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American Journal of Public Health Research
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The Implementation of a Tele-Homecare System with Service Oriented Architecture and HL7 Message Transmission Standard
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The Influence of Ethanol on Pyruvate Kinases Activity in Vivo, in Vitro, in Silico
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American Journal of Medical Sciences and Medicine
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Clinical and Histological Features in Wegener Granulomatosis
Li Cavoli Gioacchino, Passantino Rita, Bono Luisa, Tortorici Calogera, Ferrantelli Angelo, Giammarresi Carlo, Rotolo Ugo

American Journal of Infectious Diseases and Microbiology
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Dear Label_Name,

Greetings! I hope you are well.

I am writing on behalf of an international publishing house, Science and Education Publishing, USA (http://www.sciepub.com).

Science and Education Publishing (SciEP) is a multidisciplinary open access publisher of Journals covering the fields of Science, Technology and Medicine. All SciEP publications are subject to high-quality peer review, editorial and production processes and are freely available online.

Welcome to the April, 2013 edition of SciEP newsletter, providing monthly summaries of articles on science & education related topics of interest to scientist and professionals. The mission of our journals is to provide an open access forum for publishing new findings on issues pertinent to scientific and academic field. We also invite you to submit original research papers, review articles, short reports, or case studies are considered for publication in our Multidisciplinary Journals.

American Journal of Public Health Research
DOI: 10.12691/ajphr-1-1-3

The Implementation of a Tele-Homecare System with Service Oriented Architecture and HL7 Message Transmission Standard
Ching-Sung Wang

American Journal of Medical and Biological Research
DOI: 10.12691/ajmbr-1-1-2

The Influence of Ethanol on Pyruvate Kinases Activity in Vivo, in Vitro, in Silico
Lelevich Sergey Vladimirovich, Khrustalev Vladislav Victorovich, Barkovsky Eugene Victorovich, Shedogubova Tatyana Aleksandrovna

American Journal of Medical Sciences and Medicine
DOI: 10.12691/ajmsm-1-1-1

Clinical and Histological Features in Wegener Granulomatosis
Li Cavoli Gioacchino, Passantino Rita, Bono Luisa, Tortorici Calogera, Ferrantelli Angelo, Giammarresi Carlo, Rotolo Ugo

American Journal of Infectious Diseases and Microbiology
DOI: 10.12691/ajidm-1-1-2

The Curse of Lymphatic Filariasis: Would the Continual Use of Diethylcarbamazine Eliminate this Scourge in Papua New Guinea?
Anne Lanham, Lillian Mwanri

We have released issues of some journals on February 2013.
American Journal of Public Health Research
American Journal of Medical Sciences and Medicine
American Journal of Medical and Biological Research
American Journal of Pharmacological Sciences
American Journal of Clinical Medicine Research
American Journal of Nursing Research
American Journal of Cancer Prevention
American Journal of Medicine Studies
American Journal of Infectious Diseases and Microbiology
American Journal of Cardiovascular Disease Research
American Journal of Biomedical Research
American Journal of Mechanical Engineering
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Journal of Preventive Medicine
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Dear Label_Name,

Greetings! I hope you are well.

I am writing on behalf of an international publishing house, Science and Education Publishing, USA (http://www.sciepub.com).

Science and Education Publishing (SciEP) is a multidisciplinary open access publisher of Journals covering the fields of Science, Technology and Medicine. All SciEP publications are subject to high-quality peer review, editorial and production processes and are freely available online.

Welcome to the April, 2013 edition of SciEP newsletter, providing monthly summaries of articles on science & education related topics of interest to scientist and professionals. The mission of our journals is to provide an open access forum for publishing new findings on issues pertinent to scientific and academic field. We also invite you to submit original research papers, review articles, short reports, or case studies are considered for publication in our Multidisciplinary Journals.

American Journal of Public Health Research
DOI: 10.12691/ajphr-1-1-3

The Implementation of a Tele-Homecare System with Service Oriented Architecture and HL7 Message Transmission Standard
Ching-Sung Wang

American Journal of Medical and Biological Research
DOI: 10.12691/ajmbr-1-1-2

The Influence of Ethanol on Pyruvate Kinases Activity in Vivo, in Vitro, in Silico
Lelevich Sergey Vladimirovich, Khrustalev Vladislav Victorovich, Barkovsky Eugene Victorovich, Shedogubova Tatyana Aleksandrovna

