Petrography and geochemistry of the Ngaoundéré Pan-African granitoids in Central North Cameroon: Implications for their sources and geological setting

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Journal of African Earth Sciences 44 (2006) 511–529

Petrography and geochemistry of the Ngaoundere Pan-Africangranitoids in Central North Cameroon: Implications for their

sources and geological setting

R. Tchameni a,*, A. Pouclet b, J. Penaye c, A.A. Ganwa a, S.F. Toteu c

a Departement des Sciences de la Terre, Faculte des Sciences, Universite de Ngaoundere, B.P. 454, Ngaoundere, Cameroonb Institut des Sciences de la Terre, UMR 6113, Universite d’Orleans, B.P. 6759, 45067 Orleans cedex 2, France

c Centre for Geological and Mining research (CRGM), B.P. 333 Garoua, Cameroon

Received 7 October 2005; received in revised form 24 October 2005; accepted 9 November 2005Available online 25 January 2006

Abstract

The Nagoundere Pan-African granitoids in Central North Cameroon belong to a regional-scale massif, which is referred to as theAdamawa-Yade batholith. The granites were emplaced into a ca. 2.1 Ga remobilised basement composed of metasedimentary andmeta-igneous rocks that later underwent medium- to high-grade Pan-African metamorphism. The granitoids comprise three groups:the hornblende–biotite granitoids (HBGs), the biotite ± muscovite granitoids (BMGs), and the biotite granitoids (BGs). New Th–U–Pb monazite data on the BMGs and BGs confirm their late Neoproterozoic emplacement age (ca. 615 ± 27 Ma for the BMGs andca. 575 Ma for the BGs) during the time interval of the regional tectono-metamorphic event in North Cameroon. The BMGs also showthe presence of ca. 926 Ma inheritances, suggesting an early Neoproterozoic component in their protolith.

The HBGs are characterized by high Ba–Sr, and low K2O/Na2O ratios. They show fairly fractionated REE patterns (LaN/YbN 6–22)with no Eu anomalies. The BMGs are characterized by higher K2O/Na2O and Rb/Sr ratios. They are more REE-fractionated(LaN/YbN = 17–168) with strong negative Eu anomalies (Eu/Eu* = 0.2–0.5). The BGs are characterized by high SiO2 with K2O/Na2O > 1. They show moderated fractionated REE patterns (LaN/YbN = 11–37) with strong Eu negative anomalies (Eu/Eu* =0.2–0.8) and flat HREE features (GdN/YbN = 1.5–2.2). In Primitive Mantle-normalized multi-element diagrams, the patterns of all rocksshow enrichment in LILE relative to HFSE and display negative Nb–Ta and Ti anomalies. All the granitoids belong to high-K calc-alkaline suites and have an I-type signature.

Major and trace element data of the HBGs are consistent with differentiation of a mafic magma from an enriched subcontinentallithospheric mantle, with possible crustal assimilation. In contrast, the high Th content, the LREE-enrichment, and the presence of inher-ited monazite suggest that the BGs and BMGs were derived from melting of the middle continental crust. Structural and petrochemicaldata indicate that these granitoids were emplaced in both syn- to post-collision tectonic settings.� 2005 Elsevier Ltd. All rights reserved.

Keywords: Central North Cameroon; Pan-African granitoids; Geochemistry; Th–U–Pb monazite age

1. Introduction

The Central African Fold belt (CAFB) in Cameroon isdominated in its central part by abundant late-Pan-Africangranitoids (Fig. 1), which extend south of the Tchollire-

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doi:10.1016/j.jafrearsci.2005.11.017

* Corresponding author.E-mail address: [email protected] (R. Tchameni).

Banyo shear zone up to the Bertoua region and continueeastward into the Yade massif (Central African Republicand Chad). The granitoids belong to a complex batholithreferred to here as the Adamawa-Yade batholith. Theimportance of this batholith results from its location northof the late-Neoproterozoic nappe system of southernCameroon and its emplacement into a remobilisedPaleoproterozoic basement that is interpreted as part of

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Fig. 1. Geological context (A) Geology of West-central Africa and northern Brazil in a Gondwana (pre-drift) reconstruction (modified from Castainget al., 1994; Toteu et al., 2001). Dashed line boundary of Cameroon. Thick line, boundary of the two continents: (1) Phanerozoic cover; (2) Neoproterozoicformations; (3) Regions of Brasiliano/Pan-African deformation in which Paleoproterozoic basement is absent or only present as small isolated blocks; (4)Regions of Brasiliano/Pan-African deformation with large amounts of reworked Paleoproterozoic basement; (5) cratons; (6) faults. (B) Simplifiedgeological map of Cameroon (after Toteu et al., 2001) showing the location of the Ngaoundere plutonic and major lithotectonic units. KCF, Kribi-Campofault; AF, Adamaoua fault; SF, Sanaga fault; TBF, Tchollire-Banyo fault.

512 R. Tchameni et al. / Journal of African Earth Sciences 44 (2006) 511–529

an active continental margin (Toteu et al., 2001, 2004). Theimportance of the Adamawa-Yade batholith also results

from its spatially close association with major transcurrentshear zones of central Cameroon (Dumont, 1986; Ngako

R. Tchameni et al. / Journal of African Earth Sciences 44 (2006) 511–529 513

et al., 1991, 2003). These shear zones continue into NEBrazil, where they control the emplacement of late-Brasiliano (Pan-African) granitoids (Neves et al., 1996,2000; Guimaraes and da Silva Filho, 2000; Guimaraeset al., 2004).

Despite this importance, the Adamawa-Yade batholithis poorly surveyed, particularly with regard to geochemicaland geochronological studies. Data exist only for a few iso-lated massifs, which probably belong to another batholithin south-western Cameroon (Tagne-Kamga et al., 1999;Tagne-Kamga, 2003; Nguiessi Tchankam et al., 1997;Nzolang et al., 2003). In this paper, we report petrograph-ical, geochemical and geochronological data on theNgaoundere granitoids, which permit discussion of thesource and tectonic setting of the Adamawa-Yadebatholith.

2. Regional geological setting

The Adamawa-Yade massif is dominated by a NE–SWelongated regional-scale plutonic complex intrusive into aPaleoproterozoic basement and locally covered by Creta-ceous deposits (Mbere and Koum basins) and by Cainozoicvolcanic rocks of the Cameroon line. The hosting basementrocks, which now crop out as large septa within the batho-lith, comprise 2.1 Ga high-grade metasediments and orthog-neisses showing an important contribution from Archeancrust (Nd isotope and inherited zircons) and were inten-sively reworked during the Pan-African orogeny (Toteuet al., 2001). The Pan-African has also developed the late-Neoproterozoic Lom schist belt (Soba et al., 1991; Toteuet al., this volume) and numerous dextral transcurrent shearzones underlined by low- to medium-grade mylonites.

The Adamawa-Yade batholith consists of a great varietyof more or less deformed rock-types of different ages.Except for the sub-circular post-tectonic plutons, whichhave been easily mapped due to their high relief, there isa very poor cartographic distinction between the othercomponents of the batholith; thus detailed mapping ofthe batholith is necessary. The various lithologies of thebatholith include: dominant biotite and amphibole gran-ites, biotite granites, biotite, amphibole and pyroxene gran-ites, biotite and muscovite granites, diorites, gabbros, andsyenites. According to the state of deformation, theserock-types have been classified as syntectonic, late-tectonicand post-tectonic (Lasserre, 1961; Toteu et al., 2001). Thefew geochemical data that are available for the batholithare from the late to post-tectonic granitoids of the Lomregion and show a transitional composition (Soba, 1989).

Numerous Rb–Sr whole-rock and mineral ages in therange 500–600 Ma have been recorded in the Adamawa-Yade batholith in Cameroon, Chad and Central AfricanRepublic (Bessoles and Trompette, 1980). Preliminary U–Pb zircon ages on two granitoids of the Ngaoundere regionyielded 622 ± 25 Ma for the Moungel orthogneiss and verydiscordant plots with upper intercept at 635 ± 22 Ma forthe Ngaoundere granite (cf. samples 83–27 and 83–38

respectively, Toteu et al., 2001); on the other hand, thereare indications of ca. 800–1036 Ma inherited zircons insome granitoids of the same region (Toteu et al., 2001).Nd crustal residence ages from the northern border ofthe batholith at Makat are between 1.16 and 1.94 Ga witheNd at 600 Ma from �0.8 to �9.2. One sample of medium-grained biotite–muscovite granite west of Moungel yieldedan age of 1.09 Ga with eNd at 600 Ma of �1.5 (Toteu et al.,2001). The choice of the Ngaoundere region for thebeginning of the study of the Adamawa-Yade batholithwas justified by the presence of these preliminary isotopicdata.

The host rocks of the Adamawa-Yade batholith in theNgaoundere region crop out north of Ngaoundere andcomprise alternating layers of more or less migmatizedhornblende–biotite- and biotite–garnet gneisses, associatedwith mafic to intermediate orthogneisses and amphibolites(Fig. 2). The dominant structural feature of these rocks isthe presence of a steep NE–SW mylonitic foliation associ-ated with the regional-scale sinistral Tchollire-Banyo shearzone. This shear zone is the latest stage of a polyphase D1–D2 deformation associated with a high-grade Pan-Africanmetamorphism that overprinted Palaeoproterozoic relictsof granulitic assemblages (Penaye et al., 1989). The regionwas subsequently affected by WSW–ENE dextral faults ofthe CCSZ. Granitoids of the Ngaoundere region can begrouped into pre-tectonic granitoids represented by thehornblende–biotite granitoids (HBG), syn- to late-tectonicgranitoids represented by the biotite ± mucovite granitoids(BMG), and post-tectonic granitoids represented by theporphyritic biotite ± hornblende granitoids (BG).

3. Petrography of the Ngaoundere Pan-African granitoids

A total of 125 rock samples from the granitoids andenclaves were collected for petrographic studies. Mineralcompositions were determined using a Cameca SX-50 elec-tron microprobe at the CNRS-University-BRGM labora-tory of Orleans (France).

