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\DOI{10.5802/crgeos.289}
\datereceived{2024-11-19}
\daterevised{2025-03-08}
\datererevised{2025-03-18}
\dateaccepted{2025-03-20}
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\dateposted{2025-06-10}
\begin{document}

\begin{noXML}

\CDRsetmeta{articletype}{research-article}
\TopicFR{Tectonique, tectonophysique, g\'eodynamique}
\TopicEN{Tectonics, tectonophysics, geodynamics}

\shortrunauthors

\title{New K--Ar data in the northeastern edge of the Adamawa-Yad\'{e}
domain, Central African Pan-African Fold Belt: shedding light on the
transition age between  convergent and divergent tectonic regimes}

\alttitle{Nouvelles donn\'{e}es K--Ar dans la bordure nord-est du
domaine de  l'Adamaoua-Yad\'{e}, Ceinture pliss\'{e}e Pan-Africaine
d'Afrique Centrale  : mise en lumi\`{e}re de l'\^{a}ge de transition
entre les r\'{e}gimes  tectoniques convergents et divergents}

\author{\firstname{Beno\^{\i}t Joseph} \lastname{Mbassa}\CDRorcid{0000-0003-4161-1177}\IsCorresp}
\address{Institute for Geological and Mining Research, PO Box 4110, 
Yaound\'{e}, Cameroon}
\email[B. J. Mbassa]{benjo\_mbassa@yahoo.fr}

\author{\firstname{Jes\'{u}s} \lastname{Sol\'{e}}\CDRorcid{0000-0001-9600-5970}}
\address{Instituto de Geolog\'{\i}a, Laboratorio Nacional de 
Geoqu\'{\i}mica y Mineralog\'{\i}a, Universidad Nacional Aut\'{o}noma de 
M\'{e}xico. 04510 Mexico City, Mexico}

\author{\firstname{Daouda} \lastname{Dawa\"{\i}}\CDRorcid{0000-0003-0354-1876}}
\address{Department of Earth Sciences, Faculty of Sciences, University 
of Maroua, PO Box 814 Maroua, Cameroon}

\author{\firstname{Caroline} \lastname{Ngwa Neh}\CDRorcid{0009-0003-4604-9483}}
\addressSameAs{1}{Institute for Geological and Mining Research, PO Box 4110, 
Yaound\'{e}, Cameroon}

\author{\firstname{Z\'{e}non}\nobreakauthor\lastname{Itiga}\CDRorcid{0009-0006-9353-1258}}
\address{Department of Earth Sciences, University of Douala, PO Box 
24157 Douala, Cameroon}

\author{\firstname{Jean-Paul} \lastname{Sep Nlomngan}}
\addressSameAs{1}{Institute for Geological and Mining Research, PO Box 4110, 
Yaound\'{e}, Cameroon}

\author{\firstname{Patrice Augustin} \lastname{Moussango Ibohn}}
\addressSameAs{1}{Institute for Geological and Mining Research, PO Box 4110, 
Yaound\'{e}, Cameroon}

\author{\firstname{Mo\"{\i}se}\nobreakauthor\lastname{Bessong}\CDRorcid{0000-0002-4389-1988}}
\addressSameAs{1}{Institute for Geological and Mining Research, PO Box 4110, 
Yaound\'{e}, Cameroon}

\author{\firstname{Vincent} \lastname{Ngako}}
\addressSameAs{1}{Institute for Geological and Mining Research, PO Box 4110, 
Yaound\'{e}, Cameroon}

\author{\firstname{Emmanuel} \lastname{Njonfang}\CDRorcid{0000-0002-1494-3590}}
\address{Higher Teachers Training School, University of Yaound\'{e} I, 
member of the Cameroon Academy of Sciences, PO Box 812 Yaound\'{e}, Cameroon}

\begin{abstract}
The tectonic evolution of the Central African Pan-African Fold Belt in 
Cameroon has been intensively studied during the last two decades. However, 
the timing of the last stages of the Pan-African orogenic cycle is still 
not well constrained, thus leading to controversial interpretations. The 
present work provides new insights into the late Pan-African tectonic 
evolution of the Adamawa-Yad\'{e} domain. It includes a complex crustal 
assemblage of the Central African Pan-African Fold Belt, which has 
undergone a polycyclic evolution since the Palaeoproterozoic. We infer the 
late Pan-African events of this domain from in-situ dating of biotite in 
five thin sections of plutonic rocks. The dated rocks are calc-alkaline 
metaluminous to weakly peraluminous I-type granitoids, consisting of 
Qz-monzonites and granites, medium- to high-potassic, with shoshonitic 
affinities, all displaying adakitic signature. The ages obtained were 
correlated with the available geochronological data for rocks from 
post-collision events of the Adamawa-Yad\'{e} domain and the Central 
African and Brazilian orogenic belts. The new biotite age range spans 67 Myr 
geological history of the studied area, from Ediacaran (549 \textpm{} 19 Ma) to 
Early Ordovician (482 \textpm{} 6 Ma), which marks the transition between 
convergent and extensional tectonics regimes in the Central African 
Pan-African Fold Belt. 
\end{abstract}

\begin{altabstract}
L'\'{e}volution tectonique de la Ceinture pliss\'{e}e Pan-Africaine 
d'Afrique Centrale au Cameroun a \'{e}t\'{e} intensivement \'{e}tudi\'{e}e 
au cours des deux derni\`{e}res d\'{e}cennies. Cependant, la chronologie des 
derni\`{e}res phases du cycle orog\'{e}nique Pan-Africain ne demeure pas 
bien d\'{e}finie, ce qui conduit \`{a} des interpr\'{e}tations 
controvers\'{e}es. Le pr\'{e}sent travail apporte de nouvelles perspectives 
sur l'\'{e}volution tectonique Pan-Africaine tardive du domaine de 
l'Adamaoua-Yad\'{e}. Ce dernier comprend un assemblage crustal complexe de 
la Ceinture pliss\'{e}e Pan-Africaine d'Afrique Centrale, ayant subi une 
\'{e}volution polycyclique depuis le Pal\'{e}oprot\'{e}rozo\"{\i}que. Nous 
d\'{e}duisons les \'{e}v\'{e}nements Pan-Africains tardifs de ce domaine 
\`{a} partir de la datation in situ sur biotite dans cinq lames minces de 
roches plutoniques. Les roches dat\'{e}es sont des granito\"{\i}des 
calco-alcalins de type I, m\'{e}talumineux \`{a} faiblement hyperalumineux, 
compos\'{e}s de monzonites \`{a} quartz et de granites, moyennement \`{a} 
fortement potassiques, \`{a} affinit\'{e}s shoshonitiques, et pr\'{e}sentant 
toutes une signature adakitique. Les \^{a}ges obtenus ont \'{e}t\'{e} 
corr\'{e}l\'{e}s avec les donn\'{e}es g\'{e}ochronologiques disponibles pour 
les roches issues des \'{e}v\'{e}nements post-collision du domaine de 
l'Adamaoua-Yad\'{e} et des cha\^{\i}nes orog\'{e}niques d'Afrique Centrale 
et du Br\'{e}sil. La nouvelle tranche d'\^{a}ges sur biotite couvre 67 
millions d'ann\'{e}es d'histoire g\'{e}ologique de la zone \'{e}tudi\'{e}e, 
allant de l'\'{E}diacarien (549 \textpm{} 19 Ma) \`{a} l'Ordovicien 
inf\'{e}rieur (482 \textpm{} 6 Ma), marquant la transition entre les 
r\'{e}gimes tectoniques convergents et extensifs dans la Ceinture 
Pliss\'{e}e Panafricaine d'Afrique Centrale.
\end{altabstract}

\keywords{\kwd{In-situ K--Ar dating}
\kwd{Central African Pan-African Fold Belt}
\kwd{Adamawa-Yad\'{e} domain}
\kwd{\unskip\break Ediacaran}
\kwd{Early Ordovician}
\kwd{Post-collision events}}

