Orogenia Rio Doce

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Jamul ofSouth Anwicm Ewth Seicncrr.Vol. 8. No. 2. pp. 143-162, 1995Copyright 1595 Ekvier Science L&d EatthSciewu & R.sounm lnaitute

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The Rio Dote Orogeny, Southeastern BrazilM.C. CAMPOSNET0 andM.C.H. ~X+EIREDO

Instituto de Geocitncias, Universidadede Sb Paulo, Caixa Postal 11348,05422-970SzioPaulo, Brazil(Received December 1993; Rev ision s Accepted N ov ember 1994)

Abst ract - The Neoproterozoic-Eopaleozoicuperposedrogenic ystemof Southeasternrazil, which was active during theBrasiliano-Pan-African Cycle during the assembly of this sector of the Gondwana Supercontinent, includes distinct terranes suchas the GuanhBes, Cmitiba, Apia&Guaxup6 and Serra do Mar microplates and the Juiz de Fora Thrust Belt. These erogenic systemsalso affected the Reworkedborder of the Sfio Francisco Craton.The collisional or ocean plate subduction-controlled Brasiliano I Orogeny was responsible for the generation of fold belts alongthe southeasternorder f the S8o Francisco Craton, the accretion of different microplates and the formation of a magmatic arcassociated with the roots of a northwestward trending thrust belt. The Brasiliano I evolution occurred during the Neoproterozoicand by 600 Ma was alnsady in a post-erogenic stage in the Apiaf-Guaxup6 Microplate, with the intrusion of rapakivi-like grani-toids.The Rio Dote Orogeny is best characterized in the Serra do Mar Microplate by a magmatic arc, active between 590 and 570 Ma,with batholithic talc-alkaline plutonism exhibiting subduction zone components and a chemical zonation indicative of northwest-ward subduction. The collisional stage (560-530 Ma) accounted for the accretion of the Serra do Mar Microplate to the formererogenic domains. Anatexis of mostly metasediments producing peraluminous migmatites and granites, began at the talc-alkalinemagmatic arc stage and culminated during crustal thickening associated with the northwesterly piling-up of large thrust slices.The post-collisional plutonism (520-480 Ma) is characterized by plutons and dikes of mainly alkali-calcic granitoids enriched inincompatible elements.Resume - OS sistemas orog&icos superpostos Neoproteroz6icoXopaleoz6icos do sudeste brasileiro, ativos durante o CicloBrasiliano-Pan-African0 corn a amalgama@ desse setor do Supercontinente Gondwana, incluem terrenos diferentes, coma asmicroplacas GuanhHes, Curitiba, Apiai-Guaxup6 e Serra do Mar e o Cinturgo de Cavalgamento Juiz de Fora. Estes sistemasorogEnicos tamb6m afetaram a borda retrabalhada do C&on do Sb Francisco.A Orog&nese Brasiliana I, do tipo colisional ou relacionada corn subduceb de crosta oce8nica. gerou cintur&s de dobramentosna borda sudeste do C&on do Sso Francisco e a acres@o de distintas microplacas, aldm de urn arco magmltico associado &srafzes de urn cintutio de cavalgamento. A evolu#io do Brasiliano I ocorreu durante o Neoproteroz6ico e atingiu, ha cerca de 600Ma, urn est&io p6s-orog&nico corn a intrusHo de granit6ides do tipo rapakivi, na Microplaca Apiaf-Guaxup&A Orog&nese Rio Dote C melhor caracterizada na Microplaca Serra do Mar atraw% de urn arco magtitico, de 590 a 570 Ma, cornbat6litos cticio-alcalinos, apresentando componentes geoqufmicos de zona de subduc@o, cujo zoneamento quimico t sugestivode subduc@o para NW. 0 estagio colisional (560-530 Ma) foi responsavel pela acres@0 da Microplaca Serra do Mar aosdominios orog&icos precedentes. A fusb par&l de crosta continental, principalmente de metassedimentos, produziu migmatitose gmnitos peraluminosos e iniciou-se no estagio de arco magmLico cficio-alcalino, culminando durante espessamento crustalassociado ao empilhamento, para NW, de grandes fatias de cavalgamento. 0 plutonismo p6s-colisional(520-480 Ma) caracteriza-se par pldtons e diques de granit6ides, geralmente ticah-cticicos, enriquecidos em elementos incompativeis.

INTRODUCTIONRECENTGEOCHRONOLOGICALANDGEOCHEMICALDATA, ntegrated with the structural and plutonic evolu-tion, suggests the superposition, in Southeastern Brazil, oferogenic systems, in the Neoproterozoic III-Cambrian,during the assembly of the Gondwana supercontinent. Theage nomenclature used here follows Cowie et al. (1989)and our concept of orogeny comprises all subduction orcollision-related plate convergence processes (e.g., Sen-g6r, 1990), while different erogenic periods are separatedby main extensional events. These extensional regimes donot necessarily lead to the opening of new ocean basinsand the distinct erogenic periods may represent the closingstages of different arms of an ocean.

The supercontinent cycle includes rifting and break-upof an older supercontinent, followed by the assembly ofindividual cratons and magmatic arcs, ending with colli-sion and suturing events (e.g., Hartnady, 1991).

This work discusses the amalgamation stages of asupercontinent cycle related to the closing of the largeBrazilide ocean (Dalziel, 1992) which was establishedduring a major late Mesoproterozoic rifting event (e.g.,Unrug, 1992). This amalgamation involved the Congo-S%oFrancisco Craton on one side, and the Rio de la Plata,Amazonian and West Africa cratons on the other (Hoff-man, 1991; Dalziel, 1992). Particular focus will be givento the erogenic systems developed in Southeastern Brazilduring the last stages of closure of the Brazilide ocean.

Address all correspondence and reprint requests to M.C.H. Figueiredo, Institute de Geociencias,Universidade de Sgo Paulo, Caixa Postal 11348,05422-970 Sao Paula, Brazil

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1 4 4 M. CAMPOS NETO n d M.C.H. FIGUEIREDO

Belo gor iton k

440 42

22

0 100 300i0 48Fig. 1. Neoproterozoic-Cambrian erogenic systems map of Southeastern Brazil.

The Brasiliano-Pan-African Cycle appears to corre-spond to a collage (Sengar, 1990), i.e., an erogenic assem-blage of microcontinents, island arcs, accretionarycomplexes, etc. In Africa there is evidence of the superpo-sition of several erogenic systems collectively referred toas the Pan-African (e.g., Porada, 1989; Stanistreet et al.,1991; Wilson et al., 1993). We suggest that this is also thecase for Southeastern Brazil, where the Rio Dote Orogenyof the Serra do Mar Microplate is the youngest of theseerogenic systems. Another slightly older orogeny, as yetunnamed, but henceforth provisionally called BrasilianoI, will not be dealt with here in great detail because it isnot the main objective of the present work.

In Southeastern Brazil, two major Neoproterozoic-Eopaleozoic erogenic events seem to have occurred,resulting in microplates and collisional stages separated by

a large volume of plutonic rocks developed during exten-sional periods, at the end of the Neoproterozoic III (the ItuBelt of Vlach et aZ., 1990). We use the term microplates, asused by Condie (1989), for crustal fragments smaller than1O6 km* and with at least one active margin.

Several Neoproterozoic-Cambrian erogenic systems(Fig. 1) may appear in the south and at the southeasternborder of the S5o Francisco Craton (Almeida, 1977) andits passive margin-type fold belts (Araquai and Alto RioGrande belts). According to our interpretation, they aremade up of the Guanhses, Apiai-GuaxupC and Curitibamicroplates, Juiz de Fora Thrust System, and EmbuAccreted Terrane, whose main tectonic evolution is relatedto the Brasiliano I Orogeny ( Z=600 Ma), as well as theyounger Serra do Mar Microplate of the Rio Dote Orog-eny.


