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\DOI{10.5802/crgeos.282}
\datereceived{2024-07-09}
\daterevised{2024-11-12}
\datererevised{2024-11-25}
\dateaccepted{2024-11-26}
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\begin{tabular}{lccccccccccccc}
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Sample & RB2 & RB2 & RB2 & RB2 & RB2 & RB7b & RB7b & RB7b & RB7b & RB7b & RB7b & RB7b & RB7b \\
Analysis & C4 & C4 & C3 & C3 & C2 & C2 & C1 & C1 & C1 & C2 & C2 & C2 & C3 \\
Position & & r & & & & r & c & c & & & & & r\\
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\begin{document}

\begin{noXML}

\CDRsetmeta{articletype}{research-article}

\shortrunauthors

\title{The Iriba alkaline basalts, an expression of mantle-derived
Cretaceous magmatism of the Cameroon-Chad volcanic line along the
Central Africa Rift System}

\alttitle{Les basaltes alcalins d'Iriba, expression du magmatisme
Cr\'{e}tac\'{e} d'origine mantellique de la ligne volcanique
Cameroun-Tchad le long du syst\`{e}me du rift d'Afrique Centrale}

\author{\firstname{F\'{e}lix Nenadji} \lastname{Djerossem}\CDRorcid{0000-0002-8350-9237}\IsCorresp}
\address{Institut National Sup\'{e}rieur du Sahara et du Sahel
d'Iriba, D\'{e}partement des Mines, Energies Nouvelles et
Renouvelables, Tchad} 
\email[F. Djerossem]{djerossem@gmail.com}

\author{\firstname{Beno\^{i}t Joseph} \lastname{Mbassa}\CDRorcid{0000-0003-4161-1177}}
\address{Institute for Geological and Mining Research. P.O. Box 4110
Yaound\'{e}, Cameroon} 

\author{\firstname{Olivier} \lastname{Vanderhaeghe}\CDRorcid{0000-0001-9361-4631}}
\address{GET-OMP, Universit\'{e} de Toulouse, UPS, CNRS, IRD, CNES, 14
avenue E. Belin, 31400 Toulouse, France} 

\author{\firstname{Michel} \lastname{~Gregoire}\CDRorcid{0000-0001-9196-876X}}
\addressSameAs{3}{GET-OMP, Universit\'{e} de Toulouse, UPS, CNRS, IRD,
CNES, 14 avenue E. Belin, 31400 Toulouse, France}

\keywords{\kwd{Chad volcanic activity}
\kwd{Iriba basanite}
\kwd{Fractional crystallization}
\kwd{HIMU-type mantle}
\kwd{Alkaline magmatism}
\kwd{Central Africa rift system}}

\altkeywords{\kwd{Activit\'{e} volcanique du Tchad}
\kwd{Basanite d'Iriba}
\kwd{Cristallisation fractionn\'{e}e}
\kwd{Manteau de type HIMU}
\kwd{magmatisme alcalin}
\kwd{Rift d'Afrique Centrale}}

\thanks{LithoCOAC project (CNRS, France), IRN FALCoL (CNRS, France),
French Embassy in Chad through the fellowship program ``S\'{e}jour
Scientifique de Haut Niveau (SSHN)''}

\begin{abstract} 
The poorly documented volcanic rocks of the Ouaddai massif in Chad are
a continuity of the ones of the Cameroon Volcanic Line further to the
SE, located within the Central African rift system. New mineralogical
and geochemical data from the Iriba basanites (SiO\textsubscript{2}: 
41--45 wt\%) show depletion in HREE, slight negative Sm anomaly and high
LREE/HREE ratios, which is typical of OIB. The main differentiation
process is fractional crystallization with a complete lack of crustal
contamination. These features, similar to basanites exposed in southern
Cameroon, reflect the partial melting of a metasomatized subcontinental
lithospheric root reworked during the formation of the Cenozoic Central
Africa Rift System. We propose to define by Cameroon-Chad Volcanic Line
this continental scale structure controlling the emplacement of
alkaline magmas.
\end{abstract}

\begin{altabstract} 
Les roches volcaniques peu document\'{e}es du massif du Ouaddai au
Tchad sont dans la continuit\'{e} de celles de la ligne volcanique du
Cameroun plus au SE, localis\'{e}es dans le syst\`{e}me du rift
d'Afrique centrale. Les nouvelles donn\'{e}es min\'{e}ralogiques et
g\'{e}ochimiques des basanites d'Iriba (SiO\textsubscript{2}: 41--45 wt\%)
montrent qu'elles sont caract\'{e}ris\'{e}es par un appauvrissement
en HREE, une l\'{e}g\`{e}re anomalie n\'{e}gative Sm et des rapports
LREE/HREE \'{e}lev\'{e}s, typique des OIB. Le principal processus de
diff\'{e}renciation est la cristallisation fractionn\'{e}e avec une
absence totale de contamination crustale. Ces caract\'{e}ristiques,
similaires aux basanites expos\'{e}es dans les massifs volcaniques du
Cameroun, refl\`{e}tent la fusion partielle d'une racine
lithosph\'{e}rique subcontinentale m\'{e}tasomatis\'{e}e, remani\'{e}e
pendant la formation du syst\`{e}me de rift c\'{e}nozo\"{i}que
d'Afrique centrale. Nous proposons de d\'{e}signer par ligne volcanique
du Cameroun-Tchad cette structure g\'{e}ologique d'\'{e}chelle
continentale contr\^{o}lant la mise en place de magmas alcalins.
\end{altabstract} 

\maketitle
\vspace*{1pc}
\twocolumngrid

\end{noXML}

\section{Introduction}\label{sec1}

Alkaline magmatism, typical of intraplate environments including
oceanic islands, continental rifts and intracontinental volcanoes, is
generally the consequence of melting of peridotite in presence of
CO$_{2}$ \citep{Wyllie1977,Dasguptaetal2007,Baasneretal2016}, or of
recycled oceanic crust \citep{Green1973,Chase1981,Hofmannetal1986}, or
of metasomatized lithosphere
\citep{Hallidayetal1995,NiuOHara2003,Piletetal2008}. Central Africa is
made of the composite Archean to Palaeoproterozoic West Africa and
Congo cratons bordered by Pan-African belts and Variscan orogenic belts
[Figure~\ref{fig1}; \citealp{BessolesTrompette1980,Nzentietal1988}]. 

\begin{figure*}
\includegraphics{fig01}
\caption{\label{fig1}(A) Tectonic Map of Africa: Location map of the
Cameroon Volcanic Line (CVL). The main geologic features of Africa are
indicated. (B) Structural map of Central Africa Rift System 
\citep[modified after][]{Kogbe1981, Milesietal2010} showing the
Cameroon Volcanic Line (CVL) and its extension in chad. The names of
the main Early Cretaceous intracontinental rifts are indicated. SAB ${=}$ 
Sud Algerian Basin; MB ${=}$  Murzuk Basin; IB ${=}$  Iullemeden Basin; LCB ${=}$ 
Lake Chad Basin; BT ${=}$  Benue Trough; MK ${=}$  Mayo Kebbi; B ${=}$ 
Ba\"{i}bokoum; WAC ${=}$  West African Craton; CC ${=}$  Congo Craton; HN ${=}$ 
Hoggar Shield; NS ${=}$  Nigerian Shield; EAR ${=}$  East African Rift; POKH ${=}$ 
Poli--Ounianga--Kebir heavy \citep{Louis1970}.}
\vspace*{2pt}
\end{figure*}


