\makeatletter
\@ifundefined{HCode}
{\documentclass[screen, CRGEOS, Unicode, biblatex, published]{cedram}
\addbibresource{crgeos20250145.bib}
\newenvironment{Table}{\begin{table}}{\end{table}}
\newenvironment{noXML}{}{}
\usepackage[figuresright]{rotating}
\let\citep\parencite
\let\citet\textcite
\def\xcitealp#1#2{\citeauthor{#1}, \citelink{#1}{#2}}
\newcommand*{\citelink}[2]{\hyperlink{cite.\therefsection @#1}{#2}}
\def\defcitealias#1#2{}
\let\citepalias\parencite
\let\citetalias\textcite
\def\thead{\noalign{\relax}\hline}
\def\0{\phantom{0}}
\def\endthead{\noalign{\relax}\hline}
\def\tabnote#1{\vskip4pt\parbox{.95\linewidth}{#1}}
\def\botline{\\\hline}
\def\tbody{\noalign{\relax}}
\def\tsup#1{$^{{#1}}$}
\def\tsub#1{$_{{#1}}$}
\RequirePackage{etoolbox}
\usepackage{upgreek}
\def\jobid{crgeos20250145}
%\graphicspath{{/tmp/\jobid_figs/web/}}
\graphicspath{{./figures/}}
\newcounter{runlevel}
\usepackage{multirow} 
\def\nrow#1{\@tempcnta #1\relax%
\advance\@tempcnta by 1\relax%
\xdef\lenrow{\the\@tempcnta}}
\def\morerows#1#2{\nrow{#1}\multirow{\lenrow}{*}{#2}}
\let\MakeYrStrItalic\relax
\def\refinput#1{}
\def\back#1{}
\csdef{Seqnsplit}{\\}
\newcommand*{\hyperlinkcite}[1]{\hyper@link{cite}{cite.#1}}
\DOI{10.5802/crgeos.299}
\datereceived{2025-02-16}
\daterevised{2025-05-27}
\dateaccepted{2025-05-28}
\ItHasTeXPublished
\def\vspone{\vspace*{20.5pc}}
\gdef\tabonesplittabular{\hline
\multicolumn{12}{r@{}}{(continued on next page)}
\end{tabular}
\end{sidewaystable*}
\setcounter{table}{0}
\begin{sidewaystable*}
\vspone
\caption{(continued)}
\tabcolsep=3.5pt
\begin{tabular}{lccccccccccc}
\hline
& \multicolumn{7}{c}{Metabasites Lebzenia from the Tasiast} & \multicolumn{3}{c}{Sebkhet Nich} & Talhayet \\
\cline{2-8}\cline{9-11}
Sample N & DRAM 1 & DRAM 3 & DRAM 6 & DRAM 7 & DRAM 8 & DRAM 9 & DRAM 10 & M\'{e}tavolcanite & 
Amphibolite & Amphibolite & M\'{e}tavolcanite \\
\hline}
}
{
\PassOptionsToPackage{authoryear}{natbib}
\documentclass[crgeos]{article}
\usepackage[T1]{fontenc} 
\def\CDRdoi{10.5802/crgeos.299}
\def\citelink#1#2{\citeyear{#1}}
\def\xcitealp#1#2{\citealp{#1}}
\makeatletter
\def\CDRsupplementaryTwotypes#1#2{}
\newenvironment{sidewaystable*}{\begin{table*}}{\end{table*}}
\let\tabonesplittabular\relax
\let\vspone\relax
\def\hyperlinkcite#1#2{\citeyear{#1}}
\let\newline\break
}
\makeatother

\begin{document}

\begin{noXML}


\TopicFR{G\'eochimie, cosmochimie}
\TopicEN{Geochemistry, cosmochemistry}
\TopicFR{P\'etrologie, p\'{e}trophysique}
\TopicEN{Petrology, petrophysics}

\CDRsetmeta{articletype}{research-article}

\title{The metabasalts from the greenstone belt of the Lebzenia group
in the Tasiast complex, Mauritania, and their geodynamic implications}

\alttitle{Les m\'{e}tabasaltes de la ceinture de roches vertes du
groupe de Lebzenia dans le complexe de Tasiast, Mauritanie, et leurs
implications g\'{e}odynamiques}

\author{\firstname{Mohamed} \lastname{El Abd Bouha}\CDRorcid{0000-0001-6784-0918}\IsCorresp}
\address{GEO3 Laboratory, Geology Department, Faculty of Sciences, Moulay Ismail University, P.O. Box 11201 Zitoune, Mekn\`{e}s, Morocco}
\email[M. El Abd Bouha]{moustafabouha@yahoo.fr}

\author{\firstname{Houssa}  \lastname{Ouali}\CDRorcid{0000-0003-4956-3448}}
\addressSameAs{1}{GEO3 Laboratory, Geology Department, Faculty of Sciences, Moulay Ismail University, P.O. Box 11201 Zitoune, Mekn\`{e}s, Morocco}
\email[H. Ouali]{houali@yahoo.fr}  

\author{\firstname{Michel} \lastname{Jebrak}\CDRorcid{0000-0002-0473-9924}}
\address{Department of Earth and Atmospheric Sciences, University of Quebec in Montreal, Canada}
\email[M. Jebrak]{jebrak.michel@uqam.ca}

\author{\firstname{Zein} \lastname{El Arby}}
\address{D\'{e}partement de G\'{e}ologie, Facult\'{e} des Sciences et Techniques, Universit\'{e} de Nouakchott, Mauritania}
\email[Z. El Arby]{ouldarbyzein@yahoo.fr}

\thanks{EU (FP7-PEOPLE-2013-IRSES MEDYNA project)}

\shortrunauthors

\keywords{\kwd{Metabasalts}
\kwd{Greenstone belts}
\kwd{Amphibolite}
\kwd{Lebzenia Group}
\kwd{Tasiast}
\kwd{Mauritania}
\kwd{West African craton}}

