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Article de revue - Pétrologie, Géochimie
Petrogenesis and crustal evolution of the Tasiast TTG suite (SW Reguibat Shield, Mauritania). Implication for crustal growth in the West African craton
Comptes Rendus. Géoscience, Tome 353 (2021) no. 1, pp. 19-35.


The Mauritanian Tasiast unit in the Southwestern Reguibat Archean Shield (North of the West African craton = WAC) consists mainly of gneiss dated between 3.07 and 2.91 Ga. We present new field and petrographic observations combined with whole-rock geochemical data on gneisses of Tasiast to understand their petrogenesis, tectonic setting, and the evolution of the continental crust of WAC. These data provide firm evidence of two distinct orthogneisses: trondhjemite and high-K granite affinities. Geochemical characters suggest that (1) trondhjemites magma source were originally derived from polybaric partial melting at the thickened crust occurred over a range of PT conditions, covering the stability fields of garnet amphibolite and rutile eclogite. The trondhjemite composition is attributed to mixing of two major melts: one originating from eclogitic facies source region (high Nb/Ta) and the other from a garnet-amphibolite facies (low Nb/Ta) leaving garnet, amphibole, and rutile in the residue. (2) Compared to trondhjemite, the granites with high K2O and Rb contents, and low Na2O, Al2O3 and Sr contents suggest that basaltic oceanic crust was not their source material. Moreover, field relationships suggest that the granites were derived from partial melting of trondhjemites. Hence, the trondhjemite and high-K granites marked together two distinct stages during the growth of the continental crust in the Tasiast area.

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Mots clés : Trondhjemite, Granite, Amphibolite, Eclogite, Tasiast, Mauritania, West African craton
     author = {Mohamed El Abd Bouha and Houssa Ouali and Muhammad Ouabid and Hicham El Messbahi and Abdelkader Mokhtari},
     title = {Petrogenesis and crustal evolution of the Tasiast TTG suite (SW Reguibat Shield, Mauritania). Implication for crustal growth in the West African craton},
     journal = {Comptes Rendus. G\'eoscience},
     pages = {19--35},
     publisher = {Acad\'emie des sciences, Paris},
     volume = {353},
     number = {1},
     year = {2021},
     doi = {10.5802/crgeos.48},
     language = {en},
Mohamed El Abd Bouha; Houssa Ouali; Muhammad Ouabid; Hicham El Messbahi; Abdelkader Mokhtari. Petrogenesis and crustal evolution of the Tasiast TTG suite (SW Reguibat Shield, Mauritania). Implication for crustal growth in the West African craton. Comptes Rendus. Géoscience, Tome 353 (2021) no. 1, pp. 19-35. doi : 10.5802/crgeos.48.

Texte intégral

1. Introduction

Tonalite–trondhjemite–granodiorite (TTG) suites constitute an important part of outcropped Archean rocks [e.g. Martin et al. 2005], and the studies of these silicic magmatism can provide significant information about the origin and chemical evolution of the primitive continental crust [e.g., Arndt 2013; Barker and Arth 1976; Condie 2005; Dey et al. 2017; Drummond and Defant 1990; Jahn et al. 1981; Martin et al. 2005; O’Neil and Carlson 2017; Rudnick 1995]. Mineralogically, TTGs are composed mainly of quartz + plagioclase and in lesser degrees, biotite and hornblende reflecting an evolved affinity with high content of SiO2 ( >68 wt%), Al2O3 ( >15 wt%) and Na2O ( >3 wt%) and low K2O/Na2O ( <0.5) ratios [e.g., Barker and Arth 1976; Martin 1994; Martin et al. 2005; Zhai and Santosh 2011]. In addition, Archean TTGs are also characterized by steep negative rare-earth element (REE) patterns and low heavy REE abundances [Jahn et al. 1981; Martin 1994; Martin et al. 2005]. Also, the trace element concentrations of typical TTGs exhibit enrichments in incompatible elements and negative Nb–Ta anomalies [e.g., Huang et al. 2013; Martin 1994, 1999; Martin et al. 2005]. These Archean TTGs are generally interpreted to be the product of hydrous metabasalts partial melting [De Almeida et al. 2017; Foley 2008; Foley et al. 2002; Martin 1994, 1999; Martin et al. 2005; Moyen 2009; Rapp and Watson 1995; Rapp et al. 1999, 2003; Sen and Dunn 1994; Winther and Newton 1991]. However, it is difficult to distinguish between melting of garnet-bearing amphibolite and eclogite in terms of major and trace element chemistry [Foley 2008; Foley et al. 2002; Hoffmann et al. 2014; Klemme et al. 2002, 2005; Martin 1999; Rapp and Watson 1995; Rapp et al. 1999, 2003; Xiong 2006]. Furthermore, the geodynamic setting of TTG petrogenesis still remains controversial [e.g., [Moyen et al. 2017] and references therein]. Consequently, three main models are proposed for the origin of TTGs and Archean crustal growth: TTGs can be formed by (1) partial melting of subducting oceanic crust. In this model, the hotter Archean mantle is responsible for melting (rather than dehydration) of metabasalts in subducted slabs; the Archean arc magmatism is therefore TTG in nature; (2) Partial melting of amphibolite/eclogite at the base of thickened arc crust. This model is probably the dominant petrogenetic model for the formation of the TTG suite [e.g., Moyen and Martin 2012]; (3) Partial melting of wet mafic roots of an oceanic plateau [e.g. Van Kranendonket al. 2007]. These last two models are primarily supported by the lack of clear arc-related features in the Archean geology [e.g., Hamilton 1998, 2003], but it is difficult to admit a water supply at the base of the crust without evoking a subduction. In this paper, we present new field and petrographical observations, and whole-rock major and trace element data for the orthogneisses from the Tasiast Archean TTGs. The main purposes are (1) to provide new insights into the petrogenesis implications, (2) to assess the possible petrogenetic links between different orthogneisses, and (3) to understand the tectonic setting for the widespread Tasiast silicic magmatic units.