3.1. Pre-tectonic granitoids (HBG)

The HBG crop out as elongated massifs in the northernpart of the Ngaoundere region (Fig. 2). They are stronglydeformed and transformed into orthogneisses underamphibolite facies conditions and underwent both D1

and D2 regional deformations. The first one is marked bya gently dipping S1 foliation defined by elongated amphi-bole, biotite and plagioclase, sometimes associated withcompositional banding marked by alternating dark andlight layers. S1 is subsequently transposed by P2 uprightfolds with nearly horizontal fold axes. The D2 deformationcontinues with development of NE–SW C2 sinistral shearzones concordant to S2 and associated with nearly horizon-tal plunging lineations. S2 Foliation trends vary fromN55�E to N70�E and dip steeply (>70�) SE or NW. D2 isaccompanied by a progressive migmatization which out-

Fig. 2. Main geological units of the Ngaoundere granitoids and surrounding areas (Koch, 1953; Guiraudie, 1955; Lasserre, 1961) showing location ofsamples. Tertiary volcanic rocks: (1) trachyte and phonolite domes; (2) basalts. Neoproterozoic granitoids: (3) biotite ± hornblende porphyroid granitoids(BGs); (4) biotite ± muscovite granitoids; (5) hornblende biotite granitoids. Paleoproterozoic basement: (6) amphibole- and garnet gneisses; (7) faults.


lasted D2 deformation. Several N110�E dextral shear zonesoccupied by felsic dykes are also present.

The HBG are pinkish grey, medium-grained and containplagioclase (An13–29), orthoclase, quartz, hornblende (Mgratio = 0.31–0.50), and biotite (Mg ratio 0.36–0.41; TiO2

1.41–3.37 wt%). Biotite occurs locally as a replacement of

hornblende. Among the accessory minerals, titanite mayreach 5–10 vol% in the dark layers. Other accessory miner-als are magnetite, zircon, rutile and thorite. Thorite isincluded in rutile and magnetite. Some euhedral thoritecrystals were also observed in quartz. Secondary mineralsare sericite, epidote and chlorite.


3.2. Syn- to late-tectonic granitoids (BMG)

The BMG crop out as elongated intrusions into theHBG and the remobilised basement. They probably repre-sent the margin of the Leunda massif, a very large massifthat crops out north of the study area. Field evidencearound Wak village shows that BMG post-dated theHBG and were emplaced during D2 and its subsequentdevelopment. This sequence is supported by the presenceof numerous enclaves of HBG progressively digestedwithin the BMG and by the presence of ghost structures.The emplacement of BMG began during D2 (dykes alongP2 axial plane) and continued later (cross-cutting relations).The later pulses only display a magmatic layering, includ-ing schlieren and planar organization of K-feldspars. TheBMG are fine- to medium-grained with local porphyritictexture. Mineralogy consists of plagioclase (An13–27),K-feldspar (orthoclase and microcline), quartz (or myr-mekite), biotite (Mg ratio 0.32–0.51; TiO2 2.59–3.21);muscovite (Na and Ti rich) is present in some differentiatedrock types as small euhedral inclusions in feldspars or aswell-shaped flakes. Accessory minerals are represented byapatite, zircon, allanite, monazite, ilmenite, and magnetite.

3.3. Post-tectonic granitoids (BG)

The post-tectonic granitoids consist of numerous sub-circular bodies which constitute hillocks in and aroundNgaoundere town (Fig. 2). It was not possible to observethe relationship with HBG and BMG because of theCainozoic basaltic cover. Two rock types are distinguished:dominant biotite granite and hornblende–biotite monzog-ranite are both characterized by equant porphyritic texturewith megacrysts of K-feldspars. Microgranular mafic mag-matic enclaves are locally observed.

The mineralogy of the dominant biotite granite is quiteuniform and comprises potassic feldspar (orthoclase andmicrocline), plagioclase (An2–16), quartz and biotite (Mgratio 0.19–0.35; TiO2 1.98–3.83). The scarce accessory min-erals include zircon, monazite, magnetite, ilmenite, apatite,xenotime, and fluorite. Biotite occurs as dark brown orgreenish yellowish flakes. Most biotite has been altered tochlorite, or broken down to epidote, magnetite, titaniteand quartz. Apatite is the most common accessory mineral,occurring in various sizes from large anhedral to small hex-agonal crystals as inclusions in biotite, along with magne-tite. Similarly, prismatic to rounded zircon crystals arealso present and some show oscillatory zoning. Monazitesare anhedral to euhedral and are rich in Th (62,874 ppm inaverage). Xenotime occurs as small anhedral and rarely aslarge subhedral grains close to monazite. In some samples(NG1 and FC1) secondary muscovite occurs as small flakesof various dimensions. Myrmekites are observed as smallspots disseminated in some plagioclases or invading K-feldspar and plagioclase from their margins.

The hornblende–biotite monzogranite differs from thebiotite granite by the abundance of sphene and magnetite

growing into hornblende (Mg ratio = 0.70–0.94). The pla-gioclase in the monzogranites is oligoclase (An15–18), andbiotites are rich in Mg (Mg ratio 0.47–0.70; TiO2 2.57–3.43). The accessory minerals are zircon, magnetite, apatiteand sphene.

The mafic enclaves exhibit a rounded shape and igneoustexture, and they are usually surrounded by a chilled mar-gin of variable thickness. They are fine-grained and darkerthan the BG hosts. These enclaves contain the same miner-als as the hosting granitoid but in totally different propor-tions. They are broadly composed of biotite (from 40% to50%), altered plagioclase, orthoclase and quartz.

4. Geochemical characterization

Samples weighing 0.5–1.5 kg were reduced and finelychipped. Sample chips were cleaned, crushed in an agatemortar, split by quartering and finely ground. Representa-tive samples were analysed by ICP-AES and ICP-MS at theCentre de Recherches Petrographiques et Geochimiques ofNancy, France.

4.1. Classification of the granitoids

Major and trace element data for representative samplesof granitoids are listed in Table 1. We used the normativeANOR vs Q 0 diagram (Fig. 3) of Streckeisen and Le Maitre(1979) for nomenclature. HBG evolve from diorite, monzo-diorite, quartz monzodiorite to granodiorite fields. BMGand BG have a syenogranitic composition. The sampleBA-4, corresponding to a porphyritic biotite hornblendemonzogranitic facies of BG, plots in the quartz syenitefield. Based on the Al saturation index (A/CNK = molarAl2O3/(CaO + Na2O + K2O)) of Shand (1927), HBG aremetaluminous, while BG and BMG are metaluminous toslightly peraluminous. In the A/CNK vs SiO2 diagram(Fig. 4), only the most evolved BMG sample overlaps theS-type granite field; all the other rocks are in the I-typegranite field. I-type geochemical signature of HBG is sup-ported by the mineralogy (predominance of hornblende,biotite as mafic silicate minerals, and the abundance ofsphene and magnetite as accessory phases). In the AFMand (Na2O + K2O) vs SiO2 diagrams (not shown) grani-toids show a calc-alkaline trend and plot mostly in the fieldof high-K calc-alkaline series (Pecerillo and Taylor, 1976;Fig. 5).

4.2. Geochemical features

4.2.1. Pre-tectonic granitoids

The HBG show an intermediate to slightly acid compo-sition (SiO2 = 50.1–69.4%) coupled with high Al2O3 (15.5–22.4 wt%) and P2O5 (mostly > 0.30 wt %) contents, highMg values (Mg# = 29.1–38.6) and low K2O/Na2O ratios(0.3–1.1, mostly <1). Compared to other Ngaounderegranitoids, the HBG have low abundance of Rb (33–204 ppm), Th (1.5–4.9 ppm; except the sample EWA1 with

Table 1

Representative analyses of Ngaoundere granitoids. Analyses performed by ICP-AES and ICP-MS at the Centre de Recherches Petrographiques et Geochimiques (CRPG) of Nancy, France

Biotite–muscovite granitoids Biotite Granitoids Hornblende biotite granitoids

WA-5 WA-3D WA-2 WA1 WA-10 WA 8A WA6b BA-4 NG4c BA1 FC-1 FC-3 NG-1 EN1 EN4 TVO-11b EN-3 EWA-2 CK-2b CK5 EWA-5b EWA1 FT1c FT-1a CK-4b FT-1b

Major elements (wt%)

SiO2 68.98 69.78 70.18 70.21 72.18 72.5 74.46 65.21 66.12 71.49 71.84 72.22 72.23 73.13 73.71 73.84 73.93 50.11 50.93 52.33 54.94 56.73 59.68 60.49 68.18 69.44

Al2O3 14.68 14.76 14.71 15.41 14.68 16.1 13.90 15.34 14.69 14.69 13.96 13.71 14.34 13.40 13.60 14.12 13.01 17.9 17.66 22.36 17.53 15.15 16.97 17.10 15.47 15.47

TiO2 0.46 0.45 0.43 0.38 0.2 0.06 0.12 0.74 0.92 0.25 0.25 0.30 0.21 0.30 0.22 0.19 0.28 1.76 1.39 0.91 1.02 2.03 0.86 0.84 0.49 0.38

Fe2O3t 2.93 2.76 2.54 2.50 1.59 0.7 1.23 4.17 7.13 3.03 2.23 2.31 1.94 2.56 1.93 1.90 2.68 10.54 9.65 6.07 8.04 8.73 6.68 6.00 3.57 2.78

MnO <L.D. <L.D. 0.05 0.03 <L.D. 0.04 <L.D. 0.07 0.15 0.04 0.03 0.03 <L.D. 0.05 0.03 <L.D. 0.04 0.18 0.12 0.07 0.12 0.10 0.11 0.09 0.04 <L.D.

