petrogenesis of amphibolites from the neoproterozoic yaounde group

Versão online: http://www.lneg.pt/iedt/unidades/16/paginas/26/30/141 Comunicações Geológicas (2013) 100, 1, 5-13 ISSN: 0873-948X; e-ISSN: 1647-581X

Petrogenesis of amphibolites from the Neoproterozoic Yaounde Group (Cameroon, Central Africa): Evidence of MORB and implications on their geodynamic evolution Petrogénese de anfibolitos do Grupo Yaounde de idade Neoproterozóica (Camarões, África Central): Evidências de MORB e implicações para a sua evolução geodinâmica S. Owona1,*, S.P. Mbola-Ndzana2, J.E. Mpesse1, J. Mvondo-Ondoa3, B. Schulz4, J. Pfänder5, P. Jegouzo6, P. Affaton7, L. Ratschbacher5, G.E. Ekodeck1 Recebido em 05/12/2011 / Aceite em 30/08/2012

Disponível online em Setembro de 2012 / Publicado em Junho de 2013

© 2013 LNEG – Laboratório Nacional de Geologia e Energia IP

Abstract: Amphibolites from the Yaounde Group correspond to basalts that derive from subalkaline magmas (high-Fe tholeiites to basaltic andesites). Major and trace element compositions discriminate them into MORB, continental flood basalts and OIB types. The presence of 650-600 Ma (Ar/Ar-Amphibole) MORB like signature agrees with the existence of a Neoproterozoic oceanic crust inside the Central African Fold Belt to the north of Congo craton while andesitic basalts suggest the existence of a subduction environment. In these amphibolites, the Pan-African clockwise evolution encompasses prograde (375-550°C/4-8 kbar), peak (400-670°C/6-10 kbar) and retrograde (675-550°C/7-5 kbar) stages. This overall clockwise P-T path is typical of orogens that underwent crustal thickening by nappe stacking during the Central African Fold Belt continental collision like the Serigipe Belt in NE Brazil.

Keywords: Amphibolites, MORB, PT evolution, Pan-African collision, Yaounde Group, Cameroon. Resumo: Os anfibolitos do Grupo Yaounde correspondem a basaltos que derivam de magmas sub-alcalinos (toleítos com elevado Fe a basaltos andesíticos). As composições de elementos maiores e traço sugerem que sejam do tipo MORB, basaltos continentais e OIB. A presença de assinaturas do tipo MORB com 650-600 Ma (Ar/Ar-anfíbola) concordam com a existência de uma crosta oceânica Neoproterozóica dentro da Faixa de Dobramentos Centro-Africana a norte do Cratão do Congo enquanto os basaltos andesíticos sugerem a existência de um ambiente de subducção. Nestes anfibolitos, a evolução directa Pan-Africana inclui os estágios prógrado (375-550 °C; 4-8 kbar), pico (400-670 °C; 6-10 kbar) e retrógrado (675-550 °C; 7-5 kbar). Este percurso P-T directo é típico de orógenos que sofreram espessamento crustal por empilhamento de nappes durante a colisão continental da Faixa de Dobramentos Centro-Africana, tal como a Faixa Sergipe no NE do Brasil.

Palavras-chave: Anfibolitos, MORB, Evolução P-T, colisão Pan-Africana, Grupo Yaounde, Camarões. 1University of Douala, Department of earth Sciences. P.O.Box. 24157, Douala, Cameroon, 2Institute for Geological and Mining Research, C.G.M.R., P.O. Box: 333 Garoua, Cameroon, 3University of Yaounde I, Department of earth Sciences. P.O. Box: 812, Yaounde, Cameroon, 4TU-Bergakademie Freiberg, Institute of Mineralogy, D-09596 Freiberg, Germany, 5TU-Bergakademie Freiberg, Institute of Geology, D-09596 Freiberg/Sachsen, Germany, 6Université de Rennes1, Géosciences Rennes, UMR 6118, 35042 Rennes Cedex, France, 7CEREGE, CNRS-Université d’Aix-Marseille III, B.P. 80, 13545 Aix en Provence, France. Corresponding author/autor correspondente: [email protected]

1. Introduction

The Central African Fold Belt (CAFB) results from the collision between the Congo, West African, and East Sahara cratons (Fig. 1a). It includes the Pan-African nappe structure that continues further westward to northeastern Brazil (Abdelsalam et al., 2002; Da Silva et al., 2005; Neves et al., 2006; Oliveira et al., 2006). In Cameroon, the Neoproterozoic is subdivided into northern, central and southern supergroups by the Central Cameroon shear zones (CCSZ) and the Sanaga fault (SF). The southern Super Group (SCSG) comprises meta-volcano-sedimentary units, such as the Yaounde Group (YG) studied here (Fig. 1b). Protoliths of these units are considered to be deposited in passive margin environment at the northern edge of the Congo craton. The rocks of this southern domain were transported SSW-wards onto the Archean Congo craton (Mvondo et al., 2007; Nzenti et al., 1988; Owona et al., 2011a). However, many details of the Pan-African geodynamic evolution of the CAFB are still being discussed. The present study focuses on amphibolites occurring within the Neoproterozoic Yaounde Group (Fig. 1c) that recrystallized under amphibolite- to granulite-facies conditions during the Pan-African orogeny (Nzenti et al., 1984). We present new petrographical, geochemical and mineralogical data that allow us to discuss their origin and P-T evolution within the CAFB.