Figure 1.

Geological framework of the West African craton showing the location of the studied Tasiast area in Mauritania [Chorlton 2007].

2. Geological setting

Reguibat Shield is the northernmost outcrop of the West African craton (Figure 1) [e.g., Ennih and Liégeois 2008; Menchikoff 1949]. It is surrounded on almost sides by Pan-African orogenic belts and covered in the south by the extensive intracratonic sediments of the Taoudeni Basin [e.g., Cahen et al. 1984; Ennih and Liégeois 2008; Ouabid et al. 2017]. The major part of this craton is buried beneath various metamorphosed sediments, and unconsolidated superficial deposits of Sahara Atlantic coastal. Dillon and Sougy [1974] and Bessoles [1977] distinguished two provinces within the Reguibat Shield: (1) a Southwestern province composed of rocks with an age older than ca. 2 Ga (i.e., pre-Eburnean cycle); and (2) Central and North Eastern province consists of Eburnean rocks (Figure 1). The Reguibat Shield was cratonized at the end stage of the Eburnean orogeny (cs. 2.1–2 Ga) and has been stable since about 1.7 Ga [e.g., Schofield et al. 2006; Montero et al. 2014]. The Archean Shield contains grey gneiss, various acidic rocks, migmatites as well as mafic and ultramafic rocks (greenstone belt), quartzites and banded iron formations (BIF), and is characterized by regional foliations trajectories N–S to NE–SW in the southern part and NNW–SSE in its northern part [e.g., Chardon 1997]. The southern part of the Shield is subdivided into three units which are from West to East: Tasiast, Tijirit, and Amsaga; Tiris is the northernmost part. Tiris and Amsaga units have been affected by a regional granulite facies metamorphism, while Tasiast and Tijirit show greenschist or amphibolite metamorphic assemblages [Chardon 1997]. The lithologies that make up the Tasiast terrains can therefore be divided into the following three major groups [e.g., Key et al. 2008]: (1) migmatitic gneisses with amphibolite lenses and minor orthogneisses; (2) greenstone belt lithologies; and (3) post-greenstone belt granitoids (mostly TTGs), granites, and minor pegmatitic muscovite granites that are the youngest intrusions.

The Tasiast–Tijirit terrane contains rocks with Nd model ages similar to Amsaga, between 3.6 Ga and 3.1 Ga, but zircon ages that only recorded magmatic events between 2.97 Ga and 2.89 Ga and a possible thermal imprint at ca. 2.7 Ga [Chardon 1997; Key et al. 2008]. The Tiris terrane has the youngest Archean crust with Nd model ages between 3.2 Ga and 3.0 Ga, and zircon ages that reflect several magmatic events at 2.95 Ga, 2.65 Ga, and 2.48 Ga [Schofield et al. 2012]. An age U/Pb, between ca. 3.06 and 2.95 Ga obtained from inherited zircons xenocryst in three quartz veins along Tasiast thrust [Heron et al. 2016]; this age corresponds to previous ages obtained from the Tasiast–Tijirit terrane gneiss and TTG rocks. According to our field observations (Figure 1b–d), the Tasiast granitoid gneisses can be easily subdivided into grey and leucocratic suites (Figure 2b–d). The leucocratic suite appears clearly younger than grey gneiss. The field relationships between both units are clear; the leucocratic gneisses are either wrapping the grey gneisses unit or intruding them with a sharp contact. New petrographic observations of more than 30 samples from these gneiss units permit to distinguish between two types of granitoid facies: TTG gneiss and granitic gneiss.

Figure 2.

(a) Geological map of the Tasiast area [adapted from PRISM 1:500k geological map; in Heron et al. 2016]. (b–d) Field photographs illustrating lithological aspects and field relationships in the Tasiast area.