The published petrological data from the Cameroon Volcanic Line (CVL;
Figure~\ref{fig1}A) \mbox{document} a typical bimodal series, marked by
abundant mafic and felsic lavas and relatively sporadic occurrence in
intermediate terms such as mugearites and benmoreites
\citep{Marzolietal1999,Marzolietal2000}. In contrast to this general
trend, volcanoes at Bioko Island and Mount Cameroon are solely made of
basaltic lavas, and a few massifs such as Manengouba or
Bamenda-Bambouto Mountains display a complete range from mafic to
felsic rocks \citep{KagouDongmoetal2001,Kamgangetal2007,Kamgangetal2008}.
Mafic lavas composition comprises basanites, basalts, and hawaiites
while felsic lavas include trachytes, rhyolites and/or phonolites in a
given volcanic massif. According to the available geochronological
data, the oldest volcanic activity in the CVL (73 to 40~Ma) corresponds
to anorogenic complexes, while the youngest ($0.48 \pm 0.01$~Ma to
present) are well expressed in volcanic massifs along the
ocean-continent \mbox{boundary} and in the oceanic sector
\citep{Marzolietal2000,KagouDongmoetal2010}. The parental magmas
generally display OIB features consistent with mantle plume activity,
and originate from partial melting of various primitive mantle sources
influenced by components such as HIMU or FOZO
\citep{Fitton1987,Hallidayetal1990,Mbassaetal2012}. 

The CVL is marked by a NE--SW trending positive gravity anomaly in the
continuity of the ''Poli--Ounianga--Kebir heavy line'' extending from
Cameroon to Chad [Figure~\ref{fig1}B; \citealp{Louis1970}]. This
gravity anomaly marks the northern border of the Adamawa plateau 
\citep{PoudjomDjomanietal1992,PoudjomDjomani1997}. The crustal structure
beneath the Cameroon Volcanic line is marked by a 10 km thick crust
that contrasts with the 40 km thick crust along its edges 
\citep{PoudjomDjomani1997,TokamKamga2010,Eloumalaetal2014}. The mantle
transitional zone beneath the CVL is approximately located at $250 \pm
10$~km \citep{Reuschetal2010}.\looseness=-1 

The West and Central African Rift System (WCARS), extending from the
Hoggar to Cameroon and Chad, is defined as a complex set of
interconnected pull-apart, wrench and extension basins
\citep{Fairheadetal2013}, associated with two magmatic series, of
Mesozoic and Cenozoic age, respectively \citep{Yeetal2017}. It is
further delineated by Meso-Cenozoic sedimentary deposits along the
margins of intracratonic basins (southern Algeria, Murzug, Iullemmeden
and Lake Chad basins), either on Neoproterozoic and Palaeozoic
sequences or directly on the Pan-African basement 
[\citealp{Yeetal2017}; Figure~\ref{fig1}A,B]. 

In this paper, we present mineralogical and whole rock geochemical data
of the unstudied Iriba basanites in order to constrain their
petrogenesis and discuss their link with the CVL and the Central Africa
Rift. 
\vspace*{-3pt}

\section{Geological context of the west and central Africa rift
system}\label{sec2}

According to the detailed work of \citet{Genik1993,Guiraud1Maurin992,
Mchargueetal1992, Fairheadetal2013}, 
the evolution of the WCARS can be summarized as tree stages: (1) Early
Cretaceous opening of the rift marked by half basins in northern
Nigeria and wester Sudan with an E--W extension direction and opening of
the Benue trough in Cameroon and Sudan with a NE--SW direction of
extension, (2) Mid-Cretaceous sag basin marked by deposition of
sediments uncomfortably over the rift series, (3) Late Cretaceous to
present-day rift inversion marked by upright folds, reverse and
strike-slip faults and deposition of terrigenous sediments.

The Central Africa Rift System is parallel to the LVC in Cameroon and
subdivides in (i) a northern branch delineated by the Lake Chad Basin
and continuing towards the Tibesti massif, and (ii) an eastern branch
delineated by sedimentary deposits south of the Guera massif and
continuing towards the Iriba region in the Ouadda\"{i} massif
(Figure~\ref{fig1}B).
\vspace*{-3pt}


\subsection{Geological context of the central Africa rift system in
chad}\label{sec21}


The geology of Chad consists of a Precambrian basement covered by
Mesozoic to Cenozoic sedimentary sequences deposited in the Iullemmeden
and Chad intracratonic basins and in the Benue and L\'{e}r\'{e} rift
basins that are also marked by the emplacement of volcanic rocks
[Figure~\ref{fig1}B; 
\citealp{BessolesTrompette1980,Guiraud1Maurin992,Milesietal2010}]. The
volcanic rocks of Lake Chad (Hadjer Lamis), emplaced in the
Cretaceous--Paleocene transition, are composed mainly of peralkaline
rhyolites produced by fractional crystallization of parent magmas
corresponding to alkaline basaltic melts, originating from a
metasomatized mantle source \citep{Mbowouetal2012,Shellnuttetal2016}.
Based on geochronological and isotopic data, \citet{Shellnuttetal2016}
suggest that silicic volcanic rocks from Lake Chad region are related
to Late Cretaceous extensional volcanism in the Termit Basin. They
conclude that magmatism was structurally controlled by Pan-African
suture zones that were reactivated during the opening of the central
Atlantic Ocean. 

The Tibesti Volcanic Province (TVP; Figure~\ref{fig1}B) represents the
main volcanic activity in Chad. It is made of a variety of magmatic
series including (i) Miocene Plateau volcanism consisting of flood
basalts and silicic lava, with intercalated ignimbritic sheets emplaced
from 17 to 8~Ma; (ii) basaltic cinder cones and associated lava flows
emplaced from 7 to 5~Ma; (iii) Late Miocene large central composite
volcanoes located along a major NNE--SSW trending fault; (iv) Large
ignimbritic volcanoes (7--0.43~Ma); and (vi) the Tarso Toussid\'{e}
Volcanic Complex. The volcanic rocks are localized along the
southwestern NW--SE Tassilian flexure and an eastern NE--SW to NNE--SSW
major fault zone \citep{Denieletal2015}. Geochemical data from
\citet{GourgaudVincent2004}
revealed the presence of alkaline volcanic rocks
in Emi Koussi locality, forming a bimodal series made up of (1) a
silica saturated suite, composed mainly of trachytes with a few
trachyandesites and (2) a silica under-saturated suite, composed of
basalts and phonolites. Overall, available geochemical and petrological
data show that the volcanic activity is generally expressed by alkaline
to peralkaline lavas ranging from basanites to arfvedsonite ${\pm}$
acmite-bearing rhyolites, all resulting from fractional crystallization
of alkaline melts, probably originated from a metasomatized mantle
source \citep{Vicatetal2002,GourgaudVincent2004}. 

\begin{figure*}
\includegraphics{fig02}
\caption{\label{fig2}Geological map of Iriba and the surrounding area,
extracted from the geological reconnaissance map of the Equatorial
African States, Ni\'{e}r\'{e} sheet N\textdegree~ND 34 NE 0.80-E.81 
\citep{GsellSonnet1962}.}
\end{figure*}

In the northern part of Ouadda\"{i} massif, theralites and basalts are
exposed \citep{GsellSonnet1962}. The Iriba basalts (Figure~\ref{fig2})
outcrop in dome-shape or massive peak depressions from which emerge a
series of superimposed flows of limited thickness (1~to 2~m), laying on
plateau formations resulting from the weathering of basement rocks, or
Paleozoic sandstones \citep{GsellSonnet1962}. Seven (7) samples have
been selected for petrological and geochemical study.

