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\DOI{10.5802/crgeos.248}
\datereceived{2023-01-17}
\datererevised{2023-11-24}
\dateaccepted{2023-11-24}
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\dateposted{2024-04-15}
\begin{document}

\begin{noXML}

%\makeatletter
%\def\TITREspecial{\relax}
%\def\cdr@specialtitle@english{Geodynamics of continents and oceans - A tribute to Jean Aubouin}
%%\def\cdr@specialtitle@french{G\'eodynamique des continents et des Oc\'eans - Hommage \`a Jean Aubouin}
%\makeatother

\title{Temporal and spatial heterogeneity of the Ailaoshan--Song
Ma--Song Chay ophiolitic m\'{e}lange, and its significance on the
evolution of Paleo-Tethys}

\alttitle{H\'{e}t\'{e}rog\'{e}n\'{e}it\'{e} temporelle et spatiale du
m\'{e}lange ophiolitique Ailaoshan--Song Ma--Song Chay et son
importance pour l'\'{e}volution de la Pal\'{e}o-T\'{e}thys}

\author{\firstname{Wei} \lastname{Lin}\CDRorcid{0000-0001-8966-5399}\IsCorresp}
\address{State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Innovation Academy of Earth Science, Chinese Academy of Sciences, Beijing 100029, China}
\address{College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China}
\email[W. Lin]{linwei@mail.iggcas.ac.cn}

\author{\firstname{Fei} \lastname{Liu}}
\addressSameAs{1}{State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Innovation Academy of Earth Science, Chinese Academy of Sciences, Beijing 100029, China}
\addressSameAs{2}{College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China}
\address{Wuhan Center of China Geological Survey (Central South China Innovation Center for Geosciences), Wuhan 430205, China}

\author{\firstname{Yin} \lastname{Wang}}
\addressSameAs{1}{State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Innovation Academy of Earth Science, Chinese Academy of Sciences, Beijing 100029, China}
\addressSameAs{2}{College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China}
\address{Geosteering \& Logging Research Institute, Sinopec Matrix Corporation, Qingdao 266071, China}

\author{\firstname{Lingtong} \lastname{Meng}\IsCorresp}
\addressSameAs{1}{State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Innovation Academy of Earth Science, Chinese Academy of Sciences, Beijing 100029, China}
\addressSameAs{2}{College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China}
\email[L. Meng]{tone18@mail.iggcas.ac.cn}

\author{\firstname{Michel} \lastname{Faure}\CDRorcid{0000-0003-1880-8115}}
\address{Institut des Sciences de la Terre d'Orl\'{e}ans, UMR CNRS 7327, Universit\'{e} d'Orl\'{e}ans, 45067 Orl\'{e}ans Cedex 2, France}

\author{\firstname{Yang} \lastname{Chu}\CDRorcid{0000-0003-3253-9599}}
\addressSameAs{1}{State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Innovation Academy of Earth Science, Chinese Academy of Sciences, Beijing 100029, China}
\addressSameAs{2}{College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China}

\author{\firstname{Vuong Van} \lastname{Nguyen}}
\address{University of Science, Vietnam National University, Hanoi, Viet Nam}

\author{\firstname{Qinying} \lastname{Wu}}
\addressSameAs{1}{State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Innovation Academy of Earth Science, Chinese Academy of Sciences, Beijing 100029, China}
\addressSameAs{2}{College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China}

\author{\firstname{Wei} \lastname{Wei}}
\addressSameAs{1}{State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Innovation Academy of Earth Science, Chinese Academy of Sciences, Beijing 100029, China}
\addressSameAs{2}{College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China}

\author{\firstname{Hoai Luong Thi} \lastname{Thu}}
\addressSameAs{6}{University of Science, Vietnam National University, Hanoi, Viet Nam}

\author{\firstname{Tich Van} \lastname{Vu}}
\addressSameAs{6}{University of Science, Vietnam National University, Hanoi, Viet Nam}

\shortrunauthors

\thanks{This study was funded by the State Key Laboratory of
Lithospheric Evolution, Institute of Geology and Geophysics, Chinese
Academy of Sciences (SKL-Z202205) and the National Natural Science
Foundation of China (91855212 and 91855103).}

\begin{abstract}
According to detrital zircons ages spectra and Hf isotopes in the
matrix of the ophiolitic m\'{e}lange, the Ailaoshan--Song Ma--Song Chay
Indosinian suture zone can be divided into four units, namely M1, M2,
M3, and M4. Different deposition ages (310--270~Ma, 260--255~Ma,
${\sim}$245~Ma, and ${>}$255~Ma) demonstrate temporal heterogeneity.
The M1 unit is in the middle and south segments, and inner part of the
NW segment of the Ailaoshan ophiolitic m\'{e}lange, and the
southernmost part of Song Ma ophiolitic m\'{e}lange. The Silurian and
Devonian sedimentary rocks of the Indochina block are the major
provenance of the M1 unit. The M2 unit is located in the southwest part
of the NW segment of the Ailaoshan ophiolitic m\'{e}lange. At least
half of the 270--250~Ma detrital zircons might come from the South China
block. The M3 unit is mainly located in the Ailaoshan--Song Ma
ophiolitic m\'{e}lange and sourced from Indochina block. M4 is mainly
located in the Song Chay ophiolitic m\'{e}lange, and its detrital
zircons are mainly sourced from the South China block. The
strike-parallel heterogeneity refers to the different distributions
along the orogenic belt with various provenances. The
strike-perpendicular heterogeneity characterises the geometry of M1,
M2, and M3 units and is attributed to a Cenozoic positive-flower
structure of the Red River Fault showing sinistral strike-slip tectonic
event with SW-ward thrusting. These temporal and spatial
heterogeneities correspond well to the different evolutionary stages of
the eastern Paleo-Tethys.
\end{abstract}


\begin{altabstract}
D'apr\`{e}s les spectres d'\^{a}ge des zircons d\'{e}tritiques et les
isotopes Hf dans la matrice du m\'{e}lange ophiolitique, la zone de
suture indosinienne Ailaoshan--Song Ma--Song Chay peut \^{e}tre
divis\'{e}e en quatre unit\'{e}s, \`{a} savoir M1, M2, M3 et M4. Les
diff\'{e}rents \^{a}ges de d\'{e}p\^{o}t (310--270~Ma, 260--255~Ma,
${\sim}$245~Ma et ${>}$255~Ma) d\'{e}montrent une
h\'{e}t\'{e}rog\'{e}n\'{e}it\'{e} temporelle. L'unit\'{e} M1 se trouve
dans les segments moyen et sud, et dans la partie interne du segment NO
du m\'{e}lange ophiolitique d'Ailaoshan, ainsi que dans la partie la
plus m\'{e}ridionale du m\'{e}lange ophiolitique de Song Ma. Les roches
s\'{e}dimentaires siluriennes et d\'{e}voniennes du bloc indochinois
constituent la principale provenance de l'unit\'{e} M1. L'unit\'{e} M2
est situ\'{e}e dans la partie sud-ouest du segment NW du m\'{e}lange
ophiolitique d'Ailaoshan. Au moins la moiti\'{e} des zircons
d\'{e}tritiques de 270--250~Ma pourraient provenir du bloc de Chine
m\'{e}ridionale. L'unit\'{e} M3 est principalement situ\'{e}e dans le
m\'{e}lange ophiolitique Ailaoshan--Song Ma et provient du bloc
indochinois. L'unit\'{e} M4 est principalement situ\'{e}e dans le
m\'{e}lange ophiolitique de Song Chay, et ses zircons d\'{e}tritiques
proviennent principalement du bloc de Chine m\'{e}ridionale.
L'h\'{e}t\'{e}rog\'{e}n\'{e}it\'{e} parall\`{e}le \`{a} la direction
fait r\'{e}f\'{e}rence aux diff\'{e}rentes distributions le long de la
ceinture orog\'{e}nique avec des provenances diverses.
L'h\'{e}t\'{e}rog\'{e}n\'{e}it\'{e} perpendiculaire \`{a} la direction
caract\'{e}rise la g\'{e}om\'{e}trie des unit\'{e}s M1, M2 et M3 et est
attribu\'{e}e \`{a} une structure en fleur positive du
C\'{e}nozo\"{i}que de la faille de la rivi\`{e}re Rouge, qui
pr\'{e}sente un \'{e}v\'{e}nement tectonique de glissement sinistr\'{e}
avec un chevauchement vers le sud-ouest. Ces
h\'{e}t\'{e}rog\'{e}n\'{e}it\'{e}s temporelles et spatiales
correspondent bien aux diff\'{e}rents stades d'\'{e}volution de la
pal\'{e}o-T\'{e}thys orientale.
\end{altabstract}