MgO 0.63 0.52 0.75 0.51 0.36 0.05 <L.D. 1.78 1.23 <L.D. 0.4 0.34 0.12 <L.D. <L.D. 0.16 0.44 4.17 5.46 2.47 4.12 3.22 2.90 2.49 1.39 1.09

CaO 1.30 1.43 1.17 1.79 1.69 0.77 0.69 2.55 1.09 0.85 0.86 1.05 0.99 1.05 0.81 0.92 0.95 8.51 6.81 8.56 6.38 4.65 5.20 4.88 2.76 3.02

Na2O 2.50 2.7 3.37 3.62 3.18 4.65 3.06 3.73 3.95 3.71 3.45 3.86 3.71 3.51 3.28 3.19 3.48 3.78 4.09 5.05 4.30 3.11 3.91 3.86 4.28 3.57

K2O 6.63 6.77 5.93 4.89 5.59 4.61 5.65 5.59 3.24 5.84 6.23 5.25 5.75 4.85 5.57 5.47 4.45 2.22 2.67 1.36 2.44 3.56 3.11 3.55 3.12 3.64

P2O5 0.16 0.15 0.15 0.14 0.11 0.01 0.04 0.43 0.26 0.10 0.1 0.05 <L.D. 0.13 0.06 <L.D. 0.08 0.33 0.47 0.31 0.38 0.94 0.37 0.31 0.19 0.11

PF 0.79 0.54 0.58 0.44 0.46 0.67 0.68 0.58 1.15 0.36 0.49 0.70 0.48 0.68 0.51 0.33 0.48 0.99 0.79 0.63 0.69 0.59 0.70 0.59 0.43 0.53

Total 99.06 99.86 99.86 99.92 100 100.7 99.83 100.2 99.93 100.4 99.84 99.82 99.77 99.66 99.72 100.1 99.82 100.49 100.04 100.1 99.96 98.81 100.5 100.20 99.92 100.03

(A/CNK) mol 1.08 1.03 1.04 1.06 1.02 1.14 1.12 0.91 1.23 1.05 1.00 0.98 1.02 1.04 1.05 1.10 1.06 0.74 0.80 0.88 0.82 0.87 0.88 0.90 1.00 1.01

Mg# 19.29 17.31 24.71 18.48 20.10 7.35 32.18 16.09 16.62 14.06 6.43 8.56 15.43 30.54 38.61 31.14 36.29 29.08 32.55 31.56 30.20 30.35

TMonazite (�C) 944 977 943 869 879 830 829 991 939 842 873 859 939 898 861 901

TZicon (�C) 847 842 843 846 844 856 859 816 857 846 841 840 844 849 851 855 852

Norme ICPW

ANOR 12.33 13.44 12.32 21.80 18.98 12.13 8.72 20.66 16.84 9.51 9.08 12.85 12.63 13.43 10.02 12.37 13.89 65.87 58.17 81.03 59.66 44.49 51.57 47.32 40.55 39.99

Q 0 28.13 26.38 26.46 27.14 29.55 26.77 34.65 15.64 29.65 26.13 26.91 28.39 27.22 33.12 32.43 33.43 36.21 0.00 0.00 0.00 0.00 12.09 9.90 10.69 24.95 28.08

(A/NK) mol 1.30 1.25 1.23 1.37 1.30 1.27 1.25 1.26 1.47 1.18 1.12 1.14 1.16 1.22 1.19 1.26 1.23 2.08 1.84 2.29 1.80 1.69 1.73 1.68 1.48 1.58

Quartz 26.87 25.48 25.34 25.36 28.88 25.88 33.44 13.68 24.23 24.65 26.31 27.83 26.90 31.49 31.18 32.64 35.04 – – – – 9.06 7.87 8.76 22.52 25.97

Orthose 40.85 41.20 36.04 29.11 33.61 27.41 33.71 33.28 19.50 34.59 37.74 31.90 34.77 29.02 33.23 32.90 27.06 13.31 16.04 8.12 14.63 21.59 18.53 21.17 18.59 21.67

Albite 22.06 23.53 29.33 30.85 27.37 39.59 26.14 31.80 34.04 31.47 29.92 33.58 32.12 30.07 28.02 27.47 30.30 28.16 30.56 37.61 36.92 27.00 33.35 32.97 36.51 30.43

Anorthite 5.74 6.40 5.06 8.12 7.87 3.78 3.22 8.67 3.95 3.64 3.77 4.70 5.03 4.50 3.70 4.64 4.37 25.68 22.30 34.69 21.64 17.30 19.73 19.02 12.68 14.44

Corindon 1.43 0.73 0.97 1.22 0.57 2.07 1.60 0.00 3.32 0.94 0.20 0.00 0.22 0.76 0.84 1.30 0.94 – 0.00 – – 0.00 – – 0.45 0.41

Ab/An 3.84 3.68 5.79 3.80 3.48 10.46 8.12 3.67 8.62 8.65 7.94 7.14 6.39 6.68 7.57 5.91 6.94 1.10 1.37 1.08 1.71 1.56 1.69 1.73 2.88 2.11

Trace elements (ppm)

Ba 788 705 576 968 1962 511 1767 177 366 414.2 413 357 298 322 705 266 298 1329 784 784 1097 1483 2033 786 2128

Be 1.65 1.91 2.81 2.71 2.46 2.71 4.29 11.11 3.77 5.553 6.73 9.078 8.60 8.29 2.73 7.98 2.81 1.82 1.25 3.14 2.86 3.13 2.43 1.445 1.48

Co 3.79 3.52 3.37 3.98 2.74 1.18 9.97 7.69 2.81 2.281 2.72 1.81 2.70 2.01 2.42 2.76 38.5 32.7 21.5 23.6 20.7 17.6 15.5 9.96 6.69

Cr 14.27 8.57 12.2 9.04 7.65 <L.D. 48.7 14.4 <L.D. 6.10 10.9 7.98 5.35 8.23 12.8 12.3 115 170 59.1 62.0 64.2 26.1 35.73 30.12 11.2

Cs 1.02 0.90 2.41 2.42 0.579 1.35 3.56 16.73 3.80 3.454 2.85 6.08 8.04 6.13 1.24 6.97 1.68 1.73 1.00 2.08 4.27 4.32 3.39 2.24 2.57

Cu 7.35 9.92 4.32 11.1 21.1 <L.D. 17.99 <L.D. <L.D. 5.768 4.29 6.74 <L.D. <L.D. 8.07 6.83 8.80 28.9 102 14.3 13.1 52.7 61.3 24.7 23.6

Ga 26.7 26.7 27.8 23.7 20.0 20.4 23.1 40.4 26.1 24.8 26.7 25.2 29.2 25.6 21.9 26.7 23.6 23.2 23.8 25.9 28.4 23.0 21.9 18.8 15.7

Ge 1.25 1.316 1.54 1.21 1.248 1.27 1.27 2.60 1.46 1.429 1.51 1.63 1.99 1.72 1.23 1.66 1.48 1.32 1.18 1.53 1.89 1.43 1.30 0.90 0.78

Hf 9.72 8.88 11.7 10.0 4.90 5.77 7.87 35.51 12.9 7.507 8.13 5.96 13.4 9.52 5.16 10.7 4.60 6.08 2.48 4.98 10.5 6.99 4.66 4.09 2.66

Nb 23.0 22.6 41.2 21.9 10.6 17.4 18.6 107 22.0 28.51 34.8 32.3 45.17 29.4 12.0 39.4 11.1 9.32 6.18 12.8 28.3 9.84 9.11 4.71 3.18

Ni 7.59 5.53 6.36 <L.D. 5.92 <L.D. 22.13 6.45 <L.D. 5.371 <L.D. 5.36 <L.D. <L.D. 7.65 7.39 70.7 80.4 41.7 39.4 24.3 18.0 21.0 26.5 9.6

Pb 32.4 32.6 36.1 26.5 33.7 37.7 32.0 36.0 34.8 35.9 35.2 40.7 44.2 44.3 24.8 36.4 14.6 14.6 11.6 15.1 17.0 22.2 23.3 13.0 23.7

Rb 309 313 346 233 96.0 276 196 418 232 301 296 332 355 344 208 305 139 61.2 33.0 116 204 98.9 94.5 66.2 82.4

Sn 5.30 5.54 10.2 6.39 1.09 3.60 2.98 32.1 2.84 6.716 7.81 8.31 14.65 9.41 3.75 12.2 2.38 2.48 1.63 4.67 4.93 2.68 2.23 1.02 0.88

Sr 168 149 125 280 753 129 816 59.0 123 110.4 120 104 89.0 89.5 353 83.3 734 937 869 741 695 660 742 534 680

Ta 0.87 1.01 1.91 1.35 1.34 1.79 1.83 11.32 1.63 3.238 4.11 3.54 5.87 3.84 1.38 4.50 1.13 0.66 0.43 0.76 1.83 0.66 0.68 0.38 0.15

Th 79.0 83.5 79.8 46.3 41.4 33.1 30.7 215 37.0 42.07 52.9 42.6 92.4 58.2 15.0 70.4 2.22 2.45 1.95 2.19 13.15 4.92 3.01 4.30 1.52

U 2.43 2.526 6.14 5.96 2.82 5.76 5.92 25.48 4.78 7.661 8.50 7.0 14.8 12.3 1.8 15.6 1.43 0.47 1.10 1.47 3.10 2.12 1.17 1.25 0.47

V 18.4 17.3 16.4 23.0 9.84 11.3 58.7 38.1 10.3 11.75 14.0 10.0 13.5 11.2 14.1 13.9 228 196 101 162 112 131 121 49.6 46.7

Y 12.3 13.4 20.6 15.0 10.19 13.9 27.5 215.8 31.8 31.41 54.9 28.1 47.7 52.8 16.4 49.4 27.7 27.7 17.8 22.6 28.2 28.2 22.3 11.1 5.7

Zn 89.7 95.9 80.1 67.9 23.1 22.8 67.6 254 51.9 59.34 67.7 54.3 84.7 55.9 54.6 80.5 98.6 135 59.6 116 176 86.6 70.4 44.5 40.5

Zr 367 322 418 381 163 173 321 1155 491 227.4 251 180 436 297 181 340 184 246 103 201 476 289 182 162 98.1

La 127.50 196.3 136.30 64.94 77.5 32.75 73.43 162.10 149.20 55.71 82.25 66.9 134.40 82.07 55.05 81.70 21.19 40.18 23.25 28.01 108.50 39.38 43.07 29.38 16.86

Ce 257.00 395.9 279.60 129.10 153 67.29 158.70 233.30 258.10 113 157.70 124.3 254.00 160.90 101.90 164.70 51.20 93.15 49.04 67.53 234.80 72.32 74.02 53.88 30.12

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Pr

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3916

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925

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12.3

517

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17.6

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17.5

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1510

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Nd

99.0

214

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6046

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125

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58.9

314

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81.7

942

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31.4

311

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8.01

512

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8.36

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3612

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5.84

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18.8

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1.11

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50.

930.

890.

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0.65

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2.05

0.81

0.68

50.

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686

0.74

0.76

0.69

0.81

1.81

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1.98

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0.77

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0.53

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1.55

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71.

561.

550.

541.

540.

