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| Geological sketch map of Cameroon |
This paper presents new geochemical and geochronological data for the low to medium grade Rey Bouba
Greenstone Belt (RBGB), located in northern Cameroon at the northern margin of the Central African
Fold Belt (CAFB), and discusses the maximum age of volcanic activity, the maximum depositional age
of metasedimentary rocks and geotectonic implications. Geochemical data on volcanic rocks highlight
the predominance of transitional to calc-alkaline magma compositions (Zr/Y = 3.62–26.38) with mostly
andesite to basaltic andesite with an unusually high Mg concentration (>5 wt.%, high-Mg andesite), but
also basalt and trachy-andesite protoliths. Moreover, chondrite-normalized REE patterns and primitive
mantle-normalized spidergrams show enrichment of LREE relative to HREE with flat to depleted elements,
and moderate to slight negative Nb–Ta, Ti and Eu anomalies respectively; which are consistent
with a continental arc setting related to a subduction zone. U–Pb zircon LA-ICP-MS geochronology on
felsic metavolcanic or metatuff fix the maximum age for the volcanic activity in the RBGB at ca 670 Ma.
Detrital zircon grains indicate that Neoproterozoic zircon (Ediacaran to Cryogenian) are the main source
for the detritus that fed the basin, combined with minor Paleoproterozoic and Mesoproterozoic inputs.
The maximum depositional age, corresponding to the youngest graphical age peak controlled by multiple
grain ages is consistently constrained between 645 and 630 Ma, whereas the age of low grade
metamorphism weakly recorded by overgrowths on detrital zircon in the RBGB basin is around 600 Ma.
These results provide new insights into the geodynamic processes during the Neoproterozoic along the
northern margin of the CAFB of northern Cameroon, suggesting that the RBGB, where high-Mg andesite
magmatism has taken place is consistent with a continental arc-related basin
M. Houketchang Bouyoa,b,∗, Y. Zhaoa,∗∗, J. Penaye b,∗∗, S.H. Zhanga, U.O. Njel b
( note: please, you can find all details on the orginal document. see in the end of the text)
1. Introduction
The assembly of the Gondwana Supercontinent during the Late
Neoproterozoic–Cambrianinvolved closure ofthe intervening Neoproterozoic
ocean basins and subduction of a substantial volume of
oceanic lithosphere along a number of convergent margins (Collins
et al., 2007; Santosh et al., 2009, 2012; Boger, 2011; Ngako and
Njonfang, 2011). Major accretionary processes contributing to continental
growth in Africa during the Neoproterozoic have also been identified (Condie, 2003; Penaye et al., 2006; Pouclet et al., 2006;
Tchameni et al., 2006; Isseini et al., 2012). The CAFB is a collage
of Paleoproterozoic microcontinents and Neoproterozoic plutonic
and volcanic arcs attached to the Archaean Congo craton during
a Pan-African continental collision at ca 600 Ma (Toteu et al.,
2004, 2006; Van Schmus et al., 2008; Bouyo Houketchang et al.,
2009, 2013; Nkoumbou et al., 2013). Well-preserved Neoproterozoic
magmatic arcs bounded by narrow low to medium grade
volcanosedimentary schist belts have been described in northern
Cameroon and in southwestern Chad Republic (Fig. 1). These
volcanosedimentary sequences include the Poli, Bibemi-Zalbi and
Rey Bouba greenstone belts in northern Cameroon, and Zalbi and
Goueygoudoum greenstone belts in southwest Chad. In previous
works, these sequences were generally interpreted as pre-tectonic
back-arc basins intruded by or associated withthe calc-alkaline TTG
∗ Corresponding author at: Centre for Geological and Mining Research, PO Box 333, Garoua, Cameroon. Tel.: +237 696 215 566. ∗∗ Corresponding authors. E-mail address: mbouyo2100@yahoo.fr (M.H. Bouyo).
suite of the Sinassi and Mayo Kebbi Batholiths (Toteu et al., 1984,
1987, 2006; Pinna et al., 1994; Pouclet et al., 2006).
In the Cameroonian part, where most of the geological,
geochronological and isotopic data have been well-documented in
the Poli, but also in the Bibemi-Zalbi belts, the Rey Bouba belt has
been poorly studied and very little is known about its geochemistry,
age and provenance of detrital and volcanic material. The ages
and geochemistry of volcanic and sedimentary rocks from the Rey
Bouba basin are therefore crucial to better understand the tectonic
setting and geodynamic evolution of the CAFB along its northern
margin.
We present and discuss here new geochemical and laser inductively
coupled plasma mass spectrometry (LA-ICP-MS) U–Pb data
for zircon from metavolcanic and metasedimentary rocks to show that the Rey Bouba Greenstone Belt, was likely deposited in a continental
arc setting related to a subduction zone.
2. Geological setting
The NE–SW trending Rey Bouba Greenstone Belt is one of the
volcanosedimentary belts of the CAFB in northern Cameroon (Fig. 1
and Table 1). Located in the Western Cameroonian Domain, it
defines a narrow greenschist belt extending about 80 × 16 km,
closely related to the low to high grade Poli Belt and continues
beyond the Chadian border by the greenschist Goueygoudoum Belt
and the Bibemi-Zalbi Belt. Furthermore, it is located along the
Tcholliré-Banyo shear zone (TBF) interpreted as a major terrain
boundary separating the younger Neoproterozoic to Mesoproterozoic
Western Cameroonian Domain on the west side from the older
reworked Paleoproterozoic Adamawa-Yadé Domain on the east
side (Penaye et al., 1989; Pinna et al., 1994; Toteu et al., 2001, 2004;
Van Schmus et al., 2008; Bouyo Houketchang et al., 2009).
Fig. 2. Geological map of the Sinassi region showing the volcanosedimentary Rey Bouba Greenstone Belt and the associated granitoids. A schematic stratigraphic column for
the RBGB is inserted.
The RBGB mainly consists of greenschist-facies mafic to felsic
volcanic, volcanosedimentary and sedimentary rocks associated
with a set of pre-, syn- and post-tectonic granitoids and dykes.