\keywords{\kwd{Eastern Paleo-Tethys}
\kwd{Ailaoshan--Song Ma--Song Chay ophiolitic m\'{e}lange zone}
\kwd{Zircon\unskip\break U--Pb dating}
\kwd{Lu--Hf isotopic analysis}
\kwd{Paleogeography reconstruction}}

\altkeywords{\kwd{Pal\'{e}o-T\'{e}thys orientale}
\kwd{Zone de m\'{e}lange ophiolitique Ailaoshan--Song Ma--Song Chay}
\kwd{Datation U--Pb au zircon}
\kwd{Analyse isotopique Lu--Hf}
\kwd{Reconstruction de la pal\'{e}og\'{e}ographie}}

\maketitle

\twocolumngrid

\end{noXML}


\section{Introduction}

An ophiolitic suite is a sequence of temporally and spatially
associated ultramafic, mafic, felsic, and sedimentary rocks interpreted
as remnants of subducted oceanic lithosphere. It is often preserved as
a suture zone between two continental blocks 
\citep{Coleman1977,Dewey1977,Sengor1992}. Demonstrably, ophiolite
emplacement, or obduction, is a product of the subduction-accretion,
arc-continent collision, or continent-continent collision, during which
the oceanic lithosphere is incorporated into the continental margin
\citep{DilekFlower2003}. The classic ophiolite suites, such as Oman,
Cyprus, Xigaze in the Himalayas, and K\i{}z\i{}lda\u{g} (SE Turkey),
are less common in subduction zones \citep{Wuetal2014b}. Ophiolitic
m\'{e}langes (hereafter referred to as m\'{e}langes) represent
dismembered ophiolitic sequences with a chaotic mixture of sedimentary
and igneous rocks 
\citep[e.g.,][]{DilekFurnes2011,Festaetal2010,Festaetal2012}. In
detail, the m\'{e}lange is composed of ``exotic'' blocks, ``native or
intraformational'' blocks, and the sedimentary matrix 
\citep{Festaetal2010}. The exotic blocks consist of siliceous rocks
(radiolarite), pillow basalt, mafic and ultramafic rocks like the
composition of the ophiolite suites. The native blocks originate from
the disruption of a primary lithostratigraphic unit (disrupted brittle
layers). The matrix is defined as ``deformed'' or ``fragmented''
sandy-muddy rocks which are deformed ductilely relative to the blocks
\citep[e.g.,][]{SilverBeutner1980,Raymond1984}. During subduction and
collision, ophiolitic suites are often dismembered. After undergoing
different-scale shearing, they are distributed in a structural belt,
parallel to the subduction zone, thus forming a suture zone between the
collided continental\unskip\break plates.


In the same suture zone, the ophiolites usually have a relatively
uniform age, they record information about a given stage of the oceanic
lithosphere preserved in a limited tectonic context 
\citep{DilekFurnes2011,Festaetal2016,Wuetal2014b}. In the Alps, the
Neo-Tethys Ocean lasted from Middle Jurassic to Eocene, but the Alpine
ophiolite only records evolution around 160~Ma 
\citep[][and references
therein]{RosenbaumLister2005,Lietal2013,Lietal2015}.
In the Makran belt, East Iran, the geochronological age of
ophiolite is around 115~Ma, but the processes of the accretion are
still ongoing \citep{Chuetal2020}. Another significant example is the
Yarlung Zangbo ophiolite, where the geochronological investigations
indicate two peak ages at 120--130~Ma and 155--160~Ma 
\citep{Chanetal2015}. However, the timing of the Neo-Tethys closure is
generally considered to be at the beginning of the Paleocene
[${\sim}$60~Ma; \citealp{Wuetal2014a,Anetal2021}, and
references therein]. Age information from ophiolites does not closely
reflect the continuous evolution of subducted oceanic plates.


Compared to the ophiolite suite, the m\'{e}lange may provide more
information, since it also includes the materials derived from the
overlying plate, and from the subducted oceanic plate with pelagic
sedimentary rocks. Therefore, m\'{e}langes are ideal objects to
understand the evolutionary history of a disappeared ocean and its
continental margins \citep{Festaetal2010,Festaetal2012}. The matrix of
a m\'{e}lange can provide information related to the subduction
duration and various provenances of clastic material with multiple
compositions \citep[e.g.,][]{Coleman1977}. The study of the ophiolite
m\'{e}lange matrix could broaden our understanding of the process of
plate assembling  \citep{Lietal2021}.

\begin{figure*}
\includegraphics{fig01}
\vspace*{-2pt}
\caption{(A) Tectonic outline of east Asia  
\citep[modified from][]{Faureetal2014}. (B) Tectonic map of Southwest China-North
Vietnam region, modified from the 1:15,00,000 Geological map of the five
countries of southeast Asia and adjacent areas  \citep{CIGMR2006}. Nam
Co: Neoproterozoic sedimentary rocks deformed and metamorphosed in
Triassic; SC: Song Chay massif; ALMZ: Ailaoshan m\'{e}lange zone; SMMZ:
Song Ma m\'{e}lange zone; SCMZ: Song Chay m\'{e}lange zone; RRF: Red
River fault; SCF: Song Chay fault; DBF: Dien Bien Phu fault; SD: Song
Da belt; WALB: Western Ailaoshan belt; JP: Jinping zone.\label{fig1}}
\vspace*{-7pt}
\end{figure*}

Detrital zircon is a useful tool for provenance analysis to rebuild the
geological history of sedimentary basins and their surrounding source
regions. Ideally, the analysed sample would completely represent
geological information of all the possible provenances. Therefore, the
age spectra, obtained from a large number of detrital zircon grains,
can be used to assess the distribution of source rocks. Therefore, the
analysis of detrital zircon age spectra and \textit{in situ} Hf
isotopes might help to improve the understanding of the geological
evolution of the source areas by revealing unexpected, or poorly
documented, magmatic or metamorphic events 
\citep{Griffinetal2000,Condieetal2009}. Such analyses of the deformed
and\unskip\break metamorphosed m\'{e}lange matrix can furthermore help us to restore
its evolutional process.

Although displaced during the Cenozoic extrusion, the Ailaoshan--Song
Ma--Song Chay (AS--SM--SC) ophiolitic m\'{e}lange is regarded as a uniform
suture zone between South China Block (SCB) and Indochina Block (IB),
developed during the Indosinian orogeny [Figure~\ref{fig1}, 
\citealp{Faureetal2014}].
Ailaoshan, Song Ma, and Song Chay orogenic belts
 share similar litho-tectonic zones corresponding to the magmatic
arc, ophiolitic m\'{e}lange, and deformed Precambrian basement and
cover series (foreland), and duplicated by the Cenozoic sinistral Red
River Fault [Figure~\ref{fig1}; \citealp{Faureetal2014,Wangetal2022}].
However,
due to the extensive reworking during the Cenozoic extrusion of the IB,
the ophiolitic m\'{e}lange has been dismembered into three parts:
Ailaoshan, Song Ma, and Song Chay. So, the study on the Ailaoshan--Song
Ma--Song Chay ophiolitic m\'{e}lange is critical to understand the
interaction of SCB and IB. This paper summarises our previous work on
the provenance analysis of the m\'{e}lange matrix by detrital zircon
and structural analysis. Then we discuss the temporal, strike-parallel,
and strike-perpendicular heterogeneities of the AS--SM--SC ophiolitic
m\'{e}lange zone and argue for their significance for the evolution of
the Eastern Paleozoic Tethys Ocean.