3. Petrography

Grey gneisses occur as grey-dark rocks with coarse- to medium-grained texture (Figure 3a–e). They have a homogeneous minerals assemblage made of plagioclase (dominant: 45–60% vol. of the rock), quartz (25–35% vol.), biotite (5–15% vol.), and minor interstitial potassium feldspar (K-feldspar), and accessory minerals including magnetite, ilmenite, titanite, apatite, and zircon. Generally, the grey gneiss occurs with two alternating layers: quartz and plagioclase-rich layer (clear) and biotite-rich layer (dark), which give the rock its gneissic foliation. Plagioclase crystals are euhedral to subhedral (1–4 mm in size) mostly altered to albite and sericite and enclose frequently biotite and apatite. Quartz ranges from interstitial to fine aggregates (0.1–0.5 mm in size), surrounding the large plagioclase crystals. Biotite occurs strongly as pleochroic flakes which are partially altered to chlorite, titanite, and opaque minerals, or exhibits elongated crystal around plagioclase or enclosed in this mineral. K-feldspar (mainly microcline) is interstitial between quartz and plagioclase grains.

Figure 3.

Hand specimens and microscope photographs of the Tasiast gneiss types. (a–e) Grey gneisses. (f–i) Leucocratic gneisses.

Leucocratic gneisses are a medium-grained and yellow pale leucocratic rock with gneissic texture as well as the previous rock (Figure 3g–i). Major mineral phases are quartz (40–45% vol.), K-feldspar (35–40% vol.), plagioclase (10–15% vol.) and biotite ( <4% vol.). Like the previous facies, titanite, Fe–Ti oxides (magnetite and ilmenite), apatite, and zircon are common accessory minerals in these granitic rocks. Quartz occurs as polycrystalline aggregates (0.2–2 mm in size) developed around primary crystals of feldspars or as fabric of alternated quartz and feldspar-rich layers. K-feldspar (principally orthoclase) and plagioclase occurs mainly as subhedral phenocrysts (1–3 mm in size) wrapped by the fine- to medium-grained matrix composed of K-feldspar and quartz aggregates. Biotite is scarce and occurs as elongated crystals with preferential orientation parallel to the feldspar and quartz layers.

4. Analytical methods

Sixteen representative samples of the two distinct granitoid gneisses of the Tasiast unit were selected for whole-rock geochemical analyses, including eleven Grey gneiss and five leucocratic gneisses (compositions are given in Table 1). Rocks were analysed at the Instituto Andaluz de Ciencias de la Tierra (IACT, Granada, Spain). Rock powders were prepared using a jaw crusher and an agate ring mill. Whole-rock analyses of major elements and V, Cr, Co, Zr, and Ni were determined by X-ray fluorescence (XRF). Loss on ignition (LOI) was determined by drying the samples at 900 °C and is low (0.12–0.46 wt%). Trace elements (except V, Cr, Co, Zr, and Ni) were analysed using a Triple Quadrupole Agilent 8800 inductively coupled plasma mass spectrometer (ICP-MS) at the IACT laboratory. Sample digestion was performed following the HF–HClO4 digestion process described by Garrido et al. [2000]. Element concentrations were determined by external calibration, except for Hf that was calculated using Zr measured by XRF and the chondritic Zr/Hf ratio [McDonough and Sun 1995]. The compositions of the granite reference sample GS-N, analysed as unknown during the analytical runs, show good agreement with published values of this international standard [Georem database; Jochum et al. 2016].

Table 1.

Major (wt%) and trace (ppm) element concentrations for the grey gneisses in the Tasiast area