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580.

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2.78

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0.45

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1.04

51.

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939

1.52

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0.98

0.62

0.77

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0.71

0.38

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192

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1.11

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0.13

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0.21

0.38

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0.44

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0.69

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759

4.62

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1.67

1.99

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10.

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Lu

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471

0.76

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30.

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240.

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410.

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946.

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132.

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360.

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50

An

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the

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for

trac

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tsan

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EE

.

ANOR

Q'

50

40

30

20

10

00 10 20 30 40 50 60 70 80 90 100

gdmd

qmdqs qm

afg sgmg gd

HBG

BGBMG

(BA-4)

Fig. 3. Streckeisen and Le Maitre (1979) ANOR-Q 0 normative diagram.Q 0 = 1000/(Q + Or + Ab + An), ANOR = 100An/(An + Or). afg, alkalifeldspar granite; sg, syenogranite; mg, monzogranite; gd, granodiorite; qs,quartz syenite; qm, quartz monzonite; qmd, quartz monzodiorite; md,monzodiorite; d, diorite; g, gabbro.

HBG

BGBMG

Peraluminous granitoids

Metaluminous granitoids

S-type granitoidsI-type granitoids

0.5

1.1

Al 2

O3

/ (C

aO +

Na 2

O +

K2O

)

SiO2

45 50 55 60 65 70 75

(BA-4)

Fig. 4. Classification of the Neoproterozoic Ngaoundere granitoids in themolar ratio Al2O3/(CaO + Na2O + K2O) vs SiO2 diagram. The subdivi-sion between the fields of peraluminous and meta-aluminous granites isafter Chappell and White (1974) while the fields of I (igneous origin) and S(sedimentary origin) granitoids are after White and Chappell (1977).


high Th concentration, 13.1 ppm) and Zr (98–289 ppm;except the sample EWA1 with high Zr concentration,476 ppm) and high abundance of Ba (784–2033 ppm,except the sample EWA2 with low Ba, 298 ppm), and ofSr (534–937 ppm). The Rb/Sr ratios cluster around 0.04–0.24. Chondrite-normalized REE patterns (Fig. 6A) aremoderately fractionated (LaN/YbN 5.9–22.2, exceptEWA1 with LaN/YbN = 39.8). There are no Eu anomalies

45 50 55 60 65 70 75SiO2

Low-K suites

Medium-K calc-alkaline suites

High-K calc-alkaline suites

Shoshonitic suites

0

2

4

6

8

K2O

(BA-4)

HBG

BGBMG

Fig. 5. K2O vs SiO2 diagram (Pecerillo and Taylor, 1976) for the threegroups of the Neoproterozoic Ngaoundere granitoids.

Hornblende biotite granitoids

Biotite ± Muscovite granitoids

1

10

100

1000

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Biotite granitoids

A

B

C

1

10

100

1000


Sam

ples

/ C

hond

rite

Sam

ples

/ C

hond

rite

1

10

100

1000


Sam

ples

/ C

hond

rite

Fig. 6. Chondritic-normalized REE patterns for the NeoproterozoicNgaoundere granitoids. Normalizing values from Sun and McDonough(1989).


(Eu/Eu* 0.8–1.0), but one diorite (CK5) and one granodi-orite (FT1b) display a weak positive Eu anomaly(Eu/Eu* = 1.5 and 1.4). The lack of prominent Eu anoma-lies and the high Sr contents exclude important fractionalcrystallization of feldspar in the HBG petrogenetic evolu-tion (except for EWA1) or abundant feldspar in the resid-uum of the HBG source. The primitive mantle-normalizedincompatible element patterns (Fig. 7A) show enrichmentin LILE relative to HFSE. They are moderately fraction-ated with uniform negative anomalies in Nb–Ta (NbN/LaN = 0.1–0.5), Ti (TiN/GdN = 0.5–0.9). Samples CK5and FT1b show positive Sr anomalies, which may be dueto weak plagioclase accumulation, while EWA1, the mostenriched rock (e.g., P2O5 = 0.94 wt%) displays a negativeSr anomaly, probably due to plagioclase and/or apatitefractionation. The very slight Ti troughs, and lower Y,Yb, and Nb values, result in a trace element distributionpattern that is characteristic of calc-alkaline arc granitoids.

4.2.2. Syn- to late-tectonic granitoids

The BMG are acid (SiO2 = 68.9–74.5 wt%) and highlypotassic (K2O = 4.6–6.8% with K2O/Na2O > 1). They arecharacterized by low Sr contents (125–279 ppm), moderateBa contents (511–968 ppm) and high abundance of Th(33.1–83.8 ppm), Rb (233–346 ppm), Zr (163–418 ppm)and REEs (

PREE = 149–822 ppm). Rb/Sr ratios range

from 0.8 to 2.8. Chondrite-normalized REE patterns(Fig. 6B) show LREE enrichment with respect to HREE(LaN/YbN = 73–168) and strong negative Eu anomalies(Eu/Eu* = 0.2–0.5). Compared to other Ngaoundere grani-toids they are significantly depleted and fractionated inHREE (GdN/YbN = 2.1–9.2). In the primitive mantle-nor-malized incompatible element diagram (Fig. 7B), all of theBMG samples display patterns with negative anomalies ofNb–Ta (NbN/LaN = 0.2–0.5), Sr (SrN/NdN = 0.1–0.4)(except for WA10) and Ti (TiN/GdN = 0.1–0.3). The Euand Sr negative anomalies can be explained by plagioclase

fractionation. But, the Nb, Ta and some Ti negative anom-alies are inherited from the petrogenetic process.

4.2.3. Post-tectonic biotite granitoids

The biotite syenogranites are highly acid (SiO2 = 71.5–73.9 wt%) and rich in potassium (K2O = 4.5–6.2%) withK2O/Na2O > 1. They are characterized by low Ba (266–414, except for one sample, TVO11b with 705 ppm), lowSr (83–353 ppm), high Th (14.9–92.4 ppm, with 51.3in average), Y (16.4–54.9 ppm), REE (

PREE =

219–556 ppm) and Rb/Sr ratios (1.9–7.1). Chondrite-normalized REE patterns (Fig. 6C) are moderately

1

10

100

1000

Rb Ba Th Nb Ta La Ce Pr Sr Nd Sm Zr Hf Eu Ti Gd Tb Dy Y Er Yb

Sam

ples

/ Pr

imiti

ve M

antle

0.1

1

10

100

1000


Sam

ples

/ Pr

imiti

ve M

antle

0.1

1

10

100

1000

10000


Sam

ples

/ Pr

imiti

ve M

antle

Hornblende biotite granitoidsA

Biotite ± Muscovite granitoidsB

Biotite granitoidsC

Fig. 7. Primitive mantle-normalized incompatible element patterns for theNeoproterozoic Ngaoundere granitoids. Normalizing values from Sunand McDonough (1989).


fractionated (LaN/YbN = 11–37, mostly <20), and showa clear Eu negative anomalies (Eu/Eu* = 0.2–0.5) and aflat HREE profile (GdN/YbN = 1.5–2.2). In the Primitivemantle-normalized plot of incompatible trace elements,the rocks show homogenous composition with strongnegative anomalies in Ti (TiN/GdN = 0.9–0.1), Sr (SrN/NdN =0.1–0.7), Nb (NbN/LaN = 0.1–0.5) and Ba(Fig. 7C).

The fine-grained mesocratic enclave (NG4c) collected inthe biotite syenogranites shows higher Al2O3, MgO,Fe2O3t, TiO2, REE, Th, Y, Rb, Nb, Hf, Zr and lowerSiO2, Sr, Ba values than all of the other biotite syenogra-nite samples. The REE pattern is moderately fraction-ated (LaN/YbN = 5.7), with high HREE and Y contents(GdN/YbN = 1.2 and Y = 216 ppm).

Compared to the biotite syenogranite, the porphyritichornblende–biotite monzogranite facies of BG (sampleBA-4) shows an intermediate composition (SiO2 =65.2 wt%). This sample displays higher Al2O3, Mg#, Ni,Cr, Sr, Ba and Th and lower Rb than the biotite syenogra-nite. The chondrite-normalized REE pattern (Fig. 6C) ismoderately fractionated (LaN/YbN 22) with a lack of neg-ative Eu anomalies (Eu/Eu* = 0.8). The incompatible ele-ment pattern of the hornblende–biotite monzogranite ischaracterized by Nb and Ti negative anomalies and theabsence of Ba and Sr anomalies (Fig. 7C).

4.3. Thermobarometry

In the ternary Q-Ab-Or normative diagram (Fig. 8A),two samples of HBG (samples with normative quartz >10 wt%) and all the BG and BMG felsic compositions plotnear or close to the water-undersaturated (aH2O ¼ 0:5) gra-nitic minimum determined experimentally to 775 �C atP H2O ¼ 2 kbar by Holtz et al. (1992). The relatively highnormative orthoclase content of the natural granites maybe explained by pronounced H2O-undersaturated condi-tions in the source materials (Scaillet et al., 1990; Nabeleket al., 1992). Although the pressure of formation of thesegranites is unknown, the high orthoclase content, andtherefore the generation under water-undersaturated con-ditions, indicates a comparatively high temperature forthe melting process.

Assuming equilibrium with quartz, K-feldspar, plagio-clase, titanite and magnetite, the crystallization pressureof magmatic porphyroclastic hornblende (PHb) is estimatedusing an experimental calibration of the Al-in-hornblendebarometer (Johnson and Rutherford, 1989) and subsequentdiscussion (Blundy and Holland, 1990; Schmidt, 1992). Forthe HBG the pressure is between 4.5 and 5.6 kbars, and forthe BMG, the occurrence of primary muscovite in somedifferentiated samples (WA8C, WA6) places pressure limitsof the crystallization at 4–2.6 kbar, i.e. the upper crust(Gardien et al., 1995; Villa et al., 1997).