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The Rio Dote Grogeny, Southeastern Brazil 145

r-l Phancmzoic scdirncntary cows and alltahne plutonaLATE TO POST-OROGENIC HINTERLAND VOLCANO-SEDIMENTARY SEQUENCES

RIO DOCE OROGENY (600435 Ma)MAGMATIC ARCScrra do Mar Microplate

FORELAND EXOTIC FRAGMENTSPalcoprotcmzoic/ArchcanLuisAlvcs Block (LA) and Itauns Slice (I) Palcopmtemzoic Cabo Frio Window (CF)

BRASILIANO I OROGENY (700400 Ma)COLLISIONAL-TYPE SYSTEM, WITH EASTWARD SEBDUCTIONFORELAND DOMAINSPlatform cover of the NcopmtemzoicBambul-Macaubas Group

Fold and Thrust Belts

PalcopmterozoiclArchcan Sao FranciscoCraton basement tcrrancs

Ncopmterozoic Continental Margin-type Squenas: Aracuai-Rio Pardo Belt

Syn- to Post-ColIiiional Accrctcd Gtanitic Beltr-l : .. : Ncopmterozoic (7) Embu Supracrustal Tcrrane and S-type Granitic Belt

HINTERLANDDOMAINS

Curitiba MicroplateMAGIMATIC ARC WITH NORTHWESTWARDSUBDUCTION

HINTERLAND DOMAINSJuiz de Fora Thrust Belt in a talc-alkaline magmatic arc dotnatnr-l/ , _ < Ncopmtemzoic (?) high-grade supracmstal Paraiba do Sul (Ps) tenam, r/1El Palcoprotemzoic (?) infracrustal Juiz de Fora (JF) and Mantiqueira (M) terranesApiai-GuawpC Micmplate

Apial Folded Belt (A) and Socorm-GuaxupCThrust Nappc (SG)Fig. 1. (Continued) Legend of the Neoproterozoic-Cambrian erogenic systems map of Southeastern Brazil.

THE BRASILIANO I OROGENIC SYSTEMSThe Sgo Francisco Craton southern sector (Fig. l), with

its Neoproterozoic platform cover (Bambui Group), iscomposed of Archean-Paleoproterozoic granite-gneissicsequences with relicts of mafic-ultramafic rocks (Teixeiraand Figueiredo, 1991). In the beginning of the Mesoprot-erozoic, rifting occurred (Espinhaco Group) with a break-up phase and the development of a passive continentalmargin, to which was accreted a talc-alkaline volcano-sedimentary sequence forming the Mesoproterozoic AltoRio Grande Belt (Campos Neto et al., 1990; Ribeiro et al.,1990; Campos Neto, 1991; Trompette et al., 1992).

The Neoproterozoic passive continental margins corre-spond to the AraGuaf-Rio Pardo-West Congolian belts(Brito Neves and Cordani, 1991). The eastern limit of thepassive margin sequence in the Aracuaf Belt is marked by

a volcano-sedimentary sequence with basic rocks of N-MORB characteristics with Neoproterozoic (Cryogenian,850-650 Ma) Sm-Nd isochron ages (Pedrosa Soares et al.,1993). This sequence appears to be the result of an easterlyoceanic plate subducted below the crustal fragment of theGuanhbs Microplate - which had experienced rhyodac-itic volcanism in the Neoproterozoic (Tonian, 1000-850Ma) (Teixeira et al., 1990). This association developedduring a Brasiliano I collisional system at about 630 Ma,according to the ages obtained for the main metamorphicevent (Siga Jr. et al., 1982).

Another erogenic system is composed of a series ofthrust stacks with tectonic transport towards the south-southeastern margin of the SIo Francisco Craton. Thesethrust sheets, possibly related to northwestward subduc-tion, comprise crustal fragments representing portions ofan active continental margin with Cordilleran-type calc-


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146 M. CAMPOS NET0 and M.C.H. FIGUEIREDO

Fig. 2

23

24

v=wrt-A. Tectonic sketch map of the northeastern portion of the Apiai-GuaxupC Microplate and adjacent terranes - Southeastern

alkaline to alkali-calcic plutonic magmatic arc (Figueiredoet al., 1992; Heilbron, 1993; C.S. Valladares, unpublisheddata). Although poorly constrained, a Neoproterozoic III(c 650 Ma) age (Silva et al., 1987) for this magmatic arcmay be envisaged. The preserved assemblage is inter-preted as a subduction-controlled magmatic-orogeny inthe Juiz de Fora Thrust System domain, comparable to theEastern Cordillera and Subandean chains over the Brazil-ian Shield (e.g., Sengtir, 1990; Roeder, 1988).

Crustal fragments of the Juiz de Fora Thrust Systemconsist of the Mantiqueira and Juiz de Fora gray gneissand granulite series (Campos Neto and Figueiredo, 1990)probably of Paleoproterozoic age (Delhal et al., 1969;Sollner et al., 1991; K. Kawashita, unpublished data), andthe Paraiba do Sul migmatite terrane with relicts of plat-form metasediments (Sad and Dutra, 1988).

Allocthonous metasedimentary sequences overlie anArchean orthogneiss-migmatite complex cut by Mesoprot-erozoic granite dikes (Fernandes et al., 1990) in the Embu

Brazil.Accreted Terrane. These metasedimentary sequencespresent regional metamorphism and are intruded by volu-minous peraluminous plutonism at 750-700 Ma, followedby other S-type plutons associated with metaluminous por-phyritic biotite granites with late-collisional characteristicsat about 610 Ma (Vieira and Tassinari, 1988).

The Curitiba Microplate (Fig. 1) is made up of Pale-oproterozoic hornblende-biotite gneisses with subordinatemafic-ultramafic slices of unknown age, intruded by 700Ma old talc-alkaline granitoids (Basei et al., 1992).

Brasiliano I erogenic evolution in the Apiaf-GuaxupCMicroplate (Fig. 1) which can be subdivided into two maintectonic units, is characterized by the exotic terranes of theSoccorro-GuaxupC Thrust Nappe (Fig. 2A) composed ofinfracrustal units and high-grade supractustal slices (Cam-pos Neto et al., 1990; Campos Neto, 1991); and the low-grade supracrustal sequences of the Apiai Belt. Volca-nosedimentary sequences of an initial rift stage with N-MORB to E-MORB affinities grading to immature arc


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The Rio Dote Orogeny, Southeastern Brazil 147

i-l PhvKmm~c Scdrmcnwry Coven and Alkahne Plumus~0 DOCE OROGENIC DOMAINSSERRA DO MAR .MICROPLATEUpperCambnanI l PonCollis~onalGmmrcs nd Chamakil~

syn-9 Late-Collinonal l--l + Syn- O Late-CollirtonalMetalummousGnn~to~dsP9rabmUnmu mn~es

Pre-Collistonai alc-alkalincGmtwo~dsandEnd&kc SeriesSupmcmsIal emmc

BRASILIANO I OROGEMC DOMAINS

La1c4mge~c High-K CJc-alkolincGranrres Late-omgenicPenlummou Gmmtes

Syn-Omgenic ubducuon-relatedjlc-alkaline GmniloldsAPti-GuMIJpB .MCROPLATESocorroCuwupCThms Nappc