SiO2 70.92 72.72 71.98 70.11 70.69 69.92 70.62 71.91 70.38 70.68 70.19
TiO2 0.30 0.19 0.20 0.30 0.29 0.34 0.29 0.20 0.30 0.29 0.30
Al2O3 15.99 15.21 15.16 16.10 15.88 16.04 15.92 15.14 16.01 16.16 16.22
Fe2O3 1.80 1.32 1.20 2.02 1.70 2.13 1.85 1.28 1.81 1.85 1.96
MnO 0.04 0.03 0.01 0.05 0.03 0.04 0.04 0.03 0.03 0.03 0.04
MgO 0.77 0.39 0.42 0.80 0.69 0.81 0.70 0.41 0.74 0.83 0.76
CaO 2.55 1.81 1.84 2.67 2.66 2.92 2.65 1.95 2.78 2.83 2.69
Na2O 5.14 4.97 4. 73 5.46 5.08 5.06 5.20 4. 81 5.13 4.96 5.37
K2O 1.60 2.52 2.73 1.15 1.60 1.32 1.44 2.80 1.44 1.71 1.36
P2O5 0.13 0.06 0.08 0.12 0.11 0.13 0.11 0.06 0.12 0.16 0.12
LOI 0.37 0.23 0.40 0.37 0.20 0.18 0.16 0.46 0.27 0.37 0.24
Total 99.25 99.21 98.34 98.78 99.01 98.71 98.82 98.59 98.72 99.50 99.01
Mg# 46 37 41 44 45 43 43 39 45 47 43
Cs 1.11 0.49 0.52 1.63 1.38 1.23 1.72 0.55 4.27 1.57 1.40
Zr 116 130 143 132 126 137 129 134 125 128 130
Rb 32.42 69.84 67.71 30.36 41.17 37.96 39.48 91.55 67.26 58.15 56.46
Sr 342.05 402.39 372.82 304.08 477.68 473.78 377.75 438.87 385.54 516.70 313.63
Y 2.73 1.12 1.37 3.18 1.94 2.66 2.33 2.08 2.56 2.22 3.15
Nb 3.52 2.30 2.69 3.78 2.16 2.33 3.06 3.33 4.04 3.17 4.63
Ba 678.261268.041485.09 139.42 637.09 630.18 435.231264.02 458.45 878.33 226.05
La 12.12 33.39 26.95 16.87 17.73 15.72 16.63 16.95 19.37 17.68 19.40
Ce 22.98 53.80 43.31 30.63 31.68 31.01 30.20 27.02 35.79 34.83 35.32
Pr 2.40 4.48 3.94 3.03 3.17 3.05 3.04 2.37 3.37 3.27 3.50
Nd 9.03 14.15 12.14 10.92 11.41 11.36 11.29 7.62 12.11 11.92 12.42
Sm 1.33 1.73 1.58 1.52 1.66 1.74 1.57 1.12 1.66 1.53 1.69
Eu 0.55 0.47 0.55 0.38 0.55 0.63 0.42 0.50 0.44 0.62 0.37
Gd 0.99 0.94 0.95 1.12 1.03 1.15 0.97 0.79 1.03 1.00 1.21
Tb 0.12 0.11 0.10 0.14 0.12 0.14 0.13 0.10 0.14 0.11 0.16
Dy 0.56 0.33 0.37 0.66 0.45 0.62 0.49 0.47 0.56 0.43 0.71
Ho 0.10 0.04 0.05 0.11 0.08 0.09 0.08 0.07 0.09 0.07 0.11
Er 0.23 0.08 0.10 0.28 0.16 0.22 0.21 0.21 0.24 0.21 0.32
Tm 0.03 0.01 0.01 0.04 0.02 0.03 0.03 0.03 0.03 0.03 0.03
Yb 0.22 0.07 0.08 0.22 0.13 0.19 0.14 0.14 0.16 0.19 0.19
Lu 0.03 0.01 0.01 0.03 0.02 0.03 0.02 0.02 0.03 0.03
Ta 0.21 0.08 0.09 0.16 0.14 0.15 0.15 0.17 0.36 0.17 0.28
Pb 6.85 16.99 16.31 6.18 7.77 6.54 8.73 21.20 8.93 10.49 10.84
U 0.76 0.96 0.71 0.78 0.88 1.24 0.59 1.97 1.15 2.97 2.88
A/NK 1.57 1.39 1.41 1.57 1.57 1.64 1.57 1.38 1.60 1.61 1.57
A/CNK 1.08 1.07 1.08 1.07 1.06 1.06 1.07 1.04 1.06 1.06 1.07
K2O/Na2O 0.31 0.51 0.58 0.21 0.31 0.26 0.28 0.58 0.28 0.35 0.25
FMMT 2.73 1.79 1.70 2.97 2.53 3.10 2.69 1.79 2.69 2.81 2.86
Sr/Y 125.47 359.85 271.97 95.73 245.73 178.08 162.19 210.83 150.36 232.55 99.55
(La/Yb)N 38.25 333.28 233.89 52.43 92.69 56.84 81.22 83.49 83.48 63.99 71.35
Nb/Ta 16.65 30.23 29.13 24.01 14.98 15.62 20.23 19.51 11.26 18.88 16.33
Zr/Sm 87.19 75.26 90.40 86.69 75.73 78.76 82.04 120.17 75.08 83.52 76.74
Dy/Yb 2.53 4.75 4.59 2.97 3.42 3.26 3.43 3.34 3.51 2.26 3.76
West15.7052°15.6817°15.6810°15.6817°15.6813°15.6810°15.6767°15.6967°15.6894°15.7088° 15.709°

Hf had been calculated after Zr concentration. A/CNK = molar ratio of Al2O3∕(CaO + Na2O + K2O); A/NK = molar ratio of Al2O3∕(Na2O + K2O); Mg# = molar ratio of (MgO/40.3)/((MgO∕40.3) + (FeO∕71.85)) ∗100; (La/Yb)cn are chondrite-normalized values [from Sun and McDonough 1989].

5. Whole-rock geochemistry

In the anorthite (An)–orthoclase (Or)–albite (Ab) [Barker 1979; O’Connor 1965] and quartz (Qz)–Or–Ab [Barker and Arth 1976] CIPW normative diagrams, the eleven analyzed samples of grey gneiss from Tasiast region are plotted in the field of trondhjemite series while the five samples of leucocratic gneisses are classified as granites (Figure 4), which consistent with field and petrographic conclusions.

