Intracontinental subduction: a possible mechanism for the EarlyPalaeozoic Orogen of SE China

Michel Faure,1,2 Liangshu Shu,3 Bo Wang,3 Jacques Charvet,1 Flavien Choulet1 and Patrick Monie4

1Universite d�Orleans, CNRS ⁄ INSU, Universite Francois Rabelais – Tours, Institut des Sciences de la Terre d�Orleans – UMR 6113, Campus

Geosciences, 1A rue de la Ferollerie 45071 Orleans cedex 2, France; 2State Key Laboratory of Lithospheric Evolution, Institute of Geology and

Geophysics, Chinese Academy of Sciences, Beijing 100029, China; 3Department of Earth Sciences, Nanjing University, Nanjing, China;4Geosciences Montpellier, UMR 5243, Universite de Montpellier 2, 34095 Montpellier, France

Introduction

Continental subduction accounts forhigh-pressure metamorphism, ductiledeformation and melting of crustalrocks. This process is well-documentedin collision belts where magmatic arcand ophiolitic nappes show that oce-anic subduction precedes continentalone (e.g. Mattauer, 1986; Escher andBeaumont, 1997; Matte et al., 1997;Guillot et al., 2003; Faure et al.,2008). In intracontinental belts, evi-dence for an earlier, long lasting,oceanic subduction is lacking. TheCentral Asia ranges result of intra-continental subduction caused by theindentation of Asia by India after thecollision (Mattauer, 1986; Avouacet al., 1993; Burtman and Molnar,1993; Allen et al., 1999), but theseorogens do not expose their deepzones. The Pyrenees or Alice Springsbelts are examples of intracontinentalorogens where metamorphic or mag-matic domains are observed (e.g.Teyssier, 1985; Roure et al., 1989;Hand and Sandiford, 1999). Thispaper presents structural and geochro-nological data that allow us to pro-

pose an intracontinental subductionmodel accounting for the formation ofthe Early Palaeozoic Orogen of SEChina.

An outline of the geology of SEChina

The South China Block (SCB,Fig. 1A) is one of the continentalmasses that amalgamated to formEurasia. It contains several internalbelts, such as the Xuefengshan, orthe Wuyi–Yunkai Fold Belt (Fig. 1A).SCB experienced five main tectonicevents since the Proterozoic. Theearliest one corresponds to the Neo-proterozoic Jinning Belt that weldedthe Yangtze and Cathaysia Blocks toform the SCB (Guo et al., 1985,1989; Chen et al., 1991; Xu et al.,1992; Charvet et al., 1996; Shu andCharvet, 1996). From 830 to 690 Ma,the intracontinental Nanhua rift,characterized by up to 5 km of terri-geneous and volcanic rocks, superim-poses upon the Jinning Belt (Li et al.,1989; Wang and Li, 2003). There isno major sedimentary break betweenlate Neoproterozoic and Cambrianrocks, but sharp facies and thicknesscontrasts are recognized for the Cam-brian–Ordovician deposits betweenthe northern and southern domains.The former is represented by ca4.5 km of carbonate and black shale,whereas the latter consists of more

than 8 km of sandstone, siltstone andmudstone. In the southern domain,Silurian deposits are missing andDevonian conglomerates and sand-stone unconformably cover EarlyPaleozoic series (JBGMR (JiangxiBureau of Geology and MineralResources), 1984, HBGMR (HunanBureau of Geology and MineralResources), 1988; Zhao et al., 1996).Excepting the area that experiencedthe Early Palaeozoic Orogeny, theDevonian rocks cover the Silurianrocks either in direct continuity orwith a slight disconformity. TheDevonian–Carboniferous transitionis also continuous. Platform sedimen-tary sedimentation continues in thePermian, Early and Middle Triassic.A Late Triassic regional unconfor-mity argues for the Late Permian toMiddle Triassic Indosinian Orogeny(e.g. Huang, 1945; Faure et al., 1996;Shu et al., 2006; Wang et al., 2007;Lin et al., 2008). The Jurassic-Creta-ceous Yanshanian event is character-ized by NE–SW trending normal orwrench faults that controlled plutonemplacement and continental sedi-mentation (e.g. Jahn et al., 1990;Gilder et al., 1991; Faure et al.,1996; Lin et al., 2000; Zhou et al.,2006; Shu et al., 2008). In the studyarea, the post-Devonian deformationis localized along faults and does notsignificantly disturb older structures.A discussion of the Indosinian and