\section{Geological setting}

The modern Southeast Asia consists of an assemblage of a series of
continental blocks (e.g., Tarim, Qaidam, North Qiangtang, South
Qiangtang, Lhasa, India, Sibumasu, Indochina, and South China). These
microcontinents were successively rifted from northern Gondwana and
then progressively accreted to the southern margin of Laurasia during
the Late Paleozoic to Cenozoic, with the opening and closing of the
Proto-, Paleo-, and Neo-Tethys oceans 
\citep[e.g.,][]{Faureetal2014, Faureetal2018, Metcalfe2002, 
Metcalfe2011, Metcalfe2013, Metcalfe2021, Metcalfeetal2017, SoneMetcalfe2008}. 
The tectonic event related to the Eastern Paleo-Tethys Ocean closure
was called the ``Indosinian movement'' 
\citep{Deprat1914,Deprat1915,Fromaget1932,Fromaget1941}. At the
beginning of the 20th century, \citet{Deprat1915} documented
large-scale NE-directed thrusts and folds in NE Vietnam (also called
Bacbo or Tonkin belt), and reported Alpine-like nappe structures and
thin-skinned structures. The French geologist Fromaget called these
pre-Late Triassic folds as ``Indosinian fold system''
\citep{Fromaget1932,Fromaget1941}. This view was further developed by
\citet{Huang1945} and he named such an Early Mesozoic orogeny the
``Indosinian movement'' which was well accepted by Chinese geologists.
In the framework of plate tectonics, the concept of the ``Indosinian
movement'' is attributed to the Early Mesozoic collision between the
SCB and the IB recognised in southwestern China and northern Vietnam
\citep{HuangChen1987}. Presently, large-scale Triassic deformation,
magmatism, and metamorphism related to the closure of the
Paleo-Tethys, or are well documented in South China, Indochina, Qiangtang,
Lhasa, and Sibumasu blocks \citep{Metcalfe2021}. It is worth noting
that the term ``Indosinian'' is used in different ways depending on the
authors \citep{HuangChen1987}. Strictly speaking, ``Indosinian'' should
be reserved for tectonic, metamorphic, or magmatic events developed before
the Late Triassic unconformity of the continental red beds
\citep{Lepvrieretal2008}. However, ``Indosinian'' is also used to
account for tectonic or magmatic events involving late Triassic and
even Early Jurassic formations \citep{Metcalfe2021}. Indeed, the late
Triassic--Early Jurassic events are not related to the IB-SCB collision
but to the Sibumasu-IB collision or other collision. This young
collisional event can be referred to as the Cimmerian orogeny or the
Trans-Mekong one \citep{Tranetal2020}. Meanwhile, these Triassic events
were reworked by Late Mesozoic and Cenozoic tectonics 
\citep[e.g.,][]{Leloupetal2001}. Usually, the classical Indosinian
orogenic belt refers to the orogenic belt between IB and SCB, in North
Vietnam and southwest China. Combined with previous studies, our
detailed field work allows us to divide the Indosinian orogen into
three parts, namely the Ailaoshan belt in SW China, the Song Ma belt
in NW Vietnam, and the Song Chay belt in NE Vietnam. 
\citet{Faureetal2014} documented the similarities between the
Ailaoshan, Song Ma, and Song Chay belts, and united these three belts
as a single one, which was duplicated by the Cenozoic tectonics. Hence,
the AS--SM--SC belt records the evolution history of one branch of the
Eastern Paleo-Tethys Ocean between the SCB and IB.

\subsection{Ailaoshan belt}

Located in SW Yunnan Province, China, the NW--SE trending Ailaoshan belt
is divided into four litho-tectonic zones from SW to NE, namely the
Western Ailaoshan zone, Central Ailaoshan zone, Eastern Ailaoshan zone,
and Jinping zone [Figure~\ref{fig1}, \citealp{Faureetal2016}].

The Western Ailaoshan zone contains Permian to Early Triassic
mafic--intermediate felsic volcanic rocks and intrusions considered as
the products of subduction and collision \citep{Laietal2014b}.
Different types of magmatic rocks formed in various tectonic settings,
such as back-arc basins, arcs, or collision belts 
\citep{Jianetal2009a,Jianetal2009b,Laietal2014b}. The
subduction-related magmatic rocks include gabbro and quartz diorite
[\textit{ca}. 286~Ma;
\citealp{Jianetal2009a,Jianetal2009b,Fanetal2010}], Wusu-Yaxuanqiao
basalt [287--265~Ma;  \citealp{Fanetal2010,Jianetal2009a,Jianetal2009b}],
and basalt-rhyolite [249--247~Ma; \citealp{Liuetal2011}].

\begin{figure*}
\includegraphics{fig02}
\caption{Geological and structural map of the Ailaoshan (a), Song Chay
(b), and Song Ma (Thanh Hoa)~(c) belts and related cross sections 
(d--g). The locations of the cross sections are indicated on the
maps.\label{fig2}}
\end{figure*}

The Central Ailaoshan zone (Ailaoshan ophiolitic m\'{e}lange zone)
consists of blocks of serpentinized lherzolite and harzburgite, gabbro,
plagiogranite, basalt, limestone, and radiolarites enclosed in quartz 
schist, micaschist, and pelitic schist (Figure~\ref{fig1}). These
pelitic rocks are interpreted as a ``block-in-matrix'' formation
\citep{Zhong1998}. The ophiolitic blocks are~mainly exposed to the NW
of Mojiang City (Figure~\ref{fig2}a). The gabbro yields amphibole
\tsup{40}Ar/\tsup{39}Ar ages at $339.2 \pm 13.6$~Ma and $349 \pm 13$~Ma
\citep{Zhong1998,Jianetal1998}. The SHRIMP zircon U--Pb dating yields
$362 \pm 41$~Ma, $328 \pm 16$~Ma/$382.9 \pm 3.9$~Ma and $375.9 \pm
4.2$~Ma for gabbro, plagiogranite, and diabase respectively
\citep{Jianetal1998}. Major and trace element data and isotopic
compositions of~the Shuanghe gabbro suggest N-MORB type
gabbro derived~from a depleted mantle \citep{XuCastillo2004}.




The Eastern Ailaoshan zone (i.e., the Ailaoshan complex) is composed of
gneissic migmatite, augen gneiss, paragneiss, and granitic plutons 
\citep{Caietal2015}. U--Pb zircon geochronological dating documents three
magmatic stages: Neoproterozoic (850--750~Ma), Early--Middle Triassic
(250--240~Ma), and Cenozoic (30--20~Ma) 
\citep{ZhangScharer1999,Lietal2008,Qietal2010,Caietal2015}. This zone
is mostly composed of high-grade amphibolite--granulite facies
metamorphic rocks with Cenozoic ages, which is comparable to the
Day Nui Con Voi of N. Vietnam. From the viewpoint of inherited zircons,
these high-grade metamorphic rocks were exhumed to the surface from the
lower crust along the~Red River fault during the Cenozoic sinistral
strike-slip fault \citep{Tapponnieretal1990,Leloupetal1995}.

The Jinping zone is a triangular wedge located in the southeastern
Ailaoshan belt, comprising Ordovician--Carboniferous limestone,
sandstone, and mudstone. Detrital zircons of Silurian sandstone show a
SCB affinity, suggesting that the Jinping zone is the Paleozoic
sedimentary cover of the SCB \citep{Xiaetal2016}. Late Permian
ultramafic--mafic intrusive and volcanic rocks, which are parts of the
Emeishan Large Igneous Province, are exposed in this tectonic zone
\citep{Xiaoetal2004}.

\subsection{Song Ma belt}

The Song Ma belt is bounded by the Dien Bien Phu fault and the Red
River fault to the northwest and northeast, and extends over 300~km to
the Thanh Hoa area (Figure~\ref{fig1}). Four litho-tectonic units are
recognised from SW to NE in the Song Ma belt, namely the Truong Son--Sam
Nua zone, the Song Ma ophiolitic m\'{e}lange zone, the inner zone, and
the outer zone (Figure~\ref{fig1}).