\altkeywords{\kwd{Datation K--Ar in situ} 
\kwd{Ceinture Pliss\'{e}e Pan-Africaine d'Afrique Centrale}
\kwd{Domaine de l'Adamaoua-Yad\'{e}}
\kwd{\'{E}diacarien}
\kwd{Ordovicien inf\'{e}rieur}
\kwd{\'{E}v\'{e}nements post-collision}}

\maketitle

\twocolumngrid

\end{noXML}

\defcitealias{PhilpottsAgue2010}{ibid.}

\section{Introduction}

The main geological units in Cameroon consist of Archean to Late Proterozoic 
plutonic--metamorphic basement, intruded by Palaeozoic to Mesozoic 
post-Pan-African terranes and anorogenic plutonic complexes, and partially 
covered with Cenozoic to Quaternary volcano-sedimentary rocks. The basement 
includes the northern edge of the Archean Congo craton known as the Ntem 
complex, the Nyong Palaeoproterozoic unit, and the widespread Central 
African Pan-African Fold Belt (CAPFB). The geodynamic evolution of the 
Cameroonian segment of this CAPFB has been the subject of numerous models 
\citep[e.g.][]{Toteuetal2004,Bouyoetal2009,NgakoNjonfang2011};
however, a recent one by \citet{Toteuetal2022}, taking into account 
petrological and geochronological data together with structural analyses, 
involves two subductions and four main steps. The successive phases consist 
of: (1) the break-up and basin development both on the Ntem complex and on 
the northern and southern limits of the Adamawa-Yad\'{e} domain, 
synchronously with the expansion of the Poli-Ler\'{e} arc (Tonian---620 
Ma); (2) the ubiquitous pre-tectonic plutonism (800--620 Ma); 
(3) the collisional phase (${\sim}$620 Ma) was marked by a granulite 
facies metamorphism in all areas at 600 Ma; and (4) the ultimate one 
resulting in: (i) the overlapping of the Yaound\'{e}-Yangana units onto the 
Ntem complex, (ii) the accretion of the Poli-Ler\'{e} arc to 
Adamawa-Yad\'{e} domain, (iii) the extensive syntectonic magmatism partially 
controlled by local shear zones (600--580 Ma), and (iv) the emplacement of 
the Adamawa-Yad\'{e} and Poli-Ler\'{e} post-tectonic granitoids 
(${\sim}$550 Ma). The Adamawa-Yad\'{e} domain (including our study 
area) represents an asymmetric band of Precambrian basement rocks located 
East of the Tchollir\'{e}-Banyo shear zone (TBSZ) and extending from 
Cameroon (Adamawa) to the Central African Republic (Yad\'{e}) and even to 
central Southern Chad \citep{Shellnuttetal2020,Djerossemetal2021}. It 
comprises Neoproterozoic granitoids emplaced during successive orogenic 
phases \citep{Deloretal2021,Bernardetal2019} in a metamorphosed
basement.  This basement underwent two orogenic cycles: the
Eburnean--Transamazonian  around 2.1 Ga, then the Pan-African one
around 600 Ma  \citep{TankoNjosseuetal2005}. As much as the
Neo-Proterozoic events of the North equatorial  Pan-African fold belt
are\break sufficiently described and dated, little is said  about its
Paleozoic events.

The Palaeozoic Pan-African crustal evolution is marked by an extensional 
tectonic episode characterised at their early stages by tectonic, 
geophysical, and structural markers of block readjustments (such as normal 
faults, horsts, grabens, rifts, etc.), except in the case of aborted rifts. 
The Late Palaeozoic to Mesozoic tectonic regime of West--Central Africa was 
ruled by tensional stresses resulting from the reactivation of the Central 
Africa Shear Zone \citep{GuiraudMaurin1991,Moreauetal1987,Tchouankoueetal2014}
or St. Helena mantle plume activity 
\citep{Coulonetal1996} and accompanied with magmatism \citep{Maluskietal1995}. 
Nevertheless, the post-Pan-African magmatism of the CAPFB in Central 
Africa, particularly the Palaeozoic to Mesozoic magmatism in Cameroon, is 
still poorly known, although it is of great interest to understand the whole 
Pan-African orogeny. In this paper, we present evidence of early Palaeozoic 
events through new K--Ar ages obtained on biotite from magmatic rocks 
carried out in an area located East of Ngaound\'{e}r\'{e} city and bounded 
by the latitudes 7\textdegree{}19$'$ to 7\textdegree{}45$'$ North and the 
longitudes 13\textdegree{}30$'$ to 14\textdegree{}20$'$ East (Figure~\ref{fig1}c). Their 
implications on the post-Pan-African evolution of the CAPFB in Cameroon 
as well as their geodynamic significance (transition between convergent and 
extensional tectonic regimes) are discussed.

\begin{figure*}[p!]
\includegraphics{fig01}
\caption{\label{fig1}Location of the study area: (a) South
America--Africa  fit, showing cratons of western Gondwana 
\citep[modified after][]{Nevesetal2016}; (b) Geological sketch of Cameroon 
\citep[modified after][]{Castaingetal1994,Ngakoetal2003,Owonaetal2013}
with position of the study  area; (c) Simplified geological map of the
study area \citep[modified from][]{Lasserre1961} together with the
position of the studied samples.}
\end{figure*}

\section{Geological setting and review of previous dating results}

The regional geology includes Neoproterozoic igneous suites 
(Lint\'{e}-God\'{e}, Yangba-Kogu\'{e}, calc-alkaline suite of central 
Cameroon), and a vast basalt plateau crosscut by felsic domes in the 
southeastern region. The basement is underlain by gneisses and syn-to 
post-tectonic calc-alkaline granitoids, locally covered with volcanic and 
sedimentary rocks \citep{Toteuetal2001,Tchamenietal2006,Ganwaetal2008}. 
The volcanic sequence consists of mafic or felsic lavas resulting 
from the melt of a heterogeneous mantle, with possible crustal contamination 
\citep{Nkouandouetal2008,Itigaetal2014,TiabouFeudjioetal2018}. 

The local tectonic is characterised by: (1) the presence of NE--SW mylonitic 
foliation associated with the regional-scale TBSZ, known as the last stage 
of a D$_{1}$--D$_{2}$ polyphase deformation associated 
with high-grade metamorphism that affected the Palaeoproterozoic relics of 
granulitic assemblages and late tectonic granitoids \citep{Toteuetal2001, 
Bouyoetal2009}; and (2) the WSW--ENE dextral Central Cameroon 
Shear Zone. \citet{Nkonoetal2009} mentioned two main directions of lineaments: 
(i) The N65E corresponding to the Cenozoic faults, which have controlled the 
setting up of the Adamawa plateau, and (ii) the N167E direction. This local 
tectonic was segmented into four phases (D$_{1}$ to 
D$_{4}$) of deformation using together structural markers and 
geochronological data (U--Pb and Sm--Nd dating) \citep{Toteuetal2001,Toteuetal2004, 
Ngakoetal2008,NgakoNjonfang2011}; D$_{1}$ and 
D$_{2}$ mark the crustal thickening event dated between 635--615 
Ma. D$_{1}$ is represented by a low-dipping syn-migmatitic 
foliation (S$_{1}$) ascribed to the development of nappes oriented 
towards the East, whereas D$_{2}$ corresponds to the transposition 
of S$_{1}$ into conjugate steeply dipping shear zones and upright 
folds leading to the expansion of the main NE--SW foliation recording the 
local NW--SE shortening. The D$_{3}$ phase dated between 600--580 
Ma (U--Pb age; \xcitealp{Penayeetal1989}{1989}; 
\xcitealp{Toteuetal2001}{2001}) is expressed by 
deformations along sinistral shear zones trending N--S to NE--SW (e.g.\  
Tchollir\'{e}-Banyo) while the D$_{4}$ corresponds to a regression 
associated with the activation of NNE--SSW to E--W dextral shear zones (e.g.\ 
CCSZ) around 545 Ma \citep{BessolesTrompette1980}.

