Abolo et al. STD2014Vol.15.Full

Transcription

Sciences, Technologies et Développement, Volume 15, pp32-42, Février 2014
Sciences, Technologies
& Développement, ISSN 1029 - 2225
http://www.univ-douala.com/sdt/
E-mail : [email protected]
ISSN 1029 - 2225
Petrography and mineralogy of the Nlonako anorogenic complex rocks, Central
Africa : petrogenetic implications
Martin Guy Aboloa, Daniel Lamilenb, Ismaïla Ngounounoc, Dieudonné Bitomd
a Société Nationale des Hydrocarbures, BP 955 Yaoundé Cameroun,
Département des Sciences de la Terre, Faculté des Sciences, Université de Yaoundé 1, BP 812 Yaoundé, Cameroun
cEcole de Géologie et d’Exploitation Minière de l’Université de Ngaoundéré à Meiganga, BP 454 Meiganga, Cameroun
d Département des Sciences de la Terre, Faculté des Sciences, Université de Ngaoundéré, BP 454 Ngaoundéré, Cameroun
Received : May 2013
Revised: September 2013
Accepted: September 2013
Available online: Febuary 2014
b
Abstract
The Nlonako Anorogenic Complex (NAC), emplaced along the Cameroon Hot Line (CHL) is characterized by a wide range of plutonic rocks with
minor volcanic rocks. Plutonic rocks include alkali granites, syenites, microsyenites, monzonites, monzodiorites, micromonzodiorites, microdiorites,
microgabbros, anorthositic gabbros and gabbros. Volcanic rocks comprise mainly alkali rhyolites and mugearites.
In addition to textural variations, these rocks show different mineralogical compositions that could be related to the physical and chemical conditions
changes of the magma. Most importantly is the great amount of quartz in most of rocks suggesting the role of the crustal contamination during the
differentiation. Mineralogical studies reveal a labradorite + hortonolite + magnesian augite + magnesio-hornblende + hastingsitic magnesiohornblende + titaniferous biotite paragenesis in basic and intermediate rocks, and a perthitic feldspar + fayalite + ferroaugite + augite + aegirine
augite + ferrohedenbergite + aenigmatite + riebeckite + magnesian hastingsitic hornblende + ferro-edenitic hornblende paragenesis in acid rocks.
Mineralogical evolution of the main mineral phases (feldspars and clinopyroxenes) is progressive and marked by an increase of Fe2+, Mn, Fe3+, Na
and by a decrease of Mg, Al and Ca. This mineralogical evolution of the gabbro-diorite-monzodiorite-monzonite-syenite-granite alkali suite is
interpreted in terms of fractional crystallization mainly controlled by olivine, clinopyroxene and feldspar.
ISSN 1029–2225©2014 Sciences, Technologies et Développement
os
Keywords: Nlonako Anorogenic Complex, plutonic rocks, volcanic rocks, mineralogical evolution, fractional crystallization, Cameroon Hot Line,
Cameroon.
Résumé
Le complexe anorogénique du Nlonako (CAN), mis en place à la faveur de la Ligne Chaude du Cameroun se caractérise par une grande variété de
roches plutoniques largement majoritaires par rapport aux roches volcaniques. Les roches plutoniques comprennent les granites alcalins, les
syénites, les microsyénites, les monzonites, les monzodiorites, les micromonzodiorites, les microdiorites, les microgabbros, les gabbros
anorthositiques et les gabbros. Les roches volcaniques sont constituées essentiellement de rhyolites alcalines et de mugéarites.
En plus des variations texturales, ces roches montrent des variations minéralogiques vraisemblablement induites par des changements des
conditions physico-chimiques au cours du refroidissement du magma. La forte proportion du quartz dans la plupart des roches fait penser au rôle
de la contamination crustale durant la différenciation. L'étude minéralogique définit la paragénèse à labrador + hortonolite + augite magnésienne +
magnésio-hornblende + magnésio-hornblende hastingsitique + biotite titanifère dans les roches basiques et intermédiaires, et la paragénèse à
feldspath perthitique + fayalite + ferroaugite + augite + augite aegyrinique + ferrohédenbergite + aenigmatite + riébeckite + hornblende hastingsite
magnésienne + hornblende ferro-édenitique dans les roches acides. L’évolution minéralogique continue des principales phases minérales
(feldspaths et pyroxènes) est marquée par une augmentation de Fe2+, Mn, Fe3+, Na et par une diminution de Mg, Al et Ca. Cette évolution rappelle
celle d’une suite alcaline de type gabbro-diorite-monzodiorite-monzonite-syénite-granite qui est interprétée comme le résultat de la cristallisation
fractionnée essentiellement contrôlée par l’olivine, le pyroxène et le feldspath.
ISSN 1029–2225©2014 Sciences, Technologies et Développement
os
Mots clés : Complexe anorogénique du Nlonako, roches plutoniques, roches volcaniques, évolution minéralogique, cristallisation fractionnée,
Ligne Chaude du Cameroun, Cameroun.
1. Introduction
The Nlonako Massif is one of the over sixty anorogenic
complexes encountered along the Cameroon Hot Line (CHL).
It reaches an altitude of 1825 m and exhibits an elliptical form
that covers a surface area of about 71 km². Apart from a few
reconnaissance studies in the 80’s (Dumort, 1968; Tchoua,
1970, 1974; Lasserre, 1978; Cantagrel et al., 1978; Tempier
and Lasserre, 1980), no detailed study on petrography and
mineralogy has been published on the Nlonako Anorogenic
Complex (NAC) so far. 4This investigation is to present the
*Corresponding author. E-mail: [email protected]
new petrographic and mineralogical data obtained
respectively from polarizing microscope descriptions and
CAMEBAX electron microprobe analyses with the aim to
define the petrogenetic evolution of the Nlonako Massif.
2. Geological setting and previous investigations
The Nlonako Massif was emplaced along the CHL. The CHL
is an important volcano-tectonic structure which includes
volcanic massifs and anorogenic plutons trending N30°E
from Pagalu Island to Lake Chad (fig.1). Many hypotheses
have been proposed to explain its origin (Mascle, 1976;
Fitton and Dunlop, 1985; Lee et al., 1994; Burke, 2001;
Montigny et al., 2004 and Déruelle et al., 2007).