Figure 4.

Classification of Tasiast gneisses using normative An–Ab–Or [Barker 1979] and Qz–Or–Ab [Barker and Arth 1976].

Table 2.

Major (wt%) and trace (ppm) element concentrations for the leucocratic gneisses in the Tasiast area

Sample N DRT1DRT5-2 DRT8DRT12DRT13
SiO2 73.59 73.83 71.99 74.54 73.49
TiO2 0.09 0.06 0.09 0.08 0.07
Al2O3 14.66 14.26 15.08 13.39 14.60
Fe2O3 0.89 0.37 0.97 1.40 0.68
MnO 0.02 0.01 0.03 0.03 0.03
MgO 0.24 0.21
CaO 1.33 1.00 1.43 0.64 1.41
Na2O 4.47 3.56 4. 05 2.71 4.69
K2O 3.83 5.56 4.57 6.28 3.25
P2O5 0.03 0.04 0.03 0.06 0.04
LOI 0.23 0.33 0.23 0.12 0.12
Total 99.14 98.69 98.44 99.14 98.26
Mg# 35 30
Cs 0.39 0.76 0.81 0.64 0.59
Zr 80 43 85 16 57
Hf 2.16 1.16 2.30 0.43 1.54
Cr 2.00
Rb 77.96 169.18 112.84 183.48 82.50
Sr 110.94 145.14 357.85 264.50 180.57
Y 1.48 1.95 1.40 0.53 1.34
Nb 3.70 1.58 1.73 1.35 2.90
Ba 682.22 624.731451.561960.531197.09
La 7.66 13.70 21.37 6.25 8.36
Ce 12.32 25.16 40.09 10.99 14.66
Pr 1.26 2.51 3.58 0.98 1.31
Nd 4.26 8.73 12.17 3.30 4.77
Sm 0.83 1.65 1.80 0.49 0.94
Eu 0.24 0.30 0.54 0.52 0.36
Gd 0.56 1.15 1.09 0.31 0.60
Tb 0.08 0.13 0.11 0.04 0.08
Dy 0.34 0.54 0.41 0.15 0.37
Ho 0.05 0.07 0.05 0.02 0.05
Er 0.12 0.15 0.04 0.11
Tm 0.02 0.02 0.01 0.01
Yb 0.10 0.10 0.08 0.03 0.10
Lu 0.02 0.01 0.02 0.02
Ta 0.12 0.13 0.12 0.10 0.07
Pb 12.75 22.45 23.95 24.03 18.25
U 1.78 11.93 0.89 1.14 1.45
A/NK 1.30 1.30 1.27 1.19 1.20
A/CNK 1.06 1.04 1.05 1.08 1.06
K2O/Na2O 0.86 1.56 1.13 2.32 0.69
FMMT 1.15 0.40 1.20 1.37 0.71
Sr/Y 74.80 74.49 255.33 500.02 135.17
(La/Yb)N 51.15 91.29 178.40 146.12 59.11
Nb/Ta 31.83 12.61 14.27 13.30 38.67
Zr/Sm 96.96 26.02 47.27 32.67 60.76
Dy/Yb 3.26 5.22 4.93 4.94 3.80

Hf had been calculated after Zr concentration. A/CNK = molar ratio of Al2O3∕(CaO + Na2O + K2 O); A/NK = molar ratio of Al2O3∕(Na2O + K2O); Mg# = molar ratio of (MgO/40.3)/((MgO∕40.3) + (FeO∕71.85)) ∗100; (La/Yb)cn are chondrite-normalized values [from Sun and McDonough 1989].

The trondhjemitic gneisses have SiO2 contents ranging from 69.92 to 72.72 wt% and high Al2O3 contents (15.14–16.22 wt%) (Table 1). They have relatively low MgO [0.39–0.83 wt%, Mg# = Mg/(Mg + Fe) : 0.37–0.47]. Trondhjemite samples have slight enrichment in Na2O (4.73–5.46 wt%) and CaO (1,81–2.92 wt%) with low K2O contents (1.15–2.80; K2O/Na2O ∼ 0.21–0.58) and classified mostly as calc-alkaline series as shown in the diagram of Peccerillo and Taylor [1976] (Figure 5). The compositions of all the investigated trondhjemite rocks generally overlap those of the Archean TTGs as shown in Figure 4(a, b) [TTG field from Smithies and Champion 1998 and Barker and Arth 1976].