ABSTRACT

The Early Palaeozoic Orogen of SE China consists of three litho-tectonic elements, from top to bottom: a sedimentary UpperUnit, a metamorphic Lower Unit and a gneissic basement. Theboundaries between these units are flat lying, south directed,ductile decollements. The lower one is coeval with an amphib-olite facies metamorphism (M1). The belt is reworked bymigmatite–granite domes, high-temperature metamorphism(M2) and granitic plutons related to post-orogenic crustalmelting. We date here the syn-M1 ductile shearing at453 ± 7 Ma by U-Th ⁄ Pb method on monazite. Previous ages

and our new 40Ar ⁄ 39Ar ages of biotites and muscovites showthat the metamorphic rocks experienced syn-M2 exhumationfrom 440 to 400 Ma. The Early Palaeozoic Orogen of SE China isan intracontinental belt in which decollements accommodatedthe north-directed subduction of the Cathaysian continent. Thisorogen is an example of intracontinental subduction that wasnot preceded by oceanic subduction.

Terra Nova, 00, 1–10, 2009

Correspondence: Prof Michel Faure, ISTO.

UMR, CNRS 6113, Universite d�Orleans,

Bat. Geosciences, Orleans, 45067 Orleans,

France.Tel.:330238417306; fax:33023841

73 08; e-mail: michel.faure@univ-orleans.

fr

� 2009 Blackwell Publishing Ltd 1

doi: 10.1111/j.1365-3121.2009.00888.x

Yanshanian events is beyond thescope of this paper.In SE China, the existence of Early

Palaeozoic tectonics is documented bythe Devonian unconformity and LateSilurian to Early Devonian granitoidsintruding folded Early Palaeozoicrocks (e.g. Grabau, 1924; Huang,1945; Zhang et al., 1984; Guo et al.,1985; Ren and Chen, 1989). The tec-tonics of SE China have been variouslyinterpreted, namely (i) Triassic colli-sion between the Yangtze and Cat-haysia Blocks (Hsu et al., 1990); (ii)Andean-type subduction followed bycollision (Jahn et al., 1990; Ma, 2006);(iii) multiple arc collisions (Guo et al.,1989; Ma, 2006); (iv) intracontinental

belt (Ren and Chen, 1989; Charvetet al., 1996; Wang et al., 2007; Shuet al., 2008). However, these modelsdo not provide detail structural infor-mation.

The Early Palaeozoic Orogen of SEChina

The Early Palaeozoic Orogen is ex-posed from Wuyi to Yunkai in Chinaand in the Song Chai massif ofVietnam (Fig. 1A). In the study area,South of Nanchang, we recognizedfive units (Fig. 1B).The Sedimentary Upper Unit con-

sists, from top to bottom, of Middleto Late Ordovician turbidite with

slumps, disrupted sandstone bedsand pebbly mudstones. Plurimeter-scale limestone lenses scattered withinsandstone are likely to be olistoliths.Dykes or lava flows are absent.The Metamorphic Lower Unit is com-

posed of micaschist, paragneiss and asmall amount of quartzite, amphiboliteand marble. The protoliths of theserocks are terrigeneous or volcaniclasticrocks. Detrital zircons in paragneisswith 1000–700 Ma ages argue for aNeoproterozoic protolith (Wan et al.2007).The Paleoproterozoic basement con-

sists of biotite–amphibole gneiss,amphibolite, granitic–gneiss and por-phyritic granite. East of the study

