Petrography and Geochemistry of the Farchana-Hadjerhadid Granitoids (Ouaddaï Massif, Eastern Chad)

Work in the Farchana-Hadjerhadid sector is focused on petrography, with the main aim of mapping the geological formations of the eastern Ouaddaï Massif in order to identify petrographic types and gain an understanding of the geodynamic history of this area. The fieldwork showed that the Farchana-Hadjerhadid granitoids consist mainly of granites and diorites and are intersected by numerous pegmatite veins. Microscopic observation shows that the diorite is made up of plagioclase, amphibole, biotite, and quartz. The granite is composed of alkali feldspar, quartz, amphibole, plagioclase, and biotite. The granitoids in the study area are moderately differentiated (SiO2 = 55.62% to 71.67%) and belong to the medium to highly potassic calc-alkaline series. They are ferriferous type I metaluminous granitoids (A/CNK < 1.1). The parent magma was derived from the partial melting of the metabasalts, and the petrographic types evolved by assimilation and fractional crystallization.

Introduction

The outcropping basement geological formations in Chad are located between the Eastern Nigerian domain to the west, the Congo craton to the south, and the Sahara metacraton to the east [1], [2]. This basement was formed during the Pan-African orogeny that took place towards the end of the Precambrian (700–550 Ma) [3], [4]. This pan-African event represents the last active orogeny in Chad [5], [6]. The rocks formed are influenced by this event and make up the bulk of the crystalline massifs found in the Tibesti in the north, the Ouaddaï in the east, the Guera in the centre, the Mayo-Kebbi in the south-west and Baibokoum in the south [5]. The Ouaddaï Massif, in which the study area is located, lies to the east of Chad [5]. It consists mainly of migmatites and granites [6]. With the exception of data from [4], [7], [8], which are very old, very little information concerns the Ouaddaï Massif. In this paper, we focus on the petrography and geochemistry of the Farchana-Hadjerhadid granitoids (Ouaddaï Massif, eastern Chad) in order to contribute to the understanding of this portion of the Central African Pan-African chain in Chad.

Geological Context

Morphologically, the Ouaddaï Massif is a plateau at an altitude of between 800 m and 1000 m [5], [6]. It rests on a Precambrian basement, characterized by metasedimentary and metavolcanic formations (migmatite, schist, quartzite, marble) affected by low-grade metamorphism and plutonic rocks of intermediate to felsic composition, forming plutons and veins intersecting the metamorphic rocks [5] (Fig. 1). The western part of the Ouaddaï Massif is largely dominated by post-tectonic intrusive granitoids, alkaline granites, monzonites, and syenites [9] in the southern part of the Massif, metamorphic formations outcrop in abundance. This part includes formations consisting of type-S leucogranites, giving U-Pb ages on zircon of 635±3 Ma and 612±8 Ma, type-I calc-alkaline potassic granitoids, giving a U-Pb age on zircon at 538±5 Ma and metasedimentary rocks (amphibolites) corresponding to tholeiitic basalts derived from the partial melting of a depleted mantle (ƐNd 540 = 4) and green schists dated at 627±7 Ma (Th-U-Pb on monazite) [9], [10].

Fig. 1. 1:500,000 geological map drawn up by Gsell and Sonet, 1960, modified.

Materials and Methods

Materials

We used several cartographic databases for this work:

• 1:500,000 scale geological reconnaissance map of Adré, published by the Institut Equatorial des Recherches et d’Etudes Géologiques et Minières by Gsell and Sonet, 1960, based on a 1:200,000 scale photograph from the Institut Géographique National.
• This map is supplemented by Aster images GLSDEM_n013e021.tif taken on November 28, 2020.
• Samples of different petrographic units, i.e., 15 thin sections for the petrographic study, 8 samples for the geochemical study.
• A fairly diversified bibliography constitutes the fundamental material of this work.

