Petrological and Geochemical Characteristics of the Cretaceous Ngaou Boh Anorogenic Complex (Adamawa Plateau, Cameroon Line): Preliminary Constraints

Abstract

The Cretaceous Ngaou Boh anorogenic complex (NBAC) located in the far North Adamawa Plateau, the centre domain of the Cameroon Line constitutes a plutonic-volcanic ring association. The whole rock K-Ar datation yields a crystallization age of ca. 74 Ma. Plutonic rocks comprise abundant alkali feldspar granites, scarce clinopyroxene-amphibole gabbros and alkali feldspar syenites. Alkali feldspar granites are leucocratic, coarse to fine-grained; quartz and K-feldspars are the major rock-forming mineral, besides minor oligoclase, biotite and accessory phases as sphene, zircon and opaques. Alkali feldspar syenites are mesocratic coarse-grained, mainly composed of K-feldspars with small amounts of quartz and biotite. Volcanic rocks consist of a basanite-trachyte-rhyolite suite. Basanites contain olivine and diopside phenocrysts and a groundmass essentially composed of plagioclase and titanomagnetite. Biotite-clinopyroxene trachytes and clinopyroxene-amphibole rhyolites have an almost homogeneous modal composition, mainly made up of sanidine and anorthoclase microliths, scarce phenocrysts of quartz, and minor crystals of biotite, clinopyroxene (augite) amphibole (pargasite, sandagaite); Fe-Ti oxides (ilmenite, titanomagnetite) and fibreglass are often isolated in the groundmass. Plutonic rocks are alkaline, weakly metaluminous with some alkali feldspar granites displaying agpaitic or peralkaline feature. Incompatible Trace elements (HFSE and LILE) distribution and chondrite-normalized REE patterns evidence a significant petrogenetic link between clinopyroxene-amphibole gabbros, alkali feldspar syenites and alkali feldspar granites. All the analysed samples are enriched in incompatible elements, indicating melts from spinel and garnet-bearing mantle source close to OIB component. Indeed, the (Tb/Yb)N ratios of both basanites (2.3-2.5) and clinopyroxene-amphibole gabbros (1.4-1.9) suggest different parental magma sources. Alkali feldspar granites appear as residue of magma differentiation led by crystal fractionation of liquid derived from the partial melting of spinel peridotite mantle. Clinopyroxene-amphibole rhyolites and biotite-clinopyroxene trachytes (Mg#=0.0-15.4) derive through fractional crystallization from basanites (Mg#=64.3-60.1), the most primitive mafic parental melt. Both plutonic rocks and lavas trends evidence a bimodality highlighted by a pronounced ‘’Daly gap’’.

Full text

Journal of Geosciences and Geomatics, 2021, Vol. 9, No. 4, 160-176
Available online at http://pubs.sciepub.com/jgg/9/4/1
Published by Science and Education Publishing
DOI:10.12691/jgg-9-4-1
Petrological and Geochemical Characteristics of the
Cretaceous Ngaou Boh Anorogenic Complex (Adamawa
Plateau, Cameroon Line): Preliminary Constraints
Zénon Itiga1,*, Benoît Joseph Mbassa2, Rose Noël Ngo Belnoun3,
Pierre Wotchoko4, Dieudonné Tchokona Seuwui5, Sébastien Owona1,
Jacques-Marie Bardintzeff6, Pierre Wandji5, Hervé Bellon7
1Department of Earth Sciences, University of Douala, P.O. Box 24157, Douala, Cameroon
2Institute for Geological and Mining Research, Laboratory of Ore mineral Processing, P.O. Box 4110, Yaoundé, Cameroon
3Department of Earth Sciences, University of Yaoundé, P.O. Box 812, Yaoundé, Cameroon
4Higher Teacher Training College, University of Bamenda I, P.O. Box 39, Yaoundé, Cameroon
5Higher Teacher Training College, University of Yaoundé I, P.O. Box 47, Yaoundé, Cameroon
6Université Paris-Saclay, Sciences de la Terre, Volcanologie, Planétologie, UMR CNRS,
8148 GEOPS, bât 504, F-91405, Orsay, France
7Université européenne de Bretagne, CNRS UMR, 6538 Domaines océaniques, UBO-IUEM,
6 avenue Le Gorgeu, CS 93837, F-29238, Brest cedex 3, France
*Corresponding author:
Received July 23, 2021; Revised August 29, 2021; Accepted September 07, 2021
Abstract The Cretaceous Ngaou Boh anorogenic complex (NBAC) located in the far North Adamawa Plateau,
the centre domain of the Cameroon Line constitutes a plutonic-volcanic ring association. The whole rock K-Ar
datation yields a crystallization age of ca. 74 Ma. Plutonic rocks comprise abundant alkali feldspar granites, scarce
clinopyroxene-amphibole gabbros and alkali feldspar syenites. Alkali feldspar granites are leucocratic, coarse to
fine-grained; quartz and K-feldspars are the major rock-forming mineral, besides minor oligoclase, biotite and
accessory phases as sphene, zircon and opaques. Alkali feldspar syenites are mesocratic coarse-grained, mainly
composed of K-feldspars with small amounts of quartz and biotite. Volcanic rocks consist of a basanite-trachyte-
rhyolite suite. Basanites contain olivine and diopside phenocrysts and a groundmass essentially composed of
plagioclase and titanomagnetite. Biotite-clinopyroxene trachytes and clinopyroxene-amphibole rhyolites have an
almost homogeneous modal composition, mainly made up of sanidine and anorthoclase microliths, scarce
phenocrysts of quartz, and minor crystals of biotite, clinopyroxene (augite) amphibole (pargasite, sandagaite); Fe-Ti
oxides (ilmenite, titanomagnetite) and fibreglass are often isolated in the groundmass. Plutonic rocks are alkaline,
weakly metaluminous with some alkali feldspar granites displaying agpaitic or peralkaline feature. Incompatible
Trace elements (HFSE and LILE) distribution and chondrite-normalized REE patterns evidence a significant
petrogenetic link between clinopyroxene-amphibole gabbros, alkali feldspar syenites and alkali feldspar granites. All
the analysed samples are enriched in incompatible elements, indicating melts from spinel and garnet-bearing mantle
source close to OIB component. Indeed, the (Tb/Yb)N ratios of both basanites (2.3-2.5) and clinopyroxene-
amphibole gabbros (1.4-1.9) suggest different parental magma sources. Alkali feldspar granites appear as residue of
magma differentiation led by crystal fractionation of liquid derived from the partial melting of spinel peridotite
mantle. Clinopyroxene-amphibole rhyolites and biotite-clinopyroxene trachytes (Mg#=0.0-15.4) derive through
fractional crystallization from basanites (Mg#=64.3-60.1), the most primitive mafic parental melt. Both plutonic
rocks and lavas trends evidence a bimodality highlighted by a pronounced “Daly gap”.
Keywords: Ngaou Boh, Cameroon Line, Adamawa Plateau, anorogenic complex, magma differentiation, K/Ar age
dating
Cite This Article: Zénon Itiga, Benoît Joseph Mbassa, Rose Noël Ngo Belnoun, Pierre Wotchoko,
Dieudonné Tchokona Seuwui, Sébastien Owona, Jacques-Marie Bardintzeff, Pierre Wandji, and Hervé Bellon,
“Petrological and Geochemical Characteristics of the Cretaceous Ngaou Boh Anorogenic Complex (Adamawa
Plateau, Cameroon Line): Preliminary Constraints.” Journal of Geosciences and Geomatics, vol. 9, no. 4 (2021):
160-176. doi: 10.12691/jgg-9-4-1.

161 Journal of Geosciences and Geomatics
1. Introduction
The Cameroon Line (CL) is the major magmatic and
tectonic feature in Central Africa underlined by several
volcanic bodies most of Cenozoic era following the
SSW-NNE direction. The CL extends from the Pagalú
Island in the Atlantic Ocean [1] passing through the
Cameroonian territory up to Tibesti in Chad, almost more
than 1500 km long and 100 km wide. The continental
sector of the CL consists of a succession of horsts, plains
of collapse or grabens and plutonic-volcanic complexes.
The horsts correspond to the major volcanic systems
(Mount Cameroon, Mounts Bamenda, Mounts Bambouto,
Mount Manengouba, a.s.o.) while the main plains of
collapse or grabens are Kumba, Tombel, and Noun [2,3,4].
More than sixty plutonic-volcanic complexes located in
the South (e.g. Ntumbaw and Nda Ali) and in the North
(e.g. Kokoumi, Tchégui, Mboutou and Golda Zuelva),
mainly in the central sector of the CL, notably in the Tikar
plain (Pandé, Sabongari and Nana massifs) and the
Adamawa Plateau (Mayo Darlé, Hosséré Nigo, Hosséré
Guenfalabo and Ngaou Boh) have been identified.
However, numerous anorogenic complexes have not yet
been studied such as the Ngaou Boh which is the focus of
this work. Several previous works from most of those
complexes [5-14] evidence discontinuous magmatic
sequences with more or less pronounced “Daly gap” i.e.
scarce or none intermediate rocks, except the Guenfalabo
massif which forms a continuous series. Anorogenic
complexes from CL are alkaline, SiO2-saturated or
SiO2-oversaturated, but some display peralkaline
constraints such as the presence of Na-pyroxenes and
Na-amphiboles (Pandé, Nda Ali, Kokoumi). Plutonic
rocks (granite, syenite, gabbro) constantly predominate on
volcanics. The Cenozoic ages of these complexes extend
from ca.69 Ma for Guenfalabo [15] to ca.38 Ma for
Mboutou and Golda Zuelva [6]. Amongst these ring
complexes only Mayo Darlé has a mineralizing potential
borne out by the presence of high tin concentration. In this
paper we report preliminary petrographical, mineralogical,
geochemical and geochronological features relevant to the
Ngaou Boh anorogenic complex and discuss sources and
magma differentiation.
2. Geology and Regional Background
The NBAC is located within the Gapi graben at the far
SSW from the Tchabal Gangdaba volcano [16,17] in the
vicinity of the Gapi Stock [18] upon the edge of the
Adamawa Plateau (Figure 1). Ngaou Boh is a volcanic-
plutonic ring complex (6 km of the longest diameter)
rising up to 1986 m above sea level, and characterized by
a collapse at its top as a flat-bottomed depression in a
shape of caldera. Such structure has been identified in the
Nda Ali plutonic-volcanic ring complex [9]. The basement
of the NBAC is composed of Pan-African anatexite and
migmatite [19].
Figure 1. (a) Geological sketch map of the NBAC; (b) location of NBAC in the CL and (c) location of Cameroon in Africa. Main volcanic bodies are in
ash-grey and some anorogenic plutons in black. C.A.R: Central African Republic