JIJIZ DE FORA THRUST BUT

Nwmnmzoic (?I carbonate-schinquruidcmrmw eltsm he ar&a do Sul ups) crmncPaleopmwmm~c3 infmcruml seriesof Juizdc

j Mesopmtemzoic?) SupmcmsalTerrane Ford jt~ nd gray gneirrrcMantiqucUa ml termnawh high-rmddIe-grade rasIliano with inmsions of chamockites nd pmhm~noiuMamoQhlsm Bnni@=

Apmi FoldedBelt PASSIVE CONTINENTAL IMARGIN SEQUENCEMcsopmtemzw low-gmdewmne of conunenwlma&w-tye wh wndowr of mecavolcano- Aracud and Rio Pardo Fold Belts- SedimenIaryift seqenca GLAiiS ,MICROPLATEEMEU ACCREZD TERR&NX m Rootso~h~terkmdblockboundedby-, crustsbm m rhe N-NW marginof the mlcmplate

SEVERAL FORELAX DOMAINSMaopmlemzoic

- Aho RIOGrandeBe11PaleopmIcmzor

CaboFno Window

SaOFrancsco Cnton Lur AlvesSliceMassifs: Md Medina: G = GalilCia: t = Iurana: Ic = Iconh: R.S3 RiO NoVo do Sd: MF * Mud FIXUVZ d - Gdti:Gp - Cuamplri: BJ - &la Joana:ho - Angelim:SO = km QI Om: QD = Quuino-Donindia:P = PamU:ub _ ubamba:Cg = CamgUautubixN NaUvidode: P - BmgancaPaulis% p _ IpurUnr

Fig. 2. (Continued) Legend of the tectonic sketch maps of Southeastern Brazil (Figs. 2A and 2B).basalts and shoshonites (FrascB et al., 1990; Juliani, 1993)occur locally in the Apiai Belt, with zircon U/Pb andgalena Pb/Pb ages of about 1.7- 1.5 Ga (Van Schmus et al.,1986; Tassinari et al., 1990). In contrast to the Juiz de ForaThrust System which seems to contain portions of oldbelts of the SZo Francisco Craton, the Apiai Belt may havepaleogeographically formed the eastern border of the Riode La Plata Craton.

A Cordilleran-type magmatic arc, characterized bymostly high-K &c-alkaline porphyritic hornblende-biotitegranitoid batholiths (Fig. 2A), appears to be related to a

Brasiliano I subduction-related orogeny in the Apiai-GuaxupC Microplate. This high-K talc-alkaline plutonismhas U-Pb zircon ages of about 670-650 Ma (Tassinari andCampos Neto, 1988; Van Schmus, unpublished data;KrSner, unpublished data). A mangerite series (CamposNeto ef al., 1988) yields U-Pb zircon ages of around 630Ma (unpublished data by different methods by M.A.S.Basei, A. Kriiner and J.I. Wendt) appearing to define eithera high-grade metamorphic event or the final evolution ofthis magmatic arc.


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148 M. CAMPGS NET0 and M.C.H. FIGUEIREDO

= =420 Barbacena

-j Juiz d_e Fara /r

JANEIRO 43Fig. 2B. Tectonic sketch map of the Serra do Mar Microplate and adjacent terranes - Southeastern Brazil.

A late-erogenic alkali-calcic plutonism, at 610 Ma,grades to rapakivi-like Fe-Hastingsite-biotite granites with600-580 Ma (Vlach and Cordani, 1986; Janasi andUlbrich, 1991), marking the Brasiliano I post-erogenicstage and forming the northeast trending Itu granitoid belt(Vlach et al., 1990), oblique to the belt of the high-K calc-alkaline batholiths (Fig. 2A).

SERRA DO MAR MICROPLATE ANATOMYA long and linear coastal belt (Fig. 1) records the estab-

lishment of a new magmatic arc, in a plate convergence

setting, synchronous with the post-erogenic extensionalregime of the Brasiliano I domains. It corresponds to theSerra do Mar Microplate of the Rio Dote Orogenyaccreted during the Cambrian collisional event of this sec-tor of Gondwana (Fig. 3).Migmatites and granitoids predominate over the entireSerra do Mar Microplate. The granitoids form elongatedand diffuse in situ bodies and/or diapirs of garnet-biotitegranites of varying grain size and strain rate and with dom-inant schlieren and nebulitic structures. They intrude orgrade into stromatic-phlebitic migmatites. Mesosomes areperaluminous gneisses and orthogneisses, leucosomes are


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G HEmbd accrclcd tcrranc

Serra do MarI microplalci Cu balooshear zone

/Intracrustal law velocity seismiczone iEassini. 1986) 0 20 40kmFig. 3. Crustal section interpreted based on gravimetric and magnetometric data (see Fig. 2 for location).

granites and trondhjemites, and melanosomes are biotite-,garnet-biotite- or hornblende-biotite-rich. These migma-tites contain enclaves or resisters of quartzites, sometimesin kilometer-sized lenses, talc-silicate rocks, marbles andamphibolites.

Three distinct crustal segments can be recognized in theSerra do Mar Microplate (Figs. 2A and ZB): a middle-highgrade metamorphosed supracrustal terrane, imbricated bythrusts in the interior of structural windows and cut bystrike-slip shear zones; an overlying gneiss-migmatite ter-rane; and the granulite-granite-migmatite terrane whichnow occurs in the upper parts of the structural pile but wasre-equilibrated in deeper levels of the crust (e.g., Wiede-mann et al., 1987; Sluitner and Weber Diefenbach, 1989;Campos Neto and Figueiredo, 1990; FCboli and Ribeiro,1993; F&oh et al., 1993).

The supracrustal terrane is mainly composed of metap-sammite, metacarbonate and metagraywacke sequenceswith slices of stromatic migmatites with granodioriticmesosomes which appear to correspond to an older unit.The metapsammite sequence is made up of an alternationof massive beds of coarse-grained quartzites and arkosic tolithic, platy and micaceous quartzites locally with silli-manite, with gradations to quartz-schists. Subordinately italso contains layers of talc-silicatic gneisses on a decimet-ric scale. The metacarbonate sequence is characterized bymassive or layered marbles, with quartzitic and calc-sili-catic gneisses (garnet-hornblende-diopside-plagioclasegneiss) and quartz-rich talc-silicatic gneisses. The meta-graywacke sequence consists of porphyroclastic (horn-blende)-garnet-biotite gneiss containing sparse plagioclasemegacrysts, up to 2 cm in size, and laterally grading intobiotite-microcline gneisses with intercalated lenses, up to2.5 m thick, of talc-silicatic pyroxene-plagioclasegneisses.

Slices of tremolite-actinolite schists, amphibolites andhomogeneous, fine-grained granolepidoblastic horn-blende-biotite-plagioclase gneisses frequently occur. TWOmain types of protoliths can be chemically distinguished(Sad and Dutra, 1988): low-Ti and high-Mg tholeiiticbasalts; and alkaline basalts associated with arc-type calc-

alkaline andesites, suggesting the possibility of an activecontinental margin environment.

The supracrustal sequence seems to be composed offragments of platform deposits grading eastwards into adeep marine basin. The age of the supracrustal sequence isnot known, but its lower limit is constrained by the pres-ence of 2.1 Ga detrital zircons (Siillner et at., 1989), whileits upper limit is defined by the Cambrian main metamor-phic event (see later).

The gneiss-migmutite terrane is made up of (cordierite-sillimanite)-garnet-biotite gneisses, generally migmatitic,with stromatic-phlebitic structures and with leucosomaticbiotite-garnet granites, grading into bodies with porphyro-elastic nebulitic and schlieren structures. These mostlystromatic migmatites are considered to have formed byanatexis of graywacke-pelite sequences (R&go, 1989). Acordierite-rich stromatic migmatite is locally also conspic-uous.