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Abolo et al., Sciences, Technologies et Développement (Février 2014), Volume 15, 32-42
Sciences, Technologies & Développement, ISSN 1029 - 2225
Previous investigations on the NAC remain fragmentary.
Dumort (1968) described the hill as a well rounded massif
made up of syenites and gabbros. According to that author,
peralkali syenites postdate the gabbroic emplacement.
Tchoua (1970) emphasized the predominance of syenites
over the other rocks. In addition, he suggested that the
Nlonako massif was differentiated from peralkali syenitic and
granitic magmas (Tchoua, 1974). Geochemical data have
confirmed the alkali nature of the magma (Tempier and
Lasserre, 1980). Radiogenic dating K/Ar and Rb/Sr methods
indicates an age between 45.0±1 and 35±1 Ma (Cantagrel et
al., 1978) and an age of 43.3±0.3 (Lasserre, 1978)
respectively.
3. Analytical Techniques and Methods
Prior to electron microprobe studies, polished thin sections
were described under the polarizing microscope. The
nomenclature of the rocks is defined from QAP diagram (fig.
2), on both modal and normative compositions (Streckeisen,
1976; Streckeisen, 1980; Müller, 1982).
Mineral analyses of rock samples were performed on a
CAMEBAX electron microprobe at “Bundesanstaalt für
Geowissenschaften und Rohstoffe” (B.G.R), Hanover,
Germany. Selections of these analyses are shown in tables
1-6 and are given in terms of oxides of the element (wt. %).
Iron content is indicated in form of FeO, which corresponds
to the value of total iron. Chemical analyses were performed
on olivines in quartz syenites, microgabbros and
micromonzodiorites. The pyroxene types were defined after
Morimoto (1988). Although the International Mineralogical
Association (IMA) classification of Leake (1997) does not
adequately address new discoveries of new compositional
types of amphibole, this classification has been used for the
amphibole nomenclature in this work instead of the recent
classification by Hawthorne et al. (2012) which takes into
account the crystal-chemical and petrological importance of
compositional variables such as Fe2+, Fe3+, Li, and wO2contents. Our choice is based on the fact that some elements
(Li, Pb, Be or O2-) which are taken into account by this recent
classification were not determined in this study.
Figure 1. Location of the Cameroon Hot Line, showing the main plutonic and volcanic massifs. The Nlonako Alkaline
Complex is underlined. (After Déruelle et al., 2007).
33
4. Petrography
The NAC is quite different from other anorogenic
complexes of the CHL in the sense that it includes a
greater variety of petrographic facies (fig. 3). In accordance
with the chemical character of the feldspars, the
abundance of dark minerals and the texture, the diagram in
figure 2 allows recognition of the following rocks types:
fayalite alkali granites; aegirine augite alkali granites;
aegirine augite, aenigmatite and riebeckite bearing alkali
granites; pyroxene bearing quartz syenites; fayalite and
pyroxene bearing quartz syenites; amphibole bearing
quartz syenites; amphibole quartz microsyenites; quartz
syenites; quartz monzonites; monzonites; quartz
monzodiorites; quartz micromonzodiorites; amphibole
bearing micromonzodiorites; quartz microdiorites;
microdiorites; microgabbros; anorthositic gabbros and
gabbros. Beside these plutonic rocks, volcanic rocks
comprise of aegirine and arfvedsonite alkali rhyolites and
mugearites.
Figure 2. Position of plutonic rocks from NAC in Q-A-P diagram, (a): modal compositions, (b): normative
compositions.
34
Figure 3. Simplified geological map of the Nlonako Alkaline Complex.
Syenites occupy 70% of the Nlonako hill. In general, they are
grey and heterogranular. Their texture is either porphyritic
microgranular or finely granular at the border and porphyritic
granular at the centre of the mountain (fig. 4). Quartz content
of the syenites ranges from 6 to 15%. Perthitic orthoclase is
largely dominant (60 vol. %) and often occurs as thickset or
elongated prisms with different sizes. The presence of the
plagioclase (An13-An32) depends of facies. Pyroxenes are
always present in highly variable amounts and compositions:
they
include
ferroaugite,
aegirine
augite
and
ferrohedenbergite. Amphiboles are represented in small
amounts with various compositions (arfvedsonite,
ferrowinchite, ferrorichterite, kataphorite and hornblende).
Other minerals include sporadic fayalite, astrophyllite,
aenigmatite, biotite and accessory minerals (apatite, zircon
and dark minerals). Relations between these minerals
suggest a crystallization order as follows: apatite-dark
minerals-colored minerals-feldspars-quartz-astrophyllite and
biotite.
Granites are homogeneous compared with the syenites. The
texture is coarse grained. They are free of plagioclases and
contain strongly perthitic alkali feldspars (fig. 5). Quartz
appears into two generations. One early generation with
variable size; crystals are enclosed within the colored
minerals. The microcrystalline generation is made of
interstitial crystals and corresponds to the last phase of
crystallization. Ferromagnesian minerals include fayalite,
aegirine augite, riebeckite and aenigmatite. Light grey or
yellow green monzonites are porphyraceous. Plagioclase
(An28-An41) and orthoclase remain the primary mineral phase
(> 30 vol. %). Other minerals include augite, hornblende,
fayalite, biotite, quartz, apatite and ilmenite. Secondary
minerals are calcite, epidote and uralite resulting from the
alteration of the rock. The relation between minerals
suggests that apatite was formed first and was followed
respectively by ilmenite, colored minerals, feldspars and
quartz.
35
OR
FA
CPX
0,36 mm
Figure 4. Granular texture in the quartz syenite.
CPX
OR
QZ
AE
0,36 mm
Figure 5. Coarse grained alkali granite displaying strongly perthitic
alkali feldspars.
PL
PL
QZ
0,36 mm
Diorites comprise several facies according to their texture
and the abundance of quartz and amphibole. Rocks are
mostly fine grained and often slightly deformed (fig. 6). The
volume of plagioclase varies from 28.5% to 65.4% while its
composition ranges from oligoclase to labradorite (An23-An57).