It has been defined as a back-arc basin related with an oceanic
plate subduction below the southeastern continental margin of the
Adamaoua-Yadé Domain (Pouclet et al., 2006; Ngako and Njonfang,
2011). However, only very few geochronological data obtained by
Pb–Pb minimum ages on single zircon are available for the RBGB
indicating ages of 557 ± 17 Ma for the post-tectonic Vaimba granite
and 750 ± 20 Ma for the Gatougel dacitic tuff (Pinna et al., 1994).
The Poli Belt defines a pre- to syn-collisional basin developed
upon, or in the vicinity of young magmatic arcs. The filling
of the basin occurred in a docking-arc/back-arc context (Toteu
et al., 2006). It consists of low, medium- to high-grade Neoproterozoic
schists and gneisses of volcanic, volcano-sedimentary and
sedimentary origin. Metavolcanics are tholeiitic basalt and calcalkaline
rhyolite emplaced in an extensional crustal environment
(Njel, 1986; Toteu, 1990). The depositional age is constrained
between 700–665 Ma; detrital sources comprise ca. 920, 830, 780
and 736 Ma magmatic rocks (Toteu et al., 1987, 2006).
The Bibemi-Zalbi Belt extends continuously within both the
Cameroonian and Chadian territories, and is locally called Bibemi
and Zalbi Greenstone Belt, respectively. It is well studied in SW
Chad, where it is dated around 700 ± 10 Ma on metabasalt (Isseini,
2011) and 777 ± 5 Ma on epiclastite (Doumnang, 2006). Following
the assumption of Pinna et al. (1994) of an arc and back-arc basin
systemlinked to a subduction beneath theAdamawa-Yadé Domain,
Pouclet et al. (2006) interpreted the region in terms of fore-arc
basin – volcanic arc – back-arc basin that were accreted eastward to
the Adamawa-Yadé Domain, along the Tcholliré-Banyo shear zone.
This model implies the subduction and closure of a western oceanic
basin that was located between the Western Cameroonian Domain
that could have belonged to the Central Saharan Ghost Craton
(Black and Liégeois, 1993) and the remobilized Paleoproterozoic
Adamawa-Yadé Domain. All these terranes are now included in the
southern part of the Saharan Metacraton (Abdelsalam et al., 2002).
At the scale of the CAFB in Cameroon, the tectonic history is
complex and summarized by Ngako et al. (2008) and Ngako and
Njonfang (2011) in three main tectonic events related to PanAfrican
collision and post-collision evolution: (i) crustal thickening
(ca 630–620 Ma, and even 600 Ma, Bouyo Houketchang et al., 2009,
2013); (ii) left lateral wrench movements (613–585 Ma); and (iii)
right lateral wrench movement (585–540 Ma), successively; the
latter being related at the global scale to the final amalgamation of
the Gondwana Supercontinent (Alkmim et al., 2001; Meert, 2003;
Collins and Pisarevsky, 2005) during Latest Neoproterozoic-Earliest
Cambrian.
3. General sample descriptions and procedures for
geochronological and geochemical analyses
At the local scale, the geological framework of northern
Cameroon is dominated by a NE–SW extensive magmatic arc
province which includes a heterogeneous and complex plutonic
(TTG) and volcanosedimentary sequences (Fig. 2) that underwent
a polyphase deformation characterized by sub-vertical foliation,
sub-horizontal stretching lineation and folds associated with
greenschist to amphibolite facies metamorphism. In this study, we focused on greenschist facies volcanic and sedimentary components
of the RBGB from which forty five samples were collected
(Fig. 2). From metric to multi-metric thickness, mafic metavolcanic
rocks are massive, in elongated bands, flagstones, or blocks,
which commonly alternate with felsic metavolcanic, metasandstone,
metasiltstone, quartzite, schist and slaty shale locally
highly distorted. Conglomeratic layers consisting of angular to
rounded clasts of mafic volcanic, granitoid, quartzite and mineral fragments such as quartz within a finer-grained chlorite-rich
matrix are observed in Baba Sara area (Fig. 3a–d).
Thirty samples were selected for thin sections, 18 for geochemistry
and four for geochronology. In thin section, most of the rocks
examined display poorly preserved primary magmatic and sedimentary
textures superimposed by metamorphic recrystallisation
fabrics (Fig. 3e–h).
Major elements except FeO were analyzed on fused glass
discs by X-ray fluorescence spectrometry and FeO contents by
classical wet chemical analysis at the Analytical Laboratory of
the Beijing Research Institute of Uranium Geology. Trace element
concentrations were determined using inductively coupled
plasma-mass spectrometry (ICP-MS) at the Guangzhou Institute
of Geochemistry, Chinese Academy of Sciences, Guangzhou,
China. Detailed analytical methods for ICP-MS were described
by Chen et al. (2010). For most of the trace elements, analytical
precision and accuracy are better than 10%. Major element
analyses were recalculated to 100 wt.% anhydrous basis for
intercomparisons, where Fe2+ is assumed to be 80% total Fe,
with the prior conversion of reported Fe concentration (FeO or
Fe2O3) as Fe2O3t from appropriate atomic or molecular weights
(Vocke, 1999; Verma and Armstrong-Altrin, 2013). Chondrite and
primitive mantle reservoir compositions are those of Sun and
McDonough (1989). Mg-number indicating the level of evolution
of volcanic rock was defined as 100MgO/(MgO + FeO) in
mol per cent.
