International Journal of Geosciences, 2011, 2, 138-147
doi:10.4236/ijg.2011.22014 Published Online May 2011 (http://www.SciRP.org/journal/ijg)
New Constraints from Pb-Evaporation Zircon Ages of the
Méiganga Amphibole-Biotite Gneiss, Central Cameroon,
on Proterozoic Crustal Evolu tion
Ganwa Alembert Alexandre1*, Siebel Wolfgang2, Shang Kongnyuy Cosmas2, Naimou Seguem1,
Ekodeck Georges Emmanuel3
1Departme n t of Earth Scie n ces, Faculty of Sciences, University of Ngaoundere, Ngaoundere, Cameroon
2Department of Geosciences, U ni v ers i t y of Tübing en, Tübingen, Germany
3Departme n t of Earth Scie n ces, Faculty of Sciences, University of Yaounde I, Yaounde, Cameroon
Received January 24, 2011; revised March 7, 2011; accepted April 19, 2011
Abstract
The amphibole biotite gneiss (ABGn) in the Méiganga area forms part of a meta volcano-sedimentary se-
quence of the Adamawa Yade domain (AYD), Central African Fold Belt (CAFB). This sequence shows af-
finity with immature sediments (greywackes, arkoses) with intercalation of mafic lavas or tuffs. New
207Pb/206Pb zircon evaporation ages for two ABGn samples range from 1887 - 2339 Ma and from 675 - 889 Ma,
respectively. These ages and evidence from internal zircon structures indicate that igneous rocks of Archean
to Paleoproterozoic and of early Neoproterozoic age contributed to the detritus of the sedimentary sequence.
The deposition of detritus took place prior to 614 - 619 Ma which represent the syntectonic emplacement of
the Méiganga metadiorite. Leucogranites north to the Méiganga area were generated by melting of crust
identical to that which provided the source of the ABGn. The metasedimentary sequence investigated in this
study is similar to that of the southern part of the AYD and in the Borborema Province, NE Brazil. The tec-
tonic and geochronologic characteristics of the AYD in the Méiganga area support the idea that during the
Proterozoic, Central Africa and NE Brazil were part of the same continental landmass.
Keywords: Metasediment, 207Pb/206Pb Ages, Crustal Evolution, Adamawa Yade Domain, Central African
Fold Belt
1. Introduction and Geological Setting
The Adamawa – Yade Domain (AYD) is one of the three
main lithostructural units of the Central African Fold
Belt (CAFB), defined by [1] using petrographic, struc-
tural, and isotopic data in Cameroon and Central Africa
Republic (Figure 1). These authors considered the AYD
as a Paleoproterozoic basement unit that was dismem-
bered during the Pan-African orogeny. In central Came-
roon (Adamawa region), the AYD is characterized by
Pan-African granitoids intruding Paleo- to Neoprotero-
zoic gneisses which are intensively overprinted by re-
gional-scale transcurrent shear zones [1-3]. The Bafia
group to the north of Yaoundé, previously considered as
a basement tectonic slice overthrusting the Yaoundé
Group [4], is regarded as the southern extension of the
AYD in Cameroon. This interpretation is strengthened
by the presence of granulite facies assemblages retro-
gressed during the Pan-African nappe tectonics [5]. A
limited number of studies are available about the petro-
graphy, deformation history, geochemistry and geochro-
nology of the metasedimentary sequence in the southern
part of AYD [5-10]. The AYD of the CAFB has several
features in common with its equivalent in NE Brazil, the
Brasiliano/Pan-African Borborema Province, including 1)
a central position in relation to the sur- rounding cratons
[11-13], 2) a network of transcurrent shear zones, and 3)
the presence of metasedimentary sequences.