Polysynthetic twins coexist with Carlsbad and/or Pericline
twins. Rare orthoclase has crystallized in the same thickset
or elongate habit as plagioclase. Colored minerals are
present in variable amounts and comprise magnesian
hastingsite, magnesio-hornblende, augite, ferroaugite, biotite,
chlorite and olivine. Zircon, Fe-Ti oxides and apatite are the
accessory minerals. The crystallization sequence may have
started with apatite and was followed in order by Ti-Fe
oxides, zircon, colored minerals, feldspars and quartz. The
presence of orthopyroxene (ferrosilite and enstatite),
encountered respectively within quartz micromonzodiorite
and quartz diorite, is unusual for alkali series and need to be
discussed.
Gabbros are commonly grey or dark depending on the
proportions of ferromagnesian minerals. Their texture is
either microgranular, intergranular or granular (fig. 7). The
mineralogy is dominated by labradorite, bytownite, augite and
olivine. Olivine is partially transformed into iddingsite along
cracks. Other minerals comprise magnesio-hornblende,
hastingsitic magnesio-hornblende, tremolite, biotite,
kaersutite, chlorite, apatite and ilmenite. In granular gabbro,
plagioclase formed first and was followed in order by
ilmenite, clinopyroxene, biotite and secondary minerals. In
the microgabbro, the crystallization order is ilmenite-olivineplagioclase-clinopyroxene-biotite and amphibole.
Rare light grey rhyolite and dark mugearite cut the plutonic
rocks. Rhyolite displays a porphyritic microlithic or fluidal
texture where phenocrysts of anorthoclase and aegirine are
enclosed in a matrix composed of K-feldspars, quartz,
aegirine and arfvedsonite. The texture of mugearite is
microlitic or microgranular whereas constitutive minerals are
phenocrysts of plagioclase (An30) and clinopyroxene and
microcrysts of feldspars, quartz, pyroxene and opaques. In
both cases, opaques may have crystallized first and were
followed by clinopyroxene, amphibole, feldspars and quartz.
Figure 6. Slightly deformed quartz microdiorite.
5. Mineralogy
CPX
CPX
PL
CPX
0,36 mm
Figure 7. Microgranular texture in the gabbro.
5.1. Olivine
Olivine occurs in variable proportion in almost all the rock
units (up to 10%). The mineral is completely transformed into
iddingsite in granites, syenites and monzonites. In addition,
olivine is always linked to clinopyroxene. Olivine analyses
indicate manganese enrichment within the acid rocks
compared to basic rocks and very limited composition
variations within the same rock.
A compositional gap, probably due to limited number of
analyses, is observed between Fo55 and Fo7.6 (fig. 8). Olivine
in quartz syenites corresponds to fayalite (87.5-93% of
Fe2SiO4). It is typically rich in Mn (2.87%-4.6%). Calcium
36
content ranges between 0.25% and 0.38%. Tephroite and
larnite mean contents are respectively 4% and 0.5%. Olivine
is relatively rich in Mg and poor in Mn and Ca, and
corresponds either to hyalosiderite or hortonolite in
microgabbros and micromonzodiorites. Fayalite and fosterite
contents range between Fa35.9-44.1 and Fo55-63.4 respectively.
Table 1. Olivine compositions (wt% and atom per formula unit
“a.p.f.u.”).
Rock
Microgabbros Quartz
Micromonzodiorites
type
syenites
SiO2
36.09 36.43
29.08
29.07
34.47
TiO2
0.04
0.02
0.02
0.05
0.04
Al2O3
0.01
0.02
0.00
0.00
0.01
FeO
31.69 31.76
66.15
65.79
38.18
MnO
0.50
0.52
4.61
4.57
0.78
MgO
31.38 31.29
0.05
0.03
27.13
CaO
0.06
0.05
0.31
0.32
0.04
Traces
0.04
0.00
0.01
0.01
0.03
Sum
100.08
100.23
99.83
100.68
Trace elements (ppm)
Ni
280.00
0.00
40.00
40.00
200.00
Si
0.99
0.99
0.99
0.99
0.97
Ti
0.00
0.00
0.00
0.00
0.00
Al
0.00
0.00
0.00
0.00
0.00
Fe2+
0.73
0.72
1.88
1.87
0.90
Mn
0.01
0.01
0.13
0.13
0.02
Ni
0.00
0.00
0.00
0.00
0.00
Mg
1.28
1.27
0.00
0.00
1.14
Ca
0.00
0.00
0.01
0.01
0.00
Somme Y
2.02
2.01
2.02
2.02
2.06
Fosterite
63.39 63.30
0.11
0.07
55.33
Fayalite
35.91 36.04
92.78
92.81
43.68
Tephroite
0.58
0.59
6.55
6.53
0.91
Mg*
0.64
0.64
0.00
0.00
0.56
Fig. 8: Compositions of olivines of plutonic rocks in the Mg-Mn-Fe
diagram (cation %).
5.2. Pyroxenes
Clinopyroxenes constitute the most important mineral phase
after the feldspars. They are euhedral and primary. As typical
of alkaline intrusive complexes, clinopyroxene shows a wide
range of composition; several types are defined: Ca-Mg-Fe
pyroxenes in quartz syenites, micromonzodiorites,
microgabbros and gabbros, Ca-Na pyroxenes in alkali
granites and quartz syenites, and Na-pyroxenes in alkali
granites. Two evolutions are defined on the Di-He-Ac
diagram (fig. 9). The first one is continuous from
microgabbros to alkali granites and extends from calcic
augites to ferro-augites and ferro-hedenbergites. The second
evolution stretches along He-Ac join and is characterized by
a compositional gap between Di3 Hd40 Ac57 and Di3 Hd72
Ac25; also by Na enrichment at the last stages of the
differentiation. Chemical variation of pyroxenes during the
differentiation is accompanied by Mg decrease at the
expense of Fe2+ and Mn, then by Na enrichment in the last
differentiated types.
5.3. Amphiboles
Amphiboles are unequally distributed in the Nlonako rocks.
They are primary or they result from the pseudomorphosis of
pyroxenes. Yellow-green magnesian hastingsite and
magnesio-hastingsite occur in the micromonzodiorites.
Locally, these minerals form rims around calcic augites.
Ferro-edenitic hornblende and magnesian hastingsitic
hornblende are intensively corroded by felsic minerals and
occur in the quartz microsyenites.