\section{Analytical methods}\label{sec3}

Powders and thin sections of selected rock samples were prepared at the
laboratory Geosciences Environment Toulouse, (GET-OMP-University of
Toulouse 3, France) for geochemical and mineralogical analyses.
Approximately 200 to 500~g of each sample was ground in a steel jaw
crusher and then pulverized in an agate ball mill. The powders were
then digested using an alkaline fusion procedure where the powder was
mixed with lithium metaborate and melted to produce a glass pellet. The
pellet was digested in dilute nitric acid prior to analysis. Analyses
and digestions were carried out at the Service d'Analyse des Roches et
des Min\'{e}raux (SARM, CRPG, France); major elements were determined
by ICP-OES while trace elements were determined by ICP-MS following the
procedure described in \citet{Carignanetal2001}.


The major elements minerals compositions were performed at the Centre
de microcaract\'{e}risation Raimond Castaing (Toulouse, France), using
a CAMECA SX-Five Electron Probe Microanalyser. All analyzed samples
were carbon coated (15~nm thick layer, density 2.25~g/cm$^{3}$) before
being introduced into the electron microprobe. The beam conditions were
15~kV accelerating voltage and 10 or 20 nA probe current. The synthetic
and natural standards used for calibration were: albite (Na), corundum
(Al), wollastonite (Si, Ca), sanidine (K), pyrophanite (Mn, Ti),
hematite (Fe), periclase (Mg), Ni metal (Ni), Cr$_{2}$O$_{3}$ (Cr) and
reference zircon (Zr). Element and background counting times for most
analyzed elements were 10 s or 5 s (for Na and K), whereas, peak
counting times were 120 s for Cr and 80 to 100 s for Ni and 240 s for
Zr. Detection limits were 70~ppm for Cr and Zr and 100~ppm for Ni.

\section{Petrography of the Iriba basanites}\label{sec4}

The Iriba basanites, are intrusive in the Paleozoic sandstones and are
exposed as slabs or blocks within the plateau formations
(Figure~\ref{fig3}a,b). The Iriba basanites are characterized by a
microlitic porphyritic texture (Figure~\ref{fig3}c,d,f). Olivine,
which constitutes 55 vol\% of the rock, is present as sub-automorphic
to xenomorphic phenocrysts and as microlites in the fine-grained
groundmass. Olivine phenocrysts are fractured and altered into
serpentine. Clinopyroxene represents 35 vol\% of the rock and occurs
as euhedral phenocrysts with clear cleavages and signs of corrosion.
Plagioclase represents about 2--5~vol\% and occurs as euhedral crystals
with a dusty appearance due to the alteration. Opaque minerals 
(1--4~vol\%) are sub-rounded or angular and generally included in olivine
and clinopyroxene phenocrysts. Rare apatite (${<}$2~vol\%) is the only
accessory mineral. 

\begin{figure*}
\includegraphics{fig03}
\caption{\label{fig3}Different outcropping and representative
photomicrographs of Iriba basanites. (a) Blocks; (b)~slabs in the
Paleozoic sandstone and plateau formations. (c) Porphyric texture of
basanites; (d) and (e)~Olivine phenocrysts and microlitic groundmass;
(f) corroded and resorbed clinopyroxene phenocryst and microlitic
groundmass.}
\end{figure*}

\section{Mineral chemistry}\label{sec5}

\subsection{Olivine}\label{sec51}
Olivine phenocrysts are magnesian
(Fo$_{69.52\ndash 90.28}${\unskip\break}Fa$_{09.72\ndash 39.48}$) with CaO ranging from 0.04 to
0.31~wt\%. They are mostly chemically homogeneous but show slight
chemical zoning with Fe enrichment from the core to the rim (Table~\ref{tab1}).
They are characterized by low MnO (0.05--0.31~wt\%) and NiO 
(${\leq}$0.45~wt\%) contents.