This study presents new zircon evaporation ages as
well as geochemical data on amphibole-biotite gneiss
(ABGn) from the AYD. The data are used to constrain
the protolith age of the gneiss, and add knowledge to the
tectonic and geochronological evolution of the AYD in

G. A. ALEXANDRE ET AL.139
Figure 1. Geological sketch map of of the Méiganga area, East Adamawa. Inset map from [1]. Patterns are as follows: Grids,
Congo Craton (CC); dark grey; Adamawa–Yadé Domain (AYD); medium grey, Yaoundé Domain (YD); light grey, West
Cameroon Domain (WCD); heavy dots, Cameroon line; light dots, Mesozoic sediments. Square in inset map localizes the
large figure. Cameroon (C.), Central Africa Republic (C.A.R.), Central Cameroonian Shear Zone (CCSZ).
central Cameroon during the Proterozoic and its relation
to the Borborema Province.
2. Petrography of the ABGn
The ABGn (Figure 2) shows compositional banding
marked by alternating amphibole-biotite-rich layers and
quartzofeldspatic layers. It consists of green hornblende
associated with brown-greenish biotite; plagioclase crys-
tals show antiperthitic feldspar clusters, and epidote is
formed at the expense of plagioclase. Accessory minerals
are apatite, zircon, and titanite. The ABGn contains
boudins or continuous bands (0.04 to 2 m thick) of am-
phibolites with nematoblatic to nematogranoblastic tex-
tures. The amphibolites are made up of green hornblende,
biotite, plagioclase, quartz and opaque minerals. Acces-
sory minerals are titanite, apatite and zircon, while sec-
ondary minerals are chlorite, epidote and calcite.
The ABGn has been affected by four deformational
phases. Detailed studies on these phases can be seen in
[3,14].
3. Analytical Techniques
Major and trace elements were analysed by X-ray fluo-
rescence (XRF) at the University of Tübingen. Rare-
earth elements were analysed by Inductively Coupled
Plasma–Atomic Emission Spectrometry (ICP–AES) at
the Centre de Recherches Pétrographiques et Géochimi-
ques (CRPG), Vandoeuvre-lès-Nancy, France. Analytical
uncertainties are estimated at ±1% for major elements
and 5% - 10% for most trace elements.
Zircon grains were separated from 200–63-mm sieved
rock fractions by standard separation techniques (milling,
wet shaking table, magnetic and heavy liquid separation)
and finally handpicked under a binocular microscope.
Cathodoluminescence images were performed on an
electronic microscope LEO Model 1450 VP (variable
pressure) 4-Quadrant BSE-Detector working with an
accelerating voltage of 10 kV. For single-zircon Pb eva-
poration, whole zircon grains were analysed using a
double Re filament configuration [15,16]. Principles of
the evaporation method are outlined in [10,17].
4. Geochemistry
Results of geochemical analyses on selected ABGn sam-
ples are shown in Table 1. In the MgO-K2O-Na2O dia-
gram (Figure 3) of de la Roche [18], the ABGn samples
deviate from the magmatic trend and their chemical
composition shows affinity with immature detrital sedi-
ments like greywacke and arkose whereas the amphibo-
lites have chemical composition similar to basalt. The
studied samples show variable (Na2O + K2O + FeO +
MgO + TiO2) values ranging from 8 to 18 (Figure 4(a),
[19]) and molar Al2O3/(MgO+FeOtot) values between 0.7
and 2.7 Figure 4(b), [20]. These variations may imply
the heterogeneity of the protolith of the ABGn, and are in
accordance with the conclusion of [3] that the rocks of
the ABGn belong to a metavolcanosedimentary sequence.
The aluminium saturation index (A/CNK = [Al2O3/(CaO
+ Na2O + K2O) mol%]) (Table 1) varies from 0.7 to 1.1
and the Mg-number [Mg# = Mg/(Mg + FeTotal)] from
.43 to 0.50. 0

G. A. ALEXANDRE ET AL. 140
Figure 2. Geological map of Méiganga area showing the distribution of the ABGn. Legend: (1) basalt, (2) conglomerate, (3)
biotite–muscovite granite, (4) pyroxene–amphibole–biotite granite, (5) banded amphibolite, (6) amphibole–biotite gneiss
(ABGn), (7) amphibolite, (8) pyroxene–amphibole gneiss, (9) mylonite, (10) dolerite, (11, 12) schistosity, (13, 14) lineation, (15)
fracture, (16) fault, (17) suppose d faul t, (18) river, (19) road, (20) path.
