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Ž . Earth-Science Reviews 50 2000 77–111 www.elsevier.comrlocaterearscirev The origin and evolution of the South American Platform Fernando Flavio Marques de Almeida a , Benjamim Bley de Brito Neves b,1 , ´ Celso Dal Re Carneiro c, ) ´ a Department of Mining, Escola Politecnica, UniÕersidade de Sao Paulo; Alameda Franca 432, Apart. 9, 01422-000, Sao Paulo SP, Brazil ´ ˜ ˜ b Department of Geology, Instituto de Geociencias, UniÕersidade de Sao Paulo, P.O. Box 11 348, 05422-970, Sao Paulo SP, Brazil ˆ ˜ ˜ c Department of Geosciences Applied to Teaching, Instituto de Geociencias, UniÕersidade Estadual de Campinas, P.O. Box 6152, ˆ 13083-970, Campinas SP, Brazil Received 9 March 1998; accepted 14 November 1999 Abstract The South American Platform is defined as the stable continental portion of the South American plate not affected by the Phanerozoic — Caribbean and Andean — orogenic zones. It is surrounded by these orogenic zones and extends to the marginal Atlantic coast. The basement of the platform consists of Archean and Proterozoic continental crusts arranged Ž. Ž . Ž. Ž. during three main sets of orogenic events: 1 Trans-Amazonian Paleoproterozoic , 2 Late Mesoproterozoic and 3 BrasilianorPan African. The latter resulted in the consolidation of the youngest mobile belts of the platform basement. It is, Ž . by far, the main phenomenon responsible for the overall pattern of tectonic components cratonic nuclei and fold belts and the formation of the general structural framework at the time when the platform was a portion of the Gondwana supercontinent. During the Phanerozoic Eon, different cover stages were developed through six main sedimentary cratonic sequences, of which the last one is exclusive to the South American continent. The final individualization stages and their Ž respective post-Paleozoic sequences were accompanied by a series of specific intracratonic processes, both tectonic rift . Ž . basins, overprint of new structural styles in previous basins and magmatic basaltic and alkaline . The activation processes have generally been attributed to the opening of the Atlantic Ocean on the east and the Andean orogeneses on the north and Ž west. Nevertheless, a minor part of these events may have been caused by sublithospheric actions mantle-activated . processes beneath the interior of the platform. q 2000 Elsevier Science B.V. All rights reserved. Keywords: tectonics; South America; platform; basement; platform covers; Archean, Proterozoic; Phanerozoic 1. Introduction This paper aims to synthesize the present state-of- the-art of the geological knowledge on the origin and ) Corresponding author. Fax: q 55-19-289-1562. Ž . E-mail addresses: [email protected] F.F.M. de Almeida , Ž . [email protected] B.B. de Brito Neves , [email protected] Ž . C. Dal Re Carneiro . ´ 1 Fax: q 55-11-210-4958. evolution of the South American Platform, the oldest part of the South American Plate. Ž . Two of the present authors BBBN and CDRC think that it is time to update the most quoted paper on Brazilian tectonics: the classical 36-page Brazil- ian National Department for Mineral Resources Pro- Ž . duction DNPM Bulletin 241, Origin and Evolution Ž of the Brazilian Platform Origem e EÕoluc ¸ao da ˜ . Ž Plataforma Brasileira , by Almeida, 1967 following . Almeida, 1966 : this broad synthesis on the Brazilian 0012-8252r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž . PII: S0012-8252 99 00072-0
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Ž .Earth-Science Reviews 50 2000 77–111www.elsevier.comrlocaterearscirev

The origin and evolution of the South American PlatformFernando Flavio Marques de Almeida a, Benjamim Bley de Brito Neves b,1,´

Celso Dal Re Carneiro c,)´a Department of Mining, Escola Politecnica, UniÕersidade de Sao Paulo; Alameda Franca 432, Apart. 9, 01422-000, Sao Paulo SP, Brazil´ ˜ ˜

b Department of Geology, Instituto de Geociencias, UniÕersidade de Sao Paulo, P.O. Box 11 348, 05422-970, Sao Paulo SP, Brazilˆ ˜ ˜c Department of Geosciences Applied to Teaching, Instituto de Geociencias, UniÕersidade Estadual de Campinas, P.O. Box 6152,ˆ

13083-970, Campinas SP, Brazil

Received 9 March 1998; accepted 14 November 1999


The South American Platform is defined as the stable continental portion of the South American plate not affected by thePhanerozoic — Caribbean and Andean — orogenic zones. It is surrounded by these orogenic zones and extends to themarginal Atlantic coast. The basement of the platform consists of Archean and Proterozoic continental crusts arranged

Ž . Ž . Ž . Ž .during three main sets of orogenic events: 1 Trans-Amazonian Paleoproterozoic , 2 Late Mesoproterozoic and 3BrasilianorPan African. The latter resulted in the consolidation of the youngest mobile belts of the platform basement. It is,

Ž .by far, the main phenomenon responsible for the overall pattern of tectonic components cratonic nuclei and fold belts andthe formation of the general structural framework at the time when the platform was a portion of the Gondwanasupercontinent. During the Phanerozoic Eon, different cover stages were developed through six main sedimentary cratonicsequences, of which the last one is exclusive to the South American continent. The final individualization stages and their

Žrespective post-Paleozoic sequences were accompanied by a series of specific intracratonic processes, both tectonic rift. Ž .basins, overprint of new structural styles in previous basins and magmatic basaltic and alkaline . The activation processes

have generally been attributed to the opening of the Atlantic Ocean on the east and the Andean orogeneses on the north andŽwest. Nevertheless, a minor part of these events may have been caused by sublithospheric actions mantle-activated

.processes beneath the interior of the platform. q 2000 Elsevier Science B.V. All rights reserved.

Keywords: tectonics; South America; platform; basement; platform covers; Archean, Proterozoic; Phanerozoic

1. Introduction

This paper aims to synthesize the present state-of-the-art of the geological knowledge on the origin and

) Corresponding author. Fax: q55-19-289-1562.Ž .E-mail addresses: [email protected] F.F.M. de Almeida ,

Ž [email protected] B.B. de Brito Neves , [email protected]Ž .C. Dal Re Carneiro .´

1 Fax: q55-11-210-4958.

evolution of the South American Platform, the oldestpart of the South American Plate.

Ž .Two of the present authors BBBN and CDRCthink that it is time to update the most quoted paperon Brazilian tectonics: the classical 36-page Brazil-ian National Department for Mineral Resources Pro-

Ž .duction DNPM Bulletin 241, Origin and EvolutionŽof the Brazilian Platform Origem e EÕolucao da˜

. ŽPlataforma Brasileira , by Almeida, 1967 following.Almeida, 1966 : this broad synthesis on the Brazilian

0012-8252r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.Ž .PII: S0012-8252 99 00072-0

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Geology was written before the development of thenew global tectonics, with emphasis on the Precam-brian basement, as well as delineating the mostpromising research lines to be followed. We try toconsider as much as possible all the produced geo-logical information since the appearance of the two

Ž .papers of Almeida 1966, 1967 . All zones of theentire continent have been investigated under differ-ent detail levels and at present the overall picture isbetter known. The present progress on Brazilian

Žgeology may be evaluated in books Almeida and.Hasui, 1984; Schobbenhaus et al., 1984 , continen-Žtal-scale geological maps of Brazil 1984, scale

1:2,500,000, published by DNPM researchers andcolaborators; Delgado and Pedreira, 1995, scale

. Ž1:7,000,000 and South America DNPM, 1997,

.1:5,000,000, unpublished , although many maps andreports still remain unpublished. Most of the pub-lished material is in Portuguese, a fact that highlightsthe interest of such an essay. Facing the progress ofthe geological knowledge on other better-studiedcontinents, the authors believe that such a syntheticpicture may help comparisons.

Ž .In its modern concept, a platform or cratonrepresents a stable continental part of a plate, i.e., astable portion of the Earth’s crust adjoining one or

Žmore active mobile belts Sengor, 1990; Park and¨.Jaroszewski, 1994 . For South America, it means the

relatively undeformed portion of the continentallandmass during Mesozoic and Cenozoic times, not

Ž .strongly affected by the Andean and Caribbeanorogenic processes from Venezuela in the north to

Ž . Ž . Ž .Fig. 1. Situation map of: 1 the South American Platform; 2 Phanerozoic covers; 3 Andean fold belt.

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Argentina in the south at the northern border of theSierra de La Ventana fold belt.Our major objective is to update the paper of

Ž .Almeida 1967 and to evaluate if the available datasupport the initial definition of such platform. Someresearch needs seem to have been overcome, someremain, and new ones have naturally appeared. Even

Žthe expression, South American Almeida et al.,.1978 instead of Brazilian Platform, has been

changed, because the former fits the geological limitsof the stable portion of the continental plate betterŽ .Fig. 1 . In fact, the limits are surely not confinedwithin any geographical–political boundaries.The reader should keep in mind that such an

exercise involving 3.5 Ga of geological developmentand more than 10,000,000 km2 requires some preten-tiousness. So, naturally such an intention faces somerisks, as well as imperfections. All criticism is wel-come as a profitable tool for the improvement of afuture version.During more than three decades, the Brazilian

tectonic and geological knowledge has experienced agreat progress. Some models have guided such re-search since the middle of the 60’s. The formerinfluence of the geosynclinal theory was slowly re-placed by the original Plate Tectonics towards the

Žnew Global Tectonics see, among others, Moores.and Twiss, 1995, Kearey and Vine, 1996 . In Brazil,

basic nationwide mappings have been conducted bythe DNPM–CPRM2 system, the RadamBrasil Pro-ject, and state-owned mining and research institu-tions, as well as by private mineral companies andpublic universities. These latter have received mod-ern equipment for isotopic and geochemical analy-ses, geophysical prospecting, etc., and have gradu-ated hundreds of MS and PhD students. A largeamount of new data has been generated by stateinstitutions and by working agreements between uni-versities and foreign research centers. The growth ingeological research was recorded in 20 national geo-

Ž .logical congresses since 1967 with annals , a count-less regional symposia, and also in a series of papersin international periodicals.

2 CPRM — the Brazilian Company of Mineral Resources,aiming to operate as the Brazilian Geological SurÕey.

2. Geology

RelatiÕe stability is one defining characteristic ofa platform. The main stabilization phase of the SouthAmerican Platform was achieved by the transitionCambrian–Ordovician. The concept of platform sta-bility has been strengthened in the general scope of

Žnew global tectonics Brito Neves and Alkmim, 1993;.Park and Jaroszewski, 1994 , resulting in objective

Ž .e.g., absence of orogenic diastrophism and subjec-tive implications of the concept. For a given platformsome additional geological attributes have been rec-ognized — within well-defined time limits — inrelation to a previously defined mobile belt:- Antiquity is the first one, for the common factthat Archean and Proterozoic rocks usually domi-nate such basement domains.- Transitority is an essential characteristic, as itinvolves a long history of evolutionary tectonicphases.- DiÕersity of structural associations in the frame-work of the basement and of some well-defined

Ž .cover sequences Phanerozoic or older .Local occurrences of Precambrian cratonic sub-

Ž .lithospheric processes mantle-activated as well asŽrecords of cratonic tectonic activation lithosphere-

.activated are common; these are promoted by defor-mation associated with surrounding younger mobilebelts. Besides all these qualitative criteria are often-quoted geophysical characteristics, such as largelithosphere thickness, low seismicity rates, moderateto low heat-flow conditions and geothermal gradi-

Ž .ents, etc. Park and Jaroszewski, 1994 . All of themare rather well recorded in this platform.The records of the evolution of the platform

Ž .basement began in the Archean Table 1 . MostŽradiometric ages belong to the Neo-Archean 2.8–2.5

.Ga but there is also an important number of valuesŽ .of Meso-Archean times 3.2–2.8 Ga and a few of

ŽPalaeo-Archean ages 3.6–3.2 Ga, chiefly younger.than 3.4 Ga . The group of older ages tends to be

enlarged with the increasing sophistication of iso-Žtopic analyses see Table 1 and Brito Neves and

.Sato, 1998 . Fig. 2 is a situation map of the quotedgeographical names, rivers, states, the principal citiesand localities, but the reader is addressed to availabledetailed maps for a precise location.

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Table 1The main tectonic events in the basement of the South American Platform, from the younger ones to the oldest:Ž . Ž .10 Orogenic events of the Brasiliano collage, diachronous from a structural province to another. The last time interval 0.54–0.50 Ga is

characterized by escape tectonics and fissural magmatism.Ž . Ž .9 Sunsas–Aguapeı belt, southwestern part of the Amazonian region Brazil–Bolivia , low-grade volcano-sedimentary assemblages.´

Ž .- Cariris Velhos orogenic event a Wilsonian Cycle along the central part of the Borborema Province.Ž .8 Orogenic events in the southwestern part of the Amazonian region:

Ž .- GuaporerRondonianrSan Ignacio 1.45–1.30 Ga .´Ž .- Disputable orogenic events in the southwestern part of the Central Goias Massif ‘‘Uruacuano Belt’’, high-grade rocks and along the´

Ž .Espinhaco range Central Bahia and Minas Gerais, low-grade assemblages .Ž . Ž .7 Widespread events of extensional tectonics Statherian Taphrogenesis .

Ž .- The Rio Negro–Juruena Orogenesis arc plutonism in the central-western Amazonian region is the unique evidence for plateinteractions.