In the granulite-granite-migmatite terrane, gneissicgranite-diatexites predominate, with diapiric mobilizatesand with earlier stromatic migmatites, generally gradinginto granulitic rocks. The gneissic diatexites are light grayto light pink leuco- to hololeucocratic (graphite-cordier-ite)-sillimanite-garnet-biotite granites. They have porphy-roclastic to coarse-grained granoblastic to myloniticfabrics with schlieren and nebulitic structures, locallyschollen (with talc-silicatic resisters) or agmatite-diction-itic. A large volume of rocks of an enderbite series intrudeinto or are coeval with the peraluminous migmatite mobili-zates (Ri?go, 1989) The late peraluminous mobilizates cor-respond to coarse-grained (sillimanite-cordierite-rutile)-garnet granite-granodioritic diatexites, which intruded thegranulite-migmatite assemblage. The metamorphic condi-tions which affected this terrane, based on several geother-mometers and geobarometers. are of the order of 72OCand 6 kilobars (R&go, 1989; Sluitner and Weber-Diefen-bath, 1989).

The poorly studied accretionary kinematics of the Serrado Mar Microplate over the terranes of the Brasiliano Ierogenic domains suggests that geometrically, it hasadvanced northwestward. In its central portion, it was par-


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tially limited by a major dextral ductile strike-slip shearzone superimposed on northwestward thrusts. An exoticterrane, consisting of a paragneiss-quartzite sequence(Heilbron et al., 1982) associated with Paleoproterozoicorthogneisses (Zimbres et al., 1990), occurs in the CaboFrio region. This terrane is found in a refolded antiformconstituting the nucleus of a structural window (Machadoand Demange, 1990) under the override of the Serra doMar Microplate crustal slice.

A talc-alkaline batholithic plutonism (Fig. 2B)occurred in the Neoproterozoic III-Cambrian boundary,with geochemical subduction zone components. This plu-tonism indicates the microplate character of the Rio Doteerogenic system, forming a magmatic arc with an activemargin in the eastern border of the Serra do Mar Micro-plate (see later). The suture zone of this microplate cannotyet be properly defined in Southeastern Brazil, due to theas yet undefined nature of the Cabo Frio window, whichcould represent a microcontinent, or a small portion (Bra-zilian counterpart) of the Congo Craton reworked border.

THE RIO DOCE CALC-ALKALINEMAGMATIC ARC

Metamorphosed and deformed enderbitic and granitoidbatholiths, composed of basic-intermediate-acid and inter-mediate-acid talc-alkaline sequences, predominate in thenorth-central portion of the Serra do Mar Microplate (Fig.2B). They are thought to characterize a compressiveregime of ocean plate subduction coeval with a post-Bra-siliano I extensional regime in the Apiai-Guaxupe Micro-plate.

These gneissic batholiths have sharp contacts with, andintrude into, metasedimentary and migmatitic units of thesupracrustal and gneiss-migmatite terranes, while in thegranulite-granite-migmatite terrane they generally showdiffuse limits. In the gneiss-migmatite and granulite-gran-ite-migmatite terranes they cut and contain xenoliths ofperaluminous migmatites. These relations indicate a volu-minous crustal partial melting event associated with a ther-mal anomaly (steepening of the isotherms) in thismagmatic arc. Thus, the talc-alkaline batholiths are coevalwith an abundant metaluminous and peraluminous grani-togenesis, characterized by granites with biotite and (mus-covite-garnet)-biotite, as well as by sillimanite-garnet-cordierite granites. These metaluminous and peraluminousgranites occur in larger proportion in the northern portionof the Serra do Mar Microplate (Silva et al., 1987).

Geochemically, the talc-alkaline sequences are bestrepresented by norite-enderbite-charnoenderbite and gab-bro-diorite-tonalite-granodiorite batholiths. Locally, thecharnockitic rocks have subordinate patches and largerbodies of tonalite-granodiorite, with similar chemicalcomposition. A more detailed description and discussionof these sequences, including tables with their typicalcompositions, can be found in Figueiredo and CamposNeto (1993). These typical compositions were selectedfrom a data bank of over 500 samples from numeroussources, and our selection criteria were the coherence of

chemical compositions from different laboratories and theavailability of rare earth element analyses.

The Bela Joana batholith (RCgo, 1989), composed of(garnet)-clinopyroxene-orthopyroxene-(K-feldspar)-quartz-plagioclase gneisses, consists of a low-K, high-Altalc-alkaline sequence mostly intermediate but with basicenclaves that for the most part appear to be cogenetic. TheAngelim rocks (unpublished data; RCgo, 1989), composedof gneissic hornblende-biotite tonalite-granodiorites,sometimes garnet-bearing, define a less expanded calc-alkaline sequence, richer in K, LILE and REE in relationto the Bela Joana sequence. This increasing northwestwardmaturity, also shown by other massifs, such as the high-Kcharnockitic rocks of Serra da Bolivia (Fig. 2B), suggestsa NW-dipping subduction zone for the generation of thesetalc-alkaline rocks.

The hornblende-biotite gray gneisses of the MunizFreire region also consist of a low-K, high-Al talc-alkalinetrend (Figueiredo and Campos Neto, 1991), with conspic-uous cogenetic basic rocks occurring as enclaves. In thisregion gneissic (garnet-muscovite) biotite granites (seeFig. 15), tectonically intercalated with the talc-alkalinegray gneisses also occur. These differentiated and slightlyperaluminous biotite granites have chemical compositionssomewhat similar to those of the Serra dos 6rgBosbatholith gneissic granites (e.g., Puget and Penha, 1980).

The Jequitiba-type (Fig. 2B) tonalitic orthogneisses(Feboli et al., 1993) consist of a varied assemblage ofhornblende-biotite gneisses and garnet-biotite gneisses ofdioritic to granitic compositions, but mostly with a silicarange from 62 to 71%. The published geochemical dataallow the distinction of three series: a talc-alkaline series;peraluminous and HREE-depleted gneisses which appearto correspond to crustal melts; and basic-intermediategneisses enriched in Ti, K, P, Ba, Sr and REE with compo-sitions which resemble those of the late to post-collisionalgranitoids which occur in the region. The Santa Tereza-type (Fig. 2B) enderbitic gneisses (Feboli et al., op. cit.)appear to correspond to the charnockitic equivalents of theJequitiba gneisses, also showing the three differentgeochemical sequences described above.In the Guarapari region (Fig. 2B), a moderately potas-sic, high-Al intermediate talc-alkaline sequence occurswith gneissic enderbite-charnoenderbites and tonalite-gra-nodiorites which have all virtually the same chemical com-position (FCboli and Ribeiro, 1993). Sluitner and Weber-Diefenbach (1989) considered these igneous rocks asbeing derived from partial melting of graywackes, but inour opinion it is unlikely that pyroxene and amphibole-bearing charnockitic and granitoid rocks can have such anorigin.

Overall, there are great similarities in composition forall of those talc-alkaline charnockitic and/or granitoid plu-tonics. Their compositions (Fig. 4) in the RI-R2 diagram(La Roche et al., 1980) plot in the field of typical subduc-tion-related talc-alkaline sequences (Batchelor andBowden, 1985), as also indicated by the trace element dis-tributions (see later). Some of the suites have associatedcogenetic basic rocks, based on their geochemical charac-teristics, while others have essentially intermediate com-


9/20


/ 5 70 500 l o o 0 1 5 0 0 2 c m 2 5 0 0 3 a x l

Rl - 4 S l - l l ( N a + 0 - 2 ( F e t T DFig. 4. RI-R2 diagram (La Roche et al., 1980), with the tectonicdomains of Batchelor and Bowden (1985), for the talc-alkalinesequences of the Rio Dote magmatic arc. Symbols: Bela Joana(O), Angelim (+), Muniz Freire (x), Jequitiba (*), Santa Tereza(Cl) and Guarapari (A) batholiths. See text for the data sources.

