Geochemistry of the Bayonplutonic Complex – Western Cameroon

The BayonNeoproterozoic plutonic complex located in Western Cameroon intrudes gneisses of Paleo to Neo Proterozoic age. The complex is composed of gabbro, monzogabbro and monzonites frequently crosscut by trachytic and granitic veins. The primary mineral assemblages of the gabbro and monzogabbro is plagioclase (An 30 - An 69 ), clinopyroxene (En 40 – 42 Fs 12 – 18 Wo 45 - 47 ) hypersthene (En 62 – 65 Fs 34 – 37 Wo 1- 4 ) and orthoclase (Or 78 -Or 91 ) whilebiotite, magnetite, Ilmenite and apatite constitute accessory minerals. Monzonite is formed of plagioclase (An22 to An 39 ), orthoclase (Or 80 -Or 87 ),clinopyroxene (En 38-39 Fs 14-20 Wo 45-46 ), biotite and quartz. Amphiboles occur as secondary minerals. Ilmenite and apatite are accessory minerals in monzonite.The rocks are mafic to intermediate in composition (41 – 61 wt % SiO 2 ) and transalkaline with high-K and have shoshonitic features. Bayon plutonic rocks have high abundance of Ba, Sr ,V and Zr but possesses low concentrations of Rb, Sc, Y and Th.Gabbroic rocks show moderately fractioned REE patterns (La N /Lu N = 14 – 27) with none or negligible Eu anomalies. The monzonite shows also moderate fractionatedpatterns (La N /Lu N = 20 – 27) with fairly positive Eu anomalies. All the studied rocks show flat HREE features. The primitive mantle- normalized element patterns are almost homogeneous with negative anomalie in Ta, Nb, Th, P and Ti. Sm/Nd-wr-Cpx-Pl ages of the complex are 580 ± 13 Ma; 553 ± 32 Ma for the monzogabbro and 547 ± 26 Ma for the monzonite. The Nd/Sr isotopic compositions show that the Bayon plutonic rocks were generated by partial melting of subcontinental lithospheric mantle. The depleted mantle Nd model age TD M of 1.6 – 1.7 Ga indicates that the studied rocks originated by partial melting of Mesoproterozoic mantle. KEYWORDS: West Cameroon, Panafrican, Pluton, Geochemistry, partial meting


Analytical methods
Representative samples (0.5-1kg) were collected for electron microprobe (EMP), major and trace element and isotopic data analyses. Analyses were performed at the Department of Lithospheric Sciences and the Centre for Earth Sciences (Laboratory of Geochronology), University of Vienna (Austria).For mineral compositions, polished carbon-coated thin sections were analyzed with a Camera SX-100 electronmicroprobe. The operating conditions were four wavelength-dispersive spectrometers; 15 kV accelerating voltage and 20 nA beam current; 1µm beam diameter were used for pyroxenes whereas 5 µm beam were used for feldspars. Major and trace element were analysed by X-ray fluorescence (XRF) spectrometry on fused powder discs using a Phillips PW 2400. Rare earth elements (REE) were done by ICP-MS method with a Perkin Elmer ELAN 6100 DRC.
Isotopic data were obtainedat the Laboratory of Geochronology, Centre for Earth Sciences, University of Vienna. For whole rock (wr) and bulk mineral analysis, the kg-sized samples were cleaned and crushed, and then representative wr splits were taken. Apatite was concentrated using a Wilfley Rb/ 86 Sr ratios, a mean error of ±1 % was estimated, including blank contribution, uncertainties on spike composition, and machine drift; regression calculation was based on these uncertainties and the isochron calculations followed Ludwig (2003). Age calculations were based on a decay constant of 6.54 x 10 -12 a -1 for 147 Sm (Lugmair & Marti, 1978)and 1.42 x 10 -11 a -1 for 87 Rb (Steiger & Jäger, 1977); age errors were given at the 2σ level.A 143 Nd/ 144 Nd ratio of 0.511846±0.000003 (n = 38) and a 87 Sr/ 86 Sr ratio of 0.710248±0.000002 (n = 18) were determined for the La Jolla (Nd) and the NBS987 (Sr) international standards, respectively, during the period of investigation. Within-run mass fractionation for Nd and Sr isotope compositions (IC) were corrected relative to 146 Nd/ 144 Nd = 0.7219, and 86 Sr/ 88 Sr = 0.1194, respectively. Uncertainties on the Nd and Sr isotope ratios were quoted as 2σ m . For the 147 Sm/ 144 Nd and the 87 Rb/ 86 Sr ratios, a mean error of ±1 % was estimated, including blank contribution, uncertainties on spike composition, and machine drift; regression calculation was based on these uncertainties and the isochron calculations followed Ludwig (2003). Age calculations were based on a decay constant of 6.54 x 10 -12 a -1 for 147 Sm (Lugmair & Marti, 1978) and 1.42 x 10 -11 a -1 for 87 Rb (Steiger & Jäger, 1977);age errors were given at the 2σ level.