\begin{table*}
\caption{\label{tab1}Representative microprobe analyses of olivines
from Iriba lavas}
\tabcolsep2pt
\begin{tabular}{lccccccccccccc}
\thead
Sample & RB8 & RB8 & RB8 & RB8 & RB8 & RB8 & RB8 & RB3 & RB3 & RB3 & RB3 & RB3 & RB3 \\
Analysis & C1 & C2 & C3 & C4 & C5 & C5 & C5 & C1 & C3 & C4 & C4 & C5 & C5 \\
Position & & & & & r & c & & & & & & & \\
\endthead
$\mathrm{SiO}_2$ & 39.71 & 40.24 & 40.01 & 39.69 & 40.33 & 39.80 & 38.72 & 40.17 & 40.32 & 38.90 & 39.37 & 40.97 & 39.59 \\
$\mathrm{TiO}_2$ & 0.00 & 0.00 & 0.00 & 0.02 & 0.00 & 0.00 & 0.03 & 0.00 & 0.00 & 0.03 & 0.00 & 0.01 & 0.00 \\
$\mathrm{Al}_2 \mathrm{O}_3$ & 0.05 & 0.02 & 0.00 & 0.14 & 0.00 & 0.04 & 0.09 & 0.05 & 0.04 & 0.03 & 0.03 & 0.03 & 0.04 \\
FeO & 16.55 & 15.07 & 16.22 & 17.15 & 12.79 & 13.40 & 20.64 & 14.50 & 16.92 & 19.71 & 19.73 & 14.00 & 17.04 \\
MnO & 0.24 & 0.16 & 0.25 & 0.05 & 0.28 & 0.18 & 0.27 & 0.20 & 0.15 & 0.27 & 0.25 & 0.21 & 0.31 \\
MgO & 44.27 & 44.96 & 44.02 & 43.37 & 46.97 & 46.30 & 40.59 & 45.79 & 43.91 & 41.77 & 41.68 & 45.83 & 43.71 \\
CaO & 0.22 & 0.22 & 0.20 & 0.12 & 0.06 & 0.00 & 0.40 & 0.30 & 0.14 & 0.16 & 0.11 & 0.22 & 0.16 \\
Total & 101.04 & 100.67 & 100.70 & 100.54 & 100.43 & 99.72 & 100.74 & 101.01 & 101.48 & 100.87 & 101.17 & 101.27 & 100.85 \\
Si & 0.99 & 1.00 & 1.00 & 1.00 & 1.00 & 0.99 & 0.99 & 0.99 & 1.00 & 0.99 & 1.00 & 1.01 & 0.99 \\
Ti & 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 \\
$\mathrm{Al}^{\mathrm{IV}}$ & 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 \\
$\mathrm{Fe}^{2+}$ & 0.35 & 0.31 & 0.34 & 0.36 & 0.26 & 0.28 & 0.44 & 0.30 & 0.35 & 0.42 & 0.42 & 0.29 & 0.36 \\
$\mathrm{Mn}^{2+}$ & 0.01 & 0.00 & 0.01 & 0.00 & 0.01 & 0.00 & 0.01 & 0.00 & 0.00 & 0.01 & 0.01 & 0.00 & 0.01 \\
Mg & 1.65 & 1.67 & 1.64 & 1.63 & 1.73 & 1.72 & 1.55 & 1.69 & 1.63 & 1.58 & 1.57 & 1.68 & 1.64 \\
Ca & 0.01 & 0.01 & 0.01 & 0.00 & 0.00 & 0.00 & 0.01 & 0.01 & 0.00 & 0.00 & 0.00 & 0.01 & 0.00 \\
Total & 3.00 & 2.99 & 2.99 & 2.99 & 3.00 & 3.00 & 3.00 & 3.00 & 2.99 & 3.00 & 3.00 & 2.99 & 3.00 \\
Fa & 17.33 & 15.83 & 17.13 & 18.15 & 13.25 & 13.97 & 22.19 & 0.15 & 17.77 & 20.93 & 20.98 & 
{14.63} & 17.94 \\
Fo & 82.66 & 84.17 & 82.87 & 81.84 & 86.75 & 86.03 & 77.81 & 84.92 & 82.23 & 79.07 & 79.02 & 85.37 & 82.06 
\end{tabular}
{\vspace*{6pt}}
\begin{tabular}{lccccccccccccc}
\thead
Sample: & RB2 & RB2 & RB2 & RB2 & RB2 & RB7b & RB7b & RB7b & RB7b & RB7b & RB7b & RB7b & RB7b \\
Analysis & C4 & C4 & C3 & C3 & C2 & C2 & C1 & C1 & C1 & C2 & C2 & C2 & C3 \\
Position: & & r & & & & r & c & c & & & & & r \\
\endthead
$\mathrm{SiO}_2$ & 40.04 & 40.67 & 40.20 & 40.38 & 39.76 & 39.53 & 40.54 & 39.44 & 39.39 & 40.79 & 40.43 & 40.37 & 40.41 \\
$\mathrm{TiO}_2$ & 0.00 & 0.02 & 0.00 & 0.00 & 0.00 & 0.01 & 0.01 & 0.02 & 0.00 & 0.00 & 0.00 & 0.02 & 0.01 \\
$\mathrm{Al}_2 \mathrm{O}_3$ & 0.03 & 0.02 & 0.12 & 0.03 & 0.05 & 0.08 & 0.06 & 0.03 & 0.03 & 0.09 & 0.04 & 0.06 & 0.04 \\
FeO & 12.33 & 12.38 & 15.49 & 15.79 & 18.77 & 15.13 & 14.46 & 19.59 & 19.64 & 14.71 & 14.24 & 14.93 & 14.25 \\
MnO & 0.19 & 0.17 & 0.21 & 0.22 & 0.31 & 0.24 & 0.25 & 0.20 & 0.30 & 0.21 & 0.25 & 0.23 & 0.19 \\
MgO & 47.97 & 47.95 & 44.68 & 44.65 & 42.25 & 45.51 & 46.02 & 42.15 & 41.98 & 45.94 & 46.35 & 45.57 & 46.63 \\
CaO & 0.18 & 0.10 & 0.24 & 0.23 & 0.25 & 0.21 & 0.22 & 0.12 & 0.11 & 0.19 & 0.30 & 0.30 & 0.28 \\
Total & 100.74 & 101.31 & 100.94 & 101.30 & 101.39 & 100.71 & 101.56 & 101.55 & 101.45 & 101.93 & 101.61 & 101.48 & 101.81 \\
Si & 0.98 & 0.99 & 1.00 & 1.00 & 1.00 & 0.99 & 1.00 & 0.99 & 0.99 & 1.00 & 0.99 & 1.00 & 0.99 \\
Ti & 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 \\
Al$^{\text {IV }}$ & 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 \\
$\mathrm{Fe}^{2+}$ & 0.25 & 0.25 & 0.32 & 0.33 & 0.39 & 0.32 & 0.30 & 0.41 & 0.41 & 0.30 & 0.29 & 0.31 & 0.29 \\
$\mathrm{Mn}^{2+}$ & 0.00 & 0.00 & 0.00 & 0.00 & 0.01 & 0.01 & 0.01 & 0.00 & 0.01 & 0.00 & 0.01 & 0.00 & 0.00 \\\tabtwosplittabular
Mg & 1.76 & 1.75 & 1.66 & 1.65 & 1.58 & 1.69 & 1.69 & 1.58 & 1.58 & 1.68 & 1.70 & 1.68 & 1.71 \\
Ca & 0.00 & 0.00 & 0.01 & 0.01 & 0.01 & 0.01 & 0.01 & 0.00 & 0.00 & 0.00 & 0.01 & 0.01 & 0.01 \\
Total & 3.01 & 3.00 & 2.99 & 2.99 & 2.99 & 3.01 & 3.00 & 3.00 & 3.00 & 2.99 & 3.00 & 3.00 & 3.00 \\
Fa & 12.60 & 12.65 & 16.28 & 16.55 & 19.95 & 15.72 & 14.98 & 20.68 & 20.79 & 15.23 & 14.70 & 15.52 & 0.1463 \\
Fo & 87.40 & 87.35 & 83.72 & 83.45 & 80.05 & 84.28 & 85.01 & 79.32 & 79.21 & 84.77 & 85.30 & 84.47 & 85.37 
\end{tabular}
{\vspace*{6pt}}
\tabcolsep2pt\begin{tabular}{lccccccccccccc}
\thead
Sample: & RB7b & RB7b & RB7b & RB7a & RB7a & RB7a & RB7a & RB7a & RB7a & RB7a & RB7a & RB7a & RB7a\\
Analysis & C3 & C4 & C4 & C5 & C5 & C4 & C4 & C4 & C3 & C3 & C1 & C1 & C1 \\
Position: & r & r & c & r & c & r & c & m & r & c & r & c & r \\
\endthead
 $\mathrm{SiO}_2$ & 38.91 & 39.99 & 40.01 & 41.16 & 40.95 & 41.17 & 41.28 & 39.77 & 41.80 & 41.47 & 39.57 & 39.24 & 39.64 \\
 $\mathrm{TiO}_2$ & 0.00 & 0.00 & 0.00 & 0.00 & 0.01 & 0.00 & 0.00 & 0.03 & 0.00 & 0.00 & 0.02 & 0.00 & 0.02 \\
 $\mathrm{Al}_2 \mathrm{O}_3$ & 0.04 & 0.06 & 0.02 & 0.03 & 0.04 & 0.06 & 0.06 & 0.06 & 0.04 & 0.03 & 0.05 & 0.07 & 0.07 \\
 FeO & 21.59 & 14.64 & 14.71 & 9.56 & 12.29 & 12.97 & 12.28 & 15.93 & 9.80 & 9.75 & 20.15 & 18.43 & 19.67 \\
 MnO & 0.26 & 0.19 & 0.20 & 0.13 & 0.20 & 0.17 & 0.18 & 0.24 & 0.14 & 0.15 & 0.22 & 0.27 & 0.25 \\
 MgO & 40.65 & 46.61 & 45.88 & 49.84 & 47.67 & 46.57 & 47.46 & 44.29 & 49.23 & 49.35 & 41.42 & 42.89 & 42.06 \\
 CaO & 0.17 & 0.16 & 0.10 & 0.06 & 0.21 & 0.18 & 0.18 & 0.31 & 0.09 & 0.09 & 0.27 & 0.29 & 0.23 \\
 Total & 101.62 & 101.65 & 100.92 & 100.78 & 101.37 & 101.12 & 101.44 & 100.63 & 101.10 & 100.84 & 101.70 & 101.19 & 101.94 \\
 Si & 0.99 & 0.99 & 0.99 & 1.00 & 1.00 & 1.01 & 1.00 & 1.00 & 1.01 & 1.00 & 1.00 & 0.99 & 1.00 \\
 Ti & 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 \\
 $\mathrm{Al}^{\mathrm{IV}}$ & 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 \\
 $\mathrm{Fe}^{2+}$ & 0.46 & 0.30 & 0.31 & 0.19 & 0.25 & 0.27 & 0.25 & 0.33 & 0.20 & 0.20 & 0.43 & 0.39 & 0.41 \\
 $\mathrm{Mn}^{2+}$ & 0.01 & 0.00 & 0.00 & 0.00 & 0.00 & 0.00 & 0.00 & 0.01 & 0.00 & 0.00 & 0.00 & 0.01 & 0.01 \\
 Mg & 1.54 & 1.71 & 1.70 & 1.80 & 1.73 & 1.70 & 1.72 & 1.65 & 1.77 & 1.78 & 1.56 & 1.61 & 1.58 \\
 Ca & 0.00 & 0.00 & 0.00 & 0.00 & 0.01 & 0.00 & 0.00 & 0.01 & 0.00 & 0.00 & 0.01 & 0.01 & 0.01 \\
 Total & 3.01 & 3.01 & 3.00 & 2.99 & 2.99 & 2.98 & 2.98 & 3.00 & 2.98 & 2.99 & 3.00 & 3.01 & 3.00 \\
 Fa & 22.95 & 14.98 & 15.24 & 9.71 & 12.63 & 13.51 & 12.67 & 16.79 & 10.04 & 9.98 & 21.44 & 19.42 & 20.78 \\
 Fo & 77.04 & 85.02 & 84.76 & 90.28 & 87.36 & 86.49 & 87.32 & 83.21 & 89.95 & 90.02 & 78.56 & 80.58 & 79.22
\botline
\end{tabular}
\tabnote{$\mathrm{C}=$ core; $\mathrm{r}=$ rim; $\mathrm{m}=$ microlite; $\mathrm{FeO}=\mathrm{FeO}$ total.}
\vspace*{5pt}
\end{table*}