3r

8+ 2!i(3

8 1

p e r a l u m I

1 2At 2 0 3 / Cu O+ Na X ) t K2 0 )

Fig. 5. A/NK-A/CNK diagram (Maniar and Piccoli, 1989) for thetalc-alkaline sequences of the Rio Dote magmatic arc. Samesymbols as on Fig. 4.

1 0 0 L

1 0

i1 ' , , , . , , , , , l l ' 1L a c e N d S mE u G d D y Ho b Y b L u

Fig. 6. Chondrite-normalized (Boynton, 1984) REE distributionpatterns for the talc-alkaline sequences of the Rio Dote mag-matic arc. Same symbols as on Fig. 4.

positions, with scarce basic enclaves. All these lithologiesare metaluminous to slightly peraluminous (Fig. 5).

Their REE distribution patterns (Fig. 6) are moderatelyfractionated and generally present a small negative Euanomaly. Some small variations are observed, with theSanta Tereza rocks having lower REE values, while theMuniz Freire gneisses generally are richer in REE. TheBela Joana and Angelim rocks show negative Eu anoma-lies and slightly decreasing total REE contents withincreasing silica. Sluitner and Weber-Diefenbach (1989)described the rocks from the Guarapari region as exhibit-ing decreasing REE contents with differentiation and posi-tive Eu anomalies in the more evolved rocks. Many of thetalc-alkaline sequences show decreasing total REE andsmaller negative Eu anomalies with differentiation. Thesefeatures have been generally considered (e.g., Arth et al.,1978; Condie et al., 1982) to represent crystal-liquid equi-librium of the igneous protoliths and are interpreted as rep-resenting partial melting of a mafic source or fractionalcrystallization of basaltic magma. For sequences whichpresent essentially intermediate compositions with scarcebasic enclaves, such as Guarapari and Angelim, the partialmelting model is more probable, while fractional crystalli-zation may have prevailed for the Muniz Freire and BelaJoana sequences, which have conspicuous associated basicrocks. A geochemical subduction zone component, charac-terized by LILE enrichments and Nb and Ti depletions(e.g., Pearce, 1983; Condie, 1989), is evident for all thesetalc-alkaline plutonic sequences (Fig. 7).

A fluid phase with variable C0,/I-120 ratios appears tobe responsible for the charnockitic and/or granitoid char-acter of these plutonic rocks. Hypersthene-bearing darkgreen charnockitic rocks such as those of the Bela Joanabatholith probably correspond to the crystallization of CO,rich talc-alkaline magmas, while hornblende-biotite graygranitoids which occur for example in the Angelim andMuniz Freire batholiths appear to be related to HzO-bear-ing magmas. Local concentrations of carbonic or aqueousfluids would be responsible for the hybrid nature, (withboth charnockitic and gray gneissic rocks) of other intru-sions such as the Guarapari.

. l 1S r K2 ORb B a l h l a N b Ce P 2 O SZ l Hf S f n l b O2 YYbFig. 7. N-MORB-normalized (Pearce, 1983) incompatible ele-ment distribution patterns for talc-alkaline intermediate rocks ofthe Rio Dote magmatic arc. Same symbols as on Fig. 4.


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152 M. CAMPOS NETO and M.C.H. FIGIJE~RBDO

In our opinion, the Rio Dote magmatic arc evolutiondid not surpass 25 Ma (Table 1). Zircon U/Pb ages define a580-575 Ma range for the talc-alkaline series and between575 and 565 Ma for the diatexitic garnet-biotite granites.The Rb/Sr method (Table 2) yielded a larger interval,between 600 and 570 Ma. The Sr isotopic evolution ishowever, clearly distinct in these plutonic associations(Fig. 8): the talc-alkaline sequences have very low Rb/Srratios; but these ratios increase significantly in the granitesassociated with the arc metamorphic event, suggesting dif-ferent magmatic reservoirs in relation to the Rb content.

A subduction-related metamorphic event in deep por-tions of the crust appears to be preserved in a high-grademetamorphic paragenesis and in a relict foliation in theJuiz de Fora Thrust Belt domain at about 580 Ma, accord-ing to zircon U/Pb data (e.g., Sijllner et al., 1991) as wellas by the Rb/Sr system which indicates ages of 600 to 560Ma (Table 1).

A0.740-

Alrotoeac gf evolution of Tris Rios enderbiticI aneiss with L923t60Ma In a7304 - I

0.700- lMMsELI I I600 550 500 450Age (Ma.)

The evolution of this magmatic arc, with only its rootspreserved, seems to be similar to the Andean-type orogen(Sengor, 1990), where the dominant convergence is con-trolled by a Benioff zone with a moderate (normal)slope. This process may generate compressional metamor-phic core complexes (Armstrong, 1982) in a continentalmargin arc. In the Andean chain this process is recorded inthe seismic wave attenuation domains (Sengor, op. cit.)and separated from the adjacent craton by a fold-thrustbelt. It is in this hot metamorphic nucleus of strongly par-tially melted zones, that the rear-arc thrusts take root, asso-ciated with underthrusting of the hinterland infracrustaldomain.

Fig. 8. Isotopic Sr evolution diagram for the Rio Dote granitoids.Symbols: MSEL = Mantle strontium evolution line; Cl = metalu-minous to slightly peraluminous pre-collisional granites; 0 =pre-collisional enderbite; 0 = syn- to late-collisional metalumi-nous granitoids; 0 = late peraluminous granites; + = post-colli-sional granitoids.itic rocks (Fig. 2A) could correspond to partial melting ofa granulitic crust with an isotopic evolution similar to thatof the slightly older enderbitic series (Fig. 8). However,none of these rocks appear to be derived from the mucholder enderbitic gneisses of the Juiz de Fora Thrust Sys-tem, such as the T&s Rios gneiss (Fig. 8).

THE RIO DOCE COLLISIONAL GRANITOIDSAND MIGMATITES

The Rio Dote collisional erogenic stage appears tohave lasted about 30 m.y., in the Lower Cambrian, fromapproximately 560 to 530 Ma (Tables 1 and 2).

Peraluminous granitoid diapirs, with granitic and sub-ordinate granodiorite-tonalitic composition, occur abun-dantly in the Serra do Mar Microplate. Muscovite-tourmaline-bearing rocks predominate in the supracrustalterrane, while (cordierite-sillimanite)-garnet-biotite bodiesare typical of the granulite-granite-migmatite terrane.These rocks, massive to well foliated, have ages youngerthan 550 Ma and predominate at about 540 Ma, in agree-ment with the metamorphic age obtained in monazites bythe U-Pb method (Table 1).

Generally, batholithic high-K talc-alkaline, metalumi-nous granitoids, with limited compositional variation, pre-dominate in the rear arc position and even beyond themicroplate in the Embu Accreted Terrane (Fig. 2A). Theserocks have not yet been well studied but appear to corre-spond mostly to porphyritic gray biotite granite-granodior-ites, sometimes associated with muscovite and/ortourmaline-bearing S-type granites (Janasi and Ulbrich,1991).