Nanchang 28¡40 28¡40

20 km

Northern Domain

Southern Domain

Wugongshan

dome

G

N m

m

Proterozoic basement

Devonian to Early Triassic unconformable series

Pre-Devonian granitoids

Sedimentary Upper Unit Cambrian-Ordovician series

Metamorphic Lower Unit partly migmatitic (m) and anatectic granite

Decollement layer with ductile deformation

Neoproterozoic ophiolitic melange Jinning belt

Post-Jinning Neoproterozoic series

Cambrian to Carboniferous series

J-S Fault

Jianning

Lichuan Nanfeng

Jian

J

kyanite Stretching lineation with shear sense

Foliation dip and strike 50

116E 117E

27N 27N

28N 28N

115E

116E 115E 117E

50

30

Xinguo

Detachment fault

60

70

80

Cizhu 60

60

40

70

60

60

55 45

80 50

80

70

70

60

35 20

20

10 20

20

60

15 30

75

20

70

50

50 40

50 65

60 60

40

*

Fig. 2

Fig. 2

30

30

45 60

40

m

m

m

m m

m

60

30

Chongren

Nancheng

Chongyi

Xue

fengs

han

Qinling- Dabie Belt

Sichuan Basin

Yunkai

Wuyi

Mesozo

ic M

agm

atic

Bel

t

J-X F

NORTH CHINA

INDOCHINA

BLOCK

YANGTSE

BLOCK

D-S F

Songpan Ganzi Belt

Song Ma suture

Jinsha suture

Song Chai

Shanghai

(A)

(B)

Jiulingshan (B)

CATHAYSIA

405-397 Ma Ar/Ar bi/ms

453±7 Ma U/Th/Pb mz

Fig. 1 (A): Tectonic sketch of the South China Block. JLS, Neoproterozoic Jiulingshan belt; XFS, Triassic Xuefengshan Belt; JSF,Jiangshan–Shaoxing Fault. (B): Structural map of the Early Palaeozoic Orogen in the study area, south of Nanchang (JiangxiProvince). For clarity, sedimentary series and granitic plutons younger than Early Triassic have been omitted. In the SedimentaryUpper Unit, dotted lines represent bedding trajectories. In the Metamorphic Lower Unit, dotted lines outline dome structures.Boxes show the new radiometric ages given in Figs 6 and 7. U ⁄Th ⁄Pb mz: chemical age obtained on monazite (mz), Ar ⁄Ar bi ⁄ms:40Ar ⁄ 39Ar ages from biotite (bi) and muscovite (ms).

Early Palaeozoic intracontinental subduction in SE China • M. Faure et al. Terra Nova, Vol 00, No. 0, 1–9

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area, amphibolite yields SHRIMPU ⁄Pb zircon age of 1766 ± 19 Mainterpreted as the protolith age (Liet al., 2000). Detrital zircons fromparagneiss yield U ⁄Pb ages rangingfrom 3.6 to 2.5 Ga (Xu et al., 2005;Yu et al., 2006).Metatexites, diatexites and anatectic

granitoids developed at the expense ofthe Lower Unit and Proterozoic base-ment at the end of the Early Palaeo-zoic Orogeny (cf. below).Undeformed biotite granite, two

micas leucogranite and porphyriticgranodiorite, formed by crustal melting,intrude the Upper and Lower Units(Li et al., 1989; Zhou et al., 2006;Wang et al., 2007).The Upper Unit is well exposed in

