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Mineral Chemistry and Descriptive Petrology of the Pan-African High-K Granitoids and Associated Mafic Rocks from Mbengwi, NW Cameroon: Petrogenetic Constraints and Geodynamic Setting

Benoît Joseph Mbassa*, Emmanuel Njonfang, Caroline Neh Ngwa, Michel Grégoire, Zénon Itiga, Pierre Kamgang, Mfomou Ntepe, Jesús Solé Viñas, Mathieu Benoit, Jacques Dili-Rake, Ferdinand Mbossi Eddy

Institute for Geological and Mining Research, Branch for Geophysical and Volcanological Research, P.O. Box 4110, Yaoundé Cameroon

Abstract: The Mbengwi Pan-African high-K calk-alkaline I-type plutonic rocks consist of granitoids and monzodiorites. These granitoids have a rather homogeneous mineralogical composition made up of calcic amphiboles, ferromagnesian and lithio-aluminous micas, quartz, feldspars, oxides, titanite and incidentally of sulphides, magmatic epidote, apatite, zircon, chlorites and carbonates. Plagioclase compositions range from Na-albite to andesine. Micas are Mg-biotite in monzodiorites and Mg-biotite, Fe-biotite, siderophyllite, lepidomelane, muscovite and phengite in granitoids. Magmatic amphiboles are made up of Fe-hornblende, Mg-hornblende, Fe-edenite, Mg-hastingsite or edenite whereas post-magmatic amphiboles are actinolite. Trace elements analyses reveal: i) low rare earth elements content in monzodiorites (average = 199.9 ppm) compared to granitoids (average = 404.65 ppm); ii) a weak to strong fractionation ((La/Lu)N = 4.5 - 102.96), iii) an enrichment in LILE and LREE relative to HFSE and HREE, and iv) negative anomalies in Rb, K, Sr, Ti, Eu and positive ones in Th and La. Mineralogical and whole rock geochemical results reveal that magmas were produced by vapor-present partial melting of one or several heterogeneous igneous protoliths relatively rich in potassium. The investigated rocks were emplaced between 4 and 34 km deep, at pressure ranging from 1.2 to 9.4 Kbars and were not subjected to post-magmatic alterations according to their Zr/Hf values (> 20). Their differentiation process is the combined result of fractional crystallization and multiple mixing and mingling.

Keywords: Mbengwi, Northwest Cameroon, Pan-African, minerals chemistry, geochemistry

Received May 30, 2020; Revised June 22, 2020; Accepted July 01, 2020

Citation: Benoît Joseph Mbassa, Emmanuel Njonfang, Caroline Neh Ngwa, Michel Grégoire, Zénon Itiga, Pierre Kamgang, Mfomou Ntepe, Jesús Solé Viñas, Mathieu Benoit, Jacques Dili-Rake, Ferdinand Mbossi Eddy, Mineral Chemistry and Descriptive Petrology of the Pan-African High-K Granitoids and Associated Mafic Rocks from Mbengwi, NW Cameroon: Petrogenetic Constraints and Geodynamic Setting. Journal of Geosciences and Geomatics, 8 (2). 58-75. January 2020. doi:10.12691/jgg-8-2-2

1. Introduction

Granitoids, commonly known as the major components of the continental crust, are mainly associated with syn-collisional orogenic settings. Several petrogenetic models of granitoids have been evidenced including: (1) the partial melting of crustal rocks [1,2]; (2) mixing and/or mingling of maﬁc mantle melts and crustal felsic magmas [3]; and (3) differentiation of primary juvenile mantle-derived material [4,5]. The post-collisional magmatism typically generated after lithospheric thickening involves variable sources, with voluminous intrusions of metaluminous/peraluminous granitoids up to alkaline-type series, and may represent a powerful tool to deliver information on crustal processes and compositions [6]. In Cameroon, this magmatism is of great interest not only to decipher the post-Pan-African crustal evolution but also the breakup of the Western Gondwana supercontinent. Although we can mention some few studies such as the geological reconnaissance work carried out by [7] and [8] indicating the predominance of syn-tectonic potassic calc-alkaline granites and the recent specific studies done by [9,10], no thorough research work focused on the genesis of granitic rocks has yet been carried in our studied area. Therefore, new minerals chemical data, bulk rock major and trace elements contents and Sm-Nd isotopic data of the Mbengwi plutonic rocks are capitalized in this paper in order to i) constrain their geodynamic setting and ii) discuss their petrogenetic conditions.

2. Geological Framework

The Pan-African fold belt in Cameroon (Figure 1b) includes three domains [11] i) the southern domain made up of Pan-African meta-sedimentary units [12] which was thrusted onto the Archaean Congo craton towards the South [13]; ii) The central domain, located between the Sanaga fault and the Tcholliré-Banyo fault, comprises the Adamawa fault and other anastomosed faults, which constitute the Central Cameroon Shear Zone (CCSZ); and iii) The northern domain which has been affected by at least two magmatic episodes: the earlier (640- 620 Ma) corresponds to pre- to syn-tectonic calc-alkaline granitoids and the younger (≈ 580 Ma) to late tectonic granitoids [14]. The tectonic evolutionary models of the Pan-African belt in central and southern Cameroon involve an initial extension responsible of the emplacement of early Neoproterozoic sedimentary basins followed by subduction and collision between the Saõ Francisco-Congo craton, the West African craton, and the Saharan metacraton at 600 Ma [15,16], or between the Congo craton and the Saharan metacraton [17]. The resulting structures include the N70E shear zones underlined by the CCSZ, the Sanaga fault (SF) and the granulitic and migmatitic thrust sheets overriding the Congo craton [18]. Three successive tectonic events related to Pan-African collision and post-collision evolution, have been identified in Cameroon by [19] i) crustal thickening (630-620 Ma); ii) left lateral wrench movements (613-585 Ma) and iii) right lateral wrench movements (585-540 Ma) mainly marked by the CCSZ. According to their respective periods of emplacement, three main plutonic massifs have be distinguished by [20]: 1) the early pre-to syn-D₁ diorite-granodiorite strips, 2) the mostly syn-D₂, syn-kinematic porphyritic and fine-grained granites, enclosing mafic and intermediate intrusions, and 3) the late Ediacaran-Cambrian post-kinematic subcircular alkaline granitoids.