Ž .6 Trans-Amazonian Orogeneses: main deformational events of Paleoproterozoic mobile belts. Granitic plutonism.Ž .5 Some occurrences of metamorphic events, high-grade gneisses. Local juvenile rock-formation event.Ž .4 Local occurrences of metamorphic events and granite plutonism.Ž . Ž .3 High diversity of rock assemblages: orthogneisses Trondhjemites, tonalites, granodiorites, monzogranite and granites, TTG suites ,

Ž .gneissic granulites of both igneous and sedimentary origins , mafic–ultramafic complexes, noritic dikes; volcano-sedimentary pilesŽ .Grao-Para Group , greenstone belt associations, etc.˜ ´

Ž . Ž .2 Widespread occurrences of high-grade terranes, mainly orthogneisses, and the oldest known occurrences of granite-greenstone LTGassemblages.

Ž .1 Sparse occurrences of high-grade complexes.

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Fig. 2. Situation map of the quoted geographical names, rivers, main cities, states and localities.

Ž . Ž .All known up to now Archean nuclei Fig. 3have in some way been involved in the structuralframework of the mobile belts of the three majorsubsequent Precambrian orogenic events: Transama-

Ž .zonianrEburnean mainly in Paleoproterozoic times,2.2–1.8 Ga; ‘‘Late Mesoproterozoic’’, 1.3–0.95 Ga;and BrasilianorPan African, 0.9–0.5 Ga. During

such collages, plate interactions reached their cli-maxes, thus forming and successively reworking aseries of accretionary, collisional or transpressionalmobile belts, which formed different supercontinen-tal domains. A high diversity of mineral deposits as,

Ž .for instance, gold Martini 1998 was formed due tothese processes. The existing names for such super-

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Fig. 3. Archean nuclei and Paleoproterozoic mobile belts of the South American Platform.

Žcontinental collage domains are Atlantica Ledru et.al., 1994; Rogers, 1996 in the Paleoproterozoic,

Ž .Rodinia Hoffman, 1991; Unrug, 1996 at the time ofthe Mesoproterozoic–Neoproterozoic boundary, and

ŽWestern Gondwana e.g., Unrug, 1996, among oth-.ers , from the end of the Neoproterozoic to the

beginning of the Phanerozoic. There is a clear una-nimity among Brazilian Earth scientists on the con-

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cept of this later supercontinental accretion as wellas on a younger one, the Late PaleozoicrTriassicPangea.After the formation of each of these major col-

lages, there were phases of taphrogenic processesŽwith widespread continental break-up rifting, disper-

.sion, occasionally fission , thus providing the just-Žaccreted supercontinental landmass of which the

.South American Platform was part with importantsites of intracratonic sedimentation and anorogenic

Žmagmatism Almeida and Hasui, 1984; Schobben-.haus et al., 1984 . The records of lithological assem-

blages and other structural patterns for these kinds ofintracratonic Proterozoic events after the Trans-Amazonian collage are well represented in the Ama-

Ž .zonian northern part and Sao Francisco regions˜Ž . Žcentral to eastern part of the platform Brito Neves

.et al. 1995 .During the Late Paleozoic, the collage made by

the Hercynian cycle was responsible for the forma-tion of a new supercontinent, Pangea. The SouthAmerican Platform area remained relatively stableduring the subsequent break-up of Pangea, from theend of the Triassic, and during the different Creta-ceous stages of the Atlantic opening and relatedevents. The same is true in the development of thePacific and Caribbean active and transform margins,from that same time span up to now. This continentalportion has been persistently stable even if one con-siders the tectonic and magmatic processes that oc-curred in its interior, as natural cratonic responses tothe peripheral orogenies and continental break-up atthe surrounding borders.The Patagonian block, south of the Hercynian

Ž .Sierra de la Ventana northeast Argentina was notpart of the stable platform area. The Patagonianblock was diversely involved in orogenic processesduring the Phanerozoic, as can be seen by its presentshape, dimensions and position among three active

Ž .margins only one passive margin, on the east , andbecause of the relatively young thermal age of its

Žbasement mainly Mesoproterozoic and Neoprotero-.zoic .It is not an easy task to define a western limit

Ž . Ž .Figs. 1 and 3 between the platform the stable areaand the activatedrregenerated zones related to theAndean and Caribbean orogens for two main rea-sons. The lack of reliable geological and geophysical

data does not allow a good definition of the entireboundary zone. Moreover, any limit made at thisstage would be arbitrary. Generally, this westernboundary of the stable area is parallel to the western

Ž .Brazilian to western Uruguay and Paraguay border,but it is not a straight line. Actually, parts of the

ŽBrazilian territory Acre, Solimoes Basin, Pantanal˜. Žarea, etc. show some tectonic influence folding,

.shearing etc. from the Andean Chain.As mentioned before, in the basement of the

South American Platform, the BrasilianorPan-Afri-can collage succeeds two previous ones of similarextent and importance. The succession of intercon-nected Brasiliano orogeneses formed the last funda-mental tectonic and structural arrangements for thebasement of this platform. Therefore, all rocks andstructures of the Precambrian Eon, of the Neopro-terozoic and older eras, are somehow subordinated tothe framework of the Brasiliano collage.All the orogenic phases of the Brasiliano collage

are not precisely known and one should not expectthat they have been synchronic from one structural

Ž .province to another Table 1 . Some modernŽgeochronological data Chemale, 1998; Brito Neves

.and Sato, 1998 have preliminarily indicated themain events of plate interactions are ca. 750 and 600Ma, for most of the Brazilian structural provincesŽ .Almeida and Hasui, 1984 . Younger accretionary

Ž .events ca. 580–550 Ma have been detected only inŽthe southeastern part of the Platform at the Man-

tiqueira Province, Fig. 2, from Rio de Janeiro to.Espırito Santo — the Rio Doce Orogeny — and in´

Žthe southwestern part of the Platform Pampean.Province, in Argentina , but all these records need

Žadditional data. From the Neoproterozoic III ca. 590. ŽMa up to the beginning of the Ordovician ca. 500.Ma , an important group of tectonic events have

Žbeen recognized wich are connected to the latest.phases of the Brasiliano collage : collisional im-

pactogenesis, extrusion or escape tectonics, post-oro-genic collapse, etc. Many of these intracratonic post-collisional events of some provinces were coeval tothe above-mentioned youngest accretionary oroge-

Ž .nies Pampean and Rio Doce of other provinces.Stabilization of the basement structures and full

platformal conditions only appeared at the beginningof the Ordovician Period but the age for the finalindividualization of this platform is the Mesozoic

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Ž .Era after continental drift . During the Phanerozoic,pre-Ordovician Brasiliano structures provided differ-ent types of tectonic heritage for the development ofthe sedimentary platform cover in all types of basins.This is the case of the precursor Cambrian–EarlyOrdovician rifts of the Paleozoic syneclises, the ar-rangement of isopach lines in the syneclises, theinternal and external shapes of all sedimentary basinsand specially the particular case of the Mesozoic–Cenozoic rift-basins. These effects are common forall Paleozoic basins, but they are conspicuous in

Žpost-Triassic taphrogenic basins Cordani et al.,.1984 linked to the Atlantic opening.

3. The Archean

The available isotopic data for this eon can beplaced more specifically in three time intervals: 3.4–3.2, 3.0–2.8, and 2.7–2.5 Ga from the Paleo-Archeanto the Neo-Archean. The frequency order of ages isinverted due to a small critical number of isotopicdata. The values obtained up to now are not enoughto discriminate evolutionary stages for any Archeannuclei in South American. The present geographi-cal–geological areas for the Archean nuclei them-selves are relatively modest in size, even thoughthere is evidence that they were more extensive,shown by the common occurrence of reworked pro-toliths in the interior of Paleoproterozoic mobile

Ž .belts Cordani and Brito Neves, 1982 . Also, it isplausible to expect greater former extents, because ofthe embryonic stage of geochronological research in

Žthis continent RbrSr and KrAr are still are the.predominating methodologies of analyses . This sce-

nario may change in the future, with progress inisotopic research and through the use of more power-ful methods.The Archean lithostructural types, regardless of

the above-mentioned comments and age intervals,fully confirm the classical pair of terranes of the

Ž .platform basement of the world: 1 high-grade or-thogneisses of TTG-suite, granulitic orthogneisses,

Ž .mafic–ultramafic bodies; and 2 low-grade green-stone belts and similar volcano-sedimentary associa-tions. Besides these granite–greenstone terranes,other less common lithotypes have been identified in

both marine and continental environments. Amongthe less common lithotypes are: volcano-sedimentarysequences of mantle-activated rifts such as the Grao˜Para Group, Serra dos Carajas mineral province´ ´Ž .Macambira and Lafon, 1995 ; diversified sedimen-

Žtary and volcano-sedimentary sequences quartzitic,peraluminous, calc-silicate-rich, manganese and

.iron-rich rock assemblages ; as paragneisses and or-thogneisses possessing crustal affiliation, importantmigmatization processes, local mafic–ultramafic dikeswarms, etc. References for these Archean types maybe found in the Extended Abstracts of a symposiumheld in Brasilia by the Sociedade Brasileira de Ge-

Ž .ologia 1996 .Usually, larger Archean nuclei occur as central

core areas bound by Paleoproterozoic mobile belts.The size of these nuclei is variable but only a few,like the Xingu block, in the Amazonian craton, are

Ž .of large dimensions Fig. 3 . The Archean lithotypesare mainly preserved and best represented — in

Ž .order of importance — in the Amazonia Fig. 4 ,ˆŽ .Sao Francisco Fig. 5 , Goias–Tocantins, Luıs˜ ´ ´

Ž .Alves–Rio de La Plata blocks Fig. 6 . These blocksare only continental fractions of Neoproterozoicplates that played the role of cratonic areas duringthe Brasiliano collage. It is not advisable to assignfull cratonic characteristics even to these larger blocksas is usual in other continents, because of thewidespread processes of structural reworking, mag-matism and heating from the surrounding mobilebelts, specially those produced by the Trans-Amazonian collage.Smaller occurrences of Archean rock assemblages

are almost completely masked in the remote interiorŽ .of Trans-Amazonian belts Ledru et al. 1994 . They

Žare defined as basement inliers Cordani and Brito.Neves, 1982 , where rejuvenating processes are con-

spicuous and previous tectonic behavior is difficultto decipher.There are minor occurrences of Archean rocks in

the interior of all Brasiliano provinces outside theŽPaleoproterozoic structural domains including both

the above described ‘‘Archean nuclei’’ and the.Trans-Amazonian mobile belts of the Neoprotero-

Ž .zoic plates Brasiliano cratons or large ‘‘massifs’’and other large lithospheric segments. These minor

Ž .occurrences used to play two special roles: a‘‘tectonic highs’’ or local uplifted basement blocks,

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Ž . Ž .Fig. 4. A general sketch-map for the northern part of the South American Platform — 1 Guyanas and 2 Guapore shields — with´Ž .emphasis on the Archean nuclei and the surrounding Paleoproterozoic Maroni–ItacaiunasqVentuari–Tapajos, Rio Negro–Juruena mobile´

Ž .belts. The westernmost part of this Brasiliano Amazonian craton is composed of the Mesoproterozoic to Early Neoproterozoic fold belts ofŽ .San Ignacio and Sunsas–Aguapeı, in the Brazilian–Bolivian territories based on Tassinari et al., 1996 .´ ´

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Ž .Fig. 5. The central and central-eastern part of the South American Platform. The main Paleoproterozoic Trans-Amazonian mobile belts ofŽEastern Bahia and Western Bahia surround the Archean cratonic nuclei. Links among the Western Bahia Belt, Mineiro belt south of the Sao˜

. Ž .Francisco craton and the Ticunzal and Eastern Goias Paleoproterozoic occurrences have been sketched. An outline for the Neoproterozoic´Sao Francisco craton is drawn only for reference.˜

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ŽFig. 6. The ‘‘Joinville Massif’’, positioned between three Brasiliano areasrfold belts. The main central area north of Blumenau ‘‘Luıs´.Alves craton’’ is formed by high-grade Archean rocks reworked during the Trans-Amazonian collage. The Curitiba area, marginal to the

Ž .Ribeira belt ‘‘marginal massif’’ is part of the same Trans-Amazonian collage but reworked at deeper crustal levels during the BrasilianoŽ .events based on Basei et al., 1998 .

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Ž . Žand b places where Proterozoic supracrustals Meso.and Neoproterozoic in age have been locally van-

ished through drastic erosion and the basement maycrop out.It is not yet possible to discriminate Archean

cycles for the basement of the South American Plat-form due to the small number of isotopic data, andthe common reworking during later events. TheArchean terranes rarely may be considered to beautonomous structures during Paleoproterozoic at aregional scale of analysis. These terranes had short

Žperiods of stability because they were reworked at.different degrees during the successive Paleopro-

terozoic accretionary and collisional events. Amongthe available geochronological and geological datathe most important group seems to be the final

Ž .events of the Archean 2.7–2.5 Ga , ‘‘Jequie Cycle’’´and ‘‘Rio das Velhas event’’, as they have beeninformally named. At this time many continentallandmasses were consolidated, with repercussions tothe further general geological conditions of the Paleo-proterozoic.

4. The Paleoproterozoic

4.1. Distribution

Paleoproterozoic terranes predominate in most ofthe platform lithospheric blocks, with only rare ex-

Žceptions, like Pampia and Rio Apa which have.provided only Mesoproterozoic age data up to now .

