group I rocks. However, it should be noted that this alteration only affects the macroscopic appearance of the rocks. Interestingly, from a geochemical point of view, Group II rocks appear to be less affected by alteration than Group I rocks. The presence of late-magmatic hydrous minerals (ferro-edenite, biotite), whose proportion varies between the two groups, reflects the evolution of magma under variable fluid pressures and their interaction with early formed large crystals. The evolution of mineral phases is reflected by significant chemical zonation between the cores and margins of plagioclase and augite and the development of late-magmatic hydrous and hydrothermal alterations or post-magmatic metamorphism minerals on the margins.

Acknowledgments

The present work has also benefited from support from Dr. Philippe de Parseval for the microprobe analyses. We thank Mohamed Pegnalogo Ouattara from the UFR SRTM UFHB laboratory for the preparation of the thin sections.

Funding

The geochemistry analyses were funded by the Large Igneous Province (LIPs) project led by Dr. Richard E. Ernst, “LIPs-Industry Consortium for Resource Exploration (with Industry funding matched by Canadian NSERC grant CRDPJ523131-17; www.supercontinent.org).

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Cite this article as: Noura GR, Baratoux L, Ernst RE, El Bilali H, Saley ALA, Hantchi KD, Bohari AD, Ahmed Y. 2025. Mineralogical and geochemical characteristics of the NNW-NNE trending Libiri doleritic dyke swarm of the Paleoproterozoic Liptako basement (West Niger): Genesis and magmatic evolution, BSGF - Earth Sciences Bulletin 196: 18. https://doi.org/10.1051/bsgf/2025019

All Tables

Table 1

Characteristics of sampled dykes of the NNW-NNE Libiri swarm.

In the text
Table 2

Selected points are representative electron microprobe analysis results of feldspars.

In the text
Table 3

Selected points are representative electron microprobe analysis results of amphibole. Numbers of ions is based on 23 oxygens.

In the text
Table 4

Selected points are representative electron microprobe analysis results of pyroxene.

In the text
Table 5

Selected points are representative electron microprobe analysis results of biotite. Numbers of ions is based on 22 oxygens.

In the text
Table 6

Whole-rock major (wt%) and trace elements (ppm) compositions of Group I dykes. LOI = Loss On Ignition, N = primitive mantle, n = chondrite.

In the text
Table 7

Whole-rock major (wt%) and trace elements (ppm) compositions of Group II dykes. LOI = Loss On Ignition, N = primitive mantle, n = chondrite.

In the text

All Figures

thumbnail Fig. 1

Map of mafic sills and dykes in the West African Craton (Jessell et al., 2015), with modifications in red based on mapping from aeromagnetic overflights of the Liptako area dykes (ACDI, 1973).
In the text
thumbnail Fig. 2

Geological map of the Liptako area in Niger (Machens, 1973). The different trends are color coded and refer to different swarms. The dated N010° Libiri swarm at 1791 Ma (Baratoux et al., 2019) belong to Samira-Libiri sector.
In the text
thumbnail Fig. 3

Field and petrographic photos of the NNW-NNE Libiri swarm: (a and b) outcrop mode of doleritic dykes from this swarm and (c, d, e and f) microphotographs showing typical textures and representative minerals in both petrographic types. Pl: plagioclase, Opx: orthopyroxene, Cpx: clinopyroxene, Amph: amphibole, Chl: chlorite, Ap: apatite, Ox: Fe-Ti oxides, Ep: epidote.
In the text
thumbnail Fig. 4

Chemical maps of SD, SLB, TD1 and TD2 dykes belonging to both groups. Pl: plagioclase, Aug: augite, Pig: pigeonite, Hyp: hypersthene, Amph: amphibole, Kfs: K-feldspars, Ap: apatite, Gr: grain with granophyric texture, Tnt-Mg-Il: titanomagnetite-ilmenite.
In the text
thumbnail Fig. 5

Compositional diagrams of (a) feldspar, (b) pyroxene (Morimoto et al., 1988), (c) amphibole (Leake et al., 1997) and (d) biotite (Speer, 1984 and Deer et al., 1986) of boths groups. Ab: albite, Ol: oligoclase, And: andesine, La: labradorite, By: bytownite, An: anorthite. Symbols with blue circles belonging to Group I and green triangles belonging to Group II.
In the text
thumbnail Fig. 6

Diagram of variations in oxides and trace elements as a function of the Mg# parameter. Symbols with blue circles belonging to Group I and green triangles belonging to Group II.
In the text
thumbnail Fig. 7

(a) Extended primitive mantle normalized trace-element plot and (b) chondrite normalized REE plot for both groups. Primitive mantle normalizing values are from Sun and McDonough (1989) and chondrite normalizing values are from Boynton (1984). Blue and green symbols correspond to groups I and II, respectively.
In the text
thumbnail Fig. 8

(a) Total-alkali silica (TAS) (Le Bas et al., 1986) and (b) Al-FeT+Ti-Mg (Jensen, 1976) classification diagrams showing tholeiitic nature of dykes in both groups. Symbols with blue circles belonging to Group I and green triangles belonging to Group II.
In the text
thumbnail Fig. 9

Diagram Th/Nb vs. TiO2/Yb (Pearce et al., 2021) showing an inherited lithospheric modified mantle by subduction components origin of the dykes in both groups. SZLM = Subduction-modified lithospheric mantle, OIB = Oceanic Island basalts, OPB: Oceanic plateau basalts, MORB= Mid-ocean ridge basalts. Blue and green symbols correspond to groups I and II, respectively.
In the text
thumbnail Fig. 10

(a) TaN/LaN vs. HfN/SmN (La Fleche et al., 1998), (b) Nb/U vs. (Th/Nb)N (Panda et al., 2023) diagrams showing the source of contamination. A, B, C: mantle sources metasomatized by subduction-related processes. D, E: mantle sources with carbonatite metasomatism. N = primitive mantle. Blue and green symbols correspond to groups I and II, respectively.
In the text
thumbnail Fig. 11

Diagram Th/Yb vs. Ta/Yb (Pearce, 1982) showing the geodynamic setting of the NNW-NNE Libiri swarm. Th = Tholeiitic, CA = Calc-alkaline, SHO = Shoshonitic, MORB= Mid- Ocean ridge basalts. Blue and green symbols correspond to groups I and II, respectively.
In the text

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