the southwestern part of the studyarea, whereas the Lower Unit, mig-matites and basement crop out in thenorthwestern one (Fig. 1). The bulkarchitecture of the chain (Fig. 2)shows that the sedimentary UpperUnit overlies the metamorphic Low-er Unit that in turn overlies theProterozoic basement. The UpperUnit-Lower Unit boundary is awidespread flat-lying decollementzone (cf. below). The contact be-tween the Lower Unit and the Pro-terozoic basement is poorly exposed;however, near Jianning, the upperpart of the Proterozoic gneiss ismylonitized (cf. below). Migmatitesand anatectic granites form severaldomes deforming the Lower Unit.To the North, from Chongren toCizhou, the foliation dips at highangle to the North. The northernboundary of the belt is the Jiang-shan–Shaoxing fault, North of whichthe Palaeozoic series is continuousand not deformed before theDevonian.

Deformation and metamorphism inthe Early Palaeozoic Orogen

Thin skinned tectonics in thesedimentary Upper Unit

In Chongyi–Jian area (Fig. 1), the LateNeoproterozoic to Ordovician series isdeformed by E–W to NW–SE trendingfolds overturned to the S or SW(Fig. 3A). NW–SE trending fracturecleavage develops in Ordovicianrocks, and slaty cleavage is commonin Cambrian ones (Fig. 4). Bedding–cleavage relationships indicate a south-western vergence of the folds. Highangle thrusts are also observed. Openbuckle folds predominate in Ordovi-cian rocks and similar, even isoclinal,folds are found in Cambrian–Sinianrocks. A NE–SW trending stretchinglineation marked by reorientation ofdetrital particles sometimes appears inthe slates, but associated metamor-phism is absent. As a whole, the ductiledeformation increases downward.

The decollement zone

North of Xingguo (Fig. 1B), subhor-izontal slaty cleavage and N–S toN50E stretching lineation are conspic-uous (Figs 3B and 4). Asymmetricpressure shadows, sigmoidal quartzor feldspar clasts and sericite clotsindicate a top-to-the-SW shearing.These structures are coeval with seri-cite or chlorite metamorphism. Iso-clinal folds with axes parallel to thelineation formed during the ductileshearing are also found. This high-strain zone is interpreted here as aductile decollement along which thehorizontal shortening accommodatedby folding and thrusting in the sedi-mentary Upper Unit is transferred.

The synmetamorphic polyphasedeformation in the Lower Unit

Micaschist, quartzite and paragneissthat widely develop in the east of thestudy area exhibit two synkinematicmetamorphic assemblages (M1 andM2). The youngest and most con-spicuous M2 event is represented bybiotite–garnet–sillimanite–muscoviteoriented along the macroscopic foli-ation. Mica and sillimanite form aN–S to NW–SE trending minerallineation. According to geologicalmaps (JBGMR (Jiangxi Bureau ofGeology and Mineral Resources),1984), this foliation defines several20 to 30 km-scale dome structures. Inthe centre of the domes, anatecticgranite presents a foliation representedby schlierens, mafic enclaves(Fig. 3C) and preferred orientationof mica and feldspar. This foliation isparallel to the one observed in mig-matites and metamorphic rocks.Along the mineral lineation, biotiteor quartz pressure shadows and sig-moidal micas indicate contrastedkinematics depending on their loca-tion in the dome. The southern andnorthern flanks of the dome exhibittop-to-the S and top-to-the N sensesof shear respectively. In Cizhu area,the migmatitic gneiss is overlain byNeoproterozoic-Cambrian sandstoneor black shale of the Upper Unit.The boundary between these twounits is a south-dipping detachmentfault with a N–S stretching linea-tion and top-to-the-south kinematicindicators.In the Nancheng and Chongren

areas, evidence for migmatization isabsent, but the north-dipping folia-tion is deformed by north directedextensional shear bands and sigmoi-

SE SW

Paleozoic to Early Triassic cover

Neoproterozoic series

Neoproterozoic Jinning Belt

Proterozoic basement of Cathaysia

Unconformable Devonian to EarlyTriassic series

Sedimentary upper unit:Cambrian-Ordovician series

Ductile decollement

Proterozoic basement

Metamorphic lower unit

Pre-Devonian granites

Anatectic granitoids

HT metamorphic rocks

10 km

Jiangshan- Shaoxing F

Migmatite and anatectic granite

Sedimentary upper unit Metamorphic lower unit Ductile decollement layer

10 0 40 km

NE NW

Fig. 2 Synthetic cross section (located in Fig. 1) showing the bulk geometry of the Early Palaeozoic Orogen.