Geochronological data in the study area are still scarce and not yet 
integrated into a comprehensive model. A limited number of geochronological 
investigations with ages ranging from Archean to Neoproterozoic have been 
carried out in its nearest neighbourhoods, including: (i) a 2.1 ${\pm}$ 0.1 
Ga U--Pb age of the reworked metasediments and orthogneisses basement from 
Touldoro (East of Ngaound\'{e}r\'{e}) \citep{Toteuetal2001}; (ii) a 2.1 ${\pm}$ 
0.1 Ga U--Pb ages on hornblende--biotite gneisses of the Mb\'{e} region 
\citep{Penayeetal1989}; (iii) the $^{207}$Pb/$^{206}$Pb ages 
on pyroxene--amphibole gneiss from Meiganga region \citep{Ganwaetal2008} 
ranging from ${\sim}$2.6~${\pm}$~0.1~Ga for inherited Archean core 
to ${\sim}$1.7 ${\pm}$ 0.04 Ga for Palaeoproterozoic magmatic 
domain; (iv) the Th--U--Pb monazite ages on the Ngaound\'{e}r\'{e} 
biotite--muscovite granites and biotite--granite between 615~${\pm}$~27 and 
575 ${\pm}$ 8 Ma \citep{Tchamenietal2006} and (v)~U--Pb zircon ages from 
Sassa-Mbersi ranging from 725 ${\pm}$ 12 Ma for the sedimentary protolith 
to 573 ${\pm}$ 2 Ma corresponding to emplacement of pegmatite dykes along 
dextral shear zones \citep{Saha-Fouotsaetal2019}.\looseness=-1

{In Tchollir\'{e} and Banyo regions, U--Pb and Sm--Nd dating of zircons and 
garnet-whole rock pairs yield ages ranging from Palaeoproterozoic to 
Neoproterozoic, considered as dating inheritances. Zircon overgrowths and 
recrystallised domains give U--Pb ages ranging between 604 ${\pm}$ 4 and 594 
${\pm}$ 8 Ma, interpreted as corresponding to HP granulite-facies 
metamorphism, in accordance with Sm--Nd garnet-whole rock ages. Sediments 
were deposited after 620 Ma from Palaeoproterozoic to Neoproterozoic 
protoliths \citep{Bouyoetal2009}.

Palaeozoic events marking the post-tectonic granitoids from 
Adamawa-Yad\'{e} domain have been highlighted by Rb--Sr dating 
\citep{Toteuetal1986} and the discovery of Cambrian--Devonian detrital igneous zircons 
in conglomerates of the Tibati region. Whole-rock Rb--Sr dating carried 
out by \citet{Lasserre1967} and \citet{Tempieretal1981} on the granitoids gave an age 
range of 531 ${\pm}$ 19 Ma to 558 ${\pm}$ 40 Ma; subsequently, 
\citet{Toteuetal1986} obtained a Rb--Sr isochron age of 546 ${\pm}$ 9 Ma in the God\'{e} 
massif, and U--Pb ages on Baddeleyite of the dolerites veins north of 
Ngaound\'{e}r\'{e} (542 ${\pm}$ 4 Ma on dolerites of Mbaoussi; 542 ${\pm}$ 4 
Ma, 543 ${\pm}$ 5 Ma and 536 ${\pm}$ 6.4 Ma on those of Nom\'{e}).

\vspace*{-3pt}
\section{Field descriptions}

The study area includes two main lithologic units: volcanic and granitic 
rocks. Volcanic rocks outcrop as dome-shaped massifs or lava flows 
generally elongated in a preferential direction and occasionally as dykes 
intersecting granites. The lava flows are usually prismatic, massive, or 
vacuolar. 

Granitoids consist of biotite- and biotite-amphibole-granites, locally 
banded or deformed and cropping out as massifs, metric boulders, slabs, or 
domes. The banded structure is often reinforced by migmatisation. 
Fine-grained rocks are mesocratic to melanocratic and generally outcrop 
either as injections within corridors delimited by quartzo-feldspathic 
veins or as enclaves. Two local phases of deformation D$_{1}$ and 
D$_{2}$, are perceptible. The D$_{1}$ phase is 
marked by S$_{1}$ foliation dipping 30\textdegree{} toward NNE with a 
strike orientation of N100--125\textdegree{}E\break (Figure~\ref{fig2}a). This foliation is 
often straightened out and underlined by a lithologic banding materialised 
by dark ferromagnesian (amphibole ${\pm}$ biotite) layers alternating with 
clear quartz-feldspathic layers. The D$_{2}$ phase is represented 
by a mylonitic S$_{2}$ foliation striking 
N45--50\textdegree{}E and dipping 70NW to subvertical (Figure~\ref{fig2}b,c), the 
boudinage (B$_{2})$ and the (L$_{2})$ stretching 
lineation. The S$_{2}$ results from the transposition of 
S$_{1}$ (Figure~\ref{fig2}c), possibly due to dextral shear, resulting in the 
development of a new composite S$_{2}$/C$_{2}$ fabrics. 
The brittle deformation is represented by punch-through cracks, joints, and 
sub-meridian faults (Figure~\ref{fig2}d). Dry joints are sub-vertical or parallel to 
the topographic surface, with a global N--S direction. Filled joints are 
basaltic dykes or pegmatitic to fine-grained veins (Figure~\ref{fig2}d,e). Dextral 
strike-slip and ductile brittle faults are locally noticeable. The ductile 
brittle fault conveys the direction N45--50\textdegree{}E, 70\textdegree{}NW 
and is accompanied by numerous fractures oriented N135E--N150\textdegree{}E 
dipping 70--80\textdegree{}SW.

\begin{figure*}[t!]
\vspace*{2pt}
\includegraphics{fig02}
\caption{\label{fig2}Some deformation markers of the basement rocks of the 
studied area. Ductile deformation (a--c) in deformed granites: (a) 
S$_{1}$ foliation on Maboro stream bed, (b) S$_{2}$ 
Foliation taking over the S$_{1}$ with the presence of alkaline 
feldspar relics revealing the magmatic origin of the protolith, (c) Sigmoid 
boudin B2 illustrating a dextral motion. Brittle deformation (d,e): (d) 
Filled joints illustrated by a basaltic dyke crosscutting granite, (e) Punch 
fractures observed in deformed granite.}
\vspace*{2pt}
\end{figure*}

\vspace*{-3pt}
\section{Methodology}

\subsection{Samples collection and specimen preparation}

Fresh representative granitic rocks were collected in an area of about 50 km 
diameter at Ngaound\'{e}r\'{e} city vicinities in central-northeastern 
Cameroon (see Table~\ref{tab2} for their locations). Polished thin sections were 
prepared following standard procedures for petrographic observations
and  dating. Five representative samples were selected based on their
petrography  for X-ray fluorescence analyses and K--Ar dating in
Mexico at the  \textit{Laboratorio Nacional de Geoqumica y Mineraloga
(LANGEM), Instituto de Geologa, Universidad Nacional Aut\'{o}noma de
M\'{e}xico (UNAM)}. 

\vspace*{-3pt}
\subsection{Analytical methods}

Bulk rock major elements were measured using a Rigaku Primus II X-ray 
fluorescence spectrometer calibrated with international reference materials. 
The rock samples were dried at 110~\textdegree{}C for 2~h before analyses. 
To prepare the glass tablet, 0.8 g of dry powder was mixed with 7.8 g of 
Li$_{2}$B$_{4}$O$_{7}$ and heated in a gas 
furnace to fusion to obtain a pearl. Loss on ignition (LOI) was determined 
with 1 g of dry sample powder calcined at 1000~\textdegree{}C for 1~h, and 
the mass loss was subsequently determined.