The Mbengwi area, geographically bounded by latitudes 6°06′ and 5°58′ North and longitudes 9°57′ and 10°06′ East (Figure 1c), is located in the northern domain of the Pan-African fold belt in Cameroon. The studied area consists of a Pan-African basement rocks partially covered or intruded by recent alkaline formations dated at 28.2 Ma [21]. The alkaline rocks represent a bimodal series ranging from basanites to rhyolites with a gap between 50 and 62 wt% of SiO₂. The calc-alkaline Pan-African rocks typically of I-type, are metaluminous to slightly peraluminous, displaying high-K to shoshonitic features. They result from melting of the lower continental crust with variable contribution of the oceanic crust [9].

Figure 1. Geological setting of the studied area; a) location of Cameroon in Africa; b) Simplified geological map of Cameroon showing the main Pan-African domains. AF=Adamawa fault. SF=Sanaga fault. TBF= Tcholliré-Banyo fault. CCSZ= Central Cameroon shear zone. NT=Ntem complex; DS= Dja series; NS= Nyong series. 1= southern. 2= central domain. 3= northern domain; c) Geologic sketch map of the studied area

3. Sample Materials and Analytical Methods

3.1. Sample Collection and Specimen Preparation

Thirty two (n = 32) fresh rock samples of plutonic rocks were collected from various lithology, and all of them were prepared at Geosciences Environnement Toulouse, University of Toulouse 3 (France).

For petrographic observations and mineralogical analyses, standard-sized 27 × 46mm polished thin slices were prepared following standard procedures. 25 x 40 mm sections of rock material were mounted with epoxy on carrier glass, lapped with carborundum paper up to 2000 mesh to ~30 μm, and finished with diamond pastes 4-2-1-0.25μm on a cloth to achieve a mirror finish. Then after, they were coated with ~50 nm carbon prior to electron-probe micro-analysis to minimize charging in the electron beam.

For geochemical analyzes, rock powders were also obtained following standard processes. About 2kg of a rock sample collected from the field and initially cleared of its weathered parts is comminuted in a Tungsten jaws crusher to less than (<) 0.5cm, introduced into an agate mortar (previously cleaned with sea sand, water, alcohol and then dried with compressed air) and hosted into the SIEBTECHNIK brand mill for 12 minutes to obtain a fine powder. The powders obtained were then homogenized and split by a RETSCH cone-and-quartering then stored in hermetically sealed bottles for the geochemical analysis.

3.2. Analytical Methods

3.2.1. Mineral Main Element Chemistry

Mineral main element composition was determined using carbon-coated thin sections in a CAMECA S X 50 Electron probe micro analyzer of Geosciences Environnement Toulouse. The analytical conditions were 15 kV accelerating voltage and the beam size was 2×2 μm under 10 or 20 ηA, according to the resistance of the mineral to the electronic beam. Acquisition times were 10s for the peak, and 5s on both sides of the peak, for an analyzed volume of 5 μm³. Kα lines were used. Raw main and trace elements data were ZAF-corrected using the PAP-algorithm of [22].

3.2.2. Bulk Rock Major and Trace Element Geochemistry

Major elements and trace elements were analyzed by X-ray fluorescence spectrometry (XRF) at the École des Mines de St-Étienne (France) and at ALS Minerals Laboratory of Sevilla (Spain) using inductively coupled plasma-optical emission spectrometry (ICP-OES) and mass spectrometry (ICP-MS) respectively. The precision of bulk rock main elements measurements performed was usually better than 1% for both SiO₂ and TiO₂, 2% for Al₂O₃ and Fe₂O₃, and better than 4% for the other major oxides. The Standard deviation for trace elements analysis was typically below 2 wt%, 0.92 wt% for SiO₂, 326.6 ppm for TiO₂, 0.45 wt% for Al₂O₃, 0.09 wt% for Fe₂O₃, 75.55 ppm for MnO, 0.14 wt% for MgO, 0.26 wt% for CaO, 0.19 wt% for Na₂O, 753.10 ppm for K₂O and 249.50 ppm for P₂O₅. Regarding bulk rock trace element determinations, calibration was performed using international reference samples (some of which were also run as unknowns in order to determine accuracy and detection limits). Mean accuracies were generally better than 5%, and the detection limits for trace elements range from 0.01 ppm (REE) to 20 ppm (Zr).

For X-ray fluorescence analysis, pressed pallets of rock samples were prepared by mixing carefully 4.5g of homogenized sample (powered to <200 mesh ASTM size) with 0.3g wax (C₁₈H₃₆O₂N₂) as additive binder. The mixture was pressed by applying a 200 kN force to get a uniform disk of 40 mm diameter over boric acid backing in an aluminum cup. For ICP-MS), 0.1 g of <200 mesh size powdered sample was fused with a mixture of 0.15g lithium tetraborate (flux) and 0.15g of anhydrous lithium metaborate (flux) in a same Pt container. The cold mass was taken into solution with 25 ml of 8% HNO₃ then the solution was taken in 250 ml volumetric flask and made the volume up to 250 ml maintaining 4% HNO₃ medium and 10 ppb In. In XRF and ICP-MS analyses, standard reference materials W-2A [23] and GSD-7 [24] were run under the same conditions to check analytical accuracy.