2. Geological setting

The Yaounde Region consists of chlorite schists, mica schists, para- and orthogneisses, amphibolites and quartzites. Rocks of the Yaounde group include discrete bodies of metadiorites, metatonalites, and metagabbros. Other amphibolite occurrences are concordant with paraderived rocks. They form with other rock types, the YG that belongs to the South-Cameroon supergroup and the Oubanguide complex equivalent to the CAFB. The Yaounde Group has been affected by a four-stage deformation (D1 to D4). The three first stages are ductile, associated with the Pan-African high-pressure and regional metamorphism dominated by retrogression from granulite to amphibolite facies. P-T conditions range from 800 to 575°C and 12 to 9 kbar in metapelites, and from 750 to 550°C and 10 to 7 kbar in metadiorites (Nzenti et al., 1988; Mvondo et al., 2003;

Artigo original Original article

6 S. Owona et al. / Comunicações Geológicas (2013) 100, 1, 5-13

Owona et al., 2011b). The D1 overall pure shear deformation displays sub-horizontal S0/1 foliation and isoclinal F1 folds preserved only on quartzite and amphibolite. During the dominant simple shear D2 deformation, a S0/1/2 transposed foliation in metapelites and a S2 foliation in metadiorites with an associated L2 lineation are assigned to a SSW transport of the Yaounde nappe onto the Archean Ntem and Paleoproterozic Nyong complexes (Toteu et al. 2006; Mvondo et al., 2007; Mvondo Ondoa et al., 2009; Owona et al., 2011a).

Fig.1. (a) Geological sketch of the West-Central Africa and South America connection with cratonic masses and the Pan-African province of the Pan-Gondwana belt in a Pangea reconstruction modified from Castaing et al. (1994) and Ngako et al. (2003). CMR: Cameroon; CAR: Central African Republic; EG: Equatorial Guinea. NCSG: Northern Cameroon Supergroup; CCSG: Central Cameroon Supergroup; SCSG: Southern Cameroon Supergroup; CCSZ: Central Cameroon Shear Zone; SF: Sanaga Fault. Dashed lines mark the country boundaries. (b) Cameroon geological map with its main lithostructural units (modified after Ngako et al., 2003; Gnotué et al., 2000; Nzenti et al., 2006; Toteu et al., 2006): NC: Ntem complex; NyC: Nyong Complex; OC: Oubanguide Complex; YG: Yaounde Group, SG: Sanaga Group, DG: Dja Group, YoG: Yokadouma Group, SOG: Sembe-Ouesso Group. Arrows indicate the sub-N-S Yaounde nappe transport onto to NC and NyC. The location of study area is shown (c). Geological sketch of the SE of Yaounde in its contact zone with the NC and NyC. Shaded stars are Type I amphibolites and unfilled correspond to Type II amphibolites. Fig.1. (a) Esboço geológico da ligação entre a África centro-ocidental e a América do Sul com as massas cratónicas e as províncias pan-africanas da faixa pan-gondowânica numa reconstrução da Pangeia modificada a partir de Castaing et al. (1994) e Ngako et al. (2003). CMR: Camarões; CAR: República Centro-Africana; EG: Guiné Equatorial. NCSG: Supergrupo Camarões Norte; CCSG: Supergrupo Camarões Centro; SCSG: Supergrupo Camarões Sul; CCSZ: Zona de Cisalhamento dos Camarões central; SF: Falha Sanaga. As linhas a tracejado marcam as fronteiras entre países; (b) Mapa geológico dos Camarões com as principais unidades litostruturais (modificado a partir de Ngako et al., 2003; Gnotué et al., 2000; Nzenti et al., 2006; Toteu et al., 2006): NC: Complexo Ntem; NyC: Complexo Nyong; OC: Complexo Oubanguide; YG: Grupo Yaounde, SG: Grupo Sanaga, DG: Grupo Dja, YoG: Grupo Yokadouma, SOG: Grupo Sembe-Ouesso. As setas indicam o transporte sub- N-S da nappe Yaounde para o topo do NC e NyC. A localização da área de estudo é evidenciada; c) Esboço geológico do SE do Yaounde na sua zona de contacto com o NC e o NyC. As estrelas sombreadas são anfibolitos do tipo I e as não preenchidas correspondem aos anfibolitos do tipo II.

The D3 deformation is characterized by open to tight F3 folds with N-S to NE-SW trending axes parallel to L2, and shear zones such as the CCSZ (Ngako et al., 2003; Njonfang et al., 2008). D4 is a post-Pan-African deformation stage that corresponds to the brittle phase. Nd mean crustal residence ages for metasediments of the Yaounde Group (2100-1600 Ma) and meta-plutonic rocks (1900-1100 Ma) indicate a mixture of Paleoproterozoic and

juvenile Neoproterozoic sources (Toteu et al., 2006). Intrusion ages of meta-plutonic rocks are estimated at ca. 620 Ma (U/Pb on zircons, Penaye et al., 1993; Toteu et al., 1994; Pb/Pb on zircons, Owona et al., 2012). The age of the granulite facies metamorphism was estimated at 616 Ma (Sm/Nd composite WR-garnet isochrone in kyanite gneisses, Toteu et al., 1994), and at 620±10 Ma (U/Pb on rounded zircons from a metasedimentary rock, Penaye et al., 1993). The U/Th/Pb-monazite dating gave younger ages for the Pan-African metamorphic event (613 Ma to 545 Ma in the Yaounde Group metapelites, Owona et al., 2011b). Rb/Sr ages (598 - 540 Ma) on whole rocks and micas in metapelites and metadiorites are interpreted as cooling ages (Owona et al., 2012).