Methods

For the petrographic analyses, thin sections were prepared at the geology laboratory of the Abdou Moumini University in Niamey and at the geology laboratory of the University of Strasbourg, France. For the geochemical analyses, 8 samples, including 4 samples of biotite and amphibole granites (R2, R5, R6B, and R7), and 4 samples of diorites (R1, R3, R6A, and R8) were analyzed at the ACTLABS laboratory (Activation Laboratories), Ontario CANADA. Analyses were carried out using inductively coupled plasma mass spectrometry (ICP-MP) for major, trace, and rare earth elements.

Result

Petrography

The granite outcrops in domed, metric to decametric blocks to the north of Hadjer Hadid, to the east of Abouglagne, and to the south of Farchana, on the axis towards Hadjer Hadid. At the scale of the sample, the granite is a grey rock (Fig. 2a). Microscopically, the rock has a grainy texture. It is composed of plagioclase, alkali feldspar, quartz, biotite, muscovite, and zircon. Alkali feldspar (30%–35%) is pink and colorless and occurs as elongated, automorphic to sub-automorphic crystals. It varies in size from 1 mm to 3 mm and has a low relief (Figs. 2b and 2c). Quartz (25%–30%) occurs in the form of large grey patches varying in size from 1 mm to 2 mm. Small quartz crystals are sometimes found embedded in amphibole phenocrysts. The quartz has low relief and sometimes forms sub-grains (Figs. 2c and 2d). Plagioclase (20%–25%) is dark grey, sub-automorphic to xenomorphic, and varies in size from 0.2 mm to 1.2 mm, often with a polysynthetic macle (Figs. 2d and 2e). Amphibole (10%) is a green hornblende recognized by its 120° cleavage planes. It is automorphic and crystallizes as prismatic crystals between 1 mm and 2 mm in size (Figs. 2c and 2d). Biotite (5%) occurs as brown flakes in longitudinal or basal sections. It is sub-automorphic to xenomorphic. Small biotite flakes sometimes form thin bands between quartz, alkali feldspar, and plagioclase (Figs. 2b–2e).

Fig. 2. Microphotograph of granite in the study area: a) Macroscopic appearance, b) LPA with joined minerals, c) Large quartz crystals, d) Sub-grains of quartz and inclusion of apatite in plagioclase, and e) Plagioclase minerals showing polysynthetic macles.

The accessory phase of the rock consists of apatite embedded in plagioclase (Fig. 2d) and opaque minerals (Fig. 2e). The secondary phase is chlorite (Fig. 2b).

Diorite outcrops very widely around the town of Farchana, and near the village of Kourdjindi, on the road leading to Adré. It outcrops in slabs and blocks on hillsides and also in riverbeds (Fig. 3a). The diorite is sometimes cut by veins of pegmatite and quartz veins (Fig.3b). In the field, the diorite is dark grey in colour. Microscopically, the rock has a heterogranular texture (Figs. 3a and 3b). It consists of plagioclase, quartz, alkali feldspar, amphibole, biotite, pyroxene, sphene, apatite, and opaque minerals. Plagioclase (65%–70%) is a more or less altered sub-automorph. It can be recognized by its polysynthetic albite macle and is often associated with amphibole (Figs. 3e–3f). Alkali feldspar (orthoclase and microcline), whose abundance varies between 10% and 15%, is automorphic to sub-automorphic. Microcline is automorphic. It occupies large areas. It is often in the form of Scottish tissue (Fig. 3c). Orthoclase is sub-automorphic and often contains inclusions of opaque minerals. It is often in frank contact with biotite and amphibole (Figs. 3e and 3f).

Fig. 3. Mode of outcrop and microphotography of the diorite: a) Outcrop of the diorite in the form of a block, b) Pegmatite vein intersecting the diorite, c) Adjoining microcline minerals, d) Biotite and amphibole flakes, e) and f) Plagioclase minerals showing polysynthetic macles.