For K--Ar dating, the rock samples were chosen after petrographic 
observation, based on the presence of well-preserved biotite flakes. 
Samples were cut, glued with UV-cured epoxy, and polished with diamond 
paste to obtain standard thin polished sections of 27 $\times$ 46 $\times$ 
0.03 mm. The slides were mounted within an ultrahigh vacuum chamber and 
prepared for the in-situ K--Ar system. The automatised analytical system 
described in detail by \citet{Sole2014,Sole2021} is made of a noble gas mass 
spectrometer MAP 216 connected to a stainless steel ultra-high vacuum line. 
The line has two SAES GP-50 getters (one at 250~\textdegree C and the 
other at room temperature) to clean the gases evolved after ablating the 
sample. The line is also connected to an ultra-high vacuum chamber with a 
double window for transmitted light observation of the thin sections mounted 
inside. Light can be polarised to identify the crystals to be analysed. The 
ablation is done with an excimer ATL-RX laser with a wavelength of 193 
$\rmmu$m, using a beam diameter of 50 or 70 $\rmmu$m, a repetition rate of 
30--50 Hz, and 200 pulses. Fluence on samples was ${\sim} $3 J 
cm$^{-2}$, and ablation pits were about 20 $\rmmu$m depth. During 
the ablation process, a small plasma is produced, whose light is sampled by 
an optical spectrometer to obtain the K concentration. The resultant gas, 
after cleaning, is equilibrated into the mass spectrometer for Ar 
measurement. The Supplemental Material section contains a detailed 
analytical protocol and additional sample's information. Supplement S1 
includes petrographic microscope photographs of all analysed points in 
plane-polarised light and crossed Nicols. Readers can verify the ages of 
each point and their mineralogical/textural positions. It is important to 
highlight that this method has revealed that in micas from rocks subjected 
to deformation or metamorphism, a simple age gradient derived from diffusion 
theory does not explain the patterns well. The minerals in contact with the 
dated mica play a crucial role in fluid circulation and the ease of argon 
diffusion. Furthermore, age dispersions may vary based on the rock type and 
the preservation of the\break micas.

\section{Results}

\subsection{Descriptive petrography}

The studied rocks display a fine- to coarse-grained equigranular to 
heterogranular magmatic structure at a microscopic scale (Figure~\ref{fig3}a,b). The 
primary mineral phase includes quartz, alkali feldspar, plagioclase, 
amphibole, and biotite. The accessory phases consist of opaque minerals, 
sphene, apatite, and zircon. The secondary mineral phase consists of 
chlorite, epidote, and sericite crystallised at the expense of biotite, 
amphibole and alkali feldspars respectively. Quartz appears as anhedral 
medium to coarse recrystallised phenocrysts or interstitial fine grains. 
Plagioclase crystals exhibit typical twin lamellas and generally enclose 
Fe--Ti-oxides, biotite, amphibole, chlorite, or apatite, while fine 
crystals are usually included in alkali feldspars. Alkali feldspars consist 
of anhedral to subhedral microcline and orthoclase crystals, locally 
displaying cross-hatched and Carlsbad twins. The largest crystals 
(${&gt;}$1 mm) frequently show perthites and myrmekites. Amphibole 
appears as anhedral to subhedral brown to greenish pleochroic crystals 
(Figure~\ref{fig3}c,d). Phenocrystals are poikilitic 
(biotite ${+}$ apatite ${+}$ Fe--Ti-oxides), often zoned, and frequently show corroded margins, whereas 
microcrystals are interstitial. Biotite crystals are brown or greenish, with 
a regular cleavage. Fine crystals are either included in alkali feldspars 
(Figure~\ref{fig3}a) or interstitial, while phenocrysts commonly enclose apatite, 
zircon, and opaque minerals (Figure~\ref{fig3}d). Opaque minerals generally appear as 
rounded, angular, or skeletal crystals, especially concentrated in the 
interstitial phase or included in primary minerals. Phenocrysts enclose 
zircon and apatite. Zircon is euhedral with a polygonal or cubic shape and 
is regularly included in biotite or opaque minerals. Apatite mostly 
crystallises either as interstitial acicular or hexagonal microcrystals or 
as anhedral or rounded crystals included within feldspars or biotite 
(Figure~\ref{fig3}c,d--f). The more or less deformed character of the studied granitoids is 
sufficiently perceptible on certain minerals such as biotites displaying 
typical microstructures of deformation such as the elongation biotites 
flakes, the reduction of the grain size of the mica aggregates by the 
nucleation of new micas, and rarely mineral cracking or folding bands.}

\begin{figure*}[p!]
\includegraphics{fig03}
\caption{\label{fig3}Representative photomicrographs for granitic rocks from 
Ngaound\'{e}r\'{e}. (a) Medium- to coarse-grained texture in Qz-monzonite 
BJM410 and inclusion of biotite microcrystal in alkali feldspar; (b) 
Fine-grained texture in granite BJM520; (c) Marks of deformation at 
microscopical scale in granite GAM521 revealed by bending of biotite 
cleavages and feldspars cracking; (d) Biotite flakes associated with 
euhedral sphene and hornblende including apatite and opaque minerals in 
Qz-monzonite BJM410; (e) Alkali feldspar exhibiting perthite in granite 
BIN089; (f) Zoned feldspar and biotite including apatite in Qz-monzonite 
BJM069. Mineral symbols are after \citet{Kretz1983}.}
\end{figure*}

\subsection{Nomenclature and geochemical features}

The study granitoids have K$_{2}$O, Na$_{2}$O and 
Al$_{2}$O$_{3}$ contents ranging respectively between 2.8--6.3 wt{\%}, 
3.2--4.8 wt{\%} and 13--16.2 wt{\%}, and low Mg{\#} 
(13.2--25) as presented in Table~\ref{tab1}. They are potassic 
\mbox{regarding} their 
Na$_{2}$O--K$_{2}$O values (${&lt;}$2) according to  
\citet{LeBasetal1986}. In the total alkali vs.\ silica (TAS) classification 
diagram of \citet{Middlemost1994}, they plot in the Qz-monzonite and granite 
fields (Figure~\ref{fig4}A). Their normative composition is marked by (i) the presence 
of normative hypersthene (0.4--1.4 wt{\%}) in all samples, and (ii) the 
occurrence of normative corundum, except in the Qz-monzonite sample (BJM 
410) containing normative diopside (2.18 wt{\%}). Three samples including 
two granites (BJM520 and BIN089) and Qz-monzonite BJM069 plot within the 
high-K to shoshonitic fields in the K$_{2}$O vs.\ 
SiO$_{2}$ diagram of \citet{LeMaitreetal1989}, one Bt-granite 
(GAM521) and one Qz-monzonite (BJM410) occupy the field of medium potassic 
rocks (Figure~\ref{fig4}b). Considering their position in the diagram of 
\citet{ManiarPiccoli1989} and 
\citet{ChappellWhite1992} (Figure~\ref{fig4}c), the study granitoids 
are calc--alkaline, of I-type, metaluminous to weakly peraluminous (A/CNK: 
0.92--1.05; A/NK: 1.16--1.49). The slightly peraluminous granitoids 
belong to the ferroan series with Fe{\#} (FeOt/FeOt ${+}$ MgO) values ranging 
from 0.84 to 0.87 while metaluminous granitoids (Qz-monzonites) fit in the 
magnesian series (Figure~\ref{fig4}d), with Fe{\#} varying from 0.75 to 0.81 
Regarding their high Na$_{2}$O contents (3.2--4.8 wt{\%}) and 
their shoshonitic affinity (overall high 
K$_{2}$O/Na$_{2}$O ratio), the study granitoids likely 
display adakitic signature. Considering the low MgO (${&lt;}$3.0~wt{\%}) 
and Mg{\#} (mostly ${&lt;}$45) of all the rocks displaying adakitic 
signature, they can be classified as low-Mg adakitic rocks. On the MgO and 
Mg{\#} vs.\ SiO$_{2}$ discrimination diagrams, they respectively 
plot into the fields of high-silica adakite, and C-type adakites, likely 
derived from the melting of a thick lower crust (Figure~\ref{fig4}e,f).