The NW--SE trending Truong Son--Sam Nua zone is mainly composed of
Permian--Triassic sedimentary-volcanic rocks, which unconformably
overlie the Neoproterozoic to Silurian sedimentary sequence.
Carboniferous-Triassic (\textit{ca}. 310--240~Ma) plutons and volcanic
rocks have low \tsup{87}Sr/\tsup{86}Sr(i) and $\updelta$\tsup{18}O
values with high-Mg\tsup{\#} values and are I-type with positive
$\upvarepsilon_{\mathrm{Nd}}(t)$ and $\upvarepsilon_{\mathrm{Hf}}(t)$
values \citep{Qianetal2019}. With enriched Sr--Nd--Hf isotopic
compositions and depletions in Nb and Ta,  these igneous rocks are
interpreted as a continental magmatic arc related to the southwestward
subduction of the Paleo-Tethys Ocean [Figure~\ref{fig3}; 
\citealp{Houetal2019}, and references therein].
The undeformed magmatic rocks, with ages
between 240--220~Ma, are considered as formed by partial melting of
crustal rocks in a post-collisional setting  \citep{Wangetal2022}. It
is worth noting that the arc magmatism turns younger from SW to NE,
indicating a NE-ward migration of the arc\unskip\break (Figure~\ref{fig3}).


\begin{figure*}
\includegraphics{fig03}
\bcaption{\label{fig3}}{2}{Simplified late Permian to Triassic granitoids distribution
map of SW China and \mbox{N.~Vietnam}
[mostly on West Ailaoshan--Truong Son
belt; adapted from \citealp{Lietal2021}] with the age distribution
pattern inserted [ages are from
\citealp{Backhouse2004,Cromie2010,Fanetal2010,Halpinetal2016,
Hieuetal2017,Hoaetal2008,Ganetal2021,Hotson2009,Houetal2019,
Jianetal2009b,Kamvongetal2014,Laietal2014a,Laietal2014b,
Liuetal2012,Liuetal2015,Manaka2008,Qianetal2016,Qianetal2019,
Rogeretal2012,Rogeretal2014,Sanematsuetal2011,Shietal2015,
Shigetaetal2014,Tranetal2008,Tranetal2014,Wangetal2013,Wangetal2014,
Wangetal2016b,Wangetal2018}].
RRF: Red River Fault, SCF: Song Ca Fault; SMF: Song Ma fault; DBF: Dien
Bien Phu fault.}
\end{figure*}


The Song Ma ophiolitic m\'{e}lange zone is composed of micaschist and
pelitic schist, serpentinized peridotite, harzburgite, dunite,
plagiogranite, gabbro, gabbro-amphibolite, gabbro-dolerite, basalt,
chert, and limestone blocks [Figures~\ref{fig1} and~\ref{fig2}; 
\citealp{Lepvrieretal1997,Vuongetal2013}]. All these blocks, as well as
the intensively sheared sedimentary rocks, experienced greenschist- to
lower amphibolite facies metamorphism  \citep{Zhangetal2013}.

Amphibolite and gabbro from the Song Ma ophiolitic m\'{e}lange yield
ages ranging between 387~Ma to 313~Ma [Sm--Nd whole rock, 
\citealp{Vuongetal2013}]. Our new work on a diorite lens yields SIMS
zircon U--Pb age of $264.4 \pm 1.8$~Ma \citep{Wangetal2022}. The
foliated metagabbro yielded two age groups of $360.7 \pm 3.0$~Ma and
$263.2 \pm 2.3$~Ma, interpreted as inherited and crystallisation ages
respectively \citep{Wangetal2022}. Along this zone in northern Laos,
zircon U--Pb ages from the plagio-amphibolite yield a narrow age range
of 367--356~Ma as xenoliths \citep{Zhangetal2021}. Eclogite and HP
granulite are reported in this area. Zircons and monazites from
eclogite and granulite yield ages between 230~Ma and 243~Ma 
\citep{Nakanoetal2008,Nakanoetal2010,Zhangetal2013}.


To the northeast of the Song Ma ophiolitic m\'{e}lange zone, the inner
zone, or Nam Co zone, is mainly composed of paragneisses, micaschists,
quartz-schists, phyllites, amphibolites, quartzite, marbles, Paleozoic
sandstone, siltstone, and limestone [Figure~\ref{fig1}; \citealp{Liuetal2012}].
Most of these rocks underwent greenschist- to lower-amphibolite facies
metamorphism. Detrital zircons from the metasedimentary rocks in this
zone reveal a dominant age~peak at ${\sim}$850~Ma  \citep{Hauetal2018},
which suggests that the inner zone belongs to the SCB. The metamorphic
age of $234 \pm 10$~Ma is revealed by the monazite U--Th--Pb dating
\citep{Nakanoetal2010}. Meanwhile, biotite and muscovite
\tsup{40}Ar/\tsup{39}Ar dating in this zone yields the age of
250--240~Ma, recording a~Triassic thermo-tectonic event 
\citep{Lepvrieretal1997}.


Further NE, the outer zone is located between the inner zone and the
RRF (Figure~\ref{fig1}). It is composed of weakly metamorphosed Paleozoic
sedimentary rocks and Late Permian ultramafic--mafic intrusive and
volcanic rocks of the Song Da area, equivalent to the Emeishan LIP of S
China \citep{Chungetal1997}.\looseness=-1


\subsection{Song Chay belt}

The Song Chay belt develops in NE Vietnam, extends to China, and is
bounded by the Red River fault to its SW (Figure~\ref{fig1}). From the
SW to NE, this belt is divided into three litho-tectonic zones
including the Day Nui Con Voi zone, the Song Chay ophiolitic
m\'{e}lange zone, and the NE Vietnam thrust-and-nappe system
\citep{Lepvrieretal2011}.

The Day Nui Con Voi zone is bounded by the Red River and Song Chay
faults to its southwest and northeast, respectively
(Figure~\ref{fig1}). This zone is composed of upper amphibolite to
granulite-facies metamorphic rocks, forming an antiform
\citep{Leloupetal1995,Wangetal1998,Anczkiewiczetal2007}. It was exhumed
from the middle to lower crust due to the Cenozoic Red River and Song
Chay sinistral strike-slip faulting 
\citep{Leloupetal1995,Wangetal1998,Nakanoetal2018}.

The Song Chay ophiolitic m\'{e}lange zone is sporadically distributed
along the NE flank of the Day Nui Con Voi complex [Figure~\ref{fig2}b; 
\citealp{Lepvrieretal2011}]. The m\'{e}lange tectonically overlies the
Neoproterozoic to Cambrian strata. It consists of sheared and
metamorphosed sandstone and mudstone forming the matrix of
serpentinized ultramafic rocks, gabbro, gabbro-diorite, plagiogranite,
chert, and limestone as blocks. A plagiogranite yielded a zircon
crystallisation age of $356.4 \pm 2.9$~Ma, interpreted as the opening
time of the Paleo-Tethys \citep{Wangetal2022}.

The NE Vietnam thrust-and-fold system is located to the northeast of
the Song Chay ophiolitic m\'{e}lange zone, formed during the collision
of the SCB and IB [Figure~\ref{fig1}; \citealp{Lepvrieretal2011}]. It is
subdivided into an inner zone and an outer zone according to the
structural style and metamorphism. The inner zone, bounded by Song Chay
and Malipo faults, is composed of greenschist- to amphibolite facies
metamorphosed Neoproterozoic to Early Paleozoic sedimentary rocks and
the Early Paleozoic Song Chay augen gneiss massif, having experienced a
top-to-the-NE ductile shearing \citep{Wangetal2021}. Along the mineral
and stretching lineation, biotite and muscovite \tsup{40}Ar/\tsup{39}Ar
ages document a deformation age at 250--240~Ma 
\citep[][and references therein]{Wangetal2022}.
The outer zone is characterised by weakly to
un-metamorphosed Late Paleozoic sedimentary rocks that were involved in
a fold-and-thrust belt \citep{Lepvrieretal2011}. Granitic plutons
dated between 428 and 87~Ma are widely distributed 
\citep[][and references therein]{Chenetal2014}.
It is worth noting that the foliated plutons
are older than 245~Ma, while the undeformed ones are younger than
245~Ma.


\begin{figure*}
\includegraphics{fig04}
\caption{Synthetis and comparison of the cumulative probability plots
of detrital zircon U--Pb ages from the Ailaoshan--Song Ma--Song Chay
belt.\label{fig4}}
\vspace*{7pt}
\end{figure*}


\begin{figure*}
\includegraphics{fig05}
\caption{Temporal variations of $\upvarepsilon_{\mathrm{Hf}}(t)$ values
versus ages at 0--$3000$~Ma from M1, M2, M3, SW Yangtze block, and
ELIP (Emeishan Large Igneous Province).\label{fig5}}
\vspace*{7pt}
\end{figure*}


\section{The AS--SM--SC ophiolitic m\'{e}lange zone, from the viewpoint of matrix detrital zircon}

Due to the dextral offset along the Dien Bien Phu fault, a pre-Tertiary
reconstruction places the Song Ma belt to the southeast of the
Ailaoshan belt. By doing so, the tectonic units in the two belts
correlate well with each other \citep[][and references
therein]{Wangetal2022}.
Accordingly, we describe the zircon U--Pb ages and Lu--Hf
isotopic composition of the Ailaoshan and Song Ma ophiolitic
m\'{e}lange zones together. Based on a systematic work on the detrital
zircon age spectra and Lu--Hf isotopic composition of the matrix of the
m\'{e}lange, three m\'{e}lange units, namely, M1 (M\'{e}lange 1), M2,
and M3 can be distinguished (Figures~\ref{fig1} and~\ref{fig2}).