3. Materials and methods

Amphibolites samples, signed in Fig. 1c, were collected for lithogeochemistry. Major elements were determined by X-ray fluorescence spectrometry. Relative standard deviations are within 5%, and totals were within 100±1%. Rare earth elements and other trace elements were analysed by inductively coupled mass spectrometry in the Department of Geological Sciences, University of Arkansas in USA, using the protocol of Jenner et al. (1990), with standard additions, pure elemental standards for external calibration, and BIR-1 as reference material. Detailed descriptions of analytical procedures and values obtained for reference materials are given in Fan and Kerrich (1997). Detection limits are 0.01% for major elements and 0.005 to 5 ppm for trace elements. Elements analysed are given in Table 1.

Microprobe analyses (~500) of garnet porphyroblasts and coexisting amphiboles, plagioclases and opaques were performed with an electron microprobe JEOL JXA8900 RL at the Institut für Werkstoffwissenschaft at Freiberg/Saxony. The electron beam was set at 20 kV/20 nA and the matrix ZAF corrections were applied. Garnet, amphibole and plagioclase were analysed along transgranular profiles. Biotite was characterized by analyses from cores and rims. Representative slides and data are given in Fig. 2 and Table 2 (data tables including the oxide compositions can be obtained from the corresponding author).

Fig.2. Grano- to nematoblastic texture in amphibolites. Positions of garnet and amphibole zonation profiles shown in Fig. 4. XZ samples analysed for Type I amphibolites are (a) Ow140, (b) Ow245, (c) Ow247; (d) Ow253 and for Type II amphibolites, (e) Ow152 and (f) Ow116. Note radial and sigmoid Si internal schistosities in (a) and (b) discordant to matrix foliation or S0/1/2 foliation represented by amphibole, garnet, biotite, quartz and plagioclase. Amphibole (Amp), Biotite (Bt), plagioclase (Pl) and quartz (Qtz) occur in the vicinity of the garnet. Fig.2. Textura grano- a nematoblastica em anfibolitos. As posições dos perfis de zonamento da granada e da anfíbola é mostrado na Fig. 4. As amostras de orientação XZ analisadas para os anfibolitos do tipo I são (a) Ow140, (b) Ow245, (c) Ow247; (d) Ow253 e para o Tipo II (e) Ow152 e (f) Ow116. De notar a xistosidade interna radial e sigmoidal em (a) e (b) discordante em relação à foliação da matriz ou à foliação S0/1/2 representada pela anfíbola, granada, biotite, quartzo e plagioclase. Anfíbola (Amp), Biotite (Bt), plagioclase (Pl) e quartzo (Qtz) ocorrem nas proximidades da granada.

Petrogenesis of amphibolites from Cameroon 7

Table 1. Geochemical analyses of the Yaounde Group amphibolites. Type I amphibolites correspond to samples Ow140, Ow245, Ow247; Ow253 and Type II

correspond to samples Ow152 and Ow116.

Tabela 1. Análises de geoquímica dos anfibolitos do Grupo Yaounde. Os anfibolitos do Tipo I correspondem às amostras Ow140, Ow245, Ow247; Ow253 e os de Tipo

II correspondem às amostras Ow152 e Ow116.

4. Results

4.1. Petrography

Amphibolites occur in the YG as large foliated and concordant bodies (Type I) in mica schists and as centimetric boudins (Type II) parallel to the L2 lineation in paragneisses. At the outcrop and microscopic scales, the S0/1/2 foliation is outlined by differentiated mafic and quartzo-feldspathic layers, whereas the L2 mineral lineation is defined by the preferred orientation of amphiboles. Amphibolites are holomelanocrates with grano- to nematoblastic textures (Fig. 2). They are dark green to black and are foliated, forming amphibole-rich (metric) and quartz-rich layers (millimetric to centimetric). Amphibole and quartz are recognizable at the macroscopic scale.

Under the microscope, amphibolites consist mainly of light to dark green amphibole (70-75%), garnet (5-10%), quartz (5-10%), and opaque (5%). Pyroxene, biotite, muscovite, calcite, zircon and epidote are accessories. Amphibole (0.5-1 mm) defines mafic layers corresponding to the S0/1/2 transposed foliation. Its blasts (0.5-1 mm) are fine, stretched, cracked,

aligned and form the L2 mineral lineation. They display pressure shadows around garnet. Garnet includes idiomorphic blasts (0.05–1 mm) disseminated in the rocks, rich in zircon, pyroxene, opaque and epidote inclusions, which define sigmoid to helichoidal Si internal schistosity features. Quartz (0.05-1 mm) form felsic layers corresponding to the S0/1/2 foliation. It consists of recrystallized blasts showing grain boundary area reduction and sub grain reduction features. Calcite and muscovite appears in the matrix. They are aligned parallel with amphibole and quartz blasts. Amphibole is weathered to epidote. Amphibolites main mineral assemblages are the follows: (i) Pyroxene ± Biotite ± Quartz ± Opaques ± zircon inclusions in garnet poekiloblasts which defines various schemes of Si internal schistosity; (ii) Amphibole + Garnet + Calcite ± Plagioclase ± Muscovite ± Biotite ± Quartz ± Opaques that define the S0/1/2 foliation and the D1-D2 granulite/amphibolite metamorphic association, and (iii) the Quartz ± Biotite ± Epidote association that represent the epidote-amphibolite facies.