Moreover, many portions of the primary extension ofthe Paleoproterozoic units and terranes have fre-quently been masked due to the tectono-magmaticreworking resulting from subsequent Proterozoic

Ž .orogenic processes such as Rio Negro–Juruena andcollages.Palinspastic Paleoproterozoic reconstructions must

take into account different types of volcano-sedimen-tary and sedimentary basins — ‘‘dalas’’, intracra-tonic syneclises, rifts, continental margins, smalloceanic basins and arc-related basins — which de-veloped on and around the rigid substratum of theArchean landmasses. Subsequently, such basinsgradually underwent further plate interaction pro-

Ž .cesses and were reworked at different crustal levels ,

reflecting a series of accretionary and collisionalorogens between 2.2 and 1.8 Ga.There are few radiometric ages between 2.5 and

Ž .2.1 Ga Machado et al. 1996 for those early Paleo-proterozoic volcano-sedimentary basin fills, ancientrestructured contexts being even rarer. A significantnumber of nonsynchronous ages are available for themain subsequent orogenic phases from one orogenic

Žzone to another Sabate et al., 1990, Ledru et al.,´.1994 : the data are distributed from 2.2 up to 1.8 Ga,

Ž .i.e., from the end of the Rhyacian 2.30–2.05 Ga upŽ .to the end of the Orosirian 2.05–1.80 Ga periods.

ŽThe development of the Trans-Amazonian belts from.northern Amazonia to Argentina is therefore as-

sumed to be a result of a series of continuous andŽinterrelated orogenic processes accretionary and col-

.lisional , or a collage, which lasted until the end ofOrosirian time. For the ‘‘stable’’ surrounding areasŽ . Ž .Archean blocks a series of coeval and latertectonic events of continental scale took place asanorogenic and post-orogenic granitic plutonism,subvolcanic, intermediate extrusive and explosive

Žvolcanism of acid character Surumu and equivalent.groups in Guyanas and Venezuela , shearing tecton-

Ž .ics escape tectonics? accompanied by syenitic in-trusives, etc. Even thick detritical sedimentary se-quences of platform type were developed locallyŽ . Ž .Roraima group in the same time span pre-1.8 Ga ,

Žwhile elsewhere surrounding the Amazonian blocks.and others , many orogenic belts of the same collage

were active.The above described records are widespread all

over the South American Platform, offering somekind of obstacles for the acceptance of the new

Ž .IUGS time-scale Plumb, 1991 , which prescribes1.6 Ga for the end of the Paleoproterozoic. Many

ŽBrazilian geologists have suggested 1.8 Ga end of.the Orosirian period to be the end of this Protero-

zoic Era, because of the continental importance ofthe Trans-Amazonian collage.

Ž .In the Statherian period 1.80–1.60 Ga , after theTrans-Amazonian collage, widespread phenomena of

Žepeirogenesis subsequent to crustal thickening and. Žgranitogenesis and taphrogenesis Brito Neves et al.,

. Ž .1995 took place in this new super continental land-mass. Such intracratonic tectonic events — an insep-arable feature of the Paleoproterozoic platform —were characterized by extensional processes, rifting

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with formation of volcano-sedimentary basins, maficŽdike-swarms, acid to intermediate volcanism extru-

.sive, explosive, subvolcanic , mafic–ultramafic plu-tonism, etc. It is possible that the extensional pro-cesses locally led to the formation of oceanic floorŽ .Pimentel et al., 1998a,b as in the western part of

ŽGoias–Tocantins massif the Juscelandia, Indianopo-´ ˆ ´.lis and Palmeiropolis sequences and in the central-´

western part of the Amazonian region, previous sitesfor the Rio Negro–Juruena belt. For the latter, thereis strong evidence for a complete evolution of anaccretionary orogen, since the formation of anoceanic floor up to a final coalescence of magmatic

Ž .arcs during this period Tassinari et al., 1996 .ŽIn the northern part of the platform the Amazo-

.nian region, Fig. 4 , the Trans-Amazonian mobileŽbelts surround the Xingu Macambira and Lafon,

. Ž1995 and Pakaraima Archean blocks Cordani and.Brito Neves, 1982 . These belts include a series of

minor Archean crustal fragments as basement inliers.Some of them received classical names in geologicalliterature such as Adampada, Imataca, Kanuku, etc.Ž .Schobbenhaus et al. 1984 . To the west, fragmentsof the Trans-Amazonian collage are still present as

Ž .interior remnants isotopically detected in the wholeŽ .Rio Negro–Juruena belt Sato and Tassinari, 1996 ;

and even further to the west of this belt, thesefragments are being found in the basement of theMesoproterozoic belts on the Brazil–Bolivia bound-

Žary zone Lomas Maneche Group, Litherland et al.,.1986 .

These facts confirm that Trans-Amazonian col-lages have dominated the whole northern part of thisplatform. In the central and eastern-central part ofthe continent, the Trans-Amazonian mobile beltsŽ .Fig. 5 also include some internal reworked Archean

Žfragments the basement of Rio ItapicururSerrinhagreenstone belt, Jequie block, Santa Izabel gneiss-´

.granulitic ‘‘belt’’, etc. and circumscribe very largeŽ .Archean blocks, like Gaviao–Lencois central Bahia ,˜ ´

ŽCampo Belo–Claudio south of Minas Gerais, Fig.´.3 and that in the central part of the Goias–Tocantins´

Ž .massif granite–greenstone terranes domain . All ofthese Archean blocks present some kind of Paleopro-terozoic reworking.

Ž .The Eastern Bahia mobile belt III in Fig. 5 ,which is sometimes called ‘‘Atlantico’’ or ‘‘Salva-ˆdor-Juazeiro’’, probably continued northwards into

Žthe basement of Borborema Province Pernambuco–.Alagoas, Caldas Brandao and Rio Piranhas massifs ,˜

where it was deeply and diversely reworked byBrasiliano and other events. The Western Bahia mo-

Ž .bile belt II in Fig. 5 crops out in small areas,because of the extensive Mesoproterozoic and Neo-proterozoic platform covers, but there are probablysouthern links with the so-called ‘‘Mineiro belt’’Ž .Teixeira et al., 1996 of the Quadrilatero Ferrıfero´ ´area in Minas Gerais. It may also extend farther westto the eastern Goias State, where some lithostructural´

Žunits of similar nature and age occur Ticunzal.Group .

For the southeastern and southern Brazilian states,Trans-Amazonian terranes often occur as part of theinfrastructure of the Brasiliano belts. They are di-versely reworked and usually difficult to be recog-nized. Many occurrences of Trans-Amazonian rockunits have been detected in the basement of the

ŽAracuaı belt Guanhaes, Itacambira-Barrocao,´ ˜ ˜.Gouvea, blocks or rock units , Paraıba do Sul beltˆ ´

ŽJuiz de Fora, Quirino-Dorania and Cabo Frio groups,ˆ. Žblocks or rock units , in the Ribeira belt Embu

.terrane . These Paleoproterozoic occurrences areŽnoteworthy in the ‘‘Joinville Massif’’ Hasui et al.

.1975 between the Brasiliano Ribeira and Dom Feli-Ž .ciano belts, in two different domains Fig. 6 . They

are found not only in the southern marginal zone ofŽ .the Ribeira belt Curitiba Domain, Siga, 1995 , where

Trans-Amazonian orthogneisses occur as paleosomesof Brasiliano migmatites, but also all over the do-

Žmain of gneissic–granulitic rocks with mafic–ultra-.mafic bodies included of the region of Luıs Alves–´

Ž .Barra Velha Luıs Alves Craton , in Santa Catarina´Ž .state, of primary Archean ages Fig. 6 . For these

Ž .Archean domains there is evidence Siga, 1995 ofŽmesozonal Trans-Amazonian reworking isotopic re-

.setting included .Important Trans-Amazonian structures occur once

again in the southern portion of the platform, as adominant part of the Rio de La Plata Craton base-ment in central Uruguay and northern ArgentinaŽ . Ž .Fig. 7 . High-grade rocks some greenstone beltsare dominant with typical east–west structural trends,orthogonal to the bordering Brasiliano belts of the

ŽPampean province, Cordoban to the west, Ar-. Žgentina and Dom Feliciano to the east, Brazil and.Uruguay .

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Ž .Fig. 7. The Neoproterozoic blocks large, intermediate and smalland the different types of Neoproterozoic fold belts surroundingand among them. The informal classification used for these beltsis that of Condie, 1989, with some minor modifications. TheNeoproterozoic blocks of the southwest part of the figure were

Ž .diversely reworked partially regenerated by Hercynian and An-Ž .dean orogeneses AA, PA, RP , and they are out of the platform

domains. CA — Amazonian; PR — Parnaıba; SL–WA — Sao´ ˜ŽLuıs–West Africa; RN — Rio Grande do Norte Caldas Brandao´ ˜.qRio Piranhas massifsqbasement of Serido belt ; SFCKA —´

Sao Francisco–Congo–Kasai–Angola; AA — ) Arequipa–Anto-˜falla; PA — Pampia; RP — Rio de La Plata; PP — Parana-panema; KAL — Kalahari.

Therefore, for most of the central, southeasternand southern parts of the platform, the original fea-tures and structures of the Trans-Amazonian mobilebelts have been diversely fragmented; they play dif-

ferent roles as pieces of the basement of youngermobile belts, each of them within a particular levelof crustal reworking. Trans-Amazonian structures are

Žwell preserved only in Rio de La Plata Craton Dalla.Salda et al., 1998 .

Common geological features of the Paleoprotero-zoic mobile belts as final results of wide plate tec-tonic interactions may be summarized as follows:

Ž .a Supracrustal sequences of extensional basinsŽof continental basement rifts, syneclises, passive

. Žmargins, etc , with predominating clastic quartzites,.U- and Au-bearing conglomerates and chemical-

Ž .clastic composition Fe- and Mn-bearing schists ,partially to strongly involved in the Trans-Amazoniandeformation such as Parima, Kwitaro, Coeroeni, Ja-cobina, Colomi, Areiao, Minas, Ticunzal, Cantagalo˜

Ž .Groups or Supergroups , etc.Ž .b Volcano-sedimentary supracrustal sequences

of oceanic affiliation, back-arc type and similar basinsŽ .active rifts , generally attributed to greenstone belts,like those of the Barama–Mazaruni Supergroup s. l.Ž .from Venezuela to Amapa , Contendas–Mirante,´

Ž .SerrinharRio Itapicuru Bahia , etc. Thesesupracrustal rocks occur in independent basinsŽ .parallel or longitudinal to the former ones and theymay also underlie the abovementioned extensionalsequences.

Ž .c Gneissic-granulitic orthogneisses, diverselyŽ .sorted rocks, of basic tholeiitic , intermediate to

Žacid tonalitic, trondhjemitic, granodioritic and¨. Ž .granitic , from low- to high-potassium shoshonitic

content, usually strongly deformed rocks, whichoriginated during Paleoproterozoic subduction pro-cesses of oceanic realms and later collisional events.

Ž .d Basement inliers of gneiss-migmatitic andgneiss-granulitic compositions of original Archaenages entirely enclosed and submitted to the Trans-Amazonian trends, which formed microplates, ter-ranes, etc. during the evolutionary history of themobile belt.

Ž . Že Some long and linear shear zones Sadowski,.1983 , some of them accompanied by late orogenic

syenitic and granitic plutons.The Trans-Amazonian collage, as postulated here,

was responsible for the widespread agglutination ofŽ .all continental and microcontinental nuclei which

were consolidated at the end of the Archean bymeans of the Paleoproterozoic mobile belts. Larger

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Žnuclei like Pakaraima, Xingu, Gaviao-Lencois, Cen-˜ ´.tral Goias, etc., Fig. 3 and smaller ones were sub-´

mitted to different styles of tectonic activation duringthe Paleoproterozoic orogenic stages and were as-sembled together by mobile belts to compose a large

Žcontinental landmass around 1.8 Ga ‘‘Atlantica’’, as.suggested by Rogers, 1996 . The importance of this

supercontinent, which is very well documented in thebasement of South American and African platforms,transcends the present geographical limits of bothcontinents. The connection of orogenic events, actingtogether to build a final supercontinental landmass isthe reason for the use of the term collage.

4.2. Tectonic and sedimentary enÕironments postdat-ing the Trans-Amazonian collage

Different types of tectonic and sedimentary envi-ronments have diachronously succeeded the Paleo-proterozoic collage, as a physical and natural conse-quence to lithospheric thickening and growth. Thereis a remarkable set of continental-scale linked fault

Ž .system mostly normal faults and related cratogenicŽ .basins rift systems, volcanic traps, minor syneclises ,

mafic dike-swarms and, even, some continental pas-sive margins. The breaking processes of that conti-

Žnental landmass and their related sedimentary mostly. Ždetritical types , volcanic acid, intermediate, mafic

.and bimodal groups, dike swarms , volcano-sedimen-Žtary, plutonic granites, anorthosites, mafic–ultra-

.mafic bodies lithological records play a special rolein this platform. These post-Trans-Amazonian crato-genic processes are part of a global phenomenon,

Ž .and a group of their earlier events 1.8 to 1.6 Gahave recently been described as being of specialmagnitude, starting the ‘‘Statherian Taphrogenesis’’,

Ž .according to a synthesis by Brito Neves et al. 1995 .It is difficult to estimate precise time intervals for themany cratogenic tectonic events — paraplatformaland orthoplataformal — which have activated thepost-Trans-Amazonian supercontinental landmass

Žduring a large time span of about 0.9 Ga from 1.8.up to 0.95 Ga of which the Statherian period seems

to have been only the first remarkable step.In the present central and western central part of

the Amazonian block and in the western part of theGoias–Tocantins block, the extensional events seem´

to have gone beyond the usual limits of a simplecratogenic tectonics. There are reliable indications,in the first area, and still disputed data for the second

Ž .area due to the lack of accurate geochronology thatthese post-Trans-Amazonian processes evolved, fromrift to drift, and then to the formation of true oceanicbasins, and from these to orogenic belts, first in the

Ž .Upper Paleoproterozoic case of Rio Negro–JuruenaŽand later on during Mesoproterozoic times probable

.case of the ‘‘Uruacuano’’ belt .During Mesoproterozoic times, many of the Up-

per Paleoproterozoic linked fault systems and inte-rior basins such as Espinhaco–Chapada Diamantinaand the Araı ‘‘belt’’ were submitted to tectonic´inversion. These included a considerable amount ofcrustal shortening and formation of elevated oro-graphic features when they were transformed intospecial types of ‘‘ensialic’’ orogens, with characteris-tic lateral transitions to weakly deformed cratoniccovers.The sedimentary, volcano-sedimentary and mag-

matic rock assemblages developed by the StatherianTaphrogenesis are present from VenezuelaŽ .Avanavero, Pedras Pretas mafic magmatism in thenorth of the continent to the northern part of Ar-

Ž .gentina Tandilia dike-swarm . Only a minor part ofthe assemblages has remained without any tectonicoverprint, such as a flat-lying cover of the litho-spheric portions that became Brasiliano cratons. Mostof the Statherian assemblages, as already mentioned,were partially or totally reactivated in the subsequent

ŽProterozoic orogenic cycles, in Mesoproterozoic like.Espinhaco–Chapada Diamantina, Uruacuano, etc. as

Žwell as in Neoproterozoic times as part of the.Brasiliano belts . There is a special case of Stathe-

rian rock associations that were only deformed dur-Ž .ing the last Brasiliano group of events, ca. 0.6 Ga

i.e., about 1.1 Ga after their primary formation, likein the Jaguaribeano fold belt, Ceara State, Northeast´Brazil.

