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.

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.

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.

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

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.

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.

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.

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.

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.

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 (La_N/Yb_N= 5.78 – 8.03) marked by a slight enrichment of LREE patterns (La_N/Sm_N = 2.40 – 3.36) relatively to HREE (Gd_N/Yb_N= 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).

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.

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

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] ^{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.

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.

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 La_N/Yb_N ratios are between 5,78 and 8,03; and Gd_N/Yb_N 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.

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.

References