Being highly resistant to chemical and physical influences,
zircon is a particularly usefulmineralfor petro-chronological investigations
(Corfu et al., 2003; Sircombe, 2004; Vermeesch, 2004;
Andersen, 2005). In order to constrain the timing of volcanic activity
and depositional age of the RBGB, one felsic metavolcanic
tuff, VA-R 127, and three representative metasedimentary samples
including, VA-R 193, VA-R 204 and VA-R 222, were collected for
detailed study and dating. Zircon grains from each sample were
separated from 2 to 5 kg crushed rock samples by conventional
magnetic and methylene iodide liquid separation. The separated
zircon grains were handpicked and mounted in epoxy resin. The
epoxy mounts were polished to expose the mounted minerals,
carbon coated, photographed in both transmitted and reflected
light, and imaged via Cathodoluminescence (CL) using the JSM-
6510 scanning electron microscope (SEM) at Beijing Geoanalysis
Limited Company. U–Pb dating of zircon was conducted by LA-MCICP-MS
(Gehrels et al., 2008; Johnston et al., 2009; Jackson et al.,
2004) at the Isotopic Laboratory of Tianjin Centre of Geological
Survey-China. Laser sampling was performed using a UP193-FX,
and a Neptune MC-ICP-MS instrument was used to acquire ionsignal
intensities. Heliumwas applied as a carrier gas for the ablated
material. Trace element compositions of zircon were calibrated
against Nist610 using Si as an internal standard. GJ-1 standard
gem zircon from Sydney (Australia), with 207Pb/206Pb age based
on 8 TIMS determinations of 609 Ma (Jackson et al., 2004), used as
an external standard for U–Pb dating was mounted with samples
cleaned in 1 N nitric acid immediately prior to analysis to remove
surface Pb contamination, and then measured twice every eight
analyses. The counting time during the analyses is 60 s (including
the first 20 s for background). Fractionation and instrumental
mass bias are corrected by direct calibration against a zircon standard
analyzed under carefully matched conditions, using He as
the ablation gas to increase the reproducibility of the Pb/U fractionation.
Time-resolved data acquisition is employed to evaluate
zircon homogeneity and to allow selective integration of signals
to minimize common Pb contributions and Pb loss, and thus to
maximize concordance. LA-ICP-MS measured values for GJ-1show
that zircon is relatively low in Th, with mean U and Th contents
of 230 and 15 ppm, respectively. The Calibration errors (2) on
the 206Pb/238U, 207Pb/235U and 207Pb/206Pb ratio were 1.9%, 3.0%
Fig. 3. Field outcrop photographs and thin section microphotographs of various rock types of the RBGB. (a) Mafic metavolcanic rock at Mayo Rian-Mayo Vaïmba confluence;
(b) Fine-grained metasandstone in Mayo Vaïmba river; (c) Metasandstone at Baba Sara; (d) Metaconglomerate around Baba Sara; (e) Felsic metavolcanic rock (VA-R 235)
with abundant very small crystals (suggesting recrystallisation has occurred) of quartz ± plagioclase in the lighter bands, but also few epidote ± sericite ± calcite in darker
bands; (f) Felsic metatuff (VA-R 127) with quartz + feldspar embedded in argillaceous matrix; (g) Medium metasandstone (VA-R 191) showing mature sediments with well
sorted grains; (h) Metasiltstone (VA-R 166) showing a very fine-grained matrix and concentration of small muscovite flake + quartz intergrown after clasts of feldspar.
Fig. 4. CaO/Al2O3–MgO–SiO2 diagram showing geochemically unaltered nature of
most of the metavolcanic rock samples of the RBGB.
and 2.4%, respectively and propagated through the error analysis.
During the experiment, sites for dating were selected on the
basis of CL and photomicrograph images, in order to obtain as representative
a population as possible. Laser energy of 10–11 J/cm2,
a frequency of 8 Hz and a spot size of 35 m were used for the
instrument. Off-line selection, integration of background, analyte
signals, time-drift correction and quantitative calibration for
trace element analyses were performed by ICPMSDataCal (Liu
et al., 2008). Concordia diagrams, histograms overlain by probability
density distributions and weighted mean calculations were
made using Isoplot (Ver3.23) (Ludwig, 2003) to show the distribution
of the zircon populations over the full age range of all
grains (see text, Tables 3–6 and Figs. 6–8 for details on individual
samples).
4. Geochemical results
Major and trace element data of the investigated rocks from
the RBGB are presented in Table 2 (complete data set may be
accessed from the online data repository (see Appendix A)). Postdepositional
processes as well as low-temperature metamorphism
(metasomatism) are known to affect the mobility of certain elements
(Cs, Rb, Li, B, Ba, Sr, Si,K, etc.). Index of alteration also includes
the degree of hydration (i.e. Loss on Ignition which it is generally
less than 1%). Some of the analyzed samples in the present study
have Loss on Ignition (LOI) values higher than expected (Table 2).
The data were, therefore, plotted in the CaO/Al2O3–MgO–SiO2 diagram,
proposed by Schweitzer and Kröner (1985) to test alteration
effects in metavolcanic rocks. As seen, majority of the samples
plot in the field of “unaltered rocks” except few samples with
highest LOI (VA-R 223 and VA-R 127), which also show lowest
(0.35 wt.%) and highest CaO (6.39 wt.%) contents respectively
(Fig. 4; Table 2).
Both mafic to intermediate and felsic metavolcanic rocks of
the RBGB are characterized by magmatic compositions ranging
from andesite/basaltic andesite to trachy-andesite. Only one sample
clearly shows basaltic composition in the bivariate Zr/Ti versus
Nb/Y diagram (Fig. 5). In addition, they show (Zr/Y = 3.62–26.38)
ratios generally comparable to those of transitional (Zr/Y = 4.5–7.0)
and calc-alkaline volcanic compositions. On the basis of petrographic
characteristics, major elements, and chondrite and
primitive mantle normalized rare earth and trace element patterns,
they have been subdivided into three major groups (Fig. 6; Table
2): mafic to intermediate metavolcanic, felsic metavolcanic and
metasedimentary rocks.
Fig. 5. Zr/Ti versus Nb/Y classification diagram for metavolcanic rocks of the RBGB
(Table 2). Metasedimentary rocks are also plotted for intercomparisons. Compositional
fields revised by Pearce (1996) after Winchester and Floyd (1977).
4.1. Mafic to intermediate metavolcanic rocks
Mafic to intermediate metavolcanic rocks (Fig. 5) are enriched
in MgO (2.89 and 6.19 wt.%), moderate to strong fractioned with
a variable Mg-number (Mg#) between 38 and 65% (Table 2).