Petrography and Mineralogy
The Bayon plutonic complex ( Fig.1c) is made up of gabbroic rocks (alkali-gabbro, syenogabbroand monzogabbro) and monzonite. Alkali gabbro and syenogabbro are considered as Gabbro(sensulato).Representative mineral chemistry data from the three groups of Bayon plutonic complex are given in Table 1.

Gabbro
The gabbro is exposed in the West of the complex in contact with the monzogabbro. It is sometimes crosscut by granitic veins; they are rich in Fe-Ti oxides. The Gabbro (Fig.2a) shows a cumulative texture, with automorphic to subautomorphic finecoarse crystals Plagioclase(An 64 -An 69 ) forms the main primary mineral. The modal content of plagioclase is 50 -55 vol % and occurs as subhedral, rounded crystals. Some anhedral plagioclase crystals portray inclusion of apatite, zircon and iron oxides. Pyroxenes are represented by clinopyroxene  Fs 13-18 Wo 39-45 ), diopside (En 40-42 Fs 12-13 Wo 45-47 ), and pigeonite (En 61-62 Fs 32 Wo 6-7 )] and clinoenstatite (En 62-65 Fs 34-37 Wo 1-4 ). Clinopyroxenes are subeuhedral. In most samples, they are replaced by brown-green biotite or amphibole (Fig.2c). Apatite occurs as inclusions in feldspars and pyroxene or isolated in the groundmass. Iron oxides (magnetite and ilmenite) are commonly associated with pyroxene.

