Deformation of a pervasively molten middle crust: insights from … · 2007. 12. 6. · Deformation of a pervasively molten middle crust: insights from the neoproterozoic Ribeira-Arac¸uaı´

Deformation of a pervasively molten middle crust: insights fromthe neoproterozoic Ribeira-Aracuaı orogen (SE Brazil)

Alain Vauchez,1 Marcos Egydio-Silva,2 Marly Babinski,2 Andrea Tommasi,1 Alexandre Uhlein3 andDunyi Liu4

1Geosciences Montpellier, Universite de Montpellier 2 & CNRS Place E. Bataillon, 34095 Montpellier Cedex 05, France; 2Instituto de

Geociencias, Universidade de Sao Paulo, Rua do Lago, 562 Cidade Universitaria, 05508-900 Sao Paulo, SP, Brazil; 3Instituto de Geociencias

Universidade Federal de Minas Gerais, Av. Antonio Carlos, 6627 Pampulha, 31270-901 Belo Horizonte, MG, Brazil; 4Shrimp Laboratory,

Institute of Geology, CAGS, Beijing 100037, China

Introduction

Geophysical surveys suggest that themiddle crust beneath the Himalayanplateau and the Altiplano in theAndes undergoes pervasive partialmelting. The middle crust beneathTibet exhibits low seismic velocities,high attenuation and high electricalconductivity (Chen et al., 1996;McNamara et al., 1996; Nelson et al.,1996; Alsdorf et al., 1998; Unsworthet al., 2005). A pronounced satellitemagnetic low supports a Curie iso-therm (�550 �C) reached at �15 kmdeep, i.e. a mean geotherm of37 �C ⁄km for which the granite sol-idus (600–650 �C) is achieved at16–18 km (Alsdorf and Nelson,1999). Through seismic detection ofthe quartz a–b transition, Mechieet al. (2004) suggested a temperatureof �700 �C at �18 km depth. Alto-gether, these data support a>200 km wide, up to 1000 km longand >10 km thick partially molten

layer in the Tibetan middle to lowercrust.A similar picture holds for the

Altiplano and western Cordillera inthe Central Andes. Seismic and gra-vimetric data suggest a 60–70 kmthick crust (Wigger et al., 1994) char-acterized by low seismic velocitiesand high attenuation in the middlecrust (Schmitz et al., 1997; Massonet al., 1998; Dorbath and Masson,2000; Schurr et al., 2003). Fifteen totwenty per cent basaltic or andesiticmelt are required to explain both thenegative gravity anomaly and the lowseismic velocities (Schmitz et al.,1997). Geoelectromagnetic measure-ments depict an extensive zone ofhigh electric conductivity ‡40 kmthick starting at 20 km depth (Schil-ling et al., 1997; Schmitz et al., 1997).Minimum temperature at the top ofthis zone would be �650 �C (meangeotherm of 32 �C km)1; Schillinget al., 1997). Similar high conductiv-ity zones further north were alsoattributed to partial melting in thecrust (Tarits and Menvielle, 1986).These data are consistent with thehigh heat flow (100 mW m)2) in thisarea (Giese, 1994; Hamza andMunoz, 1996). They altogether sug-gest that the middle to lower Andeancrust contain large amounts of melt

as anatexites or incompletely solid-ified plutons.How is strain distributed in such a

hot lithosphere? Experiments supportthat partial melting severely weakensthe rheology of crustal rocks even for<10% melt (Rosenberg and Handy,2005). Recent models suggest that apervasively molten middle crust is soweak that it may undergo gravity-driven channel flow and lateral ext-rusion (Clark and Royden, 2000;Beaumont et al., 2001). How does apartially molten middle crust affectthe stress ⁄ strain transmission acrossthe lithosphere (Royden, 1996) andthus the mechanical coupling betweenthe seismogenic crust and the under-lying ductile lithosphere?An associated issue is the strain

distribution within the hot, partiallymolten middle crust. At low to inter-mediate temperatures, strain localiza-tion is the rule; large proportions ofstrain are accommodated in ductileshear zones. Under high tempera-ture conditions, strain localizationbecomes less efficient, large volumeof rocks are deformed homogeneouslyand faults appear as tens of kilometreswide shear zones (Vauchez andTommasi, 2003). The strength dropoccurring at the onset of melting maycause two divergent evolutions. If