\begin{sidewaystable*}
\vspone
\caption{\label{tab1}Whole rock major elements chemical analyses of the 
analysed granitoids}
\tabcolsep=4pt
\begin{tabular}{cccccccccccccccccc}
\thead
{Rock type}&amp; {Sample}&amp; {SiO}$_{2}$ &amp; {TiO}$_{2}$ &amp; {Al}$_{2}$O$_{3}$ &amp; {Fe}$_{2}$O$_{3}$T&amp; {MnO}&amp; {MgO}&amp; {CaO}&amp; {Na}$_{2}$O &amp; {K}$_{2}$O &amp; {P}$_{2}$O$_{5}$ &amp; {LOI}&amp; {Total}&amp; {A/NK}&amp; {A/CNK}&amp; {Mg{\#}}&amp; {Fe{\#}} \\
\endthead
\morerows{1}{Qz-Monzonite}&amp; {BJM410}&amp; 62.93&amp; 0.76&amp; 16.22&amp; 5.84&amp; 0.10&amp; 1.95&amp; 3.90&amp; 4.81&amp; 2.78&amp; 0.33&amp; 0.38&amp; 100.00&amp; 1.49&amp; 0.90&amp; 25.01&amp; 0.75 \\
 &amp; {BJM069}&amp; 67.58&amp; 0.55&amp; 14.97&amp; 4.27&amp; 0.05&amp; 1.12&amp; 2.08&amp; 3.71&amp; 5.03&amp; 0.23&amp; 0.42&amp; 100.00&amp; 1.30&amp; 0.98&amp; 20.83&amp; 0.79\vspace*{7pt} \\
\morerows{2}{Granite}  &amp; {BIN089}&amp; 72.01&amp; 0.35&amp; 14.09&amp; 2.28&amp; 0.04&amp; 0.35&amp; 0.86&amp; 3.19&amp; 6.34&amp; 0.07&amp; 0.42&amp; 100.00&amp; 1.16&amp; 1.03&amp; 13.19&amp; 0.87 \\
&amp; {BJM520}&amp; 70.25&amp; 0.34&amp; 14.64&amp; 3.21&amp; 0.04&amp; 0.62&amp; 0.95&amp; 3.56&amp; 5.97&amp; 0.12&amp; 0.30&amp; 100.00&amp; 1.19&amp; 1.04&amp; 16.15&amp; 0.84 \\
&amp; {GAM521}&amp; 72.52&amp; 0.52&amp; 13.01&amp; 3.57&amp; 0.06&amp; 0.83&amp; 2.18&amp; 3.40&amp; 3.32&amp; 0.13&amp; 0.48&amp; 100.00&amp; 1.42&amp; 0.99&amp; 18.81&amp; 0.81
\botline
\end{tabular}
\end{sidewaystable*}

\begin{figure*}
\includegraphics{fig04}
\caption{\label{fig4}Nomenclature and chemical features of the dated rocks. 
(a) TAS diagram of \citet{Middlemost1994}; (b) K$_{2}$O vs.\ 
SiO$_{2}$ diagram illustrating the calc-alkaline and shoshonitic 
affinities of the analysed granitoids; the subdivisions are after 
\citet{LeMaitreetal1989} and \citet{Rickwood1989}; (c) A/NK 
[molar Al$_{2}$O$_{3}$/(Na$_{2}$O ${+}$ 
K$_{2}$O)] vs.\ A/CNK [molar Al$_{2}$O$_{3}$/(CaO 
${+}$ Na$_{2}$O ${+}$ K$_{2}$O)] diagram after 
\citet{ManiarPiccoli1989}. The borderline between I-type and S-type granites is after 
\citet{ChappellWhite1992}; (d) FeO$_{\mathrm{T}}$/(FeO$_{\mathrm{T}}$ 
$+$MgO) vs.\ SiO$_{2}$ diagram; (e) MgO vs.\ SiO$_{2}$ 
diagram showing the high-silica adakite affinity of the dated granitoids. 
The respective fields in the diagram are derived from \citet{Martinetal2005}; 
(f) plots of Mg number vs.\ SiO$_{2}$. Fields of delaminated lower 
crust-derived adakitic rocks, subducted oceanic crust-derived adakites and 
thick lower crust-derived adakitic rocks were built by \citet{Wangetal2005}.}
\end{figure*}

\subsection{Biotite K--Ar ages}

About 12 to 22 analyses on different biotite crystals have been done for 
each of the five selected samples. Overall, sixty-three (63) measurements 
were carried out but only forty (40) were retained for this study because 
analyses carried out on chloritised or altered biotites were not taken into 
account due to their erroneous values. The ages obtained using the in-situ 
K--Ar method are comparable to those from Ar--Ar via laser ablation, although 
with less precision. The advantage of this method is that it allows for age 
measurements in any polished sample without the need for irradiation. 
Individual laser ablation ages obtained in each dated samples range as 
follows: GAM521 (482--501 Ma), BJM410 (471--501~Ma) BJM069 (527--562 
Ma), BIN089 (532--550 Ma) and BJM520 (527--595 Ma). Figure~\ref{fig5} shows the 
distribution of ages in the stratigraphic chart, and Table~\ref{tab2} displays the 
basic sample information. Dated samples display ages ranging from late 
Ediacaran (549~${\pm}$~19~Ma)
to Early Ordovician (482 ${\pm}$ 6 Ma), 
covering 67 Myr of regional geological history and fitting together with the 
Pan-African orogenic phase in Cameroon, as discussed below (Table~\ref{tab2}). 

\begin{figure*}[t!]
\includegraphics{fig05}
\caption{\label{fig5}In-situ K--Ar ages on biotites obtained in standard 
petrographic thin sections of five granitic rocks near Ngaound\'{e}r\'{e}, 
Cameroon. Each point corresponds to an individual laser ablation age. A 
minimum of 12 analyses per sample have been obtained, with a total of 40 
ages (see Table~\ref{tab2}). The range of ages for each sample is mainly influenced 
by later metamorphic events. At the right are photographs of some dated 
biotites, showing laser ablation craters and their ages in Ma.}
\end{figure*}