Magnesio-hornblende and magnesio-hastingsitic hornblende
are secondary minerals encountered as subhexagonal
crystals as small amount in microgabbros. Small amount of
ferro-winchite, ferro-richterite, katophorite and arfvedsonite
occur as euhedral to subhedral and skeletal phenocryts in
quartz syenites. Alkali granites and some quartz syenites
display euhedral to subhedral riebeckites. These minerals
are also interstitial in felsic minerals and are frequently
associated with clinopyroxenes. Their green or brown color
depends on Ti enrichment. The general evolution of the
amphiboles during the differentiation is marked by Ti, Fe2+,
Fe3+, Na, Mn increase, and by Mg, AlIV, Ca simultaneous
decrease, whilst Si remains constant.
5.4. Micas
Micas are Ti-rich (3.7 to 6 wt% TiO2) and their chemical
compositions correspond to phlogopite in micromonzodiorites
and microdiorites and to biotite in micromonzodiorites,
microdiorites and gabbros. Despite the limited number of
analyses, a slight progressive evolution with the
differentiation is recognized.
Fig. 9: Compositional variations of clinopyroxenes from NAC in the
Di-He-Ac diagram (diopside-hedenbergite-acmite, % cations).
5.6. Feldspars
Feldspars represent more than 50% of the total volume of the
rocks except in the amphibole micromonzodiorites (35.5%).
They appear as euhedral to subhedral crystals with different
sizes.
37
Table 2: Clinopyroxene compositions (wt. % and “a.p.f.u.” on the basis of 6 oxygens)
Quartz
micromonzodiorites
SiO2
50.60
49.62
TiO2
0.53
0.38
Al2O3
1.45
0.54
Fe2O3
3.70
3.86
FeO
12.89
27.89
MnO
0.56
0.88
MgO
12.87
16.53
CaO
18.08
1.09
Na2O
0.30
0.00
K2O
0.00
0.00
Traces
0.03
0.00
Sum
100.10
100.80
Cr
0.00
30.00
Ni
200.00
0.00
Si
1.91
1.92
AlIV
0.06
0.02
CrIV
0.00
0.00
FeIV
0.02
0.05
Sum T
2.00
2.00
Ti
0.01
0.01
Al
0.00
0.00
Cr
0.00
0.00
3+
Fe
0.08
0.06
2+
Fe
0.41
0.90
Mn
0.02
0.03
Ni
0.00
0.00
Mg
0.48
0.95
Ca
0.00
0.05
Na
0.00
0.00
K
0.00
0.00
Sum C
1.00
2.00
Mg
0.25
Ca
0.73
Na
0.02
K
0.00
Sum B
1.00
Factor
2.27
2.33
Q
1.86
1.90
J
0.04
0.00
Fe+Mg
0.89
1.86
Fe/Fe+Mg
0.46
0.49
Rock type
Micromonzodiorite
52.25
0.42
1.52
1.66
8.35
0.36
14.30
21.17
0.32
0.00
0.05
100.40
0.00
380.00
1.94
0.06
0.00
0.00
2.00
0.01
0.01
0.00
0.05
0.26
0.01
0.00
0.66
Quartz
microdiorites
50.90
51.48
0.24
0.17
0.58
0.53
3.65
2.71
23.15
23.15
0.88
0.85
19.72
20.34
1.34
1.07
0.03
0.00
0.00
0.02
0.01
0.01
100.49 100.32
50.00
0.00
1.93
0.03
0.00
0.04
2.00
0.01
0.00
110.00
1.95
0.02
0.00
0.03
2.00
0.00
1.00
0.13
0.84
0.02
0.06
0.73
0.03
0.00
1.11
0.05
0.00
0.00
2.00
0.00
0.00
0.00
0.05
0.73
0.03
0.00
1.15
0.04
0.00
0.00
2.00
0.00
0.00
0.00
1.00
2.23
1.90
0.05
0.92
0.28
0.00
2.28
1.90
0.01
1.85
0.40
0.00
2.27
1.92
0.00
1.88
0.39
Figure 10. Compositions of feldspars (mol %) from NAC in the AbAn-Or diagram.
Alkali feldspar occurs in alkali granites and quartz syenites.
This mineral always displays Carlsbad twinning. In addition, it
is characterized by a great development of perthitic texture
allowing them to be considered as hyposolvus minerals
(Tuttle and Bowen, 1958; Parsons, 1978). Chemical analyses
define an orthoclase (Or93-96) in granites. In syenites, the
composition is heterogeneous and is organized along Or-Ab
Microdiorite
Microgabbros
Alkali granites
Quartz syenites
50.76
0.35
1.07
3.19
11.71
0.33
11.13
21.15
0.41
0.05
0.01
100.15
50.51
1.14
2.51
3.22
8.60
0.32
13.74
20.61
0.34
0.00
0.00
101.01
Gabbros
50.43
1.28
2.74
3.22
6.26
0.23
14.79
20.88
0.40
0.00
0.00
100.22
52.43
0.51
1.27
2.95
6.31
0.25
15.61
20.93
0.41
0.00
0.03
100.71
49.36
1.60
3.69
3.68
5.51
0.20
14.46
20.77
0.51
0.01
0.00
99.78
50.76
1.22
0.27
30.24
2.42
0.57
0.10
5.00
11.26
0.00
0.00
101.84
48.77
1.66
0.45
25.37
11.33
2.03
0.47
1.98
8.18
1.67
0.01
110.92
48.70
0.18
0.30
4.01
21.99
1.21
3.79
20.10
0.57
0.01
0.05
100.91
48.15
0.38
0.49
4.21
21.82
1.25
3.90
19.95
0.50
0.01
0.02
100.68
40.00
0.00
1.94
0.05
0.00
0.02
2.00
0.01
0.00
0.00
0.08
0.37
0.01
0.00
0.53
0.00
0.00
1.88
0.11
0.00
0.01
2.00
0.03
0.00
0.00
1.87
0.12
0.00
0.01
2.00
0.04
0.00
80.00
1.92
0.02
0.00
0.06
2.00
0.05
310.00
0.00
1.95
0.01
0.00
0.04
2.00
0.01
130.00
0.00
1.93
0.02
0.00
0.04
2.00
0.01
0.08
0.19
0.01
0.00
0.68
30.00
0.00
1.84
0.16
0.00
0.00
2.00
0.04
0.00
0.00
0.10
0.17
0.01
0.00
0.67
0.00
0.00
1.94
0.01
0.00
0.05
2.00
0.04
0.08
0.27
0.01
0.00
0.61
0.00
250.00
1.93
0.06
0.00
0.01
2.00
0.01
0.00
0.00
0.07
0.19
0.01
0.00
0.71
0.08
0.73
0.04
0.00
0.13
1.00
0.15
0.82
0.02
0.00
1.00
2.24
1.85
0.05
0.88
0.31
1.00
0.14
0.83
0.03
0.00
1.00
2.23
1.84
0.06
0.87
0.22
1.00
0.14
0.83
0.03
0.00
1.00
2.21
1.88
0.06
0.91
0.21
1.00
0.13
0.83
0.04
0.00
1.00
2.24
1.81
0.07
0.84
0.20
0.69
0.37
0.07
0.00
0.03
0.08
0.62
0.08
2.00
0.09
0.74
0.04
0.00
0.13
1.00
0.10
0.86
0.03
0.00
1.00
2.29
1.87
0.06
0.90
0.41
0.82
0.08
0.02
0.00
0.01
0.04
0.00
0.00
1.00
0.17
0.83
0.00
1.00
2.30
0.29
1.67
0.08
0.93
0.00
0.00
0.00
0.00
2.36
0.48
1.25
0.40
0.93
1.00
0.09
0.86
0.04
0.00
1.00
2.40
1.82
0.09
0.87
0.85
1.00
0.10
0.86
0.04
0.00
1.00
2.41
1.82
0.08
0.86
0.85
join but close to the albite end-member. A hiatus is observed
between Or35 and Or93 (fig. 10).