American Journal of Medical Sciences and Medicine
DOI: 10.12691/ajmsm-1-1-1

Clinical and Histological Features in Wegener Granulomatosis
Li Cavoli Gioacchino, Passantino Rita, Bono Luisa, Tortorici Calogera, Ferrantelli Angelo, Giammarresi Carlo, Rotolo Ugo

American Journal of Infectious Diseases and Microbiology
DOI: 10.12691/ajidm-1-1-2

The Curse of Lymphatic Filariasis: Would the Continual Use of Diethylcarbamazine Eliminate this Scourge in Papua New Guinea?
Anne Lanham, Lillian Mwanri

We have released issues of some journals on February 2013.
American Journal of Public Health Research
American Journal of Medical Sciences and Medicine
American Journal of Medical and Biological Research
American Journal of Pharmacological Sciences
American Journal of Clinical Medicine Research
American Journal of Nursing Research
American Journal of Cancer Prevention
American Journal of Medicine Studies
American Journal of Infectious Diseases and Microbiology
American Journal of Cardiovascular Disease Research
American Journal of Biomedical Research
American Journal of Mechanical Engineering
American Journal of Microbiological Research
Journal of Preventive Medicine
American Journal of Food and Nutrition
American Journal of Hypertension Research
American Journal of Sports Science and Medicine
Journal of Cancer Research and Treatment
American Journal of Zoological Research
Journal of Biomedical Engineering and Technology
American Journal of Microbiological Research
Biomedical Science and Engineering

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Geochemistry and ⁴⁰ar/³⁹ar Geochronology of Amphibolites from Banefo-Mvoutsaha Area, Central Domain of the Pan-African Fold Belt in Cameroon

MVODO Hermine, NGA ESSOMBA TSOUNGUI Philomène Estelle, KOUANKAP NONO Gus Djibril*, KAMGUIA WOGUIA Brice, SIMO Pascal

Department of Geology, HTTC, University of Bamenda, P.O.Box 39 Bambili; Bamenda, Cameroon

Abstract: The Banefo-Mvoutsaha area lies along the Central Cameroon Shear Zone and belongs to the Central Domain of the Pan-African Fold Belt in Cameroon. The amphibolites of the study area are of two types: The banded amphibolites which are associated with migmatites and composed of Hbl + Bt + Qtz + Kfs + Pl ± Grt + Op; and the massive amphibolites which outcrops as discontinuous bands within the garnet bearing-gneisses; its mineral assemblage is made up of Hbl + Qtz + Kfs ± Grt ± Op. The studied amphibolites are ortho-derived, their SiO₂ contents range from 48 to 52 wt%, they are enriched in LREEs and LILEs relatively to HREE, and display negative Eu anomalies (0.65 ≤ Eu/Eu* ≤ 0.96), characteristics of alkali basaltic rocks, which belong to intraplate tholeiitic series. The banded amphibolites samples yield an Ar-Ar age of 618.14 ± 11.43 Ma, while the massive amphibolites samples ages are 547.50 ± 3.15 Ma. The 618 Ma age coincide with the emplacement age of granitoids that intruded the metamorphic basement of the area. The age of 547.50 ± 3.15 Ma is correlated to amphibolitic metamorphism events that mark the end of the Pan-African orogeny in the study area. These new Ar-Ar ages therefore point out at least two phases of amphibolitic metamorphism in the Central domain of the Pan-African Fold Belt in Cameroon.

Keywords: Pan-African, Amphibolite, Tholeiites series, Intraplate basalts, Argon-Argon

Received June 05, 2025; Revised July 07, 2025; Accepted July 14, 2025

Citation: MVODO Hermine, NGA ESSOMBA TSOUNGUI Philomène Estelle, KOUANKAP NONO Gus Djibril, KAMGUIA WOGUIA Brice, SIMO Pascal, Geochemistry and ⁴⁰ar/³⁹ar Geochronology of Amphibolites from Banefo-Mvoutsaha Area, Central Domain of the Pan-African Fold Belt in Cameroon. Journal of Geosciences and Geomatics, 13 (2). 31-43. January 2025. doi:10.12691/jgg-13-2-1