4.2. Whole-rock compositions

Type I amphibolites (samples Av14, Mb38, Mp2, Mp3, Ow15 and Ow140) show the composition of sub-alkaline basalts, with lower Fe2O3tot, TiO2, P2O5, but higher MgO, CaO and Na2O contents compared to Type II amphibolites (samples Mp1, Ow116 and Ow152) which correspond to subalkaline and high-Fe basaltic andesites in the TAS diagram (Table 1, Fig. 3). Type I amphibolites total alkali (Na2O + K2O) content ranges from 2.63 wt% to 2.89 wt% and the ferromagnesian composition (Fe2O3+MgO+TiO2) varies from 19.85 wt% to 21.65 wt% while Type II amphibolites have a wider total alkali range (1.38 wt% to 3 wt%) and a higher ferromagnesian content (23.35 wt% to 26.6 wt%). Chondrite normalized Type I and Type II rare earth elements (REE) compositions, after Hofmann (1988), display two main patterns (Fig. 3c, d). Type I shows an almost linear distribution characterized by decreasing abundances from LILE, Ba, Rb and Th down to HFSE similar to the Caribbean-Columbian Ocean plateau basalts (Hauff et al., 2000; Willbold, M. and Tracke, A., 2006). They display depleted Pb, Th and Ti and slight negative to positive (0.98-1.12) Eu anomalies. Lu/Hf ratios range from 0.23 to 0.28 while Zr/Hf ratios vary between 32.67 and 49.95 and Ti/V ratios from 19.93 to 36.30. In the FeO*-Al2O3-MgO triangular diagram (not shown), Zr versus Zr/Y diagram of Pearce and Cann (1973) and the Ti/1000 versus V plot of Shervais (1982) in Fig. 3e and f, Type I amphibolite samples Mb38, Mp2, Ow15 and Ow140 plot in the mid-ocean ridge basalts (MORB) domain whereas samples Av14 and Mp3 show affinities to continental flood basalts. Type II amphibolites display an almost linear distribution characterized mainly by decreasing abundances from Ba to Lu (Fig. 3c, d) similar to Pitcairn ocean island basalts (OIB) (Woodhead and Mcculloch, 1989). Their regular slope is indented by Pb, Sr, and Ti negative anomalies. Eu displays slight negative and positive anomalies (0.92-10.5). The Lu/Hf ratio is 0.07 while the Zr/Hf ratio varies from 38.20 to 39.3 and the Ti/V ratio ranges between 55.98 and 109.72. In the Zr versus TiO2 diagram (Fig. 3e) three Type II amphibolite samples derived from Arc lavas while four of them show affinities with MORB and two others derived from continental flood basalts (Fig. 3f).

4.3. Mineral chemistry

Garnet and amphibole chemical zonation trends were analysed. Due of usual difficulties to recognize these zonation trends in the raw zonation profiles alone, the garnet mineral data were also plotted in the grossular-pyrope-spessartine ternary diagrams (Schulz, 1993; Schulz et al., 2001) and both with amphibole in


the XCa-XMg diagrams (Martignole and Nantel, 1982). The Yaounde Group garnets are mainly almandine types. The almandine contents range from 40-48 % in Type I amphibolites and vary from 59-66% in Type II and discriminate rock type one to another. According to their zonation, almandine-garnet zonation profiles from cores to rims can be classified into 5 types, ranging from type 1 to 5 as in Yaounde group metapelites (Owona et al., 2011b). Only types 1, 3 and 5 are identified. In type 1 (Ow152), the spessartite content decreases (12.92% -

9.12%) while pyrope increases slightly (2.8% – 4.17 %) at quite constant grossular contents around 20%. Type 3 (Ow116) shows increase pyrope (3.61% - 10.48%) and grossular (9.59% - 28.72%) contents while spessartite decreases (12.26% - 1.35%). Type 5 (Ow245, Ow247) displays a decrease in pyrope (24.21% – 15.88%) and spessartite (5.28% - 1.43%), constant grossular content around 35% in Ow247 and a slight increase (27.42 – 29.98%) in Ow245 (Fig. 4a-h).

Table 2. Major element contents of garnet (Grt), amphibole (Amph) and coexistent biotite (Bt) and plagioclase (Pl) in Yaounde Group amphibolites, analysed by electron microprobe. Normalized to 12O (garnet), 24O (amphibole), 12O (biotite) and 8O (plagioclase). Mineral pairs used in geothermobarometry are as follows for the garnet-amphibole-plagioclase-quartz geothermobarometer: Sample Ow152: Grt75-Amph16, Grt75-Amph21; Sample Ow116: Grt64-Amph93, Grt60-Amph87; Sample Ow247: Grt393-Amph310, Grt387-Amph304; Sample Ow253: Amph93-Amph87. – For the Amphibole TGV geothermobarometer: Sample Ow152: Amph16, Amph21; Sample

Ow116: Amph112-Amph109; Sample Ow247: Amph307-Amph312; Ow253: Amph39-Amph17; Sample Ow245: Amph93-Amph87; Sample Ow140: Amph184-Amph194. – For the Amphibole-plagioclase-quartz geothermobarometer: Sample Ow152: Amph16-Amph21; Sample Ow116: Amph114-Amph111; Sample Ow247: Amph112-Amph109;

Ow253: Amph93-Amph87. Data are from Owona (2008)..