4.3. Rio Negro–Juruena belt

The Rio Negro–Juruena belt, where a completeevolution of an accretionary fold belt is recorded for

Žthe Statherian period, seems to be unique Tassinari.et al., 1996 . It occupies a wide space of the central

Žpart of the Amazonian block from the north Vene-

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.zuela and Brazil to the south at the small Rio ApaŽ .block Brazil–Paraguay boundary . Its lateral limits

Žare still poorly defined, both to the east Trans-.AmazonianrPaleoproterozoic belts and to the west

Ž .Mesoproterozoic belts . It is mainly composed ofgneissic and migmatitic rocks of granitic, granodi-oritic and tonalitic nature, with occasional preservedsupracrustal assemblages, which follow a dominantNW–SW trend. Granitization, migmatization phe-

Žnomena and high-grade metamorphism upper am-.phibolite facies predominate along the entire belt;

granulitic rocks are local.Important series of anorogenic plutonic rocks and

some other volcano-sedimentary assemblages ofcover rocks characterize this granitic–migmatitic beltas a result of younger Paleoproterozoic to Mesopro-

Žterozoic cratogenic episodes namely Parguazense,.Madeira, etc. . The general knowledge of this belt is

not yet satisfactory; problems still remain unsolvedas the concept of the Rio Negro–Juruena belt isderived from geochonological reconaissances ratherthan from geological fieldwork. From TassinariŽ . Ž .1981 to Tassinari et al. 1996 , the amount ofgeochonological data have increased considerably.With the abovementioned circumstances, where

the coherence of various geochronological data areŽdoubtless remarkable including Sm–Nd investiga-

.tions, Sato and Tassinari, 1996 this belt has beeninterpreted as a product of the coalescence of mag-matic arcs between 1.75–1.55 Ga, following subduc-tion processes of oceanic realms and adding juvenilecomponents to the continental lithosphere. The abovedescribed anorogenic plutonic rocks and platformcovers display ages varying from 1.6 up to 1.0 Gaand most of them postdate the main belt develop-ment.

5. The Mesoproterozoic

5.1. Distribution

The geological information on the Mesoprotero-zoic in South America is largely heterogeneous bothin quality and quantity. In many aspects, the crato-genic events show some degree of similarity withthose of the upper part of PaleoproterozoicŽ .Statherian .

The exuberance of such preserved occurrencesshould be emphasized — areas of millions of square

Ž .kilometers Schobbenhaus et al. 1984 — withoutŽany similar good expositions in the world Brito

.Neves et al., 1995 , even though some of them arefound in the Amazonian rain forest. Sedimentary andvolcano-sedimentary basins of different nature andtypes, diversified anorogenic plutonism, etc., haveprivileged large portions of this supercontinent joined

Žby the Trans-Amazonian collage before and after.Rio Negro–Juruena Orogeny whose most represen-

tative areas are in the Amazonas region and in theŽ .central-eastern part east of meridian 508W Gr. of

the continent. The pre-Neoproterozoic occurrenceswere certainly larger than the present known records.Although geological knowledge is far from com-

plete, it is possible to describe a series of cratogenicbasins, characterized by different degrees of tectonicstability which display sedimentary sequencesŽ .mostly detritical rocks , volcano-sedimentary floodbasalts and anorogenic volcano-plutonism. Strati-graphic systematizations is difficult because of thelost original lateral dimensions, erosional descontinu-ities, partial or total younger tectonic overprintsŽ .Mesoproterozoic and Neoproterozoic . Additionally,there is a natural problem to be faced: hundreds oflocal informal names were introduced by differentgeologists working at reconaissance scales. This suc-cession of cratogenic events ranges in time from theLate Paleoproterozoic up to the beginning of the

Ž .Neoproterozoic from 1.9–1.8 up to 0.95 Ga .Part of these events should represent tectonic

responses of the interior of the continental plate toŽplate interactions at their margins like Rio Negro–

.Juruena, San Ignacio, Sunsas–Aguapeı, etc. . There´ ´is another series of cratogenic events displaying evi-dence of autonomous processes of sublithospheric

Ž .activation mantle-activated areas and rifts such as‘‘Cachoeira Seca’’, ‘‘Quarenta Ilhas’’, ‘‘Nova Flo-

Ž .resta’’ all of them in the Amazonian region ,‘‘Salvador-Ilheus’’ and similar mafic magmatism´

Ž .post-Espinhaco belt Bahia , etc., which have pro-vided important tholeiitic basic and alkaline magma-tism.In the interior of some Brasiliano fold belts, away

from the cratonic domains for this cycle, part of theŽsame Paleo and Mesoproterozoic rock units of a

.previous cratogenic nature are present; some of

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Žthem have already been mapped in Riacho do Pon-.tal, Aracuaı, Brasilia belts, etc. . Many others may´

occur, but the discrimination is difficult due to thetectonic overprint of the Neoproterozoic orogeneses.

5.2. The Mesoproterozoic collage

Mesoproterozoic fold belts are a minority in arealextent among the tectonic realms of the South Amer-ican Platform. With quite few exceptions, the Meso-proterozoic mobile belts were the place and target ofstrong restructuration as basements of the Neopro-terozoic orogens, which have preferred younger

Ž .structural sites younger thermal age zones for theirdevelopment. Only Mesoproterozoic mobile belts lo-cated in the interior of Brasiliano cratonic domainscould be preserved from the widespread Neoprotero-zoic regeneration.The Mesoproterozoic collage, as first assumed

here, seems to have been completed in two mainŽorogenic phases, ca. 1.3 Ga San IgnaciorGuapore,´ ´.‘‘Uruacuano’’, ‘‘Espinhaco’’ and ca. 0.95 Ga

ŽSunsas–Aguapeı, Cariris Velhos and possibly the´.‘‘Grenvillian’’, to be mentioned . Even described as

minor in areal extent, such Mesoproterozoic mobilebelts were responsible for the agglutination of a very

Žlarge supercontinent, Rodinia according to Hoff-.man, 1991 . The Paleoproterozoic orogeneses have

already prepared wide stable cratonic areas in orderto assemble such a supercontinent, to which theMesoproterozoic mobile belts contributed as comple-mentary and final agents of agglutination.

Ž .a The westernmost remnants of the worldwidenet of Mesoproterozoic mobile belts are now foundin the basement of the Andean Chain, in the northŽ . Ž‘‘Garzon-Santa Marta’’ belt and in the south ‘‘Oc-

.cidentalia Terranes’’ , therefore they are not part ofthe South American Platform basement. Part of thesewestern belts may eventually reach the basement ofthe South American Platform in the area of thePampia block, but such discrimination is beyond thepresently available data. Probably, such belts areremnants of a greater and longer Mesoproterozoic

Ž .orogenic development the Grenvillian best repre-sented and preserved in the northern platforms.In the South American Platform, Litherland et al.

Ž .1986 have distinguished in the common Brazilianand Bolivian area in the western Amazonian block

two very well-preserved Mesoproterozoic orogenicdevelopments. The older is the San Ignacio belt,´composed of schists, meta-arkoses and paragneissesŽ .ca. 1340 Ma , pierced by a considerable amount of

Ž .granite ca. 1310 Ma and characterized by migmati-zation processes; it is a kind of central nucleus. TheSunsas–Aguapei belts surround the previous centralnucleus of San Ignacio structures and are mostly´

Žcomposed of clastic metasedimentary sequences of.rifts and passive margin settings , mature and imma-

ture rocks, with some mafic–ultramafic magmaticcontributions. The ages of the igneous rocks, related

Žto the closing orogenic events pegmatites, alkaline.rocks , are about 950 Ma. All the eastern lateral

Žportions of these belts in Brazil western portion of.the Amazonian craton is marked by anorogenic

rapakivi granites, Sn-bearing, with the same ageŽ .range from 1.5 to 0.95 Ga , which seems to charac-

terize impactogenic processes on the foreland do-main. A clearer discrimination, both in area and agerecords, between the westernmost Mesoproterozoicbelts of the Amazonia and the older one of RioˆNegro–Juruena domain is an objective for furtherresearch.

Ž .b In the central part of Brazil, along the easternborder of the Goias–Tocantins massif there is a´

Ž .group of structural features rifts and rock unitsŽ .clastics, acid to intermediate volcanics of continen-tal character, generated during the Statherian Taphro-genesis. Probably, the same cratogenic extensionaltectonic processes occur west of these rifts where

Žlarge mafic–ultramafic bodies are known Barro Alto,.Niquelandia, Canabrava as well as rock units of

Ž .oceanic affiliation Juscelandia and correlatives . Allthese structures and assemblages were submitted to

Žorogenic processes, from low- the first group, in the. Žeast to high-metamorphic grade the second group,

.in the west during Mesoproterozoic times, around1.3 Ga, probably following strong events of colli-sional interaction. These orogenic processes havebeen a common source of debate, especially becauseof the masking overprint of the Brasiliano structuraland metamorphic features. Actually, this Mesopro-terozoic belt further became part of the internal

Ž .domain thick skin of the thrust-and-fold belt ofBrasilia, during Neoproterozoic times. So, the identi-

Ž .fication of the Mesoproterozoic ‘‘Uruacuano’’ di-astrophic records is generally full of obstacles and is

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even denied by some geologists. This orogeny is afact supported by a large amount of geological andisotopic data.

Ž .c Drawing a parallel arc with the Uruacuanobelt, hundreds of kilometers to the east, the Stathe-rian linked fault system of Espinhaco and WesternChapada Diamantina — from Piauı to Minas Gerais´state — was subject to tectonic inversion, withimportant crustal shortening, producing a long linearfold belt by the interaction of ancient rifted basementblocks. It is characterized by a discontinuous foldingstyle, whose intensity increases from east to west,usually low-level regional metamorphism, raregranitic magmatism and basement reworking, and a

Žproblable age within the Ectasian Period -1.4 Ga;.)1.2 Ga . Between 1.1 and 0.9 Ga an important

Ž .event of regional mafic magmatism sills and dikestook place in previously deformed rock units, indi-cating post-tectonic extensional activity of the Meso-proterozoic tectogenesis, partially masked by thestrong overprint of Brasiliano structures mainly inthe southern parts, Minas Gerais State. For both the‘‘Uruacuano’’ and Espinhaco belts, additional data— geological and isotopic — are necessary to re-solve the remaining problems.

Ž .d In the northeastern part of the platform, in thebasement of a typical Brasiliano Province, Bor-borema, south of Patos lineament, there are notewor-thy records of accretionary orogenic processes, whichhave only recently been identified. From the south-western border of the Parnaıba Basin up to the´Atlantic coast, the lithological and structural evi-dence for this belt occupy a large WSW–ESE areaabout 800 km long and over 200 km wide. Itsnorthern segment, the Pianco-Alto Brıgida beltrter-´ ´rane, displays records of bimodal and subordinateMORB magmatism and the volcano-sedimentary as-sociation of a probable forearc basin, of ca. 1.1–1.05Ga. Southwards, the segment of the Pajeu–Paraiba´fold beltrterrane is characterized by hundreds ofcalc-alkaline sheet-like bodies, stocks, batholiths andarc-related volcano-sedimentary associations, all ofthem indicating subduction and collisional processesaround 1.0–0.95 Ga. Although still the object of

Ž .ongoing investigations Van Schmus et al., 1995 ,these are the best records for an accretionary Meso-proterozoic orogeny — Cariris Velhos events — in

Žthe continent. The Brasiliano overprint folding,

metamorphism, shearing, granite plutonism, etc.,.younger than 0.9 Ga is strong. Nevertheless, previ-

ous Mesoproterozoic processes could be recognizedand differentiated with the use of SmrNd and UrPbgeochronological methods.In general, the information rescue and the recog-

nizing degree of Mesoproterozoic structures is al-ways in inverse proportion to the level of reworkingby Brasiliano structures. It also depends on the qual-ity of the available geochronological data. In theSouth American Platform basement such work is stillbeing carried out, but it is time to consider theimportance of the Mesoproterozoic collage and itsstructures, developed during two different orogenicstages. Nowadays, when the collage and fusion of a

Ž .Mesoproterozoic supercontinent Rodinia is oftendiscussed, we realize that the improving knowledgeof this platform tends to increase its importance.