Journal of Geosciences and Geomatics 162
Field observations reveal a succession of plutonic and
volcanic rocks. The lithology of the NBAC includes from
the bottom to the summit of 1986 m: clinopyroxene-amphibole
gabbro, alkali feldspar syenite, biotite-clinopyroxene
trachyte, alkali feldspar granite and basanite. But, this
petrographic sequence is not regular everywhere because
of the absence or scarcity of some of the above-mentioned
rock types.
Plutonic rocks crop out either as chaotic metric
boulders in the shape of variable bowls or as little
intrusion shape recalling curved structures of ring dyke.
Clinopyroxene-amphibole gabbros and alkali feldspar
syenites constitute weathered and destabilized dykes,
scattered in blocks or bowls of several meters size.
Leucocratic alkali feldspar granites and ash-gray alkali
feldspar granites outcrop in the centre at altitude 1730 m
as blocks or domes. At a hundred meters from the
southeast inside edge, outcrops a ring dyke of basanite
surrounded by porphyroid alkali feldspar granite.
The lava flows occurring at altitude 1680 m have been
essentially fissural. Thus, towards the summit on the SE
flank, there is a thin basanitic outpouring, while radial
veins and rings of biotite-clinopyroxene trachyte and
clinopyroxene-amphibole rhyolite curve the outer edges of
the ESE and SW sides. This volcanism is likely the last
magmatic event during the emplacement of NBAC as
evidenced by the orientation of most volcanic veins (N100
for basanites, N35 and N105 for trachyte and rhyolite)
which are parallel to the fracturing of the whole massif.
Hosséré Nigo and Hosséré Guenfalabo, two other
anorogenic ring complexes in the vicinity of Ngaou Boh
are E-W oriented. They widely consist of plutonic silica-
under or oversaturated alkaline rocks. Hosséré Nigo (7°50
N, 12°39 E, 1750 m a.s.l.) is mainly made up of gabbro,
monzonite and syenite of 65.0 ± 0.8 Ma [14] whereas the
Guenfalabo complex (7°40 N, 12°25 E, 1650 m a.s.l.)
yields two complete suites of volcanic rocks (basanite-
rhyolite) and plutonic rocks (gabbro-granite) dated
ca.68.8±1.7 Ma [15].
3. Results
3.1. K-Ar Age Dating
Whole rock 40K/Ar40 dating were performed at the
Laboratoire de Géochronologie, Université européenne
de Bretagne, at Brest, France, following the analytical
procedure detailed by [20]. Constant values of [21] have
been used for age calculations. Uncertainties given at 1σ
have been calculated following [22].
The anorogenic complexes previously dated in CL
through Ar/Ar, K/Ar and Rb/Sr methods all yield Tertiary
ages (ca. 70-39 Ma) except the Mayo Darlé complex (ca.
73-54 Ma) [5].
Two ages dating using whole rock 40K/40Ar method
provided ca.74.21 ± 1.71 Ma and ca. 72.42 ± 1.67 Ma
with the mean of 73. 31 ± 1.71 Ma (Late Campanian)
indicating the age of crystallization of coarse-grained
alkali feldspar granite from the NBAC (Table 1). These
ages are older compared to those of the neighbouring
Hosséré Nigo (ca. 65.3 ± 0.8 Ma; [14] and Hosséré
Guenfalabo (ca. 68.8 ± 1.7 Ma [15] obtained using
respectively phlogopite 40Ar/39Ar and whole rock 40K/39Ar
dating. According to the aforementioned ages, the Ngaou
Boh should so far be considered as the oldest anorogenic
complex dated in Cameroon, since most ages available
range between ca. 60-70 Ma.
3.2. Petrography and Mineralogy
Micropobe analyses of the representative samples of
petrographic types were performed at the Université
Paris-Sorbonne, France, using a Cameca SX100
Microprobe (15 kV, 10 nA). Kα lines were used.
International geostandards were used: diopside for Si, Ca
and Mg, Fe2O3 for Fe, MnTiO3 for Ti and Mn, Cr2O3 for
Cr, albite for Na, orthoclase for K and Al. Counting times
were 10s for both peaks and background, with a 5-μm
defocused beam.
3.2.1. Volcanic Rocks
NBAC volcanic rocks are composed of a little mafic
unit (basanites) and mainly felsic unit (trachytes, rhyolites).
Basanites are melanocratic and aphanitic small blocks
issued from destabilized dyke. These rocks display
aphyric microlitic and fluid texture (Figure 2a).
Phenocrysts (11.5 vol. %; > 0.35 mm) concern
clinopyroxene whereas microphenocrysts (< 0.35 mm)
and microlites (< 0.035 mm) are composed of
clinopyroxene, olivine and plagioclase. Clinopyroxene
phenocrysts have a diopside composition (En35-41Wo46-
50Fs13-15) [23] (Table 2). Some sections sometimes cluster
together into rosettes or aggregates. Olivine (chrisotile)
microphenocrysts are frequently embayed with Fe-Ti
oxides. The ferrotitanium oxides generally form anhedral
titanomagnetite microphenocrysts (mol % usp: 0.61-0.58;
Ilm: 0.39-0.42 mol %) (Table 3) scattered within the
groundmass (88.5 vol. %). Traces of skeletal nepheline
microphenocrysts often occur. Plagioclase microlites
(labradorite-andesine, An54-39) (Table 4), the most
abundant mineral phase of the groundmass, are locally
oriented.
Biotite-clinopyroxene trachytes outcrop as decametric
whitish or ash-gray coloured blocks. They have a
porphyritic microlitic texture (Figure 2b and c).
Phenocryst phases (15.8 vol. %) are composed of rods,
chopsticks or subhedral anorthoclases. But the most
developed phenocrysts (0.9 × 0.9 mm) are always
anhedral. Biotite microphenocrysts (0.25 x 0.2 mm), most
often altered, occur as lamellas often including sanidine
microlites. Primary Fe-Ti oxides form granular
microphenocrysts sparse in the groundmass. This
groundmass (84.2 vol. %) is essentially made up of
sanidine microlites and slight reddish-brownish glassy
matrix. Within the groundmass, augite constitutes some
interstitial anhedral microcrystals slipped in between
sanidine microlites.
Clinopyroxene-amphibole rhyolites, gray-coloured,
occur at the foot of the Ngaou Boh SW slope as a dyke.
The porphyritic texture is marked by the irregular
appearance of whitish millimetric alkali feldspars
phenocrysts. This rhyolite is atypical due to the abundant
spherulitic and radiating alkaline feldspar rods. The
phenocrysts (16.5 vol. %) are mainly sanidine,
anorthoclase, quartz and amphibole (Figure 2d). Sanidine