They have low to medium SiO2 (50.95–63.00 wt.%) contents and
relatively high TiO2% (0.65–1.03), MgO% (2.89–6.19), and CaO%
(0.35–8.86) respectively with an average of (0.83, 4.50, and
4.64 wt.%; Table 2). The relatively high concentrations of transition
metals V (up to 332 ppm), Cr (249 ppm), Ni (162 ppm), Cu
(236 ppm), Sc (28 ppm) and total REE (313 ppm) are noticeable
(Table 2). On chondrite and primitive mantle normalized diagrams,
they display enrichment in LREE and slight depletion in HREE with
no Eu anomaly (Fig. 6a and Table 2); but also erratic distribution
in the mobile LILE (Rb, Ba, K) and negative anomalies in Nb-Ta and
Ti (Fig. 6b). Inter-element ratios Nb/Ta and (La/Yb)N give values of
0.18–0.58 and 1.92–39.00 (with an average of 10.62), respectively.
4.2. Felsic metavolcanic rocks
Felsic metavolcanic rocks display the highest SiO2
(69.51–75.37 wt.%) and lowest MgO (0.41–1.46 wt.%) contents
(Table 2). They are also characterized by low concentrations oftransition
metals: Ti (0.11–0.49 wt.%), Sc (4–13 ppm), V (14–62 ppm),
Cr (2–22 ppm), Ni (1–12 ppm) and total REE (33–68 ppm). Felsic
metavolcanic rocks exhibit enriched LREE and flat to depleted
HREE patterns (Fig. 6c). The trace element patterns of this group
are relatively enriched in LILE (Rb, Ba, K) and have negative
anomalies in Nb-Ta, P and Ti (Fig. 6d); but variable Eu anomalies
(Eu/Eu* = 0.60–1.31; Table 2). Nb/Ta and (La/Yb)N ratios are
between (0.13–0.38) and (1.45–28.35 with an average of 12.48)
respectively.
4.3. Metasedimentary rocks
Metasedimentary rocks, compared with the previous
groups show variable contents of SiO2 (58.61–66.86 wt.%),
Mg (2.36–3.71 wt.%); transition metals Ti (0.57–1.00 wt.%), Sc
(13–20 ppm), V (88–131 ppm), Cr (65–114 ppm), Ni (34–64 ppm)
and total REE (65–157 ppm) (Table 2) between the highest values
for mafic metavolcanic rocks and the lowest for felsic metavolcanic
rocks and vice versa. However, Ba and Zr have the highest contents,
reaching respectively up to 1739 ppm and 212 ppm. The chondrite
normalized plots show enrichment in LREE relative to HREE which
have a flat pattern and slight fractionated or negative Eu anomaly
(Fig. 6e; Table 2). Regarding the primitive mantle normalized
diagram, they show enrichment in LILE (Rb, Ba, K) and negative
anomalies in Nb-Ta and Ti (Fig. 6f). Their Nb/Ta and (La/Yb)N ratios
are respectively (0.33–0.78) and (2.66–8.35 with an average of
5.61).
Fig. 6. Chondrite and primitive mantle normalized diagrams for (a and b) Mafic to intermediate metavolcanic rocks, (c and d) Felsic metavolcanic rocks and (e and f)
Metasedimentary rocks.
5. Geochronological results
In order to give an adequate, accurate and precise feature of the
age distribution ofthe volcanic and sedimentary rocks of RBGB, several
hundred grains of zircon were selected randomly from each
sample. Among the four samples selected for dating, 318 zircon
crystals were individually analyzed in three samples, and only 26
crystals saved for the fourth; while a maximum of 120 representative
target spots from each sample were located using CL images.
In total, 394 analytical spots were performed on 344 zircon grains.
5.1. Sample VA-R 127: Felsic metatuff
This sample is interpreted as a felsic metavolcanic rock or
metatuff comprising a mixture of fragments and euhedral to subeuhedral
fine- to medium-grained quartz and feldspar crystals
embedded in a fine-grained matrix. Collected near Vaïmba village
(Fig. 2), and generally displaying a grey colour or grey yellowish to
brownish weathered surfaces, the rock has very fine-grained illite,
sericite and chlorite associated with sub-angular to sub-rounded
quartz, and minor feldspar, epidote and oxides. Late stage microfractures
or veinlets filled by calcite are also locally observed.
Zircon grains from this sample are light-pink to brownish in
colour. They are commonly prismatic, but occasionally rounded or
fragmentary in shape, possibly related to different populations. The
grain size varies from ca 80 to 230 m, the majority being between
ca 105 and 115 m. On CL images (Fig. 7a), all the grains display
igneous features with oscillatory or sector zoning. In most cases,
grains show different colours in grey, with faint, broad zoning, and
in fewer cases, they are unzoned. Overgrowths are very rare to
absent.
A number of 110 points were analyzed in situ on 107 grains.
Most of the analyzed zircon grains (92%) show 232Th/238U ratios
higher than 0.2 (Table 3, complete data set may be accessed from
the online data repository (see Appendix A)), pointing to a magmatic
origin (Williams and Claesson, 1987). From the data, two
main groups of concordant and discordant zircon (Fig. 8a and Table
3) are observed. The concordant group is very clustered (inset
on Fig. 8a) and display three distinct sub-groups or populations
on the probability density distribution and histogram diagrams
(Fig. 8b), with a high concentration of 206Pb/238U ages at ∼670 Ma
(33 analyses; median = 668 ± 8.5 [1] Ma), at ∼700 Ma (27 analyses;
median = 697 ± 3.1 [1] Ma) and at ∼760 Ma (5 analyses;
median = 758 ± 2.8 [1] Ma). The discordant group shows signifi-
cant episodic loss of lead which may be related either to analytical
issues or to relatively high U contents of analyzed zircon grains
(dark domains, Fig. 7a: 127–3 and Table 3: 127–3: 713 ppm, 22:
371 ppm, 85: 476 ppm, 94: 253 ppm, 98: 451 ppm, etc.). However
they yield an upper intercept of 1431 Ma and lower trajectory to ca
0 Ma (Fig. 8a).
5.2. Sample VA-R 193: Schistose sandstone
RockVA-R193 is a light- tomedium-grey, schistose sandstone or
metasiltstone from Baba Sara, showing a very fine-grained groundmass
of silica and clayey minerals in association with clasts of
quartz, pyrite and iron oxides. The unit is locally affected by recrystallisationfabrics
due to intense deformation, and interbedded with
felsic metavolcanic rock and quartzite bands, and commonly contains
lithic fragments giving it the appearance of conglomeratic
volcanosedimentary schist.