1. Introduction

Collisional orogenic belts are worldwide associated to amphibolitic metamorphic facies from medium to high grades [1,2,3,4]. Amphibolites are therefore used to characterize orogenies which help: 1) to reveal the nature of the parent rocks; 2) to constraint the different tectonic setting and metamorphism phases; 3) and to time the tectono-metamorphism events [5,6,7]. The panafrican orogeny formations in Cameroon (Figure 1) are made up of wide range of magmatic and metamorphic rocks, including amphibolites of various metamorphic grades and ages [8,9,10,11]. The Banefo-Mvoutsaha area located in the southern part of the Cameroon Central Shear Zone comprises metamorphic rocks intruded by granitic plutons (Figure 2). The Banefo-Mvoutsaha granitoid massif is made up of syn- to post-tectonic, high-K calc-alkaline rocks showing chemical composition of granite, quartz-monzonite and granodiorite [12]. Geochronological data from U-Pb zircon dating yielded an age of 602 ± 1.4 Ma for the coarse-grained meta-granites [13]. These granitic bodies are elongated N70°E and were emplaced during and after the D₂ deformation phase along the Cameroon Central Shear Zone (CCSZ). Amphibolites outcrop in Bafeno-Mvoutsaha area and are still poorly investigated [14]. In this paper we present the whole rock geochemical data and the ⁴⁰Ar/³⁹Ar geochronology data of the amphibolites from the study area to constraint their protoliths, the geotectonic setting and the ages of amphibolitic metamorphism in the Pan-African Fold Belt in Cameroon.

Figure 1. Geological reconstruction of African Panafrican domains and NE Brazil showing the continuity of the Sergipano Belt and North Equatorial Fold Belt [modified After 15,16,18,19]. ASZ: Adamawa shear zone; CCSZ: Central Cameroon Shear Zone; BOSZ: Bétaré-Oya shear zone; Pe: Pernambuco shear zone; Pa: Patos shear zone SF: Sanaga Fault; TBF: Tibati-Banyo Fault. Dash lines represent the boundary of Cameroon

Figure 2. Geological map of Cameroon with the three main domains of the Pan-African North-Equatorial Fold Belt [modified After 15], (1) southern domain corresponding to the Yaoundé series thrust on the Congo Craton, (2) central domain, (3) northern domain; granitoids suites are not further distinguished. CCSZ: Central Cameroon Shear Zone; DS: Dja series; NS: Nyong series; NT: Ntem complex; SF: Sanaga Fault; TBF: Tibati-Banyo Fault. Location of study area is marked by a star

2. Geological Setting

The Neoproterozoic belt in Cameroon is widely known as the Pan-African North Equatorial Fold Belt (PANEFB) [15,16]. This belt is a prolongation in the African continent of the Braziliano-Panafrican Belt in South America across the Atlantic (Figure 1) [17,18,19]. In Cameroon, the Neoproterozoic belt is made up of three distinct geodynamic domains [20,21,22,23,24]: the northern domain and the southern domain which are separated by the central domain (Figure 2).

The northern domain consists of restricted 830 Ma metavolcanic rocks of tholeiitic and alkaline affinities associated with metasediments and widespread 630-660 Ma calc-alkaline orthogneisses whose age is interpreted as a major episode of continental accretion [25]. A Paleoproterozoic crustal source in this region is attested by the presence of 2 Ga old inherited zircons in the granitoids within the orthogneisses [25].

The southern domain comprises Neoproterozoic metasedimentary units such as the Yaoundé, Mbalmayo and Ntui-Betamba series. The protoliths of these units were deposited in a passive margin environment at the northern edge of the Congo Craton, and were metamorphosed under high-P conditions (T = 750-800°C, P = 9-13Kb) at 616 Ma [15,23]. A Pan-African meta-igneous rock assemblage comprising alkaline ultramafic to metagabbros and amphibole-bearing alkaline orthogneisses [23] was also recognized in association with these Neoproterozoic units. The rocks of this southern domain were thrusted onto the Archean Congo Craton towards the south [26].

Figure 3. Geological map of the Banefo-Mvoutsaha massif [modified after 12] showing sample locations

The central domain in which the present study area is located (Figure 2, Figure 3) is situated between the Sanaga fault to the South and the Tibati-Banyo fault to the North [27]. The metamorphic basement of the central domain consists of an assembly of fragments of Paleoproterozoic continental crust, recrystallized under high-grade granulite facies (850-900°C, 10-12Kb) at ca 2100 Ma, and associated with Neoproterozoic amphibolite and gneiss [5,8,22,24]. This basement is intruded by widespread Neoproterozoic syntectonic plutonic rocks of high-K, calc-alkaline affinities [12,13,20,21,24,28,29,30,31,32,33]. In the central domain polycyclic metamorphism and three phases of deformation related to metamorphic recrystallisation under low to high grade amphibolite facies conditions have been identified.