\begin{sidewaystable*}
\vspone
\caption{\label{tab2}Biotite K--Ar dating results of granitic rocks
from the  study area}
\begin{tabular}{ccccccc}
\thead
{Rock type} &amp; \multicolumn{2}{c}{Qz-monzonite} &amp; &amp; \multicolumn{3}{c}{Granite} \\
\cline{2-3}
\cline{5-7}
{Sample code} &amp; BJM410 &amp; BJM069 &amp; &amp; BIN089 &amp; BJM520 &amp; GAM521 \\
\xxbotline
\morerows{1}{Location}&amp; 07\textdegree{}37$'$17.3$''$N &amp; 07\textdegree{}33$'$51.3$''$N  &amp; &amp; 07\textdegree{}37$'$35.5$''$N &amp; 07\textdegree{}38$'$34.9$''$N &amp;  07\textdegree{}34$'$20.4$''$N\\
          &amp; 13\textdegree{}46$'$05.5$''$E &amp; 13\textdegree{}54$'$49.3$''$E  &amp; &amp; 13\textdegree{}55$'$03.1$''$E &amp; 14\textdegree{}16$'$10.4$''$E &amp; 13\textdegree{}40$'$43.3$''$E\\
\endthead
{Altitude (m)}&amp; 1320&amp; 867&amp;&amp; 889&amp; 789&amp; 1138 \\
\cline{2-3}
\cline{5-7}
\parbox{2cm}{\centering {Mineralogical assemblage}} &amp; 
\raisebox{2.5em}{}\parbox{3cm}{\centering Qz $+$ Kfs $+$ Pl $+$ Bt $+$ Spn $+$ Amp $+$ Zrn $+$ Ep $+$ Ox}&amp; 
\parbox[b]{3cm}{\centering Qz $+$ Kfs $+$ Pl $+$ Bt $+$ Spn}&amp;&amp; 
\parbox[b]{3cm}{\centering Qz $+$ Kfs (Mc) $+$ Pl $+$ Bt $+$ Spn $+$ Ox}&amp; 
\parbox{3cm}{\centering Qz $+$ Kfs (Mc) $+$ Pl $+$ Bt $+$ Amp $+$Spn $+$ Zrn $+$ Ep $+$ Ox}&amp; 
\parbox{3cm}{\centering Qz $+$ Kfs $+$ Bt $+$ Amp $+$ Pl $+$ Spn $+$ Zrn $+$ Ox}\vspace*{3pt}\\
\cline{2-3}
\cline{5-7}
\morerows{9}{\parbox[t]{3cm}{\centering Individual laser ablation age (Ma)}} &amp; 483 &amp; 556 &amp; &amp; 532 &amp; 543 &amp; 501 \\
&amp; 496 &amp; 543 &amp; &amp; 524 &amp; 529 &amp; 484 \\
&amp; 482 &amp; 562 &amp; &amp; 518 &amp; 543 &amp; 500 \\
&amp; 501 &amp; 560 &amp; &amp; 522 &amp; 546 &amp; 482 \\
&amp; 498 &amp; 530 &amp; &amp; 522 &amp; 527 &amp; \\
&amp; 471 &amp; 540 &amp; &amp; 505 &amp; 593 &amp; \\
&amp; 490 &amp; 527 &amp; &amp; 524 &amp; 575 &amp; \\
&amp; 488 &amp; 541 &amp; &amp; 522 &amp; 563 &amp; \\
&amp; 482 &amp;     &amp; &amp; 527 &amp;     &amp; \\
&amp; 498 &amp;     &amp; &amp; 517 &amp;     &amp; \\
Average age (Ma) &amp; $495 \pm 10$ &amp; $521 \pm 17$ &amp; &amp; $538 \pm 18$ &amp; $549 \pm 19$ &amp; $482 \pm 6$
\botline
\end{tabular}
\tabnote{For mineral symbols, Qz ${=}$ quartz; Kfs ${=}$ Potassic feldspar,
Pl ${=}$ Plagioclase; Bt ${=}$ biotite; Spn ${=}$ sphene;\break Amp ${=}$ Amphibole;
Zrn  ${=}$ zircon; Ep ${=}$ epidote; Ox ${=}$ oxide; Mc ${=}$ microcline.}
\end{sidewaystable*}

\section{Discussion}
\subsection{Laser K--Ar dating}

Geochronometers can be classified based on their diffusive blocking 
(closure) temperature. The U--Pb method applied to zircon and other trace 
minerals is typically restricted to high and ultrahigh-temperature events 
(\textgreater 600~\textdegree{}C), such as the crystallisation of igneous 
rocks, migmatites, and the so-called high-grade metamorphism. However, 
medium and low-temperature events (\textless 600~\textdegree{}C) that 
separate similar blocks from different geological evolutions, can sometimes 
be difficult or impossible to detect with the U--Pb method. 

The laser K--Ar (or Ar--Ar) method is sensitive to temperatures below ${\sim }
$600 \textdegree C, deciphering medium to low-grade metamorphism, 
cooling, or hydrothermal circulation, contrary to the U--Pb method. The 
in-situ K--Ar method gives ages of different parts of crystals, like the 
Ar--Ar laser-probe method \citep{Kelleyetal1994,Mulchetal2002, 
Pickersgilletal2020}, which can span a range of ages larger for one 
sample than the classical K--Ar, which shows only the weighted mean of all 
crystal ages measured. In this sense, the age range of each sample can be 
more comparable to the ages obtained with the Ar--Ar step heating approach. 
In-situ geochronology (e.g., K--Ar, Ar--Ar, Rb--Sr, U--Pb, Sm--Nd, Lu--Hf) 
applied to petrographic sections give spatial information not attainable by 
conventional techniques (bulk analyses, mineral separation). This added 
knowledge overcomes the fact that the precision of each analysis is lower 
due to the very small amount of sample analysed during laser\break ablation. 

These medium-temperature geochronometers could discriminate rocks that 
experienced a different geological history even if the protoliths were the 
same or the samples were spatially grouped by tectonic activity. Moreover, 
the direct dating of thin sections allows checking the petrology of samples 
and selecting the appropriate minerals to be dated, producing a strong test 
on ages. If better precision and further interpretative control are needed, 
the Ar--Ar method will be more appropriate, assuming the additional 
analytical effort. Other low-temperature thermochronometers like fission 
tracks or U--Th--He will complement the interpretation of the\break tectonic 
history of Precambrian samples in cratonic regions worldwide. Nevertheless, 
the K--Ar (or Ar--Ar) methodology preserves, in general, a more detailed 
record of cooling or overprinting. These ages may indicate the time since 
the mineral (re)crystallisation. Still, because most igneous and metamorphic 
rocks can form at moderate to high temperatures, most dates indicate the 
length of time since the material dropped below some critical temperature 
known as closure temperature at which diffusion of argon out of the sample 
became negligible \citep[e.g.,][]{PhilpottsAgue2010}. The K--Ar dating method 
differs from other common methods by involving a decay\break product that is an 
inert gas, not affected by initial isotopic ratios, as is the Rb--Sr method. 
Even at moderately low temperatures, this gas is a fugitive component (i.e., 
has very low solubility) and is typically not incorporated in minerals 
\citepalias[e.g.,][]{PhilpottsAgue2010}. 

\subsection{Geodynamic significance and implications on the
Post-Pan-African evolution}

Two major facts are remarkable with regard to the Post-orogenic ages 
presented in Table~\ref{tab3}: (1) Few Paleozoic igneous events have been reported in 
the Adamawa-Yad\'{e} domain; and (2) the K--Ar ages presented in this study 
(549 ${\pm}$ 19 to 482 ${\pm}$ 6 Ma) are among the youngest obtained so far 
in the CAPFB. As much as these Paleozoic ages linked to the Pan-African 
orogeny have been interpreted either as emplacement ages particularly for 
the granitoids from northern Cameroon \citep{Lasserre1967,Tempieretal1981, 
Toteuetal1986}, or erroneously ascribed to the extension of 
post-collisional metamorphism and magmatism related to the late collision 
phases between the Saharan metacraton, the S\~{a}o Francisco-Congo and West 
African cratons (despite the absence of a regional Cambrian metamorphic 
event recorded in the CAPFB) by \citet{Kanouoetal2021}, their magmatic source 
remains\break unidentified.

The age range obtained in the present work overlaps the period of Late 
Cambrian post-orogenic magmatism recorded in K\'{e}kem 
\citep{Lemdjouetal2022} (see Table~\ref{tab3}), and is consistent not only with the maximum emplacement 
age for post-tectonic granitoids in northern Cameroon (554.3 ${\pm}$ 5.7 
Ma; \citet{Toteuetal2022}, but also with the Rb--Sr ages (550--500 Ma) 
recorded in post-tectonic alkaline granites emplaced subsequent to the 
uplift of the Pan-African fold belt \citep{Toteuetal2001}. However, assuming 
that: (1) Post-tectonic granitoids in the Adamawa-Yad\'{e} domain were 
emplaced between 604 ${\pm}$ 8 and 544 ${\pm}$ 16 Ma \citep{Toteuetal2022}, (2) 
most of plutonic intrusions throughout Nigeria, Cameroon, and A\"{\i}r were 
emplaced between 645 and 580 Ma and (3) a large proportion of them intruded 
at about 580 Ma \citep{Liegeoisetal1994,EkwuemeKroner1998,
Ferreetal1998}, the new ages range presented in this work is too 
young to be considered as the crystallisation age of magmas. They likely 
correspond to the late stages of the Pan-African Orogeny. Therefore, the 
dated granitoids are likely the fingerprints of late- to post-orogenic 
events in the area. They have intruded the early Pan-African metamorphic 
units in the late stages of the Brasiliano/Pan-African orogenic evolution 
later to the 640--580 Ma compressional tectonic regime of the\break CAPFB.