\altkeywords{\kwd{M\'{e}tabasaltes}
\kwd{Ceintures de roches vertes}
\kwd{Amphibolite}
\kwd{Groupe de Lebzenia}
\kwd{Tasiast}
\kwd{Mauritanie}
\kwd{Craton Ouest-Africain}}


\begin{abstract}
Samples of metabasalts collected from the Lebzenia Group, located in
the Tasiast region of Mauritania, reveal a complex geochemical
signature that highlights magmatic processes related to tectonically
active environments. The analyses show low concentrations of
incompatible elements such as Nb, TiO\textsubscript{2}, and P\textsubscript{2}O\textsubscript{5},
combined with marked negative anomalies in Nb, Ta, and Ti. These
characteristics, along with an enrichment of light rare earth elements
(LREE) and Th, are typical of a transitional calc-alkaline affinity
commonly associated with volcanic arcs. The high Th/Nb ratios, as well
as the presence of specific geochemical anomalies, suggest an
interaction between an enriched mantle source and a modified
continental crust. This interaction reflects partial mantle melting
processes at significant depths, influenced by crustal materials or
fluids from intense tectonic activity. Samples such as DRAM7 and DRAM8
show a transitional calc-alkaline affinity while exhibiting
similarities to tholeiitic basalts of mantle origin. These geochemical
data shed light on the diversity and complexity of the tectonomagmatic
processes that contributed to the formation of the Lebzenia Group. This
study enhances the understanding of the geodynamic evolution of the
Tasiast region and its role in crustal growth processes during the
Archaean, emphasising the contribution of mantle--crust interactions in
magma genesis.
\end{abstract}

\begin{altabstract}
Les \'{e}chantillons de m\'{e}tabasaltes collect\'{e}s dans le groupe
de Lebzenia, situ\'{e} dans la r\'{e}gion de Tasiast en Mauritanie,
pr\'{e}sentent une signature g\'{e}ochimique complexe qui met en
\'{e}vidence des processus magmatiques li\'{e}s \`{a} des
environnements tectoniquement actifs. Les analyses r\'{e}v\`{e}lent de
faibles concentrations en \'{e}l\'{e}ments incompatibles tels que Nb,
TiO\textsubscript{2} et P\textsubscript{2}O\textsubscript{5}, accompagn\'{e}es d'anomalies
n\'{e}gatives marqu\'{e}es en Nb, Ta et Ti. Ces caract\'{e}ristiques,
associ\'{e}es \`{a} un enrichissement en terres rares l\'{e}g\`{e}res
(LREE) et en Th, sont typiques d'une affinit\'{e} calco-alcaline
transitionnelle, souvent associ\'{e}e aux arcs volcaniques. Les
rapports Th/Nb \'{e}lev\'{e}s, ainsi que la pr\'{e}sence d'anomalies
g\'{e}ochimiques sp\'{e}cifiques, sugg\`{e}rent une interaction entre
une source mantellique enrichie et une cro\^{u}te continentale
modifi\'{e}e. Cette interaction refl\`{e}te des processus de fusion
partielle du manteau \`{a} des profondeurs significatives,
influenc\'{e}s par des mat\'{e}riaux crustaux ou des fluides issus
d'une activit\'{e} tectonique intense. Des \'{e}chantillons comme DRAM7
et DRAM8 montrent une affinit\'{e} calco-alcaline transitionnelle tout
en pr\'{e}sentant des similitudes avec les basaltes thol\'{e}iitiques
d'origine mantellique. Ces donn\'{e}es g\'{e}ochimiques mettent en
lumi\`{e}re la diversit\'{e} et la complexit\'{e} des processus
tectonomagmatiques ayant contribu\'{e} \`{a} la formation du groupe de
Lebzenia. Cette \'{e}tude enrichit la compr\'{e}hension de
l'\'{e}volution g\'{e}odynamique de la r\'{e}gion de Tasiast et de
son r\^{o}le dans les processus de croissance crustale durant
l'Arch\'{e}en, en soulignant l'importance des interactions
manteau-cro\^{u}te dans la gen\`{e}se magmatique.
\end{altabstract}

\maketitle

\twocolumngrid

\end{noXML}

\defcitealias{Pittfieldetal2005}{ibid.}
\defcitealias{Pearce2008}{ibid.}

\section{Introduction} 

The study of Archaean greenstone belts, in particular their formation
and geodynamic context, raises complex questions and is the subject of
intense debate among geologists. One of the main points of discussion
concerns the role of subduction-related tectonic processes similar to
those observed in the Phanerozoic, in the formation of these belts
\citep{Bedard2006,Bedard2013,PolatKerrich2006,Bedard2013,Wyman2013}.
The mechanisms of crustal growth, whether influenced by subduction or
mantle plume-related processes, have profound implications for Earth
evolution and metallogenesis during the Archaean.



In the Tasiast region, Mauritania, the Lebzenia Group, still poorly
studied, has been interpreted, along with the neighbouring Talhayet and
Sebkhet Nich formations, as a volcanic arc formed in a subduction
context after a phase of continental-to-oceanic rifting, with typical
enriched MORB signatures \citep{Pittfieldetal2005}.

There is a general consensus that a horizontal tectonic framework led
to the terrane accretion events documented in the western part of the
Superior Province 
\citep{Goodwin1981,Percivaletal2006,Stottetal2010,Bedard2013,Wyman2013}. 
In this context, the present study aims to investigate in detail the
geochemical characteristics of the Lebzenia Group metabasites. These
rocks show a notable similarity with the mafic volcanic rocks (basalts)
of Ungava, located north of the Superior Province, Quebec
\citep{Lamothe2007,Corriganetal2009}, as well as to those of the
Wawa-Abitibi terrane, the Tanzanian craton, and the Dharwar craton in
India, which are reminiscent of modern volcanic arcs
\citep{Polat2009,WymanKerrich2009,Lodgeetal2014}.