\subsection{Clinopyroxene}\label{sec52}

The selected pyroxenes analyses are reported in Table~\ref{tab2}.
Clinopyroxenes (Cpx) are Ca- and Mg- rich (Wo$_{38.8\ndash
53.7}$--En$_{33.4\ndash 43.2}$) with variable Fe composition
(Fs$_{12.4\ndash 23}$). According to the classification of
\cite{Morimotoetal1988}, the Cpx of Iriba basanites have a composition
of diopside (Wo$_{47.22\ndash 49.41}$En$_{33.37\ndash
38.01}$Fs$_{12.36\ndash 17.76}$) and  augite (Wo$_{38.75\ndash
43.07}$En$_{38.23\ndash 43.22}$Fs$_{13.71\ndash 23.02}$)
(Figure~\ref{fig4}). Some clinopyroxene crystals display Wo contents
${>}$ 50\%. Similar Cpx with Wo ${>}$ 50\% are commonly found in CVL
such as basanites from Bamenda Mountains  \citep{Kamgangetal2008},
Mbengwi \citep{Mbassaetal2012} and Mount Bambouto \citep{Wandjietal2000}. 
The phenocrysts are variably rich in  Al$_{2}$O$_{3}$ (2.96--9.94~wt\%)
and TiO$_{2}$ (0.4--4.49~wt\%) with the highest contents of both Al and
Ti in the crystals rims. Their Mg\# vary from 66 to 88. 

\begin{figure}
\includegraphics{fig04}
\caption{\label{fig4}Ca-pyroxenes of Iriba basanites in Wo-En-Fs
ternary diagram of \cite{Morimotoetal1988}. Wo ${=}$ Wollastonite;
En ${=}$ Enstatite; Fs ${=}$ ferrosilite.}
\end{figure}

\begin{table*}
\caption{\label{tab2}Representative microprobe analyses of
clinopyroxenes from Iriba lavas}
\begin{tabular}{lccccccccc}
\thead
Sample & RB3 & RB2 & RB2 & RB2 & RB2 & RB2 & RB2 & RB7a & RB7a\\
Analysis & C2 & C3 & C2 & C2 & C2 & C2 & C1 & C2 & C1\\
Positions && c &&&&& b && b\\
\endthead
SiO$_2$ & 51.19 & 48.16 & 50.75 & 49.95 & 49.90 & 50.31 & 50.08 & 45.67 & 43.29\\
TiO$_2$ & 1.17 & 2.08 & 0.47 & 0.70 & 0.64 & 0.66 & 0.68 & 3.31 & 4.49\\
Al$_2$O$_3$ & 2.96 & 6.89 & 3.91 & 5.19 & 4.94 & 4.88 & 4.88 & 8.13 & 9.94\\
FeO & 8.40 & 7.29 & 13.44 & 10.48 & 10.85 & 10.08 & 10.17 & 7.26 & 7.31\\
MnO & 0.17 & 0.17 & 0.42 & 0.22 & 0.24 & 0.22 & 0.23 & 0.14 & 0.03\\
MgO & 15.16 & 12.65 & 12.92 & 11.28 & 11.62 & 11.42 & 11.51 & 11.97 & 11.05\\
CaO & 21.02 & 22.88 & 18.22 & 22.98 & 22.21 & 22.82 & 22.77 & 24.32 & 24.44\\
Na$_2$O & 0.56 & 0.74 & 0.55 & 0.82 & 0.81 & 0.72 & 0.78 & 0.36 & 0.45\\
Total & 100.63 & 100.86 & 100.68 & 101.62 & 101.21 & 101.11 & 101.10 & 101.16 & 101.00\\
Si & 1.89 & 1.78 & 1.90 & 1.86 & 1.86 & 1.87 & 1.87 & 1.70 & 1.62\\
Ti & 0.03 & 0.06 & 0.01 & 0.02 & 0.02 & 0.02 & 0.02 & 0.09 & 0.13\\
Al$^{\mathrm{IV}}$ & 0.11 & 0.22 & 0.10 & 0.14 & 0.14 & 0.13 & 0.13 & 0.30 & 0.38\\
Al$^{\mathrm{VI}}$ & 0.02 & 0.08 & 0.07 & 0.08 & 0.08 & 0.09 & 0.08 & 0.06 & 0.06\\
Fe$^{3+}$ & 0.09 & 0.10 & 0.05 & 0.12 & 0.12 & 0.08 & 0.10 & 0.12 & 0.14\\
Fe$^{2+}$ & 0.17 & 0.12 & 0.37 & 0.21 & 0.22 & 0.24 & 0.22 & 0.10 & 0.08\\
Mn$^{2+}$ & 0.01 & 0.01 & 0.01 & 0.01 & 0.01 & 0.01 & 0.01 & 0.00 & 0.00\\
Mg & 0.84 & 0.70 & 0.72 & 0.62 & 0.65 & 0.63 & 0.64 & 0.66 & 0.62\\
Ca & 0.83 & 0.91 & 0.73 & 0.91 & 0.89 & 0.91 & 0.91 & 0.97 & 0.98\\
Na & 0.04 & 0.05 & 0.04 & 0.06 & 0.06 & 0.05 & 0.06 & 0.03 & 0.03\\
Total & 4.03 & 4.03 & 4.01 & 4.03 & 4.03 & 4.02 & 4.03 & 4.04 & 4.04\\
Wo & 43.07 & 49.41 & 38.75 & 48.86 & 47.22 & 48.81 & 48.55 & 52.02 & 53.66\\
En & 43.22 & 38.01 & 38.23 & 33.37 & 34.37 & 33.99 & 34.14 & 35.62 & 33.76\\
Fs & 13.71 & 12.58 & 23.02 & 17.76 & 18.41 & 17.20 & 17.31 & 12.36 & 12.58
\botline
\end{tabular}
\xxtabnote{C ${=}$ core; b ${=}$ rim; FeO ${=}$ FeO total.}
\vspace*{-3pt}
\end{table*}