3. Analytical Methods

Fresh representative samples were sent to ACME Analytical Laboratories Ltd, Vancouver, Canada for whole rock geochemical analysis. Whole rock analyses for major and trace elements were carried out by Inductively-Coupled Plasma Mass Spectrometry (ICP-MS) from pulps. 0.2g of rock powder was fused with 1.5g LiBO2 and then dissolved in 100MM3 5%HNO3. The REE contents were determined by ICP-MS from pulps after 0.25g rock-powder was dissolved with 4 acid digestions. Analytical uncertainties vary from 0.1% to 0.04% for major elements; from 0.1 to 0.5% for trace elements; and from 0.01 to 0.5ppm for rare earth elements.

The Ar-Ar geochronology was performed at the Department of Isotope Geochemistry, Vrije Universiteit de Boelelaan, Amsterdam, The Netherlands. For the two samples (T21G9 and T22A) of amphibolites, hornblende grains measuring as 400-500 microns were used. The ⁴⁰Ar/³⁹Ar measurements were accomplished on an MAP-215-50 mass spectrometer in VU University Amsterdam. The experimental methods follow those developed at Amsterdam by [34].

4. Results

4.1. Petrography

The amphibolites investigated in Banefo-Mvoutsaha area are subdivided, based on their field occurrences and structure into two types. The first one is massive and outcrops as centimetric and discontinuous bands (Figure 4A). The second type is the banded amphibolites which is interbedded with migmatitic gneisses and garnet gneisses (Figure 4E). Those bands are parallel to the foliation within the garnet-bearing gneisses.

4.1.1. Massive Amphibolites

The massive amphibolites are very fine to fine grained. They are dark green in color and show poorly-developed foliation (Figure 4A). They occur within garnet-gneisses as lenses of 30 cm - 5 m long and 10 - 20 cm width. It is no easy to see minerals with necked eye, while in microscope, the rock displays granolepidoblastic heterogranular microstructure comprising amphibole, quartz, feldspar, pyroxene, rutile and opaque minerals which are preferentially oriented following the same direction. Amphibole crystals which are the most abundant minerals, are fish-amphibole according to their form (Figure 4B). Depending on their color, two types of amphibole crystals are observed: greenish-light (Figure 4C) and brownish crystals (Figure 4D). The brownish amphibole crystals are secondary resulting from destabilization of pyroxene. Some of these minerals contain opaque minerals inclusions. Quartz and feldspar fill the interstices between amphibole crystals following the same direction as amphibole fish crystals (Figure 4B, Figure 4C). Garnet is very low in the rock, as well as pyroxene crystals, which destabilizes into amphibole crystal. They also follow the same orientation as amphibole, quartz and feldspar. The accessory phase is made of opaque minerals and rutile. Opaque minerals are anhedral primary and secondary minerals (Figure 4B). Mineral assemblages are Hbl + Qtz + Kfs + Px ± Grt ± Op (primary assemblage) and Hbl + Px ± Op (secondary assemblage). They are respectively characteristics of high-grade and low-grade amphibolite facies.

Figure 4. Photographs and micrographs of Banefo-Mvoutsaha amphibolites. A) Occurrence of massive amphibolites interbedded with migmatitic gneisses and garnet-bearing gneisses. B) heterogranular granolepidoblastic texture. Noted the orientation of the minerals and the presence of amphibole-fish. C) LPNA micrograph showing oriented amphibole, quart, garnet and pyroxene. D) LPA micrograph showing destabilization of pyroxene into brownish amphibole. E) Occurrence of banded amphibolites. Note the foliation. F) Granoblastic heterogranular texture. Quartz is rounded. Stocky and subhedral plagioclase crystals. G) LPNA micrograph showing a pile of biotite flake. H) LPA micrograph

Figure 5. A) Chondrite normalized REE distribution diagram for the Banefo-Mvoutsaha amphibolites. The normalizing chondrite values are from . B) Primitive mantle-normalized multi-element patterns. The normalizing chondrite values are from

4.1.2. Banded Amphibolites

The banded amphibolites are greyish in color. As well as the massive amphibolites, they display very fine to fine grain. The foliation is more expressive (Figure 4E), and the alternance of quartzo-feldspathic bands and ferromagnesian bands is clearly observed. The rock is intimately associated with migmatites. In thin section, the rock displays a granoblastic heterogranular texture consisting of quartz, biotite, amphibole, feldspar, garnet and plagioclase (Figure 4F). Minerals are poorly deformed, differently from the massive amphibolites. Quartz and biotite are the most abundant minerals in this rock. Quartz is mostly rounded (Figure 4E), occurring as anhedral mineral (Figure 4F). Biotite flakes are primary minerals and are scattered in the rock. Amphiboles are anhedral and primary minerals. The percentage of the garnet is lower than in the other rock. Stocky plagioclase crystals are subhedral and lurking in the matrix. The mineral assemblage is made of Hbl + Bt + Qtz + Kfs + Pl ± Grt + Op Which is characteristic of low-grade amphibolite facies. Along micro-shear planes in the study area, the banded amphibolites have been deformed into ultramylonites and pseudotachylites fault rocks.