The new laser biotite K--Ar ages may correspond to metamorphism or 
deformation subsequent to the emplacement of the dated rocks. The Late 
Ediacaran ages might provide insight into the timing of late 
Adamawa-Yad\'{e} kinematics and probably the uplift and cooling of the 
CAPFB. Otherwise, the Adamawa uplift and cooling of the CAPFB are 
post-metamorphic and post-tectonic events that mark the end of convergence 
and the beginning of crustal extensional. Thus, Cambrian intrusions 
associated with these events mark the transition between convergence and 
divergence in CAPFB. 

As presented in Table~\ref{tab3}, all the Cambrian to early Ordovician ages, are 
mostly linked to Post-Pan-African events in Cameroon. Moreover, it is 
noteworthy that in Cameroon, the earliest Pan-African molasses (generally 
defined as post-collision detrital sediments) in CAPFB to date are the 
Mangbai-type through \citep{Beaetal1990}, the syn-trough deposits 
approximated Rb--Sr age of interleaves volcanic is ca 585 Ma 
\citep{Montes-Laueretal1997}. Therefore, in absence of additional mineralogical and 
structural/tectonic data, the new K--Ar ages (549 ${\pm}$ 19--482 ${\pm}$ 
6 Ma) here presented, can more likely be ascribed to the late stage of the 
Pan-African orogeny in CAPFB as it is the case for the Cambrian--Ordovician 
ages of the Brazilian Belt or NE--Nigeria, than to an independent event such 
as rifting or strike-slip. Rifting and/or strike-slip in 
Cambrian--Ordovician times were most likely related to further isostatic 
evolution of CAPFB, in response to the equilibration of the Pan-African 
thickened crust during nappes\break stacking.

\begin{sidewaystable*}
\vspone
\caption{\label{tab3}Post-Pan-African ages of some localities from the Cameroon}
\tabcolsep8pt
\begin{tabular}{ccccc}
\thead
{Locality}&amp; {Age (Ma)}&amp; {Rock type}&amp; {Method}&amp; {Reference} \\
\endthead
Tibati-Banyo&amp; ${\sim}$598 to ${\sim}$568&amp; Gneiss&amp; U--Pb zircon&amp; \citet{Bernardetal2019} \\
Djerem-Mb\'{e}r\'{e} basin&amp; 540--371&amp; Conglomerates&amp; U--Pb zircon&amp; \citet{Bernardetal2019} \\
K\'{e}kem&amp; 576 ${\pm}$ 4 &amp; Alkaline gabbro&amp; LA--ICP--MS U--Pb&amp; \citet{Kwekametal2013} \\
Bouar&amp; 566.4 ${\pm}$ 7.0&amp; Granite&amp; U--Pb zircon&amp; \citet{Toteuetal2022} \\
B\'{e}a&amp; 562.7 ${\pm}$ 2.1&amp; Amphibolite&amp; U--Pb zircon&amp; \citet{Toteuetal2022} \\
Rocher de Loup&amp; 519${\pm}$ 19 &amp; Nepheline syenite&amp; U--Pb zircon&amp; \citet{Nsifaetal2013} \\
Guider&amp; 593 ${\pm}$ 4&amp; Syenite&amp; U--Pb zircon&amp; \citet{Dawaietal2013} \\
Dschang&amp; 421.3 ${\pm}$ 3.5&amp; Basalt&amp; $^{40}$Ar/$^{39}$Ar&amp; \citet{Tchouankoueetal2014} \\
Maham&amp; 404.2 ${\pm}$ 3.2--417 ${\pm}$ 8&amp; Basalt &amp; K/Ar and$^{40}$Ar/$^{39}$Ar&amp; \citet{Tchouankoueetal2014} \\
K\'{e}kem&amp; 486 ${\pm}$ 15--497 ${\pm}$ 14 &amp; Gabbro&amp; U--Pb zircon$^{\mathrm{}}$&amp; \citet{Lemdjouetal2022} \\
God\'{e}&amp; 546 ${\pm}$ 9&amp; Granite&amp; Rb/Sr WR isochron&amp; \citet{Toteuetal1986} \\
Balch\'{e}&amp; 440 &amp; rhyolite&amp; Whole rock K/Ar&amp; In \citet{Lasserreetal1977} \\
Mangbai basin&amp; 373${\pm}$ 10--425 ${\pm}$ 12&amp; Volcanic rocks &amp; Whole rock K/Ar &amp; \citet{Beaetal1990} \\
Mamfe&amp; ${\sim}$536--${\sim}$449&amp; Granite and gneiss&amp; U--Pb zircon&amp; \citet{Kanouoetal2021}
\botline
\end{tabular}
\end{sidewaystable*}

The entirely Ediacaran age (558--630 Ma) of the late Pan-African 
plutonism phase of the Eastern Adamawa region including our study area as 
stated by \citet{Bernardetal2019} and \citet{Deloretal2021} is questioned in the 
present work, because the new ages obtained evidence that this phase might 
be longer and undoubtedly extends until the end of the Cambrian. A relevant 
fact on similar ages in nearby blocks and domains must be mentioned first. 
Although some retrogression processes may be recorded, no regional 
metamorphism has been signalled during the uplift. The new ages correspond 
to K--Ar on biotite of grain sizes from ${\sim}$1 mm to 
${\sim}$0.5 mm, so if we apply the theory of diffusion to this 
mineral using the experimental data, the temperature of closure (assuming 
only Fickian diffusion controlled by temperature) is 350--400~\textdegree{}C. 
Most of the geochronology of Cameroon, particularly Precambrian terranes, 
has been done with the U--Pb geochronometer on zircon, which has a very high 
temperature of closure, magmatic in general. For age analysis, a density 
diagram has been created for each sample, assuming a 2{\%} variation for 
each Gaussian curve of each age. Uncertainty in K--Ar ages has been 
calculated using standard methods that rely on normal error propagation. 
Precision is derived from analytical argon measurements taken with the mass 
spectrometer, and potassium from measurements taken with the optical 
spectrometer. Accuracy is established through the analysis of internal 
standards. The set of curves, which has been fitted with one or more 
functions to determine the centre and standard deviation, is displayed in 
Figure~\ref{fig6}. BIN089 is unimodal with a centre at 523 ${\pm}$ 17 Ma. The 
dispersion appears to be associated with analytical uncertainty and the 
cooling of the sample below the biotite closure temperature, nominally 
between 300~\textdegree{}C and 400~\textdegree{}C. BJM069 shows a main peak at 
543 ${\pm}$ 20 Ma and a secondary one at 515 ${\pm}$ 20 Ma. We consider the 
first peak to represent the sample age and the second to indicate a 
potential ``reset'' of some analysed points. BJM410 can be regarded as having 
an age of 495 ${\pm}$ 17 Ma, with a slight curve deformation that could be 
divided into two at 492 ${\pm}$ 17 and 501 ${\pm}$ 12 Ma. The difference is 
not significant, and the first age is accepted as valid. BJM520 presents a 
main peak at 541 ${\pm}$ 16 Ma, accompanied by a small adjacent peak at 581 
${\pm}$ 20 Ma. The fact that the secondary peak is older suggests an excess 
of argon in some of the analysed micas. Thus, we consider the first age to 
be the best estimate. GAM521 has a single, well-defined peak at 482 ${\pm}$ 
13 Ma. Therefore, the oldest spot obtained on the Amph--Bt-granite BJM520 
(549 ${\pm}$ 19 Ma) could be considered to be closest to the intrusion and 
the lower ages reported in this work, although similar to some early 
Palaeozoic late Pan-African intrusions cited above, can be interpreted as 
the final epoch of cooling of the analysed rocks or a last disturbance of 
the K--Ar geochronometer on biotite. Therefore, this range (from 538 ${\pm}$ 
18 Ma to 482 ${\pm}$ 6 Ma) likely corresponds to either cooling by uplift 
fluid movements due to tectonic events or kinematic disturbances rather than 
a long period of\break intrusions. 