Recent studies of Archaean greenstone belts suggest that they formed in
various tectonic environments, including subduction zones, volcanic
arcs, and back-arc basins, illustrating the geological complexity of
these regions \citep{Condie1981,deWit1998,Polatetal1998}. The
presence of complex lithological associations, such as tholeiitic
basalts, komatiites, and ultramafic rocks, along with evidence of
high-pressure metamorphism, indicates that diverse tectonic processes
operating during the Archaean \citep{Wyman1999,KerrichPolat2006,Furnesetal2009}.

We aim to contribute to this debate by studying the Tasiast greenstone
belt in Mauritania. Similar to those of the Archaean cratons in Canada,
Australia, and West Africa, these belts are known for their rich gold
deposits \citep{PolatKerrich2002,SylvesterAttoh1992}. The
Tasiast belt, in particular, hosts one of the largest orogenic gold
deposits in West Africa, with over 10 million ounces of confirmed gold
\citep{Stuart2010}. This not only highlights the economic value of the
region but also its importance in international geological and
mineralogical discussions.

Orogenic gold deposits, such as those at Tasiast, are generally
associated with greenstone belts, which have been shown to form in
tectonically active environments, involving processes such as
subduction, continental collision, and mantle plumes 
\citep{SternHanson1991,Windley1995}. This region frequently attracts
the attention of geologists at international conferences, such as those
organised by the Geological Society of America or the International
Union of Geological Sciences, where the geodynamic and economic
implications of greenstone belts in Archaean cratons are discussed.

A key question regarding the Tasiast belt is to understand its
tectono-magmatic origin. Do the processes that shaped these rocks
reflect a tectonic regime similar to modern volcanic arcs, dominated by
subduction, or do they show distinct geochemical signatures resulting
from alternative processes, such as mantle plumes or back-arc
environments? Answering this question is essential for reconstructing
the tectonic and thermal conditions of the Archaean, a key period in
Earth's evolution.

\section{Materials and methods}

In the Tasiast region, eight samples were collected from the greenstone
belts for in-depth studies of their petrographic composition and
detailed analyses of their major and trace elements. These samples were
carefully selected to represent the geological diversity of the region
and were collected from strategic locations to obtain a representative
picture of the chemical composition of the study area. Table~\ref{tab1} lists
the collected samples, along with their respective GPS coordinates
(Figure~\ref{fig1}). Whole-rock analyses, were done at the Andalusian Institute
of Earth Sciences in Armilla, Granada, Spain, using two analytical
techniques: Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and
Wavelength Dispersive X-ray Fluorescence Spectrometry (WD-XRF). The
ICP-MS method was used to determine the trace element composition of
the samples. The Wavelength Dispersive X-ray Fluorescence Spectrometry
(WD-XRF) was used to analyze the major elements present in the samples.



\begin{figure*}
\vspace*{-4pt}
\includegraphics{fig01}
\vspace*{-6pt}
\caption{\label{fig1}(a) The geographical distribution of cratons in Africa 
\citep{Beggetal2009}. (b) Geological framework of the West African 
craton showing the location of the studied Tasiast area in Mauritania
\citep{Heronetal2016}. (c) The main litho-tectonic units of the
Reguibat shield \citep[modified from][]{Pittfieldetal2005} (d) Section  
(east--west) of the Chami sector, Tasiast--Tijirit Terrane.}
\end{figure*}