On the other hand, according to the petrographic observations and in contrast to previous gneiss, the granitic gneisses are silica rich with SiO2 contents ranging between 71.99 wt% and 74.54 wt% and they are characterized by moderate Al2O3 concentrations (13.39–15.08 wt%). Moreover, they are classified as high-K calc-alkaline to shoshonitic series with high K2O (3.25–6.28 wt%), moderate Na2O (2.71–4.69 wt%), and low CaO (0.64–1.43 wt%) concentrations (Figure 5; Table 2). Furthermore, all the investigated samples of trondhjemite and granite gneisses are peraluminous [mol. Al2O3∕CaO + Na2O + K2O = A∕CNK > 1 and mol. Al2O3∕Na2O + K2 O = A∕NK > 1; [Maniar and Piccoli 1989]; Figure 6; Tables 1 and 2]. In contrast to the trondhjemite samples, the compositions of the studied leucocratic gneisses contrast starkly with Archean TTGs, and show a common trend for calc-alkaline granites (Figure 4a, b).

The primitive mantle-normalized trace element spider and chondrite-normalized REE patterns are shown in Figure 7. The Tasiast trondhjemitic gneisses are enriched on high field strength elements (HFSE) such as Rb, Ba, U, Pb, and Sr (Sr/Sr ∼ 1.36–2.93) and strongly depleted in Nb (Nb/Nb ∼ 0.06–0.27), Ta (Ta/Ta ∼ 0.07–0.52), and Ti (Ti/Ti ∼ 0.38–0.66) with high Sr/Y (95.7–359.8). They have highly variable Nb/Ta (11.26–30.23) ratios. On the chondrite-normalized REE diagrams, trondhjemites show similar moderate REE concentrations (𝛴REE ∼ 50.86–109.61 ppm) with high fractionations between LREE and HREE [(La/Yb)cn ∼ 38.25–333.28], high fractionations between the HREE [(Gd/Yb)cn : 3.72–6.40)], and negligible to positive Eu anomalies (Eu/Eu ∼ 0.75–1.55). On the other hand, the granitic gneisses are characterized by low REE concentrations (𝛴REE ∼ 23.11–81.33 ppm), significant variations in REE patterns in terms of fractionation [(La/Yb)cn ∼ 51.15–178.40; [(Gd/Yb)cn ∼ 4.50–10. 89)], and Eu anomaly (Eu/Eu ∼ 0.62–3.83). Spider diagrams show an enrichment of HFSE (e.g., Rb, Ba, U, and Pb), positive Sr anomaly (Sr/Sr up to 4.20), and a strong negative anomaly of Nb (Nb/Nb ∼ 0.01–0.12), Ta (Ta/Ta ∼ 0.18–0.45), and Ti (Ti/Ti ∼ 0.15–0.35). All samples have high Sr/Y ratio ( ∼74–500).

Figure 5.

K2O versus SiO2 [Peccerillo and Taylor 1976] (symbols are as in Figure 4).

6. Discussion

6.1. Petrogenesis of trondhjemitic gneisses

Generally, the TTG suites (trondhjemite, tonalite, and granodiorite) represent the major component of the Archean juvenile continental crust [Jahn et al. 1981; Laurent et al. 2014; Smithies et al. 2019]. Our new field and petrographic observations, and geochemical investigations confirm the widespread of trondhjemite in the Mauritanian Tasiast Archean unit. This unit is previously dated at about 3.07–2.91 Ga [Chardon 1997; Heron et al. 2016; Key et al. 2008; Figures 2, 7]. The Tasiast trondhjemitic studied samples have typical Archean TTGs signatures (Figure 4a, b). In detail, these rocks are silica and alumina rich (SiO2 > 68 wt%; Al2O3 ⩾ 15 wt%) and have low FMMT contents (Fe2O3tot+MgO+MnO+TiO2<5 wt%) [e.g., Martin 1994]. The high Na2O and low K2O (Figure 5 and Table 1), resulting in a strong enrichment in plagioclase and absence of alkali feldspar as shown petrographic observations, suggest a derived mafic source rather than felsic one for these granitoids [e.g., Moyen 2009]. The low Rb/Sr ratio and pronounced negative Nb, Ta, and Ti anomalies, and a strong fractionated displaying similar degrees of fractionation for both LREE and HREE, show that Tasiast trondhjemites are similar to most TTG [e.g., Condie 1989, 1993; Jahn 1994; Martin 1994; Martin et al. 2005; Tarney et al. 1982]. In addition, high A/NK (1.38–1.61), A/CNK (1.04–1.08), Sr/Y (95.73–359.85), and (La/Yb)cn (38.25–333.25) ratios characterize high-Al TTGs and adakites [Figure 8(a, b); e.g., Martin 1999; Martin et al. 2005].