6. The Neoproterozoic

The early beginning of Neoproterozoic first wit-nessed the last orogenies of the previous era and the

Žconsequent fusion of continental landmasses prior to.950 Ma . The subsequent scenery, still in the Tonian

period, all over Western Gondwana, was character-ized by diachronous taphrogenic processes graduallycompleting the fission of the Mesoproterozoic super-

Ž .continent diachronously up to 750 Ma that installeda new cycle of global tectonics, the Brasiliano itself.Two major groups of tectonic components thenstarted to interact.

Ž .a Neoproterozoic blocks, large, intermediate andsmall lithospheric fragments derived from the break-out of the previous supercontinent, which start towork out as rigid domains, such as plates, mi-

Ž .croplates, microcontinents, ‘‘terrranes’’, etc. Fig. 7 .Ž .b Different evolutionary stages of the Brasiliano

Ž .fold belts or the Neoproterozoic basins , positionedinter and intra these Neoproterozoic blocks. A practi-

Ž .cal and elementary way to represent and to classifysuch ‘‘basins’’rorogenic belts is based on their pre-inversion lithostratigraphic records. These tools arecapable of showing original paleogeographic envi-ronments and tectonic settings. Some of these pri-mary ‘‘basins’’ — syneclises, rifts, rift systems,aulacogens, gulfs, oceanic branchs, small oceanic

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Žand oceanic basins which display several informalnames as ‘‘Adamastor’’, ‘‘ANEKT’’, ‘‘Brasilides’’,

.‘‘Goiano’’, ‘‘Ribeirao da Folha’’, etc. could only˜preserve some of their original characteristics, evenafter the many stages that have gradually trans-formed them into fold belt segments. Naturally, someother ‘‘basins’’ or part of ‘‘basins’’ offer moredifficulties to identify their original characteristics.

(6.1. The Neoproterozoic blocks ‘‘the probable sons)of Rodinia’’

The major fragments of the Upper Mesoprotero-zoic supercontinent fission have worked out as platesŽ .or ‘‘subplates’’ , whose remaining continental por-tions were transformed into the so-called Brasiliano

Žcratons Amazonian, Sao Luıs–West Africa, Sao˜ ´ ˜.Francisco–Congo, Rio de la Plata, etc. , as tenta-

tively shown in Fig. 7, outlining the end of theNeoproterozoic collage. All these fragments weresomewhat reworked during the Brasiliano events,and these phenomena were especially more relevantfor the small blocks, with variable intensity fromshallow to deep crustal levels. Besides the ampleexposition of basement rocks usually defined as cra-tons, ‘‘massifs’’, basement ‘‘highs’’, etc., there is inthe infrastructure of the Brasiliano belts other direct

Ž .and indirect evidence including isotopic data ofimportant portions of the pre-Neoproterozoic base-ment, which were severely reworked and are nowpart of gneissic–migmatitic complexes.

Ž .In fact, when these blocks are figured out Fig. 7there is a certain amount of implicit subjectivity inaspects such as number, size and shape. For instance,along the periphery of the major blocks it was not

Žpossible to represent all the parcels involved and.reworked as basement of the circumscribing fold

belts, under both thin-skin and thick-skin structuralconditions.As fractions of the Mesoproterozoic superconti-

nent fission, these blocks are composed of segmentsof the Trans-Amazonian and Upper Mesoproterozoiccollages, especially of the former one. It is necessaryto consider that some blocks were substantially mod-ified and that some others have increased in area byNeoproterozoic granitogenesis, like Goias–Tocantins´Ž .whose western border is a Brasiliano magmatic arc ,Pernambuco–Alagoas, Rio Piranhas, etc. These new

features may give the false impression that they aregenerally larger than their previous dimensions, andonce again this brings problems to their correctgraphic representations.The segments of pre-Neoproterozoic collage oc-

curring in the internides of the Brasiliano fold beltsgenerally present evidence of regeneration: tectonic,thermal-metamorphic, compositional, etc. and some-times all of them together. When these lithologicalunits became ductile they may be mixed up withNeoproterozoic gneissic–migmatitic domains. Some-times, when mostly submitted to brittle tectonics

Žthey may appear as remarkable rigid flakes, slivers,.overthrusted fractions, etc. local structural features.

The discrimination of the whole pre-Neoproterozoiclithostratigraphic contexts in the interior of the manyBrasiliano fold belts is an open question. This isclear as much as these contexts are positioned farfrom the cratonic domains, at distal sites.The behavior of the Neoproterozoic blocks during

the orogenic Brasiliano processes varied to someextent, as Neoproterozoic plates and microplatesŽ .subplates , as microcontinents, as internal structural‘‘highs’’ of fold belts, etc. Besides, they were thebasement for ensialic sedimentary basins, both ininterior and continental margins. It is necessary toadd the role of ‘‘terranes’’, for some of these smallerblocks, in the sense of erratic blocks considerably

Ždisplaced from their original positions Troia–Taua,´.Guanhaes, probably Luıs Alves block, etc. . Marginal˜ ´

parts of some major blocks were occupied by Brasil-Žiano continental magmatic arcs - like west of

Goias–Tocantins, south of Pernambuco–Alagoas,´.east and west of Pampia, etc. and thrust-and-fold

Žbelts Sao Francisco Peninsula as a whole, southeast-˜.ern part of Amazonia, etc. . Therefore, the placing ofˆ

Ž .Neoproterozoic blocks Fig. 4 , as descendants of asupercontinental fission, has natural deficiencies anduncertainties.There is evidence that the interaction of some

Neoproterozoic blocks involved consumption ofoceanic realms positioned among them, thus generat-ing accretionary orogenies and subsequently colli-sional and transpressional orogenic types. When anoceanic basin was not present, the interactions of theblocks caused deformation of the continental sedi-mentary or volcano-sedimentary piles between themwith deep reworking of the basement underlying

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such basins. The available geochronological dataindicate important interactions, general convergenceactivities with subduction, first in the Criogenian

Ž . Žperiod ca. 750 Ma , in the Neoproterozoic III ca..600 ma , and in some particular cases, from the end

of Neoproterozoic III to the early beginning of theŽCambrian 580–540 Ma. From one province to an-

other, such interactions and their orogenic events arenot synchronous.Centripetal convergence of the Neoproterozoic

blocks may be assumed for the orogenic processesand tectonic consolidation of most of the provinces,like Borborema, Tocantins, etc. In all provinces the

Ž .arrrangement of blocks, as figured out Fig. 7 , afterthe Brasiliano orogeny, had a last component of

Žlateral displacement along linear shear zones linea-.ments , which seems to be connected with collisional

Ž . Žimpactogenesis and late-collisional escape tecton-.ics events. These shear zones, present in all Brasil-

iano provinces, are additionally responsible forŽseveral varied volcano-sedimentary post-tectonic,

. Žpull-apart basins and intrusions alkaline granites,.granodiorites diachronously formed from one

province to another, from ca. 590 up to ca. 500 Ma.ŽThese associated events of sedimentation immature

.continental clastic plus varied volcanism and anoro-Žgenic plutonism are considered together Alpha se-

.quence as representative of an important tectonictransition stage, preceding the general conditions oftectonic stability of the Ordovician period, duringwhich the platform started to achieve striking stabil-

Ž .ity and to develop their first real cratonic maturecover sequences.

6.2. Origin, classification and arrangement of theBrasiliano fold belts

The early origin of the Brasiliano fold belts isrelated to the formation of sedimentary andvolcano-sedimentary sites led up by the TonianTaphrogenesis, which broke up the Mesoproterozoic

Ž .supercontinent Rodinia . Subsequent convergent ac-tivities started to take place and led to the subductionof oceanic realms and the tectonic inversion of the

Ž .different systems of original Tonian and youngerbasins, continental rifts systems, interior syneclises,proto-oceanic basins, etc. The final picture was thecollision and transpressional movements conducted

by the convergence of the Neoproterozoic blocks.All these combined processes developed a complete

Ž .new global cycle Brasiliano , whose consequencewas the agglutination of a newer supercontinental

Ž .landmass Western Gondwana , Neoproterozoic toCambrian in age, which joined together only some ofthe many descendantsrfragments of Rodinia.To cover some important aspects of the Brasiliano

Ž .cycle the classification scheme of Condie 1989 waschoosen, because lithostratigraphic composition andtectonic mobility are prevailing factors among aseries of variables which allow good inferences aboutthe earlier tectonic settings of these Neoproterozoic

Ž .fold belts Fig. 7 . First, the two remarkable mainkinds of Proterozoic rock associations, QPCsdiamictite–quartzite–pelite–carbonate associationŽsyneclises, continental margins, miogeosynclines,

.etc., of proximal domains of the fold belts andŽvolcano-sedimentary association latu sensu litho-

sphere-activated and mantle-activated rifts, forearcand backarc basins, of distal domains of the fold

.belts are present in the far interior of most foldbelts. For this second more general case, three differ-ent subtypes of rock associations are distinguishedhere: BVACs bimodal volcanic–arkose-con-glomerate, turbidites; ‘‘Greenstone’’s large propor-tions of volcanic with both calc-alkaline and tholei-itic affinities, absence of komatiites; and also specialand local associations with remnants of oceanic floor

Žand subduction complexes ‘‘ophiolites and related.deep-sea sediments’’ . Another subsidiary type of

fold belts included here are those which constitutemagmatic arcs, which are usually situated onmarginal parts of the Neoproterozoic blocks, as pre-viously mentioned.The present distribution of the Brasiliano fold

belts, post-collage and post-Mesozoic continentaldrift, may be described as forming four main struc-

Žtural provinces: Borborema northeast of the. Ž .platform , Tocantins central part , Mantiqueira

Ž . Žsoutheast and south and Pampean southwest of the.platform , which have already been named and dis-

Ž .cussed by Almeida et al. 1981 . This is a kind ofpreliminary and useful geographical–geological clas-sification, even with recognized paleogeographic andtectonic connections among these provinces.Another possible approach for the arrangements

of these belts is attractive, as that emphasizing the

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chelogenic character of the belts surrounding theŽmajor Neoproterozoic blocks which have acted as

.seed-nuclei , namely: Peri-Amazonian, Peri-WestAfrica, Peri-Franciscan, Peri-Rio de la Plata, Peri-Kalahari, etc. Regarding such classification and dis-tribution, it is necessary to remark that generally the

Ž .rock assemblages as above discussed are arrrangeddisplaying lateral gradations from the Neoprotero-zoic blocks themselves to the interior of the foldbelts. This means, from more stable types, miogeo-

Žclines and similar proximal environments QPC rock.associations to more unstable types, distal environ-

Ž .ments BVAC, ‘‘Greenstone’’, ‘‘Ophiolites’’ , withthe rock associations increasing volcanic componentsto the more distal parts. The regional metamorphismand folding phases also usually displays the samepolarity, with the gradual intensification of bothfeatures towards the distal domains, far from theNeoproterozoic blocks.The informal names used to designate fold belts

may lead to problems and deserve comments, be-cause sometimes the influence of a geographicalpoint of view may be hiding some important geolog-ical facts about close relationships and previous con-tinuities among the fold belts. For instance, there arefold belts positioned among different blocks thatmay be up to 5000 km long, with no fundamentaldiscontinuities to be discussed, like that from the

Ž .Rockelides lateral to the West Africa block up toŽCordoba in Argentina western part of Rio de La´

.Plata . Different names have been used along thisŽbelt Araguaia, Tocantins, Paraguay, Cordoban or

.East Pampean, etc. in order to cover the manydifferent geographic segments rather than to statetrue different geological aspects themselves. Another

Žexample is the case of the fold belt or group of fold. Žbelts surrounding the S. Francisco peninsula just a

part of the Sao Francisco–Congo–Kasai–Angola˜.Craton which exihibits a litho-stratigraphical and

structural coherence along thousands of kilometers.First, with proximal facies of ancient continental

Ž .passive margins QPC assemblages and their lateralpassage to distal and deeper proto-oceanic andoceanic sites, up to some local ophiolitic remnants.Second, this peri-continental paleogeographic con-text was transformed into a more or less continuousarrangement of centripetally convergent thrust-and-

Ž .fold belts, with thin-skin domains proximal areas

Ž .laterally giving way to to thick-skin distal areadomains. Nevertheless, instead of a general unique

Ž .name like peri-Sao Francisco, for example many˜local geographical names based mostly on physio-

Žgraphic aspects Rio Preto, Brasılia, Rio Grande,´.Aracuaı, Rio Pardo, Sergipano, etc. were used to´

designate the same long peripheral belt, hiding theimportance of such mutual and global relationships.The initial tectonic settings for these fold belts

were rather variable, as a function of the nature ofŽthe basement, the extension factor of the Tonian

.event , the relative position to the Neoproterozoicblocks, sedimentary sources and volcanism, etc. Thesame is true for inversion tectonic conditions, inmany different interactive conditions, obliquity of

Ž .the convergence, intensity and type B or A ofsubduction, aspects of crustal shortening, etc. Somefold belts show evidence of tectonic inheritance fromthe previous Mesoproterozoic and Paleoproterozoic

Žframeworks, like in the Borborema province from. ŽCariris Velhos trends , Aracuaı from Espinhaco and´

.Trans-Amazonian trends , the northern part of Man-Ž .tiqueira from Trans-Amazonian trends and so on.