\begin{sidewaystable*}
\vspone
\caption{\label{tab1}Major (wt\%) and trace element (ppm) analyses of metabasites 
from the greenstone belt, collected from the Lebzenia formation in the
Tasiast complex}
\tabcolsep=3.5pt
\begin{tabular}{lccccccccccc}
\thead
& \multicolumn{7}{c}{Metabasites Lebzenia from the Tasiast} & \multicolumn{3}{c}{Sebkhet Nich} & Talhayet \\
\cline{2-8}\cline{9-11}
Sample N & DRAM 1 & DRAM 3 & DRAM 6 & DRAM 7 & DRAM 8 & DRAM 9 & DRAM 10 & M\'{e}tavolcanite & 
Amphibolite & Amphibolite & M\'{e}tavolcanite \\
\endthead 
SiO$_{2}$ &  \053.04\0 &  \054.85\0 &  51.57 &  58.47 &  46.91 &  50.74 & 50.23 &  48.79 &  49.72 &  53.87 &  46.31 \\
Al$_{2}$O$_{3}$ &  \015.44\0 &  \013.91\0 &  15.78 &  13.74 &  14.24 &  16.62 & 15.65 &  22.77 &  14.20 &  14.99 &  15.07 \\ 
Fe$_{2}$O$_{3}$  &  \0\08.53\0 &  \0\09.75\0 &  11.08 &  8.14 &  18.13 &  10.24 &  12.19 &  \08.03 &  12.78 &  8.46 &  13.56 \\ 
Fe$_{2}$O* &  \0\07.68\0 &  \0\08.78\0 &  9.97 &  7.32 &  16.31 &  9.21 &  10.98 & \07.23 &  11.50 &  7.61 &  12.21 \\ 
CaO  &  \010.93\0 &  \0\09.44\0 &  10.57 &  9.34 &  10.32 &  12.23 &  11.89 & 13.51 &  12.11 &  11.80 &  12.56 \\ 
MgO  &  \0\06.77\0 &  \0\05.85\0 &  5.86 &  5.18 &  5.26 &  6.88 &  6.94 &  \03.72 & 6.68 &  4.21 &  9.95 \\ 
Na$_{2}$O  &  \0\02.78\0 &  \0\03.43\0 &  3.46 &  3.25 &  1.81 &  1.26 &  1.15 & \01.68 &  1.69 &  1.62 &  0.95 \\ 
K$_{2}$O  &  \0\00.16\0 &  \0\00.16\0 &  0.12 &  0.16 &  0.29 &  0.17 &  0.19 & \00.05 &  0.07 &  1.17 &  0.15 \\ 
TiO$_{2}$  &  \0\00.68\0 &  \0\00.69\0 &  0.67 &  0.65 &  1.05 &  0.40 &  0.41 & \00.50 &  0.83 &  0.76 &  0.72 \\ 
MnO  &  \0\00.12\0 &  \0\00.17\0 &  0.15 &  0.13 &  0.23 &  0.19 &  0.19 &  \00.10 & 0.19 &  0.19 &  0.26 \\ 
P$_{2}$O$_{5}$  &  \0\00.08\0 &  \0\00.35\0 &  0.02 &  0.10 &  0.07 &  0.05 &  0.04 & \00.02 &  0.05 &  0.06 &  0.06 \\ 
LOI  &  \0\00.59\0 &  \0\00.46\0 &  0.51 &  0.39 &  0.73 &  0.50 &  0.62 &  \00.70 & 0.8 &  3.2 &  0.2 \\ 
Cr &  \085.00\0 &  \055.00\0 &  95 &  44 &  104 &  10 &  39 &  - &  - &  - &  -\\ 
Zr  &  \078.00\0 &  \079.00\0 &  116 &  87 &  39 &  34 &  28 &  25.00 &  39.00 &  47.00 &  26.00 \\ 
Hf &  \0\02.106 &  \0\02.133 &  3.132 &  2.35 &  1.05 &  0.92 &  0.76 &  \00.9\0 & 1.4 &  1.4 &  1 \\ 
Ni  &  \068.00\0 &  \058.00\0 &  67 &  32 &  26 &  45 &  56 &  \05.60 &  20 &  26 &  30 \\ 
Rb &  \0\02.07\0 &  \0\04.04\0 &  2.62 &  5.37 &  2.81 &  1.84 &  7.57 &  \01.50 &  1.10 &  41.00 &  1.40 \\ 
Ba &  176.01\0 &  189.75\0 &  191.60 &  169.62 &  288.46 &  85.81 &  123.80 &  38.00 &  24.00 &  287.00 &  31.00 \\ 
Th &  \0\03.98\0 &  \0\06.00\0 &  3.33 &  1.40 &  1.72 &  4.76 &  1.45 &  \00.10 & 0.10 &  0.60 &  0.10 \\ 
U &  \0\00.72\0 &  \0\00.87\0 &  0.84 &  0.29 &  0.36 &  0.78 &  0.50 &  \00.05 & 0.05 &  0.50 &  0.05 \\ 
Nb &  \0\02.83\0 &  \0\04.10\0 &  0.50 &  3.48 &  1.20 &  1.40 &  2.79 &  \01.81 & 1.60 &  1.90 &  1.50 \\ 
Ta &  \0\00.29\0 &  \0\00.30\0 &  0.32 &  0.12 &  0.13 &  0.27 &  0.15 &  \00.10 & 0.10 &  0.10 &  0.10 \\ 
\tabonesplittabular
La &  \0\05.22\0 &  \042.94\0 &  31.39 &  3.39 &  3.69 &  7.48 &  4.44 &  \01.70 & 2.40 &  4.00 &  1.80 \\ 
Ce &  \013.02\0  &  \078.41\0 &  17.10 &  7.35 &  8.75 &  16.10 &  9.40 &  \04.10 &  6.20 &  9.00 &  5.80 \\ 
Pb &  \0\03.31\0 &  \0\01.49\0 &  1.10 &  2.57 &  2.49 &  1.63 &  1.37 &  \00.20 & 0.10 &  1.30 &  0.40 \\ 
Pr &  \0\01.64\0 &  \0\08.37\0 &  13.21 &  0.87 &  1.02 &  2.02 &  1.30 &  \00.62 & 0.87 &  1.10 &  0.93 \\ 
Nd &  \0\06.98\0 &  \030.03\0 &  53.99 &  3.53 &  4.32 &  8.84 &  5.45 &  \03.50 & 5.30 &  5.80 &  5.00 \\ 
Sm &  \0\01.91\0 &  \0\06.18\0 &  12.04 &  0.95 &  1.19 &  2.42 &  1.43 &  \01.10 & 2.00 &  2.10 &  1.90 \\ 
Eu &  \0\00.69\0 &  \0\00.88\0 &  3.28 &  0.36 &  0.47 &  0.91 &  0.53 &  \00.52 & 0.68 &  0.77 &  0.79 \\ 
Gd &  \0\02.48\0 &  \0\07.50\0 &  10.45 &  1.23 &  1.55 &  3.10 &  1.93 &  \01.40 & 2.80 &  2.20 &  2.50 \\ 
Tb &  \0\00.41\0 &  \0\01.09\0 &  1.50 &  0.21 &  0.26 &  0.49 &  0.32 &  \00.27 & 0.48 &  0.45 &  0.47 \\ 
Dy &  \0\02.87\0 &  \0\07.05\0 &  8.85 &  1.51 &  1.88 &  3.45 &  2.21 &  \01.70 & 3.40 &  2.90 &  3.30 \\ 
Y &  \017.23\0   &  \039.71\0 &  34.21 &  8.63 &  10.74 &  20.36 &  12.73 & 11.00 &  20.00 &  17.00 &  18.00 \\ 
Ho &  \0\00.66\0 &  \0\01.49\0 &  1.64 &  0.33 &  0.40 &  0.76 &  0.50 &  \00.35 & 0.62 &  53.00 &  0.70 \\ 
Er &  \0\02.00\0 &  \0\04.21\0 &  4.41 &  0.99 &  1.24 &  2.30 &  1.48 &  \01.20 & 2.10 &  1.70 &  2.00 \\ 
Tm &  \0\00.28\0 &  \0\00.56\0 &  0.63 &  0.15 &  0.18 &  0.33 &  0.21 &  \00.16 & 0.03 &  0.22 &  0.28 \\ 
Yb &  \0\01.91\0 &  \0\03.46\0 &  4.00 &  1.00 &  1.17 &  2.13 &  1.41 &  \01.30 & 2.00 &  1.40 &  1.90 \\ 
Lu &  \0\00.30\0 &  \0\00.53\0 &  0.61 &  0.16 &  0.19 &  0.33 &  0.22 &  \00.16 & 0.24 &  0.21 &  0.28 \\ 
Mg\# &  \0\00.61\0 &  \0\00.54\0 &  0.51 &  0.56 &  0.37 &  0.57 &  0.53 &  \00.48 & 0.51 &  0.50 &  0.59 \\ 
La/Yb &  \0\02.73\0 &  \012.42\0 &  7.84 &  3.40 &  3.14 &  3.52 &  3.14 &  \01.31 &  1.20 &  2.86 &  0.95 \\ 
Th/Yb &  \0\02.08\0 &  \0\01.73\0 &  0.83 &  1.40 &  1.46 &  2.24 &  1.02 &  \00.08 & 0.05 &  0.43 &  0.05  
\botline
\end{tabular}
\tabnote{Mg\#\ ${=}$ molar ratio of (MgO/40.3)/((MgO/40.3) ${+}$ (FeO/71.85)) ${*}$ 100.}
\tabnote{Melt compositions used in melt interaction modeling.
The values correspond to the compositions of the Sebkhet Nich and
Talhayet formations within the greenstone belt, based on datasets
published by \citet{Pittfieldetal2005}.}
\end{sidewaystable*}