ABSTRACT

Pervasive melting of the middle crust, as inferred in Tibet andthe Altiplano, probably influences the deformation of thelithosphere. To constrain strain distribution in a pervasivelymolten crust, we analysed the deformation in an erodedanalogue of these orogens. The Ribeira-Aracuaı orogen (SEBrazil) comprises a stack of allochthons containing largevolumes of anatectic and magmatic rocks. The upper allochton(�300 km long, 50–100 km wide and >10 km thick) involvesperaluminous diatexites and leucogranites resulting from par-tial melting of the middle crust. It overlies another allochthoncontaining huge early- to syn-collisional plutons intruding

metasediments. Both anatexites and magmatic intrusions dis-play a pervasive strain-induced magmatic fabric. Homogeneousstrain distribution suggests inefficient localization. U–Pb agesof �575 Ma imply that anatexite melting was synchronous tothe early- to syn-collisional magmatism. Similarity in agesmagmatic and solid-state fabrics indicates that intrusions andanatexites deformed coherently with solid-state rocks whilestill molten, in response to a combination of gravity-driven andcollision-driven deformation.

Terra Nova, 19, 278–286, 2007

Correspondence: Dr A. Vauchez, Geo-

sciences Montpellier, CNRS, Universite

de Montpellier II, UMR 5243, Place E.

Bataillon, cc 060, 34095 Montpellier Cedex

05, France. Tel.: +33 467 14 38 95; fax:

+33 467 14 36 03; e-mail: vauchez@dstu.

univ-montp2.fr

278 � 2007 Blackwell Publishing Ltd

doi: 10.1111/j.1365-3121.2007.00747.x

melting only affects limited volumewithin the crust, strain is preferentiallyaccommodated in the partially moltendomains, leading to localization(Rosenberg and Handy, 2000). Onthe other hand, if partial meltingpervasively affects the middle crust, ahomogeneous strain distribution maybe expected.Many ancient orogens display vol-

umes of synorogenic migmatites largeenough to represent analogues of thepartially molten crust in active areas.Deformation analysis in these largeanatectic domains and in contiguousdomains where pervasive melting didnot occur should provide clues on themechanical behaviour of a pervasivelymolten middle crust. We present pre-liminary results on the kinematics andstrain repartition in a segment of theneoproterozoic Ribeira-Aracuaı oro-gen (SE Brazil) that underwent HT-LPmetamorphism, pervasive partial melt-ing and widespread magmatism duringcollisional deformation. This resultedin development of a �300 km long,50–100 km wide and >10 km thickanatectic domain overlying allochtho-nous units containing abundant syn-collisional magmatism. We showthat: (1) the partially molten domainas well as the early- to syn-collisonalmagmatic intrusions deformed coher-ently with solid-state rocks beforecomplete solidification, and (2) strainis homogeneously distributed, suggest-ing inefficient strain localization.

Geological setting

The Ribeira, Aracuaı and WesternCongo belts form an orogen>1000 km long and �500 km wideresulting from the final amalgamationof the Gondwana super-continent(Fig. 1). The convergence betweenthe African and South American con-tinents during the Neoproterozoicpossibly involved the closure of anoceanic basin bounded eastward by anactive margin (Pedrosa-Soares et al.,1998, 2001). Dating using variousgeochronometers supports that colli-sion began after 600 Ma, lasted until�520 Ma (e.g. Silva et al., 2005), andwelded together continental litho-sphere of contrasted age and origin(Brueckner et al., 2000).From north to south, the Ribeira-

Aracuai orogen displays a changein dominant deformation regime

(Trompette, 1994; Vauchez et al.,1994). The southern domain is char-acterized by transpressional deforma-tion involving coeval or slightlydiachronous thrusting normal to thebelt and dextral, orogen-paralleltranscurrent movements (Trompette,1994; Egydio-Silva et al., 2002; Sch-mitt et al., 2004). Deformation in thenorthern domain is characterized byHT thrusting of allochthonous unitson the Sao Francisco craton margin(Cunningham et al., 1998; Oliveiraet al., 2000). The change in dominantdeformation regime (Fig. 1) is spa-tially associated with the bending of

the belt around the southern termin-ation of the Sao Francisco craton(Vauchez et al., 1994) where transcur-rent and thrust fabrics coexist (Egy-dio-Silva et al., 2005).