\subsection{Zircon U--Pb ages and Lu--Hf isotopic composition of the Ailaoshan--Song Ma ophiolitic m\'{e}lange zone}

The M1 unit is mainly exposed in the NW to the central segment of the  
Ailaoshan ophiolitic m\'{e}lange zone and the easternmost part of the
Song Ma zone (Thanh Hoa area) (Figures~\ref{fig1}, \ref{fig2}a,~c).
The analysed samples from the matrix consist of
sheared siltstone (AS391, AS405), sandstone (AS397, AS420, AS147),
sericite schist (AS103), siltstone (AW06), sublithic arenite (YN-54),
and mylonitic siltstone \mbox{(YN-41)} in the Ailaoshan belt and sandstone
(TH05) in the Thanh Hoa area [Table~\ref{tab1} and Figure~\ref{fig2}c; 
\citealp{Xiaetal2016,Liuetal2023}]. In these samples, the
detrital\unskip\break zircon
age distribution shows two significant peaks at ${\sim}$435~Ma and
${\sim}$960~Ma and five secondary peaks of  ${\sim}$790~Ma,
${\sim}$1110~Ma, ${\sim}$1650~Ma, ${\sim}$1870~Ma, and ${\sim}$2500~Ma
(Figure~\ref{fig4}). The M1 unit has several Late Paleozoic zircons with a
cluster at 355--375~Ma (1.3\% of the total measurement), indicating the
maximum depositional age of the matrix. Meanwhile, these Phanerozoic
zircons yield $\upvarepsilon_{\mathrm{Hf}}(t)$ ranging from ${-}$25.4 to
${+}$12.0, and seventy-two percent are negative, which have the
T\tsub{\mathrm{DM}}\tsup{2} (two-stage model) ages ranging from 750 to
3960~Ma (Figures~\ref{fig5} and~\ref{fig6}).

\begin{figure*}
{\vspace*{1pt}}
\includegraphics{fig06}
{\vspace*{1pt}}
\caption{Temporal variations of T\tsub{\mathrm{DM}}\tsup{2} versus ages at 
0--$3000$~Ma from M1, M2, M3, and ELIP (see Figure~\ref{fig4} for
relevant statistics data).\label{fig6}}
{\vspace*{1pt}}
\end{figure*}

\begin{table*}
\caption{Summary of the measurements of youngest age of detrital zircon
grains in Ailaoshan--Song Ma--Song Chay belt\label{tab1}} 
\begin{tabular}{ccccccc}
\thead
\parbox[t]{5pc}{\centering Ophiolitic m\'{e}lange unit} 
& Sample & YSG & YPP & YC1$\upsigma$ ($2+$)$^{\mathrm{a}}$ 
& YC2${\upsigma}$ ($3+$)$^{\mathrm{a}}$ 
&\parbox[t]{6pc}{\centering  Probable depositional age}\vspace*{2pt} \\
\endthead
\morerows{10}{M1}  & AS391 & 251.4 $\pm$ 2.9 & 435 & 414.1 $\pm$ 3.3 (2) & 418.7 $\pm$ 3.9 (3) & \morerows{10}{(355?) 310--270?}  \\
 & AS397 & 355.0 $\pm$ 3.9 & 430 & 357.7 $\pm$ 3.7 (4) & 360.0 $\pm$ 3.8 (5) &  \\
 & AS405 & 358.3 $\pm$ 5.1  & 435 & 423.2 $\pm$ 4.9 (6) & 423.3 $\pm$ 5.1 (10) &  \\
 & AS420 & 394.7 $\pm$ 4.3 & 435 & 415.8 $\pm$ 5.1 (2) & 423.6 $\pm$ 4.8 (8) &  \\
 & AS423 & 414.4 $\pm$ 3.8 & 425 & 417.5 $\pm$ 4.2 (4) & 424.6 $\pm$ 4.6 (13) &  \\
 & TH05 & 263.3 $\pm$ 2.2 & 439 & 370.4 $\pm$ 2.6 (2) & 435.6 $\pm$ 3.7 (17) &  \\
 & AS103 & 371.4 $\pm$ 3.9 & 432 & 435.5 $\pm$ 4.6 (12) & 435.5 $\pm$ 4.6 (12) &  \\
 & AS147 & 373.7 $\pm$ 3.8 & 425 & 436.4 $\pm$ 4.6 (6) & 428.6 $\pm$ 4.5 (8) &  \\
 & AW06 & 261.5 $\pm$ 2.47 & 460 & 444.3 $\pm$ 5.9 (2) & 450.1 $\pm$ 5.5 (3) &  \\
 & YN-54 & 427.2 $\pm$ 6.7 & 440 & 441.5 $\pm$ 6.0 (9) & 449.2 $\pm$ 6.6 (11) &  \\
 & YN-41 & 399.7 $\pm$ 5.1 & 440 & 401.3 $\pm$ 4.7 (2) & 435.1 $\pm$ 5.5 (15)\vspace*{4pt} &  \\
\morerows{3}{M2}  & AE02 & 229 $\pm$ 4.6 & 250 & 259.8 $\pm$ 3.2 (3) & 254.6 $\pm$ 2.8 (5) & \morerows{3}{260--255}  \\
 & AW07 & 239 $\pm$ 2.2 & 260 & 258.3 $\pm$ 2.9 (18) & 257.3 $\pm$ 2.8 (19) &  \\
 & AW08 & 242 $\pm$ 2.8 & 260 & 260.0 $\pm$ 2.7 (39) & 259.5 $\pm$ 2.7 (41) &  \\
 & AW10 & 232.4 $\pm$ 2.4 & 260 & 258.3 $\pm$ 2.5 (7) & 258.3 $\pm$ 2.5 (7)\vspace*{4pt} &  \\
\morerows{5}{M3}  & AW17 & 237 $\pm$ 3 & 245 & 243.3 $\pm$ 2.5 (47) & 244.2 $\pm$ 2.5 (49) & \morerows{5}{${\sim}$245 (250--245)}  \\
 & AW20 & 236 $\pm$ 3 & 245 & 244.9 $\pm$ 2.4 (29) & 244.9 $\pm$ 2.4 (29) &  \\
 & 13YZ06 & 236 $\pm$ 2 & 245 & 246.9 $\pm$ 2.4 (31) & 246.9 $\pm$ 2.4 (31) &  \\
 & TH03 & 223.2 $\pm$ 11.0 & 240 & 242.6 $\pm$ 6.2 (20) & 242.6 $\pm$ 6.2 (20) &  \\
 & TH11 & 117.0 $\pm$ 21.3 & 247 & 249.3 $\pm$ 6.7 (99) & 249.3 $\pm$ 6.7 (99) &  \\
 & TH12 & 224.4 $\pm$ 4.8 & 245 & 244.7 $\pm$ 4.8 (69) & 244.7 $\pm$ 4.8 (69)\vspace*{4pt}  &  \\
\morerows{3}{M4}  & TK242B & 271.2 $\pm$ 2.5 & 550 & 308.3 $\pm$ 2.6 (3) & 308.3 $\pm$ 2.6 (3) & \morerows{3}{310--255}  \\
 & TK210 & 295.3 $\pm$ 1.9 & 490 & 431.9 $\pm$ 3.5 (2) & 424 $\pm$ 3.4 (4) &  \\
 & TK207B & 236.4 $\pm$ 1.7 & 490 & 460.9 $\pm$ 4.0 (2) & 484.7 $\pm$ 3.4 (9) &  \\
 & SM209 & 255 $\pm$ 4 & 460 & 255.5 $\pm$ 4.0 (2) & 464.2 $\pm$ 7.0 (6) &  
\botline
\end{tabular}
\tabnote{Notes: YSG: youngest single grain age; YPP: youngest graphical
age peak; YC1${\upsigma}$ ($2+$): youngest 1${\upsigma}$ grain cluster;
YC2${\upsigma}$ ($3+$): youngest 2${\upsigma}$ grain cluster.
\newline
$^{\mathrm{a}}$ Numbers of grain ages in clusters in parentheses.} 
\vspace*{8pt}
\end{table*}





The M2 unit located in the NW segment of the Ailaoshan ophiolitic
m\'{e}lange zone, close to Permian strata following the previous
geological map (Figure~\ref{fig2}a). The previous analytical work
contains the quartz arenite (AE02), sandstone (AW07, AW08), and
quartz-arenite (AW10) belonging to the M2 unit\unskip\break [Table~\ref{tab1} and
Figure~\ref{fig2}a; \citealp{Xiaetal2020}]. Detrital zircons analysis
reveals that the M2 unit ages cluster around 240--270~Ma, peaking at
${\sim}$260~Ma with secondary peaks at ${\sim}$410~Ma, ${\sim}$755~Ma,
${\sim}$950~Ma, ${\sim}$1450~Ma, ${\sim}$1850~Ma, and ${\sim}$2470~Ma
[Figure~\ref{fig4}; \citealp{Xiaetal2020}]. The $\upvarepsilon_{\mathrm{Hf}}(t)$
of the zircons with 240--270~Ma ages range from ${-}$28.9 to ${+}$8.1,
clustering around ${-}$1.0 to ${+}$7.0 and showing a mantle-derived
material contribution (Figure~\ref{fig5}). The T\tsub{\mathrm{DM}}\tsup{2} ages
of these zircons range from 0.67~Ga to 2.53~Ga with a significant
Neoproterozoic peak of ${\sim}$0.85~Ga\unskip\break (Figure~\ref{fig6}).