Terra Nova, Vol 00, No. 0, 1–9 M. Faure et al. • Early Palaeozoic intracontinental subduction in SE China

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dal biotite porphyroblasts indicate atop-to-the-North shearing (Fig. 3E).This post-folial deformation devel-oped at the end of M2 event,as indicated by Ar ⁄Ar dating (cf.below).Furthermore, relict porphyroblasts

of staurolite, kyanite, garnet, andbiotite inclusions in garnet (JBGMR(Jiangxi Bureau of Geology and Min-eral Resources), 1984) argue for anearly M1 metamorphism. Locally, sil-limanite develops at the expense ofkyanite. The high temperature M2event overprints the medium pressureM1one. The pressure–temperature

evolution from medium to high-pressure conditions (11–12 kb, 550–600 �C) to high temperature ones(4–4.4 kb, 570–650 �C) has alreadybeen determined in the Wuyi area(Zhao and Cawood, 1999). As the M1assemblages are mostly erased by theM2 event, the M1-related microstruc-tures are rare. Kyanite defines a N–Strending lineation, but the kinematicsis unclear. As a hypothesis, we suggestthat the syn-M1 lineation is coevalwith the lineation observed in thedecollement zones. The thermo-baro-metric estimates proposed by Zhaoand Cawood (1999) and our own

dating (cf. below) are used to con-struct a P-T-t path for the Lower Unit(Fig. 5).

Deformation of the Proterozoicbasement

A few hundred of metres below thecontact with the Metamorphic LowerUnit, the basement gneiss is penetra-tively foliated and contains a N–S

0.5 mm

N S

xenolith

X

L

S N

L

(A)

(D)

(B)

(C) (E)

Fig. 3 Some tectonics features of the pre-Devonian orogen of SE China in Jiangxi.(A) nearly isopach fold in Cambrian sandstone S. of Jinggangshan. (B) Stretchinglineation in the decollement layer N. of Xingguo. (C) anatectic granitoids nearLichuan, the magmatic foliation is defined by biotite schlierens and preferredorientation of biotitic boudins; note also a gneiss xenolith the foliation of which isoblique to the magmatic one. (D) Proterozoic gneiss with a conspicuous planar andlinear fabric related to the Early Paleozoic tectonics, N. of Jianning. (E) Sigmoidalbiotite showing a top-to-the-N sense of shear attributed to the late orogenicexhumation, S. of Chongren. This sample is the one dated by 40Ar ⁄ 39Ar (cf. Fig. 7).

S0 pole (n = 24)S1 pole (n = 12)

Sedimentary upper unit near Chongyi

S1 pole (n = 20)L1 strike (n = 50)

Ductile decollement N. of Xinguo

S1-2 pole (n = 37)L2 strike (n = 15)

Cizhou Dome(C)

(B)

(A)

Fig. 4 Stereographic plots (Schmidtlower hemisphere projection) of thestructural elements related to the EarlyPalaeozoic orogeny of SE China. (A)poles of bedding (S0) and cleavage (S1)planes in the sedimentary Upper Unitnear Chongyi. (B) cleavage poles (S1)and strike of stretching lineation in theductile decollement layer North of Xing-guo. C: pole of composite foliation(S1-2) and M2 mineral lineation (L2)mainly represented by sillimanite andbiotite around the Cizhou dome.