Strain distribution in the partiallymolten middle crust of thenorthern Ribeira-Aracuaı orogen

The northern Ribeira-Aracuaı orogenunderwent HT-LP synkinematic meta-morphism, westward thrusting, andwidespread magmatism (e.g. Oliveiraet al., 2000). It comprises a variety ofmetamorphic rocks and magmatic

Fig. 1 Schematic reconstitution of the position of Africa and South America beforeSouth Atlantic opening. Shaded areas represent domains where collision occurredbefore �600 Ma. They involve cratonic domains (white crosses) and neoproterozoicbelts (white lines). In between, the Ribeira-Aracuaı-West Congo orogen resulted froma collision younger than 600 Ma. The main structural directions in the Ribeira-Aracuaı orogen show the curvature of the belt associated with the termination of theSao Francisco craton (SFC). The area delimited in the northern part of the Ribeira-Aracuaı orogen (stippled box) represents the location of Fig. 2. Arrows show theconvergence between Africa and South America. Small black arrows show the mainkinematics. CC, Congo craton; BB, Brasilia belt; SP, Sao Paulo; RJ, Rio de Janeiro.

Terra Nova, Vol 19, No. 4, 278–286 A. Vauchez et al. • Deformation of a pervasively molten middle crust

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� 2007 Blackwell Publishing Ltd 279

intrusives, but its most striking featureis the presence of a large anatectic unitoverlying a stack of west-vergingnappes. Three main allochthonousdomains compose the belt (e.g. Oliveiraet al., 2000): the western, central andeastern domains, which are thrust ontothe HT para-autochthonous metasedi-mentary cover of the Sao Franciscocraton (Figs 2 and 3). These threedomains display consistent kinematics,but varyingmelt volumes during defor-mation.

Western domain: distributed high-temperature solid-state thrusting

The western domain of the Aracuaıbelt is composed by HT mylonites(Cunningham et al., 1998), whichform a >5 km thick subhorizontalshear zone (Fig. 3). These mylonitesmainly derive from sedimentary prot-oliths. They are injected by abundantsynkinematic leucocratic melts thatfrequently contain garnet and cordi-erite. Foliations dip gently eastward;lineations trend consistently close toEW (Fig. 2). Numerous centimetre to10 m scale shear sense criteria indi-cate reliable top-to-West shearing(Fig. 4).HT-LP conditions during the

mylonitic deformation are attested bybiotite, garnet, prismatic sillimanite,(cordierite) mineral assemblages andby the systematic absence of musco-vite. PT conditions of �750 �C and�600 MPa were estimated from therim compositions of biotite–garnet,garnet–plagioclase and garnet–cordi-erite mineral pairs (Petitgirard, 2005).These values probably represent post-peak metamorphism conditions assuggested by core-rim Fe–Mg gradi-ents in garnet crystals and by tem-perature estimates (�800 �C) obtainedusing biotite inclusions and the hostgarnet core.These mylonites overlie para-auto-

chthonous metasediments (mainlygneiss and Al-rich quartzite) thatbelong to the Sao Francisco cratonand were also mylonitized underHT-LP conditions. The transitionfrom the para-autochthonous metase-diments to the mylonitic unit is pro-gressive and the boundary between thetwo domains is not sharply defined.Together the western unit and theuppermost metasediments of the cra-ton form a HT-LP shear zone >5 km

Fig. 2 Simplified geological map modified from �Projeto Leste� (Oliveira et al., 2000)showing the four main domains in the northern Ribeira-Aracuaı orogen. The �easterndomain� (1) or uppermost allochton comprises a thick (‡10 km) layer of diatexites andanatectic granites (a) representing the partially molten middle crust, which is topped bymigmatitic kinzigites (b). The �central domain� comprises pre- to syn-collisionalmagmatic complexes (2 = Galileia batholith; 3 = Sao Vitor Tonalite) intruded in HTmetasediments (5). The �western domain� (6) involves metasedimentary and meta-igneous mylonites thrust upon the para-autochthonous metasedimentary cover of theSao Francisco craton (7). A late generation of porphyritic granitoids associated withcharnockites (4) intruded the stack of allochthonous units. 8 = foliation and lineationmeasured in the field; 9 = foliation and lineation deduced from anisotropy ofmagneticsusceptibility measurements (8 and 9 together summarize measurements from morethan 500 localities). GV, Governador Valadares; TO, Teofilo Otoni. A–B = locationof the cross section of Fig. 3. Top left insert: Location map showing the main cratonicdomains (SFC, Sao Francisco craton; CC, Congo craton; AC, Amazon craton; WAC,West African craton) and neoproterozoic mobile belts in eastern South America (SA)and western central Africa (AF). Square East of the SFC is the area studied.