The Zr and REE contents of the melt and the mineralcompositions of zircon and monazite can be used as inde-pendent chemical geothermometers for estimating the tem-perature of the magma (Watson and Harrison, 1983; Rappand Watson, 1986; Montel, 1993). We applied these ther-mometers to the BMG and BG to estimate the minimumtemperature of the melts. The solubility model of Montel(1993) yields monazite saturation temperatures of 830–977 �C for BMG (average of 907 �C) and 842–939 �C (aver-age of 900 �C) for BG. The zircon saturation temperatures(Watson and Harrison, 1983; Rapp and Watson, 1986)

1050

1000

950

900

850

800

7508007006005004003002001000

(La + Ce + Nd) ppm

T˚C

(M

onaz

ite s

atur

atio

n) NG4C (enclave)B

(BA-4)

OrthoseAlbite

Quartz

830850

M

A

850

Fig. 8. (A) Composition of the Neoproterozoic Ngaoundere granitoidsplotted in the ternary normative Q-Ab-Or diagram determined experi-mentally by Holtz et al. (1992) (at P H2O ¼ 2 kb and aH2O ¼ 0:5). M,eutectic minimum; 830, 850, liquidus temperatures (�C). (B)(La + Ce + Nd) plotted against the T �C estimated from the monazitethermometry (Montel, 1993) for the Neoproterozoic BMG and BG.Symbols as in Fig. 5.


range from 842 to 859 �C (average of 848 �C) for BMG andfrom 816 to 855 �C (average of 848 �C) for BG. The fine-grained mafic enclave (NG4c) exhibits higher monaziteand zircon saturation temperatures (991 and 857 �C) com-pared to the biotite granite host rock. The monazite andzircon saturation temperatures are 777 and 738 �C respec-tively for the sample BA-4, the porphyritic hornblende–biotite monzogranite. The plot of the sum of LREE (La,Ce, Nd) against temperature, calculated using the monazitesolubility method (Fig. 8B), shows a good positive correla-tion. This suggests that concentration of LREE in theserocks may be controlled by the crystallization of monaziteand other accessory phases (allanite) that concentrateLREE.

5. Chemical Th–U–Pb monazite dating

BG and BMG yielded enough monazite for chemicalTh–U–Pb dating. Two samples of BG (EN-4 and NG-3)

and two samples of BMG (WA3D and WA10) were col-lected. Analyses were carried out using a Cameca SX-100electron probe microanalyser at the BRGM-CNRS-Universite d’Orleans laboratory (France), on in situ grains(NG3, WA-3D and WA-10) and on grains mounted inresin (EN4). The analytical procedure was described bySuzuki and Adachi (1991, 1994), Montel et al. (1996),Cocherie et al. (1998), and Cocherie and Albarede(2001). Average weighted ages calculation was performedusing the Isoplot/exe program (Ludwig, 2000) andrefined with Th/Pb vs U/Pb isochron plot (Cocherie andAlbarede, 2001). The analytical results are given inTable 2.

5.1. Biotite–muscovite granitoids (BMG)

Two BMG samples (WA3D and WA10) were investi-gated. Monazite is fresh and associated with allanite. Atotal of 35 chemical spot ages on three grains was obtainedfrom sample WA3D. The calculated age histogram displaysa Gaussian distribution indicating a homogeneous popula-tion. Spot ages range from 880 ± 55 to 1008 ± 65 Ma withan average weighted age of 926 ± 12 Ma (Fig. 9A), not dif-ferent from the 926 ± 12 Ma obtained with the Th/Pb vsU/Pb plot. Monazites from the sample WA10 are very smallin size. Its average age, calculated directly from the fourindividual ages is 615 ± 27 Ma (Fig. 9B). This age is verydifferent from that obtained for sample WA3D and indi-cates the presence of monazites of various ages in the BMG.

5.2. Biotite granitoids (BG)

Two samples (samples NG3 and EN4) of biotite graniteswere investigated for monazite dating. The monazite occursas inclusions in feldspar or biotite, where it is associatedwith zircon. A total of 44 chemical spot ages on four mon-azite grains was obtained from sample NG3, and a total of54 spot ages on three monazite grains from sample EN4.The age histogram for monazites from each sample definesa Gaussian distribution indicating a monogenetic popula-tion in each sample. The average weighted ages are648 ± 14 Ma for EN4 (Fig. 9C) and 610 ± 10 Ma forNG3 (Fig. 9d) respectively. The precision of these agesusing the Th/Pb vs U/Pb plot (Cocherie and Albarede,2001) gives 652 ± 10 and 610 ± 17 Ma for EN4 and NG3respectively.

5.3. Interpretation of the results

Field observations indicate that the HBG and BMG arerespectively pre- tectonic and syn-tectonic while the BGsare post-tectonic. The zircon age of 622 ± 25 Ma obtainedon the HBG, the age of younger monazites in BMG(615 ± 54 Ma), and in BGs (610 ± 10 Ma) are consistentwithin the error limits with the age of the syntectonic grani-toids of northern Cameroon (Toteu et al., 2004). We inter-preted these ages as the emplacement age of the HBG and

Table 2Analytical data for monazite from biotite granitoids and biotite ± muscovite granitoids

NUM Age(Ma)

Error(Ma)

U(ppm)

Error(%)

Th(ppm)

Error(%)

Pb(ppm)

Error(%)

Th*(U)(ppm)

Error(%)

U/Pb Error(%)

Th/Pb Error(%)

Corr

Sample WA3D (biotite muscovite granite)

1 916 49 960 15.63 115,340 2.00 4930 3.04 118,615 2.38 0.195 18.67 23.396 5.04 0.1602 901 57 590 25.42 91,790 2.00 3830 3.92 93,800 2.50 0.154 29.34 23.966 5.92 0.1363 913 52 920 16.30 106,430 2.00 4536 3.31 109,568 2.41 0.203 19.61 23.466 5.31 0.1704 884 51 870 17.24 108,760 2.00 4476 3.35 111,720 2.40 0.194 20.59 24.297 5.35 0.1645 933 65 320 46.88 80,180 2.00 3439 4.36 81,273 2.60 0.093 51.24 23.314 6.36 0.0846 973 56 410 36.59 99,970 2.00 4478 3.35 101,376 2.48 0.092 39.93 22.324 5.35 0.0787 964 61 520 28.85 88,450 2.00 3950 3.80 90,231 2.53 0.132 32.64 22.392 5.80 0.1158 918 52 680 22.06 109,220 2.00 4644 3.23 111,540 2.42 0.146 25.29 23.517 5.23 0.1239 890 52 670 22.39 106,260 2.00 4379 3.43 108,541 2.43 0.153 25.81 24.265 5.43 0.13110 917 52 980 15.31 106,180 2.00 4554 3.29 109,524 2.41 0.215 18.60 23.314 5.29 0.18011 922 53 600 25.00 105,170 2.00 4486 3.34 107,218 2.44 0.134 28.34 23.444 5.34 0.11412 880 55 530 28.30 98,040 2.00 3980 3.77 99,843 2.47 0.133 32.07 24.633 5.77 0.11713 974 53 690 21.74 109,580 2.00 4954 3.03 111,946 2.42 0.139 24.77 22.119 5.03 0.11514 1008 65 430 34.88 81,380 2.00 3797 3.95 82,859 2.59 0.113 38.83 21.433 5.95 0.10015 927 61 800 18.75 85,210 2.00 3697 4.06 87,942 2.52 0.216 22.81 23.047 6.06 0.19016 914 51 830 18.07 111,550 2.00 4744 3.16 114,381 2.40 0.175 21.23 23.514 5.16 0.14617 1007 60 630 23.81 90,650 2.00 4248 3.53 92,816 2.51 0.148 27.34 21.337 5.53 0.12818 921 51 760 19.74 109,850 2.00 4699 3.19 112,444 2.41 0.162 22.93 23.378 5.19 0.13520 906 58 740 20.27 90,850 2.00 3836 3.91 93,372 2.49 0.193 24.18 23.684 5.91 0.16921 899 51 860 17.44 109,800 2.00 4594 3.27 112,730 2.40 0.187 20.71 23.902 5.27 0.15722 910 52 760 19.74 105,860 2.00 4477 3.35 108,452 2.42 0.170 23.09 23.644 5.35 0.14423 931 57 830 18.07 92,920 2.00 4047 3.71 95,755 2.48 0.205 21.78 22.962 5.71 0.17724 981 54 1010 14.85 104,670 2.00 4818 3.11 108,135 2.41 0.210 17.96 21.723 5.11 0.17325 892 53 610 24.59 103,010 2.00 4248 3.53 10,5087 2.45 0.144 28.12 24.249 5.53 0.12426 925 56 650 23.08 96,940 2.00 4160 3.61 99,159 2.47 0.156 26.68 23.303 5.61 0.13527 927 65 340 44.12 79,670 2.00 3400 4.41 80,831 2.60 0.100 48.53 23.432 6.41 0.09128 902 63 510 29.41 80,220 2.00 3350 4.48 81,958 2.58 0.152 33.89 23.946 6.48 0.13729 936 64 410 36.59 81,670 2.00 3530 4.25 83,071 2.58 0.116 40.83 23.136 6.25 0.10430 891 61 470 31.91 83,280 2.00 3430 4.37 84,880 2.56 0.137 36.29 24.280 6.37 0.12331 959 63 450 33.33 84,540 2.00 3750 4.00 86,081 2.56 0.120 37.33 22.544 6.00 0.10732 887 64 530 28.30 78,280 2.00 3220 4.66 80,084 2.59 0.165 32.96 24.311 6.66 0.14933 979 66 230 65.22 80,370 2.00 3610 4.16 81,159 2.61 0.064 69.37 22.263 6.16 0.05734 954 65 420 35.71 79,400 2.00 3500 4.29 80,838 2.60 0.120 40.00 22.686 6.29 0.10835 961 63 460 32.61 82,970 2.00 3690 4.07 84,546 2.57 0.125 36.67 22.485 6.07 0.11136 899 60 550 27.27 86,180 2.00 3590 4.18 88,054 2.54 0.153 31.45 24.006 6.18 0.137

NUM Age(Ma)

ErrorAge(Ma)

U(ppm)

ErrorU (%)

Th(ppm)

ErrorTh (%)

Pb(ppm)

ErrorPb (%)

Th*(U)(ppm)

ErrorTh* (%)

U/Pb ErrorU/Pb (%)

Th/Pb ErrorTh/Pb (%)

Corr

Sample WA10 (biotite muscovite granite)