In terms of acidic magma origin, many experimental and geochemical studies concluded that most TTGs are derived from a hydrated crust, although physical conditions are still debated [e.g., Foley et al. 2002; Hoffmann et al. 2011; Rapp et al. 2003]. The TTG melting occurred over a range of PT conditions, covering the stability fields of garnet amphibolite and rutile eclogite. There are two common tectonic scenarios to produce TTG magmas: (1) melting of subducted oceanic crust; and (2) melting at the base of a mafic crust [e.g., Condie 2005; Drummond and Defant 1990; Hoffmann et al. 2011, 2014; Martin and Moyen 2002; Martin et al. 2005, 2014; Moyen 2011; Moyen and Martin 2012; Smithies 2000]. Subduction-related TTG magmas tend to possess high Mg#, Cr, and Ni contents due to interactions between the TTG melts and the mantle wedge [e.g., Martin and Moyen 2002; Martin et al. 2005; Moyen and Martin 2012; Smithies 2000]. Anyway, the studied Tasiast TTG units have low MgO concentration (0.39–0.83 wt%, Mg# = 37–47) indicating an absence of interaction of studied TTG magmas with peridotite in the mantle wedge [e.g., Condie 2005; Martin et al. 2005; Rapp et al. 1999; Smithies 2000]. Therefore, it is more likely that the Tasiast trondhjemites were generated at the base of a thick mafic crust. The resulted melt involving basalts or amphibolites under various PH 2O conditions below 10 Kbar were tonalitic in composition, without any trondhjemitic affinity; the garnet is not stable in these conditions [e.g., Beard and Lofgren 1989, 1991; Helz 1976; Holloway and Burnham 1972; Rushmer 1991]. According to field and petrographic observations, and geochemistry data, there are no tonalitic rocks among Tasiast gneiss units [Figures 24; e.g, Key et al. 2008]. Also, we notice that the majority of studied Tasiast TTG samples have a positive or no Eu anomalies accompanied by pronounced Sr anomalies (Figure 7a, b) that could reflect melting at pressures above the plagioclase stability field. Furthermore, the high Sr abundances (304–516 ppm) and low Y ( <3.18 ppm) generating the high Sr/Y ratios in the trondhjemite rocks of Tasiast region (Figure 8a), indicating that the plagioclase was not in the residue and the depth of melting is above plagioclase stability field [e.g., Martin et al. 2005]. In addition, the Tasiast trondhjemitic gneisses display a pronounced enriched LREE relative to HREE patterns with high (La/Yb)cn and low Ybcn (Figures 7b, 8b, Table 1), which are consistent with garnet in the residue [e.g., Martin et al. 2005].

Figure 6.

Investigated Tasiast granitoid gneisses in A/NK versus A/CNK diagram [Maniar and Piccoli 1989] (symbols are as in Figure 4).

Figure 7.

Primitive mantle-normalized trace elements patterns (a, c) and chondrite-normalized REE (b, d) of the trondhjemites, and granites in the Tasiast area [Sun and McDonough 1989], (symbols are as in Figure 4).

Figure 8.

Sr/Y–Y adapted from Martin et al. [2005] (a) and La/Ybcn–Ybcn diagrams adapted from Martin [1999] (b) for Tasiast trondhjemites and granites. (Symbols are as in Figure 4).

Figure 9.

Zr/Sm versus Nb/Ta magma source composition discriminating plot for the trondhjemites from Tasiast terrain [Hoffmann et al. 2011]. Amphibole/melt partition coefficients of Foley [2008] (blue arrow); amphibole/melt partition coefficients of Klein et al. (1997) (green arrow).

Moreover, melts with pure amphibole in the residue would have low Nb/Ta, Gd/Yb, and Dy/Yb ratios, and high Zr/Sm ratios [e.g., Foley et al. 2002; Rapp et al. 2003; Zhou et al. 2014]; while the amphibole and garnet as a residual phases will not only effectively raise the Sr/Y, La/Yb, and Zr/Sm ratios with variable Nb/Ta ratio, but also increase the Gd/Yb and Dy/Yb ratios in the produced melts [e.g., Davidson et al. 2007; Macpherson et al. 2006]. The Tasiast trondhjemites have variable Nb/Ta (11.26–30.23), high Gd/Yb (4.5–1.4), Dy/Yb (2.3–4.7), and Zr/Sm ( >75) indicating that the TTG gneisses were derived by partial melting of a source with garnet and amphibole in the residue. These characteristics make the trondhjemite rocks of Tasiast unit different from the low-pressure TTGs described by Moyen and Martin [2012].