163 Journal of Geosciences and Geomatics
is the most abundant mineral phase, present either as
elongated phenocrysts or medium sized laths.
Anorthoclase exceptionally constitutes thickset chopsticks
and prisms. Quartz euhedral phenocrysts are the most
developed mineral phase (up to 1.20 x 1.36 mm).
Amphibole phenocrysts generally constitute skeletal
crystals often replaced by chlorite in the core and oxide at
the border. The groundmass (79 vol. %) includes in
addition of sanidine and anorthoclase microlites, rods and
strips plagioclase altered into sericite, little amount of
augite, microlites, acicular or anhedral Fe-Ti oxides, and
quite abundant light brown glass (4.5 vol. %). The latter
also display sometimes elongated fibres commonly
arranged into spherulites.
Table 1. Whole rock 40K/40Ar age dating of coarse-grained alkali feldspar granite from NBAC
Sample
Analysis
Age (Ma)
36Ar Exp.(10-9 cm3)
40Ar* (%)
40Ar*/g(10-7cm3)
K
2
O (%)
I34
B6663-6
72.42 ± 1.67
2.231
94.4
110.800
4.65
I34 B6677-4 74.21 ± 1.71 1.042 90.5 113.600 4.65
Mean
73.31 ± 1.71
Figure 2. Photomicrographs of representative NBAC plutonic and volcanic rocks (a) Microlithic porphyritic texture in basanite HG5. The
microphenocrysts of olivine and augite are embedded in a groundmass of microlites of plagioclase, Fe-Ti oxide and glass. (b) Porphyritic microlitic
texture with a fluid tendency in a trachyte (sample TG20). In the center, sanidine and anorthoclase microlites encompass a quartz porphyrocrystal.
(c) Trachyte (sample I21) more or less altered. (d) Equigranular and porphyritic microlitic texture with corroded euhedral quartz phenocryst in rhyolite
I33. Microlites of sanidine and anorthoclase, and Fe-Ti oxide granules essentially compose the fined matrix. (e, f) Fine to coarse-grained equigranular
textures in alkali feldspar granites (I30, I35) showing sometimes altered alkaline feldspars. (g) Coarse-grained and intersertal texture in clinopyroxene-
amphibole gabbro TG23. Pseudomorphose of augite crystals into serpentine or uralite and plagioclase minerals in the process of being sericitized.
(h) Heterogranular coarse-grained texture in a syenite (I38). Alkaline feldspars are mainly perthitic and myrmekitic. Au: augite; Bt: biotite; Fk: alkaline
feldspar; Hb: hornblende; Ol: olivine; Ox: Fe-Ti oxide; Myr: myrmekite; Spr: spherulite; Qzt: quartz

Journal of Geosciences and Geomatics 164
3.2.2. Plutonic Rocks
The NBAC plutonic rocks are made up of leucocratic
granite, ash-gray amphibole ± biotite granite, syenite and
gabbro.
The NBAC leucocratic alkali feldspar granites are
whitish, heterogranular and coarse-grained rocks mainly
consist of alkali feldspar, quartz, plagioclase, green
hornblende, biotite and Fe-Ti oxides (Figure 2e). Alkali
feldspar (41 vol. %), the predominant mineral, rarely
exhibits the Carlsbad twin and consists of orthoclase
and microcline. Plagioclase appears as euhedral
megaphenocrysts (4-6 mm) and microphenocrysts (0.8 x
0.5 mm). The rims of phenocrysts are usually indented or
altered into sericite or epidote while the microphenocrysts
are fully saussuritized. Compositions range from
oligoclase (Or7-3Ab85-76An19-9) to andesine (Or4Ab70An26)
(Table 4). Quartz (25 vol. %) crystallized as interstitial
anhedral microcrystals. Biotite (3.33 vol. %) appears as
greenish or brownish platelets more or less chloritized or
oxidized. Biotite phenocrysts always enclose apatite and
zircon, whilst microphenocrysts are sometimes included
into alkali feldspar. The oxides (2.33 vol. %) are
secondary minerals pseudomorphous after hornblende and
biotite. They are chemically composed of titanomagnetite
(mol % Ilm: 0.97-0.93) (Table 3). Amphiboles
(1.66 vol. %) are green hornblende euhedral crystals
of various shapes and sizes (chopsticks, needles and
tabular) often altered into opaque or chlorite. Chemically,
they are sadanagaite (Figure 3; Table 5). The accessory
mineral phases (2.12 vol. %) are made up of calcite,
zircon and apatite, which are usually included either into
biotite crystals or in plagioclase and hornblende (zircon
only).
The ash-gray amphibole ± biotite granites are generally
exposed as a dome in the innermost part of the NBAC.
They locally constitute enclaves (≤ 30 cm size) within
leucocratic granite and syenite. Quartz (30 vol. %)
occupies the interstitial spaces, where it forms anhedral late
microcrysts. Scarce laths and large euhedral crystals of
plagioclase (15-20 vol. %) are clustered with biotite minerals.
Biotite lamellas (3.3 vol. %) are moderate chloritized or
oxidized. The phenocrysts locally enclose apatite and
zircon while the smallest crystals are included in alkali
feldspar (Or89-63Ab11-37). Green hornblende (1.66 vol. %)
constitutes euhedral microcrysts of various shapes (chopsticks,
needles and platelets) frequently twinned. The pseudomorphic
replacement of biotite by chlorite is common; additionally,
biotite usually encloses quartz and plagioclase crystals.
Opaque minerals (2.3 vol. %) are mostly secondary minerals,
resulting from the alteration of biotite and green hornblende.
Their chemical composition corresponds to ilmenite (mol
% Ilm: 0.93-0.97). Zircon occurs as elongated prisms or
granules generally included into biotite (Figure 2e).
The Ngaou Boh alkali feldspar syenite mainly outcrops
as metre-sized boulders in the outer flanks, eastern and
inner western sides. At the NW and SW flanks syenite occurs
as a “ridge”. In general, this rock type is little fresh. In some
places, the outcrops present tabular-shaped crystals of
feldspar of several centimetres (2.5 cm long and 1.5 cm
wide) (Figure 2h). Its coarse grained texture essentially
consists of porphyrocrysts of orthoclase (Or70 Ab30) (70
vol. %) and medium grains of perthites (Or51-45Ab48-53).
Rare subeuhedral amphibole phenocrysts (pargasite and
sadanagaite) (Table 5; Figure 3) are found in that rock.
The biotite flakes are surrounded by quartz and feldspar while
millimetric quartz crystals (<10 vol. %) are most scattered.
Figure 3. Classification diagram for the NBAC amphibole from [24]