Fig. 3 Schematic cross section from the parautochnous metasediments of the SaoFrancisco craton westward, to the anatectic allochthonous unit eastward. This sectionshows from W to E: the dominantly granulitic (1) and metasedimentary (2) crust ofthe Sao Francisco craton; the western, the lower allochthonous domain involvingdominantly metasedimetary (3) and tonalitic (4) mylonites; the central domain thatcomprises metasediments (5), and pre- to syn-collisional intrusives, especially the SaoVitor tonalite (6) and the Galileia batholith (7); the eastern, pervasively anatecticupper allochton, which comprises minor metasediments (8), dominant anatexites (9)intruded by syn- to late granitoids (10). The western domain is topped by kinzigiticmylonites (11). Surfaces above the topography schematically represent the dominantfoliation and lineation.

Deformation of a pervasively molten middle crust • A. Vauchez et al. Terra Nova, Vol 19, No. 4, 278–286

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thick that accommodated homogen-eously the westward translation of theallochthonous middle crust over thecraton, representing therefore its basalcontact.

Central domain: coherent deformationof synkinematic magmatic bodies andmetasedimentary host rocks

The central domain involves hugeamounts of igneous rocks emplacedwithin metasediments (Fig.2). Twomain magmatic bodies: the Sao VitorTonalite (576 ± 5 Ma; Noce et al.,2000) to the North and the Galileiabatholith to the South (594 ± 6 to576 ± 4 Ma; Nalini, 1997; Naliniet al., 2000), are in continuity, form-ing a domain >250 km long and�50 km wide in which magmaticrocks and metasediments are imbri-cated (Figs 2 and 3). The Galileiaand Sao Vitor plutons displaya pronounced magmatic fabric(Figs 5 and 6), marked by the orien-tation of biotite and feldspar crystalsthat parallels the solid-state fabric inthe metasediments. Despite a strongshape-preferred orientation at macro-to meso-scales, intracrystalline defor-mation features are not observed inthin sections. Solid-state deformationis limited to scarce metric-scale shearzones. These observations support

that the Sao Vitor and the Galileiaplutons deformed coherently withtheir country rock before completesolidification.Close to the contact with the west-

ern domain, the magmatic foliation inthe Sao Vitor tonalite dips gentlyeastward. Lineations, observed in thefield or deduced from anisotropy ofmagnetic susceptibility (AMS) mea-surements, trend close to EW. East-ward, the dip of the magmaticfoliation and the plunge of the linea-tion progressively increase from gentleto moderate then to steep. Furthereastward, entering the Galileia batho-lith, the magmatic foliation becomessubvertical (Fig. 3). Subvertical andsubhorizontal lineations coexist, sug-gesting transpression-induced strainpartitioning.

Eastern domain: thrusting in apartially molten middle crust

This domain consists of an anatecticunit �300 km long, 50–100 km wideand >10 km thick (Figs 2 and 3).Rather homogeneous lithology rangesfrom peraluminous garnet–biotitediatexites to leucogranites. Garnet isubiquitous, frequently in large pro-portion and locally forming elongatedcumulates. Prismatic sillimanite andcordierite in equilibrium with garnetare frequent and sometimes formclusters. Muscovite is absent, exceptas alteration product. These anatexitesdisplay clear magmatic textures,although some felsdpars and garnetsdisplay evidence of corrosion, beingprobably inherited from the sourcemetasediments.At the outcrop scale, a penetrative

fabric is marked by alignment of maficminerals (especially biotite) in theleucogranites or by alternating bio-tite-rich and leucogranitic layers inthe migmatites (Fig. 7). This well-developed fabric might be sometimes

(a)

(b)

W

W

Fig. 4 Top-to-the-west shear criteria inmylonitic paragneisses intercalated withleucocratic melt veins in the westernmylonitic allochthonous domain. (a)Ten-metre scale asymmetric lensesof amphibole-bearing gneiss developedaround mafic boudin (picture length ¼10 m). (b) Asymmetric lenses resultingfrom boudinage of leucocratic veinsinjected in biotite-rich mylonites. Coex-istence of boudinaged and non-boudin-aged veins suggests several generationsof melt intrusion. Hammer is �30 cmlong.