Early Palaeozoic intracontinental subduction in SE China • M. Faure et al. Terra Nova, Vol 00, No. 0, 1–9

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4 � 2009 Blackwell Publishing Ltd

trending mineral lineation marked bybiotite clots and amphibole needles(Fig. 3D). Quartz and feldspar formelongated ribbons alternating withmafic minerals. Asymmetric K-feld-spar porphyroclasts indicate a top-to-the-S sense of shear.

Geochronological constraints

Available radiometric dating and ournew data allow us to settle the timingof the main events of the Early Palae-ozoic Orogeny. The post-tectonicgranitoids between Jingganshan andChongyi yield U ⁄Pb zircon ages of387 ± 4 Ma, 397 ± 4 Ma and 434± 5Ma (JBGMR (Jiangxi Bureau ofGeology and Mineral Resources),1984; Li et al., 1989; Li, 1994; Shenet al., 2008). The M2 metamorphismand coeval anatexis is estimatedaround 450–420 Ma from U ⁄Pbzircon ages of Cizhu (432 ± 4 Ma),Lichuan (444 ± 4 Ma), and graniteseast of the study area (447 ± 2 Ma,Wan et al., 2007; Chen et al., 2008;Shu, unpublished). The Cizhu anatec-tic granite, coeval with the M2 event,yields a muscovite 40Ar ⁄ 39Ar age at421 ± 8 Ma (Shu et al., 1999).Presently, the M1 metamorphism is

undated therefore to place time con-straints on the early metamorphicevent, monazite was dated by thechemical U-Th ⁄Pb method (Cocherieand Albarede, 2001). The analysedquartz–biotite–garnet micaschist,located near Chongren (Fig. 1),formed during the M1 event. M2sillimanite is absent. U, Th and Pb

contents measured by EPMA in tenmonazite grains are plotted in theTh ⁄Pb vs. U ⁄P diagram (Fig. 6). Theanalytical points define a regressionline close to the 453 ± 7 Ma isochroninterpreted as the age of the M1 event.

40Ar ⁄ 39Ar laser probe, single grainstep-heating dating was carried outon a muscovite–biotite pair fromthe same sample where those micasare deformed by top-to-the-N shear

bands. The analytical procedure isdescribed in Brichau et al. (2008).Data are reported in Table 1. Bothmicas provide relatively concordantage spectra for a large percentage ofthe argon released with very fewevidence of argon loss or excess argon.Plateau ages of 397 ± 4 Ma and405± 4 Ma have been calculated forbiotite and muscovite respectively(Fig. 7). These ages are interpreted todate final cooling of this micaschistthrough a temperature window ofabout 400–300 �C as the unit wasprogressively exhumed at the end ofthe M2 event.

Tectonic interpretation and apossible geodynamic evolutionmodel

The above described tectono-meta-morphic events are restricted to theSouth of the Jiangshan–Shaoxing fault(JSF, Fig. 1), North of which, thesedimentary series is continuous fromLate Neoproterozoic to Early Trias-sic, deformed by NE–SW strikingupright folds before the Late Triassicunconformity (JBGMR, 1984). Silu-rian rocks, concordant with Ordovi-cian ones, are covered by Middle

Fig. 5 P-T-t path of the Metamorphic Lower Unit. Time constrains are thoseprovided in this paper (cf. Figs 6 and 7), P and T conditions are adapted from Zhaoand Cawood (1999).

50

0

10

20

30

40

60

0 4 8 12 16

SC 831 10 grains

MSWD = 1.7, n = 74

453 Ma

Age U-Pb = 455 + 49-52 Ma

Age Th-Pb = 452 + 14-14 Ma

Age U-Th-Pb = 453 ± 7 Ma

U/Pb

Th

/Pb 10 m

Fig. 6 U-Th ⁄Pb isochron diagram of monazite from a biotite–garnet micaschistbelonging to the metamorphic Lower Unit (location 27�42¢20, 115�59¢26). Analyticalprocedure and data reduction are provided in Cocherie and Albarede (2001). Thecalculated regression line (thick line) is similar to the theoretical isochronon (dashedline). The thick lines represent the error envelopes. The calculated age at the centroidis 453 ± 7 Ma, which is interpreted as the age of the M1 metamorphism.