5. 207Pb/206Pb Geochronology
Representative zircon grains were studied in two samples
(Me5, NY1) of the ABGn (Figure 5). Zircon grains of
sample Me5 are short and oval in shape with smooth
crystal faces. Grain b1 shows a high luminescence rim
with truncated oscillatory zoning, surrounding a dark
grey core domain. The other grains are dominated by less
luminescence domains. Zircon grains from sample NY1
vary in shape. Grains b4 and b5 show dark cores sur-
rounded by homogeneous rims with faint oscillatory
zoning. This behaviour is attributed to the differential
absorption of contaminants during crystallization [21] or
to the effects of deformation during subsequent recrys-
tallization [22]. Grain b3 is made up of a high lumines-
cence rim, free of zonation, and a composite core frag-
ment. This core is similar to the core of grain b1. Zircon
grains portraying truncation of oscillatory zoning or re-
gions with fading oscillatory zoning are typical of mag-
matic zircons modified by high-grade metamorphism
[23]. Table 2 shows the analytical data obtained from
evaporation of representative zircon grains of samples
Me5 and NY1. In the first sample, U/Th ratios (1.2 - 2.9)
decrease with increasing evaporation temperatures. The
207Pb/206Pb ages are Neoproterozoic, and increase with
increasing evaporation temperature from 675  7 Ma to
889  2 Ma. Sample NY1 portrays U/Th ratios of 2.0 to
5.8, decreasing with increasing evaporation temperature.
The 207Pb/206Pb ages of NY1 vary from 1888  2 Ma to
2339  3 Ma. High U/Th ratios of the two ABGn sam-
ples indicate that the zircons were derived by erosion of
an igneous protoliths [24]. The ABGn was affected by
the same metamorphic event as the metadiorite of the
Méignanga area which was dated at 614 - 619 Ma [3]. It
is likely that the deposition age is therefore older than
619 Ma. The youngest age (675 Ma) of sample Me5

G. A. ALEXANDRE ET AL.141
Table 1. Geochemical composition of the amphibole-biotite gneiss from the Méiganga area. (b.d.l. = below detection limit).
Samples NY2 Me5 Me7 PNg2 Moo ZBo-1
SiO2 58.25 61.66 59.41 74.88 67.88 63.12
TiO2 0.75 0.48 0.92 0.294 0.50 0.63
Al2O3 15.23 20.07 16.12 12.90 15.45 15.44
Fe2O3 8.54 3.18 7.23 2.16 4.24 6.36
MnO 0.12 0.04 0.12 0.03 0.07 0.1
MgO 3.80 1.20 3.59 0.76 1.68 2.81
CaO 7.33 2.76 5.69 2.77 3.59 5.33
Na2O 3.72 4.21 3.35 3.91 4.59 4.18
K2O 1.75 6.33 2.59 1.12 1.80 1.32
P2O5 0.28 0.22 0.26 0.06 0.12 0.18
LOI 0.73 0.75 0.92 0.38 0.64 0.67
Sum 100.75 101.47 100.48 99.43 100.81 100.32
Na2O + K2O 6.71 10.54 5.94 5.02 6.39 5.50
Na2O/K2O 4.25 0.66 1.29 3.50 2.54 3.17
A/CNK 0.7 1.1 0.9 1.0 1.0 0.9
Mg# 0.47 0.43 0.50 0.41 0.44 0.47
Ba 907.4 3362 984.3 730.4 1190.5 616.5
Co 49.99 31.6 41.5 58.1 13 20.6
Cr 92.46 106.6 164 93.4 90.6 51.7
Ni 44.82 53 70.5 27.9 50.9 40.2
Rb 41.6 114.7 78.9 18.8 45.7 24.9
Sr 546.5 853.7 558.6 549.4 524.3 591.3
V 143.7 40.98 146.5 25.5 63.5 123.7
Y 34.44 5.93 26.8 b.d.l. 12.3 15.2