Amphibole (15%–20%) is sub-automorphic to xenomorphic, up to 5 mm in size. It is greenish in colour, which is characteristic of green hornblende. It has a low relief. The amphibole shows quartz ocelli (Figs. 3d–3f). Biotite (5%–8%) is brown in colour, poorly cleaved, and small in size, appearing as more or less oriented elongated flakes. It is often associated with amphibole (Figs. 3c–3e). Quartz (2%–5%) is less abundant and occurs in xenomorphic crystals. It is recrystallization quartz included in the amphibole and forms an association with it (Figs. 3c–3f). Pyroxene (2%–3%) is very rare in the rock. It occurs in the form of destabilized crystals whose relics can be seen around the amphibole, showing quartz ocelli and associated with plagioclase, sphene, and opaque minerals (Fig. 3c). Quartz (2%–5%) which is less abundant, occurs in small xenomorphic crystals. It is recrystallisation quartz included in the amphibole and forms an association with it (Figs. 3d and 3f).

Geochemical Characterisation

Geochemical analyses on total rock were carried out on eight (08) samples, including four (04) diorite samples and four (04) granite samples. The data obtained on major elements, trace elements, and rare earths are shown in the appendix (Table I).

Table I. Analyses of Major (WT%) and Trace Elements (PPM) of Granitoids Farchana-Hadjerhadid Sector

Element	R1	R2	R3	R5	R6A	R6B	R7	R8
SiO2	56.17	69.39	56.24	67.81	60.01	68.2	71.67	55.62
Al2O3	14.97	13.93	15.46	14.05	14.05	15.14	11.95	14.99
Fe2O2t	12	4.3	9.36	4.55	9.59	3.18	5.52	11.31
MnO	0.129	0.078	0.113	0.049	0.109	0.032	0.076	0.126
MgO	2.66	1.55	3.35	0.66	2.12	0.32	0.24	2.82
CaO	6.45	5.2	6.17	2.15	5.56	1.66	0.477	6;73
Na2O	2.92	4.42	3.36	3.47	2.88	2.74	4.22	2.72
K2O	1.83	0.64	2.47	5.25	2.02	6.26	4.22	1.55
TiO2	2.602	0.422	2.073	0.569	2.074	0.375	2.74	2.746
P2O5	0.39	0.14	0.67	0.2	0.36	0.25	0.25	0.33
LOI	0.38	0.49	0.15	0.35	0.86	0.6	0.27	0.47
TOTAL	100.5	100.6	100.4	98.9	99.63	100.1	98.73	98.4
Sc	13	7	16	3	12	4	6	12
Be	1	2	2	3	2	<1	1	1
V	204	47	183	43	163	49	10	200
Ba	555	415	904	1386	531	1104	2172	323
Sr	499	809	641	259	424	212	185	485
Y	26	15	21	8	22	9	14	15
Zr	283	166	307	349	259	265	559	137
Cr	30	40	50	30	<20	20	<20	20
Co	40	34	10	29	9	25	8	4
Ni	60	20	50	<20	30	<20	<20	70
Cu	50	<10	50	60	50	150	120	50
Zn	130	50	110	120	140	90	100	140
Ga	23	16	24	26	24	24	19	22
Ge	2	1	2	1	2	1	1	2
As	<5	<5	<5	<5	<5	<5	<5	2
Rb	36	8	81	169	66	195	94	44
Nb	19	8	19	17	17	7	6	18
Mo	<2	<2	<2	<2	<2	<2	<2	<2
Ag	0.8	0.5	1.1	1.4	0.9	1	2	0.5
In	<2	<2	<2	<2	<2	<2	<2	<2
Sn	2	2	3	2	2	1	1	1
Sb	<0.5		<0.5	<0.5	<0.5		<0.5	<0.5
Cs	<0.5	0.5	0.9	0.5	0.7	<0.5	0.8	0.5
Hf	6.7	4.2	71.7	8.9	6.7	7	10.6	3.5
Ta	1.3	0.8	1.3	1.2	1.3	0.3	0.3	1.2
W	1.3	1.3	1.2	1.3	0.3	0.3	<1	2
Ti	0.1	<0.1	0.3	0.6	0.3	0.8	0.4	0.1
Pb	6	9	13	21	8	31	13	8
Bi	<0.4	<0.4	<0.4	<0.4	<0.4	<0.4	<0.4
Th	4.6	8	12	23.7	7.5	33.9	8.2	4.1