Fig. 5 Typical outcrop of the Sao Vitortonalite (central domain) displaying awell-developed magmatic foliationmarked by the preferred orientation ofplagioclase and biotite and by an elon-gated, more mafic enclave. Coin dia-meter is 2 cm.

Fig. 6 Granodiorite outcrop from theGalileia batholith (central domain)showing a sub-vertical magmatic foli-ation marked by the alignment of maficenclaves. The long axis of the enclaves issub-horizontal, suggesting a strike-slipcomponent of deformation. Hammer is�30 cm long.

Fig. 7 Two typical examples from theanatectic middle crust outcropping with-in the eastern domain. (a) Anatecticleucogranite displaying a magmatic foli-ation marked by preferential orientationof mafic minerals. (b) Diatexite display-ing intercalations of leucocratic melt andnarrow biotite-rich layers. In bothfacies, centimetre-scale garnets (g) arepresent in the leucosome, but the diatex-ite is richer in garnet. Scale on (a) is1 cm.


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mistaken for a solid-state, gneissicfoliation. However, quartz–feldsparaggregates have a typical graniticmicrostructure (Fig. 8). Quartz doesnot display significant intracrystallinedeformation, like undulose extinctionor subgrains, and systematically pre-serves interstitial shapes (Fig. 9). Thisindicates that this fabric developed ina magmatic mush before full crystal-lization. Evidence of HT solid-state

reworking is limited to rare metric-scale shear zones.Lineations in migmatites are usually

difficult to observe (Ferre et al., 2003;Egydio-Silva et al., 2005). Whereobserved in the field or deduced fromAMS measurements, lineations,marked by alignment of mafic miner-als, span from NW–SE to NE–SWwith a statistically dominant NE ori-entation. This dispersion is partiallydue to late kilometre-scale open folds,but it may also reflect a vertical pureshear (possibly gravity-driven) com-ponent of the deformation, leading todispersion of lineation within the folia-tion plane.At the regional scale, the western

boundary of the anatectic unit isslightly oblique to the structural grainof the belt. As a result, the anatexitesoverlie the central domain to the southand are in direct contact with thepara-autochthonous metasedimentarycover of the craton to the north.Eastward, the anatectic crust sheet

is topped by migmatitic kinzigites(Fig. 1) containing biotite, garnet,prismatic sillimanite and poikiloblas-tic cordierite suggesting metamorphicconditions of �820 ± 30 �C and650 ± 50 MPa (Munha et al., 2005).Numerous shear criteria in kinzigitessuggest a top to W or SW shearing.Well exposed in the southern andnorthern part of the studied area, theyare often buried beneath coastal sedi-ments in the central part. U–Pb datingof zircons from the kinzigites suggestsa maximum age of �630 Ma for

sediment deposition (Noce et al.,2004). Thermochronology estimates(Munha et al., 2005) suggest that tem-perature remained >700 �C until480 Ma.

Dating anatexis

An important issue is to determinewhether partial melting in the easterndomain was coeval with graniteemplacement and deformation in thecentral domain. The metasedimentaryorigin of the anatexites imposes to usein situ techniques and careful selectionof the crystals. Zircon crystals fromthe leucocratic anatexites were separ-ated for cathodoluminescence (CL)imaging and SHRIMP U–Pb dating.CL imaging and zircon U–Pb isotopedating were performed at the Instituteof Mineral Resources, Chinese Acad-emy of Geological Sciences, and in theSHRIMP II at the Beijing SHRIMPLaboratory, respectively. U–Pb iso-tope data were collected in sets of fivescans throughout the masses; a refer-ence zircon TEM (417 Ma) wasanalysed every fourth analysis. Meas-ured U, Th and Pb abundances andPb isotope ratios were normalizedusing the reference zircon SL13(572 Ma) values. Common Pb wascorrected using the measured 204Pb.Data were processed following Comp-ston et al. (1992) using the ISOPLOTprogram (Ludwig, 2001).The analysed zircon crystals range

from 220 to 530 mm in length, areacicular and euhedral. CL imagingshows well developed oscillatory zon-ing; rare inherited cores were observedbut not analysed (Fig. 10). Analysedzircons yielded concordant data

Fig. 8 Microstructure of the anatectic leucogranite from the western domain. Left:polarized light microphotograph showing the well-developed magmatic foliationmarked by alignment of biotite- and garnet-rich layers. Right: same thin sectionunder crossed-polarized light, showing a typical magmatic fabric with interstitialquartz crystals.