The analytical data set furthers our understanding of the mineralogical
and chemical composition of the greenstone belts in the Tasiast region,
thereby laying the foundation for additional studies of the geology and
geodynamic evolution of this fascinating~area.\looseness=1


\section{Geological setting}

The Reguibat Ridge, located in the northern part of the West African
Craton (Figure~\ref{fig1}a), is one of the major geological structures
in the region. It extends over approximately 1500~km along a
northeast-southwest axis, with a width ranging between 250 and 400~km.
It is mainly exposed in Mauritania, but also extends into Morocco and
Algeria (Figure~\ref{fig1}b). The ridge is bounded by several major
geological units: the Central African Mobile Belt to the east, the
Mauritanides and the Senegal-Mauritanian sedimentary basin to the west,
as well as the Taoudeni and Tindouf basins to the south and north,
respectively \citep{Bessoles1977}. Previous geological studies have
revealed the dominance of Archaean rocks in the northwestern part of
the Reguibat Shield, while Palaeoproterozoic formations predominate
elsewhere. This distinction has led to the subdivision of the region
into two provinces: a southwestern province composed of pre-Eburnean
orogenic rocks (older than 2~Ga) and a central and northeastern
province characterised by Eburnean-age formations 
\citep{DillonSougy1974,Cahenetal1984}.

The Reguibat Ridge is further divided into two major terranes: the
Choum--Rag el Abiod Terrane to the east and the Tasiast--Tijirit
Terrane to the west (Figure~\ref{fig1}c). The latter, located in
northwestern Mauritania, consists of granitic complexes, gneisses, and
greenstone belts \citep{ElAbdBouhaetal2021}. The main greenstone belts
in this region, from east to west, include Tijirit, Ahmeyim, Sebkhet
Nich, Kreidat, and Chami (Figure~\ref{fig1}c). Two additional,
smaller structures, Lebzenia and Hadeibt Lebthein\^{i}y\'{e}, are
grouped under the term ``Lebzenia Greenstone Belt''
\citep{Bronneretal1985,Roccietal1990}. These formations are mainly
composed of mafic metavolcanic rocks, ultramafic lithologies, and
metasediments, with less frequent occurrences of intermediate to felsic
volcanic rocks (Figure~\ref{fig1}c). The lithological units of these
greenstone belts, collectively referred to as the ``Lebzenia Group'',
include the Talhayet, Tijraj, Aou\'{e}oua, and Sebkhet Nich formations,
each named after its type locality. The linear orientation of the
greenstone belts is largely influenced by major tectonic events. To the
west, they follow the stromatic structure of the surrounding migmatitic
gneisses, while to the east, they are aligned along a NNE--SSW axis,
parallel to the Trans-Iguidi Shear Zone, which marks the eastern
boundary of the Tasiast--Tijirit Terrane. This shear zone plays a
crucial role in the tectonic structuring of the region, controlling
lithological contacts and deformation episodes \citep{Pittfieldetal2005}. 