\subsection{Feldspars}\label{sec53}

Feldspar crystals are scarce in the studied basanites and correspond to
Na-rich plagioclase (An$_{2.09}$Ab$_{80.10}$Or$_{17.80}$)
(Table~\ref{tab3}). 

\begin{table*}
\caption{\label{tab3}Representative microprobe analyses of feldspars
and opaque minerals from Iriba lavas}
\tabcolsep7pt
\begin{tabular}{ccccccc}
\thead
\multicolumn{2}{c}{Feldspars} && \multicolumn{4}{c}{Opaque minerals}
\\\cline{1-2}\cline{4-7}
Sample & RB2-C3 && Sample & RB3 & RB2 & RB7a\\
Analysis & C3 && Analysis & C5 & C2 & C3\\
Position & r && Position &&&\\
\endthead
SiO$_2$ & 68.55 && SiO$_2$ & 0.77 & 0.00 & 0.14\\
Al$_2$O$_3$ & 19.71 && TiO$_2$ & 15.81 & 0.05 & 0.97\\
FeO & 0.15 && Al$_2$O$_3$ & 1.63 & 0.02 & 46.49\\
CaO & 0.46 && FeO & 71.32 & 78.18 & 20.30\\
Na$_2$O & 9.74 && Fe$_2$O$_3$ & 7.93 & 8.69 & 2.26\\
K2O & 3.29 && Cr$_2$O$_3$ & 0.32 & 0.01 & 12.24\\
Total & 101.90 && MnO & 0.59 & 0.14 & 0.18\\
Si & 11.91 && MgO & 1.50 & 0.00 & 17.29\\
Al & 4.04 && NiO & 0.01 & 0.50 & 0.25\\
Fe total & 0.02 && Total & 99.88 & 87.59 & 100.12\\
Ca & 0.09 && Ti & 0.39 & 0.00 & 0.02\\
Na & 3.28 && Al & 0.06 & 0.00 & 1.16\\
K & 0.73 && Fe$^{2+}$ & 0.32 & 0.00 & 0.00\\
Total & 20.07 && Fe$^{3+}$ & 1.64 & 1.96 & 0.36\\
Or & 17.80 && Mg & 0.07 & 0.00 & 0.54\\
Ab & 80.10 && Total & 2.48 & 1.96 & 2.08\\
An & 2.09 && xMg & 3.58 & 0.00 & 60.08\\
&&& xFe & 95.61 & 99.82 & 39.57\\
&&&  xMn & 0.80 & 0.18 & 0.35
\botline
\end{tabular}
\xxxtabnote{$r=\mathrm{rim}$.}
\vspace*{15pt}
\end{table*}


\subsection{Opaque minerals}\label{sec54}

Opaque minerals (Table~\ref{tab3}) are spinels
(Mg$_{3.58\ndash60.08}$Fe$_{39\ndash99}$Mn$_{0.18\ndash0.80}$). They
correspond to ulvospinel, magnetite and magnesioferrite according to
the classification of \cite{HaggertyTompkins1983} (Figure~\ref{fig5}).
The samples RB2 and RB7a are characterized by TiO$_{2}$ contents 
${>}$0.20~wt\% (0.95--15.81~wt\%) with Fe$^{2+}$/Fe$^{3+}$ ratios
${<}$2. These characteristics are indicative of their non-mantelic
origin according to \citet{Kamenetskyetal2001}. They are also
characterised by Cr\# (100Cr/(Cr+Al)) ranging between 11.24 and 25.12
and are fairly NiO-rich (0.25--0.50~wt\%) except RB3 which is NiO-poor
(0.01~wt\%).

\begin{figure}
\includegraphics{fig05}
{\vspace*{-2pt}}
\caption{\label{fig5}Composition of opaque minerals after 
\citet{HaggertyTompkins1983}.}
{\vspace*{-2pt}}
\end{figure}


\section{Whole rock geochemistry}\label{sec6}
\subsection{Major elements}\label{sec61}
Chemical analyses (major and trace elements) of Iriba lavas are
reported in Table~\ref{tab4}. SiO$_{2}$ contents range from 41 to
46~wt\% and TiO$_{2}$ contents from 2.15 to 2.77~wt\%, with high
K$_{2}$O+Na$_{2}$O (4.97--6.59~wt\%) and MgO contents expressed by
Mg\# (100 Mg/{[}Mg + Fe{]}) ranging between 56 and 65. According to the
TAS diagram [after \citealp{LeBasetal1986}; Figure~\ref{fig6}], they
are classify as basanites. All the samples are alkaline with a sodic
tendency according to criteria defined by \citet{IrvineBaragar1971} and
\citet{Middlemost1975}. In the major elements Harker diagrams
(Figure~\ref{fig7}a), Al$_{2}$O$_{3}$ and K$_{2}$O contents are
positively correlated with SiO$_{2}$ while MgO, TiO$_{2}$, CaO and
P$_{2}$O$_{5}$ display a negative correlation.

\begin{table*}
\caption{\label{tab4}Whole rock composition of Iriba basanites\vspace*{-3pt}}
\fontsize{9.3}{10}\selectfont
\begin{tabular}{lccccccc}
\thead
Sample & RB2 & RB3 & RB4 & RB5 & RB6 & RB7 & RB8\\
\endthead
SiO$_2$ & 45.53 & 43.50 & 42.38 & 44.73 & 42.66 & 41.60 & 44.29\\
Al$_2$O$_3$ & 13.91 & 13.15 & 13.09 & 13.69 & 13.14 & 12.91 & 13.54\\
Fe$_2$O$_3$t & 12.58 & 12.92 & 13.84 & 12.63 & 13.98 & 13.39 & 14.23\\
MnO & 0.21 & 0.20 & 0.25 & 0.18 & 0.23 & 0.19 & 0.20\\
MgO & 8.86 & 10.41 & 9.38 & 10.15 & 9.01 & 12.27 & 8.92\\
CaO & 9.57 & 11.31 & 10.58 & 9.85 & 10.67 & 11.27 & 10.34\\
Na$_2$O & 4.74 & 4.10 & 5.11 & 3.62 & 5.21 & 4.02 & 3.67\\
K$_2$O & 1.78 & 1.05 & 1.48 & 2.03 & 1.20 & 0.79 & 1.30\\
TiO$_2$ & 2.15 & 2.70 & 2.67 & 2.51 & 2.77 & 2.72 & 2.55\\
P$_2$O$_5$ & 0.67 & 0.66 & 1.23 & 0.61 & 1.13 & 0.83 & 0.97\\
Total & 100.00 & 100.00 & 100.01 & 100.00 & 100.00 & 99.99 & 100.01\\
Mg\# & 58.42 & 61.65 & 57.46 & 61.57 & 56.22 & 64.63 & 55.56\\
ppm&&&&&&&\\
Rb & 36.92 & 25.88 & 24.18 & 42.39 & 67.47 & 50.01 & 50.87\\
Sr & 969.28 & 742.93 & 1214.25 & 829.19 & 1189.9 & 887.15 & 985.83\\
Ba & 592.63 & 476.46 & 665.51 & 446.71 & 627.85 & 597.08 & 510.53\\
V & 175.43 & 244.85 & 203.58 & 211.12 & 201.81 & 236.67 & 181.67\\
Cu & 58.23 & 60.31 & 45.12 & 57.50 & 45.22 & 56.24 & 53.44\\
Cr & 48.63 & 57.48 & 45.92 & 53.86 & 50.72 & 59.68 & 54.30\\
Co & 470.93 & 578.19 & 393.64 & 659.70 & 477.27 & 548.05 & 306.75\\
Ni & 271.06 & 316.59 & 191.63 & 372.14 & 281.87 & 349.86 & 207.41\\
Sc & 18.63 & 23.76 & 19.29 & 22.17 & 18.81 & 23.74 & 16.58\\
Zn & 122.23 & 105.36 & 130.63 & 105.44 & 189.20 & 104.97 & 148.10\\
Y & 25.41 & 20.28 & 31.83 & 21.46 & 30.58 & 25.42 & 22.96\\
Zr & 227.56 & 194.78 & 374.16 & 214.35 & 328.14 & 164.37 & 241.31\\
Hf & 4.72 & 4.37 & 7.52 & 4.65 & 6.73 & 3.95 & 5.19\\
Ta & 3.30 & 3.42 & 6.66 & 3.37 & 5.93 & 3.09 & 3.64\\
Nb & 50.96 & 49.53 & 94.20 & 47.24 & 84.81 & 44.96 & 53.44\\
La & 40.07 & 34.26 & 69.95 & 33.21 & 67.28 & 39.93 & 48.51\\
Ce & 79.29 & 67.99 & 134.75 & 66.12 & 130.58 & 81.20 & 96.50\\
Pr & 9.19 & 8.01 & 15.29 & 7.79 & 14.85 & 9.71 & 11.15\\
Nd & 36.30 & 32.07 & 58.65 & 31.32 & 57.17 & 39.25 & 44.43\\
Sm & 7.59 & 6.73 & 11.15 & 6.62 & 11.05 & 8.33 & 9.15\\
Eu & 2.50 & 2.16 & 3.46 & 2.15 & 3.45 & 2.68 & 2.92\\
Gd & 6.61 & 5.65 & 9.03 & 5.76 & 9.05 & 7.11 & 7.57\\
Tb & 0.98 & 0.81 & 1.26 & 0.83 & 1.25 & 1.01 & 1.04\\
Dy & 5.44 & 4.44 & 6.84 & 4.57 & 6.76 & 5.56 & 5.36\\
Ho & 1.01 & 0.81 & 1.26 & 0.85 & 1.22 & 1.02 & 0.92\\
Er & 2.47 & 1.90 & 2.99 & 2.04 & 2.84 & 2.40 & 2.04\\
Tm & 0.33 & 0.24 & 0.40 & 0.27 & 0.37 & 0.31 & 0.25\\
Yb & 2.00 & 1.47 & 2.42 & 1.61 & 2.17 & 1.83 & 1.40\\
Lu & 0.29 & 0.20 & 0.35 & 0.23 & 0.30 & 0.25 & 0.18\\
Th & 4.98 & 4.09 & 8.73 & 4.27 & 8.15 & 3.36 & 5.06\\
U & 1.32 & 1.17 & 2.71 & 1.21 & 2.28 & 0.95 & 1.36
\botline
\end{tabular}
\end{table*}