In the central portion of the Set-t-a do Mar Microplate,(Fig. 2B) (cordierite)-sillimanite-garnet-biotite, mostlystromatic and diatexitic (S-type granites) migmatitesoccur, as well as the sillimanite-garnet-biotite-cordierite-rich banded aluminous gneisses (considered here asmostly stromatic migmatites) and S-type granites (FCboliet al., 1993).

In the southern portion of the Serra do Mar Microplate(Fig. 2A) ample elongated massifs of porphyritic horn-blende-biotite monzonite-granites crop out, frequentlywith charnockitic facies, in association with mangeritesand hypersthene-bearing quartz syenites and charnockitesin a high-K and low-Ca series (e.g., Gasparini and Manto-vani, 1979). They are generally foliated rocks grading toextensive mylonitic belts. Their age is about 550-540 Ma(Table 2) and they may be related to Caledonian I-type(sensu Pitcher, 1982) syn- to late-collisional series.

These lithologies present mostly granitic compositionswith subordinate tonalitic-granodioritic compositions (Fig.9) and are peraluminous (Fig. 10). More details of theirtypical compositions can be found in Figueiredo and Cam-pos Neto (1993). Their REE distribution patterns (Fig. 11)exhibit moderately fractionated LREE, slightly fraction-ated HREE and generally negative Eu anomalies, alsoindicating their crustal derivation. A few samples showHREE depletion, while others have positive Eu anomalies,suggesting variation in the residual assemblage during par-tial melting of their source, mostly metasediments.

The typical Sr isotopic behavior of these granitoids is A good example of the slightly peraluminous, leuco-compatible with a predominantly crustal origin (R,, = cratic biotite granites and (muscovite-garnet)-biotite gran-0.710-0.713) and it is possible that the Ubatuba chamockites occurs in the Muniz Freire area.


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I cl 7 ! I0 500 lala 1!500 2Oxl 2500 3txo

R = 4S - ll(Na + KI - 2(Fe + TOFig. 9. Rl-R2 diagram (La Roche et al., 1980), with the tectonicdomains of Batchelor and Bowden (1985), for the Rio Dote pera-luminous migmatites and granites. Symbols: Sgo Fidelis stro-matic migmatites A; Catalunha S-type granites 0; DomingosMartins aluminous gneisses V; Colatina S-type granites Cl; andMuniz Freire biotite gneisses *. See text for the data sources.

3[ ;Metaluminous PeraluminousI I

I1 2

Ak203/(CaO + Na20 + Kx>)Fig. 10. A/NK-A/CNK diagram (Maniar and Piccoli, 1989) forthe Rio Dote peraluminous migmatites and granites. Same sym-bols as on Fig. 9.

,Itf

L

1 I cLaoe Nd SmEuGd DyHoB YbLuFig. 11. Chondrite-normalized (Boynton, 1984) REE distributionpatterns for the Rio Dote peraluminous migmatites and granites.Same symbols as on Fig. 9.

These rocks are silica rich (about 74-78% SiO,) andhave BEE distributions (Fig. 11) with LREE enrichments,large negative Eu anomalies and relatively undepletedHREE (Figueiredo and Campos Neto, 199 1). They appearto correspond to I-type crustal melts coeval to the intrusionof the voluminous talc-alkaline magmas.

THE RIO DOCE COLLISIONAL STRUCTURESIn the Serra do Mar Microplate, the main structures are

generally NE trending, and correspond to northwestwardsub-horizontal ductile shear slices, with or without associ-ated folding, followed by an also northwestward thrustpile. They are superposed by large strike-slip shear belts.These main structures continue beyond the boundaries ofthe microplate, although with less intensity, mostlytowards the Ssio Francisco Craton.

The principal tectonic foliation corresponds to a S2 inthe supracrustal terrane, and partly in the gneiss-migmatiteterrane, while it is a primary tectonic foliation (Syl) in thegranitoids and diatexites. The relict St foliation, whenpresent, may represent either a deformational structuregenerated during the subduction-related compressionalregime, or as product of continuous ductile deformation.

This S2 foliation defines the axial plane of asymmetricisoclinal folds, outlined by pre-kinematic leucosomes orby intrusive contacts between granitoids (Fig. 12) andranges from decimeter to meter scale. These folds are gen-erally compatible with upper limb parasitic foldings ortransposed structures associated with bands of strong duc-tile shearing with west-northwestward transport.

This principal foliation was deformed by asymmetricrecumbent folding, in the IC sub-class, and admitted anaxial plane S, (Sy2) foliation with biotite (garnet andbiotite in the granulite-granite-migmatite terrane). Thisfolding is found in variable scales (Figs. 13 and 14), wasmostly cylindrical (Fig. 14C), and generally is oriented inthe NE-SW direction with NW vergence. The S, foliationgrades into mylonitic belts, with kinematic indications fornorthwestward transport, such as sigmoidal elongation oflate-kinematic garnet porphyroblasts in decimeter scaleshear bands (Fig. 13D). The NW transport produced up to2 km thick lens-shaped sub-horizontal slices similar to aduplex thrust system (Figs. 14 and 15).

These thrust systems tear the folds or deform them intoconical structures, with curvilinear hinges, with rotation ofthe B-axes and the constructed conical fold generated linestowards the transport direction (Fig. 14A). Another localfoliation, generated by pressure-solution (strong spacedcleavage) in the biotite zone, is developed in belts in theinterior of the allocthonous piles. Locally, this foliation isthe axial plane of conical drag folds which affect either themetamorphic banding (St/S,) or the S, foliation(Fig.14B). This local foliation is the result of northwest-ward transport of the thrust slices. Small brittle-ductilestructures are also frequently associated with the latestages of this thrust system evolution, in upper portions ofthe crust (Fig. 13C). All these structures are also well rep-resented in the Juiz de Fora Thrust System (Fig. 16).


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Csgan

lggatae

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Csgon

gae

Fg1L

SmsesuueaoaewhhmnoaoeaeohcoeA=aymconodwhmnoaoaSaapaneoma

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aymconcpeaoeodnaoyounodmwhSaapaoaompegaodnuoohMuzFeebhhFg1RgSmse

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13/20

The io Duce Orogeny, Southeastern Brazil 155

-Constructed comcal fold Qenerared lme

/ Supracrustal terrane I Gneiss-mlgmatlte terrace IS - fahation in

C 0I IN$OW (Garnet- muscovite).I

Biotite granite /(Hornblende) Biotite tonalite-

granodiorite S6iE- Juiz de Fom Supracrus?ol II I terrane terrane .

S -foliation i;l I\S - follotion in I

Granulite -qanitemigmatite ter r aneFig. 14. (Top) Sema do Mar Microplate thrust pile profile (see Fig. 2B for location). Fig. 15. (Bo~fmn)Muniz Freire and Guandu calc-alkaline batholiths profile (see Fig. 2B for location).

The large strike-slip shear zones (Figs. 2A and 2B),which occur at the boundaries as well as in the interior ofthe different superposed erogenic systems, appear to havehad a longer evolution in the Rio Dote Orogeny. Theemplacement of small plutons of alkali-calcic and high-Ktalc-alkaline granitoids in the Apiai-GuaxupB Microplateseems to be controlled by these shear zones. These grani-toids yield Rb-Sr and Pb-Pb ages between 580 and 520 Ma(Vlach and Cordani, 1986; Tassinari and Campos Neto,1988), indicating that these large shear zones were activealready before the Rio Dote collisional event.The Aldm Paraiba and Gus+ shear zones (Figs. 2Band 16), which cut parts of the boundary between the Serrado Mar Microplate and the Juiz de Fora Thrust System,show superposed movements. They generate foliations,under biotite zone metamorphic conditions, related to sin-istral transport subsequent to higher grade dextral trans-port (Fig. 16D).