Detrital grains of zircon range in size mostly from ca 110 to
200 m, with pale pink to yellowish colour. Grains are euhedral to
sub-euhedral, mostly displaying oscillatory or sector zoning typical
of igneous rocks (Fig. 7b). Moreover, some individual crystals show
strong variations in the development of zoned domains, where one
large uniform central zone is succeeded by much finer oscillatoryzoned
bands.
A total of 120 spots on 105 zircon grains were analyzed, with
94% indicating 232Th/238U ratios higher than 0.2 (Table 4, complete
data set may be accessed from the online data repository
(see Appendix A)). The U–Pb isotope data obtained from the zircon
grains show a wide range of ages almost all concordant (115
analyses) between 529 ± 3 Ma and 906 ± 5 Ma (Fig. 8c) with a
peak at 645 Ma representing a dominantly Neoproterozoic source
(Fig. 8d). The presence of Late Mesoproterozoic and Paleoproterozoic
components is also indicated by two zircon ages at 1016 ± 6 Ma
(193–32), and 2077 ± 12 Ma (193–119) respectively. Five discordant
data (193–10, 12, 48, 52 and 104; Table 4) are not considered.
5.3. Sample VA-R 204: Chl-schistose sandstone
Sample VA-R 204 is a greenish grey to dark green, fine-grained
chlorite-bearing schistose sandstone or volcanoclastic schist, collected
near the river Mayo Godi, about 6 km NW from Baba Sara.
The rock consists essentially of quartz and chlorite in a cryptocrystalline
matrix. It is locally strongly deformed and interbedded with
well-foliated slaty schists.
Zircon grains are euhedral to sub-euhedral, and commonly vary
in size from ca 135 to 180 m and up to 235 m in places. They are
mostly pale pink and display morphologies and internal structures
characterized by more or less well-preserved magmatic growth
zoning (Fig. 7c). These grains for the most part have less blunted or
eroded terminations, suggesting a short distance of transportation
prior to deposition, while very few show thin overgrowths.
Some 44 laser ablation analyses were performed on 26 crystals.
232Th/238U ratios range from 0.03 to 1.67 with the majority (80%)
higher than 0.2 (Table 5, complete data set may be accessed from
the online data repository (see Appendix A)). Data are plotted in
Fig. 8e. Most analyses are concordant defining three main groups
with 206Pb/238U ages ranging from 585 ± 4 to 876 ± 6 Ma (36 analyses,
insert Fig. 8e) for the first group, one analysis at 1259 ± 10 Ma
for the second group, and the third group at ca. 2000 Ma (four
analyses). A predominant Neoproterozoic source is shown by a
broad probability density distribution from Ediacaran to Tonian
with maximum peaks at 663 and 700 Ma, but also at 682, 630 and
600 Ma (Fig. 8f). This sample also confirms the presence of older
Mesoproterozoic and Paleoproterozoic sources for the sediments.
5.4. Sample VA-R 222: Ferruginous metasandstone
The rock specimen is brown to ocher, probably related to its
dominantly ferruginous cementingmaterial.Itis afine- tomediumgrained
ferruginous metasandstone, collected upstream of the
Mayo Beoulpir river. Minerals identified in thin section include
principally sub-angular to sub-rounded quartz, but also plagioclase,
calcite, pyrite, iron oxides, epidote, illite, chlorite and indistinct
grains of brownish altered ferromagnesian mineral. Folded veinlets
of quartz and calcite are also observed.
Detrital crystals of zircon are prismatic to round, and pink, yellowish,
or brown in colour. Crystals are sub-euhedral to euhedral,
range in size from ca 75 to 200 m, with the majority between
110 and 150 m, and mostly show a well-developed oscillatory to
sector zoning (Fig. 7d). The differences in morphology and colour
among zircon may reflect distinct sources, and the small degree of
abrasion suggests a proximal provenance.
A whole of 120 measurements were performed in situ on 106
zircon grains, with 99% displaying 232Th/238U ratios higher than 0.2
(Table 6, complete data set may be accessed from the online data
repository (see Appendix A)). Concordia diagrams (Fig. 8g) show
scattered discordant points with relatively high U contents (dark
domains, Fig. 7d: 222–13 and 26; Table 6: 222–13: 172 ppm, 26:
Fig. 7. Cathodoluminescence images of zircon from representative metavolcanic and metasedimentary samples of the RBGB basin: (a) felsic metatuff, VA-R 127; (b) schistose
sandstone, VA-R 193; (c) chlorite-bearing schistose sandstone, VA-R 204 and (d) ferruginous metasandstone, VA-R 222. The figure also shows circles indicating analyzed
spots (not at scale), with numbers representing 206Pb–238U age and spot position listed respectively in Tables 3–6.
227 ppm, 29: 156 ppm, 30: 240 ppm, 65: 327 ppm, 117: 326 ppm,
etc.) possibly due either to analytical issues or to common lead or
extreme lead loss; and concordant points (56 analyses, diagram
insert) all yielding Neoproterozoic 206Pb/238U ages from 574 ± 4
to 893 ± 6 Ma. The combined probability density distribution and
histogram diagrams on concordant points show a maximum peak
at 660 Ma, but also at 740, 795 and 848 Ma (Fig. 8h).