Geochemical analyses of the metavolcanic rocks from these greenstone
belts \citepalias{Pittfieldetal2005} suggest that the early mafic and
ultramafic sequences were emplaced in a continental rift setting.
Banded iron formations (BIFs) and siliciclastic metasediments are
interbedded within these units or concentrated in the central basins of
the greenstone belts, as observed in the Chami Belt. The greenstone
belts primarily consist of volcanic and sedimentary rocks affected by
low- to medium-grade metamorphism. Metamorphosed basalts and gabbros
are dominant, accompanied by amphibolites, schists, and banded-iron
formations. Ultramafic rocks, although less common, occur as elongated
lenses, particularly in the Sebkhet Nich Belt. In the Chami Belt,
banded iron formations are extensively developed and are associated
with metasedimentary schists that have undergone metamorphic
transformations leading to mineral assemblages including actinolite,
chlorite, quartz, and sericite \citepalias{Pittfieldetal2005}. These belts
are often crosscut by well-defined shear zones that have influenced the
deformation of various lithological units. Petrographic studies of the
Chami Belt, conducted during gold exploration campaigns by ``Normandy
la Source'', have identified mafic and ultramafic metavolcanic schists
enriched in amphiboles, chlorite, epidote, albite, and quartz
\citepalias{Pittfieldetal2005}. Chlorite results from the alteration of
ferromagnesian minerals such as amphiboles, micas, and pyroxenes.
Felsic metavolcanic formations are rare and occur as small lenticular
bodies, particularly in the Kreidat Belt, where massive, homogeneous,
quartz-rich, pale green rocks have been observed. Metabasalts are
widespread and predominantly occur as plagioclase-rich amphibolites.
Some outcrops exhibit similarities with the metabasalts of Akjoujt,
with frequent quartz-carbonate replacements. Certain metabasalts
display ``onion skin'' weathering structures, indicative of post-volcanic
alteration, while others show trachytic textures characterised by the
alignment of prismatic plagioclase crystals 
\citepalias{Pittfieldetal2005}. Amphibolites represent a key 
\mbox{component} of
the greenstone belts and include metabasalts, meta-andesites, and
metadiorites. The northern segment of the Chami Belt is dominated by
amphibolites, which appear to correspond to altered metabasic rocks and
intermediate metavolcanic formations. A similar lithological
arrangement is observed in the eastern part of the Kreidat Belt.
Ultramafic rocks, although less abundant, are widely distributed as
lenses aligned with the regional foliation. They are generally
schistose, except in certain areas where they occur as massive bodies,
such as in the western part of the Sebkhet Nich Belt
\citepalias{Pittfieldetal2005}. The greenstone belts of the Tasiast region
exhibit significant lithological and structural diversity, reflecting a
complex geological history characterised by ancient volcanic and
sedimentary processes, followed by multiple episodes of metamorphism
and tectonic deformation. Their study provides important insights into
the geodynamic evolution of this segment of the West African Craton.


\section{Petrography}

The metabasalts of the greenstone belts form distinctive landforms,
including low hills and elongated ridges (Figure~\ref{fig2}a,b). They
range in colour from green to black, and in structure from fissile to
massive. The alteration of ferromagnesian minerals results in brownish
to rust-coloured stains on the surface (Figure~\ref{fig2}a,b). The
mineralogical composition generally gives these rocks a dark shade,
from dark gray to black. However, samples that have undergone more
intense metamorphism may have lighter or greenish hues due to the
development of secondary amphiboles and chlorite
(Figure~\ref{fig2}a). A preferential mineral alignment is often
observed, forming distinct foliated structures (Figure~\ref{fig2}a).
In addition, the presence of pyroxenes and amphiboles, sometimes
visible to the naked eye, gives the rocks a granular texture.


\begin{figure*}
\includegraphics{fig02}
\caption{\label{fig2}Photographs of the metabasalts from the greenstone belt in the
Tasiast area. (a--b) Highly Fractured metabasalt Coated with Desert
Varnish---Lebzenia Group. (c) The metabasalts are composed mainly of
plagioclase and clinopyroxene, with secondary amphibole formed by the
alteration of pyroxenes.}
\end{figure*}


Under the microscope, these metabasalts are primarily composed of
plagioclase and clinopyroxene, with secondary amphiboles resulting from
the alteration of pyroxenes (Figure~\ref{fig2}c). The matrix consists of
fine-grained minerals, mainly plagioclase and clinopyroxene, associated
with opaque phases. Accessory minerals such as epidote, zoisite,
chlorite, sphene, zircon, apatite, and ilmenite are also present.

Various textures are observed, reflecting the magmatic and metamorphic
processes that have affected these rocks. The porphyritic texture is
the most common, characterised by plagioclase and clinopyroxene
phenocrysts embedded in a fine-grained matrix (Figure~\ref{fig2}c).
Some samples also exhibit a sub-ophitic texture, where clinopyroxene
crystals partially enclose plagioclase grains (Figure~\ref{fig2}c).
The preferential mineral orientation observed in certain sections
suggests deformation and recrystallisation processes related to the
tectonic evolution of the region (Figure~\ref{fig2}a).

\section{Geochemistry}

The metabasalts of the Lebzenia Group, located within the Tasiast
Complex, provide key insights into the geological evolution of this
region. Their geochemical analysis reveals a wide range of basaltic
compositions, ranging from transitional to calc-alkaline, based on the
Th/Yb vs.\ Zr/Y ratios, using the magmatic affinity diagram of 
\citet{Bedard2009} (Figure~\ref{fig4}). 

These metabasalts have SiO$_{2}$ contents ranging from 
46.91 to 51.57 wt\%, with a relatively high magnesium index (Mg\#),
ranging from 37 to 61. They also have notable concentrations of
TiO$_{2}$(0.40 to 1.05~wt\%), Fe$_{2}$O$_{3}$t (7.32 to 16.31 wt\%),
CaO (9.34 to 12.23~wt\%), and MgO (5.18 to 6.94 wt\%), distinguishing
them from the basaltic metavolcanites of the Sebkhet Nich and Talhayet
formations (Table~\ref{tab1}). These characteristics place most of the
samples within the basalt-andesite fields on the Zr/TiO$_{2}$ vs.\ 
SiO$_{2}$ diagram (Figure~\ref{fig3}).



\begin{figure}
\includegraphics{fig03}
\vspace*{-.6pc}
\caption{\label{fig3}Zr/TiO$_{2}$ vs.\ 
SiO$_{2}$ diagram for the  Tasiast area the metabasalts  
\citep{WinchesterFloyd1977}.}
\vspace*{-.8pc}
\end{figure}


\begin{figure}
\includegraphics{fig04}
\caption{\label{fig4}Discrimination of the metabasalts of the Lebzenia Group in the
Tasiast area based on trace elements, using the Th/Yb vs.\ Zr/Yb diagram
of \citet{Bedard2009}. The rocks are predominantly calc-alkaline.
The greenstone belts of Talhayet and Sebkhet Nich are presented for
comparison with our data (symbols are as in Figure~\ref{fig3}).}
\end{figure}