Tabela 2. Conteúdo em elementos maiores da granada (Grt), anfíbola (Amph), biotite coexistente (Bt) e plagioclase (Pl) nos anfibolitos do Grupo Yaounde analisados por microssonda electrónica. Normalização para 12O (granada), 24O (anfíbola), 12O (biotite) e 8O (plagioclase). Pares de minerais utilizados para a getotermobarometria são os

seguintes para o geotermobarómetro granada-anfíbola-plagioclase-quartzo: Amostra Ow152: Grt75-Amph16, Grt75-Amph21; Amostra Ow116: Grt64-Amph93, Grt60-Amph87; Amostra Ow247: Grt393-Amph310, Grt387-Amph304; Amostra Ow253: Amph93-Amph87. Para o geotermobarómetro TGV da Anfíbola: Amostra Ow152:

Amph16, Amph21; Amostra Ow116: Amph112-Amph109; Amostra Ow247: Amph307-Amph312; Ow253: Amph39-Amph17; Amostra Ow245: Amph93-Amph87; Amostra Ow140: Amph184-Amph194. – Para o geotermobarómetro anfíbola-plagioclase-quartzo: Amostra Ow152: Amph16-Amph21; Amostra Ow116: Amph114-Amph111; Amostra

Ow247: Amph112-Amph109; Ow253: Amph93-Amph87. Dados de Owona (2008).


Fig.3. Typology of Yaounde amphibolite in TAS diagram. In geochemical diagrams, labels 2, 14, 15, 38, 140 correspond to samples Mp2, Av14, Mb38, Ow15 and Ow140. Labels 3 and 14 correspond to samples Mp3 and Av14. Labels 1, 116 and 152 design samples Mp1, Ow116 and Ow152. (c, d) Spider diagrams of Types I and II amphibolites normalized to chondrite, compared to Caribbean-Columbian Ocean plateau and Pitcairn OIB (Hauff et al., 2000; Hofmann, 1988; Willbold and Stracke, 2006; Woodhead and Mcculloch, 1989). (e) TiO2 vs. Zr diagram (Pearce and Cann, 1973) discriminating diagram applied to Type I and II amphibolites. Tectonic environments of Types I and II amphibolites discriminate MORB, arc and within plate basalts. (f) V vs. Ti/1000 (Shervais, 1982) discriminating diagram for basalts applied to Type I and II amphibolites; the plot has MORB, continental flood basalts and OIB or alkali basalts interpreted as high-Fe andesite basalts. Fig.3. Tipologia do anfibolito Yaounde no diagram TAS. Nos diagramas geoquímicos, as referências 2, 14, 15, 38, 140 correspondem às amostras Mp2, Av14, Mb38, Ow15 e Ow140. As referências 3 e 14 correspondem às amostras Mp13 e Av14. As referências 1, 116 e 152 referem-se às amostras Mp1, Ow116 e Ow152. (c, d) Spidergrams dos anfibolitos do tipo I e II normalizados ao condrito, comparados com o plateau oceânico Caribenho-Columbiano e o OIB Pitcairn (Hauff

et al., 2000; Hofmann, 1988; Willbold & Stracke, 2006; Woodhead & Mcculloch, 1989). (e) Diagrama discriminante TiO2 vs. Zr (Pearce & Cann, 1973) aplicado aos anfibolitos do Tipo I e II. Os ambientes tectónicos dos anfibolitos do Tipo I e II discriminam basaltos MORB, arco e do interior da placa. (f) diagrama discriminante V vs. Ti/1000 (Shervais, 1982) para basaltos aplicado aos anfibolitos do Tipo I e II; a projecção tem MORB, basaltos continentais e OIB ou basaltos alcalinos interpretados como basaltos andesíticos com elevado Fe.

Leake et al. (1977) classification used for amphibole nomenclature (Fig. 5a) reveals that amphiboles in the study have a tschermakitic hornblende composition (Mp1, Ow116 and Ow253) with Si and Mg contents of 6.24-6.43 p.f.u. and 1.88-2.59 p.f.u. respectively. They correspond to magnesio-hornblende with Si and Mg contents of 6.27-6.63 p.f.u. and 2.29-2.59 p.f.u., respectively, for sample Ow14. They range from ferro-tschermakite-hornblendes to tschermakite-hornblendes in samples Mp2, Mp3 and Ow152 with Si contents varying from 6.19 to 6.34 p.f.u. and Mg contents ranging from 1.18-2.52 p.f.u.. As for samples Ow253 and Ow247, amphiboles vary from tschermakite-hornblendes to tschermakites with Si compositions of 6.25-6.37 p.f.u. and Mg contents of 2.31-2.44 p.f.u. Figures 5b-g display XMg, XFe, XCa and XMn variations in amphiboles suggesting the zonation of their profiles from cores to rims. In this study variations in these elements from cores to rims evolved inversely, with most decreasing slightly, confirming Mg-Fe as well as Ca-Mn exchanges in bulk composition. Plagioclase An(5-31) ranges from albite to andesine. Opaques are ferrotitanium types with TiO2 content that ranges from 53 to 55 % wt and FeO* constant at ca.47 wt%. The biotite in sample Ow152 is characterized by XMg constant around 0.33.

Fig.4. (a-h) Zonation profiles from cores-to-rims (c, r) of single garnet porphyroblasts from Types I (a-d) and II (e-h) amphibolites in Almandine - 40 % (due to scale, Alm-40), grossular (Grs), pyrope (Prp) and spessartite (Sps) components, see Table 1. Evolution of garnet compositions in grossular (Grs), pyrope (Prp) and spessartite (Sps) components. Arrows indicate the overall core-to-rim evolution, assembled from several analysed porphyroblasts in a sample; analyses used for thermobarometry in Fig. 2 are marked with numbers. See Fig. 1c for locations of samples. Fig.4. (a-h) Perfis de zonamento do núcleo para os bordos (c, r) de um único profiroblasto de granada dos anfibolitos de Tipo I (a-d) e II (e-h) em Almandina 40% (devido à escala Alm-40), e componentes grossulária (Grs), piropo (Prp) e espessartina (Sps). As setas indicam a evolução geral do núcleo para o bordo, reunindo a informação de vários porfiroblastos numa única amostra; as análises usadas para a termobarometria na Fig. 2 estão marcadas com números. Ver a Fig. 1c para a localização das amostras.