Terra Nova, Vol 00, No. 0, 1–9 M. Faure et al. • Early Palaeozoic intracontinental subduction in SE China

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Devonian deposits with a disconfor-mity lower than 20�. Therefore, theJSF appears as a major tectonicboundary for the Early Palaeozoic

Orogen. Furthermore, Palaeozoic arc-type plutons or volcano-sedimentaryrocks are missing.The metamorphic Lower Unit and

the basement are reworked by a high-temperature and melting event. Thetop-to-the-north kinematics observedin the northern part of the belt coevalwith the M2 event develops during theexhumation of the M1 metamorphicrocks. The Upper and Lower Unitscannot be considered as nappes, as

younger and less metamorphosedrocks always superimpose upon olderones. Thus, the boundaries betweenthe three elements are decollementzones rather than thrust contacts.The Jiangshan–Shaoxing Fault does

not represent an ophiolitic suture zone,but a tectonic �scar� corresponding tothe place where a piece of continentallithosphere of Cathaysia has been sub-ducted to thenorthbelowanother pieceof the same continent (Fig. 8). A tec-tonic scenario is proposedhere (Fig. 8).From the LateNeoproterozoic toMid-dle Ordovician, the site of the EarlyPalaeozoic Orogen was occupied by asubsiding trough corresponding to theNanhua Rift and to the Early Palaeo-zoic facies transition.In the Late Ordovician, c. 460–

450 Ma, two main, south-directed,ductile decollement zones accommo-date North–South shortening. Theupper decollement, devoid of meta-morphism, separates the Upper andLower Units. The lower one separates

Table 1 Analytical data of laser probe, single grain step-heating dating of muscovite and biotite from the biotite garnet micaschist

sample.

Step 40 ⁄ 39 38 ⁄ 39 37 ⁄ 39 36 ⁄ 39 (E-3) F39Ar released %40* 40* ⁄ 39K Age Ma ±1 SD Ma

Muscovite (J = 0.009063)

1 22.601 0.023 0.00952 3.631 4.71 95.19 21.51 321.36 1.74

2 21.863 0.012 0.85786 12.203 5.17 83.73 18.32 277.06 9.43

3 27.263 0.012 0.03142 0.000 12.81 99.94 27.25 398.19 1.05

4 27.434 0.013 0.01446 0.403 24.70 99.51 27.30 398.85 1.25

5 27.797 0.013 0.01136 0.020 40.16 99.93 27.78 405.08 1.12

6 27.786 0.012 0.00000 0.541 55.38 99.37 27.61 402.90 1.12

7 27.140 0.014 0.00000 0.000 57.29 99.94 27.12 396.54 1.96

8 27.896 0.012 0.00548 0.405 75.79 99.52 27.76 404.88 0.83

9 28.752 0.011 0.03005 1.097 82.22 98.83 28.41 413.40 1.93

10 28.100 0.012 0.01981 0.528 89.95 99.40 27.93 407.08 1.57

11 28.396 0.014 0.02165 0.361 94.32 99.57 28.28 411.58 1.74

12 36.082 0.013 0.04335 1.276 100.00 98.92 35.69 505.64 2.36

Total age: 405.9 ± 3.7

Biotite (J = 0.009063)