Apart from alkali granites, plagioclase exists in all rocks, with
a wide variation in composition from An12 to An75. Oligoclase
predominates in quartz microsyenites, quartz syenites and
microdiorites. Its composition varies slightly within the same
mineral (An26-An29) and the Or content remains high.
Oligoclase and andesine with calcic core (An53-An57) occur in
micromonzodiorites. The composition of plagioclase is quite
homogeneous (An54-An64) in gabbros. Labradorite, often with
bytownite core, occurs in microgabbros. Chemical evolution
of plagioclases is progressive and continuous from basic
rocks to acid rocks. The differentiation process is
characterized by an impoverishment of Ca at the enrichment
in Na, in other words, K-feldspar becomes more important
than plagioclase.
5.7. Aenigmatite
Aenigmatite is present as red prisms with variable
dimensions in alkali granites (7%) and in quartz syenites
(0.5%). Chemical analyses were carried on quartz syenites.
The structural formula indicates aenigmatite (s.s.)
composition of 65 to 78.3 mol. %, Fe-aenigmatite
composition of 19.8 to 29.6 mol. % and rhoenite composition
of 1.9 to 6.4 mol. %.
38
Table 4. Mica compositions (wt% and “a.p.f.u.” on the basis of 22 oxygens).
Table 3. Amphibole compositions (wt% and “a.p.f.u.” on the basis of 23 oxygens).
Rock
Micromonzodiorites
type
SiO2
41.75
TiO2
3.74
Al2O3
10.70
Fe2O3 4.33
FeO
10.61
MnO
0.27
MgO
12.38
CaO
11.25
Na2O
2.68
K2O
0.90
Traces 0.08
Sum
98.70
Cr
50.00
Ni
580.00
Si
6.16
AlIV
1.84
CrIV
0.00
FeIV
0.00
Sum T 8.00
Ti
0.41
AlIV
0.03
Cr
0.00
3+
Fe
0.48
2+
Fe
1.31
Mn
0.03
Ni
0.01
Mg
2.72
Sum C 5.00
Ca
1.78
Na
0.22
Sum B 2.00
Na
0.55
K
0.17
Sum A 0.72
Factor 8.87
CANA 2.00
NAB
0.22
NAKA 0.72
Si
6.16
AlIV
1.84
MGFE 0.68
42.34
3.26
10.43
4.42
10.24
0.21
12.70
11.15
2.68
0.96
0.03
98.42
190.00
0.00
6.25
1.75
0.00
0.00
8.00
0.36
0.06
0.00
0.49
1.26
0.03
0.00
2.79
5.00
1.76
0.24
2.00
0.53
0.18
0.71
8.87
2.00
0.24
0.71
6.25
1.75
0.69
Quartz
micromonzodiorites
44.46
44.52
1.62
1.59
6.86
7.18
10.20
9.39
10.04
11.26
0.38
0.32
11.58
11.19
10.76
10.96
1.50
1.55
1.15
1.12
0.04
0.08
98.58
99.15
Quartz
syenites
46.23 47.91
1.69
1.40
1.40
0.86
9.10
6.98
29.26 30.67
1.44
1.32
0.04
0.36
5.27
4.14
4.90
6.35
1.16
1.25
0.02
0.00
100.50 101.24
Quartz
microsyenites
42.04 42.33
1.01
1.01
7.18
7.07
8.42
8.68
18.50 17.50
1.79
1.96
6.43
6.69
10.62 10.47
2.46
2.34
1.28
1.21
0.05
0.00
99.78 99.26
0.00
330.00
6.61
1.20
0.00
0.19
8.00
0.18
90.00
50.00
7.35
0.26
0.00
0.38
8.00
0.20
130.00
280.00
6.49
1.31
0.00
0.20
8.00
0.12
0.95
1.25
0.05
0.01
2.57
5.00
1.71
0.29
2.00
0.14
0.22
0.36
8.93
2.00
0.29
0.36
6.61
1.20
0.67
80.00
500.00
6.60
1.25
0.00
0.14
8.00
0.18
0.91
1.40
0.04
0.01
2.47
5.00
1.74
0.26
2.00
0.19
0.21
0.40
8.91
2.00
0.26
0.40
6.60
1.25
0.64
0.70
3.89
0.19
0.00
0.01
5.00
0.90
1.10
2.00
0.41
0.23
0.64
9.55
2.00
1.10
0.64
7.35
0.26
0.00
0.00
0.00
7.55
0.16
0.00
0.29
8.00
0.17
0.53
4.04
0.18
0.00
0.08
5.00
0.70
1.30
2.00
0.64
0.25
0.89
9.46
2.00
1.30
0.89
7.55
0.16
0.02
0.78
2.39
0.23
0.00
1.48
5.00
1.76
0.24
2.00
0.49
0.25
0.74
9.28
2.00
0.24
0.74
6.49
1.31
0.38
0.00
0.00
6.54
1.29
0.00
0.18
8.00
0.12
0.83
2.26
0.26
0.00
1.54
5.00
1.73
0.27
2.00
0.43
0.24
0.67
9.28
2.00
0.27