165 Journal of Geosciences and Geomatics
Table 2. Representative electron microprobe analyses of clinopyroxene. Structural formulas calculated on the basis of 4 cations and 6 oxygen
anions
Sample
Basanite HG5
Gabbro TG24
Analysis
1
2
4
5
7
9
10
12
21
117
123
124
125
126
SiO2
45.05
49.26
44.17
45.03
44.65
45.02
43.59
44.90
50.56
50.54
52.55
52.30
52.02
49.84
TiO2
4.18
2.32
4.48
3.79
3.46
3.39
4.76
3.19
1.23
0.34
0.40
0.53
0.49
0.05
Al2O3
7.26
4.43
8.79
7.32
8.04
6.68
9.09
8.00
3.09
4.53
3.57
3.35
3.41
4.80
Cr2O3
0.06
0.00
0.17
0.01
0.23
0.01
0.01
0.24
0.18
0.12
0.11
0.08
0.19
0.03
FeO
8.12
8.33
8.24
8.24
8.71
7.57
8.23
8.25
7.56
18.27
11.87
12.89
13.08
19.95
MnO
0.17
0.20
0.04
0.14
0.12
0.12
0.12
0.08
0.16
0.47
0.27
0.20
0.17
0.50
NiO
0.02
0.05
0.04
0.02
0.05
0.01
0.03
0.00
0.01
0.00
0.03
0.05
0.00
0.03
MgO
12.17
13.30
11.70
12.15
11.77
12.03
11.18
11.87
14.17
10.72
14.77
14.40
14.21
10.51
CaO
22.18
22.18
22.38
22.33
22.72
23.16
22.05
22.37
22.32
10.20
12.52
12.03
12.00
10.09
Na2O
0.67
0.71
0.80
0.72
0.67
0.75
0.70
0.74
0.62
0.47
0.35
0.25
0.20
0.46
K2O
0.07
0.06
0.01
0.06
0.01
0.07
0.03
0.03
0.00
0.03
0.02
0.04
0.04
0.05
Total
99.96
100.83
100.82
99.81
100.41
98.82
99.80
99.66
99.91
95.69
96.46
96.11
95.81
96.30
Si
1.70
1.83
1.66
1.70
1.69
1.72
1.65
1.70
1.89
1.98
1.99
2.00
2.00
1.96
Al4+
0.30
0.17
0.34
0.30
0.31
0.28
0.35
0.30
0.11
0.02
0.01
0.00
0.00
0.04
Al6+
0.02
0.03
0.05
0.03
0.04
0.02
0.06
0.06
0.02
0.19
0.15
0.15
0.15
0.19
Cr
0.00
0.00
0.01
0.00
0.01
0.00
0.00
0.01
0.01
0.00
0.00
0.00
0.01
0.00
Ti
0.12
0.06
0.13
0.11
0.10
0.10
0.14
0.09
0.03
0.01
0.01
0.02
0.01
0.00
Fe
0.26
0.26
0.26
0.26
0.27
0.24
0.26
0.26
0.24
0.60
0.38
0.41
0.42
0.66
Mn
0.01
0.01
0.00
0.00
0.00
0.00
0.00
0.00
0.01
0.02
0.01
0.01
0.01
0.02
Ni
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Mg
0.69
0.74
0.65
0.69
0.66
0.69
0.63
0.67
0.79
0.63
0.84
0.82
0.81
0.62
Ca
0.90
0.88
0.90
0.91
0.92
0.95
0.89
0.91
0.89
0.43
0.51
0.49
0.49
0.43
Na
0.05
0.05
0.06
0.05
0.05
0.06
0.05
0.05
0.04
0.04
0.03
0.02
0.01
0.03
K
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Sum
4.04
4.03
4.05
4.05
4.06
4.06
4.04
4.05
4.03
3.92
3.93
3.92
3.92
3.94
XMg
0.73
0.74
0.72
0.72
0.71
0.74
0.71
0.72
0.77
0.51
0.69
0.67
0.66
0.48
SumOx
2.27
2.23
2.25
2.27
2.27
2.30
2.28
2.28
2.24
2.36
2.28
2.30
2.31
2.37
Altot
0.32
0.19
0.39
0.33
0.36
0.30
0.41
0.36
0.14
0.21
0.16
0.15
0.15
0.22
Number_Ox
6.00
6.00
6.00
6.00
6.00
6.00
6.00
6.00
6.00
6.00
6.00
6.00
6.00
6.00
Fs
14.21
14.07
14.31
14.28
14.99
13.10
14.78
14.00
12.55
36.82
22.28
24.15
24.56
39.25
Wo
48.64
46.85
49.59
48.78
49.40
50.46
50.00
50.00
46.45
25.67
29.43
28.45
28.50
24.80
En
37.15
39.08
36.10
36.94
35.61
36.45
35.22
36.00
41.00
37.51
48.29
47.40
46.94
35.95
Table 3. Representative electron microprobe analyses of Fe-Ti oxides. Structural formulas calculated on the basis of 3 cations and 4 oxygen
anions for ulvöspinel-magnetite solid solutions and on the basis of 2 cations and 3 oxygen anions for ilmenite-hematite solid solutions. Usp:
ulvöspinel; Mgt: magnetite; Ilm: ilmenite; Hem: hematite
Rock
Basanite
Granite
Sample
HG5
I30
I32
Type
Usp-Mgt
Usp-Mgt
Usp-Mgt
Usp-Mgt
Ilm-Hem
Ilm-Hem
Ilm-Hem
Ilm-Hem
Ilm-Hem
Ilm-Hem
Analysis
11
15
16
19
107
63
71
73
74
72
SiO2
0.04
0.14
0.06
0.05
0.09
0.04
0.16
0.13
0.07
17.49
TiO2
22.47
23.47
23.64
23.21
47.79
48.65
48.20
49.31
49.00
35.61
Al2O3
2.28
2.73
2.84
3.09
0.05
0.00
0.02
0.02
0.03
7.02
Cr2O3
0.41
0.41
0.42
0.46
0.05
0.06
0.02
0.13
0.05
0.16
FeOt
75.32
71.19
73.24
73.17
46.26
42.82
43.92
42.65
43.39
31.99
MnO
0.98
0.86
0.86
0.80
3.35
3.91
3.46
3.88
4.36
2.67
MgO
1.94
3.36
3.45
3.76
0.03
0.04
0.06
0.00
0.00
0.03
CaO
0.10
0.15
0.12
0.16
0.06
0.07
0.04
0.27
0.01
0.17
Na2O
0.12
0.07
0.05
0.00
0.00
0.00
0.00
0.00
0.00
0.00
K2O
0.02
0.08
0.06
0.03
0.00
0.00
0.00
0.00
0.00
0.00
NiO
0.09
0.14
0.05
0.09
0.00
0.00
0.00
0.00
0.00
0.01
Total
103.77
102.63
104.79
104.83
97.68
95.59
95.86
96.39
96.91
95.13
Si
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.43
Ti
0.58
0.61
0.60
0.59
0.92
0.96
0.95
0.97
0.96
0.65
Al
0.09
0.11
0.11
0.12
0.00
0.00
0.00
0.00
0.00
0.20
Fe+3
0.72
0.65
0.67
0.69
0.14
0.07
0.09
0.05
0.08
-0.37
Cr
0.01
0.01
0.01
0.01
0.00
0.00
0.00
0.00
0.00
0.00
Fe+2
1.45
1.41
1.40
1.37
0.85
0.87
0.87
0.88
0.86
1.02
Mn
0.03
0.03
0.02
0.02
0.07
0.09
0.08
0.09
0.10
0.06
Mg
0.10
0.17
0.17
0.19
0.00
0.00
0.00
0.00
0.00
0.00
Ca
0.00
0.01
0.00
0.01
0.00
0.00
0.00
0.01
0.00
0.00
Na
0.01
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Total
3.01
3.01
3.00
3.00
2.00
2.00
2.00
2.00
2.00
2.00
Fe2O3 wt. %
27.82
24.87
26.33
27.12
7.43
3.47
4.48
2.70
4.07
-19.98
FeO wt. %
50.28
48.82
49.55
48.77
39.57
39.70
39.88
40.22
39.73
49.97
Total
104.70
106.10
107.00
107.10
98.40
95.90
96.30
96.50
97.30
93.00
Number Ox onion
4
4
4
4
3
3
3
3
3
3
Usp mole %
0.58
0.60
0.59
0.61
Mgt mole%
0.42
0.40
0.41
0.39
Ilm mole %
0.93
0.96
0.96
0.97
0.96
0.95
Hem mole%
0.07
0.04
0.04
0.03
0.04
0.05