The last large compressive deformations, already underlower grade metamorphic conditions, correspond to anti-forms and synforms with different scales but which areassociated with structures with an average wavelength ofabout 3 to 5 km.

These cylindrical, NE to ENE-oriented structures (Fig.16D, E, F) exhibit a crenulation or spaced cleavage (S,)and axial plane white-mica and/or biotite schistosity. Theyoccur in a convergent fan towards the inner arch of theantiform structures. Many of those folds were re-orientedor correspond to NNE-oriented en echelon folds, near thestrike-slip shear zones (Fig. 16D). These folds are alsoflattened in the transpressive domains between these shearzones (Itaperuna region in Fig. 16), with width/wavelengthratios greater than 1. On the other hand, evidence of brit-tle-ductile shear zones in extensional regimes can also befound in some arrays of strike-slip faults, such as in theregion of the Guandu batholith (Fig. 15).


14/20


S-folroltonm a dextralstrike -slip sheor 20~

I uiz de Fara thrust bell : Reworked boundaryBonded enderbilic aneiss Serra da Mar mcroolok\ Gronulile- Gramle- Gnelss-Miamottte/ FSerro da Mar microplateOf IAngelim Bela Joana

Fig. 16. A link between the hinterland Juiz de Fora Thrust Belt and the Serra do Mar Microplate, through the AlCm Paraiba dextralstrike-slip shear zone (see Fig. 2B for location)

In the Apiai-GuaxupC Microplate and in the Alto RioGrande Belt (Fig. 2A), this folding was continuous, withan axial trend commonly oriented towards ENE-WSW(Campos Neto, 1991), and coeval with a northwestwardstransport resumption of the Socorro-Guaxupk ThrustNappe, bending the subjacent Alto Rio Grande Belttowards the transport direction (Fig. 2A).

THE RIO DOCE LATE- TO POST-COLLISIONALPLUTONISM

Many late- to post-tectonic plutons, with ages rangingfrom 520 to 480 Ma (Tables 1 and 2), occur in the Serra doMar Microplate, with varied composition, mostly alkali-calcic, generally with associated basic rocks. Many corre-spond to zoned plutons, with evidence of magma min-gling/mixing and are generally enriched in K, P, Ba, Sr andZr (e.g., Bayer et al., 1986; Wiedemann et al., 1986, 1987;Junho, 1993). See Figueiredo and Campos Neto (1993) fortypical compositions and a more detailed discussion.

Recently published geochemical data, including REE(Ftboli et al., 1993; Ftboli and Ribeiro, 1993), for late- topost-tectonic plutons and granitic dikes indicate mostly analkali-calcic trend for these rocks. Many of these plutonshave a monzogabbro-quartz monzonite-granite composi-tional trend, with post-collisional uplift and late-erogeniccharacteristics (Fig. 17) and are essentially metaluminous(Fig. 18). They show high contents of LILE, P, Zr and REEwith an alkali-calcic to shoshonitic character. They are

strongly enriched and fractionated in REE with small neg-ative Eu anomalies (Fig. 19).

Some of these late- to post-collisional plutons, such asCastelo and Santa AngClica, have ample evidence ofmagma mingling/mixing relations (e.g., Wiedemann et al.,1987; Schmidt-ThomC and Weber-Diefenbach, 1987) andit is interesting to note that these plutons generally haveassociated talc-alkaline intermediate rocks. On the otherhand, other plutons, for instance the Garrafgo and AracCmassifs, correspond to alkali-calcic monzonitic series andshow strong inflections in their compositional trends insome variation diagrams (e.g., Al,O, Harker diagrams), atabout 65% SiO, (e.g., data in Ftboli et al., 1993). Thissuggests that the differentiation of these alkali-calcicsequences was mainly controlled by crystal-liquid equilib-rium processes, like fractional crystallization.

Granitoid dikes which intrude into the Angelim tonal-itic gneisses have compositions of monzogabbro-quartzmonzonite-granite and are also REE enriched and fraction-ated (Fig, 19), with the difference that the monzogabbro ismuch less fractionated and the more differentiated rocksexhibit increasing REE fractionation.

All these rocks also show incompatible element distri-butions with conspicuous geochemical subduction zonecomponents (Fig. 20) with LILE enrichments and deple-tion of Nb and Ti, probably reflecting metasomatic enrich-ment of the mantle sources during the precedingsubduction environment, as suggested by the LILE andLREE-enriched associated basic rocks.


15/20

The Rio Dote Orogeny, Southeastern Brazil

01!, I I :0 loo0 15(302X025003oooR1=4S-ll(Na+KJ-2(Fe+T)

I /1 2

Al203/CaO + Na20 + K20)Fig. 17. (Left) Rl-R2 diagram (La Roche et af., 1980), with the tectonic domains of Batchelor and Bowden (1985), for the Rio Dotelate- and post-collisional plutonism. Symbols: Itaipavas quarry dikes 0; Garrafao pluton 0; Arac& pluton A, Domingos Martins dikes +;Iconha pluton V; Rio Novo do Sul pluton *; Pidma dikes x. See text for data sources. Fig. 18. @i&t) A/NK-A/CNK diagram (Maniarand Piccoli, 1989) for the Rio Dote late- to post-collisional plutonism. Same symbols as in Fig. 17.

1 1La Ce Nd SmEuGd e/ Ho& Yb LuFig. 19. Chondrite-normalized (Boynton, 1984) REE distributionpatterns for the Rio Dote late- to post-collisional plutonism.Same symbols as on Fig. 17.

*l 1 Sr K2ORb BaTh To Nb CeF2OSZr Hf SmTO2Y YbFig. 20. N-MORB-normalized (Pearce, 1983) incompatibleelement distribution patterns for the acid rocks of the Rio Dotelate- to post-collisional plutonism. Same symbols as on Fig. 17.

CONCLUSIONSThe Gondwana supercontinent cycle is characterized in

Southeastern Brazil by the Brasiliano-Pan-African Cycle,which lasted from the Neoproterozoic to the Lower Ordov-ician with superposed erogenic events in distinct domains.

The final stage of plate convergence of the BrasilianoI orogeny is probably marked in the Apiai-GuaxuptMicroplate by an A-type (sensu Pitcher, 1982) rapakivi-like plutonism at 600-580 Ma.

The first erogenic event of the Brasiliano-Pan-AfricanCycle in Southeastern Brazil appears to have occurredafter the onset of a MORB-type tholeiitic magmatism withages of about 850 Ma (Pedrosa Soares et al., 1993), in theAraquaf Belt, approximately coeval to a platform epiconti-nental basin deposition (Bambui Group, Fig. 1).

The Rio Dote Orogeny is well characterized in theSerra do Mar Microplate with the piling up of three differ-ent crustal segments in a hinterland domain in relation to anorthwestward dipping ocean crust subduction zone.

The geochronological data for the Serra do Mar Micro-plate indicate a separation in time of the main erogenicsteps (Fig. 21), from the pre- to the post-collisional stages.

The Brasiliano I erogenic systems were of the colli- The Rio Dote magmatic arc erogenic stage occurredsional type or controlled by ocean plate subduction. They between 590-570 Ma with talc-alkaline granitoid andwere responsible for the generation of fold belts in passive charnockitic batholiths. This data shows clear geochemicalcontinental margins (Araqiai-Rio Pardo), related to the subduction zone components, associated with a volumi-southeastern border of the S%o Francisco Microcontinent, nous metaluminous and slightly to strongly peraluminousand active margins in distinct microplates (e.g., Guanhbs) plutonism with characteristics of magmatic arc roots. Dur-or in the eastern border of the Rio de La Plata Craton ing this period, the Apiai-GuaxupC Microplate domain was(Apiai-Guaxupe). relatively stable under probable extensional conditions, but


16/20

158 M. CAMPOS NET0 and M.C.H. FIGUE IREDO

Table 1. U-Pb Geocbronology in the Serra do Mar Microplate.