\begin{figure}
\includegraphics{fig06}
\caption{\label{fig6}Total alkali-silica diagram 
\citep[after][]{LeBasetal1986}. The dark line separates the alkaline and the
subalkaline domain, according to \citet{IrvineBaragar1971}.}
\end{figure}

\begin{figure*}
\includegraphics{fig07}
\caption{\label{fig7}(a) Major elements (wt\%) versus SiO$_{2}$
distribution; (b) Trace elements distribution (Th versus Ni, Nb, Co, Zr,
La, Cr, Ta and V) of the Iriba basanites.}
\end{figure*}

\subsection{Trace elements}\label{sec62}

Compatible or slightly incompatible trace elements concentrations are
variable: Sc (17--24~ppm), V (175--245~ppm), Cr (46--60~ppm), Co
(307--660~ppm), Ni (192--372~ppm), Cu (45--60~ppm) and Zn (105--189
ppm) (Table~\ref{tab4}). In the trace elements $x$--$y$ diagrams
(Figure~\ref{fig7}b), highly incompatible elements such as Nb, Zr, La
and Ta show positive trends with each other, while compatible trace
elements such as Ni, Cr, Co and V show negative trends. REE element
contents (Figure~\ref{fig8}a) show enrichment in LREE (14.37 ${\leq}$
(La/Yd)$_{\mathrm{N}} \leq 24.82$) compared to MREE and HREE ($2.73 \leq$ 
(Gd/Yb)$_{\mathrm{N}}\leq 4.46$). 

\begin{figure*}
\includegraphics{fig08}
\vspace*{-5pt}
\caption{\label{fig8}(a) Chondrite normalized REE paterns and (b)
primitive mantle normalized trace elements diagrams of Iriba basanites.
The normalization values are after \citet{SunMcdonough1989}.}
\vspace*{-5pt}
\end{figure*}

\section{Discussion}\label{sec7}
\subsection{Magmatic differentiation of the Iriba basanites}\label{sec71}
Given their low silica contents, their high Ni (207--372~ppm) and Cr
(45--597~ppm) contents most of the Iriba basanites, might be considered
as crystallized from a primary magma as defined by \citet{Freyetal1978},
eventhough they display a MgO content (8.6--12 wt\%; Mg\# ${\leq}$65) 
slightly below the one of the primary magmas. These characteristics
may be consistent with an evolution through fractional crystallization
with or without crustal contamination. The relative contributions of
these processes can be deciphered from the analysis of major and trace
elements compositions. The negative correlations of CaO concentration
with SiO$_{2}$ and of Ni, Cr, Co and V with Th 
(Figure~\ref{fig8}a,b), suggest fractionation linked to the
crystallization of olivine, clinopyroxene, oxides and feldspar. Olivine
and clinopyroxene fractionation are further suggested by the linear
trend in the Sr versus Ba diagram [\citealp{Franzetal1999}, 
Figure~\ref{fig9}b]. The positive correlations of Al$_{2}$O$_{3}$ with
SiO$_{2}$ and that of Zr with Th likely imply a fractionation of
plagioclase and zircon respectively. Fractional crystallization is also
consistent with (i) the enrichment in LREE, (ii)~the similarity of
hygromagmatophilic elements ratios such as Zr/Nb, Th/Hf and La/Ta,
(iii) the parallelism of the REE patterns (Figure~\ref{fig8}a,b),
and (iv) the \mbox{horizontal} trend of the studied basanites in the Sr versus
Rb diagram of Xu et al, 2007 (Figure~\ref{fig9}a). Nevertheless, the
relatively high Ni (192--350~ppm) and Co (307--660~ppm) contents of the
basanites points to a low degree of fractional crystallization of the
parental magma. 


\begin{figure*}
{\vspace*{4pt}}
\includegraphics{fig09}
\vspace*{5pt}
\caption{\label{fig9}(a) Illustration of fractional crystallization
trend in Rb versus Sr diagram \citep{Xuetal2007} and (b) illustration of
olivine and clinopyroxene fractionation in the Sr versus Ba diagram of
\citet{Franzetal1999}.}
\vspace*{5pt}
\end{figure*}

The range of P$_{2}$O$_{5}$ contents of the studied basanites  (0.6 ${<}$
P$_{2}$O$_{5}< 1.9$) points to a complete lack of crustal
contamination. This is confirmed in the K$_{2}$O/Th versus La/Ta
diagram (Figure~\ref{fig10}a,b), where the Iriba lavas fall within
the field of uncontaminated basalts. Such features are similar to that
of some lavas from northern Cameroon, notably the Ngaound\'{e}r\'{e}
basaltic lavas [0.9 ${<}$ P$_{2}$O$_{5}<1.3$;
\citealp{Nkouandouetal2010}]; [0.4 ${<}$ P$_{2}$O$_{5}<$ 1.4;
\citealp{Fitton1987}] and the Kapsiki basalts [0.6 ${<}$ P$_{2}$O$_{5}<
1.2$; \citealp{Ngounounoetal2000}].