Monzogabbro
The Monzogabbro occupy large domain of the area. They are the most ubiquitousrock in the area. They are surrounded by gabbro and in some placescrosscut by pegmatitic granite veins and small trachytic veins. They are fine to coarse grained. The rocks show heterogranular to granular texture (Fig.2d). Plagioclase (An 30 -An 58 ) is thedominant mineral (35 -40 vol %), and appears either as automorphic to subautomorphic phenocrysts with apatite, biotite and opaque inclusion; or automorphic small crystals often included in K-feldspars. Myrmekite appears in some crystals (Fig.2e).
Orthoclase (Or 84 -Or 91 ) forms automorphic plates and are dominantly perthitic and GEOCHEMISTRY OF THE BAYONPLUTONIC COMPLEX -WESTERN CAMEROON 75 poikilitic with clinoenstatite, apatite and ilmenite inclusions.Feldspathoidsusually show cracks (Fig.2e). Xenomorphic coarse crystals of biotite are poikilitic (Fig.2f), frequently associated with pyroxenes and Fe-Ti oxides. The clinopyroxenes forming about 10 vol% of the rocks consist mostly of augite (En 39-42 Fs 14-17 Wo 42-44 )and diopside (En 42-49 Fs 13-15 Wo 46 ). The coarse grains are subhedral and usually associated with biotite. Glomerophyric association of augite-plagioclase is sparse in all samples. Orthopyroxene (clinoenstatiteEn 55-63 Fs 33-40 Wo 1-2 ) is the most abundant pyroxene; it occurs as acicular small euhedral crystals associated with biotite. The largest crystals of orthopyroxene show small inclusions of plagioclase, biotite, Fe-Ti oxides, and apatite. A few quartz crystals are also found as inclusions in plagioclase, biotite and orthoclase phenocrysts. Magnetite is the dominant oxide, with minor ilmenite. Zircon appears as inclusions in biotite and apatite appears as prism in the groundmass or as inclusions in the plagioclase.         The feldspar chemical composition of all rocks is calculated on the basis of eight oxygen; fixing the total number of cation to 5. The structural formulae of pyroxene are calculated on the basis of six oxygen's fixing the total numbers of cation to 4. The structural formulae of ferric and ferrous iron contents were calculated on the basis of stoichiometry using the method of Lindsley (1983)      A monzonite body occurs in the north and the south of the gabbroic rocks and shows heterogranular to granular texture. The monzonite (Fig.2g)is made up of plagioclase, K-feldspar (orthoclase), clinopyroxene, biotite and quartz. Accessory minerals include titanite, zircon, apatite, opaque minerals. Amphibole occurs as secondary mineral. The feldspar in monzonite is dominated by the presence of euhedral to subhedral tabular plagioclase phenocrysts (An 22 -An 39 ) (25-40 vol. %), sometimes weakly zoned and partially resorbed into sericite. Euhedral K-feldspar (Or 80 -Or 87 ) usuallyshows cracks.Some crystals contain euhedral to subhedral inclusions of plagioclase or albite lamellae and others have perthitic intergrowths. Clinopyroxenes have a weak pale yellow green pleochroism. In term of composition, they are diopside (En 38-39 Fs 14-15 Wo 45-46 ) and augite. (En 38-39 Fs 15-20 Wo 39-45 ) (Table1).Some clinopyroxenes are completely replaced by amphibole (Fig.2h), in some samples and are recognizable only by their characteristic morphology. Biotite (20 vol %), in monzonite is ferroan. Itis the most mafic mineral in the monzonite with a strong reddish brown-yellowish brown pleochroism. They occur as poikilitic plates with inclusion of apatite, Ilmenite and zircon, or insmall flakes. Quartz (8 vol %) forms subeuhedral and polygonal crystals with undulatory extinction. Iron oxides represented by ilmenite and magnetite occur as inclusion in pyroxene.

Geochemistry
Representative chemicaldata of eighteen samples from Three groups of Bayon plutonic complex are given in Table 2.

Trace and Rare Earth elements
Trace and Rare Earth element content of the representative studied rocks are show in In the chondrite normalized Rare Earth Elements (REE) diagram (Fig. 5a, 5b), all the Bayon gabbroic rocks exhibit Light Rare Earth Elements (LREE) enrichment with (La N /Yb N =14.10-20.37) and without Eu anomalies (Eu/Eu*=0.86 to 1.12). In the primitive mantle normalized trace element spiderdiagram (Fig. 6a, 6b), Bayon gabbroic rocks characterized by enrichment in large ion lithophile elements (LILE) such as Ba, Sr but depleted of Th, P, Ce, Nb and the high-field-strength elements (HFSE) such as Ta and Ti. Positive anomalies in Sr, K, Smand Ba were noted.

GEOCHEMISTRY OF THE BAYONPLUTONIC COMPLEX -WESTERN CAMEROON
Monzonitesamples are characterized by REE content ranging from 184.54 to 224.17ppm with LREE more enrichedcompared to HREE (Fig. 5c). This is expressed by La N /Lu N =20. 63 -26.98 and La N /Yb N =19.62-24.81. The LREE and HREE showed slight fractionation with La N /Sm N =3.30-3.64 and Gd N /Yb N =3.46-3.64. The moderate positive Eu (Eu/Eu*=1.07-2.07) anomaly is dominant in all the monzonite samples (Fig. 6c). The patterns revealed the depletion in Nb, Th, Ce, P and Ti and positive anomaly at Ba, and K.These small to negligible Eu anomalies and the high Sr contents exclude important fractional crystallization of feldspar in the Bayon plutonic rocks petrogenetic evolution.