Zn 72.41 56.84 77.6 b.d.l. 48.5 58.4
Zr 229.2 534.6 187.3 184.4 227.5 148.4
K 14543 52970 21500 9264 14967 10957
La 109.2 54.29 42.7 30 63.2 53.9
Ce 203.7 100.7 74.5 b.d.l. 102.1 59.5
Pr 22.06 11.13

G. A. ALEXANDRE ET AL. 142
Nd 78.16 39.92 38.7 9.7 34.2 28.8
Sm 12.17 5.65 3.7 b.d.l. 6.3 5.1
Eu 2.62 1.37 1.5 1.2 1.6 1.7
Gd 8.70 3.11
Tb 1.21 0.34
Dy 6.53 1.41
Ho 1.12 0.22
Er 3.34 0.57
Tm 0.47 0.08
Yb 3.02 0.55 2.3 0.2 0.7 1.2
Lu 0.45 0.10
Hf 5.28 10.04
Ta 0.56 0.32
W 248.2 202.3
Pb 13.79 38.98
Th 13.38 9.34 8.4 3 14 1.1
U 0.71 0.26 b.d.l. 6.4 4.9 1.7
Nb 28.56 2.93 b.d.l. b.d.l. b.d.l. b.d.l.
Be 2.70 1.37
Cs 0.36 1.17
Cu 8.04 79.72
Ga 21.45 22.02
Ge 1.51 0.96
Mo 1.51 1.24
Sn 1.86 1.02
∑REE 487.27 225.38
K/Rb 373.25 458.33 272.49 492.76 327.50 440.05
Rb/Sr 0.04 0.13 0.14 0.03 0.09 0.04
Th/U 21.50 35.39 0.47 2.86 0.65
Sr/Y 15.87 143.87 20.84 42.63 38.90
LaN/YbN 24.55 67.30 24.55
Eu/Eu* 0.75 0.96 0.75

G. A. ALEXANDRE ET AL. 143
Table 2. Zircon evaporation data including radiogenic 207Pb/206Pb ratios and corresponding 207Pb/206Pb ages for samples Me5
and NY1.
Sample and zircon num
b
er
(a,b,c,d,e = Temp. Step)
Evap.
Temp ˚C
No. of
ratios U/Th ratio206Pb/208Pb ratio204Pb/206Pb ratio207Pb/206Pb isotope ratio
207Pb/206Pb age
(Ma) 2 error
Me5-1a 1380 114 2.60 8.64 0.000102
0.064825  127 768.7  4.1
Me5-1b 1400 114 1.53 5.00 0.000050 0.068293  106 877.5  3.3
Me5-1c 1420 114 1.49 4.87 0.000041 0.068362  097 879.6  2.9
Me5-3a 1400 113 1.57 5.22 0.000202 0.063386  099 721.3  3.3
Me5-3b 1420 112 1.30 4.27 0.000105
0.066173  096 811.9  3.0
Me5-3c 1440 112 1.20 3.94 0.000101 0.067232  108 845.0  3.4
Me5-4a 1400 110 2.32 8.00 0.000249 0.063961 195 740.4  6.5
Me5-4b 1420 111 1.27 4.19 0.000139 0.067693  130 859.2  4.0
Me5-4c 1440 108 1.14 3.72 0.000088
0.068612  217 887.1  6.6
Me5-5a 1400 114 1.92 6.26 0.000060 0.064358  132 753.5  4.3
Me5-5b 1420 107 1.59 5.21 0.000060 0.066254  195 814.4  6.2
Me5-5c 1440 114 1.35 4.41 0.000048
0.068689  075 889.4  2.3
Me5-6a 1420 98 2.94 9.87 0.000131 0.062028  201 675.1  7.0
Me5-6b 1440 109 1.44 4.72 0.000106 0.066721  196 829.1  6.2
NY1-1a 1410 113 3.36 11.52 0.000042 0.124573  147 2022.8  2.1
NY1-1b 1440 114 3.28 11.15 0.000013 0.130093  066 2099.3  0.9
NY1-1c 1470 114 2.04 6.97 0.000013 0.149428  229 2339.4  2.6
NY1-2a 1430 103 5.77 19.85 0.000028
0.121856  116 1983.7  1.7
NY1-2b 1460 81 5.08 17.44 0.000028
0.122150  127 1988.0  1.9
NY1-2b 1460 30 5.08 17.44 0.000028 0.123358  125 2005.5  1.8
NY1-4a 1430 114 2.02 6.87 0.000028 0.134196  109 2153.7  1.4
NY1-4b 1460 109 1.88 6.43 0.000028 0.143420  210 2269.0  2.5
NY1-4c 1490 114 1.82 6.24 0.000028 0.147719 147 2319.7  1.7
NY1-5a 1430 107 4.28 15.76 0.000174
0.115491  133 1887.6  2.1
NY1-5b 1460 105 3.74 12.78 0.000030 0.119954  177 1955.6  2.6
NY1-5c 1490 107 4.29 15.82 0.000174 0.120504  171 1963.8  2.5
could be a post-depositional metamorphic age or a mix-
ing age between pre-depositional and post-depositional