3.2.3. Sr and Nd Isotopic Analyses

Sr and Nd isotopic measurements were eluted after HF/HNO₃ digestion using Eichrom Sr-Spec, Thru-Spec and Ln-Spec resins. The measurements were performed on a Finnigan 261 multicollector thermal-ionization mass spectrometer. Sr isotopic ratios were corrected for mass fractionation with normalization to ⁸⁶Sr/⁸⁸Sr = 00.1194. Replicate analyses of the NBS 987 standard yielded an average ⁸⁷Sr/⁸⁶Sr value of 0.710255±0.00002. Nd isotopic data were corrected for mass fractionation by normalization to ratio ¹⁴⁶Nd/¹⁴⁴Nd = 00.7219. Replicate analyses of the La Jolla Nd standard yielded an average ¹⁴³Nd/¹⁴⁴Nd value of 0.511850±0.00001.

4. Results

4.1. Field Data and Petrography

Three main lithology types are observable: granitoids, mafic intrusions and incidentally micaschists. The studied granitoids (monzonite, Qz-monzonite, granodiorite, and granites) are medium- to coarse-grained while the mafic enclaves (monzodiorites) are microgranular. The granitoids are almost ubiquitous and mostly occupy high altitudes. They outcrop either as decametric to hectometric slabs (Figure 2a) as elongated massifs, as domes (Figure 2b), or as metric boulders of variable shapes (Figure 2h). These granitoids are locally intersected by various veins and usually enclose mafic intrusions.

Many kinematic markers such as foliation, schistosity, shears, stretching lineations, fractures and folds are perceptible on granitic rocks. Two types of foliations were identified: i) a gneissic foliation materialized by a rough alternating of light quartzo-feldspathic and dark ferromagnesian layers and ii) a granulometric or textural-type foliation due either to the succession of parallel aplitic veins within the medium to coarse grained granite, or to an alternating pegmatitic layers and medium-grained granite (Figure 2c). The schistosity, globally oriented N-S is outlined by the alignment of micas flakes into the foliation planes or by the stretching of quartzo-feldspathic ribbon boudins conferring them an eyed appearance. This schistosity is locally internal, and rather oblique to the boudins stretching (Figure 2e). Shears are sinistral or dextral, non-coaxial, and tend to reorient the internal structures (Figure 2d). They are responsible of the formation of tension veins. The folding is evidenced by the presence of aplitic folded veins within porphyritic granitoids and the twisting of quartzo-feldspathic boudins (Figure 2f and Figure 2g) while faults are observable on the quartzo-feldspathic veins (Figure 2i).

At microscopic scale, the studied rocks are mainly characterized by a mylonitic structure, reflected by the presence of quartz (Qz) and feldspars (Fsp) megacrysts molded by microcrystals (Qz, Fsp, and mafic minerals). Three crystal sizes are identifiable: large (>1 mm), medium (1-0.5 mm), and small (≤0.4 mm) crystals. The primary mineral assemblage consists of amphibole (Amp), biotite (Bt), alkali-feldspar (Afs), plagioclase (Pl) and Qz.

In granitoids quartz appears both as interstitial anhedral fine grains usually filling cracks affecting Fsp or Amp and as medium to coarse grains. The phenocrysts are generally recrystallized with undulating extinction and often enclose Pl, Bt or Afs.

Plagioclases are tabular-shaped, often zoned and frequently display bent twin lamellas and deformational kink bands or cross twinning in some granites. The less developed crystals are usually included in Afs while the phenocrysts often enclose Amp, Bt, chlorite (Chl), opaque minerals, zircon (Zrn) or apatite (Ap). Afs are anhedral to subhedral and exhibit distinct cross-hatched and Carlsbad twins in some granites. The megacrysts show a dusty surface due to alteration and are poekilitic (pyrite + Bt + Zrn + opaque minerals + Pl + Ap) with frequent spark perthites and myrmekites.

Amp has subhedral to anhedral habit and is mostly from 0.7 x 1.5 to 0.1 x 0.3 mm in size. Crystals are generally pleochroic brown to green or bluish, frequently zoned, and some crystals have corroded margins. Microcrystals are interstitial while the phenocrysts commonly include Bt, Ap and opaque minerals.

Bt occurs as brown or greenish fine flakes, displaying a regular longitudinal cleavage. Fine crystals are interstitial or included in Fsp while phenocrysts commonly enclose Ap, Zrn, and opaque minerals. Muscovite (Ms) and phengite (Ph) crystallize at the expense of Amp in two-mica granites and appear as elongated flakes frequently cleaved and regularly included in Afs.

Figure 2. Some field observations of the studied rocks: a): Slab-shaped granitoid; b) Dome outcropping; c): textural or granulometric foliation; d): Sinistral shear: e): symetrical boudin, parallel to the foliation but with an oblique internal schistosity suggesting a dextral motion; f): fold illustrated by a flexure of a granite vein; g) fractured tension vein; h - i): Normal fault observed in a granite boulder (the direction and the amplitude of the displacement are perceptible on the quartzo-feldspathic vein)

The accessory phase consists of opaque minerals, titanite (Spn), Ap, Zrn and prehnite (Prh). Opaque minerals generally appear as rounded, angular or skeletal grains, preferentially concentrated in the interstitial microcrystalline phase or included in Fsp, Bt or Amp, Qz or Spn. Phenocrysts enclose zircon and apatite and probably resulted from Spn and Bt alteration. Spn is euhedral to subhedral in shape and occurs as frequently zoned phenocrysts or as inclusion in Fsp. Zrn is acicular, polygonal, cubic to rounded, shaped and frequently included in opaque mineral or Bt. Ap has elongated to hexagonal shape or occurs as prismatic to acicular microcrystals mostly included in spinel (Spl), Bt and Fsp. Brown or green interstitial epidote (Ep) is restricted to some granites. Prh crystallizes as interleaving lenses within Bt or Ms. Due to late alteration processes, Bt is locally replaced by Chl, Amp by Chl, Ep or calcite (Cal), iron-oxides by goethite, while sericite needles crystallized at the expense of Fsp.