Journal of Geosciences and Geomatics 166
Table 4. Representative electron microprobe analyses of feldspars. Structural formulas calculated on the basis of 5 cations and 8 oxygen anions
Rock type
Basanite
Gabbro
Granite
Sample
HG5
TG24
I30
I32
I34
Analysis
3
6
13
14
118
119
120
122
114
117
121
123
124
125
81
83
109
110
114
SiO2
53.26
55.16
53.05
55.06
51.21
51.40
51.22
50.23
64.46
63.51
64.19
61.4
63.18
65.82
64.23
65.27
66.38
65.83
68.26
Al2O3
28.23
25.99
29.47
28.30
30.95
30.93
31.16
31.38
21.13
22.04
20.89
23.75
22.65
20.52
18.34
18.69
16.92
18.67
16.58
FeOt 0.36 0.67 0.69 0.64 0.16 0.17 0.18 0.28 0.04 0.06 0.16 0.34 0.07 0.09 0.19 0.12 0.69 0.42 0.86
CaO
10.85
8.40
11.27
9.52
13.20
13.29
13.35
13.86
2.68
3.55
2.01
5.39
3.81
1.80
0.05
0.15
0.00
0.00
0.07
Na2O
5.20
5.39
5.11
6.09
3.67
3.67
3.72
3.20
9.93
8.96
9.88
8.16
8.40
9.77
1.21
4.11
3.01
6.53
4.91
K2O 0.49 2.81 0.39 0.46 0.10 0.21 0.13 0.09 0.59 1.21 1.19 0.77 0.90 1.17 14.90
10.94 10.77 8.23 7.93
BaO
0.00
0.23
0.34
0.14
0.00
0.00
0.00
0.00
0.38
0.45
0.05
0.90
0.00
0.12
0
0
0.06
0.00
0.09
Total
98.40
98.66
100.30
100.21
99.29
99.74
99.76
99.02
99.21
99.78
98.37
100.71
99.01
99.29
98.94
99.30
97.82
99.68
98.70
A 2.75 2.71 2.70 2.69 2.75 2.74 2.74 2.77 2.68 2.68 2.70 2.67 2.68 2.67 2.79 2.74 2.82 2.70 2.77
Si
2.44
2.49
2.39
2.46
2.34
2.34
2.33
2.31
2.87
2.83
2.88
2.73
2.82
2.92
2.99
2.98
3.12
2.95
3.15
Al
1.52
1.38
1.56
1.49
1.67
1.66
1.67
1.70
1.11
1.16
1.11
1.25
1.19
1.07
1.01
1.01
0.94
0.99
0.90
Fe
0.01
0.03
0.03
0.02
0.01
0.01
0.01
0.01
0.00
0.00
0.01
0.01
0.00
0.00
0.01
0.00
0.03
0.02
0.03
Ca
0.53
0.41
0.54
0.46
0.65
0.65
0.65
0.68
0.13
0.17
0.10
0.26
0.18
0.09
0.00
0.01
0.00
0.00
0.00
Na 0.46 0.47 0.45 0.53 0.33 0.32 0.33 0.29 0.86 0.77 0.86 0.70 0.73 0.84 0.11 0.36 0.27 0.57 0.44
K
0.03
0.16
0.02
0.03
0.01
0.01
0.01
0.01
0.03
0.07
0.07
0.04
0.05
0.07
0.88
0.64
0.64
0.47
0.47
Ba
0.00
0.00
0.01
0.00
0.00
0.00
0.00
0.00
0.01
0.01
0.00
0.02
-
0.00
0.00
0.00
0.00
0.00
0.00
Total 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.02 5.01 5.03 5.01 4.97 5.00 5.00 5.00 5.00 5.00 5.00
Or
2.82
15.56
2.22
2.59
0.59
1.22
0.75
0.56
3.29
6.79
6.65
4.35
5.33
6.67
88.76
63.21
70.18
45.33
51.33
Ab
45.15
45.37
44.06
52.28
33.30
32.90
33.27
29.31
84.16
76.47
83.92
70.1
75.7
84.71
10.97
36.06
29.82
54.67
48.27
An 52.03 39.07
53.72 45.13 66.11 65.88
65.99
70.13 12.55 16.74
9.43 25.57 18.97
8.62 0.27 0.73 0.00 0.00 0.40
Table 5. Representative electron microprobe analyses of amphibole. Structural formulas calculated on the basis of 13 cations eCNK (except Ca,
Na and K) and 23 oxygen anions
Rock type
Granite
Sample I30
I32
I37
Analysis 105 112 120 66 67 69 70 75 76 77 79 84 111 115
SiO2
42.71
42.53
42.92
45.09
43.46
43.77
44.18
43.66
44.06
44.39
43.37
44.40
46.90
46.79
TiO2 2.13 1.98 1.83 1.58 1.98 1.71 1.75 2.14 1.89 1.63 2.15 1.52 1.42 1.49
Al2O3
7.28
6.82
6.95
5.79
6.74
6.77
6.42
6.84
6.64
6.09
6.95
5.78
1.22
1.40
Fe
2
O
3
10.20
10.02
9.60
8.56
7.80
7.40
7.01
7.09
7.91
8.27
7.35
9.02
4.18
5.08
FeO
16.41
16.28
17.16
17.41
17.40
18.02
17.62
18.25
17.61
17.16
18.08
17.55
30.46
30.11
MnO
0.85
0.75
0.70
0.42
0.51
0.57
0.46
0.57
0.53
0.61
0.50
0.55
1.44
1.67
MgO 6.96 7.01 6.68 7.36 7.31 7.02 7.31 6.86 7.01 7.29 6.96 6.90 0.32 0.34
CaO
9.81
9.59
9.91
9.66
9.95
9.97
9.63
9.79
9.55
9.73
9.71
9.44
4.30
4.62
Na2O
2.18
2.23
1.94
2.05
2.14
2.12
2.24
2.18
2.27
2.05
2.35
2.18
5.77
5.71
K2O 0.97 0.94 0.92 0.73 0.84 0.86 0.84 0.87 0.78 0.74 0.89 0.74 1.34 1.21
Total
99.50
98.15
98.61
98.63
98.18
98.29
97.68
98.35
98.24
97.94
98.30
98.06
97.30
98.42
Si
6.50
6.56
6.59
6.86
6.67
6.72
6.79
6.70
6.75
6.81
6.66
6.83
7.65
7.56
Al
IV
1.31 1.24 1.26 1.04 1.22 1.23 1.16 1.24 1.20 1.10 1.26 1.05 0.23 0.27
AlVI
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Ti
0.24
0.23
0.21
0.18
0.23
0.20
0.20
0.25
0.22
0.19
0.25
0.18
0.17
0.18
Fe
3+
1.17 1.16 1.11 0.98 0.90 0.85 0.81 0.82 0.91 0.96 0.85 1.04 0.51 0.62
Fe2+
2.90
2.10
2.21
2.23
2.23
2.31
2.27
2.34
2.26
2.20
2.32
2.26
4.16
4.07
Mn
0.11
0.10
0.10
0.05
0.07
0.07
0.06
0.07
0.07
0.08
0.06
0.07
0.20
0.23
Mg 1.58 1.61 1.53 1.67 1.67 1.61 1.68 1.57 1.60 1.70 1.59 1.58 0.08 0.08
CaB
1.60
1.58
1.63
1.58
1.64
1.64
1.59
1.61
1.57
1.60
1.60
1.55
0.75
0.80
NaB
0.40
0.42
0.37
0.42
0.36
0.36
0.41
0.39
0.43
0.40
0.40
0.45
1.25
1.20
KA 0.19 0.18 0.18 0.14 0.16 0.17 0.16 0.17 0.15 0.14 0.17 0.14 0.28 0.25
NaA
0.24
0.25
0.21
0.18
0.27
0.27
0.25
0.26
0.24
0.21
0.30
0.21
0.58
0.59
Total cation
15.43
15.44
15.39
15.32
15.44
15.44
15.42
15.43
15.40
15.35
15.47
15.35
15.86
15.84
X
Mg
0.43
0.43
0.41
0.43
0.43
0.41
0.43
0.40
0.42
0.43
0.41
0.41
0.02
0.02
Si+Na+K
6.93
6.99
6.98
7.18
7.11
7.16
7.21
7.13
7.14
7.16
7.14
7.18
8.51
8.40
Ca+Al
IV
2.91 2.82 2.89 2.61 2.85 2.86 2.75 2.85 2.77 2.70 2.86 2.60 0.99 1.07
The clinopyroxene-amphibole gabbro occurs as
boulders at the NE external flank at the height 1700 m. It
has coarse grained texture with a cumulative tendency
(Figure 2g). The paragenesis consists mainly of
plagioclase, commonly weathered clinopyroxene and
brown hornblende. Plagioclase (65.2 vol. %) forms
labradorite large phenocrysts (Or0-1Ab29-33An65-70)
interlocking with each other or enclosing the hornblende
crystals. The widest crystals reach 1.1 x 1.2 mm. They are
frequently zoned and display mechanic twinning and kink

167 Journal of Geosciences and Geomatics
bands. Green hornblende (26.8 vol. %) appears as both
euhedral phenocrysts (1.1 x 0.8 mm) and as interstitial
anhedral crystals, which often alter into chlorite or epidote.
Some of the crystals form inclusions in plagioclase.
Clinopyroxene (6.8 vol. %) subhedral phenocrysts
yielding the bluish or brownish colour are usually replaced
by either serpentine or rarely uralite. They are augite in
composition (En36-48Wo25-30Fs22-40) (Table 2) according to
the classification of [23].
4. Whole Rock Geochemistry
The analytical data were yielded at the CRPG, Nancy,
France, using ICP-OES (Inductively Coupled Plasma-
Optical Emission Spectrometry) for major elements and
ICP-MS (quadrupole Mass Spectrometry) for trace
elements. For whole rock analyses, samples have been
first pulverized in a steel crusher and then milled to a very
thin powder in an agate mill. About 300 mg of powder
was fused with LiBO2 and dissolved in HNO3.
4.1. Nomenclature and Classification
The bulk rock composition (major and trace elements)
and the CIPW normative composition of NBAC samples
given in Table 6 reveal: (1) the occurrence of normative
quartz (9.0-46.6 wt. %) in almost all felsic samples;
(2) the occurrence of normative corundum (2.9-8.6 wt. %)
both in all felsic volcanic rocks and in all granites (0.9-2.2
wt. %) and the lack of normative acmite in all the samples
except one (I37); (3) mafic volcanic and plutonic rocks are
marked by the occurrence of normative nepheline
(4.1-15.5 wt. %) and normative olivine (10.6-21.8 wt. %);
(4) the normative hypersthene in almost all felsic plutonic
rocks samples ≤ 4.5 wt.%.
In the modified total Alkali vs. Silica (TAS)
classification diagram of Le Maitre ([25]; Figure 4)
plutonic unit from Ngaou Boh is made up of
clinopyroxene-amphibole gabbro, alkali feldspars syenite
and alkali feldspars granite whereas the volcanic suite is
composed of basanite, trachyandesite, trachyte and
rhyolite. The two groups are characterized by a “Daly gap”
between mafic and felsic rocks, illustrating a lack of
intermediate terms. All the analyzed NBAC rocks plot
over the alkaline-subalkaline boundary [26] except two
samples: gabbro TG24 and rhyolite I35.
The agpaitic index [A.I = (Na2O + K2O)/Al2O3 mole %]
is mainly < 1 (0.19-0.97) except for two samples (granite
I34 and syenite I37= 1) which can be considered as
peralkaline. Basanites, clinopyroxene-amphibole gabbros
and granitic rocks are metaluminous (0.50≤ ASI ≤ 0.99)
while all felsic volcanic rocks are peraluminous (ASI ≥
1.03-1.32) ([27,28]; Figure 5). The ASI is Alumina
Saturation Index: molar % Al2O3/CaO + Na2O + K2O.
The NBAC gabbros and granitoids plot in the field of I-
type granite ([29]; Figure 5). However, granitoids display
characters of A-type granite and they plot in the domain of
ferroan rocks whereas gabbros are assigned to magnesian
rocks ([30]; Figure 6a). Moreover, in the MALI vs. silica
diagram ([30]; Figure 6b) clinopyroxene-amphibole
gabbros exhibit calcic affinity; syenites are strictly alkalic
while granites display mainly alkali-calcic and alkalic
features.
Table 6. Major elements (wt. %), trace elements, rare earth elements (ppm) and CIPW norm (wt. %) data for representative samples of the
NBAC. The samples concerned by this investigation are quite fresh; those with LOI (loss on ignition) > 3% have been excluded. Fe2Ot = Fe2O3
total. For CIPW norms calculation, the values of Fe2O3/FeO ratios are indicated
Rock type Basanite
Trachyandesite Trachyte Rhyolite Gabbro
Syenite
Granite
Sample
I31
HG5
HG16
I42
TG21
TG20
I33
TG23
TG24
I38
I37
I32
I30
I34
SiO2 42.65 42.73 42.21 61.66 69.55 70.14 75.46 48.77 50.15 60.99 61.23 66.38 66.91 68.23
TiO2 3.66 3.00 2.74 1.44 0.12 0.15 0.20 0.30 0.29 0.44 0.95 0.72 0.58 0.24
Al2O3 14.02 13.00 12.24 14.68 16.07 15.32 10.65 22.98 25.86 18.81 15.25 14.88 14.71 14.67
Fe2O3t 12.41 12.58 12.82 6.31 2.87 2.70 4.78 5.52 3.57 2.74 4.48 4.84 5.00 3.84
MnO
0.18
0.21
0.21
0.08
0.00
0.04
0.00
0.05
0.04
0.73
0.15
0.12
0.08
0.13
MgO
9.44
10.8
11.66
2.73
0.00
0.25
0.00
5.25
2.92
0.64
2.86
0.90
0.73
0.05
CaO
11.31
9.73
11.72
4.13
0.40
0.50
0.00
11.47
13.24
1.60
2.39
2.05
1.84
0.40
Na
2
O
3.24
3.42
3.59
3.35
3.66
4.78
3.49
4.10
2.75
5.83
5.01
4.47
4.39
5.55
K2O
1.73
1.72
1.15
3.90
4.98
4.92
4.25
0.98
0.36
6.06
6.50
4.19
4.65
5.01
P
2
O
5
0.68 1.01 0.85 0.46 0.03 0.03 0.00 0.02 0.06 0.18 0.27 0.26 0.21 0.04
LOI 0.67 0.87 0.15 0.87 2.00 0.95 1.10 1.52 1.02 1.98 0.32 1.03 0.78 0.63
Total 99.99 99.06 99.34 99.62 99.68 99.77 99.93 100.96 100.26 100.04 99.41 99.83 99.88 98.75
Fe2O3/FeO 0.15 0.15 0.15 0.40 0.40 0.40 0.40 0.35 0.35 0.35 0.40 0.40 0.40 0.40
CIPW norm wt %
Quartz
0
0
0
16.6
42.4
27.17
46.62
0
2.20
9.03
0
20.86
19.90
18.50
Plagioclase
24.41
26.25
15.44
40.33
10.51
33.43
16.42
68.44
80.35
38.48
40.47
39.78
39.97
42.70
Orthoclase
10.66
10.60
7.30
23.85
29.58
29.15
25.12
5.94
2.27
35.96
40.30
25.20
27.84
29.68
Corundum
0
0
0
0.41
8.61
3.48
2.86
0
0
4.74
0
2.19
1.35
0.93
Nepheline
11.59
9.49
15.48
8.75
0
0
0
4.11
0
0
0
0
0
0
Diopside 23.43 17.21 30.64 0 0 0 0 4.66 1.41 0 6.45 0.00 0 0
Hypersthene 0 0 0 0 1.76 2.24 2.83 0 9.00 4.37 4.22 4.47 4.31 2.50
Olivine 16.18 21.77 19.18 0 0 0 0 10.60 0 0 1.42 0 0 0
Acmite 0 0 0 0 0 0 0 0 0 0 1.69 0
0
Ilmenite 6.95 5.70 5.20 2.73 0.23 0.28 0.38 0.57 0.55 0.84 1.80 1.37 1.10 0.46