zircon domains. It seems most likely to us, that this age
post-dates the deposition of the sediments.
6. Discussion
6.1. Age and Provenance of the Protolith
Geochemical characteristics of the studied samples show
that the ABGn comes from a sedimentary sequence
whereas the protolith of the intercalated amphibolites
was igneous, probably representing mafic lava, or tuff.
Thus, it is likely that the whole complex represents an
ancient volcano-sedimentary sequence. The ABGn shows
affinity with detrital immature sediments (Figure 3),
indicating that the detritus was transported only over a
short distance. These sedimentary deposits were proba-
bly formed during alternate phases of volcanic activity.
The zircon crystals of the ABGn show relics of mag-
matic structures (truncated and faint oscillatory zoning)
and their U/Th content also militates for a magmatic ori-

G. A. ALEXANDRE ET AL. 144
Figure 3. Geochemical characteristics of the samples from
the ABGn (this study) and amphibolites [3] in the MgO-
K2O-Na2O diagram [18]. Solid line shows the compositional
trend of plutonic rocks with rhyolite (Rh), granite (Gr),
granodiorite (Go), quartz diorite (qD), diorite (D) basalt (B),
gabbro (G). Dashed contours delimit the field of shales (Sh),
greywackes (WK), and arkoses (Ark).
gin. The 207Pb/206Pb zircon evaporation ages obtained for
ABGn sample NY1 (1887 - 2339 Ma) fit in the range of
those reported by Ganwa et al. [17] for the neighbouring
pyroxene amphibole gneiss (1685-2602 Ma). The oldest
age (2339 Ma) of sample NY1 could be interpreted as a
mixing age of Paleoproterozoic and Archean zircon do-
mains; this is strengthened by the presence of dark cores
in some zircon crystals (Figure 5). It seems possible that
erosion of the pyroxene amphibole gneiss provided de-
tritus for the sedimentary sequence of the ABGn. Zircon
ages obtained from sample Me5 (675 - 889 Ma) indicate
that early Neoproterozoic plutonic rocks also contributed
to the detritus of the sedimentary sequence. The meta-
sedimentary sequence of the southern AYD was derived
by erosion of Mesoproterozoic (1617 Ma) to Paleopro-
terozoic (2289 - 2351 Ma) plutonic rocks [9,10]. Zircon
grains of samples Me5 and NY1 yield ages in the same
range as those of a two mica granite (sample Man and Mi,
Figure 6) north of the study area [3,14]. It is likely that
this Neoproterozoic leucogranite was formed by melting
of the crustal material similar to the inferred protolith of
the ABGn. Leucogranites generated by melting of Pa-
leoproterozoic crustal rocks have been also described in
the Serrinha–Pedro Velho Complex (Borborema Pro-
Figure 4. Geochemical characteristic of the ABGn in (a) the (Na2O + K2O + FeO + MgO + TiO2) vs ((Na2O + K2O)/(FeO +
MgO + TiO2)) [18] and (b) molar CaO/(MgO+FeOtot) vs molar Al2O3/(MgO + FeOtot) [19] diagrams. One sample has a molar
CaO/(MgO + FeOtot) of 1.0 and is not shown in the diagram.