\begin{figure*}
\includegraphics{fig10}
\caption{\label{fig10}Position of the studied basanites in (a) Th/Yb
versus Ta/Yb \citep[from][]{Pearce1982,Pearce1983} and 
(b)~K$_{2}$O/Th versus
La/Ta diagrams. (c) Gd/Yb versus La/Yb diagram 
\citep[after][]{Yokoyamaetal2007} illustrating the partial melting of Iriba
basanites. The curves at Grt 4\% and 8\% correspond to the garnet
content in the source \citep{Hallidayetal1995}. CVL ${=}$ Cameroon
Volcanic Line. (d) Nb/Y versus Zr/Y diagram of \citet{Weaver1991} and
\citet{Condie2005}. OIB ${=}$ Ocean Island Basalts, OPB ${=}$ Ocean Plateau
Basalts, PM ${=}$ primitive mantle, DEP ${=}$ highly depleted mantle,
REC ${=}$ recycled component; HIMU ${=}$ high $^{238}$U/$^{204}$Pb mantle
source, EM1 and EM2 enriched mantle sources.}
\end{figure*}




\subsection{Source of the Iriba basanites and conditions of partial
melting}\label{sec72}

The Iriba basanites display characteristics of intraplate magmas
(Figure~\ref{fig10}a,b), generated by partial melting of an
enriched mantle source. A similar intraplate signature is displayed by
alkaline continental basalts exposed in Emi-Koussi in Chad 
\citep{GourgaudVincent2004}
and by alkaline basalts from Mayo
Oulo-L\'{e}r\'{e}, Babouri-Figuil, located at the Cameroon-Chad border 
\citet{Ngounounoetal2000}. The geochemical signatures of the Iriba
basanites point to a higher degree of partial melting (2--4\%) than the
ones from Baossi-Warack [0.5--2\%; \citealp{Tiabouetal2018}] and from
Mt Cameroon [1--2\%; \citealp{Yokoyamaetal2007}] but lower than that of
basanites from Mbengwi [5--8\%; \citealp{Mbassaetal2012}]. 

Assuming that MREE/HREE ratios are sensitive to the amount of residual
garnet in the source, the range of the studied Iriba basanites
(Tb/Yb)$_{\mathrm{N}}$ ratios (2.22--3.37) is higher than 1.7 and is consistent
with the presence of garnet in the source, as proposed by
\citet{Wangetal2002}. This interpretation is corroborated by the
position of the Iriba basanites in the Gd/Yb versus La/Yb diagram
(Figure~\ref{fig10}c), which indicates partial melting of a lherzolitic
mantle containing less than 8\% of garnet, except for sample RB8,
characterized by high levels of Gd and La but low levels of Yb. The
ranges of some incompatible trace elements ratios such as Zr/Nb, La/Nb,
Ba/Nb, Rb/Nb and Ba/La of Iriba basanites (Table~\ref{tab5}) are
typical of Ocean Island Basalts (OIB) defined by 
\citet{ZindlerHart1986}.
This OIB affinity is also confirmed in the Nb/Y versus Zr/Y diagram of
\cite{Weaver1991}, with an imposing role for recycled components and
the HIMU-EM1 mantle poles, except one sample plotting close to the
enriched EM2 pole (Figure~\ref{fig10}d).

\begin{table*}
\caption{\label{tab5}Comparative incompatible elements contents of
Iriba alkaline basanites with those of the crust and certain mantle
reservoirs \citep{ZindlerHart1986}}
\begin{tabular}{lccccc}
\thead
& Zr/Nb    & La/Nb       & Ba/Nb       & Rb/Nb      & Ba/La\\
Iriba basanites     & 3.6--4.5 & 0.69--0.90  & 7.0--13.2  & 0.25--1.11 & 9.33--14.95\\
\endthead
Continental crust   & 16.2     & 2.2         & 54          & 4.7        & 25\\
Primitive mantle    & 14.8     & 0.94        & 9           & 0.91       & 9.6\\
N-MORB              & 46.2     & 1.07        & 1.7--8.0    & 0.36       & 4\\
E-MORB              & 14.07    & 1.05        & 4.9--8.5    & nd         & nd\\
HIMU--OIB             & 3.2--5.6   & 0.66--0.77    & 4.9--6.9    & 0.35--0.38   & 6.8--8.7\\
EM1--OIB             & 4.2--11.5  & 0.86--1.19    & 11.4--17.8  & 0.88--1.77   & 13.2--16.9\\
EM2--OIB             & 4.5--7.3   & 0.89--1.11    & 7.3--13.3   & 0.59--0.85   & 8.3--11.3
\botline
\end{tabular}
\vspace*{8pt}
\end{table*}

\subsection{The Cameroon chad volcanic line}

The Iriba basanites are similar to alkaline magmatic series emplaced
along the Central Africa Rift System including in the north of the
volcanic rocks of Lake Chad and the Tibesti Volcanic Province. The
influence of Cretaceous--Paleocene to Cenozoic mantle-derived magmatism
initially identified in Cameroon and the western part of Chad might
thus be extended northeastward, up to the northern part of Ouadda\"{i}
massif, on both sides of the positive gravity anomaly of the
``Poli--Ounianga--Kebir heavy line''. We proposed to defined this large
SW--NE magmatic province as the Cameroon Chad Volcanic Line (CCVL). The
Cretaceous--Paleocene to Cenozoic magmatic activity of the Cameroon-Chad
Volcanic Line appears to be controlled by the opening of the Central
Africa Rift System localized along the edges of Archean cratonic nuclei
marked by tectonic \mbox{reworking} and accretion of juvenile Paleoproterozoic
to Neoproterozoic crust. 

\section{Conclusion}\label{sec8}

In this contribution, we present the first petrographic, mineralogical
and geochemical study of the Iriba basanites located in the northern
part of the Ouadda\"{i} massif. The Iriba basanites are composed of
magnesian olivine, Ca-rich clinopyroxenes \mbox{(diopside} and augite), albite
and spinel. They display sodic and alkaline affinities that are
attributed to magmatic differentiation of a primary mantle-derived
magma by fractional crystallization. The geochemical signatures of
these basanites indicate an affinity with OIB and are consistent with a
generation by a low \mbox{degree} of partial melting (2--4\%) of HIMU-type
mantle source containing residual garnet (4--8\%), without any crustal
contamination. The Iriba basanites are similar to an alkaline magmatic
series emplaced along the Central Africa Rift System, active from the
Cretaceous--Paleocene to the Cenozoic, belonging to the so-called
Cameroon Chad Volcanic\break Line (CCVL).
\vspace*{-5pt}

\section*{Declaration of interests}
The authors do not work for, advise, own shares in, or receive funds
from any organization that could \mbox{benefit} from this article, and have
declared no affiliations other than their research organizations.

\section*{Funding}

The research has been financially supported by LithoCOAC project (CNRS,
France),  the IRN FALCoL (CNRS, France) and the participation of the
French Embassy in Chad through the fellowship program ``S\'{e}jour
Scientifique de Haut Niveau\break (SSHN)''. 

{\vspace*{-2pt}}

\section*{Acknowledgements}

{\vspace*{-2pt}}

We thank Philippe de Parseval for
his assistance with electron microprobe analyses. Fabienne de Parseval
is also thanked for the confection of rocks thin sections. We
gratefully acknowledge several anonymous reviewers for their critical
and constructive comments of the manuscript.

{\vspace*{-2pt}}

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