0.67
6.54
1.29
0.41
Microgabbros Alkali granites
45.65
0.65
9.86
6.33
8.04
0.16
13.91
12.07
2.09
0.08
0.00
98.84
44.24
0.75
10.54
6.23
8.99
0.13
13.13
12.20
2.12
0.09
0.01
98.45
0.00
70.00 90.00
0.00
6.45 6.33
1.55 0.00
0.00
1.66
8.00 8.00
0.08 0.96
0.26
0.00
0.68 1.32
1.10 2.38
0.02 0.34
0.00
2.85 0.01
5.00 5.00
1.91 0.00
0.09 2.00
2.00 2.00
0.51 0.43
0.02 0.00
0.52 0.43
8.76 9.33
2.00 2.00
0.09 2.00
0.52 0.43
6.45 6.33
1.55 0.00
0.72 0.00
6.58
1.42
8.00
0.07
0.26
0.00
0.69
0.97
0.02
2.99
5.00
1.86
0.14
2.00
0.45
0.01
0.46
8.67
2.00
0.14
0.46
6.58
1.42
0.76
40.80
8.18
0.02
25.52
18.31
2.56
0.05
0.01
8.07
0.02
0.01
103.55
40.75
8.18
0.04
24.64
18.50
2.25
0.11
0.01
7.90
0.00
0.00
102.37
0.00
0.00
6.38
0.01
0.00
1.61
8.00
0.96
1.29
2.42
0.30
0.00
0.03
5.00
0.00
2.00
2.00
0.40
0.00
0.40
9.41
2.00
2.00
0.40
6.38
0.01
0.01
Rock
Quartz
type
microdiorites
SiO2
37.77
38.31
TiO2
4.94
4.72
Al2O3
12.38
12.31
FeO
14.63
14.46
MnO
0.08
0.03
MgO
15.80
16.03
CaO
0.02
0.00
Na2O
0.08
0.16
K2O
9.67
9.74
Traces
0.05
0.04
Sum
95.43
95.80
Cr
80.00
240.00
Ni
310.00 60.00
Si
5.60
5.65
Al
2.17
2.14
Cr
0.00
0.00
Mg
Fe3+
0.23
0.20
Sum
8.00
8.00
Al
Ti
0.55
0.52
Fe3+
0.13
0.15
2+
Fe
1.45
1.43
Mg
3.50
3.53
Mn
0.01
0.00
Ni
0.00
0.00
Sum
5.65
5.63
Ca
0.00
0.00
Ba
Na
0.02
0.05
K
1.83
1.83
Sum
1.86
1.88
Factor
8.92
8.86
35.44
4.76
13.31
23.65
0.11
10.47
0.02
0.15
9.02
0.02
96.95
36.20
4.92
12.92
22.60
0.04
10.86
0.00
0.21
9.17
0.02
96.94
Quartz
micromonzodiorites
39.31
37.34
4.28
4.28
11.93
11.90
20.37
20.60
0.12
0.05
11.72
12.19
0.00
0.00
0.17
0.20
9.18
9.26
0.04
0.01
94.13
95.83
0.00
130.00
5.38
2.38
0.00
20.00
110.00
5.46
2.30
0.00
150.00
180.00
5.60
2.17
0.00
0.24
8.00
0.24
8.00
0.54
0.36
2.40
2.37
0.01
0.00
5.69
0.00
0.04
1.75
1.79
9.12
Microdiorites
Micromonzodiorite Gabbros
36.56
4.85
14.43
14.85
0.04
15.36
0.02
0.52
8.86
0.01
95.50
36.31
6.13
13.50
17.64
0.09
12.79
0.00
0.18
9.46
0.01
96.11
35.84
5.83
13.48
17.66
0.12
12.50
0.00
0.36
9.24
0.10
95.14
70.00
0.00
5.65
2.12
0.00
0.00
80.00
5.41
2.52
0.00
50.00
0.00
5.48
2.40
0.00
0.12
0.00
820.00
5.47
2.43
0.00
0.10
0.22
8.00
0.23
8.00
0.07
8.00
0.56
0.34
2.28
2.44
0.01
0.00
5.63
0.00
0.50
0.30
2.10
2.70
0.02
0.00
5.62
0.00
0.49
0.29
2.08
2.75
0.01
0.00
5.62
0.00
0.54
0.29
1.47
3.39
0.01
0.00
5.70
0.00
0.06
1.77
1.83
9.07
0.05
1.81
1.86
9.27
0.06
1.79
1.85
9.09
0.15
1.67
1.82
8.89
8.00
0.00
0.70
0.00
2.23
2.76
0.01
0.00
5.69
0.00
0.00
0.05
1.82
1.87
9.07
8.00
0.00
0.67
0.00
2.25
2.74
0.02
0.01
5.69
0.00
0.00
0.11
1.80
1.91
9.17
39
Table 5: Feldspar compositions (wt% and “a.p.f.u.” on the basis of 8 oxygens)
Rock
Quartz
Quartz
Quartz
Quartz
type
microsyenites micromonzodiorites Micromonzodiorites microdiorites Microdiorites
Alkali granites syenites
Microgabbros Gabbros
SiO2
64.86
64.79 63.75 64.91 64.64 65.26
53.56
61.65
54.17
55.37 52.91 50.20 61.92 61.61 48.33 48.73
52.18 51.99
Al2O3
17.67
17.76 19.20 21.40 18.46 21.68
29.31
24.31
28.90
27.92 29.35 31.47 23.75 24.05 32.18 32.10
29.75 30.15
Fe2O3
0.84
0.87 0.23
0.17
0.16
0.29
0.35
0.27
0.22
0.44 0.50
0.49
0.42
0.22
0.43
0.36
0.36 0.28
CaO
0.47
2.59
0.00
2.76
11.79
5.86
11.01
10.48 12.33 14.54
5.10
5.07 15.43 15.04
12.78 12.99
Na2O
0.75
0.62 4.83
9.19
0.64
9.68
4.78
8.08
5.12