The M3 unit is exposed in the SE segment of the Ailaoshan ophiolitic
m\'{e}lange and the Thanh Hoa area (Figure~\ref{fig2}). It forms the
main part of the m\'{e}lange zone (Figure~\ref{fig1}). The M3 unit
includes siltstone (AW17, 13YZ06), and mudstone (AW20) in the Ailaoshan
belt, and siltstone (TH03), schistose tuff (TH11), and schistose
sandstone (TH12) in the Thanh Hoa area [Table~\ref{tab1} and
Figure~\ref{fig2};  \citealp{Xuetal2019,Lietal2021}]. The detrital
zircons of the M3 unit yield a significant age peak at ${\sim}$245~Ma
with three secondary peaks at ${\sim}$360~Ma, ${\sim}$770~Ma, and
${\sim}$1045~Ma (Figure~\ref{fig4}). The dominant zircons at 230--260~Ma
have $\upvarepsilon_{\mathrm{Hf}}(t)$ values ranging from ${-}$21.9 to
${+}$10.1 and T\tsub{\mathrm{DM}}\tsup{2} ages from 0.55~Ga to 2.62~Ga,
respectively (Figures~\ref{fig5} and~\ref{fig6}). Besides, more than
eighty-four percent of the 230--260~Ma zircons have negative
$\upvarepsilon_{\mathrm{Hf}}(t)$ values from ${-}$17 to ${-}$5
(Figure~\ref{fig5}), indicating that the M3 detrital zircons are
sourced from the continental crust. Although both M2 and M3 units have
Late Permian to Early Triassic detrital zircons, the different Hf
isotopic compositions of these zircons allow distinguishing the M3 unit
from the M2 one (Figure~\ref{fig5}).

\subsection{Zircon U--Pb ages and Lu--Hf isotopic composition of the
Song Chay ophiolitic m\'{e}lange zone}

Along the Red River fault, four samples were analysed from the matrix
of the Song Chay ophiolitic m\'{e}lange, including sheared
meta-sandstone (TK242B, TK210, TK207B) and a lithic sandstone (SM209)
(Figure~\ref{fig2}a). To keep consistency, this m\'{e}lange zone will
be referred to as M4 in the following. The analytical results show peak
ages at 315~Ma, 470~Ma, 610~Ma, 730~Ma, 770~Ma, 965~Ma, 1780~Ma, and
2450~Ma, which are significantly different   from M1, M2, and M3 units\unskip\break
(Figure~\ref{fig4}). In these samples,  the
$\upvarepsilon_{\mathrm{Hf}}(t)$ values of the detrital zircons range
from ${-}$28.2 to ${+}$10.8, and the 
T\tsub{\mathrm{DM}}\tsup{2} ages range from
0.97~Ga to 3.87~Ga \citep{Wangetal2021}.


\section{Discussion}
\subsection{Deposition age of the AS--SM--SC ophiolitic m\'{e}lange
from the viewpoint of the detrital zircon age distribution}

As formed in the turbidity deposits, it is difficult to determine the
biostratigraphic age of the m\'{e}lange matrix using the fossils
contained in the strata \citep{CaridroitdeWever1984,YuTekin1996}.
Moreover, most of the m\'{e}langes experienced intensive deformation
and metamorphism during subduction, collision, and exhumation.
Therefore, the matrix mostly is generally represented by
pelitic schist, micaschist, and even paragneiss 
\citep{Festaetal2010,Festaetal2012}. Hence, it is difficult to
constrain the depositional age of the matrix of the m\'{e}langes.

To determine the maximum depositional age of the sedimentary rocks,
\citet{DickinsonGehrels2009} tested four alternate measurements: YSG:
{Y}oungest {S}ingle {G}rain age with 1$\upsigma$
uncertainty, but inherent lack of reproducibility diminishes confidence
in its reliability because some individual YSG ages might be spurious
due to lead loss or grain of unknown origin; YPP: {Y}oungest
graphical age {P}eak on an age-probability {P}lot-controlled 
by more than one grain age (single-grain age peaks ignored),
the crest of the discrete youngest age peaks visible on
age-distribution curves; YC1${\upsigma}$ ($2+$): {Y}oungest
{$1\upsigma$} grain {C}luster (two or more grains)---provides 
a numerical measure of youngest grain age incorporating a
similar minimal reproducibility as YPP; and YC2${\upsigma}$ ($3+$):
{Y}oungest {2$\upsigma$} three or more grains
{C}luster---provides a more statistically robust and conservative
measure of youngest age. In fact, from the viewpoint of our analytical
work on the zircon from the matrix of the m\'{e}lange, all these
estimations have significant uncertainties (Table~\ref{tab1}). The
maximum depositional age should also consider regional geology
constraints.

According to the YSG age of the M1 unit, the feasible depositional age
ranges from 251~Ma to 427~Ma. However, the YPP, YC1{${\upsigma}$
($2+$), and} YC2${\upsigma}$ ($3+$) give depositional ages at around
430--440~Ma, 357--441~Ma, and 360--449~Ma, respectively
(Table~\ref{tab1}). The detrital zircons of the M1 unit are mainly
supplied from the Silurian and Devonian substratum, and Truong Son--Sam
Nua arc-related felsic magmatism  \citep{Lietal2021,Liuetal2023}. The
deposition time of M1 is restricted to \textit{ca.} 310~Ma, which
corresponds to the initial development stage of the magmatic arc 
\citep[e.g.,][]{Zhangetal2021}. The Truong Son--Sam Nua arc activity can be
divided into two stages, A1 and A2, between 310--270~Ma and 
265--240~Ma with peaks at 286 and 248~Ma, and a gap between 270--265~Ma
(Figure~\ref{fig3}). If we consider that the A2 signal is significant
in M2, and absent in M1, the depositional age of the M1 should be
constrained between 310~Ma and 270~Ma (Table~\ref{tab1}).

The possible depositional age of the M2 unit suggested by the YSG is
between 242--229~Ma (Table~\ref{tab1}). However, all the YPP,
YC1${\upsigma}$ ($2+$) and YC2${\upsigma}$ ($3+$) have a consistent age
of 260--255~Ma (Table~\ref{tab1}). According to \citet{Cawoodetal2012},
if the deposition forms at the edge of converging plates, such as the
trench, and the age of detrital zircons have a dominant age peak, then
the largest proportion of ages is close to the depositional age.
Considering the provenance analysis, a possible deposition time of the
M2 unit is constrained between 260~Ma and 255~Ma (Table~\ref{tab1}).

The YSG age of the M3 unit is 237--117~Ma, while the YPP,
YC1{${\upsigma}$ ($2+$), and} YC2${\upsigma}$ ($3+$) give ages between
240~Ma and 250~Ma clustering around 245~Ma (Table~\ref{tab1}). In the
fold-and-thrust unit of the Song Ma belt, the biotite and muscovite
yield \tsup{40}Ar/\tsup{39}Ar ages of 250--240~Ma corresponding to the
timing of the collision \citep{Lepvrieretal1997}. The matrix of the
m\'{e}lange should be deposited earlier than this age. Accordingly, the
possible depositional age of the M3 unit is constrained at 245~Ma
(Table~\ref{tab1}).