4.2. Mineral Chemistry

4.2.1. Feldspars

Selected analyses of feldspars correspond both to Afs and Pl in the Or-Ab-An ternary diagram (Figure 3). Alkali feldspars compositions range from Ab to Or in almost all the granitoids except in one sample of two-mica granite (E₁₄₅) where only Ab is found. Pl compositions range from andesine to oligoclase in monzonites; andesine to Ab in Qz-monzonites; oligoclase in granodiorites; oligoclase to Ab in Amp+Bt- granites, Bt-granites, two-mica granites (with dominance of oligoclase) and in the leucocratic granite (with high proportion of albite). In monzodiorites Afs is Or, while Pl are more calcic than those found in granitoids with compositions ranging from oligoclase to andesine with labradorite (An53.9) in the core.

The Fsp compositions evolution during the differentiation is characterized by: 1) an overall decrease of CaO, Al₂O₃, increase of Na₂O, and rather constant K₂O in plagioclase series; and 2) little decrease of K₂O coupled with an increase of Na₂O contents of Afs. The plagioclase zoning observed is marked by a reduction of Ca contents from the core to the rim of crystals. This depletion commonly ends in some crystals by a compositional evolution from andesine at the rim to labradorite in the core (e.g. E₆₃), or from andesine in the core to oligoclase at the rim of crystals (e.g. E₁₂₇). However, a reverse evolution marked by an increase of the Ca contents from the core to the rim is also observed in some crystals. A significant change in K₂O and Na₂O contents in Afs on either both sides of macle plane, or from the core to the rim of the crystals is also noticed.

4.2.2. Micas

The analyzed micas belong to the Fe-Mg and Li-Al series according to [25].

The Mg-Fe series is made up of: i) Mg-Bt in monzodiorites, monzonites, Qz-monzonites, granodiorites, and Bt-granites; ii) Fe-Bt in Qz-monzonites, Bt-granites, Amp+Bt-granites and two-micas granites; iii) siderophyllite in two-micas granites and iv) lepidomelane in Amp+Bt- granites (Figure 4a-c).

Figure 3. Ternary plots of feldspars compositions from Mbengwi plutonic rocks after [73]. (a) feldspars from monzodiorites, (b) feldspars from intermediary granitoids, and (c) feldspars from granites

Figure 4. Plot of the micas from Mbengwi in the (Mg - Li) vs (Fet + Mn +Ti-AlIV) discriminating diagram of [25] (a - c); (d) the Fe²⁺/(Fe²⁺ + Mg) vs. Altotal (a.p.f.u.) coordinates of the Ann - Phl - siderophyllite - Eas quadrilateral. Tetrahedral micas are subdivided after [26] and [27]; and (e) the Al₂O₃ vs. MgO tectono-discriminating diagram of [74]. The Li contents in (a - c) were computed using the equation LiO₂ = 0.289SiO₂ - 9.658 [25]. Discrimination trends of Bt in (d) from calc-alkaline and peraluminous granitoids are after [74]. Symboles are after [75]

In the XFe vs. AlT variation diagram of [26] completed by [27] (Figure 4d), micas from granitoids plot within the fields of Fe-poor Bt (meroxene), lepidomelane and peraluminous micas while those from monzodiorites mainly occupy the field of meroxene. The micas FeOT/(FeOT+MgO) ratio ranges between 0.47 and 0.61 in monzodiorites and from 0.57 to 0.99 in granitoids. XMg varies from 0.39 to 0.52 in monzodiorites and ranges between 0.01 and 0.43 in granitoids. The AlT contents gradually increase from meroxene (average: 2.80 a.p.f.u.) to peraluminous micas (average: 3.12 a.p.f.u.). The studied Bt mostly have typical compositions of calc-alkaline environments (Figure 4e) and the use of the TiO2 - Fe*O(FeO + MnO) -MgO ternary diagram of [28] (not shown) led us to distinguish primary Bt mainly present in granites from late-magmatic Bt found in monzodiorites and some monzonites.

Li-Al micas are found only in granites and include both Ms and Ph. Muscovites compositions are characterized by XFe and Na/(Na + K) ratios ranging from 0.72 to 1 and from 0.02 to 0.1 respectively. Compared to Ms, Ph crystals are poor in Al₂O₃ (30.72-30.75 wt%), and rich in MgO (0.97-1.07 wt%) and FeO (4.34-4.26 wt%).

4.2.3. Amphiboles

The plot of selected compositions of Amp in the discrimination diagram of [29] (Figure 5a) confirms that almost all of the analyzed Amp are of magmatic origin. According to the classification scheme of [30] the studied amphiboles belong to calcium amphibole subgroup. They are characterized by a large range of Mg# [100 Mg/(Mg+Fe²⁺): 1.81-81.70], with Si between 6.05-7.92 a.p.f.u, and total alkali contents ranging from 0.01 to 0.81 a.p.f.u. Primary amphiboles are composed of Mg-hornblende in monzodiorites, monzonites, Qz-monzonites, granodiorites and granites and pargasite in monzodiorites, Qz-monzonites, granites and rarely in monzonites. Amphibole with tremolite composition found in a monzodiorite is post magmatic field (Figure 5b). The overall evolution of amphiboles composition both in the granitoids and monzodiorites involves a decrease of MgO, CaO, and SiO₂, in addition to an increase of FeO, Al₂O₃, MnO, and K₂O contents.