\begin{figure*}
\includegraphics{fig06}
\caption{\label{fig6}Density diagram of dated samples individual K--Ar ages. The 
numbered horizontal bars indicate the ages (including the margin of error) 
presented in Table~\ref{tab3}, listed in sequential order, starting from the first 
line of the table. Some large age ranges from Table~\ref{tab3} have not been 
included.}
\end{figure*}

\subsection{Comparison and correlation with the adjacent Brazilian Belt}

The tectonic evolution of both the CAPFB and the Brazilian fold belt is 
characterised by three main tectonic events involving collision, 
post-collision, and crustal extension, suggesting that both formed a single 
belt before they drifted in Mesozoic, although the sequential limits of the 
above events are still controversial 
\citep{NgakoNjonfang2011,Toteuetal2022}. 
The correlation of the CAPFB with the Brasiliano fold belt (Figure~\ref{fig7}) 
has allowed highlighting some similarities between the two belts both in 
terms of the orogenic cycle and their tectonic evolution 
\citep{DeWitetal2008}. This is the reason why we propose to make a summary comparison of the 
K--Ar ages obtained in the present work with some published data of the 
post-collision episodes of the east coast of Brazil, although the 
Adamawa-Yad\'{e} domain does not seem to extend overseas. The 
Cambro-Ordovician post-collisional phase of the Brasilano fold belt is 
characterised by intense magmatic activity clearly visible both in the SE 
and NE of Brazil and even in the central zone \citep{Castroetal2009,Castroetal2012, 
Teixeira2005} and Rio Grande do Norte \citep{Nascimentoetal2015}.

\begin{figure*}[t!]
\vspace*{3pt}
\includegraphics{fig07}
\vspace*{3pt}
\caption{\label{fig7}Generalised geological map of NE-Brazil and West-central
Africa  in a Gondwana (pre-drift) configuration  
\citep[modified from][]{Toteuetal2001}. The blue dashed outline indicates the
boundary of Cameroon. TBF ${=}$  Tchollir\'{e}-Banyo Fault (in Cameroon);
SF ${=}$ Sanaga Fault (in Cameroon);  PL ${=}$ Pernambuco Lineament (in
Brazil); AF ${=}$ Adamawa Fault (in Cameroon).  The white star
approximately locates the study area. ``B/PA with PP'' states  to
regions of Brazilian-Pan-African belt with large amounts of reworked
Palaeoproterozoic basement; ``B/PA without PP'' refers to regions of 
Brazilian-Pan-African belt in which Palaeoproterozoic basement is
absent  or poorly represented as small isolated blocks.}
\vspace*{3pt}
\end{figure*}

In SE-Brazil, Cambrian to early Ordovician post-collisional magmatism has 
been dated in Ara\c{c}uai-Ribeira belt precisely in the region of Espirito 
Santo. The U--Pb ages obtained on zircon, monazite and titanite range from 
525 ${\pm}$ 3 Ma to 480.7 ${\pm}$ 6.1 Ma \citep{Mendesetal2005,Valerianoetal2011,
deCamposetal2004,deCamposetal2016}. These ages were associated with the 
third phase of anatexis \citep{deCamposetal2004} and correspond to the 
emplacement and crystallisation ages. During this post-collisional 
Cambro-Ordovician phase, two distinct magmatic events were highlighted in 
this SE-Brazilian region: (i) an earlier of Cambrian age (${\sim}$ca.512 Ma) 
post-dating the end of the foremost collisional phase, then (ii) 
the late one of Ordovician age (${\sim}$at ca.486 Ma) post-dating 
the end of the second collisional episode. In the Parana and Parnaiba 
basins, the post-collision Ordovician \mbox{magmatism} was emplaced in an 
extensive regime and associated with the Trans-Brasiliano Shear Zone 
\citep{BritoNevesetal2014,Heilbronetal2018}.

In the NE Brazil, the Palaezoic magmatism has been identified in the 
northernmost continuation of the Trans-Saharan Belt known as the
Brasilian  Serid\'{o} belt (520 ${\pm}$ 3.0--518 ${\pm}$ 1.8 Ma; 
$^{40}$Ar/$^{39}$Ar ages; \citet{Araujoetal2005}), and  widely
distributed in the Borborema Province (BP) particularly in the 
Pianc\'{o}-Alto Brigida domain (540 ${\pm}$ 5 Ma; U--Pb apatite dating;
\citet{Dhuimeetal2003}), the Prata Complex (534 ${\pm}$ 3 Ma--533
${\pm}$ 4  Ma; U--Pb zircon SHRIMP; \citet{Hollandaetal2010}), and
numerous plutons  including the Serra do Velho Zuza (538 ${\pm}$ 23 Ma;
U--Pb zircon TIMS),  Serra do Pereiro (543 ${\pm}$ 7 Ma; U--Pb zircon
TIMS)  \citep{Guimaraesetal2004}, Marinho Pluton (550 ${\pm}$ 3 Ma--527
${\pm}$ 6 Ma; U--Pb zircon  SHRIMP; \citet{Guimaraesetal2012}), and
Queimadas (550 ${\pm}$ 6 Ma; U--Pb  Zircon SHRIMP;
\citet{Almeidaetal2002}) In the Serid\'{o} belt it was ascribed  to the
likely post-peak metamorphism age associated with gold  mineralisation
\citep{Araujoetal2005} while in the BP which is generally  considered
as an extension of the CAPFB in pre-drift reconstructions of the 
Atlantic Ocean 
\citep{Guimaraesetal2009,Oliveiraetal2006,Toteuetal2001} (see
Figure~\ref{fig7}), the Cambrian ages were interpreted as cooling age 
following regional metamorphism associated with granitic emplacement at
ca.  580 Ma in Pianc\'{o}-Alto Br\'{\i}gida belt
\citep{Dhuimeetal2003}. Thus,  despite slight shifts, similarities
emerge between the K--Ar ages obtained  in the present work and those
of the post-collision episodes from eastern  coast of Brasilian belt. 

\section{Conclusions}

This study reports new in-situ K--Ar geochronological data on thin sections 
of five granitic rocks from Eastern Ngaound\'{e}r\'{e} in Cameroon. The 
analysed calk-alkaline I-type plutonic rocks consist of Qz-monzonite and 
granite. Their dating provided significant ages ranging from late Ediacaran 
(549 ${\pm}$ 19 Ma) to Early Ordovician (482 ${\pm}$ 6 Ma). They are among 
the youngest ages published in the\break Pan-African Adamawa-Yad\'{e} domain. 
These new ages obtained mark the extensional regime in the earliest stages 
of the Adamawa uplift between the late Ediacaran to Early Ordovician as well 
as the end of the CAPFB orogen, implying no later disturbance of the argon 
system in biotite after ${\sim}$480 Ma. 

\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*{Acknowledgments}
The authors deeply thank Teresa Pi Puig and Rufino Lozano of the LANGEM 
(UNAM, Mexico) for the X-ray diffraction and whole rock analyses presented 
in this paper, then Professor Alembert Ganwa (Ngaound\'{e}r\'{e} University) 
for constructive discussions. We also thank Professor Rigobert Tchameni and 
Michel Villeneuve for their significant and constructive comments, and the 
editorial assistance from Michel Faure.

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\section*{Supplementary data}

Supporting information for this article is available on the journal's
website under \printDOI\ or from the author.

\CDRsupplementaryTwotypes{supplementary-material}{\cdrattach{crgeos-289-suppl.pdf}}

\printbibliography
\refinput{crgeos20240932-reference.tex}

\end{document}
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