79 604 60 1120 13.39 71,620 2.00 2048 7.32 75,343 2.56 0.547 20.72 34.964 9.32 0.46380 594 57 1230 12.20 75,700 2.00 2131 7.04 79,785 2.52 0.577 19.23 35.518 9.04 0.48181 626 53 1160 12.93 84,540 2.00 2491 6.02 88,403 2.48 0.466 18.95 33.935 8.02 0.40182 630 53 940 15.96 86,880 2.00 2553 5.88 90,011 2.49 0.368 21.83 34.033 7.88 0.327

NUM Age(Ma)

Error(Ma)

U(ppm)

Error(%)

Th(ppm)

Error(%)

Pb(ppm)

Error(%)

Th*(U)(ppm)

Error(%)

U/Pb Error(%)

Th/Pb Error(%)

Corr

Sample EN4(biotite granite)

2 622 57 5920 2.53 56,900 2.00 2142 7.00 76,607 2.14 2.764 9.54 26.569 9.00 0.9044 633 58 5490 2.73 56,780 2.00 2137 7.02 75,073 2.18 2.569 9.75 26.567 9.02 0.8965 737 77 2500 6.00 48,670 2.00 1898 7.90 57,071 2.59 1.317 13.90 25.644 9.90 0.7727 613 55 4980 3.01 63,050 2.00 2194 6.84 79,616 2.21 2.270 9.85 28.736 8.84 0.8788 683 68 3000 5.00 54,800 2.00 1994 7.52 64,836 2.46 1.505 12.52 27.487 9.52 0.8059 696 59 5490 2.73 57,140 2.00 2366 6.34 75,526 2.18 2.320 9.07 24.146 8.34 0.87610 653 58 5880 2.55 55,400 2.00 2201 6.82 75,022 2.14 2.672 9.37 25.171 8.82 0.89911 673 50 6680 2.25 68,730 2.00 2759 5.44 91,060 2.06 2.421 7.68 24.913 7.44 0.86713 612 57 5780 2.60 56,120 2.00 2070 7.25 75,345 2.15 2.792 9.84 27.107 9.25 0.90714 665 54 5180 2.90 66,580 2.00 2512 5.97 83,885 2.18 2.062 8.87 26.507 7.97 0.85315 535 66 3350 4.48 52,260 2.00 1519 9.87 63,334 2.43 2.205 14.35 34.403 11.87 0.89316 650 57 6050 2.48 56,650 2.00 2247 6.68 76,836 2.13 2.693 9.16 25.216 8.68 0.89817 693 53 7000 2.14 63,100 2.00 2698 5.56 86,537 2.04 2.594 7.70 23.385 7.56 0.87819 655 50 6510 2.30 70,000 2.00 2703 5.55 91,730 2.07 2.408 7.85 25.894 7.55 0.869

(continued on next page)


Table 2 (continued)

NUM Age(Ma)

Error(Ma)

U(ppm)

Error(%)

Th(ppm)

Error(%)

Pb(ppm)

Error(%)

Th*(U)(ppm)

Error(%)

U/Pb Error(%)

Th/Pb Error(%)

Corr

20 724 47 6200 2.42 82,350 2.00 3364 4.46 10,3161 2.08 1.843 6.88 24.483 6.46 0.80221 633 58 3600 4.17 65,650 2.00 2212 6.78 77,645 2.33 1.627 10.95 29.673 8.78 0.81722 654 55 4690 3.20 66,980 2.00 2431 6.17 82,633 2.23 1.929 9.37 27.550 8.17 0.84523 705 58 4540 3.30 63,770 2.00 2508 5.98 78,986 2.25 1.810 9.28 25.426 7.98 0.83024 821 64 3920 3.83 60,730 2.00 2744 5.47 73,995 2.33 1.429 9.29 22.133 7.47 0.769

25 648 51 3300 4.55 80,570 2.00 2670 5.62 91,578 2.31 1.236 10.16 30.172 7.62 0.73226 631 53 3350 4.48 76,170 2.00 2478 6.05 87,330 2.32 1.352 10.53 30.733 8.05 0.76327 611 45 4690 3.20 89,200 2.00 2881 5.21 104,799 2.18 1.628 8.41 30.966 7.21 0.79528 613 45 6100 2.46 84,410 2.00 2885 5.20 104,702 2.09 2.115 7.66 29.262 7.20 0.84429 554 43 6610 2.27 83,790 2.00 2626 5.71 105,674 2.06 2.517 7.98 31.911 7.71 0.87730 633 48 6580 2.28 73,370 2.00 2713 5.53 95,294 2.06 2.426 7.81 27.048 7.53 0.86932 622 51 5120 2.93 72,890 2.00 2515 5.96 89,934 2.18 2.036 8.89 28.983 7.96 0.85133 561 46 7020 2.14 73,980 2.00 2446 6.13 97,234 2.03 2.870 8.27 30.242 8.13 0.89834 591 69 1230 12.20 58,010 2.00 1651 9.08 62,094 2.67 0.745 21.28 35.131 11.08 0.58335 679 60 5410 2.77 55,260 2.00 2241 6.69 73,353 2.19 2.414 9.47 24.662 8.69 0.88536 597 55 5260 2.85 61,710 2.00 2124 7.06 79,185 2.19 2.477 9.91 29.056 9.06 0.89237 598 59 5450 2.75 54,600 2.00 1952 7.69 72,707 2.19 2.793 10.44 27.977 9.69 0.91138 697 70 4020 3.73 48,610 2.00 1947 7.71 62,074 2.38 2.065 11.44 24.970 9.71 0.87139 762 63 5690 2.64 52,970 2.00 2477 6.06 72,130 2.17 2.297 8.69 21.383 8.06 0.87140 660 58 5740 2.61 56,140 2.00 2234 6.71 75,306 2.16 2.570 9.33 25.132 8.71 0.89342 655 57 2490 6.02 73,090 2.00 2399 6.25 81,401 2.41 1.038 12.28 30.464 8.25 0.68645 728 68 4070 3.69 51,960 2.00 2154 6.96 65,627 2.35 1.889 10.65 24.119 8.96 0.84946 649 137 1980 7.58 22,150 2.00 839 17.88 28,756 3.28 2.360 25.45 26.396 19.88 0.91547 632 46 3070 4.89 95,980 2.00 3021 4.97 106,208 2.28 1.016 9.85 31.776 6.97 0.66148 675 86 3140 4.78 38,060 2.00 1475 10.17 48,558 2.60 2.129 14.95 25.801 12.17 0.88849 637 91 3120 4.81 34,530 2.00 1287 11.66 44,929 2.65 2.425 16.46 26.833 13.66 0.91150 650 75 2870 5.23 46,840 2.00 1649 9.10 56,415 2.55 1.741 14.32 28.411 11.10 0.84751 657 59 3800 3.95 63,480 2.00 2251 6.66 76,165 2.32 1.688 10.61 28.199 8.66 0.82452 628 61 3510 4.27 59,950 2.00 2023 7.41 71,640 2.37 1.735 11.69 29.635 9.41 0.83753 699 63 3690 4.07 59,060 2.00 2249 6.67 71,421 2.36 1.641 10.74 26.265 8.67 0.81855 673 60 3150 4.76 64,880 2.00 2287 6.56 75,410 2.39 1.378 11.32 28.374 8.56 0.77456 629 77 3200 4.69 43,850 2.00 1541 9.74 54,508 2.53 2.077 14.42 28.460 11.74 0.88357 665 88 3950 3.80 33,790 2.00 1406 10.67 46,985 2.50 2.810 14.47 24.040 12.67 0.92658 618 88 3430 4.37 34,920 2.00 1286 11.67 46,334 2.58 2.668 16.04 27.162 13.67 0.92359 721 101 1830 8.20 35,130 2.00 1342 11.18 41,272 2.92 1.363 19.37 26.172 13.18 0.79460 634 70 3260 4.60 49,870 2.00 1731 8.67 60,732 2.47 1.884 13.27 28.814 10.67 0.86161 675 65 3170 4.73 57,810 2.00 2079 7.22 68,408 2.42 1.525 11.95 27.811 9.22 0.80662 614 68 3260 4.60 52,300 2.00 1743 8.60 63,145 2.45 1.870 13.21 30.002 10.60 0.85963 675 50 2420 6.20 87,860 2.00 2916 5.14 95,950 2.35 0.830 11.34 30.126 7.14 0.59564 663 136 1950 7.69 22,560 2.00 867 17.31 29,073 3.28 2.250 25.00 26.035 19.31 0.908

Sample NG3 (biotite granite)

1 624 52 3570 4.20 77,520 2.00 2511 5.97 89,406 2.29 1.422 10.18 30.871 7.97 0.7762 605 45 2730 5.49 98,460 2.00 2927 5.12 107,536 2.29 0.933 10.62 33.638 7.12 0.6353 580 42 3680 4.08 102,330 2.00 2987 5.02 114,539 2.22 1.232 9.10 34.255 7.02 0.7214 569 35 2370 6.33 138,290 2.00 3738 4.01 146,146 2.23 0.634 10.34 36.993 6.01 0.4796 574 52 2220 6.76 79,920 2.00 2253 6.66 87,282 2.40 0.985 13.41 35.474 8.66 0.6727 607 47 2870 5.23 90,800 2.00 2739 5.48 100,342 2.31 1.048 10.70 33.146 7.48 0.67910 639 108 2830 5.30 27,780 2.00 1068 14.04 37,213 2.84 2.649 19.34 26.006 16.04 0.92611 612 101 2560 5.86 31,320 2.00 1096 13.69 39,835 2.82 2.337 19.55 28.590 15.69 0.91012 651 88 1400 10.71 43,350 2.00 1408 10.65 48,022 2.85 0.994 21.37 30.785 12.65 0.69313 535 110 2300 6.52 27,740 2.00 847 17.70 35,343 2.97 2.714 24.22 32.739 19.70 0.93214 604 50 3810 3.94 78,000 2.00 2461 6.09 90,665 2.27 1.548 10.03 31.690 8.09 0.79815 623 46 2620 5.73 97,240 2.00 2971 5.05 105,962 2.31 0.882 10.77 32.733 7.05 0.61516 620 44 3170 4.73 101,160 2.00 3114 4.82 111,710 2.26 1.018 9.55 32.489 6.82 0.65917 573 48 2440 6.15 87,210 2.00 2453 6.12 95,300 2.35 0.995 12.26 35.555 8.12 0.67019 558 64 1850 8.11 60,990 2.00 1681 8.92 67,117 2.56 1.100 17.03 36.271 10.92 0.72220 588 59 2130 7.04 67,310 2.00 1967 7.63 74,381 2.48 1.083 14.67 34.223 9.63 0.71121 649 60 1910 7.85 69,990 2.00 2233 6.72 76,362 2.49 0.855 14.57 31.339 8.72 0.62322 613 60 2120 7.08 67,540 2.00 2056 7.30 74,592 2.48 1.031 14.37 32.854 9.30 0.69223 619 65 2340 6.41 59,150 2.00 1863 8.05 66,938 2.51 1.256 14.46 31.744 10.05 0.75924 601 51 2600 5.77 81,480 2.00 2437 6.15 90,121 2.36 1.067 11.92 33.433 8.15 0.69425 634 50 2450 6.12 86,710 2.00 2707 5.54 94,874 2.35 0.905 11.66 32.030 7.54 0.63126 621 50 2570 5.84 85,780 2.00 2636 5.69 94,335 2.35 0.975 11.53 32.540 7.69 0.65929 592 135 2130 7.04 21,660 2.00 764 19.62 28,734 3.24 2.787 26.67 28.337 21.62 0.936