Fig.5. (a) Classification of amphiboles according Leake et al. (1997) and (b-g) their XCa-XMg-XFe-XMn zonations. Amphiboles analysed derived for Type I from samples (b) Ow140, (c) Ow245, (d) Ow247; (e) Ow253 and for Type II, samples (f) Ow152 and (g) Ow116. Fig.5. (a) Classificação das anfíbolas de acordo com Leake et al. (1997) e (b-g) o seu zonamento XCa-XMg-XFe-XMn. As anfíbolas analisadas derivam dos anfibolitos do Tipo I das amostras (b) Ow140, (c) Ow245, (d) Ow247; (e) Ow253 e do Tipo II, das amostras (f) Ow152 e (g) Ow116.

4.4. Geothermobarometry

When compiled in the grossular - pyrope - spessartite garnet and amphibole ternary diagrams, it is obvious that each of the single garnet and amphibole zonation gives a characteristic chemical evolution trend that represent a segment or period of crystallization along an overall temperature-pressure evolution (Fig. 6). In each sample, the cores of zoned garnet/amphibole, its plagioclase and biotite inclusions, as well as cores of zoned matrix minerals and plagioclase should represent an early stage of the metamorphic evolution. These zonations give evidence that at least the Fe, Mg, Mn and Ca components were not homogenized at high temperatures but reflect garnet and amphibole growth zonation trends (Owona et al., 2011b). Accordingly, the garnets labelled as types 1-3 characterized by the increase of Mg and increase then decrease of Ca should have crystallized during increasing temperatures and at

increasing/decreasing pressures as suggested by pyrope and grossular zonations in garnet, respectively (Fig. 6a). Zonations labelled as types 4-5, characterized by the decrease of Mg, increase of Mn and also Ca should have crystallized at decreasing temperature and pressure as in Yaounde group metapelites (Owona et al., 2011b; Fig. 6a). They could be explained by cooling decompression crystallization that is marked by retrogressive small-scale cation-exchange in the outer garnet and amphibole rims (Bhattacharya et al., 1982; Martignole and Nantel, 1982; Spear, 1993). Amphiboles should have parallel evolution, crystallizing during increasing or decreasing temperatures and pressure as suggested by the XMg and XCa zonations (Fig.5b-g). They are labelled as types 1 to 5 as garnets. Amphiboles labelled as types 1-3 characterized increase of Mg and increase then decrease of Ca suggesting their crystallization during increasing temperatures and at increasing/decreasing pressures (Fig. 6b). Zonations labelled as type 5, characterized by the decrease of Mg, increase of Ca and Mn should have crystallized at decreasing temperature and pressure (Fig. 6b).

Such localised equilibria within low-variance assemblages allow the evaluation of P-T changes for the consecutive stages of garnet and amphibole growth by geothermobarometry. Garnet and amphibole were used with coexisting or not coexisting biotites for the garnet-biotite Fe-Mg, garnet-amphibole Fe-Mg and amphibole geothermometers, and the garnet-plagioclase-amphibole-quartz Ca-net-transfer geobarometer based on Garnet + amphibole + plagioclase + biotite and Amphibole + plagioclase + biotite mineral equilibra observed in Yaounde Group amphibolites (Table 2). Figures 7a-c display individual P-T paths of Types I and II amphibolites determined by the (i) garnet-amphibole-plagioclase-quartz (Gerya et al., 1997; Kohn and spear, 1989, 1990; Perchuck and Lavrent’eva, 1983; Perchuck et al., 1985; Perchuck, 1991; Johnson and Rutherford, 1989), (ii) amphibole-plagioclase-quartz (Perchuck et al., 1985; Perchuck and Lavrent’eva, 1983; Perchuck, 1991; Gerya et al., 1997) and (iii) TGV amphibole-equilibra thermobarometry (Gerya et al., 1997; Holland and Blundy, 1994); while figure 7d combines P-T paths in the Pan-African geodynamic model. The geothermobarometric estimates include a minimum error of ±50 °C and ±1 kbar as suggested by many authors after several calibrations (Holdaway, 2001; Wu and Cheng, 2006); based on an internally consistent thermodynamic data set (Holland and Powell, 1990; Powell and Holland, 1994), with the activity models for garnet given by Ganguly et al. (1996) and for plagioclase as proposed by Powell and Holland (1994).

The (i) Garnet-amphibole-plagioclase-quartz thermobarometer gives high pressure-medium temperature P-T paths (Fig. 7) with garnet labelled types 1-3 (Fig. 6a). Type 1 (Ow152, Ow140) almandine garnets crystallized at increasing temperature and pressure from 372 °C, 8.64 kbar to 515 °C, 9.16 kbar and from 460 °C, 7.5 kbar to 530 °C, 7.95 kbar (Fig. 7). Type 3 garnet (Ow116, Ow247) crystallized at increasing temperature (430 °C to 540 °C, then 620 °C to 650 °C) and decreasing pressures (9.4 kbar to 7.4 kbar then 5.8 kbar to 5.2 kbar).