Conversely, some fold belts seem to have originatedstraight from the first structural lines of the Tonianrifting, without any apparent influence of basement

Ž .trends such as those of Araguaia east of Amazoniaˆand Rockelides, the western part of Sergipano belt,Rio Pardo belt, etc. The natural diversity of foldbelts resulting from the Brasiliano collage is stillchallenging a synthesis.Moreover, different kinds of granitization pro-

cesses have accompanied all the phases of evolutionof the Brasiliano fold belts up to the completion of

Ž .this collage and postdating it Cambrian period .Examples are the many gneissic–migmatitic com-plexes and products of migmatization processes thatcan be seen in Borborema and Mantiqueira provinces,which mostly include basement rocks and Neopro-terozoic supracrustals and which open a series ofproblems for stratigraphic classification. These show

Žspecial geodynamic conditions high isothermal gra-.dients for the Brasiliano, which is in agreement with

Žwidespread isotopic rejuvenating phenomena Rb–Sr.and K–Ar systems, mainly for most of the pre-

Neoproterozoic rock associations.The general structural trends of the South Ameri-

can Platform and its final geographical–geological

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shape, still as part of the Western Gondwana, weredominated by the network of shear zones, as alreadymentioned, which was followed by a series of other

Žlithogenetic activities sedimentary, volcanic and plu-.tonic , which lasted until the end of the Cambrian.

Among these shear belts, the position, role and be-Žhavior of the Transbrasiliano Lineament Schob-

.benhaus et al., 1984 , which transversely intersectsthe entire platform, from NNE to SSW and in sodoing practically divides two distinct arrangementsfor Neoproterozoic blocks and fold belts, should bementioned. To the west, the large Amazonian blockwith its peripheral belts is the dominating feature. To

Žthe east of it, the number of blocks of different.sizes and types of fold belts is greater, and the array

Žof the Brasiliano collage is much more complex see.Fig. 7 . There is considerable evidence for the poly-

cyclical movements along this continental shear zone,from Precambrian to Recent times, and the same istrue for many others lineaments.The primary displacements promoted along the

shear zones is not known well enough because mostof the studies have offered more qualitative thanquantitative results, mainly of reconnaissance map-ping. Some authors estimate hundreds of kilometers

Žfor strike–slip movement for the case of Trans-.brasiliano and Patos lineament, for instance have

partially been confirmed by structural, isotopic andgeophysical studies of the adjacent domains. Duringall the Phanerozoic Eon, but especially in the Meso-zoic and Cenozoic eras, these shear belts behavedlike polycyclical sites or zones of tectonic move-

Žments huge vertical displacements have been con-.firmed , as preferential sites of the tectonic heritage

as shown by the analysis of all sedimentary basinsŽ .see Cordani et al., 1984 .

7. The Phanerozoic platform cover

The stages of the litho-structural development ofthe cratonic cover of South American Platform vary

Ž .considerably and are well recorded Fig. 8 . Fromthe Cambrian period onwards they comprise count-less volcano-sedimentary and sedimentary coversŽ .some of them associated with plutonic suites stages.

ŽThe first of these or the transition stage Almeida,.1967 corresponds to the Alpha sequence, which will

be described below. When tectonic stabilization wasreached, after the Cambrian Period, volcanism wasalmost completely absent during the calmer andlonger post-Cambrian stabilization stage. During thissecond major stage, since the first half of the Ordovi-

Žcian period, true cratonic sequences the concept of.Sloss, 1988 , composed of marine and continental

sediments, started to develop successively until Tri-assic and Jurassic times.The best representation for these sequences are

now in the larger Gondwana or Paleozoic syneclises:Ž 2 . Ž 2 .Solimoes 600,000 km , Amazonas 400,000 km ,˜Ž 2 . Ž 2 .Parnaıba 700,000 km , 1,100,000 km and´

Ž 2 .Chaco–Parana 600,000 km , this last mostly in´ŽArgentina. The stratigraphy of these basins as well

.as the coastal younger ones have been revised a fewŽ .years ago, as follows: Solimoes Eiras et al., 1994 ,˜

Ž . ŽAmazonas Cunha et al., 1994 , Parnaıba Goes and´ ´. Ž .Feijo, 1994 , Acre Feijo and de Souza, 1994 , Parana´ ´ ´

Ž .and Chaco–Parana Milani et al., 1994 . The range´of thicknesses varies from 3000 to 5000 m from onedepocenter to another. The sedimentary fill of Acre

Ž 2 .basin 200,000 km , to the west of the Solimoes˜syneclise, includes equivalents of these sequences,but its evolution presents some similarities to thoseof the subandean basins. The sequences are alsopresent in the bottom of many interior and coastalMesozoic–Cenozoic rifts such as ParecisrAlto XinguŽ 2 .500,000 km Araripe, Tucano–Jatoba, Barreiri-´

Ž .nhas, Sergipe–Alagoas, etc. Fig. 8 .The cratonic sequences are separated from each

other by interregional unconformities; they goŽthrough five major tectono-sedimentary cycles six,

.if the Alpha sequence is also included , agreeingŽ .with Soares et al. 1974, 1978 . These sequences

correspond to successive major events of the plat-form surface lowering below the regional base level

Žand its subsequent uplift. Each of these cycles des-.ignated by a Greek letter from alpha to zeta is an

assembly of stratigraphic groups and formations, evenof isolated beds, in some cases, between regionalunconformities. Many problems and obstacles are

Žcommon for thickness estimates after previous ero-.sional events of the former sedimentary column, as

changes of thickness due to compacting, post-deposi-tional geometric modifications, poorly definedandror insufficient chronological data, etc. Never-theless, this approach seems to be the most practical

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Fig. 8. General records of the post-Paleozoic activation of the South American Platform, with emphasis on the following: 1. Archean andŽ . Ž . Ž . Ž .Paleoproterozoic domains: 1, 2 Guyana and Guapore shields; 3 Sao Francisco craton; 4 Rio de la Plata Craton covered ; 2.´ ˜Ž . Ž . Ž . Ž . Ž . Ž .Sedimentary coÕer, including A Subandean basins; B Solimoes basin; C Amazonas basin; D Parnaıba basin; E Parana basin; F˜ ´ ´

Ž . Ž .Chaco-Parana basin; G Parecis basin; H Alto Xingu basin; 3. Andean belt; 4. Exposed Upper Jurassic–Lower Jurassic Õolcanic rocks;´(5. Main dike swarms; 6. Triassic alkaline rocks; 7. Upper Cretaceous alkaline rocks; 8. Lower Cretaceous alkaline granites and Õaried

)Õolcanism ; 9. Tertiary alkaline rocks; 10. Southernmost boundary of the platform.

way to synthesize all the cover stages of the platformand its Phanerozoic tectonic history.

The paraplatformal Alpha sequence actually rep-Ž .resents deposits late to post-tectonic and associated

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magmatism of the then recently agglutinated Gond-wana continent, rather than a real cratonic sequence.After this stage of transition in the general gradualand diachronous tectonic conditions, from mobilebelts to stable cratonic domains, the Paleozoic Gond-wana basins from the Ordovician up to Jurassictimes received the deposition of four true cratonic

Ž .sequences Beta, Gamma, Delta and Delta-A , ofmarine and continental environments, with their nat-ural particuliarities from one basin to another. Dur-ing these times, the closest plate border was the

Ž .Pacific margin Zalan, 1991 , and its complex his-´tory of accretion and microcollision caused manybase-level changes and influenced these sedimentarycratonic covers and their unconformities.In post-Paleozoic times the interior of the plat-

form started to be intensively activated due to tec-tonic processes of formation of the present activeand passive margins of the South American conti-nent. This complex actiÕation stage and its corre-sponding sedimentary and volcano-sedimentary

Ž .recording ‘‘Epsilon sequence’’ show important dif-ferences and variations, both in physical space and in

Žtime, i.e., from the Upper Paleozoic first recordings.in the northern part of the platform to Upper Creta-

ceous. In the Guyana shield, at the northern part ofthe platform, Triassic rifts and mafic dike swarmsare synchronous to the opening of the North Atlantic.In the southern part of the platform, important traps

Žof basaltic magmatism Serra Geral Group and rela-. 2tives , over 1,000,000 km , were mostly formed in

early Cretaceous times. Along the coastal Atlanticarea this stage of evolution presents first a series ofrifts with associated mafic magmatism, followed byproto-oceanic domains, gulfs and the alike, near themiddle part of Cretaceous. Actually, Epsilon se-quence does not strictly follow the general require-ments for a cratonic sequence. However, this is apractical and useful way to group a series of interre-lated episodes and their lithogenetic products, at leastdue to expositive reasons.

Ž .The last cover sequence Zeta mainly assemblesthe sedimentary phases connected with the individu-alization of the South American Platform, i.e. thosehaving the Atlantic shoreline as the regional baselevel. It is indigenous and it was formed pari passuwith the last geomorphological evolution of thisplatform. It is also representative of younger stages

Ž .of sedimentation and slight magmatism when tec-Žtonic quietness started to be restored a gradual and

.diachronous restabilization , from the Upper Creta-ceous to the Present, replacing the previous dramaticevents of the actiÕation stage.

7.1. Transition stage — Alpha sequence

ŽThis sequence includes sedimentary mostly im-.mature continental clastic , volcano-sedimentary

Žacid to intermediate rocks and some mafic magma-. Ž .tism are common with plutonic anorogenic rocks,

from the Neoproterozoic III up to the end of theCambrian, diachronous from one ‘‘basin’’ to an-other, following the different steps and times ofconsolidation of the four main structural Brasilianoprovinces. It is naturally complex and diversifiedfrom place to place, and generally fills rifts andpull-apart basins of modest sizes, which often cropout on the periphery of the syneclises. A basalunconformity to Neoproterozoic lithostructural rockunits is common and some of them reveal constraintsof a previous larger size, preceding the Phanerozoicerosional phases. A generally well-marked upperunconformity is defined with the Ordovician andror

ŽSilurian sediments Beta or Gamma sequence, the.latter being the most common case .

In the northern part of the platform, representativerock units for this sequence have not yet been recog-nized. Alkaline ultramafic bodies occur along the

Ž .axis precursor rift system of the AmazonasŽ .syneclise, Cambrian in age ca. 500 Ma , attributed

to late-Brasiliano impactogenesis led by the AraguaiaŽ .belt Peri-Amazonian .The best representations of cratonic cover rocks

Žare found in the molassic foredeeps Alto Paraguai,.Lagarto-Tobias Barreto, Itajaı, CamaquarGuaritas´ ˜

Ž .and some minor intradeeps Jua, part of Camarinha´of the Brasiliano provinces. Similar deposits are also

Ž .present in post-collisional extrusion pull-apartbasins, always associated to extensional phases of

Žthe major shear belts Jaibaras, Cococi, Piranhas,.Camarinha, etc. ‘‘basins’’ and ‘‘subbasins’’ , whose

previous larger size may be inferred from their lo-cally preserved remnants. Volcano-sedimentary se-quences filling these basins are hundreds to thou-sands meters thick, mostly with immature clastic

Ž .deposits plus volcanic rocks and discontinuous

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folding style. The brittle structures dominate. Graniticplutonism is rare, except in the case of the ‘‘Joinville

Ž .Massif’’ the northern part of the Luıs Alves block ,´where an important suite of alkaline to peralkalinegranites make the main elevations of the Serra do

Ž .Mar Almeida and Carneiro, 1998 , from Sao Paulo˜to Santa Catarina states, in close association withmany occurrences of these ‘‘basins’’.These types of ‘‘basins’’ are related to the pres-

Ž .ence of shear belts basin-forming tectonics andthey were protected against younger erosion episodesdue to the close presence of a Lower Paleozoic covermostly belonging to the Gamma sequence. The num-ber and size of the ‘‘Eo-Paleozoic’’ occurrencesdecreases sharply where these conditions are notfound.Moreover, there is preliminary evidence of larger

and thicker areas of the occurrence of such basinalrocks beneath the Parnaıba and Parana syneclises,´ ´occupying the precursor triggering grabens for devel-opment of these Paleozoic basins. Cambrian depositsof this type have been found by deep wells alongtheir main depositional axes.

7.2. The Stability stage

The relative tectonic quietness from OrdovicianŽup to Upper Jurassic at some places, these tectonic

conditions stopped in Triassic times, and even before. Žthem was defined as the stability stage Almeida,

.1966, 1967 . It was developed under orthoplatformalconditions, when continuous and mature cover se-

Žquences of marine mostly, but not exclusively,.Lower Paleozoic and continental provenances could

widely be formed, even beyond the present erosionalboundaries of the major syneclises and rifts.Unconformities of an interregional character are

used to limit successive cratonic cover sequences:Ž . Žthe Beta Ordovician–Silurian , Gamma Devo-

. Žnian–Lower Carboniferous , Delta Upper Carbonif-. Ž .erous–Triassic and Delta-A Triassic–Jurassic are

composed of mature sediments. Magmatism wasŽpractically absent during this long stage over 350

.Ma in some basins , though its end is diachronousand generally marked by Mesozoic basaltic magma-tism, locally starting to appear at the end of the

Ž .Permian period Amazonas Basin .