Sm-Nd isotopes
The Sm-Nd data for the Bayon plutonic rocks are presented in Table 3. In samples BA5 and mineral fractions, the Sm/Nd-wr-Cpx-Pl isochron yielded 580 ± 13 Ma with the initial 143 Nd/ 144 Nd ratio of 0.511577 ± 0.000011 and a (MSWD) = 0.107 (Table 3, Fig.7a). From sample F8, the wr and handpicked minerals (two clinopyroxenes fractions Cpx1 and Cpx2; plagioclase and apatite) define an isochron corresponding to an age of 577 ± 49 Ma and the initial 143 Nd/ 144 Nd ratio of 0.511572 ± 0.000040 and a MSWD =0.107 (Table 3, Fig. 7b). wr sample BA4, pyroxene and plagioclase isochron age yielded 553 ± 32 Ma with an initial   (Table 3, Fig. 7c). The wr, clinopyroxene and plagioclase fractions from the MBA2 sample give an isochron age of 547 ± 26 Ma, an initial 143 Nd/ 144 Nd ratio of 0.511470 ± 0.000019 and a MSWD = 5.1 (Table 3, Fig. 7d). The slight deviation of the whole rock from the mineral isochron may results in a large age error and could probably be related to random Nd isotope perturbation via high temperature hydrothermal fluids related to the crystallization of hydrous phases such as hornblende and phlogopite. The single sample F14 whole rock composition analysed using DM parameters of Michard et al. (1985)gives a TDM model age at ca. 1600 Ma for monzogabbro. Since they have similar Nd isotopic values, it is possible that they originate from the same magmatic episode. No Sm-Nd isochron can be obtained because of the very small variations in the isotopic values. The negative initial εNd values that range between -6 and -9, suggest the dominanceofenriched crustal component of the protolith.  =0.1967=0. (Faure, 1986.T Dm were calculated based on present day DM values of ( 143 Nd/ 144 Nd) DM =0.513114and( 147 Sm/ 144 Nd) DM =0.222. (Michard et al., 1985).I CHUR (580 Ma) =0.511890

Geochemical evolution
Geochemical studies of Bayon plutonic rocks show two groups: gabbroic rocks (gabbro l.s., and monzogabbro) and monzonite. All these rocks have the same mineral assemblage; however, little quartz occurs in the monzonite and in some Monzogabbro samples. Major and trace elements variations show a continuity between gabbroic rocks and monzonite. These rocks showed the least scatter of data( Fig. 3)with negative correlations between SiO 2 and MgO, Fe 2 O 3 , CaO, P 2 O 5 , MnO, TiO 2 . Alternatively, K 2 O, Al 2 O 3 and Na 2 O correlated positively with SiO 2 . The important feature in the data is the presence of little gap at 56.70 -59.58 % SiO 2 separating the gabbroic rocks from monzonite in two distinct groups. The increase in Na 2 O content and the decrease in MgO, Fe 2 O 3 , CaO, TiO 2 and P 2 O 5 at 45.52 wt% SiO 2 showed that fractionation of mafic mineral took place in the early stages of crystallization. In gabbro, the corresponding mineral was pyroxene. The higher value of P 2 O 5 in some gabbros samples could be linked to the presence of the apatite phase. The high levels of total alkalis and aluminium in the monzogabbro

GEOCHEMISTRY OF THE BAYONPLUTONIC COMPLEX -WESTERN CAMEROON
and monzonite can be explained by the presence of alkali-feldspar. Higher TiO 2 and Fe 2 O 3 are due to iron oxide, which is represented by ilmenite and magnetite. The abundance of TiO 2 decreased with increase in SiO 2 , which could imply crystallization of titaniferous magnetite indicating relatively high fO 2 in the melt. According to the total alkali content (∑alkali = 4.30 -9.73wt %), the (Na 2 O+K 2 O) vs SiO 2 diagram (Fig. 5a) (TAS diagram with fields after Middlemost, 1997), the studied rocks exhibit a trans-alkaline character and showa positive correlation between SiO 2 content and the alkalis. It is noted also that the Bayon plutonic rocks plot within the high-K to shoshonitic fields. The similarities of REE and multi-elements patterns for gabbroic rocks and monzonite associated with some trends in Harker diagrams suggest that those lithologies are cogenetic and that fractional crystallization could have played an important role in the generation of the magma. The homogeneity of geochemistry and isotopic significance of mafic and intermediate Bayon rocks show that they have close genetic relationships. The Bayon plutonic rocks which are enriched in LILE including K, Rb, Sr and Ba relative to the HFSE especially Zr, Nb and Y can be compared to the shoshonitic association which main characteristic are: high total alkalis (Na 2 O+K 2 O>3), low TiO 2 (<1.3wt%), high contents of LILE(Ba, Sr, Rb), low Nb and no Fe enrichment (Morrison, 1980).The presence of coupled Ta, Nb and Ti negative anomalies in spider diagram could be an indication of the contribution of subduction related components. The little scatter of data in major, trace and isotopic composition diagrams could be explained by the contamination of primitive melts by crustal components (Huppert and Sparks, 1985).