Fig. 9 Two examples of interstitialquartz crystals in anatexites from thewestern domain. Feldspars display evi-dence of dissolution and may be partlyinherited. Scale is the same for bothpictures.

Fig. 10 Cathodoluminescence imagesof a selection of zircon crystals fromthe anatectic granite (AR548; easterndomain). The numbers refer to analysesshown in Table 1, and the circles markthe spots analysed by SHRIMP.


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(Table 1 and Fig. 11). A weightedmean 206Pb ⁄ 238U age of574.9 ± 3.3 Ma (MSWD = 1.46;P = 0.15) was calculated from the11 analyses clustered on the concordiadiagram. It is interpreted as the age ofthe melt crystallization in the anatecticunit.This age fits the age range of the

batholiths of the central domain (SaoVitor and Galileia batholiths, Naliniet al., 2000; Noce et al., 2000), whichwere deformed before complete cry-stallization. This similarity in age hastwo important implications: (1) hugevolumes of middle crust either werepartially molten or contained incom-pletely solidified magmatic stocks atthe time of the deformation, and (2)deformation in the central and easterndomains was simultaneous andoccurred around 580–570 Ma.

Strain repartition

A recurrent problem faced in thetectonic analysis of rocks deformedunder HT conditions, especially in thepresence of melt, is the lack of reliablemethods to quantify strain. Based onfield and thin section observations, afew lines of evidence may however bedrawn:There are no significant strain chan-

ges between adjacent rocks deformedin solid or magmatic state as shown bythe striking continuity between themagmatic deformation in the SaoVitor tonalite and the mylonitic defor-mation in the western domain.No evidence of strain localization

along the contacts between the threedomains is observed. The units aredefined on their dominant lithologyrather than on the existence of straingradients across basal faults.The kinematics in the three units is

coherent. The main peculiarities are:(1) the transpressional deformationregime in the central domain, wheresteeply dipping foliations dominateand horizontal and vertical lineationscoexist, and (2) the more dispersedand poorly defined lineation in theanatectic unit, and its SW–NE dom-inant trend.Strain appears as homogeneously

distributed over the entire allochtho-nous pile. Temperature conditionsrecorded in themetasedimentarymylo-nites at the base of the pile (�750 �C)are close to those in the kinzigitic Tab

le1

SHRIM

PzirconU–Pbisotopedata

forCarlosChagasGranite(A

R548).

Spo

t

% 206P

bc

pp

m

U

pp

m

Th

232Th

⁄238U

pp

m206P

b*

206P

b⁄

238U

Ag

e

207P

b⁄

206P

bA

ge

208P

b⁄

232Th

Ag

e

238U

⁄206P

b*

(%)

207P

b*

⁄206P

b*

(%)

20

7P

b*

⁄2

35U

(%)

20

6P

b*

⁄2

38U

(%)

Erro

r

corr

ecti

on

AR

548-

1.1

0.08

315

450.

1524

.856

4.5

±5.

052

8±

3756

8±

2110

.93

±0.

930.

0579

5±

1.7

0.73

1±

1.9

0.09

152

±0.

930.

478

AR

548-

2.1

0.40

186

740.

4115

.157

9.9

±5.

961

6±

3854

1±

1310

.62

±1.

10.

0603

±1.

80.

783

±2.

10.

0941

2±

1.1

0.51

4

AR

548-

3.1

0.30

326

390.

1226

.658

3.9

±5.

448

8±

3656

3±

4010

.55

±0.

970.

0569

1±

1.6

0.74

4±

1.9

0.09

481

±0.

970.

511

AR

548-

4.1

0.20

526

570.

1142

.057

1.5

±5.

053

1±

2654

3±

2110

.787

±0.

910.

0580

3±

1.2

0.74

2±

1.5

0.09

270

±0.

910.

614

AR

548-

5.1

0.16

235

460.

2018

.656

7.8

±5.

562

4±

5459

9±

1810

.86

±1.

00.

0606

±2.

50.

769

±2.

70.

0920

8±

1.0

0.37

7

AR

548-

6.1

0.19

279

430.

1622

.557

9.0

±5.

563

9±

3161

1±

1910

.64

±0.

990.

0609

8±

1.4

0.79

0±

1.8

0.09

398

±0.

990.

565

AR

548-

7.1

0.23

339

490.

1527

.658

1.7

±5.

165

0±

4963

0±

2310

.590

±0.

920.