The analysis of Harker diagrams (Figure~\ref{fig5}) highlights
well-defined geochemical trends, illustrating characteristic magmatic
processes. A gradual decrease in CaO and MgO contents as SiO$_{2}$
increases reflects a fractional crystallisation process dominated by
the formation of calcium-rich plagioclases and clinopyroxene.
Furthermore, the relative stability of TiO$_{2}$ content despite the
increase in SiO$_{2}$ and the decrease in Fe$_{2}$O$_{3}$
concentrations with SiO$_{2}$ enrichment suggest the fractionation of
ferromagnesian minerals.


\begin{figure*}
\includegraphics{fig05}
\caption{\label{fig5}Harker Diagrams for Samples Collected from the Lebzenia Group
in the Tasiast area (symbols are as in Figure~\ref{fig3}).}
\end{figure*}


The rare earth element (REE) spectra show moderate and homogeneous
fractionation, with an enrichment of light rare earth elements (LREE)
relative to medium (MREE) and heavy (HREE) rare earth elements
(Figure~\ref{fig6}a), illustrated by the ratios [(La/Sm)N ${=}$
1.7--4.4, (La/Yb)N ${=}$ 1.9--8.6, and (Gd/Yb)N ${=}$ 1.02--2.16]. As for
the trace element spectra, normalised to the primitive mantle
(Figure~\ref{fig6}a), they show an enrichment in Th. There is a large
variation in the La/Nb ratios (0.97 to 62.28), a depletion in Nb--Ta
(Nb/Nb* ${=}$ 0.05--0.30), and a slight negative anomaly in the Zr--Hf
level (Figure~\ref{fig6}a).




\begin{figure*}
\includegraphics{fig06}
\caption{\label{fig6}(a) Primitive mantle-normalized normalized trace element
patterns of the Tasiast metabasalts, based on the normalisation values
proposed by \citet{McDonoughSun1989}. (b) Normal and enriched  
mid-ocean ridge basalt (N-MORB, E-MORB) and ocean island basalt (OIB)
from \citet{McDonoughSun1989}; Calc-Alkaline Basalt (CAB), average
of Sunda, Flores and Mandiri, from \citet{Stolzetal1990}; Island Arc
Tholeiite (IAT) from Tonga, after \citet{TurnerHawkesworth1997}
(symbols are as in Figure~\ref{fig3}).}
\end{figure*}


\section{Discussion} 

\subsection{The magmatic genesis of the Lebzenia Group metabasalts}

Basaltic samples from the greenstone belts of the Lebzenia Group,
located in the Tasiast region of Mauritania, exhibit geochemical
characteristics indicative of a complex magmatic genesis. The low
concentrations of incompatible elements such as Nb, TiO$_{2}$, and
P$_{2}$O$_{5}$ (Figure~\ref{fig7}b) are typical of magmas formed in
environments associated with volcanic arcs  
\citep{Defantetal1992,Sajonaetal1996,Kepezhinskasetal1997}. The samples
are enriched in large-ion lithophile elements (LILE) and depleted in
high-field strength elements (HFSE) relative to light rare earth
elements (LREE), while exhibiting negative Nb-Ta anomalies,
characteristic of igneous rocks associated with volcanic arcs
(Figure~\ref{fig6}a)
\citep{McCullochGamble1991,PearcePeate1995,Kelemenetal2007}. However,
sample DRAM2 shows a strong negative anomaly, indicating magma that has
undergone plagioclase fractionation (Figure~\ref{fig6}a). In
addition, the LREE enrichment observed in samples DRAM3 and DRAM6
suggests a differential melting process, involving a garnet-rich
peridotite source in which HREE was preferentially trapped, changing
the chemical composition of the resulting rocks 
\citep{Kayetal1994,HaschkeGunther2003}.


\begin{figure*}
\vspace*{2pt}
\includegraphics{fig07}
\vspace*{4pt}
\caption{\label{fig7}(a--c) \citet{Pearce2008} diagrams illustrating variations in
mantle source composition (Nb/Yb), the influence of subduction or
crustal contamination (Th/Tb), and melting depth (TiO$_{2}$/Tb). (b)
P$_{2}$O$_{5}$ (wt\%) versus TiO$_{2}$ (wt\%) after  
\citet{Defantetal1992}. A ${=}$ Archaean crust, from \citet{Pearce2008}.
The generic assimilation--fractional crystallisation (AFC) curves are
from  \citet{Pearce2008} for N-MORB and more enriched magmas, each
contaminated with Archaean crust. Th ${=}$ tholeiitic; Alk ${=}$ alkaline;
MORB ${=}$ mid-ocean ridge basalt; OIB ${=}$ ocean island
basalt.}
\end{figure*}


Two main hypotheses can explain these characteristics: (1) An active
subduction context: The magmas may have originated from mantle melting
in a magmatic arc environment, related to either oceanic or continental
subduction 
\citep{McCullochGamble1991,PearcePeate1995,Kelemenetal2007}. (2)
Crustal contamination: The interaction of magmas with pre-existing
continental crust could explain the high Th/Nb ratios (${>}$0.2) and the
negative Nb, Ta, and Ti anomalies \citep{Lightfootetal1990,Pearce2008}.

The geochemical signatures of the Lebzenia Group metabasalts can be
interpreted in terms of different tectonic environments. Although Nb--Ta
anomalies are typical of arc magmas, the lack of direct evidence for an
active subduction zone suggests an environment of mature continental
rifting or a thinned continental margin \citep{Wilson1989}. However,
the specific data from the Lebzenia Group basalts allow us to rule out
one of these two hypotheses.

Th/Nb ratios greater than 0.2, consistent with \citet{Pearce2008}
criteria, indicate that these samples have undergone significant
crustal contamination, likely from a Th-enriched Archaean crust.
Similar geochemical features have been identified in peri-Atlantic
Triassic dolerites \citep{Marzolietal2019}, as well as in Cambrian
metabasites associated with the opening of the Iapetus Ocean 
\citep{Oualietal2001,Oualietal2003}. 