1 39.752 0.040 0.27762 129.511 0.45 3.74 1.49 24.14 29.28

2 27.571 0.064 0.13137 45.445 0.60 51.27 14.14 217.50 22.03

3 26.697 0.012 0.00000 1.795 3.16 97.95 26.15 383.71 2.63

4 26.871 0.013 0.00000 0.167 6.31 99.76 26.81 392.36 2.15

5 26.988 0.014 0.00290 0.818 17.50 99.05 26.73 391.36 1.18

6 27.270 0.014 0.00967 0.000 21.82 99.94 27.26 398.26 1.37

7 27.010 0.014 0.00968 1.685 26.85 98.10 26.50 388.29 1.88

8 27.209 0.015 0.00000 0.000 29.46 99.94 27.19 397.44 2.15

9 27.476 0.015 0.00515 0.662 37.26 99.23 27.27 398.39 1.29

10 26.842 0.016 0.01016 0.000 40.26 99.94 26.83 392.64 2.60

11 27.493 0.014 0.00892 1.938 47.80 97.86 26.91 393.67 1.30

12 27.626 0.016 0.00993 0.034 52.62 99.91 27.60 402.79 1.52

13 27.216 0.015 0.00070 0.251 88.35 99.67 27.13 396.56 0.73

14 27.377 0.016 0.00472 0.027 93.87 99.92 27.35 399.55 1.72

15 27.078 0.014 0.01674 0.538 100.00 99.36 26.90 393.65 1.48

Total age: 393.7 ± 3.5

200

300

400

500

0 20 40 60 80 100

Biotite

AG

E (

Ma)

397 +/– 4

0 20 40 60 80 100

39Ar cumulative (%)

39Ar cumulative (%)

405 +/– 4

200

300

400

500

Muscovite

AG

E (

Ma)

Fig. 7 Laser probe, single grain step-heating dating of muscovite and biotitefrom the same biotite garnet micaschistsample belonging to the metamorphicLower Unit (location 27�42¢20,115�59¢26). Plateau ages of 397 ± 4Ma and 405 ± 4 Ma have been cal-culated for biotite and muscovite respec-tively. These ages are interpreted to datefinal cooling of this micaschist througha temperature window of about 400–300 �C.

Early Palaeozoic intracontinental subduction in SE China • M. Faure et al. Terra Nova, Vol 00, No. 0, 1–9

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the metamorphic Lower Unit from thebasement. These high strain zones cor-respond to rheologically weak inter-faces such as Neoproterozoic pelites orthe contact between the crystallinebasement and the overlying sedimen-tary rocks. The northern boundary ofthe deformed block is the JSF scar. Thesynsedimentary structures in the Ordo-vician turbidite argue for a tectonicinstability that might reflect the onsetof the deformation.In Early Silurian, (c. 445–430 Ma),

the deep part of the orogen is ex-humed coevally with the developmentof the high temperature M2 metamor-phism and anatexis. The detail oftemperature rise is not settled yet.The erosion of the belt provided thematerial for the Silurian sandstone inthe northern domain as indicated byNW-directed scour marks (Ronget al., 2003). However, the 40Ar ⁄ 39Arages suggest that exhumation contin-ued up to Early Devonian.

Conclusion

The Early Palaeozoic Orogen of SEChina lacks ophiolites, magmatic arc,subduction complexes, high-pressuremetamorphism, thus it cannot be acollision belt. Conversely, ductiledecollements, medium pressure ⁄mediumtemperature metamorphism andcrustal melting are conspicuous. Thestructural, metamorphic and sedimen-tary elements presented here characterizean intracontinental orogen controlledby the northward subduction of Cat-haysia. Pre-orogenic sedimentary andstructural inheritances play certainlyan important role in the localizationand initiation of continental subduc-tion (e.g. Cloetingh et al., 2008).

Acknowledgements

This research has been supported byNational Natural Science Foundation ofChina (No. 40634022). M. Faure acknowl-edges also the Chinese Academy ofSciences funding n�KZCX1-YW-15-1 andthe State Key Laboratory of LithosphereTectonic Evolution, Institute of Geologyand Geophysics, Academy of Sciences,Beijing for the material support andenlightening scientific discussions.

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Received 19 February 2009; revised versionaccepted 18 May 2009

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