6. Discussion
6.1. Age of the volcanic activity
U–Pb zircon LA-ICP-MS geochronology on a felsic metavolcanic
rock or metatuff (sample VA-R 127; insert Fig. 8a) is used
to constrain the timing of magmatic activity within the RBGB. On
CL images (Fig. 7a), zircon grains from VA-R 127 show typical
oscillatory growth or sector zoning and are characterized by high
232Th/238U ratios (Table 3) indicating a magmatic origin. Concordant
zircon grains (65 analyses) reported on Fig. 8 a (insert) and b
define three peaks (ca 760, 700 and 670 Ma) which we interpreted
as the times of different magmatic sources in the region. They indicate
that the magmatic activity in the RBGB is prominent and took
place in the Neoproterozoic. The peak at ∼760 Ma reflects an earlier
inherited magmatic event which is consistent with the Pb/Pb
age of 750 ± 20 Ma on Gatouguel dacitic tuff (Pinna et al., 1994),
whereas the youngest peak at ∼670 Ma fix the maximum age for
the volcanic activity in the RBGB.
From previous studies in the Poli Belt by Toteu (1990) and
Toteu et al. (2001, 2006), a wide range of Neoproterozoic detrital
zircon (Tonian–Cryogenian) at 700, 780, 830, and 920 Ma have
been recorded (Table 1), some of which correspond to well identified
magmatic rocks originating from neighbouring areas (e.g.,
Goldyna metarhyolite, medium-grade schists and underlying volcanic
sequence). In the Chadian part of the Bibémi-Zalbi Belt,
Isseini (2011) constrains a crystallization U–Pb age on metabasalt
at 700 ± 10 Ma, quite different from the previous U–Pb age of
777 ± 5 Ma derived from an epiclastite by Doumnang (2006).
The distribution of ages implies at least three major periods
or episodes, representing formation times of volcanic rocks at the
northern margin of the CAFB: (1) an early volcanic activity from ca
920 to 830 reported only in the Poli Belt,(2) a second from ca 780 to
700 reported in the Poli and Bibémi-Zalbi belts, and (3) a third volcanic
event around 670 Ma or younger reported in the RBGB belt.
These results show that the volcanic activity is older in the Poli and
Bibémi-Zalbi belts and younger in the RBGB.
6.2. Provenance of the sediments
LA-ICP-MS U–Pb detrital zircon dating presented here provides
new constraints onthe geological evolutionoftheRBGB, dominated
by Neoproterozoic zircon with some Paleoproterozoic, Mesoproterozoic,
and Early Cambrian inputs. Over 70% of zircon grains have
Neoproterozoic (Ediacaran to Tonian) ages ranging from ca 574 to
ca 906 Ma (samples VA-R 193, VA-R 204 and VA-R 222), with major
Fig. 8. U–Pb concordia and probability density distribution-histogram diagrams for detrital zircon analyses from representative metavolcanic and metasedimentary samples
of the RBGB basin: (a and b) felsic metatuff, VA-R 127; (c and d) schistose sandstone, VA-R 193; (e and f) chlorite-bearing schistose sandstone, VA-R 204 and (g and h)
ferruginous metasandstone, VA-R 222.
peaks at 645, ca 660 and 700 Ma (Fig. 8d, f and h) but also at 600,
630, ca 680, 740, ca 800 and ca 850 Ma (Fig. 8f and h); indicating
that the Neoproterozoic (Ediacaran to Cryogenian) was the main
source for the Rey Bouba basin. These ages correspond well with the
U–Pb and Ndmodel ages ofthe juxtaposed NeoproterozoicWestern
Cameroonian Domain (granitoids and volcanosedimentary sources
at ca 595, 600, 615, 630, 660, 693, 736, 750, 780, 830, and 920 Ma;
Table 1; Toteu et al., 2001, 2004, 2006; Bouyo Houketchang et al.,
2009; Dawaï et al., 2013) as well as the Pan-African Mayo Kebbi
Batholith or magmatic arc of SW Chad (ca 640, 665 and 740 Ma;
Table 1; Penaye et al., 2006; Isseini et al., 2012). Looking at the
high proportion (more than 90%) of 232Th/238U ratios higher than
0.2, this implies that the predominant sources of Neoproterozoic
detritus that fed the RBGB basin were derived from erosion of the
adjacent magmatic arc and Western Cameroon Domain or Block. In
this regard, our data clearly demonstrate that northern Cameroon
volcanosedimentary basins at the northern margin of CAFB are
younger than the magmatic arc, and not intruded by the latter
as suggested by previous works (Pinna et al., 1994; Penaye et al.,
2006; Pouclet et al., 2006). However, there is also minor input of
Paleoproterozoic (Orosirian to Rhyacian) zircon at ca 2000 Ma and
2077 ± 12 Ma, recorded on samples VA-R 193 (Fig. 8c) and VA-R
204 (Fig. 8e). Rare Mesoproterozoic (Stenian to Ectasian) zircon at
1016 ± 6 Ma (sample VA-R 193; Fig. 8c) and 1259 ± 10 Ma (sample
VA-R 204; Fig. 8e), and very rare (less than 1%) Early Cambrian (Fortunian)
zircon at 529 ± 3 Ma (sample VA-R 193; insert Fig. 8c) also
occur.
Paleoproterozoicdetrital zirconreportedfrommetavolcanosedimentary
schist of the RBGB may be correlated with the nearby
gneisses of Adamawa-Yadé Domain (e.g. Mayo Makat and Mayo
Kout gneisses at 2218 ± 14 Ma, and also Mbé gneiss at 2014 ± 7 Ma,
Penaye et al., 1989; Mayo Kout orthogneiss at 2039 ± 26 Ma, Bouyo
Houketchang et al., 2009; Table 1). The Mesoproterozoic zircon may
be regarded as evidence of eroded rocks that remain unknown
or hidden, since the area is not mapped in detail. The Neoproterozoic
ages at ca 600 Ma (sample VA-R 204, Fig. 8f) agree with
tectonometamorphic ages previously reported from volcanosedimentary
basins within the CAFB (zircon U–Pb at 620 ± 10 Ma,
Yaoundé: Penaye et al., 1993; composite Sm–Nd isochron at
616 Ma, Bivouba-Yaoundé: Toteu et al., 1994; Pb–Pb zircon evaporation
at 611 ± 20 Ma, Yaoundé: Stendal et al., 2006; zircon U–Pb
at ca 600 Ma, Lom: Toteu et al., 2006; Sm–Nd garnet-whole rock at
628 ± 68 Ma, Bafia: Numbem Tchakounté et al., 2007; zircon U–Pb
and Sm–Nd garnet-whole rock at ca 600 Ma, Banyo and Tcholliré:
Bouyo Houketchang et al., 2009, 2013; monazite U–Th–Pb at
622 ± 43 Ma, Boumnyebel-Yaoundé: Yonta-Ngouné et al., 2010; at
613 ± 33 Ma, Yaoundé: Owona et al., 2011). The Early Cambrian
ages are likely to be minimum estimates for the formation of RBGB
or may represent post-metamorphic cooling.