In the M4 unit, the YSG give possible depositional ages of 295--236~Ma
(Table~\ref{tab1}). However, the YPP gives older ages between 460~Ma and
550~Ma (Table~\ref{tab1}). Besides, the YC1$\upsigma$ ($2+$) and
YC2${\upsigma}$ ($3+$) give ages of 255--460~Ma and 308--484~Ma
respectively. Since the youngest age cluster is around 310~Ma
(${\sim}$5.4\%), the deposition age should be younger than the Late
Carboniferous. Considering that the deformation and metamorphism
occurred earlier than the Late Triassic in the Song Chay ophiolitic
m\'{e}lange zone \citep{Lepvrieretal2011}, we suggest that its maximum
deposition age is older than ${\sim}$255~Ma \citep{Wangetal2021}.

Overall, the detrital zircon age analyses suggest that the m\'{e}lange
matrix has different depositional times in the AS--SM--SC belt
(Table~\ref{tab1}). It looks like the m\'{e}lange matrix is
characterised by temporal heterogeneity.

\begin{figure*}
{\vspace*{2pt}}
\includegraphics{fig07}
{\vspace*{1pt}}
\caption{Paleogeographic reconstruction of SCB and IB at 260~Ma. Arrow
indicated paleo-current.\label{fig7}}
\end{figure*}


\subsection{Provenance and strike-parallel heterogeneity of the
different ophiolitic m\'{e}lange units}

In the AS--SM--SC belt, the matrix in different areas has different
depositional times and provenances, manifesting obvious strike-parallel
heterogeneity, namely, variations along the NW--SE trend, parallel to
the strike of the orogenic belt \citep{Linetal2022}.

As the main peaks of M1 unit, the spectrum of~435~Ma and 960~Ma also
appear in the \mbox{Paleozoic} sedimentary rocks of IB and the Cathaysia block
[Figure~\ref{fig4}; \citealp{Xiaetal2016,Wangetal2016a}]. However, the
Hf isotopic composition of the M1 unit is more like that of the IB
Paleozoic strata rather than the Cathaysia block ones, particularly to
the Silurian and Devonian sedimentary rocks of the Eastern Simao in the
IB \citep{Wangetal2016a,Xiaetal2016,Liuetal2023}. In particular, as the
major age group (400--500~Ma), detrital zircons from the M1 unit
and Silurian and Devonian sedimentary rocks in the IB share the same
age spectrum, similar $\upvarepsilon_{\mathrm{Hf}}(t)$ values, and
T\tsub{\mathrm{DM}}\tsup{2} ages \citep{Liuetal2023}. Accordingly, the
above-mentioned strata of the IB are the major provenance of the M1
unit (Figure~\ref{fig7}). In the Ailaoshan--Song Ma suture zone, gabbro,
diabase, and anorthosite which were dated to \textit{ca.}\ 
${\sim}$370~Ma were interpreted as emplaced during the initial opening of the
Paleo-Tethys Ocean  \citep{Jianetal2009b}. Considering this magmatism,
we can argue that contemporary felsic magmatism acts as the potential
provenance for the M1 unit, providing the youngest detrital zircons
around 370~Ma.




Except for a younger prominent cluster with ages of 270--240~Ma (peak at
260~Ma), the M2 and M1 units are correlatable in terms of detrital zircon
spectrum (Figure~\ref{fig4}). Consequently, the provenance of the M2
unit is supposed to be from the magmatic arc (Truong Son--Sam Nua) and
Paleozoic strata of the IB \citep{Xiaetal2020}. However, the
$\upvarepsilon_{\mathrm{Hf}}(t)$ values (${-}$28.9 to ${+}$8.1) and model
ages (2.53~Ga to 0.67) from the dominant age spectrum (270--240~Ma) are
like the zircons in ELIP (Emeishan Large Igneous Province), rather than
the Late Permian--Middle Triassic magmatic zircons from Western
Ailaoshan--Truong Son--Sam Nua magmatic arc zone (Figures~\ref{fig5}
and~\ref{fig6}). Hence, we argue that the 270--240~Ma detrital zircons,
at least half of them (positive $\upvarepsilon_{\mathrm{Hf}}(t)$
values, 52\%), might come from the ELIP, i.e., the passive margin of
SCB. Particularly, if we restore the initial location, before the
sinistral strike-slip displacement of the Red River fault, the ELIP
becomes located close to the Ailaoshan belt\unskip\break (Figure~\ref{fig7}).

In the M3 unit, the detrital zircons have a dominant peak around 245~Ma
(Figure~\ref{fig4}). Meanwhile, the $\upvarepsilon_{\mathrm{Hf}}(t)$
values (more than 84\% are negative) and model ages of these 260--230~Ma
zircons are consistent with the widespread 255--245~Ma arc-related
magmatic rocks of IB (A2 in Figure~\ref{fig3}). Therefore, this
dominant cluster was directly supplied by the overriding IB
(Figure~\ref{fig7}). Like the M1 unit, the signal of 360~Ma is also
sourced from the magmatic rocks related to the opening of the
Paleo-Tethys Ocean in M3 unit. Besides, the Precambrian detrital
zircons of the M3 unit show two peaks of 770 and 1045~Ma
(Figure~\ref{fig4}). The typical ages of 440~Ma and 960~Ma of IB signature
spectrum are missing in the M3 unit, which suggests that the Paleozoic
sequences in IB may not have supplied the material of the M3 unit. However,
the 770~Ma peak appears as a major age group of the Paleozoic
sedimentary rocks in the southwest margin of the SCB
\citep{Lietal2021}. Accordingly, it could be concluded that the
provenance of M3 units is from both magmatic arcs of IB and Paleozoic
sedimentary rocks in the SCB  [Figure~\ref{fig7};
\citealp{Lietal2021}].

The matrix M4 unit shows two peaks at 310~Ma and 460~Ma. There is no
obvious 310~Ma peak in the sedimentary rocks of SCB and IB, while
${\sim}$300~Ma magmatic rocks were reported in the Truong Son--Sam Nua
zone \citep{Kamvongetal2014}. The 460--440~Ma zircons are widely
distributed in the Paleozoic strata of the IB and Paleozoic granite of
the SCB \citep{Wangetal2011,Wangetal2016a}. Their
$\upvarepsilon_{\mathrm{Hf}}(t)$ values are almost negative and
resemble the magmatic zircons of Paleozoic granite in the SCB but
different from the Paleozoic strata of the IB \citep{Wangetal2021}. The
Neoproterozoic detrital zircons are grouped into predominant 800--700~Ma
and subordinate 650--550~Ma and 1000--900~Ma (Figure~\ref{fig4}). The
group of 800--700~Ma is the main peak of the sedimentary rocks of the
SCB, while it is the subordinate peak in the IB \citep{Sunetal2009}. In
the northwestern to the western margin of the SCB, the Neoproterozoic
to Paleozoic strata have abundant 650--550~Ma detrital zircons, but a
few ones are preserved in the Paleozoic strata of the IB 
\citep{Wangetal2016a,Xiaetal2016}. Both in the SCB and IB, the
Paleozoic strata contain a multitude of 1000--900~Ma zircons.
Considering that the Neoproterozoic zircons represent more than 60\% of
the analysed grains, we suggest that the zircons of the M4 unit are
dominantly supplied from the SCB, with a small number of Late
Carboniferous (${\sim}$310~Ma) zircons sourced from the magmatic arc in
the IB \citep{Wangetal2021}.

As shown in Figure~\ref{fig7}, the matrix of the AS--SM--SC ophiolitic
m\'{e}lange manifests obvious strike-parallel heterogeneity.
Importantly, our results demonstrate that the detrital materials of the
m\'{e}lange matrix could be provided by both the active continental
margin of the IB and the passive continental margin of the SCB
(Figure~\ref{fig7}).