The (ii) Amphibole-plagioclase-quartz thermobarometer determines low pressure-middle temperature P-T paths represented by amphiboles labelled types 1 to 5 (Fig. 6b). Types 1-3 amphiboles (Ow152, Ow140) crystallized at increasing temperature and pressure from 600 °C, 6.2 kbar to 640 °C, 6.85 kbar and from 620 °C, 4.8 kbar to 640 °C, 5.5 kbar (Fig. 7). Type 5 amphiboles (Ow140, Ow247) crystallized at decreasing temperature and pressure, from 660 °C, 6.3 kbar to 630 °C, 5.8 kbar and from 680 °C, 5 kbar to 630 °C, 4.6 kbar.

The (ii) TGV amphibole-equilibrium thermobarometer determines middle pressure-middle temperature P-T paths


represented by type 1 amphiboles too (Fig. 6b). Type 1 amphiboles (Ow116, Ow152) crystallized at increasing temperature and pressure from 610 °C, 7.5 kbar to 640 °C, 8 kbar and from 630 °C, 7.3 kbar to 670 °C, 8.5 kbar (Fig. 7). Type 5 amphiboles (Ow245, Ow253, Ow247, and Ow140) crystallized at decreasing temperature and pressure, from 670 °C, 7.7 kbar to 630 °C, 7.1 kbar; from 670 °C, 7.8 kbar to 620 °C, 6.5 kbar and; from 620 °C, 6.7 kbar to 695 °C, 6.1 kbar (Fig. 7).

Fig.6. Semi quantitative P-T-evolution trends of (A) isobaric heating, (B) isothermal decompression and (C) decompression/cooling models in grey boxes expressed by XCa-XMg after Martignole and Nantel (1982) for garnet (a) and amphibole (b) in amphibolites. In comparison of garnet in metapelites that display trends 1 to 5 (Owona et al., 2011b); one can distinguish prograde trends 1 and 3 with increasing and decreasing XMg and zonation trend 5 attributed to retrogression. The zonation trends of retrogressive cation exchange in the margins of porphyroblasts are only shown for sample Ow247 and omitted for the other samples. The P-T data presented in Fig. 7 was calculated by involving the garnet and amphibole analyses represented as points in the XCa vs. XMg coordinates. Fig.6. Padrões de evolução P-T semi-quantitativos de (A) aquecimento isobárico, (B) descompressão isotérmica e (C) modelos de descompressão/arrefecimento em caixas cinzentas expressos pelo XCa-XMg segundo Martignole & Nantel (1982) para a granada (a) e anfíbola (b) nos anfibolitos. Comparando a granada nos metapelitos que mostra as tendências 1 a 5 (Owona et al., 2011b); observam-se as tendências prógradas 1 a 3 com subida e descida de XMg e a tendência de zonamento 5 atribuída à retrogradação. As tendências de zonamento de troca catiónica retrógrada nas margens dos porfiroblastos são apenas observadas na amostra Ow247 e encontram-se omitidas nas outras amostras. Os dados P-T apresentados na Fig. 7 foram calculados envolvendo as análises de granada e anfíbola representadas pelos pontos nas coordenadas XCa vs. XMg.

5. Discussion and conclusion

Geochemistry has discriminated Types I and II amphibolites of the YG and CAFB. Type I is derived from sub-alkaline basalts. Their spider diagram displays a slight negative Nb anomaly. They are depleted in Th, Pb, Sr and Ti. Their Ti/V ratios (20-50) define MORB compositions similar to Caribbean-Columbian Ocean plateau basalts (Hauff et al., 2000; Shervais, 1982; Srivastava et al., 2004; Willbold and Stracke, 2006). Zr/Hf and Lu/Hf ratios in terrestrial rocks that determine mineral–melt partition coefficients for HFSE (Table 1, Pfänder et al., 2007), combined in coordinate diagrams (Shervais, 1982; Pearce and Norry, 1979), suggest MORB for Type I amphibolite protoliths.

Fig.7. Syndeformational and overall clockwise P-T path from the Yaounde Group amphibolites. (a-c) P-T path segments from Types I and II amphibolites in the Yaounde Group. (d) Summary of P-T data and D1-D2 syndeformational P-T path sections from both, Types I and II amphibolites that recorded the same tectonothermal event. Note the overall clockwise P-T evolution given by the P-T paths synthetic diagram. The stability field are the kyanite and sillimanite ones. Staurolite (St+, St), cordierite (Cd+) univariant lines after Spear (1993). Fig.7. Percurso P-T sin-deformacional e directo dos anfibolitos do Grupo Yaounde. (a-c) segmentos do percurso P-T dos anfibolitos do Tipo I e II do Grupo Yaounde. (d) Sumário dos dados P-T e secções dos percursos P-T sin-deformacionais D1-D2

de tanto os anfibolitos do Tipo I e II que registaram o mesmo evento tectonotérmico. De notar a evolução P-T directa evidenciada pelo diagrama sintético P-T. Os campos de estabilidade são os da cianite e da sillimanite. As linhas univariantes da estaurolite (St+, St), cordierite (Cd+) encontram-se segundo Spear (1993).