Source and tectonic setting
A question always emerging from the study of rocks is whether magma source were located in the subcontinental lithospheric mantle (SCLM) or in the asthenosphere. Determinationofthe source of the magma composition is highly difficultbecause there are several mantle components (DMM, HIMU, EM1, EM2).Some authors (Coish and Sinton, 1992)have used La/Ta and La/Nb ratios to distinguish lithospheric (La/Ta > 22; La/Nb > 1.5) fromasthenospheric sources which would be characterized by La/Ta < 22; La/Nb < 1.5. The Bayon rocks are characterized by ratios (La/Ta = 36-80; La/Nb=3-5). On the other hand, the nature of the igneous source can be constrained using the geochemical and isotopic signatures of plutonic rocks. The Nb/Zr vs Nb/Ba diagram ofHopper and Hawkesworth, (1993) (Fig. 8)shows that the gabbroic rocks (gabbros s.l. and monzogabbro) and monzonite originated from partial melting of an enriched subcontinental lithospheric mantle(SCLM). The studied samples are also most radiogenic and are enriched in large ion lithophile elements; they have high 87 Sr/ 86 Sr and low 143 Nd/ 144 Nd and; the Nd T DM model age ranges around 1.6 to 1.7 Ga. According to Zindler and Hart, (1986), the sample which has these signatures (high 87 Sr/ 86 Sr and low 143 Nd/ 144 Nd), originates from enriched mantle (EMII).The spider diagrams of the Bayon plutonic rocks are almost similar. The gabbroic rocks and monzonite are high -K calc-alkaline to shoshonitic Itype granite; however High-K calc-alkalinerocks often occur in the continental arc setting or the late collision setting; sometimes they evolve to shoshonitic composition or peralkaline in the final stage of the orogeny(Liegeois et al., 1998).  Hopper and Hawkesworth (1993).SCLM = subcontinental lithospheric mantle, A = asthenosphere.
Trans-alkaline characters as well as high-K to shoshonitic enrichments suggest that Neoproterozoic magmatism initially with calc-alkaline characters evolved towards alkalinity at the end of the Precambrian; this evolution predates alkaline affinities of the magmatism of the region during the Tertiary after a gap (absence of magmatic activity) running from the Cambrian to the Tertiary.Potassic contents of rocks in the Bayon plutonic complex are compatible with these age data since the incompatible element composition of the gabbroic rocks shows close similarities with those of the magmatic arc granitoids, e.g. the enrichment in LILE and LREE with depletion at Nb and Ti (Pearce et al., 1984).All the Bayon plutonic rocks areenriched in Ba and Sr. This enrichment is probably inherited from such an enriched mantle (Chen et al., 2002) (Table 4) and low 143 Nd/ 144 Nd with the Nd T DM model age ranging from 1.6 to 1.7 Ga.In the Sr -Nd correlation diagram (Fig. 9b), it is evident that all the analysed rocks plot in the right lower quadrant which reflects the enriched sources, suggesting that the magma from these rocks originated from the same source as mentioned above.The T DM ages of the studied rocks range from 1.6 to 1.7 Ga with negative εNd (600) between -6.1 to -9.2. This result agrees with the remnants of Mesoproterozoic crust in this area of CAFB. The Nd and Sr isotopic from Bayon plutonic rocks (Ngo Belnoun, 2008) indicate the slow differential cooling age of the intrusive rocks.The tectonic discrimination diagrams for granitoids Rb vs Y+Nb (Peace, 1996) shows that all the samples clearly plot in the field of post collisional granite (Fig. 9a). In the Zr vs (Nb/Zr) N diagram (Fig. 9b) ofThiéblemont and Tegyey (1994), all the studied samples plot again in the field of collision zone rocks. All these characteristics and high-K calc-alkaline affinity are consistent with an orogenic collision setting (Liégeois et al.,1998).The diagram of Frost et al., 2001(Fig. 4b) clearly indicates that the rocks are magnesian and the plot in field ofthe diagram ofPearce et al., (1984), shows that the studied rocks are situated in the field of orogenic granitoids.