4.2.4. Accessory and Secondary Minerals

Opaque minerals are mainly oxides and incidentally pyrite. In granitoids, the Fe-Ti oxides include magnetite (Mag), ilmenite (Ilm), hematite (Hem) and more rarely rutile (not shown), whereas Mag and Ilm are found in monzodiorites. All magnetite crystals are chemically characterized by lower TiO₂ contents (TiO2 < 1.7 wt%), negligible Cr₂O₃ (< 0.2 wt%); YFe³⁺ [100Fe³⁺/(Fe³⁺+Cr+Al)] ratios ranging from 99.55 to 100; Fe²⁺# [100Fe²⁺/(Fe²⁺+ Mg): 99.35 - 100]; Mg/Fe²⁺ (≤ 0.005); Mg# (≤ 0.20), and Fe³⁺/(Fe3+ + Fe²⁺) from 0.65 to 0.67. Ilmenite is richer in MnO (2.3-8.2 wt%) than magnetite (<0.6 wt%) or hematite (<0.3 wt%). All the analyzed Hem encompass very low apparent SiO₂ (<0.2 wt%).

Titanite chemical compositions are characterized by XAl [Al/(Ti+Al+Fe³⁺)] and XFe3+[Fe³⁺/(Ti+Al+Fe³⁺)] ratios ranging from 0.03-0.26 and 0.01-0.06, respectively. Spn crystals found at Mag rims or crystallized at Bt-ilmenite contacts are the most depleted in Ti, the most aluminous and frequently, the most calcic. Some Spn crystals exhibit euhedral shape reflecting their magmatic origin, although their Al₂O3 contents (0.85-6.87 wt%) are of secondary Spn according to [31].

Epidotes have a composition of pistacite with [100Fe³⁺/(Fe³⁺+Al)] ratio varying from 25.8 to 31.6. They are rich in Al₂O₃, FeO and CaO with negligible amounts of TiO2 (< 0.2 wt%), Cr₂O₃ and Mn₂O₃.

Chlorites compositions include ripidolite, picnochlorite and diabantite according to the classification chart of [32] (not shown). They can be considered as iron-rich (XFe: 1.22-2.74) with AlIV values ranging from 1.22 to 2.74 a.p.f.u and (Ca+Na+K) < 0.5 a.p.f.u.

Carbonates are scarce and mostly composed of calcite (97.60-98.07 wt%), although the presence of low contents of siderite (< 0.7 wt%), magnesite (≤ 0.4 wt%), and rhodochrosite (≤ 1.5 wt%) are noticeable.

Figure 5. Amphiboles chemistry in a) the Ca+Na+K (apfu) versus Si (apfu) discriminative diagrams evaluating the primary vs secondary trend of Amp from Mbengwi rocks after [29]; b) the C(AlVI+Fe³⁺+2Ti) vs A(Na+K+2Ca) diagram, following the terminology of [30]

4.3. Whole Rock Geochemistry

Bulk-rock major and trace elements geochemical analyses of representative rock samples from the studied rocks are given in Table 1. Loss on ignition is mostly lower than 2 wt%.

Table 1. Bulk-rock main element oxide (in wt%) and trace element contents (in ppm; μg·g⁻¹ug/) measured by XRF in representative plutonic samples from Mbengwi. LG = leucocratic granite

Figure 6. Chemical classification of the studied rocks in: A) the compositional TAS diagram of [33] B) K₂O vs. SiO₂ diagram illustrating the high-K calc-alkaline and shoshonitic affinities of the Mbengwi plutonic rocks. Subdivision after [35] and [76]; C) A/NK [Al₂O₃/ (Na₂O + K₂O)] vs. A/CNK [Al₂O₃/(CaO + Na₂O + K₂O)] diagram after [36]. The boundary between type-I and type-S granites is after [37]

4.3.1. Major Elements Data

In the classification diagram of [33], the mafic enclaves plot in the monzogabbro and monzodiorite fields, whereas the granitoids samples fall in the monzonite, Qz-monzonite, granodiorite and granite fields (Figure 6A). Due to both the lack of Cpx and the nature of plagioclase they have, all the samples plotting in the monzogabbro field were considered as monzodiorites.

The granitoids samples from Mbengwi area have overall high K₂O (1.7-7.1 wt%), moderate high Na₂O (2.2-5.5 wt%) and moderate Al₂O₃ (12.1-17.1 wt%) contents and low Mg# (6.1 - 40.7). Their CaO/Al₂O₃ molar ratios range from 0.06 to 0.68 and [(Na₂O + K₂O)/Al₂O₃] vary from 0.59 to 1.02. According to [34], almost all granitoids are potassic regarding their Na₂O- K₂O value (< 2) except the granodiorites. Nearly all the studied rocks plot within the high-K to shoshonitic fields in the K₂O vs. SiO₂ diagram of [35], except granodiorites and one sample of Bt-granite (E₂₀) which occupy the field of medium potassic rocks (Figure 6B). Given their position in the diagram of [36] and [37] (Figure 6C), the studied granitoids are mainly metaluminous to weakly peraluminous in composition. Only a Qz-monzonite sample containing normative acmite (E₁₂₅) displays a molar (Na+K) > Al and could be considered as peralkaline according to [38]. Apart from a two-mica granite sample (E₆₁₀) that plots in type-S (A/CNK >1.1) field, these granitoids are mostly of type I. Al₂O₃, TiO₂, Fe₂O3t, MgO, MnO, CaO and P₂O₅ are negatively correlated contrary to K₂O which has a positive correlation with SiO₂ both for granitoids and associated monzodiorites (not shown).

Monzodiorites have moderate K₂O (2.2-3.7 wt%), moderatly high Na₂O (3-4.8 wt%), and high Al₂O₃ (15.4-19.4 wt%) contents. They are mainly potassic ((Na₂O-K₂O < 2) and plot within the high-K to shoshonitic fields (Figure 6B). Their CaO/Al₂O₃ molar ratios vary from 0.66-1.06 while [(Na₂O + K₂O)/Al₂O₃] vary from 0.47 to 0.62, with Mg# ranging between 42.1 and 58.9. They are metaluminous and of type I (A/CNK: 0.63-0.84) (Figure 6C).