In total rock, diorites have the SiO₂ content of intermediate rocks (55.62%–60.01%). They have higher Al₂O₃ values than granites (14.05%–15.46%) and Fe₂O₃t (9.36%–12%). However, they have low levels of TiO₂ (2.074%–2.74%), MnO (0.109%–0.129%), MgO (2.12%–3.35%), CaO (5.56%–6.73%), Na₂O (2.72%–2.92%), K₂O (1.55%–2.47%) and P₂O₅ (0.33%–0.67%). Alkali sums are relatively low (4.27 < Na₂O + K₂O < 4.9%). Na₂O/K₂O ratios vary between 1.36 and 1.75. Granites have higher SiO₂ contents than diorites (67.81%–71.67%). They have high levels of Al₂O₃ (11.95%–15.4%), Fe₂O₃t (3.18%–5.52%), K₂O (0.64%–6.26%) and Na₂O (2.74%–4.42%). On the other hand, they have low levels of TiO₂ (0.375%–2.74%), MnO (0.032%–0.078%), MgO (0.24%–1.55%), CaO (0.477%–5.2%) and P₂O₅ (0.09%–0.25%). Alkaline sums are high (5.06 < Na₂O + K₂O < 9.65%); Na₂O/K₂O ratios vary between 0.54% and 0.90%.

In the diagram presented by Irvine and Baragar [11], the sum of alkalis = f (SiO₂) (Fig. 4a), except for sample R6B which belongs to the alkaline series, the granitoids studied belong to the sub-alkaline series. The samples analysed have the composition of intermediate to acid rocks. The diorites generally occupy the diorite domain, and the granites are plotted in the granite, granodiorite, and syenite domain. In K₂O vs. SiO₂ diagram (Fig. 4a) from the classification of [12], the granitoids belong to the calc-alkaline series that is moderately to strongly potassic and weakly shoshonitic (Fig. 4b).

Fig. 4. Chemical classification Na₂O + K₂O vs. SiO₂ from [13], adapted to plutonic rocks specifying the limit between alkaline series and subalkaline series from [11] (a), Chemical affinities of rocks from the Farchana-HadjerHadid sector specifying the Subdivisions of subalkaline rocks (b), Diagram A/NK vs. A/CNK (in molar %) from [13]. A/CNK = Al₂O₃/(CaO + Na₂O + K₂O) and A/NK = Al₂O₃/(Na₂O + K₂O) (c), Fe + Mg + Ti vs. Mg/Mg + Fe (cationic) diagram from [15], distinguishing associations (d).

In the diagram (A/CNK vs. A/CK) presented by Maniar and Piccoli [13], the Farchana-Hadjer-Hadid granitoids have an A/CNK ratio 1.1 and the A/NK ratio >1 (mole %). This diagram indicates that the Farchana-Hadjer-Hadid granitoids are metaluminous to weakly, so, they are type I granitoids (Fig. 4c) and belong to the ferriferous associations (Fig. 4d).

The Harker diagrams (Fig. 5) of the oxides Al₂O₃, Fe₂O₃t, P₂O₅, CaO, MgO, MnO, TiO₂, Na₂O and K₂O as a function of SiO₂ show more or less well-defined evolutionary trends in granites and diorites. Concentrations of oxides such as Al₂O₃, Fe₂O₃, CaO, MgO, TiO₂, MnO, and P₂O₅ decrease from diorites to granites as silica content increases. On the other hand, K₂O values increase with silica content.

Fig. 5. Hacker diagrams showing the evolution of oxides as a function of silica. It is the same legend as Fig. 4.