In the primitive mantle-normalized trace element diagram (Figure 7a), the trondhjemites are strongly depleted in Nb and Ta, which are consistent with rutile or/and amphibole in the residue [e.g., Martin et al. 2005; Zhou et al. 2014]. Consequently, these mineral phases involve in controlling the HFSE ratios such as Nb/Ta, Zr/Hf, and Zr/Sm because these elements pairs have different partition coefficients in different residual mineral phases [e.g., Foley et al. 2002; Jenner et al. 1994; Klemme et al. 2002; Rubatto and Hermann 2003; Xiong et al. 2007]. So these ratios can be used as powerful evidence to discriminate between different source regions [e.g., Hoffmann et al. 2011]. Rutile and amphibole are two diagnostic minerals in eclogite facies and amphibolite facies conditions, respectively [Foley et al. 2002; Rapp et al. 2003]. Therefore, the Nb/Ta and Zr/Sm ratios can be used to recognize the conditions of melting: eclogite facies or amphibolite facies [Hoffmann et al. 2011]. Foley et al. [2002] investigated the distribution of HFSE in TTG liquids and concluded that the low Nb/Ta and high Zr/Sm ratios in Archean TTG preclude an eclogitic residue but are consistent with a garnet-bearing amphibolite. Both residues cannot explain the compositional trends found for trondhjemites of Tasiast. The variable Nb/Ta (11.26–30.23) and high Zr/Sm (75.08–120.17) ratios clearly in favour of polybaric melting occurred over a range of PT conditions, covering the stability fields of garnet amphibolite and rutile eclogite as illustrated in Figure 9. Therefore, we attribute the TTG compositions to mixing of two major melts: the first is originating from eclogitic facies source regions (high Nb/Ta) and the second is formed from a garnet-amphibolite source (low Nb/Ta).

6.2. Petrogenesis of granitic gneisses

Compositional differences between granitic and trondhjemitic gneisses in the Tasiast area are remarkable. In contrast to the trondhjemite, the granites have high K2O and Rb, and low Na2O, Al2O3 and Sr contents (see Figures 57 and Table 2). Unlike TTGs (K2O < 2.84), a high K2O (K2O ∼ 3.3–6.34) content in granites suggest that basaltic oceanic crust was not their source material [e.g., Smithies 2000]. According to Patino Douce [2005], the partial melting of anhydrous TTG within the middle or lower crust could produce granitic melts. The depleted HREE, Y and high Lacn∕Ybcn ( ∼51–178) exhibited by studied Tasiast granite samples indicate the role of high fractions of residual garnet in their sources or an HREE-depleted source similar to that produced by the trondhjemites (Figures 7, 8, Table 2). In addition, the contribution of Watkins et al. [2007] highlighted that the melting of TTGs with low K2O/Na2O ( <0.3) could not generate high-K granites. As the Tasiast trondhjemites have an average value of K2O/Na2O at about 0.36 (0.21–0.58), these trondhjemites seem to be the main suitable source of the granites studied here. Furthermore, the field relations between the trondhjemite and the granite clearly support this hypothesis (Figure 2).

6.3. Continental evolution in the Tasiast Archean unit

Archean TTGs show secular variation in composition indicating change in geodynamic setting of their formation or cooling of the Earth [e.g., Martin and Moyen 2002; Smithies 2000]. The geochemistry data of the Tasiast trondhjemite suggest that the most reasonable mechanism for the origin of these rocks is the partial melting at the base of a thickened mafic crust covering the stability fields of garnet amphibolite and rutile eclogite (see Section 6.1). On the other hand, the trace element variations of granitic rocks were probably controlled mainly by source compositions and melting conditions rather than tectonic settings [e.g., Wu et al. 2003]. Their geochemical characteristics are likely inherited from the trondhjemite and cannot be used for discriminating its tectonic settings.

In addition, the Tasiast unit occurs as Archean continental crust and is dominated by grey gneiss complexes (mainly trondhjemites, dated at about ∼3.07–2.91 Ga [Chardon 1997; Heron et al. 2016; Key et al. 2008]. Hence, the emplacement of these rocks is envisaged to represent the growth of new continental crust by partial melting of mantle that generate oceanic tholeiites (now amphibolites and eclogites), followed by the partial melting of these metabasalts to constitute the parental magma of studied Tasiast trondhjemitic gneisses. On the other hand, the Tasiast granitic gneisses are produced afterwards by partial melting of trondhjemitic gneisses (see Section 6.2). The age of granitization is unknown but it seems to be younger than 2.91 Ga and can be linked to an early stage of new crustal growth in the West African craton. These granites should be probably similar to the syn-tectonic high-K granites of the Tiris complex (Mauritania) that yielded an age of about 2.56–2.48 Ga [Schofield et al. 2012]. This comparison is strengthened by the recognition in Tasiast terrains of an important thermal imprint at that time [Chardon 1997; Key et al. 2008].

7. Conclusion

Field and petrographic observations, and geochemistry data from the Tasiast Archean unit presented in this study allowed to formulating the following conclusions:

(1) Tasiast area consists mainly of trondhjemitic gneisses which host the high-K granites.

(2) Geochemical composition of the Tasiast trondhjemites suggests that they are juvenile continental crust generated by polybaric melting of metabasalts occurred over a range of PT conditions, covering the stability fields of garnet amphibolite and rutile eclogite, and the melt has occurred without any mantle wedge involvement.

(3) Field relationships and trace element composition provide a genetic link between the trondhjemite and the high-K granitic gneisses.

(4) Our new field and petrographic observations, and geochemical data highlight at least two stages of magmatic events in the Tasiast Archean unit as follows: (1) a trondhjemite gneiss which is the earliest known gneiss in the region; (2) high-K granite which was generated by a partial melting of trondhjemites.


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