Table 2 (continued)

NUM Age(Ma)

Error(Ma)

U(ppm)

Error(%)

Th(ppm)

Error(%)

Pb(ppm)

Error(%)

Th*(U)(ppm)

Error(%)

U/Pb Error(%)

Th/Pb Error(%)

Corr

30 682 129 2790 5.38 21,430 2.00 943 15.91 30,763 3.02 2.959 21.28 22.725 17.91 0.94031 626 138 2480 6.05 19,940 2.00 793 18.91 28,199 3.19 3.126 24.95 25.131 20.91 0.94732 742 143 2070 7.25 20,950 2.00 934 16.07 27,909 3.31 2.217 23.31 22.442 18.07 0.90533 538 115 2920 5.14 23,820 2.00 807 18.59 33,475 2.90 3.620 23.73 29.528 20.59 0.95834 654 128 2630 5.70 22,050 2.00 907 16.54 30,828 3.05 2.900 22.24 24.316 18.54 0.93935 631 124 2300 6.52 24,100 2.00 901 16.64 31,762 3.09 2.552 23.17 26.741 18.64 0.92436 767 137 2300 6.52 21,740 2.00 1020 14.70 29,488 3.19 2.254 21.22 21.307 16.70 0.90637 744 120 2450 6.12 25,850 2.00 1144 13.12 34,088 3.00 2.143 19.24 22.606 15.12 0.89638 554 132 2300 6.52 21,530 2.00 724 20.72 29,145 3.18 3.178 27.24 29.744 22.72 0.94939 618 131 2010 7.46 23,330 2.00 833 18.00 30,019 3.22 2.412 25.46 27.998 20.00 0.91842 602 57 2060 7.28 71,890 2.00 2132 7.03 78,737 2.46 0.966 14.32 33.716 9.03 0.66843 603 52 2410 6.22 80,440 2.00 2400 6.25 88,450 2.38 1.004 12.48 33.523 8.25 0.67544 644 50 2090 7.18 87,470 2.00 2738 5.48 94,440 2.38 0.763 12.65 31.941 7.48 0.57045 652 49 2500 6.00 90,960 2.00 2917 5.14 99,303 2.34 0.857 11.14 31.185 7.14 0.60746 643 46 2460 6.10 97,720 2.00 3066 4.89 105,923 2.32 0.802 10.99 31.874 6.89 0.57947 593 44 3180 4.72 99,060 2.00 2925 5.13 109,622 2.26 1.087 9.85 33.869 7.13 0.68648 622 46 2320 6.47 98,290 2.00 2966 5.06 106,013 2.33 0.782 11.52 33.136 7.06 0.57349 583 49 2940 5.10 83,380 2.00 2443 6.14 93,137 2.32 1.204 11.24 34.136 8.14 0.73150 649 48 2590 5.79 92,540 2.00 2956 5.07 101,181 2.32 0.876 10.87 31.303 7.07 0.61351 631 48 2710 5.54 90,920 2.00 2840 5.28 99,948 2.32 0.954 10.82 32.009 7.28 0.64652 587 45 2790 5.38 96,240 2.00 2782 5.39 105,501 2.30 1.003 10.77 34.590 7.39 0.664

Analytical errors are <5% for major elements and in the range 5–10% for trace elements and REE.

B

520

540

560

580

600

620

640

660

680

700

Age

Ma

Sample WA10: Mean = 615±27 [4.4%] 95% conf.

Wtd by data-pt errs only, 0 of 4 rej.

MSWD = 0.39, probability = 0.76

A

780

820

860

900

940

980

1020

1060

1100

Age

Ma

Sample WA3D: Mean = 926±12 [1.3%] 95% conf.



C

400

500

600

700

800

900

Age

Ma

Sample EN4: Mean = 648±14 [2.1%] 95% conf.



350

450

550

650

750

850

950

Age

Ma

Sample NG3: Mean = 610.5±9.8 [1.6%] 95% conf.



D

Fig. 9. Plots of average weighted age using individual ages and errors (2r) for the monazite from biotite muscovite granitoids (A, sample WA3D; B,sample WA10) and for biotite granitoids (C, sample NG3; D, sample EN4).


Partial melts from metagreywacke sources

Partial melts from metabasic to tonalitic sources

Partial melts from metapelitic sources

14

12

10

8

6

4

2

00 0.2 0.4 0.6 0.8 1.0 1.2 1.4

molar CaO/(MgO +FeOt)

mol

ar A

l 2O

3 / (

MgO

+ F

eOt)

HBG

BGBMG

Fig. 10. Plots of the Neoproterozoic Ngaoundere granitoids in a molarAl2O3/(MgO + FeOt)–CaO/(MgO + FeOt) diagram (Altherr et al., 2000),with composition fields of partial melts deriving from experimentaldehydratation–melting of various source rocks (Wolf and Wyllie, 1994;Gardien et al., 1995; Partino Douce and Beard, 1995, 1996; Singh andJohanneses, 1996).


BMG. As a consequence, the ca. 926 ± 12 Ma monazites ofBMGs are inherited grains. Similarly, the 652 ± 10 Mamonazites of the post-tectonic BGs also correspond toinherited grains.

To understand the significance of the 610 ± 10 Mamonazites of BG, we recall the U–Pb results for bulk zirconanalyses of the same granite (83–38, Toteu et al., 2001)which yielded very discordant ages with an upper interceptat 635 ± 22 Ma. One additional data point for a single zir-con from the same sample (unpublished data from Toteu)is slightly above the concordia with a 206Pb/238U age of575 ± 8 Ma. This suggests that the 635 ± 22 Ma interceptdoes not correspond to the emplacement age, but ratherrepresents a mean age of mixed populations that includeinherited and young syn-magmatic zircons. This meansthat the emplacement age of the Ngaoundere biotite gran-ite is probably close to 575 Ma, identical to the individuallower monazite spot ages. As a consequence, monazitteswith the 610 ± 10 Ma ages may be interpreted as inheritedgrains.

Our data also give some indications of the age of thesource for BMG and BG. The inherited monazites withages of ca. 926 Ma recorded for BMG and the 1.09 GaTDM age on a similar granite west of Moungel indicatean early Neoproterozoic source for the protolith. Ages ofthis range are also known on detrital zircons from metase-diments in Cameroon (Toteu et al., this volume) and in NEBrazil where they are related to the Cariris Velhos event(Van Schmus et al., 1995; Santos et al., 1997).

The presence of younger inherited monazites and zir-cons in BG also suggests the contribution of a Neoprotero-zoic source to the protolith. Although country rocks for theNgaoundere granitoids are of Paleoproterozoic age, noindication of monazite or zircon of this age has beenrecorded. This is an indication that inherited monazitesand zircons are probably from the source, rather than fromcontamination by the country rocks. In conclusion, thepresent study strongly suggests a major contribution of aNeoproterozoic source to the generation of the Ngaoun-dere granitoids.

6. Discussion

6.1. Source rock characteristics

The Ngaoundere granitoids display mineralogical andchemical features of I-type granites (Fig. 4) and belong tohigh-K calc-alkaline suites (Fig. 5). Considering the Baand Sr abundances (Table 1), the HBG correspond to thetypical high-Ba–Sr granitoids, though BG and BMG arelow-Ba–Sr granitoids (Tarney and Jones, 1994; Fowleret al., 2001; Quian et al., 2003). Major and trace elementdata of the HBG high-Ba–Sr granitoids agree with differen-tiation of a mafic magma from an enriched subcontinentallithospheric mantle (Fig. 10), with possible crustal assimila-tion, as shown by the high Th/Yb and Rb/Th ratios com-pared to the Rb contents. The HREE undepleted patterns

indicate melting of a source with a residual HREE-richphase such as garnet.

The BG and BMG trace element patterns (Fig. 7) arealso characterized by strong Th- and LREE-enrichment.They may derive from partial melting of igneous proto-liths (Fig. 10), but at a temperature above monazite andzircon saturation temperatures (>750 �C). The presenceof inherited monazite (ca. 926 Ma), the presence ofnumerous enclaves of HBG progressively digested withinthe BMG, and the presence of ghost structures are allthe consequence of a significant crustal contribution.BG that show weak Ta negative anomalies and lowNb/Ta ratios (Fig. 7) may have originated from a lowerto middle crust of mafic to intermediate composition withinherited low Nb/Ta ratio (metagreywackes?) (Fig. 10),because these Nb–Ta abundances cannot be explainedeither by titanite–rutile fractionation or by the presenceof these phases as source residue (Green and Pearson,1987).

BMG are characterized by the presence of enclaves ofHBG that occur mostly along the margin of the intrusion.This indicates that enclaves were incorporated during theemplacement and do not represent restites from the source.This is confirmed by the geochemical data (Figs. 6, 10, and11), which indicate that BMG and HBG intrusions origi-nated from a distinct source.

6.2. Tectonic settings and tentative geodynamic context

The overall geochemical features of the Ngaounderegranitoids are compatible with the compositions of

ORG

WPG

syn-COLG

VAG

Y + Nb

Rb

1000

100

10

11 10 100 1000

HBG

BG

BMG

Fig. 12. (Y + Nb) vs Rb discrimination diagram for the NgaoundereNeoproterozoic granitoids after Pearce et al. (1984). Syn-COLG, syn-collision granite; VAG, volcanic arc granite; WPG, within-plate granite;ORG, ocean ridge granite.