4.2. Whole Rock Geochemistry

Seven fresh samples of amphibolites (massive and banded) were collected and analyzed for whole-rock majors and trace elements, and their compositions are listed in Table 1.

Table 1. Geochemical data of selected samples of amphibolites from Banefo-Mvoutsaha area. Analyses performed at Acme lab, Vancouver, CANADA

Table 2. Summary of Ar-Ar geochronology data of samples T21G9 and T22A

4.2.1. Major Elements

Amphibolite samples are characterized by SiO₂ content ranging between 48 wt% and 52 wt%. The percentage of Al₂O₃ is high (16.36 – 17.25 wt%), except for the sample T21G9 which shows a relatively low percentage (13.86 wt%). As well as Al₂O₃, samples show low Fe₂O₃ values (10.53 – 12.43 wt%), while the T21G9 sample displays 16.15 wt% value. The rock present low values of MgO (2.86 – 5.69 wt%) and CaO (4.93 – 6.13 wt%), except for T21G9 sample (9.30 wt% for CaO). TiO₂ contents range from 1.52 wt% to 2.94 wt%. The averages A/CNK molar (Al₂O₃/ CaO + Na₂O + K₂O), and A/NK molar (Al₂O₃/ Na₂O + K₂O) values are 1.22 and 2.47 respectively.

4.2.2. Trace Elements

The Banefo-Mvoutsaha amphibolites sample display variable high field strength elements (HFSE: e.g Nb, Ta, Zr, Hf) and large-ion-lithophile elements (LILE; e.g., Ba, Rb, K and Sr) concentrations (Table 1). Ba concentrations are from 436 to 721ppm, Rb (59.6 – 109.6ppm), and Sr from 122.2 to 515.1 ppm.

Samples are enriched in LILE (Rb, K), and relatively weakly depleted in the other elements. On the chondrite-normalized rare earth element (REE) diagram from [35], all the samples REE pattern are weakly fractionated (LaN/YbN= 5.78 – 8.03) marked by a slight enrichment of LREE patterns (LaN/SmN = 2.40 – 3.36) relatively to HREE (GdN/YbN= 1.39 – 1.85). The samples display a negative Eu anomaly with Eu/Eu* ratio values ranging between 0.65 and 0.96 (Figure 5A), which highlight feldspar fractionation in the rock, confirmed by the negative anomalies on Sr. On the multi-elements chondrite-normalized plots from [36], Rb, K, Nd and Zr show positive anomalies, and Th, Sr and Ti display negative anomalies except for the sample T21G9 (Figure 5B).

4.3. Ar-Ar Geochronology

The summary of Ar-Ar geochronology data is represented in Table 2. Two representative samples from the massive amphibolites (T21G9) and the banded amphibolites (T22A) have been subjected to dating.

Figure 6. Graphic representations of Ar-Ar data of massive amphibolites sample (T21G9): (A and B) patterns of cumulative 39Ar released %, (C and D) normal and inverse isochrons respectively)

Figure 7. Graphic representations of Ar-Ar data of banded amphibolites sample (T22A): (A and B) patterns of cumulative 39Ar released %, (C and D) normal and inverse isochrons respectively)