7.2.1. The Beta sequenceThe Beta sequence is formed by continental sedi-

ments presenting transitions to fossiliferous marinedeposits, thus characterizing the first important ma-rine trangression on to the recently consolidated

Žplatform, from west to east Solimoes-Amazonas,˜. ŽParana basin and from south to north Parnaıba´ ´

.basin . Its lower limit is very well marked by thepost-Cambrianrpre-Ordovician unconformity and itsupper surface limit is represented by the Eo-De-vonian unconformity.In the Solimoes basin, the Benjamin Constant˜

Formation is the oldest marine sedimentary record-ing for this sequence and of all Brazilian basinsŽ .sandstones and black shales, at subsurface . All

Ž .syneclises and many interior some coastal riftspresent a fair representation for this sequence, withcontinental and subsequent predominant marine sedi-ments. Among the former can be mentioned someminor occurrences of shallow marine and glacial

Ždeposits in Amazonas, Parnaıba and Parana Assine´ ´.et al., 1994 syneclises. The sediments of this se-

quence often crop out in the periphery of the basins,but most of them occur under subsurface conditions

Ž .as in the Parana basin Assine et al., 1998a,b .´

7.2.2. The Gamma sequenceThis sequence is present in all Brazilian syneclises

as well as in the main paleozoic rift systems. Itcorresponds to a complete transgressive–regressivemarine cycle, from the Early Devonian to the end ofthe Lower Carboniferous, limited at the top and thebottom by two important interregional unconformi-ties.In the Solimoes basin there are recordings of˜

marine and glacio-marine sediments. In the Ama-zonas basin the recording of a complete sedimentarycycle begins and ends with deltaic-fluvial sediments,successively passing throughout neritic, euxinic,glacio-marine and neritic facies. In the Parnaıba´syneclise and in the Tucano–Jatoba rift system,´recordings are also complete and similar to that ofAmazonas, but with remarkable erosional uncon-formity and stratigraphical vacuity at its top. In theParana syneclise the recordings for the first phases of´

Ž .the transgressive cycle Parana Group , from con-´glomerates and clear sandstones to black shales, are

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very well documented, but a strong erosion eventremoved the sediments of the regressive phase. Thesame rock units of Parana are found in the bottom of´the rift systems of the Parecis basin.This is the most general stage of the Lower

Paleozoic covering in the South American Platform,under stable tectonic conditions and strong marineinfluence. The huge erosional unconformity of its topis a platform milestone that was attributed to theinfluences of the orogenic Hercynian events on the

Ž .western margin of the continent the Andean Chain .

7.2.3. The Delta sequence and the Delta-A ‘‘subse-quence’’The last Paleozoic tectonic-sedimentary cycle has

a complex evolution, limited by an Upper PermianrEo-Triassic unconformity. A climatic and paleogeo-graphic differentiation can be observed in the sedi-mentary recordings of this sequence, from the north-

Žern semi-arid conditions, fluvial and marine sedi-. Ž .ments to the southern glacial, glacio-marine basins.

The Solimoes and Amazonas syneclises display˜continental sediments with marine intercalations, withtypical and important evaporitic deposits. In theParnaıba basin, this sequence is characterized by low´subsidence rates, shallow marine to continental sedi-ments and progressive evidence for desertic environ-ments from their basal beds to the top.Permo-Carboniferous glacial deposits are widely

Ž .distributed in Gondwana Eyles, 1993; Smith, 1997 .In the Parana syneclise the delta sequence is charac-´terized by thick glacial deposits with marine interca-

Ž .lations five different horizons . Semi-arid to aridconditions are recorded only near the top of thesequence. The final phase of the tectono-sedimentarycycle is composed of terrigenous sediments, fromlitoral and shallow marine zones whose upper limitreached the Triassic ages. Similar rock assemblagesare present in the Parecis basin, and there are alsosome remnants of this sequence preserved in other

Ž .interior Tucano–Jatoba and even in some coastal´Ž .rifts Sergipe–Alagoas .

ŽThe subsequence Delta-A was proposed Soares.et al., 1974, 1978 to assemble the sediments of

desertic environments prevailing at the top of theprevious sequence, characterizing wide continental

Ž .conditions geocracy of Pangea , from the Early

Triassic period. Sometimes this subsequence is sepa-Ž .rated from the lower main sequence Delta by local

unconformities. This subsequence represents one ofthe largest deserts in the history of the Earth whichcovered areas of the post-Hercynian supercontinent.

Ž .This post-Triassic part Delta A of the same generalŽ .development since the end of Lower Carboniferous

of the cycle has been separated as a subsequence, butthis is not a generally accepted concept. The end ofthis subsequence is diachronous and it is related with

Žthe early breakout of the supercontinent from the.Permian to Lower Cretaceous , marked by rifting

processes and basaltic magmatism. This new seriesof tectonic-sedimentary realms then developed werebest defined as part of the Epsilon sequence.

7.3. The post-Paleozoic actiÕation — Epsilon andZeta sequences

As previously emphasized, the South AmericanPlatform became individualized as the western part

Ž .of Pangea West Gondwana in the Cretaceous, andsince then it has been separated from the African

Ž .Platform. The initial taphrogenic rift systems, gulfsprocesses preceding such a drift were generally initi-ated in the Triassic period, even a little earlier, it had

Žits culminating in the Lower Cretaceous ca. 120–130.Ma , with diachronic continental manifestations

Ž .tectonic and lithogenetic processes all over thecontinent. These phenomena were first described as

Ž .‘‘Wealdean reactivation’’ Almeida, 1967 and laterŽ .on as ‘‘Mesozoic activation’’ Almeida, 1972 . Most

of the papers on such a complex subject consideredit as reflecting the continental drift, but it is neces-sary also to add and to remember the influence of the

Žcoeval processes of orogenetic interactions subduc-.tion, microcollision, faults on the Andean and

Caribbean margins of the plate.A review of these processes will follow the differ-

Žent geographical–geological areas different struc-.tural provinces , from north to south, which have

different behavior in terms of the most importantfeatures.

7.3.1. The Guyana shieldThis was the first region of the platform where the

processes of activation took place, especially that ofbasic magmatism. Eo-Triassic up to Eo-Jurassic

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Ž .ca. 225; ca. 180 Ma dike swarms, NNW–SSEŽ .trending are common Cassipore diabases from´

Amapa up to the French Guyana territory, and from´there to the continental boundary. Dikes of the samenature and age are also common in the states of

Ž .Roraima, Para and Amazonas Brazil as well as in´Suriname and Guyana. Such processes of magma-tism and rifting are considered to be related to theopening of the North Atlantic, which was effectiveall over the Guyana Shield.

ŽThe Takutu rift attributed to the same tectonic.cause is a NE–SW trending semi-graben about 3000

km long, 30–50 km wide, which developed fromŽ . Ž .Roraima Brazil to Guyana along older reactivated

Proterozoic structures of the Guyana Central Pre-Ž .cambrian shear zone Costa et al., 1991 . The gen-

eral structure seems to be controlled by normal faults— reactivated along the main trend of the shear zone— and by some NW–SE trending transcurrent faults.The preserved volcano-sedimentary pile exhibitsabout 1500 m of basaltic flows, which dated from180 up to 150 Ma — the Apoteri Formation —

Žcovered by 5500 m of clastic sediments minor.clastic–evaporitic , mainly siltstone red-beds and

deltaic sandstones of the Late Jurassic to LowerCretaceous ages. Mesozoic alkaline plutonism is alsopresent, as the examples of Catrimani syenite of

Ž .Roraima Fig. 8 , with age ca. 100 Ma and thealkaline ultramafic and carbonatitic province of Seis

Ž .Lagos, in Amazonas state Rio Negro valley , whichwas tentatively attributed to the Upper TriassicrEo-Cretaceous ages. A very thick Nb-bearing duricrustcharacterizes the Seis Lagos region, from whichsamples of the fresh bedrock have not yet beenobtained.

7.3.2. The Amazonas basinThe Amazonas syneclise itself is usually divided

into three major subbasins, as follows: Solimoes˜Ž .Alto Amazonas , Medio Amazonas and Baixo Ama-´zonas. All of them received continental sedimentsŽ .Alter do Chao Formation from the Albian stage to˜

Ž .the Upper Cretaceous Turonian stage and severalCenozoic covers. The same is true for the western

ŽAcre basin, at the boundary of the platform west-.ward of Amazonas , under the Andean zone of influ-


Countless diabase sills and dikes intruded into thePaleozoic lithostratigraphic pile, with a known maxi-mum thickness of 809 m observed in the Solimoes˜

Ž . 3subbasin. Aires 1983 estimated about 340,000 kmfor the whole volume of intrusive magmatic material,pointing out that the total thickness of the sills islarger under two main general conditions: where thebasement of the basin is deeper and where the totalthickness of post-diabase beds is larger. Moreover,these large thicknesses of basaltic magmatism aregenerally situated where the major Precambrian shearzones cross cut each other. The oldest known sills

Žare from the beginning of the Triassic period ca..250 Ma ; therefore, coeval to the above-mentioned

Cassipore diabase. A younger episode of sill and´dike intrusion is known from the end of Jurassic tothe Neocomian stage.The influence of the Andean orogenesis on this

region has been less investigated and is little known,partially due to of the considerable extent and thick-

Ž .ness of the younger cover sequences Zeta . Influ-ences and tectonic responses for these Andeanorogenesis are recognized in the Solimoes Basin,˜

Ž .situated between the structural arches of Purus eastŽand Iquitos this latter separating Solimoes from˜

.Acre-Pastaza, a sub-Andean basin and having as aPrecambrian basement rocks of the Rio Negro–Juruena belt. Detected faults and folding systems ofLate-Jurassic and Eo-Cretaceous ages have affectedreal Paleozoic cratonic sequences, and the sourcesfor such stress fields have their origin in the subduc-tion processes of the Nazca plate. There is a veryimportant fault zone, the ‘‘megasuture of Solimoes’’˜Ž .Caputo and Silva, 1991 , about 1000 km long,developed by a system of faults N70–80E trending,which is a good example of intraplate tectonicsderived from the active margin.

(7.3.3. The Guapore shield south of Amazonas-´)Solimoes basin˜

Once again, the most conspicuous events of thepost-Paleozoic activation for this southern part of theAmazonas basin are recorded by the occurrences ofdiabase sills and dikes, also piercing rocks of thebasement and many others of Proterozoic andPhanerozoic evolution. Basaltic flows are present inboth Mato Grosso and Rondonia states, with Jurassicˆ

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and Cretaceous ages. The well-known basaltic flowsof the southwestern part of this shield, Tapirapua and˜Anari, are the northernmost occurrences of magmaticrocks affiliated with those of the Parana Basin´Ž .Epsilon sequence . They are covered by Cretaceous

Ž .sandstones of the Parecis Formation Zeta sequence ,which is correlated to the Bauru Group, to the south,in the Parana Basin. Some kimberlite intrusions,´Cretaceous in age, are present in this western part ofthe shield, in the states of Mato Grosso and Rondonia.ˆ

7.3.4. The northeast regionDuring Late-Jurassic and Eo-Cretaeous two main

graben systems, SW–NE trending, were formed inŽthis region, divided by basement tectonic highs Lima

.Filho et al., 1996, among others . The Afro-BrazilianŽdepression Central Atlantic, with CaborPer-

nambuco, Sergipe–Alagoas, Reconcavo–Tucano–.Jatoba basins and Bahia–Espirito Santo basins and´

ŽAraripe–Potiguar from west Pernambuco to the Rio.Grande do Norte coastal area, Equatorial Atlantic ,

whose sedimentary remnants are trapped and pre-served in the bottom of the deeper grabens, receivingmany informal names of basins and subbasins. They

Ž .present recordings for pre-rift Paleozoic sequences ,Ž .rift and gulf stages Epsilon sequence before the

Žmarine sediments of Atlantic provenances Zeta se-.quence , from different Upper Cretaceous stages

Ž .post-Albian .In the interior of the Parnaıba syneclise, there are´

widespread continental sediments associated to rift-Žing and basaltic magmatism ‘‘Alpercatas amphi-

.clise’’, after Goes and Coimbra, 1996 . Over 150 m´Žof fluvial-lacustrine deposits Corda and Pastos Bons

.Formation were deposited at that time. Triassic andJurassic basaltic flows and diabase intrusions occur

Žon the western part of the basin ‘‘Mosquito’’ mag-.matism , while younger Eo-Cretaceous sills, dikes

and minor basaltic flow are present in the easternŽ .part ‘‘Sardinha’’ magmatism , associated with some

clastic sedimentary rock units. The first Mesozoicoccurrence of marine-related or marine sedimenta-tion is recorded in this area, as well as in thepreviously mentioned rift system about the middle

Ž .part of Cretaceous after the Albian–Aptian stages .The Brasiliano Borborema Province, part of the

shield to the east of the Parnaıba syneclise, presents´many occurrences of basaltic magmatism, mainly

intrusions of tholeiitic nature. The relationship be-tween the basaltic magmatism of the Borborema

ŽProvince and their ancient tectonic features activated.in the Mesozoic time is not always clear. Occur-

rence of tholeiitic basalt magmatism generally coin-cides with fault lines that formed the sedimentarybasins, a reality both for the interior and for thecontinental margin areas.There are several hypotheses for these occur-

rences of magmatism as that based on the clockwiseŽmovement of the South American plate Francolin

. Žand Szatmari, 1987 , following a rotation pole about.398W Gr., 78S which caused N–S extension stresses

in this part of the plate and E–W compression at theŽ .same time Neocomian in the African counterpart

Ž . Žthe area around Cameroon . The so-generated or.similar stress fields have thus reactivated many

Precambrian fault lines of the Borborema Province,according to the different stages of the continentaldrift. They initiated a series of Mesozoic grabens andfavored intrusion of diabase dikes around 120–130Ma, especially from central and south Ceara to Rio´Grande do Norte State, which present general E–Wtrends, parallel to the southern border of Potiguar

ŽBasin Ceara MirimrCabugi magmatism and related´.events .