10 100 1000 100000.1

1

10

(Nb

/ Zr)

N

Zr

C

B

A

HBG

BMG

BG

Fig. 13. Zr vs (Nb/Zr)N diagram of Thieblemont and Tegyey (1994) forthe Neoproterozoic Ngaoundere granitoids: (A) subduction-zone mag-matic rocks, (B) collision zone rocks; (C) alkaline intra-plate zone rocks.Normalization values from Sun and McDonough (1989).

1 10Rb

100 10001

10

100

Rb/

Th

AFC

CC

FCHBG

BMG

BG

Fig. 11. Rb vs Rb/Th diagram for the Ngaoundere Neoproterozoicgranitoids. AFC, assimilation–fractional crystallization; CC, crustalcontamination; FC, fractional crystallization.


calc-alkaline magmas of orogenic domains. In the Y + Nbvs Rb discrimination diagram (Pearce et al., 1984;Fig. 12), the HBG clearly plot within the volcanic-arcgranite field, while BG plot in the within-plate granitefield. The BMG plot in the field of syn-collisional gran-ites. On the Zr vs (Nb/Zr)N diagram (Fig. 13) of Thieble-mont and Tegyey (1994), most HBG plot from thesubduction to collision zone fields while BMG plot inthe collision zone field, and BG overlap the collisionand intra-plate fields. These characteristics and theirhigh-potassic calc-alkaline compositions are consistentwith a continental collision setting (Liegeois et al., 1994,1998; Toteu et al., 2004). Therefore, some within-plategranite features observed in the BG (Fig. 12) may beinterpreted as an evolved trend of calc-alkaline suites, as

shown by the trace element patterns (Fig. 7C). These geo-chemical features are similar to those documented fornumerous syn- to late-collisional Pan-African granitoidsof western Cameroon (Nguiessi Tchankam et al., 1997;Tagne-Kamga, 2003; Nzolang et al., 2003). Comparedto the granitoids of NE Brazil, the HBG with lowerFeOt/(FeOt + MgO) ratios (60.70), Nb, Y, Yb, andK2O are similar to the normal calc-alkaline and high-Kcalc-alkaline granitoids (Conceicao and Itaporanga types),whereas the BG and BMG that have higher contents ofalkalis, lower contents of Ba and Sr, high FeOt/(FeOt + MgO) ratios (P0.70), and pronounced Eu anom-alies, are similar to the calc-alkaline to transitional grani-toids of Solidao and Cerra Blanca type (Guimaraes et al.,1998; Guimaraes and da Silva Filho, 2000).

A BRGM (Bureau de Recherches Geologiques et Minie-res d’Orleans) database, including a compilation of analy-ses of recent Andean lavas (Thieblemont, 1999), was usedto test further the analogy between the NgaoundereNeoproterozoic granitoids and Andean magmatic rocks.Averaged analyses were calculated (Table 3) using onlythe highly potassic calc-alkaline lavas from the centralAndes. For comparison with the HBG, the Andean aver-age was calculated in the 60% 6 SiO2 6 68% interval,based on �400 determinations of major elements and100–400 determinations for trace elements, depending onthe elements. For many elements (Hf, Zr, and most ofthe REE in particular), this average shows low standarddeviations (Table 3), which makes it possible to assign arepresentative composition at the scale of the centralAndes. The average compositions of Andean rocks andHBG show strong analogies (Fig. 14), but also show differ-ences among some trace elements, such as Ba and Sr. Theseelements show high standard deviations, however, andcannot be used for comparison with other granitoid suites.

Table 3Average chemical analyses for the hornblende biotite granitoids and biotite granitoids

Neoproterozoic Central Andes average Neoproterozoic Central Andes averageNgaoundereAverage HBG(n = 9)

Average calc-alkalinelavas SiO2 = 60–68%

Ngaoundere Average BG (n = 9) Average calc-alkalinelavas SiO2 > 70%

Average Standard deviation n Average Standard deviation n

Major elements (wt%)SiO2 58.09 63.2 1.78 418 71.95 72.74 2.16 89Al2O3 17.29 16.27 0.73 383 14.02 14.13 1.25 83TiO2 1.08 0.8 0.17 382 0.30 0.26 0.14 83Fe2O3t 6.89 4.96 1.09 391 2.53 1.66 0.8 83MnO 0.104 0.08 0.04 383 0.03 0.06 0.03 82MgO 3.03 2.14 0.61 383 0.36 0.49 0.32 84CaO 5.64 4.53 2.35 383 1.11 1.58 0.81 84Na2O 3.99 3.78 0.57 383 3.55 3.66 0.79 84K2O 2.85 3.11 0.53 413 5.44 4.31 0.97 89P2O5 0.38 0.39 0.08 326 0.11 0.08 0.06 75LOI 0.66 1.17 0.51

Total 100.00 99.92

Trace elements (ppm)Ba 1191 869 289 322 545 626 350 56Co 20.73 18 9 261 3.28 9 14.4 42Cr 63.73 37 31 262 12.48 10 9 38Hf 5.22 5.41 0.65 148 9.01 4.07 0.84 30Nb 10.5 12.8 6.4 167 29.12 18.4 7.9 24Ni 36.83 16 11 290 5.32 9 21 47Rb 99.54 109 42 377 285.26 177 95 71Sr 732.64 558 180 399 209.85 260 190 71Ta 0.74 1.27 0.73 127 3.33 1.67 1.55 30Th 3.96 13.9 7.4 176 49.02 18.3 10.5 38U 1.39 4.16 2.9 143 8.71 7.3 5.33 30V 127.48 107 30 225 17.49 23 18.4 27Y 21.24 17.5 5.4 160 37.77 16.4 10.3 35Zr 215.53 195 35 241 302.66 158 68.4 49La 38.87 39.00 9.7 184 86.74 28.2 10.1 35Ce 80.67 80.00 22.1 149 165.92 53.3 17.9 36Pr 9.90 17.60Nd 38.96 35.00 9.8 103 59.71 21.8 8.9 29Sm 7.24 6.4 1.5 120 10.62 4.5 1.4 28Eu 1.91 1.4 0.31 189 0.904 0.85 0.49 35Gd 5.52 5.03 1.4 24 7.91Tb 0.756 0.6 0.14 179 1.167 0.4 0.15 32Dy 4.05 3.00 0.73 11 6.53Ho 0.729 1.219Er 1.978 1.2 0.32 10 3.532Tm 0.283 0.544Yb 1.846 1.4 0.54 130 3.645 1.3 0.78 28Lu 0.284 0.2 0.07 115 0.546 0.2 0.15 15

The average composition (and standard deviation) of the high-K calc-alkaline andesite, dacite and rhyolite from the central Andes is given for comparison(BRGM geochemical database on Andean magmatic rocks) (Thieblemont, 1999) and was calculated using a number of measurements that varieddepending on the element.


For comparison with BG, the Andean average was cal-culated by using only the most acid rocks (i.e. SiO2 > 70%)and for an age range extending to the late Miocene. Theprimitive mantle-normative patterns of BG show someanalogies to the Andean rocks (Fig. 14), although BG aremuch more differentiated. It is concluded that the Ngaoun-dere granitoids in central North Cameroon that are part ofthe Adamawa-Yade batholith may be the roots of a conti-

nental margin that was active during the late Neoprotero-zoic (Toteu et al., 2004). In pre-drift reconstructions, thisactive continental margin may be correlated with the Cen-tral Tectonic Domain or Transverse Zone of the Borbor-ema province in NE Brazil (Ebert, 1970; Van Schmuset al., 1995), which is characterized by abundant late Neo-proterozoic plutonism and development of continental-scale shear zones (Neves et al., 1996).

1

10

100

1000


Sam

ples

/ P

rim

itiv

e M

antl

e

HBGAnde (SiO2=60-68%)Andes (SiO2 >72%)BG

Fig. 14. Primitive mantle-normalized incompatible element patterns forthe average HBGs and BGs compared to the Central Andes average ofcalc-alkaline lavas. Normalizing values from Sun and McDonough (1989).


7. Conclusions

Petrography, geochemistry and new geochronologicalwork carried out in the Ngaoundere region on the Adam-awa-Yade batholith revealed three main types of Pan-African granitoids: the hornblende–biotite granitoids(HBG), the biotite ± muscovite granitoids (BMG), andthe biotite granitoids (BG). Monazite dating is not conclu-sive but strongly indicates the presence of pre-magmaticmonazite inheritances at ca. 926 Ma.

Geochemical data indicate that the Ngaoundere Pan-African granitoids are calc-alkaline, high-K and of I-type.The HBG are of the high-Ba–Sr sub-type and were gen-erated by differentiation of mafic magmas derived froman enriched subcontinental lithospheric mantle, with sub-stantial crustal contamination. By contrast, the BGs andBMG are of a low-Ba–Sr sub-type and were possiblyderived from melting of middle continental crust. Differ-ences in these granitoid groups can be explained by vari-ations in the source composition, the melting conditionsand the degree of mineral fractionation. The large-scalemelting of the source rocks could have been promotedby high heat flow during Neoproterozoic deformationand/or by the upward migration or underplating ofmantle-derived magmas generated during a convergentphase.

The chemical signature of the HBG shows strong anal-ogies with the magmatic suites of recent active marginsand, more precisely, with calc-alkaline rocks of the centralAndes. These analogies suggest a setting above a subduc-tion zone at an active margin. Generation of the BMGby crustal melting may have occurred subsequent to a col-lisional stage, with emplacement along SW-NE left-lateralstrike slip faults. Then, the BG plutons were emplaced ina post-orogenic setting.

Acknowledgements

RT thanks the Agence Universitaire de la Francophonie(AUF) for the post-doc scholarship and the Institut desSciences de la Terre d’Orleans (ISTO) for accommodationand support of all the laboratory work. The authors aregrateful to O. Rouer, G. Drouet and C.Gille for their assis-tance during the electron microprobe analyses. The paperbenefited from critical reviews by I.P. Guimaraes and D.Gasquet and from English improvement by J.J.W. Rogers.They are gratefully acknowledged. This work is a contribu-tion to IGCP-470.

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Petrography and geochemistry of the Ngaoundéré Pan-African granitoids in Central North Cameroon: Implications for their sources and geological setting