The results of Ar release during stepwise heating and inverse isochrones are given on Figures 6 and 7 for T21G9 and T22A respectively. Sample T21G9 yield an Ar-Ar age of 547.50 ± 3.15 Ma (Figure 6A), the patterns of cumulative ³⁹Ar released %, are relatively flat, and constant from 10% to 100% ³⁹Ar released (Figure 6B). Sample T22A yielded an Ar-Ar age of 618.14 ± 11.43 Ma (Figure 7A), the patterns of cumulative ³⁹Ar released %, show a significant variation in the age plateau, and the results of this sample do not give a line in various isochron diagrams, variation of loss and gain ³⁹Ar released is noted (Figure 7B). Data from T21G9 crushing experiments in ³⁹Ar/³⁶Ar vs. ⁴⁰Ar/³⁶Ar diagram define a homogenous line on the normal isochron for massive amphibolites samples (Figure 6C), but show a disparity line for the banded amphibolites. From 0 to 60 for ⁴⁰Ar/³⁶Ar values, the line is homogen, and the disparity start from 70 up to 200 for ⁴⁰Ar/³⁶Ar values (Figure 7C). In the inverse isochron diagram, the difference is considerable between the two amphibolites. The results of the apparent age spectrum plots for these stepwise-crushing experiments are presented in ³⁶Ar/⁴⁰Ar vs. ³⁹Ar/⁴⁰Ar diagrams (Figures 6D and 7D). The lines are not homogeneous, but the gap is wider in the banded amphibolites (Figure 7D). The ³⁶Ar/⁴⁰Ar values range from 0.0020 to 0.0030 above, while for T21G9 sample, the gap is slight and the lines are very close (Figure 6D). The age spectrum is a convenient way to present the data in terms of progressive gas release [34]. It should be noted that the gas is released by progressive crushing and not by progressive heating as it is more commonly the case in ⁴⁰Ar/³⁹Ar dating experiments.

5. Interpretation and Discussion

5.1. Petrogenesis of Banefo-Mvoutsaha Amphibolites

The positions of studied amphibolites are shown in the TiO₂-Ni discriminating diagram for ortho and para amphibolites of [37] and the Zr/TiO₂ vs Ni diagram indicate that they are orthoderived, with an igneous source for the parental rocks (Figures 8A and 8B). The same results are described in the center domain of CAFB in Makénéné [5], the Fomopéa pluton [11], in Kombé II [8], and in the Congo craton from Akom II [38], Mewongo [7], and Toko-Nlonkeng [39] areas. The geochemical classification of extrusive rocks diagram using Nb/Y vs. Zr/Ti adapted from [40] shows the volcanic origin of those rocks (Figure 8C), where the main samples plot in alkali-basalts field, except one sample falling within the basalts field, probably due to some contamination of the magma source. Alkali basalts are characterized by highest Na₂O and K₂O content than the basic basalts, and low SiO₂ contents (48.35 – 51.13 wt%) [41,42], they are located behind the arc. However, they are not likely to form at depths shallower than 50-60 km [43,44]. This hypothesis is confirmed by the high values in Ni (11 – 87.4 ppm) and Co (25.6 – 38.6 ppm) which are evidence of a (deep) mantle source [45]. Amphibole and rutile as residual minerals can be used to constrain this source. Indeed, rutile and amphibole are index minerals which usually contain abundant Nb and Ta. In the rutile mineral composition, Ta is relative to Nb, and the experimental results showed that, if minor rutile appears in the residual source, the Nb/Ta ratios in the coexisting partial melts will increase [46]. On the other hand, Nb is more compatible than Ta in amphibole and small amount of amphibole as residual mineral could lead to lower Nb/Ta ratios [46]. All samples of massive and banded amphibolites display lower Nb/Ta ratios (16.65 – 18.72), indicating the compositional source of amphibole minerals.

The degree of contamination can be estimate through certain chemical parameters. For example, the major elements TiO₂ is a well-founded discrimination between arc and spreading ridge basalts [47, 48]. He is relatively immobile during alteration. Low TiO₂ content suggests that the protolith is in an arc system [47]. But the TiO₂ content of the study amphibolites is low to medium (1.52 – 2.94 wt%), this needs others explanations to give a conclusion. In the same vein, ratios of some trace elements can constrain the crustal contamination in basaltic rocks. Those which are affected by crustal contamination exhibit La/Ta = 22 and La/Nb = 1.5 [49]. The studied amphibolites display low values of La/Ta and La/Nb, respectively ranging from 14.9 – 19.9 and 0.84 – 1.06, and this confirm that the role of contamination during magma evolution has been minimal. In addition, incompatible trace elements such as Ta, Yb and Th are considered to determine crustal contamination. The crustal contamination affects Th more than Ta and Yb. The contamination shows high Th/Yb values [50]. The studied amphibolites show low values of Th/Yb (1.1 – 1.8), and this suggests no or minimal crustal contamination for Banefo-Mvoutsaha amphibolites parent rocks.