0613

±2.

30.

798

±2.

50.

0944

3±

0.92

0.37

5

AR

548-

8.1

0.22

287

400.

1422

.756

5.8

±6.

856

5±

3558

5±

2410

.90

±1.

30.

0589

5±

1.6

0.74

6±

2.0

0.09

17±

1.3

0.61

7

AR

548-

9.1

0.30

251

320.

1320

.157

3.7

±5.

556

3±

4757

6±

3510

.74

±1.

00.

0589

±2.

20.

756

±2.

40.

0930

7±

1.0

0.42

0

AR

548-

10.1

0.31

272

860.

3321

.957

5.9

±5.

552

6±

6456

0±

2710

.70

±0.

990.

0579

±2.

90.

746

±3.

10.

0934

6±

0.99

0.32

3

AR

548-

11.1

0.39

228

480.

2218

.557

8.7

±5.

455

9±

4655

3±

3010

.65

±0.

980.

0588

±2.

10.

761

±2.

30.

0939

2±

0.98

0.42

4

Erro

rsar

e1)

sigm

a.Pb

can

dPb

*in

dica

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mm

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port

ions

resp

ectiv

ely.

Usi

ngm

easu

red

20

4Pb

for

com

mon

Pbco

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Unc

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the

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are

1sig

ma.


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material that tops the anatectic unit(Uhlein et al., 1998, Munha et al.,2005), suggesting that a large volumeof crust was submitted to similarsynkinematic HT conditions. Thenorthern Ribeira-Aracuaı orogenrepresents therefore a case of �weaklithosphere� similar to those documen-ted in SW Finland (Cagnard et al.,2006b), although the large-scalekinematics (thrusting over the edgeof a craton) is different.

Conclusions

The northern Ribeira-Aracuaı orogenis characterized by aHT-LP (>700 �C,600 MPa) metamorphism suggesting a�35 �C km)1 geotherm, pervasive par-tial melting of the middle crust, and�Himalayan-type� thrusting of the allo-chthonous pile onto the Sao Franciscocraton. It may therefore represent aneroded analogue of the crust beneaththe Tibet and Altiplano plateaus.The three allochthonous domains of

this collisional orogen display varyingmelt volumes during deformation.The western domain deformed underHT solid-state conditions, the centraldomain comprises a huge volume ofmagmatic rocks (tonalite, granodior-ite, granite…) emplaced in, anddeformed coherently with, metasedi-ments before full crystallization, and

the eastern domain is composed ofanatectic rocks, the mineral assem-blages of which suggest formationthrough HT melting of metasedi-ments. SHRIMP U–Pb dating sup-ports that partial melting in theeastern domain and intrusion of mag-mas in the central domain were coevalat �580–570 Ma. These observationssuggest that, during the collision, alarge volume of crust was molten(>10% melt) and hence had a lowstrength. This low strength is probablythe reason for the observed homogen-eous strain distribution. There is noobvious strain localization, even at thecontacts between the various units andsub-units that are defined on litholog-ical rather than tectonic criteria.The eastern, anatectic domain dis-

plays: (1) a basal contact oblique tothe gross structure of the belt, (2) asharp transition from steep foliationsin the central domain to flat foliationswithin the anatexites, and (3) a dis-persed lineation, with a statisticallydominant NE orientation rather thanEW as in the western domain. Thesedifferences might indicate partialdecoupling at the base of the anatecticdomain, perhaps due to a gravity-driven pure-shear component. Partialdecoupling might have been favouredby coeval escape tectonics in thetranspressive central and southern

Ribeira belt, where dextral orogen-parallel wrench faulting dominated atca. 580 Ma (e.g. Silva et al., 2005).This escape tectonics might have actedas a �mobile boundary� substitutingthe �free� boundary necessary in ana-logue models to generate gravity-driven lateral flow (Cagnard et al.,2006a). This suggests a model combi-ning collision-driven and gravity-driven deformation controlled by thefar-field strain regime rather thanlocal gravity-driven lateral flow ofthe middle crust as invoked for Tibet(e.g. Beaumont et al., 2001).

Acknowledgements

Funding by FAPESP, CAPES, COFECUBand INSU-DyETI National Programis acknowledged. We are indebted toS. Pacheco Neves, R. Schmitt, S. Sie-gesmund and an anonymous reviewerfor their suggestions and constructivecriticism.

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Received 29 November 2006; revised versionaccepted 4 June 2007


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