5.48 4.30
3.15
8.41
8.31
2.64
2.74
4.39 4.15
K2O
15.79
16.20 8.73
1.14 15.79
0.78
0.18
0.51
0.18
0.19 0.41
0.24
0.78
0.91
0.17
0.16
0.28 0.25
Sum
99.90 100.24 97.20 99.38 99.69 100.45
99.97
100.69
99.59
99.88 99.79 100.09 100.37 100.17 99.19 99.13
99.73 99.81
Si
3.00
3.00 2.96
2.88
2.86
2.87
2.42
2.72
2.45
2.50 2.41
2.29
2.75
2.74
2.23
2.25
2.38 2.37
Al
0.96
0.97 1.05
1.12
1.13
1.12
1.56
1.27
1.54
1.49 1.57
1.69
1.24
1.26
1.75
1.74
1.60 1.62
3+
Fe
0.03
0.03 0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01 0.02
0.02
0.01
0.01
0.01
0.01
0.01 0.01
Sum
4.00
4.00 4.02
4.01
4.00
4.00
4.00
4.00
4.01
4.00 4.00
4.00
4.00
4.00
4.00
4.00
3.99 3.99
Ca
0.02
0.12
0.13
0.13
0.57
0.28
0.53
0.51 0.60
0.71
0.24
0.24
0.76
0.74
0.62 0.63
Na
0.07
0.06 0.43
0.79
0.82
0.83
0.42
0.69
0.45
0.48 0.38
0.28
0.72
0.72
0.24
0.24
0.39 0.37
K
0.93
0.96 0.52
0.06
0.04
0.04
0.01
0.03
0.01
0.01 0.02
0.01
0.04
0.05
0.01
0.01
0.02 0.01
Sum
1.00
1.01 0.97
0.98
1.00
1.00
1.00
1.00
0.99
1.00 1.00
1.00
1.01
1.01
1.01
1.00
1.03 1.01
Factor 2.78
2.78 2.79
2.67
2.65
2.64
2.72
2.66
2.72
2.71 2.73
2.74
2.66
2.67
2.77
2.77
2.74 2.74
Or
93.31
94.50 53.05
6.58
4.33
4.37
1.03
2.87
1.03
1.09 2.35
1.39
4.37
5.13
0.99
0.93
1.57 1.41
Ab
6.69
5.50 44.57 80.84 82.64 82.63
41.86
69.33
45.23
48.07 37.79 27.80 71.63 70.96 23.41 24.56
37.76 36.09
An
0.00
0.00 2.38 12.58 13.03 13.00
57.12
27.80
53.74
50.84 59.85 70.81 24.00 23.90 75.60 74.51
60.68 62.50
Table 6: Ilmenite (a), magnetite (b) and aenigmatite (c) compositions (wt%)
Rock
Qz
Qz
type
microdiorite Microdiorite syenite Gabbro
TiO2
49.96
51.84
50.91
48.72
Al2O3
0.00
0.00
Fe2O3
4.95
1.07
3.16
6.56
FeO
42.26
45.47
42.90
42.74
MnO
1.02
1.01
2.85
0.90
MgO
0.91
0.05
0.00
0.08
Traces
0.06
0.04
0.04
0.05
Sum
99.15
99.49
99.86
99.04
Cr
250.00
0.00 250.00
20.00
Ni
50.00
0.00
0.00
0.00
Zn
100.00
360.00
0.00 380.00
2+
Fe
1.79
1.93
1.82
1.83
Mg
0.07
0.00
0.00
0.01
Mn
0.04
0.04
0.12
0.04
Zn
0.00
0.00
0.00
0.00
Ni
0.00
0.00
0.00
0.00
Fe3+
0.10
0.02
0.06
0.13
Sum A
2.00
2.00
2.00
2.00
Ti
1.90
1.98
1.94
1.87
Al
0.00
0.00
Cr
0.00
0.00
0.00
0.00
Fe3+
0.09
0.02
0.06
0.13
Sum B
2.00
2.00
2.00
2.00
Factor
3.05
3.05
3.04
3.07
a
Rock
Qz
Qz
Qz
type
micromonzodiorite microsyenite Gabbro
microdiorite
TiO2
2.51
8.48
1.44
1.94
Al2O3
0.78
0.37
1.46
1.26
Fe2O3
63.33
52.12
63.54
63.47
FeO
33.46
36.76
32.33
32.48
MnO
0.13
1.92
0.03
0.08
MgO
0.02
0.02
0.18
0.25
Traces
0.68
0.25
1.35
0.60
Sum
100.91
99.92
100.32
100.08
Cr
70.00
10.00
510.00
180.00
Ni
260.00
300.00
40.00
410.00
V
3230.00
0.00
7800.00
2800.00
Zn
1270.00
1660.00
1000.00
900.00
Mg
0.00
0.00
0.01
0.01
Fe2+
0.92
0.69
0.94
0.92
Ni
0.00
0.00
0.00
0.00
Mn
0.00
0.06
0.00
0.00
Ti
0.07
0.24
0.04
0.06
Zn
0.00
0.01
0.00
0.00
Sum A
1.00
1.00
1.00
1.00
Al
0.03
0.07
0.07
0.06
Cr
0.00
0.00
0.00
0.00
Fe2+
0.14
0.08
0.08
0.11
Fe3+
1.81
1.82
1.82
1.82
V
0.01
0.03
0.03
0.01
Sum B
2.00
2.00
2.00
2.00
Factor
2.28
2.28
2.28
2.29
b
5.8. Fe-Ti oxides
Absent from alkali granites, Fe-Ti oxides occur in the rest of
the rocks in amounts up to 10%. The minerals crystallized
after the plagioclase in gabbros, elsewhere, it is primary.
Chemical analyses indicate the presence of ilmenite (46.8 <
TiO2 % <51.8) and magnetite (88.8 <Fe2O3+FeO % < 97).