\subsection{Strike-perpendicular heterogeneity}

Our results document a strike-perpendicular heterogeneity of the
AS--SM--SC ophiolitic m\'{e}lange with variations along the NE--SW trend,
transverse to the orogenic belt. In the field, the M1 unit is located
to the NE of the M2 unit in the northwestern segment of the Ailaoshan
ophiolitic m\'{e}lange zone (Figure~\ref{fig2}a). The NE-dipping of the
foliation indicates that the M1 unit is geometrically above M2. The
Ailaoshan complex formed by Cenozoic upper amphibolite to granulite
facies metamorphic rocks also overlies the M1 (Figure~\ref{fig2}d).
Thus, the M1 and M2 units and the Ailaoshan complex appear as inverted. In the
central segment of the Ailaoshan ophiolitic m\'{e}lange, the M3 unit,
located to the northeast of the M1 unit overlies this unit
(Figure~\ref{fig2}a). On the contrary, the high-grade metamorphic rocks
of the Ailaoshan complex, are located above the M3 unit
(Figure~\ref{fig2}e). Further SE, at the coastal end of the Song Ma
belt (Thanh Hoa area), the M1 unit overlies the M3 unit, where the M2
unit and high-grade metamorphic rocks are missing (Figure~\ref{fig2}c,~g).
The presently inverted geometry of the M1, M2, and M3
units and the Ailaoshan complex can be attributed to the Cenozoic
transpressional tectonics related to a strike-slip deformation
(Figure~\ref{fig2}). 


\subsection{Implications of the paleogeography and the tectonic
evolution of the eastern Paleo-Tethys} 

It is well acknowledged that from the Devonian, several continental
blocks, such as SCB, IB, Sibumasu, North Qiangtang, and South
Qiangtang, were separated from Gondwana, drifted to the north, and
collided with Laurasia. This rifting was coeval with the opening and
closure of the oceanic basins ascribed to several branches of the
Paleo-Tethys. In the study area, the AS--SM--SC Ocean, opened between SCB
and IB, was one of the branches of the Paleo-Tethys. As mentioned
above, the M1, M2, M3, and M4 units contain detrital zircon age
clusters between 380 to 355~Ma that may be ascribed to the oceanic
magmatism related to the initial opening of the Paleo-Tethys
(Table~\ref{tab1}; Figures~\ref{fig4} and~\ref{fig8}A).


\begin{figure}
{\vspace*{4pt}}
\includegraphics{fig08}
{\vspace*{4pt}}
\caption{Tectonic evolution between the SCB and IB from Late Devonian
to Cenozoic times.\label{fig8}}
{\vspace*{-4pt}}
\end{figure}


During the Late Carboniferous to Early Permian (310--270~Ma), this
branch of the Paleo-Tethys subducted beneath the IB and formed the
NW--SE striking Jiangda-Weixi, Western Ailaoshan--Truong Son--Sam Nua
magmatic arc (A1 in Figure~\ref{fig3}) along the north margin of IB
[Figure~\ref{fig7};  \citealp{Liuetal2012,Wangetal2018}]. During this
period, the Silurian and Devonian strata, that is, the cover of the IB,
rather than the magmatic arc provided detritic material for the M1
unit [Figure~\ref{fig8}B1;  \citealp{Lietal2021}]. Meanwhile, the
Paleozoic sedimentary rocks of the passive continental margin of the
SCB acted as the main source of sediments in the Song Chay ophiolitic
m\'{e}lange zone (Figure~\ref{fig8}B2).

During the 270--265~Ma period, a magmatic quiescence occurred in the
Western Ailaoshan--Truong Son--Sam Nua zone (Figure~\ref{fig3}). At that
time, the ELIP and related acid magma erupted to the SW of the SCB,
providing abundant detritic materials for the M2 unit
(Figure~\ref{fig8}C). Subsequently, in the Western Ailaoshan--Truong
Son--Sam Nua zone, the second-stage arc-related magmatism (260--250~Ma)
occurred (A2 in Figure~\ref{fig3}, D1 in Figure~\ref{fig8}). A change in the
subduction angle is suggested by the northeastward magmatic arc
migration (Figure~\ref{fig3}). During this period, the provenance of
the matrix of the M3 m\'{e}lange was supplied by the A2 magmatic arc
(D1 in Figure~\ref{fig8}).

\begin{figure*}
\includegraphics{fig09}
\caption{Global plate reconstruction in Early Triassic times of the
Paleo- and Neo-Tethys Ocean evolution 
\citep[modified from][]{Zhuetal2021}.\label{fig9}}
\end{figure*}


At approximately 250~Ma, the SCB subducted southwestward beneath the
IB, accompanied by large-scale regional metamorphism and deformation
that continued up to 240~Ma \citep{Lepvrieretal1997}. In the SCB, the
top-to-the-NE shearing and related fold and thrust deformation were
developed. During this period, the M1, M2, and M3 units were thrust
upon the Neoproterozoic--Early Mesozoic sedimentary cover
(Figure~\ref{fig8}D2). Afterwards, a large-scale magmatism occurred at
240--220~Ma, representing a post-collisional stage [Figure~\ref{fig8}E;
\citealp{Rogeretal2014}].\looseness=-1

In the Cenozoic, the India-Eurasia continental collision led to the
eastward extrusion of the IB along the sinistral strike-slip Red River
and Dien Bien Phu faults  \citep{Tapponnieretal1990,Leloupetal1995}.
Accompanied by this strike-slip deformation, the southwestward
thrusting caused an inverted geometry of the M1, M2, and M3 units and
the Ailaoshan complex (i.e., strike-perpendicular heterogeneity, 
Figure~\ref{fig8}F).

The AS--SM--SC belt is duplicated by the Cenozoic strike-slip Red River
and Dien Bien Phu faults \citep{Faureetal2014}. Such a Cenozoic
superimposition forms a complex architecture, that is, the
juxtaposition of several orogenic belts or suture zones (Figure~\ref{fig7}).
Accordingly, we think of the Paleo-Tethys as an ocean surrounded by
microcontinents and lacking islands, rather than an archipelagic ocean
as the present Pacific Ocean (Figure~\ref{fig9}). It should be noted that the
passive continental margin of the SCB acted as a significant provenance
area for the matrix of the m\'{e}lange (Figure~\ref{fig7}). Furthermore, the
Paleo-Tethys is a ``narrow'' or ``limited'' ocean, like the present
Black Sea (Figure~\ref{fig9}). This interpretation could improve the understanding
of the Late Permian to Early Triassic paleogeographic reconstructions
(Figure~\ref{fig9}). Several lines of evidence, including the Early Triassic
acidified/sulfurised/overheated ocean causing the slow recovery after
the mass extinction, the lack of porphyry copper deposits in the
Paleo-Tethys, and the analogous Lopingian quadruped fauna in the
northern margin of the North China block and the IB (Laos), also
support this conclusion \citep{RichardsSengor2017,Huetal2021,Liuetal2021}.

\section{Conclusion}

Based on the structural analysis and regional tectonics, the review of
detrital zircons age distribution and Hf isotopes in the m\'{e}lange
zone between the SCB and IB allows us to draw the following conclusions:
\begin{enumerate}[(1)]
\item The geochronological data of the detrital zircons from the matrix of
the AS--SM--SC ophiolitic m\'{e}lange zone document a temporal
heterogeneity between M1, M2, M3, and M4 units at 310--270~Ma,
265--250~Ma, 245--240~Ma, and 310--255~Ma, respectively.

\item The different components and provenances of each unit reflect a
strike-parallel heterogeneity. The M1 unit was mainly sourced from the
Paleozoic sedimentary rocks of the IB. The main provenance for the M2
unit is Emeishan large igneous province. The magmatic arc developed in
the IB provided the materials for the M3 unit, and the detrital
materials of the M4 were mainly sourced from the SCB. The Cenozoic
strike-slip deformation led to an inverted geometry of the M1, M2, and
M3 units, accounting for a strike-perpendicular heterogeneity straight
to the strike of the orogenic belt.

\item The temporal, strike-parallel, and strike-perpendicular
heterogeneities help to decipher the time-space evolution of the
Paleo-Tethys. The M1, M2, M3, and M4 units contain information from
different evolutionary stages, likely recording the comprehensive
history of the ancient oceanic basin. Importantly, our results
demonstrate that both the active continental margin of the IB and the
passive continental margin of the SCB supplied significant material for
the m\'{e}lange matrix, indicating that the Paleo-Tethys Ocean was a
``narrow'' or ``limited'' ocean rather than the archipelagic ocean
proposed before.
\end{enumerate}

\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}
This study was funded by the State Key Laboratory of Lithospheric
Evolution, Institute of Geology and Geophysics, Chinese Academy of
Sciences (SKL-Z202205) and the National Natural Science Foundation of
China (91855212 and 91855103). The editor O.~Fabbri and two reviewers
are thanked for their constructive comments and suggestions, which lead
to a significant improvement in our manuscript. Mr. J.~Y.~Li is
acknowledged for his help during the fieldwork.

\CDRGrant[CAS]{SKL-Z202205}
\CDRGrant[NSFC]{91855212}
\CDRGrant[NSFC]{91855103}

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