Implications for regional geodynamics
The Bayon plutonic complex compared to some massifs in West Cameroon shows that it is less potassic than the other studied complexes (Fig. 10).Comparative studies of incompatible trace elements versus primitive mantle of Sun and McDonough (1989)of data from the Bayon plutonic complex and some massifs of West Cameroon (Bafoussam, Ngondo) show similarities in terms of negative anomalies in Nb, Ce, Ti (Fig. 11)which represent signatures of subductional and collisional events. Such evidenceis also found when the Fomopéa granitoids are compared to the Bayon intrusive rocks. The Bayon plutonic rocks that have high content of alkali, Ba and Sr are also similar to the calc-alkaline to transitional granitoids of the Solidao type (Guimarães et al., 1998;Guimarães and da Silva Filho, 2000). Fig. 9. Tectonic diagrams for Bayon plutonic rocks. a) Rb vs (Y+Nb) with discrimination fields after Pearce(1996). WPG=within plate granites; ORG= oceanic ridge granites; VAG= volcanic arc granites; Syn-COLG= syn-collisional granites; Post-COLG= post -collisional granites. b) Zr vs (Nb/Zr) N diagram of Thiéblemont and Tegyey (1994) for Bayon granitoids. A= subduction-zone magmagtic rocks; B= collision zone rocks; C= alkaline intraplate zone rocks. Normalization to primitive mantle value from Sun and McDonough (1989).  et al.,(1989) and Peccerillo and Taylor (1976). Dash line = Ngondo plutonic rocks; Black circle = Bangangte syenite; dotted line = Fomopea plutonic rocks. This diagram shows that Bayon plutonic complex has almost the same geochemical affinity.

CONCLUSION
The Bayon plutonic complex is composed of gabbroic (gabbro s.l. and monzogabbro) and monzonite rocks.The major minerals areplagioclase, augite, diopside, pigeonite, clinoenstatite, and biotite. Orthoclase is found in monzogabbro and monzonite, whilst quartz is found only in the monzonite samples. Accessory minerals common in the gabbroic and monzonitic rocks are ilmenite, magnetite, apatite, titanite and zircon. Geochemicaldata indicate that the Bayon plutonic rocks are transalkaline, metaluminous, magnesian, I-type and have high-K to shoshonitic affinities.The primitive mantle normalized trace element Nb, Ti, Ta, negative anomalies.Chondrite-normalized rare earth elements patterns indicate the enrichment ofLREE and flat patterns of HREE. εNd (600) vs 87Sr / 86 Sr at 600 Ma diagram suggesting that the studied rocks were derived from enriched mantle with a little continental crust contamination. In all the discrimination diagrams, all the rocks studied fall within the collision zone. Geochemical variation of the Bayon plutonic rocks suggests that fractional cystallisation and crustal contamination may have taken place during the evolution of the magma. The Bayon plutonic rocks were generated by differentiation of mafic magma derived from enriched subcontinental lithospheric mantle. The Sr-Nd isotopic composition indicates that the plutonic rocks have been produced by partial melting of a subcontinental mantle supported by their initial 87 Sr/ 86 Sr(600Ma). The Bayon plutonic rocks were emplaced in a subduction to collison tectonic environment. All isotopic ages from this study are almost identical; they are cooling ages and date most probably uplift linked to wrench tectonic event at ca 560Ma.