Furthermore, the fractionation occurs during the formation of the protoliths of those amphibolites. The REE patterns are marked by TiO₂, Sr and Eu anomalies (Figure 5). The TiO₂ anomaly in multi-element diagram suggests a Ti-oxides fractionation. The Sr anomaly in multi-element diagrams indicates plagioclase fractionation [51]. This fractionation of plagioclase is confirmed by the presence of Eu anomaly (Eu/Eu*: 0.65 – 0.96) in the chondrite normalized REE diagrams of Banefo-Mvoutsaha amphibolites, which indicate a plagioclase-depleted crustal source or fractionation during magmatic differentiation. The K₂O + Na₂O vs. K₂O/(K₂O+Na₂O) diagram [52] displays an assimilation and fractional crystallization trend (Figure 9). We can see through this plot, the role of the fractionation in the rock’s emplacement. The combination between low MgO value and moderate to high Fe₂O₃ contents also suggest fractional crystallization, but the fractional crystallization of Mg-rich minerals (i.e., pyroxene) which is typical of tholeiitic magmas [53]. In the same way, the parental basaltic magma from which the Banefo-Mvoutsaha amphibolites were derived is inferred to belong to the tholeiitic series as shown by the SiO₂ vs. FeOt/MgO diagram (Figure 10) of [54], like in the Mewongo area [7]. All the samples of Banefo-Mvoutsaha amphibolites plot in the tholeiitic series area and display a tholeiitic trend as demonstrated in the Mg# vs. SiO₂/Al₂O₃ diagram, showing the primitive basalts field and differentiation [55] and mineral fractionation trends (Figure 11) adapted after [56].

Figure 8. Geochemical classification diagrams of Banefo-Mvoutsaha amphibolites. A) Studied amphibolites position in the TiO₂-Ni discriminating diagram for Ortho and para-amphibolites . B) Zr/TiO₂/Ni plot diagram of metavolcanic rocks [from 36]. C) Zr/Ti vs. Nb/Y Plot of amphibolites protolites

Figure 9. K₂O+Na₂O vs. K₂O/(K₂O+Na₂O) diagram of igneous rocks [adapted from 52] showing alteration trend of Bafoussam amphibolites

Figure 10. Discrimination diagrams showing the affinity of Banefo-Mvoutsaha amphibolites. A) FeOt/MgO vs. SiO₂ Plot of . B) The AFM discrimination plot showing tholeiitic affinity of the Bafoussam amphibolites

Figure 11. Mg# vs. SiO₂/Al₂O₃ diagram showing the primitive basalts field and differentiation and mineral fractionation trends (after )

5.2. Age of protholith

From the Ar-Ar data, the flat cumulative patterns and the well-defined normal isochron of sample T21G9 means that sample were less disturbed, which is traduced by no significant loss or gain of Argon. The age of 547,50 ± 3,15 Ma may be correlated to an amphibolitic metamorphism phase related to the tectono-metamorphic events marking the end of Pan African orogeny. The important variation in the patterns of sample T22A, coupled to the fact that results are not defining an isochron, are the evidences of significant loss in Ar, this means samples were probably affected by multi phases metamorphism. The age of 618,14 ± 11,43 Ma yielded by T22A of the study area has been obtain by [15,23,57], (U-Pb / zircon age) [24,29,31,58,59,60]. The amphibolites are well documented in the CAFB in Cameroon during the Neoproterozoic [5,8,11,10,40]. [10] showed that amphibolites from the Yaoundé Group (Cameroon, Central Africa) have a basaltic affinity. They are dated between 650 and 600 Ma, and confirm the existence of a Neoproterozoic oceanic crust within the CAFB. The studied amphibolites are younger than the Makénéné amphibolites in the CAFB and the Mewongo amphibolites in the Congo craton which display an Archean age [5,7].

This age of 618 Ma traduces the local peak of metamorphism which is a high-grade amphibolite to granulite facies. At the regional scale this recurrent age of 618 Ma obtained by different method confirm that the Pan African Fold Belt in Cameroon is homogeneous regarding the ages of events. The Ar-Ar ages of the studied amphibolites characterize a polyphase metamorphism during the Pan African Orogeny in Banefo-Mvoutsaha.