6.3. Age of metamorphism
Although very rare to absent on CL images (Fig. 7), younger
metamorphic overgrowth rims around older magmatic cores are
locally present and characterized by 232Th/238U ratios lower than
0.2 (less than 9% of the entire samples; Tables 3–6). Except for
a few very young ages (e.g. 288 Ma or 432 Ma, VA-R 127–3 and
127-24 respectively; Table 3), which may be due to calibration
problems or lead loss after deposition, about 67% of this metamorphic
population yield Late Neoproterozoic ages, most often
discordant. However, the youngest and best example to constrain
the age of metamorphism is given by sample VA-R 204-24 with
a concordant 206Pb/238U age of 600 ± 4 Ma, and 232Th/238U ratio
of 0.03 (Table 5). This result is consistent with the ca 600 Ma age
of metamorphism obtained from high pressure granulites (Bouyo
Houketchang et al., 2009, 2013) in Banyo and Tcholliré (Poli Belt)
0
10
20
30
40
50
60
500 600 700 800 900 1000 1100
Age (Ma)
Number
All of the metasedimentary
rocks; n=207
645
Fig. 9. Synthesis diagram displaying the combined probability density distributionhistogram
for the geochronological data from the three metasedimentary samples.
within the CAFB of north-central Cameroon (Table 1). This is the
most direct evidence for the timing of continental collision.
6.4. Maximum depositional age of the RBGB basin
The maximum depositional age for Precambrian stratigraphic
units lacking preserved biostratigraphic age control such as fossils
is commonly constrained by the youngest U–Pb ages of zircon
grains in populations of detrital zircon (Jones et al., 2009). As mentioned
in previous sections, Neoproterozoic ages around 600 Ma
and Early Cambrian ages are best interpreted as tectonometamorphic
ages and post metamorphic cooling respectively, and
therefore, they were not considered in the detrital zircon populations.
The maximum depositional age for each sample is defined
here as the 206Pb/208U age of the youngest dated 99.5% to 100.5%
concordant zircon at 621 ± 5 Ma in the case of sample VA-R 204-
23 (Table 5; Fig. 8e); 631 ± 4 Ma in the case of sample VA-R 222-16
(Table 6; Fig. 8g), and 626 ± 4 Ma in the case of sample VA-R 193-13
(Table 4; Fig. 8c). Our results show somewhat different youngest
ages from one sample to another, and so individually are not yet
considered to represent actual depositional ages. Dickinson and
Gehrels (2009) tested the research strategy of using youngest U–Pb
ages of detrital zircon to constrain the maximum depositional ages
of strata containing zircon grains by comparing a total of 5365
concordant or nearly concordant U–Pb ages of detrital zircon in
58 samples of Mesozoic sandstone from the Colorado Plateau and
adjacent areas with depositional ages known independently from
biostratigraphy. Their analysis confirms that using youngest detrital
zircon ages to constrain maximum depositional age is a valid
procedure, but indicates that results vary somewhat depending
upon how the youngest grain age is specified. Moreover, they add
that in general, the youngest-age measurements based on multiple
grain ages are more consistently compatible with depositional
ages.
In our approach to constrain the maximum depositional age,
we consider the geochronological data from the three metasedimentary
samples as a composite sample of detrital zircon, in
order to do statistical comparisons of similarity between zircon
populations within the RBGB. From a total of 284 spots on 237
grains analyzed, 207 are considered significant in terms of concordance
and have been used to construct a probability density
distribution-histogram diagram (Fig. 9; Tables 4–6). From Fig. 9,
a single peak controlled by multiple ages is defined at 645 Ma,representing the main provenance source of the detrital zircon, as
well as the youngest probability density distribution peak. In light
of Figs. 8f and 9, the maximum depositional age can be consistently
constrained between 645–630 Ma, whereas 600 Ma represents the
age of low grade metamorphism recorded by the detrital zircon
overgrowths from the RBGB basin.
6.5. Petrogenesis of volcanic and sedimentary rocks
Knowledge of the tectonic setting of an ancient basin is
very important for understanding the geodynamic evolution and
associated mineral resources, as well as the paleogeographic environment.
Although there are many discrimination diagrams for
ultrabasic and basic tectono-magmatic, and sedimentary (Bhatia,
1983; Roser and Korsch, 1986; Armstrong-Altrin and Verma, 2005)
settings in the literature with success rate sometimes controversial
for a specific geodynamic context, very few or almost none exist
for intermediate composition rocks. In this work, we constrain the
RBGB basin tectonic setting based on new sets of multidimensional
tectonic discrimination diagrams for intermediate magmas from
five tectonic settings (island arc-IA, continental arc-CA, continental
rift-CRand ocean island-OI combined together, and collision-Col) of
Verma andVerma (2013), but also for sedimentary rocks from three
tectonic settings (arc, continental rift, and collision) of Verma and
Armstrong-Altrin (2013); all based on worldwide examples with
high success rates of about 63–100%.
In two different sets of five discrimination diagrams for intermediate
rocks designed to discriminate among five tectonic settings
based on major-elements (Fig. 10A, accessible from the online data
repository (see Appendix A)) and immobile trace-elements (Fig.
10B, accessible from the online data repository (see Appendix A)),
about 80% of our samples (mafic to intermediate metavolcanic
rocks) fall in the arc tectonic setting field consistent with the dominantly
transitional to calc-alkaline andesite to basaltic andesite
geochemical signatures of the studied rocks (Fig. 5). Similarly,
in the new multi-dimensional major-element based diagrams for
tectonic discrimination of siliciclastic sediments from three main
tectonic settings, the RBGB metasedimentary rocks mostly plot in
the arc tectonic setting field for high-silica (Fig. 11a, accessible from
the online data repository (see Appendix A)) and low-silica (Fig.