Journal of Geosciences and Geomatics 168
Rock type
Basanite
Trachyandesite
Trachyte
Rhyolite
Gabbro
Syenite
Granite
Sample I31 HG5 HG16 I42 TG21 TG20 I33 TG23 TG24 I38 I37 I32 I30 I34
Magnetite
2.70
2.74
2.78
3.65
1.67
1.57
2.77
2.80
1.81
1.39
1.75
2.81
2.90
2.23
Hematite
0
0
0
0
0
0
0
0
0
0
0
0
0
Apatite
1.58
2.34
1.97
1.07
0.07
0.07
0
0.05
0.14
0.42
0.63
0.6
0.49
0.09
Trace (ppm)
Cr 425.00 393.10 688.10 85.06 0 0 0 129.95 49.10 0 0.02 6.05 0 0
Co 45.90 48.61 52.98 16.73 0.47 0.33 0.47 39.12 17.40 0.41 12.00 5.93 4.36 0.27
Ni 144.00 336.3 252.00 39.47 0 0 0 54.52 18.95 0 48.00 0 0 0
V 270.00 193.4 244.10 106.50 0 0 0 74.00 57.39 7.76 96.00 26.73 13.20 0
Cu
44.30
68.03
71.10
21.22
0
4.01
0
5.75
21.65
0
0
0
0
0
Zn
109.00
124.5
119.80
114.90
91.80
86.79
237.00
55.38
30.85
147.60
214.80
130.40
121.00
214.80
Ga
20.40
21.43
18.68
26.15
38.10
39.04
43.4
18.45
19.90
29.27
23.70
28.32
31.00
43.20
Ge
1.40
1.30
1.51
1.578
1.62
2.27
2.48
1.00
1.53
1.78
2.00
1.61
1.78
2.19
As
0
1.30
0
0
0
0
0.84
0
0
7.65
0
0
0
0
Be 1.68 2.37 1.60 2.78 5.26 7.43 12.8 0 0 5.57 21.00 9.24 5.98 6.53
Cs 0.38 0.378 0.64 2.41 0.81 0.72 0.65 0.56 0 1.43 7.70 1.14 0.85 0.49
Rb 38.80 44.66 94.77 190.10 134.90 128.60 195.00 14.35 5.66 161.60 291.90 160.90 142.00 105.00
Sr 843.00 1069.00
951.10 545.30 72.77 63.40 6.80 769.89 710.10 101.50 981.90 197.00 165.00 13.69
Ba 546.10 533.8 626.70 1016.00 139.4 132.10 20.60 159.57 172.90 186.60 2324.00 550.90 411.00 110.8
Zr 277.00 348.8 253.80 401.40 550.00 525.20 2866.00 13.45 28.70 1029.00 650.90 514.60 539.00 1065.00
Hf
6.14
7.40
5.58
9.25
17.53
15.95
66.30
0.32
0.73
21.00
12.90
12.83
14.8
24.36
Ta
5.02
5.62
6.26
0.87
13.71
13.09
20.20
0.28
0.27
16.31
4.70
7.86
7.35
13.65
Nb
67.10
81.68
91.86
13.30
175.90
172.70
262.00
1.30
2.58
226.6
47.2
60.98
90.7
180.20
Y
29.20
34.66
26.76
15.35
39.33
61.03
92.40
3.34
3.99
41.72
20.28
72.81
52.8
76.93
Mo 2.34 3.63 3.27 1.42 4.26 1.37 0.71 0 0 21.51 6.67 2.89 4.79 22.67
Cd 0 0.37 0.34 0.28 0 0.00 0.82 0 0 1.02 0.50 0.49 0.52 0.83
In 0.10 0.11 0.11 0.00 0.17 0.15 0.24 0 0 0.00 0.19 0.12 0.12 0.24
Sn 2.00 2.51 1.65 3.55 9.38 8.79 17.50 0 0 2.91 4.00 4.79 7.03 7.33
Sb 0 0.11 0.12 0.24 0 0 0.24 0.35 0 0.61 0 0 0 0
La 45.60 59.2 68.56 97.29 100.1 321.20 27.3 3.20 5.86 159.80 86.8 55.58 89.60 88.99
Ce
92.80
120.80
128.70
188.30
147.20
262.80
53.00
6.35
9.05
258.80
198.50
113.50
180.00
262.30
Pr
11.80
14.61
14.50
19.60
27.40
77.47
6.50
0.81
1.20
24.59
14.12
13.94
20.80
24.74
Nd
48.60
59.07
54.85
66.12
92.38
262.10
23.60
3.43
5.53
73.70
50.19
54.20
76.10
89.69
Sm
9.92
11.70
9.90
9.39
17.55
47.43
6.14
0.81
1.16
10.24
12.60
11.87
15.10
18.89
Eu
3.28
3.71
3.30
2.05
1.35
3.38
0.36
0.72
0.79
2.31
1.63
2.29
2.35
1.66
Gd 8.75 9.83 8.04 5.94 10.77 28.60 9.60 0.76 0.96 7.66 10.51 11.45 11.90 14.94
Tb 1.13 1.37 1.08 0.73 1.9 4.26 2.34 0.10 0.16 1.18 0.91 1.91 1.90 2.54
Dy 5.89 7.105 5.63 3.36 10.27 20.44 18.90 0.69 0.84 6.81 11.12 11.51 10.70 15.15
Ho 1.07 1.22 0.95 0.52 1.69 2.92 4.41 0.11 0.19 1.30 2.50 2.24 1.92 2.88
Er
2.73
3.14
2.38
1.36
4.52
7.13
12.20
0.29
0.38
3.97
8.98
6.41
5.14
8.28
Tm
0.37
0.43
0.31
0.18
0.67
0.96
1.99
0.06
0.11
0.60
0.29
0.96
0.74
1.23
Yb
2.23
2.67
1.92
1.11
4.46
6.04
12.70
0.31
0.38
4.21
5.70
6.48
4.85
8.31
Lu
0.33
0.39
0.29
0.16
0.62
0.80
1.72
0.03
0.06
0.66
0.85
0.93
0.73
1.21
Bi
0
0
0
0.11
0
0
0.21
0.01
0
0
0
0
0
0
W 0.41 0.78 1.05 0.83 2.63 2.21 0.51 0.28 0.00 5.19 5.70 2.30 1.97 0.83
Pb 2.47 3.64 3.68 11.48 6.78 6.04 27.50 5.01 2.25 12.70 7.58 7.90 8.96 8.10
Th 4.35 6.27 7.97 35.74 25.33 23.78 35.20 0.35 0.57 23.55 55.20 15.72 17.70 16.72
U 1.22 1.80 1.98 2.85 5.84 4.57 9.72 1.05 0.10 5.99 4.85 4.94 3.84 5.77
∑REE 234.49 295.25 300.42 396.10 420.88 1045.53 180.76 17.67 26.67 555.84 404.70 293.26 421.83 540.81
Eu/Eu*
1.05
0.52
0.56
0.78
0.28
0.26
0.14
2.75
2.22
0.76
0.42
0.59
0.52
0.29
(La/Yb)
N
13.89
2.31
3.72
59.33
15.25
36.13
1.46
7.01
10.48
25.77
10.34
5.83
12.55
7.27
(Dy/Lu)N
1.81
81.25
87.21
2.10
1.66
2.56
1.10
2.30
1.40
1.46
1.24
1.25
1.31
1.03
Mg#
60.11
62.97
64.32
46.11
0
15.36
0.00
65.33
61.80
31.50
55.88
26.94
22.50
2.47
A/CNK 0.50 0.52 0.43 0.85 1.32 1.09 1.03 0.80 0.89 0.99 0.78 0.95 0.94 0.96
A/NK 1.95 1.74 1.71 1.50 1.41 1.16 1.03 2.94 5.26 1.17 1.00 1.25 1.20 1.00
A.I 0.51 0.57 0.58 0.67 0.71 0.86 0.97 0.34 0.19 0.85 1.00 0.80 0.83 1.00
Tsat zircon 607 629 555 822 929 900
541 601 873 878 957 846 937
Tsat apatite 757 842 788 1006 806 804
493 606 981 959 813 925 882