Normative composition of monzodiorites (not shown) is marked by the lack of normative Qz alongside the occurrence of olivine (2-18.9 wt%) and nepheline (3.5-6wt%). Granitoids are characterized by i) the occurrence of normative acmite in one Bt-granite sample (E₁₂₅) and ii) the presence of normative corundum, except in samples containing normative diopside.

4.3.2. Trace Elements Data

The whole rock trace elements analyses of granitoids are characterized by moderate Sr (73.3-683 ppm), Y (3.4-51.9 ppm) contents and high Ba (203-1720 ppm), Rb (80-238 ppm), Nb (3.3-33.7 ppm), and Zr (73-809 ppm) contents, when compared with most I-type granitoids, and the average continental crust. The moderate Sr and high Rb contents of these rocks give rise to moderate to high Rb/Sr values (average = 0.73).

Monzodiorites exhibit high Sr (346-1040 ppm; average = 697 ppm), Rb (104.5-180.5 ppm; average = 133 ppm), Ba (300 - 2010 ppm; average= 888 ppm), Nb (9.9-27.2 ppm; average = 15.5 ppm) and Zr (64-697 ppm; average = 217 ppm), but moderate Y (19.5-65.4 ppm; average = 32 ppm) contents, compared with the average continental crust.

Figure 7. Harker diagram of some trace elements. The grey field includes monzodiorite samples

V, Sr and Ni show a compatible behavior over the entire range, while W, Pb and Th appear incompatible (Figure 7). Co, Zn, Y, Nb and Ta change their behavior from monzodiorites to granitoids and could be considered as transitional. Due to the marked depletion in Rb of these granitoids, their K/Rb ratios (117.2-792.7; average = 267.1) are close both to the average of magmatic rocks (230; [39]) and the chondritic values (242; [40]). The granitoids suite exhibit significantly high Rb/Sr ratios (0.14-3.14) compared to the monzodiorites characterized by rather low ones (0.11 - 0.52).

Chondrite normalized incompatible multi-element patterns of both groups (Figure 8) exhibit: i) an enrichment in almost all the analyzed elements relative to chondrite except P; ii) enrichment in LILE compared to HFSE, iii) negative anomalies in Rb, K, Ta, Sr, P and Ti; iv) positive anomalies in Th, La and Ce for all the granitoids and in Y and Tb only in few Bt-granites and Amp+Bt-granites. However, monzodiorites are less enriched in incompatible LILE and HFSE relative to granitoids.

Figure 8. Chondrite-normalized multi-element diagrams using the chondrite normalizing values of [77]

Figure 9. Chondrite-normalized REE diagrams using the chondrite normalizing values of [77]

The granitoids samples have high REE contents (average ∑REE = 232 ppm), with normalized La and Lu values ranging between 40-1548 and 4-37 respectively. The REE fractionation is more pronounced for LREE compared to HREE and the degree of LREE fractionation in granitoids (average (La/Sm)N = 6.77) is higher than the one from monzodiorites (average (La/Sm)N = 2.53). The situation differs for HREE as both granitoids (average (Gd/Yb)N = 3.64) and associated monzodiorites (average (Gd/Yb)N = 2.67) are moderately to fairly fractionated given their almost flat patterns. However, Bt-granites are strongly fractionated according to their average (La/Lu)N = 42.64 and (Gd/Yb)N = 7.75 values. Overall, both granitoids and monzodiorites display enriched, parallel to sub-parallel REE patterns, characterized by moderate to steep slopes (Figure 9) because of their enrichment of the LREE relative to middle and HREE ((La/Yb)N; 4.9-121.6). The REE patterns exhibit negative Eu anomalies except for E₂₀ (Bt-granite sample) with rather variable ranges (Eu/Eu*: 0.08-1.24) for granitoids and (0.46-1.02; average = 0.84) for monzodiorites.

5. Discussion

5.1. Origin of Magmas and Petrogenetic Constraints

The chemistry of granitic rocks is mostly controlled by the composition of the source. Granitoids can result either from fractional crystallization of mantle derived magma, or from partial melting of crustal rocks. Concerning the production of high-K magmas in converging tectonic environments such as the Pan-African fold belt in Cameroon, two main processes have been identified to explain: 1) in continental arc environments, parental mantle magmas enriched in fluids could be contaminated by crustal material during ascent [41]; 2) in syn- to post-collisional environments, crustal rocks could melt following decompression after peeling at the base of the lithosphere [42,43]. Compositional differences of magmas produced by partial melting of those different protoliths under variable conditions of melting are shown in Figure 10. In this diagram, several observations can be made: i) The parental magmas of the studied plutonic rocks probably originated by partial melting of meta-igneous source with a negligible contribution of a metapelite; ii) The magma of the monzodiorites, monzonites and two Qz-monzonites (E₄₂ and E₆₇) derived from partial melting of amphibolite; iii) Granodiorites magmas derived from partial melting of dacite and tonalite; iv) Granites and Qz-monzonites are mostly derived from the partial melting of a metagreywacke and incidentally a metapelitic source (E₁₂, E₆₈, E₆₉).

The abundance of hydrous minerals especially Amp and Bt in monzodiorite and some granitoids, suggests that their parental magmas were relatively water-rich. According to [44], this water is probably provided by major thrust or shear zones that concentrate and channel fluids in the crust. Furthermore, the presence of Spn+ Mag + Qz paragenesis alongside amphiboles having medium to higher Mg# in addition to the very low MgO contents of Mn-ilmenite imply relatively high oxygen fugacity [45]. As stated by [42], the high-K calc-alkaline to shoshonitic and metaluminous characters of the studied rocks involve a metaluminous and relatively K-enriched source. The metaluminous to weakly peraluminous feature as well as the presence of Mag and Spn as main accessory minerals in the studied plutonic rocks suggest that their magma may have been formed by partial melting of a basic component according to [46].