5.3. Tectonic Setting and Geodynamic Implication

The Nb/Yb vs. Ba/Yb diagram [61] is useful to constrain the geodynamic settings of the protolith of the mafic metavolcanic rocks. According to this diagram, the studied amphibolites plot in the MORB array (Figure 12A), suggesting that the immobile trace element Nb could be used to separate the different types of ocean-floor basalts [62]. They are N-MORB related for the majority of samples, but they also display back arc basin basalts (BABB; Figure 12B) and Arc-Volcanic (Figure 12C) features. The protolith is alkali basalt, and alkali basalts often occur in a subduction area resulting from a collision between a continental and oceanic crusts [63]. The ternary MgO-FeOT-Al₂O₃ diagram of studied amphibolites after [64] displays a continental trend (Figure 13A). They are Continental Island Arc basalts, according to CaO-Na₂O-K₂O ternary plot of [65] (Figure 13B). On the others geotectonic discrimination diagrams that use immobile trace elements during metamorphism, all the samples belong to the same geotectonic setting which is intraplate context. This context is shown by the binary diagram Zr-Zr/Y of [66] in which the protoliths of studied amphibolites are intraplate basalts (Figure 14A). This is supported by the discrimination geotectonic Nb–Zr–Y [62] and the Th-Hf/3-Ta of [66] ternary diagrams. Indeed, the samples are plotted on AI and AII, which correspond to within plate alkali basalts and within plate tholeiitic basalts according to geochemistry results (Figure 14B). Furthermore, the Figure 14C confirms the same results.

6. Conclusion

From the petrography, the whole-rock geochemistry and Ar/Ar dating of the Banefo-Mvoutsaha amphibolites, the main findings are as follow:

1). Two distinct types of amphibolites outcrop in Banefo-Mvoutsaha area, the banded amphibolites and the massive amphibolites.

2). Both amphibolites are ortho-derived, their protoliths are alkali basalts which belong to tholeiitic series.

3). The studied samples are enriched in LREE relatively to HREE. The LaN/YbN ratios are between 5,78 and 8,03; and GdN/YbN values are from 1,39 to 1,85. The samples display a negative Eu anomaly with Eu/Eu* values comprised between 0,65 and 0,96. In general samples are enriched in LILE, and relatively weakly depleted in others elements.

4). The Ar/Ar geochronology reveals that all samples have Pan-African age. The banded amphibolites age is 618,14 ± 11,43 Ma and the massive amphibolites age is 547,50 ± 3,15 Ma. The older metamorphic event at 618 Ma match with the emplacement ages of granites that intruded the metamorphic basement of Banefo-Mvoutsaha area. The youngest metamorphic event at 547 Ma is related to the tectono-metamorphic events which mark the end of the Panafrican orogeny in the study area. Therefore, Panafrican orogeny in the study area presents multiphase metamorphism with at least two amphibolitic phases.

5). The Banefo-Mvoutsaha amphibolites are the volcanic input of a volcano-sedimentary protoliths of the metamorphic basement of the area. The massive and banded amphibolites are metabasites and have a basaltic composition. They are N-MORB related and display back arc basin basalts and Arc-Volcanic characteristics. They follow a continental arc trend and correspond to within plate alkali and within plate tholeiitic basalts.

Figure 12. Tectonic setting discrimination diagrams of the Banefo-Mvoutsaha amphibolites. A) Nb/Yb vs. Ba/Yb diagram (after ). B) Y vs. La/Nb diagram showing the distribution of the main amphibolites within back-arc basin basalts (BABB) field, (after ). C) La/Nb vs. Nb/Th tectonic discrimination diagram showing the plots of the Penjween amphibolites within arc field, (after )

Figure 13. (A) MgO-FeOT-Al₂O₃ plot after . B) CaO-Na₂O-K₂O ternary diagram showing sandstones compositions from oceanic Island Arc, Active Continental Margin tectonic settings

Figure 14. Geotectonic discrimination diagrams of the Banefo-Mvoutsaha amphibolites. A) Zr/Y vs Zr of indicating geotectonic context of studied amphibolites. B) Nb–Zr–Y of . WPA (AI, AII): Within-plate Alkalic Basalt; WPT (AII, C): Within-plate Tholeiitic Basalts; P MORB (B) plume mid-ocean ridge basalts; N MORB (D): normal mid-ocean ridge basalts; VAB (C, D): Volcanic Arc Basalts; WPB, Within-Plate Basalts; E-MORB, Enriched Mid-Ocean Ridge Basalts. C) Geotectonic diagram Th-Hf/3-Ta showing the positions of Banefo-Mvoutsaha amphibolites within the WPT/WPA domain

ACKNOWLEDGEMENTS

The Ar-Ar geochronology presented here was possible through financial support and technical assistance of Prof Jan WIJBRANS from the Department of Isotope Geochemistry, Vrije Universiteit, de Boelelaan 1085, 1081HV Amsterdam, The Netherlands. We are grateful to Prof Jan WIJBRANS.

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