169 Journal of Geosciences and Geomatics
Figure 4. Total Alkali vs. Silica diagram ([25] modified) for the NBAC rocks and some anorogenic complexes from the CL for comparison: Nda Ali [9],
Hosséré Nigo [14], Hosséré Guenfalabo [15], Nana and Sabongari [10,11]
Figure 5. NBAC rocks in the molar % A/NK (Al2O3/Na2O+K2O) vs. molar % A/CNK (Al2O3/CaO+Na2O+K2O) diagram of [27]. Peraluminous and
metaluminous fields after [27]; fields of I-type and S-type granitoids after [29]. Mafic rocks are also plotted for comparison
Figure 6. Plutonic rocks from NBAC subdivided into: (a) ferroan (A-type granite) and magnesian types; (b) alkalic, alkali-calcic, calc-alcalic and calcic
in the MALI (CaO wt. %- (NaO2 wt. %+K2O wt. %)) against silica diagram [30]

Journal of Geosciences and Geomatics 170
4.2. Major Elements Characteristics
Basanites display low SiO2 (42.2-42.7wt. %) and high
Al2O3 (12.2-14.0 wt. %), Fe2O3t (12.4-12.8 wt. %) and
CaO (9.7-11.7wt. %). The K2O/Na2O ratio (~ 0.5)
indicates their sodic character, which is different with the
majority of the studied plutonic-volcanic massifs of the
CL, rather potassic [3,4]. Biotite-clinopyroxene trachytes
and clinopyroxene-amphibole rhyolites have high SiO2
(69.6-75.5 wt. %), Al2O3 (10.7-16.1wt. %), low Fe2O3t
(2.7-4.8 wt. %), CaO (0.0-0.5 wt. %), moderate Na2O
(3.5-4.8wt. %) and K2O (4.3-5.0 wt. %). K2O/Na2O ratios
range from 1.03-1.36 reflects its potassic character.
Gabbros have low to moderate SiO2 (48.8-50.2 %), Fe2O3t
(3.6-5.5 wt. %), MgO (2.9-5.3 wt. %), high Al2O3 (23.0-
25.9 wt. %) and CaO (11.5-13.2 wt. %) contents. In the
other hand, high but narrow variation in SiO2 (61.0-68.2
wt. %), Al2O3 (14.7-18.8 wt. %), Fe2O3t (2.7-5.0wt. %)
and low CaO (0.4-2.4 wt. %) characterize the NBAC
felsic plutonic rocks. The K2O/Na2O ratios range from 0.9
to 1.3. The molar Mg number [Mg#= (molar 100 x
Mg/(Mg+Fe2+)] up to 66 for all the samples decreases
with the increasing of SiO2.
The major elements variations diagrams (Figure 7)
display: i) linear positive trends for Na2O, K2O, negative
for CaO, MgO and Al2O3; and ii) a lack of correlation for
the other oxides Fe2O3, TiO2, P2O5 with the increasing of
SiO2.
Figure 7. Harker variation diagrams for some major elements
4.3. Trace Elements Characteristics
Compatible elements of clinopyroxene-amphibole gabbros
and mafic lavas such as Ni (19.0-336.3 ppm), Co (17.4-
53.0 ppm), Cr (49.1-688.1 ppm) and V (57.4-270.0 ppm)
have relative high contents. Conversely, granitoids and
felsic lavas contents of incompatible elements are higher
than those of compatible elements: Ba (159.6-2324.0
ppm), Zr (514.6-2866.0 ppm), Rb (134.9-291.9 ppm), Sr
(6.8-981.9 ppm) and Y (20.3-92.4 ppm) (Table 6).
Trace elements of all NBAC rocks as Ta, Nb, Rb
display clear chemical trends with Th, and there is a lack
of correlation between Sr, Zr, and La with Th (Figure 8).
Compatible transition elements (Ni and Cr) decrease with
the increasing of Th might reflect the fractionation of
olivine and clinopyroxene.

171 Journal of Geosciences and Geomatics
Figure 8. Harker variation diagrams for some trace elements
Figure 9. Chondrite normalized REE patterns of plutonic rocks (a) and volcanics (b) from NBAC (Chondrite values after [31]. ACC=Average
Continental Crust after [32]); OIB composition from [33]. Pyrolite normalized spidergrams of plutonic rocks (c) and volcanics (d) from NBAC (Pyrolite
values after [31]. ACC=Average Continental Crust [32]; OIB and N-MORB compositions from [33]

Journal of Geosciences and Geomatics 172
Chondrite-normalized patterns of NBAC alkali
feldspars granites and alkali feldspars syenites show a
pronounced enrichment in light rare earth elements (LREE)
relative to heavy rare earth elements (HREE) while
clinopyroxene-amphibole gabbros are characterized by a
moderate enrichment in LREE relative to HREE
(Figure 9a). The overall (La/Yb)N ratio (5.83-25.77)
highlights the moderate to strong fractionation of
LREE/HREE whereas (DyN/Lu)N ratio (1.03-1.46)
suggests the lower MREE/HREE fractionation. The I34
sample has the highest LREE enrichment (~500 times)
relative to chondrite (Figure 9a). The ∑REE of granites
vary very few (293.26-540.81ppm). All their REE patterns
have slight negative Eu anomalies (Figure 9a) evidenced
by Eu/Eu* < 1 (0.30-0.60). The REE patterns of the
Ngaou Boh gabbros (∑REE=17.67-26.67 ppm) are very
different from those of granites; they have moderate
LREE/HREE with (La/Yb)N ratios ranging from 7.01 to
10.48 and show extreme positive Eu anomalies (Eu/Eu* =
2.22-2.75) indicative of cumulus plagioclase. The
pyrolite normalized incompatible trace elements profiles
(Figure 9c) of plutonic rocks exhibit slight spikes in Rb,
Th, Zr and strong negative anomalies at K, P, Ti.
The chondrite normalized patterns of biotite-clinopyroxene
trachytes and clinopyroxene-amphibole rhyolites display
prominent negative Eu anomalies (Eu/Eu*=0.14-0.28)
indicative of the significant fractionation of feldspars
(Figure 9b). The (La/Yb)N (1.46-36.13) and (Dy/Lu)N
(1.10-2.56) ratios respectively testify the low enrichment
of LREE/HREE and MREE/HREE. Chondrite-normalized
REE patterns of basanites (Figure 9b) display slight LREE
enrichment relative to HREE ((La/Yb)N= 2.31-13.89)
illustrated by an almost flat profile for heavy rare earth
elements. In other words, the shape of the patterns of
basanite describes a profile of moderate slope from LREE
to HREE, which reflects an enrichment of 90 to 200 times
compared to the chondrite. Rhyolites and trachytes are
characterized by high REEs contents, with ∑REE ranging
between 180.96 and 1045.53 ppm.
The pyrolite-normalized spidergrams of NBAC
trachytes display strong negative K, P and Ti anomalies
and moderate negative Ba and Sr (Figure 9; c and d). K,
Ba and Sr anomalies reflect significant fractionation of
alkali feldspars, while Ti anomaly is symptomatic of Fe-Ti
oxides fractionation.
5. Discussion and Concluding Remarks
5.1. Geodynamic Setting
The NBAC rocks have an affinity with Within-Plate
Granites (WPG) except clinopyroxene-amphibole gabbros
and biotite-clinopyroxene trachytes in the Rb vs. Y+Nb
geotectonic diagram [34] (Figure 10a). According to the
Ce-Y-Nb discriminating diagram of [35] all the studied
granitoids are of A-type granite [36] and plot both in the
A1 and A2-subtype fields (Figure 10b): A1 is known as
related to a post-collision environment or a typical
anorogenic environment with the possible presence of
"rapakivi-type granite". Moreover, a synthetic review of
the position of NBAC rocks in the (Nb/Zr)N vs. Zr
discriminating diagram ([37]; Figure 10c), and the Th/Hf
vs. Ta/Hf, Th/Ta vs. Yb and Th/Ta vs. Ta/Yb variation
diagrams of [38,39]; Figure 10d-f) leads to the conclusion
that an alkaline and typically anorogenic or post-
collisional magmatism characterizes the NBAC and
results in a relic signature of the subduction type. The
studied NBAC samples plot in four different fields: within
plate zone (WPG), the post-collision zone (PCG), the
within plate volcanic zone (WPVZ) and the active
continental margin (ACM) field.
5.2. Magma Sources
It is recognized that mafic magmas have two main
sources: the subcontinental lithosphere mantle (SCLM)
and the upper mantle. [42] characterized these two sources
by the La/Ta> 22 and La/Nb> 1.5 ratios for magmas from
the SCLM; La/Ta < 22 and La/Nb< 1.5 ratios for those
originated from the upper mantle. Therefore, the La/Ta
(8.8-10.9) and La/Nb (0.68-0.75) ratios indicate an
asthenospheric source for basanites parent magmas. The
same ratios La/Ta (10.4-21.7) and La/Nb (2.3-2.5), on the
other hand, evidence a hybrid source (both the upper
mantle and the SCLM) for gabbros parent magmas.
Basanites are characterized by moderate MgO (9.44-
11.66 wt. %) and compatible elements contents (Ni:
144.00-336.30 ppm, Cr: 393.10-688.10 ppm). Such values
are relatively sufficient to be regarded as indicators of
primary liquids directly resulting from the mantle melting
according to [43]. These basanites nevertheless represent
primary melt that has early undergone differentiation
process. The overall studied samples have OIBs features
(Figure 9); this is testified by their average La/Nb (0.71)
and Nb/Ta (14.2) ratios similar to the respective OIB
values which are 0.77 and 17.7 [34].
At least two distinct magmatic sources have controlled
the formation of the NBAC rocks and the two subsequent
series (Figure 11). The clinopyroxene-amphibole gabbro-
alkaline syenite (trachyte)-alkaline granite- suite
originates from the partial melting of upper mantle spinel
peridotite (with slightly higher (La/Yb)N ratios:7.0-36.1;
average~21.6). Indeed, mantle origin is ascribed in general
to metaluminous, alkaline or peralkaline granitoids.
Such rocks match with peralkaline and alkaline
granitoids (PAG) or within plate granite (WPG) and
A-type granitoids [31,45]. The NBAC plutonic rocks are
mainly metaluminous (A/CNK<1) with geochemical
features of I-type granites. Such igneous rocks have been
described as related to partial melting of a magmatic
protolith [46,47]. [36] established that A-type granites
derived from transitional or alkaline mafic mantle sources
while [48] argued mantle-derived intrusions inferred
partial melt of the lower crust for the petrogenesis of
A-type granites.
On the other hand, the basanite-trachyandesite-rhyolite
suite originates from the upper mantle garnet peridotite
(Figure 11) (with lower (La/Yb)N ratios: 1.46-13.83;
average~7.6). The relative narrow Nb/Ta (4.64-9.55;
average~ 7.1) and high La/Nb (0.57-1.86; average~ 1.22)
ratios of clinopyroxene-amphibole gabbros also confirm
that they derive from a mantle source distinctly different
from that of basanitic rocks, although that source
compositionally close to OIB component according the
pyrolite-normalized spidergrams (Figure 10). Moreover,