Figure 5. Cathodoluminescence images of representative zircon crystals from the ABGn.

G. A. ALEXANDRE ET AL.145
Figure 6. Histogram showing the distribution of radiogenic 207Pb/206Pb ratios obtained from evaporation of zircons from
ABGn (samples Me5, NY1).
vince, NE Brazil; [25]).
6.2. Deposition Age and Evolution of the ABGn
The ABGn shows the same solid state deformation as
syntectonic diorite plutons for which emplacement ages
between 614 and 619 Ma were obtained [3]. It appears
that the age of metamorphism in the Méiganga area is
Neoproterozoic. Therefore, sedimentation must have
started prior to 619 Ma with episodes of volcanic activity.
After this period, the basin and the whole region was
subjected to a regional solid state transformation with
four deformational phases [3]. The D1 deformational
phase is present only in the ABGn whilst the D2 defor-
mational phase, which is the major deformational phase
in the AYD, is present both in the ABGn and the meta-
diorite.
6.3. Comparison with the Borborema Province,
NE Brazil
A common feature between the CAFB and the Brasili-
ano/Pan-African Borborema province is the occurrence
of metasediments outcropping in their central domain. In
the Borborema province, metasediments are found in the
Cachoeirinha, Alto Pajeú, Alto Moxotó and East Per-
nambuco belts. They consist of metapelites, metagrey-
wackes and associated bimodal volcanic rocks, which
were metamorphosed under variable conditions [26,27].
As for the CAFB, Neoproterozoic depositional ages (660 -
620 Ma) were determined for the metasediments in the
Borborema province [26,27]. Like in the AYD, the de-
tritus of the metasedimentary sequences in the Bor-
borema province was derived from Archean to Neopro-
terozoic sources. Zircon ages up to 3275 Ma have been
reported for a quartzite sample from the Cachoeirinha
belt [28]. Similarly, a metasedimentary complex in the
AltoMoxotó belt yielded Sm-Nd ages varying from 2.0
to 3.0 Ga [29]. In the East Pernambuco belt, U-Pb data
for detrital zircons from paragneiss exhibit ages ranging
from 3320 to 665 Ma [30].
7. Conclusions
Based on the present findings and relevant previous
studies [3,14,16], the tectonic and geochronologic evolu-
tion of the AYD in the Méiganga area can be summa-
rized as follows: 1) 2.1 - 1.8 Ga – formation of Paleo-
proterozoic juvenile crust, 2) 889 - 675 Ma – generation
and emplacement of early Neoproterozoic granitic melts,
3) > 619 Ma – erosion, transportation and deposition of
the detritus of the former magmatic rocks accompanied
by episodes of volcanism, 4) 619 - 614 Ma – syntectonic
magmatism and metamorphism of the sedimentary se-
quence, 5) 601 - 558 Ma – intrusion of late to post-tec-
tonic granites.
The evolution summarized above is closely linked to
that of the Borborema province, NE Brazil [29]. Fur-
thermore, the existence of Archean zircon inheritances in
both Central Cameroon and NE Brazil [17,29,31-35]
suggests that these two regions share a similar geological
history since the Paleoproterozoic.
8. Acknowledgements
The authors thank Dr Hartmut Schulz (University of
Tübingen) for CL images. The first author is highly in-
debted to the German Academic Exchange Service (DA-
AD) for support of his research stay at University of
Tübingen (Germany). Thanks go to two anonymous re-
viewers for their critical reviews of the manuscript.
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