11b, accessible from the online data repository (see Appendix A))
contents. Moreover, the chondrite and primitive mantle normalized
diagrams show respectively enrichment in LREE relative to
HREE and moderate to slight negative Nb-Ta, Ti and Eu anomalies
for all analyzed samples (Fig. 6); compatible with a subduction
zone. Thus an arc setting related to convergence and subduction
can be clearly inferred for the northern margin of the CAFB during
the Neoproterozoic (645–630 Ma), and therefore indicates the
depositional environment of the RBGB basin prior to collision at ca
600 Ma.
6.6. Geodynamic interpretation
As mentioned above, mafic to intermediate metavolcanic rocks
from the RBGB show geochemical features similar to those of wellstudied
high-Mg andesite (HMA) from Mitsuiyama (Okoppe area
of North Hokkaido in Japan, with MgO contents ranging from 4.9
to 7.9 wt.%, Ayabe et al., 2012). These include: andesitic composition
(Fig. 5) with high contents of MgO (2.89–6.19% or Mg-number
between 38–65% with an average of 52; Table 2), high concentrations
of transition metals V (up to 332 ppm), Cr (249 ppm), Ni
(162 ppm), Cu (236 ppm) and Sc (28 ppm), relatively low (La/Yb)N
ratios between 1.92–39.00 with an average of 10.62, depleted rare
earth elements (Fig. 6), and also high field strength elements (Nb,
Ta, Ti), which are indicators of arc volcanic rocks developed in continental
setting (Figs. 5, 10 and 11). This is supported by the detrital
sources that fed the RBGB which are derived predominantly from
erosion of the adjacent Neoproterozoic Mayo-Kebbi and Sinassi
juvenile magmatic arcs of the Western Cameroonian Domain and
from the Paleoproterozoic Adamawa-Yade Domain. Therefore, the
RBGB may correspond to an extensional basin developed upon or
behind a Neoproterozoic magmatic arc. Fig. 12 (accessible from the
online data repository (see Appendix A) illustrates a subductionrelated
geodynamic setting model for the RBGB (Fig. 12A) in the
network of Neoproterozoic magmatic arcs (Fig. 12B) of northcentral
Cameroon in the CAFB.
Although the andesitic rocks from the RBGB are Mg-rich, their
geotectonic setting isquitedifferentfromthat oftheprimitivehighMg
andesite as defined by Wood and Turner (2009) or by Ayabe
et al. (2012) in the Okoppe volcanic field of North Hokkaido, Japan.
Finally, it seems evident that there are different types of HMA in
subduction-related settings; some are primitive and result from
interaction of a melt derived from subducted oceanic basaltic crust
and the overlying mantle wedge peridotite (Shiraki et al., 1978;
Meijer, 1980; Crawford et al., 1981; Tatsumi and Ishizaka, 1982; Xu
et al., 2000) and others aremore evolved and resultfrominteraction
of melt derived from subducted oceanic crust and the continental
magmatic arc.
7. Conclusions
The main objective of our work was to acquire new geochemical
and geochronological data from the low to medium grade volcanosedimentary
Rey Bouba Greenstone Belt (RBGB) of northern
Cameroon at the northern margin of the Central African Fold Belt
(CAFB), in order to contribute for a better knowledge and understanding
of tectonic settings and geodynamic evolution during its
Precambrian history.
The overall results of our work show that:
- Petrogenesis of mafic to intermediate metavolcanic rocks from
the RBGB is consistent with the dominantly transitional to calcalkaline
andesite to basaltic andesite, indicating geochemical
features that are similar to typical high-Mg andesite including:
andesitic composition with high contents of MgO (2.89–6.19% or
Mg-number between 38 and 65%), high concentrations of transition
metals V (up to 332 ppm), Cr (249 ppm), Ni (162 ppm),
Cu (236 ppm) and Sc (28 ppm), relatively low (La/Yb)N ratios
between 1.92 and 39.00 with an average of 10.62, depleted rare
earth elements, and also in high field strength elements (Nb, Ta,
Ti), which are suitable signatures of arc volcanic rocks;
- U–Pb zircon LA-ICP-MS geochronology on felsic metavolcanic or
metatuff fix the maximum age for the volcanic activity in the
RBGB at ∼670 Ma, thus younger than in the neighbouring Poli
and Bibémi-Zalbi belts;
- the dating of detrital zircon indicate that the main source for
the detritus that fed the RBGB is Neoproterozoic (Ediacaran to
Cryogenian) with minor Paleoproterozoic and Mesoproterozoic
inputs from the neighbouring Mayo Kebbi-Sinassi magmatic arc
and Adamawa-Yade Domain;
- the age of low grade metamorphism weakly recorded by younger
metamorphic overgrowth rims around older magmatic cores,
characterized by 232Th/238U ratios lower than 0.2 is 600 Ma;
- the maximum depositional age of the RBGB, corresponding to the
youngest graphical age peak controlled by multiple grain ages is
consistently constrained between 645 and 630 Ma.
Our data therefore provide new insights into the geodynamic
processes during the late Neoproterozoic, suggesting thattheRBGB,
where high-Mg andesite magmatism has taken place is consistent
with a continental arc setting related to a subduction zone.
Source:
M. Houketchang Bouyo, Y. Zhao, J. Penaye, S.H. Zhang, U.O. Njel, 2015.Neoproterozoic subduction-related metavolcanic and metasedimentary rocks from the Rey Bouba Greenstone Belt of north-central Cameroon in the Central African Fold Belt: New insights into a continental arc geodynamic setting.
Precambrian Research 261:40-51.
Les activités
humaines ont-elles perturber l'écosystème Terre au point que nous aurions
entamer une nouvelle ère géologique ? L'Anthropocène, dont il est question, est
illustré par une vidéo originale et spectaculaire diffusée à la veille de la
conférence "Planet Under Pressure" qui a eu lieu fin mars 2012 à
Londres.