173 Journal of Geosciences and Geomatics
clinopyroxene-amphibole gabbros tend to be more
primitive in regard with lower content in alkalis (NaO2,
K2O) and higher contents in CaO than basanites (Table 6).
Overall, the mantle origin is commonly reported for
almost all the studied anorogenic complexes of the CL
([4,10] and references therein).
Figure 10. Geotectonic setting for felsic rocks and mafic rocks also plotted for comparison. (a) Rb vs. Y+Nb diagram [34] (b) Ce-Y-Nb discriminating
diagram after [35]: A1= field of Oceanic Island basalts, A2= field of Post orogenic rocks or Island Arc basalts; (c) (Nb/Zr)N vs. Zr diagram [37].
Normalizing values from [31]. A= magmatic rocks related to subduction zone (VAG); B= Granitoids associated to collision zone (PCG); C= Alkaline
rocks from intraplate zone (WPG); D= Syn-collisional granites. (d) Th/Hf vs. Ta/Hf diagram: different domains after [40,41], OA= Oceanic arcs,
ACM= Active continental margins, WPVZ= Within plate volcanic zone, WPB= Within plate basalts, MORB= Mid-ocean ridge basalts; (e) Th/Ta vs.
Yb diagram: discriminating lines after [38]; (f) Th/Ta vs. Ta/Yb diagram: line separating the two domains after [39]
Figure 11. (Tb/Yb)N vs. Th diagram distinguishing the melting of garnet peridotite from that of spinel peridotite [44]. Normalizing values from [32]

Journal of Geosciences and Geomatics 174
5.3. Magma Differentiation Processes
5.3.1. Fractional Crystallization/Crustal
Contamination?
The major and most trace elements behavior allows to
decipher the petrogenesis and magma processes of the
NBAC. The linear positive or negative correlations of
major elements in the Harker variation diagrams such as
Na2O, K2O, Fe2O3t, CaO and MgO vs. SiO2 wt %
(Figure 7) corroborates the fractionation of main mineral
phases during the differentiation process, and evidence
two magmatic series. Thereby, trachyandesites, biotite-
clinopyroxene trachyte and clinopyroxene-amphibole
rhyolites might likely related to basanites by the
fractionation of alkali feldspars with regard to their
pronounced negative Eu anomalies (Eu/Eu*= 0.14-0.80)
whereas alkali feldspar syenites and alkali feldspar
granites derived from mafic parental liquid, with
clinopyroxene-amphibole gabbro as primitive liquids,
by the fractionation of alkali feldspars, plagioclases,
ca-pyroxenes and amphiboles.
The LILE vs. HFSE (Rb, Pb, U, vs. Th) and HFSE vs.
HFSE (Nb, Zr vs. Th) diagrams underline a positive linear
correlation passing through the origin (Figure 8). Such
correlations are consistent with magma evolving by
fractional crystallization mechanism [49]. Whatever are
those prior analyses, the awaited Sr-Nd-Pb isotopic
compositions, undoubtedly must unravel these features.
The almost perfect parallelism between the chondrite-
REE normalized patterns of alkali feldspar syenites and
those of alkali feldspar granites (Figure 9a and Figure 9b)
correlated to the constant variation of the trace elements
ratios, for instance (La/Yb)N (5.8-25.8), is also the result
of a magmatic differentiation marked by fractional
crystallization [50]. The magmatic affinity and the
diversity of the NBAC rocks, though, evidences several
mantle components and the involvement of differentiation
processes. Therefore, the parallelism of REE chondrite-
normalized patterns of mafic and felsic lavas testifies their
co-genetic link despite the apparent “Daly gap”, except
the rhyolite I35 which could be either the result of partial
melt of a crustal component or of crustal contamination of
mafic magma during its ascension [16].
However, the lack of significant depletion in Nb, Ta, P
and Ti combined to the faintly enrichment in Ba and Th
suggest that mantle-derived parent magmas were not
strongly modified by crustal materials (Figure 9c and
Figure 9d). No constraint of magma mixing characteristics
has been found (zoned crystals or enclaves). Consequently,
fractional crystallization is the main differentiation
process responsible of the NBAC magmatic evolution in
spite the apparent “Daly gap” [17,51].
5.3.2. Thermobarometry
The determination of P and T crystallization conditions
was achieved using zircon and apatite saturation
thermometers of [52] and [53]. The estimated temperature
and pressure are presented in Table 6. We consider that
zircon is not liquidus phase for basanites contrary to
apatite which might crystallize from the liquidus or at
temperature below. On the other hand, assuming that
clinopyroxene-amphibole gabbros are more or less
cumulative phases, their zircon and apatite saturation
temperatures are not significant. Trachytes with higher
zircon saturation temperatures (930-900°C) than those of
apatite (806-804°C) begin to crystallize as liquidus,
indicating that zircon saturation has been achieved. The
granitoids have a wide range of zircon saturation
temperatures (957-846°C) that are lower or higher than
those of apatite (981-925°C), showing that they are either
liquidus phases or have crystallized slightly below
liquidus. The ilmenite-magnetite and ilmenite-hematite
geothermobarometers [54,55] indicate the following
temperatures ranges: 734-770°C for basanite and
598-925°C for alkali feldspar granite. The same
geothermobarometer allows estimating oxygen fugacities
ranging from -16 to -17 (basanite) and -22 to -12 (alkali
feldspar granite).
Finally expected isotopic, petrological and
geochronological (notably U-Pb) data will undoubtedly
provide more precision in the knowledge of the detailed
NBAC petrogenetic model.
Acknowledgments
This paper is dedicated to late Junior Désiré Nolla who
passed in glory in July 2017 for his encouragements and
friendly support during the early moments of this work.
The authors are grateful to the Ministère des Affaires
Étrangères of France for the financial support of one of
the mission works on NBAC and for providing a
scholarship to I.Z. for his PhD study at the Université
Paris-Sud Orsay (now Paris-Saclay).
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