The diorites are moderately enriched in REE (∑REE = 125.89–265.91 ppm). The spectra show an enrichment in light rare earths, LREE, compared with heavy rare earths, HREE ((La/Yb)_N = 13.15–21.54). LREEs are more fractionated ((La/Sm)_N = 2.43–3.48) compared to HREEs which are not very fractionated ((Dy/Yb)_N = 1.12–6). The REE spectra (Fig. 6a) show very weak negative europium (Eu) anomalies Eu/Eu^* = (0.86–0.94). Granites are moderately enriched in REE (∑REE = 154.51–415.27 ppm). REE spectra show an enrichment of LREE compared with HREE ((La/Yb)_N = 12.83–172.72). LREE are less fractionated ((La/Sm)_N = 2.43–2.98), compared with HREE ((Dy/Yb)₂ = 1.56–13.37). The REE spectra in granites have very low negative anomalies in Europium (Eu/Eu^* = 0.63–0.89). The multi-element spectra of diorites normalized to MORB show positive anomalies in Rb, Ba, and Th and weak negative anomalies in Nb − Ta (Fig. 6b), samples from granites show positive anomalies in K, Rb, Ba, and Th and negative anomalies in Ta, Nb, P₂O_5, and TiO₂.

Fig. 6. Spidergram of REE of diorite and granite normalized with respect to chondrites from de [14] (a), Spidergram of multi-elements of diorite and granite normalized with respect to MORB values from [16] (b).

Discussion

Source

The diagram (A/CNK vs. A/CK), the plotted samples have an A/CNK > 1.1 ratio and the A/NK > 1 ratio (Fig. 7a). This diagram indicates that the samples from the study area are metaluminous to slightly hyperaluminous, are iron-bearing and belong to type I granitoids. These parameters are similar to those of the Guera Massif granites studied by [17]–[21] and the Mayo-Kebbi granitoids studied by [3], [22]. According to the Al₂O₃/(MgO + Fe₂O₃t) vs. CaO/(MgO + Fe₂O₃t) diagram (in molar proportion) [23], (Fig. 7a), the samples analyzed are in the range of rocks whose magma is produced by partial melting of metabasalts and/or metatonalites. This result is consistent with Type I granites, illustrated by the A/CNK-A/CN molar diagram and the presence of accessory minerals such as sphene and apatite. This result is similar to those of the granitoids of the Ouaddai Massif described by [10]. The continuous evolution of the rocks from basic to intermediate to acidic terms suggests that the Farchana-Hadjer-Hadid granitoids have the same magmatic source, whose magma derived from the melting of metabasalts evolved by assimilation and fractional crystallisation.

Fig. 7. Nature of the source of the granitoids in the study area (a), Geotectonic environment discrimination diagrams from [16] showing the position of the samples in the different geodynamic contexts. VAG: volcanic arc granitoids; Syn-COLG: Syn-collisional granitoids; WPG: intra-plate granitoids; ORG: mid-ocean rift granitoids (b) and (c).

Geodynamic Background to the Formation of the Massif

The geodynamic context of the emplacement of the Farchana-Hadjer-Hadid plutonic Massif takes into account petrographic and geochemical characteristics.

In the Y vs. Nb and (Yb + Ta) vs. Rb tectonic discrimination diagrams (Figs. 6c and 7b) of [16], the granitoids of the study area occupy the domain of volcanic arc granitoids. Highly potassic calc-alkaline magmatism is typical of the post-collisional tectonic context but can occur in active continental margins [2], [24]. The Farchana-Hardjer-Hadid granitoids show a moderately to strongly potassic calc-alkaline affinity similar to that of late-to post-orogenic granitoids previously affected by subduction phenomena [24]. In the Rb vs. (Yb + Ta) discrimination diagram, these granitoids are grouped together in the upper part of the diagram, suggesting a post-collisional context.

Conclusion

The aim of this work was to make a contribution a contribution to the petrography and geochemistry of the granitoids of the Farchana-Hadjerhadid sector.

In petrographic terms, the study area is made up of plutonic formations represented mainly by diorites and granites. The diorites are made up of amphibole, biotite, plagioclase, pyroxene, and quartz. Granites are made up of orthoclase, plagioclase, biotite and quartz. The granitoids in the study area are moderately differentiated, belonging to the medium to highly potassic calc-alkaline series. They are ferriferous type I metaluminous granitoids (A/CNK < 1.1).