This interpretation favours the presence of oceanic remnants and suggests ophiolite origin of the talc schists in the Ngoung, Lamal Pougue, and Bibodi Lamal localities of the YG (Nkoumbou et al., 2006). MORB protolith would corroborate the existence of the Serigipe Neoproterozoic Ocean crust hypothesis in the Pan-African Mobile Zone geodynamic models (Trompette, 1994). Their andesitic affinity can be related to crustal contributions. Compared to Type I, REE patterns of Type II amphibolites corroborated by Zr/Hf, Lu/Hf and Ti/V ratios > 50 in terrestrial rocks and the Ti/1000 vs. V and TiO2-MnOx10-P2O5x10 plots suggest their similarities with Pitcairn ocean island basalts (Shervais, 1982; Mullen, 1983; Woodhead and Mcculloch, 1989; Srivastava et al., 2004; Pfänder et al., 2007). Despite the above concordant conclusions, this OIB nature is not confirmed in several discriminating diagrams such as TiO2 vs. Zr (Pearce and Cann, 1973; Fig. 3e), FeO*-Al2O3-MgO (Pearce et al; 1977; not shown), 2Nb-Zr/4-Y (Meschede, 1986; not shown) and Zr vs. Zr/Y (Pearce and Norry, 1979; not shown). Type II amphibolites can be considered as high-Fe basaltic andesites overlapped by crustal contaminated basaltic andesites indicated by Fe2O3*, Al2O3 and TiO2 enrichments (Fig. 2d, Shervais, 1982), a model similar to the that in a back arc environment as suggested for the central Eastern Desert of Egypt (e.g. Abd El-Naby and Frisch, 2006). They suggest the existence of a subduction environment and the related andesitic volcanism. The mineralogy and geothermobarometry corroborate this collisional crustal thickening environment under the overthrusting of the Yaounde Nappe during the Pan-African tectonothermal event as well as the collision between the West African,?Saharan meta- and Congo cratons (Abdelsalam et al., 2002; Owona et al., 2011b) and not by exhumation driven by crustal extension, as has been shown by Michard et al. (1993) and Wheeler and Butler (1994). This correlation can be extended to the Neoproterozoic Sergipano belt in NE Brazil related to the collision between the Francisco-West African cratons, with similar tectonic evolution like the CAFB (Cameroon, Africa) (Da Silva et al., 2005;


Neves et al., 2006; Oliviera et al., 2006). Between the (i) garnet-amphibole-plagioclase-quartz (Gerya et

al., 1997; Kohn and spear, 1989, 1990; Perchuck and Lavrent’eva, 1983; Perchuck et al., 1985; Perchuck, 1991; Johnson and Rutherford, 1989), (ii) amphibole-plagioclase-quartz (Perchuck et al., 1985; Perchuck and Lavrent’eva, 1983; Perchuck, 1991; Gerya et al., 1997) and (iii) TGV amphibole-equilibra thermobarometry (Gerya et al., 1997; Holland and Blundy, 1994), the garnet-amphibole-plagioclase-quartz thermobarometer gives mainly high-P-T value because of the presence of garnet that crystallized before amphibole. Types 1-2-3-4-5 garnets typified by XCa vs. XMg (Owona et al., 2011b) for P-T predictions in section 4.4 and figure 8 are confirmed for types 1, 3 and 5. Type 1 garnets define increasing temperature (375 °C to 530 °C) and pressure (7.5 kbar to 9.16 kbar) phases. Type 3 determine increasing temperature (430 °C to 650 °C) but decreasing pressures (9.4 kbar to 5.2 kbar) stages. Type 5 crystallized at decreasing temperature and pressure, from 680 °C to 630 °C and 6.3 kbar to 4.6 kbar. Type 1 amphiboles crystallized at increasing temperature and pressure from 610 °C and 7.5 kbar to 670 °C and 8.5 kbar. Type 3 amphiboles with increasing temperature and decreasing of pressure is not confirmed with amphibole as garnets. Type 5 defines decreasing temperature and pressure, from 695 ° and 7.8 kbar to 630 °C and 6.1 kbar. These P-T paths display an overall clockwise metamorphism (Fig. 7d). The prograde phase (375 °C and 4 kbar to 550°C 8 kbar), associated with the D1-D2 deformation occurred under isobaric compressional heating environment and peaked (420-640°C and 9.5-5.2 kbar) under heating decompression conditions. The retrograde phase (675-575°C and 7.5-5 kbar) related to the D3-D4 deformation happened under isothermal decompression and isobaric cooling conditions. This high pressure-middle temperature, followed by the low pressure-middle temperature metamorphism corresponds to the Pan-African metamorphism (616-586 Ma, U/Th/Pb-Monazite, Owona et al., 2011b; Sm/Nd-Garnet, Toteu et al., 1994). Its cooling phase extends until 540 ± 5 Ma in surrounding metapelites (Rb/Sr-WR-Ms-Bt, Owona et al., 2012) and ca. 535 Ma in amphibolites (Ar/Ar-Hbl, Owona, 2008). This overall clockwise metamorphism; registered in Types I and II amphibolites is similar to numerical modelling collisional crustal thickening (England and Thompson, 1984; Davy and Gillet, 1986; Spear, 1993) such as in the YG and CAFB (Owona et al., 2011b). It is related to continental collision recorded in above Types I and II amphibolites from the Neoproterozoic Yaounde Group and CAFB (Cameroon, Central Africa), comparable to the one that occurred in the Neoproterozoic Sergipano belt in NE Brazil (Da Silva et al., 2005; Neves et al., 2006; Oliviera et al., 2006).

Acknowledgements

The authors are grateful to the DAAD (German Academic Exchange Office) for financial support toward Owona’s stay in Freiberg (Germany); to the members of the Laboratory of Tectonophysics, Instittute für Geologie for geochemical analyses; to M. Göbbels and A. Renno for electron microprobe analyses at the TU-Bergakademie Freiberg, Sachsen, Germany. The constructive reviews by C. K. Shang, anonymous reviewers are also gratefully acknowledged.

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petrogenesis of amphibolites from the neoproterozoic yaounde group

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