UNIT AGE IN MA UPPERINTERCEPT REF.

POST-COLLISIONAL GRANITOIDSzircon Porphyritic charnockite of Padre Paraiso type SOS + 5Zircon Alla&e granite of Santa Angtlica Pluton 513+8SYN- TO LATE-COLLISIONAL GRANITOIDS AND METAMORPHIC ROCKSR Zircon Cordierite-garnet-sillimanite banded gneis\ 533 f 106 1883+848Mona&e Garnet-biotite granodiotite diatexltic gnetss 538i II&COll Granite-granodiotite nebulitic gnew s41 * 50ZKOll Quartzite 545+12 1864+ 1234R Zircon Garnet-biotite granite diatexitlc gneiss 551*9PRE-COLLISIONAL PERALUMINOUS GRANITOIDSP Zircon Garnet-biotite granite gneiss 563.1% 9P Zircon Garnet-biotite granodiorite gneiss 514 ?r 82P Zircon Cordierite-garnet-sillimantte diatexite 575 * IO 2101 f 162PRE-COLLISIONAL CALC-ALKALINE GRANITOIDSP Zircon Charnockite and enderbite of Guaraparr type s14+ I4P Zircon Granodiorite of Muniz Freire Batholith 580 f 28

23

55154

445

53

P Zircon has a prismatic shape, while R Zircon has a rounded shape with new growth faces. References: 1 - Delhal et al.(1969); 2 - Siga Jr. (1986); 3 - SBllner et al. (1987); 4 - SRllner ef al. (1989); 5 - Siilhter et al. (1991).

Table 2. Rb-Sr Geochronology on Granitoids of the Rio Dote Orogeny.

UNIT A;;N (87Sr/86Sr) REF .

POST-COLLISIONAL GRANITOIDSGranite of ltinga type 480+ IO 0.7 17Chamockite of Padre Paraiso type 520 + 20 0.7112SYN- TO LATE-COLLISIONAL PERALUMINOUS AND METALUMINOUS GRANITOIDSSillimanite-garnet-biotite granite 505 + 35 0.715Sillimanite-garnet-biotite granite 535 * 15 0.709(Hornblende)-bioti te porphyritic granitoid ofcaraguatatuba type 543 + 16 0.712Charnockite and Hypersthene syentte of Ubatuba type 551+5 0.7101Biotite granitoid of Galileia type 550 + 20 0.713PRE-COLLISIONAL METALUMINOUS TO SLIGHTLY PERALUMINOUS GRANITOIDS(Garnet)-biotite porphytitic foliated granite of Medina Batholith 571+4 0.708(Garnet)-biotite porphyritic fohated granitoid 57.5 + 10 0.706(Garnet)-biotite porphytitic foliated granitoid of Medina Batholith 580 f 40 0.711PRE-COLLISIONAL CALC-ALKALINE GRANITOIDSEnderbite of Bela Joana type 6OO+lO 0.708Chatnockite of Itarama type 600 0.708

I5

36

References: 1 - Besang et 01. (1977); 2 - Gasparint and Mantovani (1979); 3 - Batista (1984); 4 - Litwinski (1985); 5 - SigaJr. (1986); 6 - Silva etaI. (1987); 7 - Angeli (1988) ; 8 Tassinari (1988).


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600,

cf 540g.-

: 520-

The Rio Dote Grogeny, Southeastern Brazil

,

II1500- Post-collisional plutonism II I II I

0 Calc-alkaline granitoid series @ Metaluminous granitoids0 Peraluminous granitoids 0 Metamorphism ages+ + Rb-Sr method F -- -4 U-Pb method

Fig. 21. Geochronological data for the Rio Dote erogenic evolution. See Tables 1 and 2 for data sources. The metaluminous granitoidsinclude metaluminous to slightly peraluminous pre- to syn-collisional biotite granodiorite-granites and post-collisional alkali-calcicplutons.within a stress field compatible with the beginning of theclosing of a residual ocean basin in more easterly domains.

The collisional erogenic stage occurred in the LowerCambrian, between 560 and 530 Ma, generating a gneis-sosity folded in a recumbent style or in antiformal nappes,ruptured in horizontal ductile shear zones with west-north-westward transport in the hinterland domain, emplacingthe Serra do Mar Microplate over reactivated BrasilianoI erogenic segments. The resumption of movement alonglarge directional ductile shear zones was followed by highamplitude antiformal and synformal folding. The mainuplift of the Apiai-Guaxupe Microplate and EmbuAccreted Terrane occurred during this period and migrated(Fig. 22) from the westerly (at 555 Ma) to the easterly ter-ranes (515 Ma).

The late- and post-erogenic plutonism lasted up to theLower Ordovician, from 520 to 480 Ma, when the generaluplift of this erogenic system occurred (Fig. 22), while thefinal uplift of the Brasiliano I domains occurred slightlyearlier, in the Cambro-Ordovician boundary.

The plate convergence of the Rio Dote Orogenyappears to have migrated in time, towards the west-south-western sector of Gondwana. Contemporaneously with theRio Dote magmatic arc stage, divergent plate regimesprevailed in the Corumbi Group in western Brazil (Boggi-ani et al., 1993) and in the Pampean Cycle (Rapela et al ,

1992), in western Argentina. In this last region, the inver-sion of passive to active margin occurred during the RioDote collision.

The proposed superposition of orogenies in Southeast-em Brazil appear to be also typical of the development ofthe Neoproterozoic-Eopaleozoic fold belts from the south-central region of Africa, such as the progressively youngerLufilian Arc and Zambezi belt, with an erogenic episode at820 Ma, and the Kaoko and Damara belts, with an orogenyoccurring at 600-450 Ma (e.g., Porada, 1989; Stanistreet etaZ., 1991; Wilson er al., 1993).

Instead of an opening of a small Neoproterozoic oceanbasin between Africa and South America, the AdamastorOcean (Hartnady et al., 1985), we adopted the concept ofthe large Brazilide Ocean (Dalziel, 1992) among the Riode La Plata-Amazonia-West Africa cratons and the SloFrancisco-Congo Craton. The closing stages of this largeocean are responsible for the erogenic superposition dur-ing the amalgamation of this sector of the GondwanaSupercontinent.Aclinowledgements-Financial support for research in SE Brazil hasbeen provided, over the years, by CNPq, FAPESP and FINJS? The manu-script was greatly improved by revisions, discussions and suggestions byIan McReath, Benjamim Bley de Brito Neves and Umberto Giuseppe.Cordani. We thank Hubems Porada and an anonymous reviewer for theirhelpful comments and suggestions.


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160 M. CAMPOSNETO and M.C.H. FIGUEIREDO

mm Socorro -Guaxupethrust nappe domain (43 1

Embu terrone (33)

Migration of moin postRio Dote uplift

Southern Serro do Mar microplote (20)

Whole rock or K-Feldspar dotaAmphibole data

550 600 650 700 750Age in Ma B.F?

Fig. 22. K-Ar data for the Rio Dote and Brasiliano domains in the region of the crustal section shown in Fig. 3. Data from numeroussources, synthesized in Cordani and Teixeira (1979). Additional data from Tassinari et al. (1985), Vlach and Cordani (1986) andTassinari (i988). ,

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