The samples with transitional calc-alkaline characteristics exhibit a
distinct trend of assimilation and fractional crystallisation (AFC),
suggesting a potential influence on the Archaean crust or contaminants
(Figure~\ref{fig7}a). These potential contaminants show enrichment in
Th, high values of light rare earth elements relative to heavy rare
earth elements, and negative Nb--Ta--Ti anomalies. Samples DRAM7 and
DRAM8 plot within the tholeiitic OIB field on the TiO$_{2}$/Yb vs.\ 
Nb/Yb diagram (Figure~\ref{fig7}c). These observations suggest a
mantle melting event occurring at greater depths, probably in an
asthenospheric mantle influenced by a mantle plume \citep{Pearce2008}.

We propose that the basalts of the Lebzenia Group originated from the
asthenospheric mantle but were contaminated by a Th-rich Archaean crust
during their ascent. The formation of the Lebzenia Group does not
require the presence of a volcanic arc or a subduction zone.

\subsection{Comparison with other metabasalts in the Tasiast region}

Our samples exhibit geochemical profiles similar to those observed in
modern volcanic arcs. These profiles are characterised by an enrichment
of light rare earth elements (REE), elevated concentrations of thorium
(Th), and negative anomalies in Nb--Ta ${\pm}$ Ti
(Figure~\ref{fig6}a,b). These characteristics are typical of magmas
generated in subduction zones, where the mantle source is enriched by
fluids derived from the crust.

In contrast, the formations of Talhayet and Sebkhet Nich display
geochemical characteristics more akin to those of modern N-MORB. These
rocks are characterised by relatively low concentrations of
incompatible elements such as Nb, Ta, Th, and La (Figure~\ref{fig6}a)
and are distinguished by a low Nb/La ratio as well as a very low Th/Nb
ratio, suggesting that they have not been influenced by crustal
contamination. In addition, the REE patterns of these formations show a
flat slope, with almost no Nb--Ta anomaly, in sharp contrast to the
characteristics of arc magmas (Figure~\ref{fig6}a,b).

Modern N-MORB, although formed in environments similar to those of
E-MORB, are derived from a more depleted mantle source 
\citepalias{Pearce2008}. The geochemical differences observed in our samples
suggest that the Lebzenia Group metabasalts have a distinct geological
history from the rocks of the classical-formations studied.

The exact age of the metabasalts of the Lebzenia Group remains
uncertain, but it could be comparable to that of the schistose
metadacites with plagioclase phenocrysts of the Aou\'{e}oua Formation.
These rocks exhibit a geochemical signature typical of volcanic arcs,
characterised by a pronounced negative Ta--Nb anomaly, indicating a
subduction-related magmatic setting \citep{Pittfieldetal2005}. They also
have a high SiO$_{2}$ content (73.8\%), reflecting a significant degree
of magmatic differentiation, which accounts for the elevated
concentrations of Zr and Hf \citepalias{Pittfieldetal2005}. Their geochemical
profile is marked by an enrichment in light rare earth elements (LREE),
suggesting an extended fractional crystallisation process
\citepalias{Pittfieldetal2005}. These metadacites have been dated to
approximately 2.968~Ga \citep{Pittfieldetal2005,Kayetal2008}, providing
a potential temporal framework for the associated basaltic formations.


Furthermore, our results highlight specific geochemical anomalies that
may indicate particular formation processes or tectonic contexts unique
to the Tasiast region, suggesting a distinctive geological dynamic for
this area.



\section{Conclusion} 

The geochemical analysis of the basaltic samples from the Lebzenia
Group in the Tasiast region reveals a complex magmatic history, with
signatures indicative of significant mantle melting and crustal
contamination. The observed enrichment in light rare earth elements
(LREE), elevated thorium (Th) concentrations, and negative Nb--Ta
anomalies suggest a magmatic origin in an environment influenced by
subduction processes. However, the presence of crustal contamination,
as evidenced by high Th/Nb ratios and distinct calc-alkaline
characteristics, suggests a more complex tectonic setting, possibly
involving a mature continental rift or a thinned continental margin,
rather than a traditional volcanic arc.

The comparison with modern N-MORB and E-MORB further supports the idea
of a mantle source which produced a basalt that was enriched during
ascent through a Th-rich Archaean crust, rather than originating from
an active subduction zone. These findings, coupled with the distinct
geochemical anomalies in the Lebzenia Group basalts, provide new
insights into the tectonic evolution of the Tasiast region, and suggest
that the region may have experienced unique processes of magmatic
differentiation and crustal interaction.


The results highlight the need for further studies to better understand
the complex interactions between mantle processes and crustal
contamination in the formation of the Lebzenia Group basalts. Our
results open new perspectives on the geological dynamics of the region,
which could contribute to a broader understanding of the tectonic and
magmatic processes in the Archaean and Proterozoic.


\section*{Declaration of interests}

The authors do not work for, advise, own shares in, or receive funds
from any organization that could benefit from this article, and have
declared no affiliations other than their research organizations.


\section*{Acknowledgements}

This article was funded by the FP7-PEOPLE-2013-IRSES MEDYNA project:
Maghreb-EU research staff exchange on geodynamics, geohazards, and
applied geology in Northwest Africa 2014-2017, WP3 - Deep structures
and mantle processes. We would like to express our deep gratitude to
Carlos J. Garrido, co-coordinator of MEDYNA, for providing access to
the analytical facilities of the Department of Petrology, Geochemistry,
and Geochronology (PGG) at IACT-CSIC in Granada. We also warmly thank
all the members of the PGG for their invaluable help in sample
preparation and their assistance during the laboratory work.

\CDRGrant[EU]{FP7-PEOPLE-2013-IRSES}

\back{}

\printbibliography
\refinput{crgeos20250145-reference.tex}

\end{document}