6. Discussion
The primary magma source of the Nlonako rocks is basaltic,
as demonstrated by the presence of gabbros and
Qz
syenite
Rock type
SiO2
38.90
TiO2
6.63
Al2O3
0.69
Fe2O3
15.45
FeO
31.08
MnO
1.21
MgO
0.00
CaO
0.69
Na2O
7.82
K2O
0.00
Traces
0.04
Sum
102.50
Cr
0.00
Ni
330.00
Si
5.49
Al
0.11
Cr
0.00
3+
Fe
0.40
Sum T
6.00
Ti
0.70
Fe3+
0.30
Sum Ti
1.00
Fe3+
0.95
Fe2+
3.67
Mn
0.14
Mg
0.00
Sum Y
4.76
Ca
0.10
Na
2.14
K
0.00
Sum X
2.24
Facteur
8.48
Rhoenite
5.19
Aenigmatite
65.21
Feaenigmatite
29.60
c
microgabbros. These rocks include metaluminous
(Na2O+K2O<Al2O3) and peralkaline (Na2O+K2O>Al2O3)
suites. Sodium in the melt was high enough to allow
crystallization of arfvedsonite and aegirine in peralkaline
rocks under both reduced and oxidized conditions (Markl et
al., 2010, Marks et al., 2011). In particular, the presence of
titanite+magnetite +quartz indicates oxidizing oxygen fugacity
conditions whereas the presence of aenigmatite and ilmenite
indicates that the rocks were relatively reducing (Shellnutt
and Lizuka, 2010). In addition to textural variations, these
40
rocks display evidence for mineralogical variations which
could be related to changes in the physical and chemical
conditions of the magma. The presence of perthitic feldspars
demonstrate that the rocks were generated at depth from hot
and dry magma. Moreover, the absence of free plagioclases
in alkali granites is linked to the fact that the rocks have
crystallized at relatively high temperature, under hypersolvus
conditions (Bard, 1980). Perthitisation took place after the
crystallization of alkali feldspars. The occurrence of
orthopyroxenes (ferrosilite and enstatite) respectively in
quartz micromonzodiorite and quartz microdiorite is unusual
in the alkaline series. It indicates the interaction between the
magma and the country rocks as suggested in Mount
Mboutou (Parsons et al., 1986).
The mineralogical study defines in the basic and intermediate
rocks a labradorite + hortonolite + magnesian augite +
magnesio-hornblende + hastingsitic magnesio-hornblende +
titaniferous biotite paragenesis. For the acid rocks, the
paragenesis is perthitic feldspar + fayalite + ferroaugite +
augite + aegirinic augite + ferrohedenbergite + aenigmatite +
riebeckite + magnesian hastingsitic hornblende + ferroedenitic hornblende. The main substitution observed during
the evolution of the olivine is Mg 1 Fe2+ exchanges within
Fa-Fo series; but it can also be accompanied by Fe2+ 1 Mn
substitution within Fa-Te series. Both Fe and Mn increase
with the differentiation while Mg decreases. The first
evolution of pyroxenes in figure 9 suggests low oxygen
fugacity during crystallization (Larsen, 1976) and is controlled
by the substitution Ca Mg 1 Ca Fe2+, Mn2+ ; whereas, the
second one can be related to peralkalinity. This evolution
suggests high oxygen fugacity and is controlled by the
substitution Ca Mg Fe2+ Mn 1 Na Fe3+ Al3+ Al Ti (Yagi,
1966). The passage from calcic amphiboles to alkaline
amphiboles corresponds to the diminution of the temperature
during the crystallization (Fabries, 1978). The main
substitution which results is: Na(B) Fe3+(C) 1 Ca(B) Mg(C).
The evolution of micas is similar to the one observed in
Ntumbaw (Ghogomu, 1984). It shows Fe increase follow by
Mg decrease. Fe/Fe+Mg varies from 28% to 54%. The main
substitution here is Mg2+1 Fe2+. In general, the mineralogical
evolution of the main mineral phases (olivines, feldspars and
clinopyroxenes) is progressive and is marked by an increase
of Fe2+, Mn, Fe3+, Na and by a decrease of Mg, Al and Ca.
This mineralogical evolution and the importance of K-feldspar
in comparison with plagioclase during the differentiation are
compatible with the gabbro-diorite-monzodiorite-monzonitesyenite-granite alkali suite. Therefore, it appears that the
fractional crystallization is the only processes that generated
acids rocks from basic rocks in the NAC. However, the
mineralogical study has highlighted the quartz enrichment in
some rocks suggesting the minor influence of crustal
contamination at the end of the differentiation. This is
confirmed by initial 87Sr/86Sr values of 0. 7055 and 0.7056
obtained respectively in syenites (Lasserre, 1978) and in
granites (analyses from “BGR Laboratory, Germany). In the
field, the predominance of felsic rocks over mafic rocks
seems to contradict the proposed fractional crystallization.
This can be explained either by erosion not yet reaching
down into more basic-dominated levels or by density
differences promoting the rise of acid more than basic liquids.
7. Conclusions
Petrographical and mineralogical topics considered in this
paper help to complete earlier, fragmentary investigations on
the petrogenesis of Nlonako hill and to discuss some issues
that are raised. The NAC comprises wide range of
predominantly plutonic rocks with minor volcanic rocks. The
plutonic rocks form a complete petrographic suite, some
associated with microgranular equivalents, which extend
from gabbros to alkali granites through diorites,
monzodiorites, monzonites and syenites. Most of these rocks
indicate high content in quartz in addition to textural and
mineralogical variations. The occurrence of orthopyroxene
(ferrosilite and enstatite) in the Nlonako massif suggests an
interaction between the magma and the basement during the
differentiation. In spite of the wide range of petrographic
types with various compositions encountered in the massif,
an hypothesis invoking the existence of two magmas (an acid
one and a basic one) is to be excluded. The rocks were
differentiated at depth from a hot and dry magma by
fractional crystallization processes mainly controlled by
olivine, clinopyroxene and feldspar. The assimilation of
continental crust has affected this process to some extent.
Acknowledgements
Martin Guy ABOLO gratefully acknowledges all anonymous
reviewers for providing constructive comments on the
manuscript, the German Academic Exchange Service
(Deutscher Akademischer Austausch Dienst “DAAD”) for
providing him with a six months grant in Germany and the
Bundesanstaalt für Geowissenschaften und Rohstoffe
(B.G.R) in Hanover, Germany for performing all the analyses
related to this study.
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