Figure 10. Plot of the Mbengwi plutonic rocks in the molar Al₂O₃/(MgO + FeOT) vs. CaO/(MgO + FeOT) [80]; The composition fields of the magmas produced by experimental melting of various sources are from [78] and [79]

5.1.1. Magma Source

Chemically, as we have mentioned before, the Mbengwi granitoids are weakly peraluminous, with high total alkali content (mostly > 8.1 wt%), low FeOT/MgO (very few > 5) and low Mg#. The low Mg# in addition to their high-K calc-alkaline geochemical features, rule out an origin from the direct melting of a peridotitic mantle source [47], and a mixing between crust- and mantle-derived melts [48]. The crustal origin of the studied rocks already evidenced by [9] is to some extent, confirmed by Rb/Ti and Rb/Sr values. The difference in Rb/Ti ratio values of the monzodiorites (0.01-0.03; average: 0.02), and those of the host granitoids (0.2-1; average: 0.129) suggests that the rock types were derived from different magma sources according to [49]. The Rb/Sr ratios of the granitoids suite fall within a range of 0.15-3.14, with an average of 0.73; while the same ratios range from 0.11- 0.52 with an average of 0.24 for monzodiorites. Such Rb/Sr ratios preclude any participation of mantle materials in the genesis of these rocks since mantle materials possess very low Rb/Sr ratios, (0.1-0.01; [39]). Therefore, the monzodiorites parental magma may probably represent non-differentiated cumulative parts of the granitoids magma.

5.1.2. P and T Crystallization Conditions

Generally, many igneous rocks lack mineral assemblages suitable for thermobarometric analysis. Either the thermodynamic variance is high, or, the phases are characterized by component exchanges easily reset during slow cooling or later thermal events. However, it remains possible to determine crystallization or emplacement temperature and pressure conditions in many granitic intrusions through mineral equilibria less susceptible to subsequent readjustment. We have used plagioclase-hornblende thermobarometry (temperature by [50,51]; pressure by [52] and [53]) and also taken into account the total Al content of Bt to estimate the crystallization pressures for granites lacking Amp, using the equation: P (kbar) = 3.03 x AlT - 6.53 (±0.33) from [54]. Therefore, the depth of solidification was appraised from a pressure-depth relationship of 1 kbar ≈ 3.6 km. The minimum pressure is estimated at 1.22 Kbars in a monzodiorite whereas the maximum pressure is 9.4 Kbars in Amp+Bt- granite, meaning that Mbengwi plutonic rocks crystallized at depths ranging from 4.4 to 33.8 km. The corresponding crystallization temperatures range from 764°C to 537°C in granitoids while those from monzodiorites crystallized between 789°C and 685°C.

The oxidizing or reducing nature of the magmatic environment had been deciphered using XMg (Mg/Mg+Fe²⁺) values of Bt according to [55]. Biotites that crystallized under oxidizing conditions tend to show higher XMg values compared to those formed in a reducing environment [56]. The XMg of Bt from monzodiorites (0.50-0.67) are significantly higher compared to that of Bt from granitoids (0.02-0.58); and therefore, might be consistent with their moderately to strongly oxidized nature.

5.2. Magmatic Differentiation

The Mbengwi plutonic rocks show a relative large range of major and trace elements compositions, as well as Sr-Nd isotopic compositions. Several processes could therefore be envisioned to explain their genesis such as magma mixing and/or fractional crystallization.

5.2.1. Magma Mixing

In basement rocks, the process of magma mixing is commonly evidenced by the occurrence of dark enclaves of varying sizes [57]. Several field observations, petrographic feature (Figure 11a-d) in addition to mineralogical and geochemical evidences are taken into account to decipher the magma mixing.

The presence of multiple round or elliptical fine-grained melanocratic enclaves along with their chilled margins against granitoids is therefore likely a strong evidence for the involvement of magma mixing during the differentiation of the Mbengwi plutonic rocks. The change in the shape of fine-grained mafic enclaves from round to elliptical, the lack of residual minerals (e.g. andalusite, sillimanite, cordierite and garnet) in addition to their relative high hornblende and biotite contents compared to that of the neighboring host granitoids are typical signatures of enclaves formed by injections of mafic material as magma globules into the host granitic magmas during mixing process [58,59]. The contacts between enclaves and host rocks which are not clear, but chilled, are indicative of a limited contamination of these enclaves by the host rock and conversely [57]. The presence of felsic veins within enclaves provides further evidence of magma mixing during the formation of these enclaves.

Figurer 11. Macroscopic sizes and shapes of the mafic intrusions and their relationship with the host rock

Numerous microscopic petrographic features also highlight the mixing. The reverse zoning observed in some plagioclase crystals is a disequilibrium texture that indicates magma mixing. The presence of resorption surfaces in plagioclases and corroded margins of some Amp crystals are somewhat consequences of magma mixing. This particular chemical corrosion is suggestive of disequilibrium system resulting from chemical and/or thermal changes during melting process and possibly points out magma mixing event [60]. Furthermore, the presence of acicular Ap crystals in some monzodiorites has been interpreted by [61] as suggestive of quenching occurred consequently to a mixing of a hot maﬁc melt with a cooler granitic magma.

Amp crystals of the granitoids are slightly richer in Fe³⁺, Si, and Mg than those of monzodiorites. This has been used by [62] as an argument to highlight the importance of magma mixing; although the observed compositional similarity of the plagioclase and Bt phenocrysts from monzodiorites and their host granitoids indicate that the studied monzodiorites were mainly equilibrated with their hosts.

Figure 12. Geochemical and AFC discrimination diagrams of the Mbengwi plutonic rocks. (a) FeOt vs. MgO; (b) Ti/Zr vs. Yb/Hf diagram [65] indicative magma mixing in the evolution of the studied rocks; (c) Sr vs. SiO₂; (d) Eu/Eu* vs. SiO₂; (e-f) SiO₂ vs. ⁸⁷Sr/⁸⁶Sr(i), ¹⁴³Nd/¹⁴⁴Nd(i); (g-h) ⁸⁷Sr/⁸⁶Sr(i) vs. MgO (wt%.) and Sr (ppm)

The use of the FeO -MgO diagram (Figure 12a) reveals that, the monzodiorites do not plot along the fractional crystallization trend, but rather near the mixing line as mafic end-member, while the host granitoids plot as the felsic end-member, with a reasonable linear relationship between the two that reﬂects a magma-mixing trend, similar to those of rocks with hybrid origin [63,64]. The role of mixing in the genesis of the Mbengwi plutonic rocks has been furthermore examined using the Yb/Hf ratio plotted against Ti/Zr (Figure 12b; e.g. [65]). The increasing linear correlations between these ratios are all features expected for magma mixing.

5.2.2. Fractional Crystallization

Fractional crystallization process is well evidenced both in the Harker diagrams and in oxides vs. MgO diagrams. Straight curves symbolize the fractionation of a constant mineralogical assemblage, while slope failures usually indicate a change in the composition of the assemblage [66]. Plots of selected major oxides and trace elements show well-defined negative or positive correlations with increasing SiO₂ content (Figure 7), which reveal fractionation of common mineral phases in the granitic magmas. Both the granitoids and monzodiorites are characterized by the decrease in MgO, CaO, Al₂O₃, and Fe₂O3T contents, as well as the increase in K₂O contents, with increasing silica suggesting that fractional crystallization was dominated by Amp and calcic plagioclase. The high (La/Yb)N ratio values in granitoids (average ≈ 36) contrary to the lower ones in the monzodiorites (average ≈ 11) respectively reflect their strongly and moderately fractionated nature. The observed negative Ti anomaly is related to fractionation of Ti-bearing phases (Ilm, Spn and/or titanomagnetite), while the negative P anomaly likely results from Ap separation. According to [66], fractional crystallization could be incomplete if the residual liquid (or part of it) remained trapped within the crystal matrix. This is overall reflected in our case by the presence of a protomylonitic texture in coarse-grained granitoids marked by the presence of large phenocrysts molded by fine interstitial crystals.

The decreases of Sr contents and Eu/Eu* values with silica rising (Figure 12c-d) in addition to the depletions in Rb, K and Sr, and the general negative Eu anomalies are principally related to feldspar fractionation, particularly in felsic magmas, as Eu is highly compatible in plagioclase and K-feldspars [67]. Such negative Eu anomalies are most likely attributed to the removal of feldspar from the melt by crystal fractionation or the partial melting of a rock in which feldspar is retained in the source. Thus, the difference in the extent of the negative Eu anomalies observed within granitoids indicates that the fractionation of feldspars was more dominant in the generation of Amp+Bt-granites. One Bt-granite sample (E₂₀) displays a slight positive Eu anomaly. This sample showing a small positive Eu anomaly has the lowest LREE and HREE abundances; therefore the small positive Eu anomaly simply results from depletion of LREE and HREE through fractionation of accessory phases.

During the evolution of the studied rocks, fluids fractionation might have been an important process in regards to their average value of the Nb/Ta ratio (~17.5 in monzodiorites; ~15.2 in granitoids) according to [68].

In order to assess the role of assimilation and fractional crystallization (AFC) vs. fractional crystallization (FC) processes in the genesis of the studied rocks, Sr and Nd isotopic ratios were plotted against SiO₂, MgO and Sr (Figure 12 e-g). ⁸⁷Sr/⁸⁶Sr(i) and ¹⁴³Nd/¹⁴⁴Nd(i) ratios remain nearly constant with varying SiO₂ (Figure 12e-f), and the same holds true for ⁸⁷Sr/⁸⁶Sr(i) vs. MgO and Sr (Figure 12h-g). This indicates that the magmatic evolution of the studied rocks was mostly controlled by FC rather than AFC processes.

5.3. Geodynamic Significance

Mbengwi granitoids show many geochemical similarities such as high alkali contents and high XFeOT (≥ 0.70), low Ba and Sr contents, and pronounced Eu anomalies with various high-K syn- to post-tectonic Pan-African granitoids from many localities in Cameroon (e.g. [69]) and with Neoproterozoic transitional to calc-alkaline granitoids from NE Brazil [70].

Generally, high-K calc-alkaline granitoids are related to subduction zones, as it is the case of Pan-African granites of eastern Nigeria, emplaced during a collision-subduction of the eastern edge of the West African craton ([71] and references therein). Several works on the Central African Pan-African chain [72] suggest that it was emplaced in a subduction context (640 Ma; [19]), distinct to the granites from Nigeria that were emplaced in a continent-continent collision context at around 620±10 Ma. This subduction context of our Pan-African rocks is justified by their calc-alkaline nature, negative anomalies in Nb, light LREE-enriched patterns and weak fractionation of MREE and HREE.

6. Conclusions

Mineral chemistry and bulk-rock data of the Mbengwi Pan-African high-K calc-alkaline I-type plutonic rocks have been capitalized to investigate their petrogenetic conditions. The leading conclusions are as follows:

1. The studied plutonic rocks are enriched in LILE and depleted in HFSE. They display ƩREE-enrichment (average ≈ 366.3 ppm), and strongly LREE-fractionated patterns ((La/Yb)N ratios average ≈ 40) with an overall negative anomalies in Rb, Sr, P, Ti, Eu, and positive ones in Th and La.

2. The granitoids and monzodiorites result from partial melting of several heterogeneous protoliths relatively rich in K. The studied rocks were emplaced between 4 and 34 km deep, at pressure ranging from 1.2 to 9.4 Kbars.

3. Their differentiation might be a combined result of fractional crystallization or/and mixing (mingling) processes with negligible assimilation.

Acknowledgements

Authors acknowledge the constructive comments of anonymous reviewers and are also grateful to the editors for the diligent editorial handling of the manuscript.

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