ŽIn the coastal area, south of Recife CaborPer-.nambuco basin it is necessary to mention the Ipo-

juca volcanism of probable Albian age, which com-prises rhyolites, trachytes, basalts, different types oftuffs, etc. of the same span of time as the alkaline

Ž .granite of Cabo Santo Agostinho ca. 100 Ma , theunique Phanerozoic anorogenic granites of the plat-form. In the domains of the Potiguar basin, there aresome local occurrences of alkaline basalts of

Ž .Oligocene to Miocene ages between 45 and 29 Ma ,cutting across most of the sedimentary pile andprevious Mesozoic basaltic occurrences. During theOligocene some alkaline necks were positioned inthe surroundings of Fortaleza, in Ceara State.´

7.3.5. The Sao Francisco Craton and its eastern˜Aracuaı Fold Belt´The Sao Francisco Craton behaved as a very˜

stable area during the events of the post-Paleozoicactivation. Rifting and sedimentation processes are

Žpresent in its western part Eo-Cretaceous, connected.with the Parnaıba Basin and along the Atlantic´

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Coastal Area. In spite of the importance of verticaltectonics, the occurrences of mafic magmatism aresurprisingly scarce and local.

ŽIn the area of the Serra do Espinhaco Proterozoiccover sequence of the craton, partially reworked by

.the Brasiliano Aracuaı Fold Belt , at Minas Gerais,´diabase dikes have isotopic ages from 220 up to 170

Ž .Ma Dossin et al., 1995 . Towards the coastal areaŽthere is a linear Precambrian shear belt found in

high grade rocks of the distal segment of the fold.belt , NNW–SSE trending, about 200 km long, from

Ž .Vitoria Espirito Santo to the north–northwest,which was reactivated and it was penetrated bydiabase dikes ca. 170 Ma.

7.3.6. The Parana and Chaco–Parana Basins and´ ´surrounding areasContinental sediments with low subsidence rates

characterize the evolution of the Parana Basin in´ŽBrazil Piramboia and Rosario do Sul formations,´ ´

.Delta-A sequence and homologous basins inŽUruguay, Paraguay and Argentina Chaco–Parana

.Basin . From Late Jurassic to Lower Cretaceous,these basins accumulated widespread, but not very

Žthick, aeolian deposits Botucatu Formation in Braziland their stratigraphic correlatives in the surrounding

.countries .The Botucatu Formation is outstanding in the

Ž .Parana Basin Almeida, 1953 for the large distribu-´tion and relatively uniform thickness. Dune fields arerecognized in the sandstone beds. They were affectedfrom the Neocomian onwards by the widespreadbasic magmatism of Serra Geral Formation. Takinginto account the South American and African basins

Žtogether, this area of desert sedimentation Delta-A. 2subsequence was over 2,000,000 km , and it may be

Ž .classified as the ‘‘Dala’’ table land basin typeŽ .Botucatu Dala which was immediately succeededby the mafic magmatism and related unstable eventsof the Epsilon sequence.In the Parana Basin domal structures and linear´

arches are common, parallel or not to the limits ofthe basin. Many were formed during the activationstage. Some, like the N–S Asuncion Arch, on the´west of the main basin, presented a general trend forupward movements from Devonian times. The PontaGrossa Arch has been pointed out as a wide domalstructure prevailing before the continental drift; its

Ž .present features Zalan et al., 1991 were developed´after the Triassic period. Many other structural posi-tive areas with upward movements appeared with theactivation processes: The Paranaıba Arch, Rio Grande´Arch, Bom Jardim Arch, the Serra do Mar features,and others. These areas of positive tectonic move-ments are responsible for the main outline of thebasin after the Jurassic and Cretaceous times; theyhad an important role on the emplacement of thealkaline intrusions.The first important magmatic episodes were

marked by emplacement of alkaline rocks of EasternParaguay during the Eo-Jurassic or even Late Trias-sic times. Diabase dikes of Late Jurassic gave way tothe basaltic flows of the Serra Geral during the EarlyCretaceous. The whole area occupied by the basalticmagmatism is over 1,200,000 km2; with local

Ž .isolithic lines including sills that may reach up to1700 m. The estimated total volume of magmaticmaterial is about 1,000,000 km3. This is, by far, oneof the most important occurrences of flood basalts ofthe world, both in terms of size and of volume. Thefirst flows of basaltic material are intertrapped witheolian sediments of the Botucatu Formation, at dif-ferent stratigraphic levels. Similar lava flows are also

Ž .present in Uruguay Arequita Formation , ParaguayŽ .and Chaco–Parana, in Argentina Russo et al., 1980 .´

The magmatic processes were relatively very fast,most of them probably lasting around 10 millions of

Žyears, between 138 and 127 Ma after Turner et al.,.1994 . It is still possible that these processes were

Ž .still faster, according to Renne et al. 1992 , only 3Ž .or 2 million years based on ArrAr data . Most of

the basaltic flows present tholeiitic characteristics,without olivines. Only in the southern states of Brazilare there occurrences of tholeiitic andesites, rhyolites

Ž .and rhyodacites Melfi et al., 1988 . In the southernŽpart of Brazil as well as in Etendeka Namibia,

.Africa , the volcanic rocks are intercalated with ap-proximately a half hundred meters of rhyolitic lavaflows. The basalt-rhyolitic suite of the Palma type, at

Žthe Parana State, southern Brazil Piccirillo et al.,´.1988b and the equivalent low-TiO suite in Etendeka2

Ž .Erlank et al., 1984 are in correspondence both inage and composition. After an isotopic study Piccir-

Ž .illo et al. 1988a have concluded that the origin ofthe volcanism in the Parana Basin is related to a´distinct mantelic source, of lithospheric type, and not

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to any mantelic isotopically homogeneous material,Ž .of asthenospheric type e.g., mantle plume . Turner

Ž .et al. 1996 also inferred that more than one sourcein the lithospheric mantle has formed the volcanism.The Tristan da Cunha plume according to theseauthors was greatly passive in the process, contribut-ing with the heat that made easier the remobilizationof the ancient lithospheric material. The associationof the basalts to acid lavas seems to support theirconclusions.Diabase dikes are known in the Parana basin and´

all the surrounding crystalline areas of its basementŽ .see Almeida et al., 1996 . The mafic dikes of theSerra do Mar, between Sao Paulo and Rio de Janeiro˜Ž .Fig. 8 are associated to lamprophyres and otheralkaline rocks, following NE–SW structural trends

Ž .inherited from the basement Mantiqueira Province .Another remarkable group of mafic dike swarms arethose parallel to the main axis of the Ponta Grossaarch, with a general NW–SE trend, that presentLower Cretaceous ages, from the litoral area up tothe interior of the Parana basin itself where they´

Žoccur cutting across previous basaltic flows Ussami.et al., 1991 .

The alkaline magmatism is rather variable in termsof rock types, saturated and unsaturated rocks, leucoand melanocratic rocks; it includes some ultrabasicrocks, carbonatites and kimberlites. The favorite sitesare found in a sedimentary basin interior, not farfrom their external boundaries, mainly in upwardtrending zones and reactivated fault lines. The ar-rangement of the intrusive bodies used to vary, someof them have remarkable linear features, like that

Ž .from Jaboticabal interior of the Parana Basin to´Ž .Cabo Frio Rio de Janeiro coastal area and possibly

Žup to the Saldanha da Gama bank on the continental.shelf , performing a curved zone of about 1500 kmŽ .long about 60 km wide , where 26 varied alkaline

intrusions are positioned, also including some occur-rences of volcanic flows in the interior of the Guan-

Žabara, Volta Redonda and Rezende grabens Rio de. Ž .Janeiro state . Almeida 1991 proposed that such

Ž .alkaline bodies Fig. 8 formed a small circle relatedto the rotation of the South American plate duringthe opening of the South Atlantic from the Campa-

Ž .nian to the Eocene stage from 87 up to 42 Ma .The Parana basin presented low rates of subsi-´

dence after the episodes of basaltic magmatism,

probably related to thermal effects. During the LateŽ .Cretaceous Santonian–Maastrichtian , a new and

autonomous basin was born. It is an interior syneclisefilled up by the Bauru Group, about 300 m of

Ž .continental sediments Zeta sequence , with somebasal volcanic intercalations, whose depocenters co-incide with the area of maximum thickness of theunderlying Serra Geral basalts. To the north, somemarginal elevations separate this basin from its co-

Ž .eval and probably homologous Alto San Franciscanbasin, which also belongs to an Upper Cretaceousage.

7.3.7. The South–southeast region of BrazilThis is a region where the activation phenomena

were much more intensive, including special phasesof vertical movements up to now which form un-usual landscapes. The region placed on the continen-tal shelf to the slope facing the Santos and Camposbasins has as structures of the Ribeira Fold BeltŽ .central part of Mantiqueira Province as basement.The Santos Basin is among the largest and deepestsedimentary basins of the South Atlantic continentalmargin. Its depocenters locally reach depths of 13km. By its turn, the adjacent uplifted continentalarea, between Santa Catarina and Rio de Janeirostates, is a striking plateau with a huge line of

Žescarpments facing the sea usually called Serra do.Mar , whose normal elevations are between 1500

and 800 m, the highest peaks reaching 2200 m.Along the Serra do Mar there are some normal

fault zones, parallel to the previous structures of theBrasiliano fold belt, a series of half-grabens, tilted tothe NNW, and filled up by Cenozoic continentalsediments. This group of sedimentary basins is pre-

Ž .sent from the Parana Curitiba basin up to Rio de´Janeiro states, with preserved sedimentary piles ofhundreds of meters, associated to some alkaline mag-matism. The generative taphrogenetic process beganin the Paleogene period, when the plateau reached a

Žgeneral altitude around 1200 m heights below those.at present , informally known as ‘‘Japi Surface’’, of

Upper Cretaceous to Lower Eo-Tertiary ages. Such aplateau was formed by the erosion and levelling ofpreexisting large region with many basaltic and alka-line rocks.The origin and evolution of the Serra do Mar, its

rift systems and magmatism and the Santos and

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Campos basins still present a series of questions toŽ .be unraveled Almeida and Carneiro, 1998 . The first

Žmagmatic events are those of diabase dikes NE.trending of Late Jurassic ages. During the Neoco-

mian there were the Serra Geral flow basalts pro-Ž .cesses, whose extent to South Africa has been

discussed. Stratigraphic data from the Santos BasinŽ .Chang et al., 1992 and other inferences allows tosuppose the start of rising processes of the Serra do

Ž .Mar at about 90 Ma Turonian stage . Based on thefission method in apatites, Vignol-Lelarge et al.Ž .1994 , confirmed this age value, because they gotan age of 86 Ma for the upward movement of thearea of the Ponta Grossa Arch. They also consider

Žthe possibility of a general denudation event 2.5 km.thick during the Coniacian and Santonian stages

Ž .immediately post-Turonian . This range of ages co-incides with events of volcanic alkaline activity inthis south–southeast region.The first extensional processes recorded for this

Ž .region Eo-Cretaceous are marked by the beginningof the rift phase of Santos Basin, when many oldfault lines placed in the Ribeira fold belt have gainedlistric characteristics. With the progress of thestretching stress fields, the Santos Basin started to be

Ž .formed and filled up at the beginning by sedimentsrelated to those of Botucatu Formation as well aswith basaltic flows. After that, the Serra do Mar

Ž .started its ascending movements ca. 90 Ma . Theerosional processes of the ascending source area andthe weight of the volcanic-sedimentary pile in thebasin have probably composed a self-sustained stressfield that gradually increased both the elevation ofthe coastal area and the subsidence of the basin atthe seashore. The eastwards block movements led bylistric faults were responsible for the formation ofthe half-graben basins in the continental area, accord-ing to the orientation of maximum stretching, as wellas by the usual dip sense of the Tertiary beds there

Ž .present Melo et al., 1985 .

7.3.8. Southeast of UruguayDuring Jurassic times, in the southeast of Uruguay

Ž .southeast of Rio de La Plata block tectonic troughswere developed and filled up first with Eo-Creta-

Žceous marine and later by continental sediments the.rest of Cretaceous , reaching total thicknesses near

Ž .2000 m Bossi, 1996 . The main area, the Santa

Lucia trough, exihibits SE–NW general trends. Inthis case, basaltic flows, equivalent to those of SerraGeral, cover the Precambrian basement, precedingthe first taphrogenic movements that formed such thebasin.

7.3.9. Salado BasinThe Salado Basin, in the Buenos Aires province

in Argentina, is close to the southern limit of theSouth American Platform. It is positioned betweenelevated blocks of the Martin Garcia island of Parana

Ž . Žriver to the NNE and the Sierras Buenaerensis at.SSW , completely hidden by Cenozoic sediments.

The Rio Salado trough was formed after the flowbasaltic processes of Lower Cretaceous age and wasfirst filled up by continental sediments. Since the endof the Cretaceous period, and during Cenozoic timestotal thickness of more than 6000 m of alternatingcontinental and marine sediments developed. Thistypical sedimentary basin of the Activation stage isnow integrated to the general environment of thePampean plains.

8. Concluding remarks

The basement of the South American Platformwas generated by three complex Proterozoic col-lages, all succeeded by processes of taphrogenesisand dispersion. The first collage took place during

Ž .the Paleoproterozoic Trans-Amazonian , and agglu-Žtinated several Archean microcontinental nuclei of

.Neo-Archean age . Long accrettionary and colli-sional mobile belts developed in this period whichculminated with the formation of a large continentallandmass, around 1.8 Ga. This collage was followedby many intracratonic breaking and magmatic

Ž .episodes Statherian Taphrogenesis .During the mid and late Mesoproterozoic, a sec-

ond collage event consolidated sparsely distributedmobile belts according to different climaxes of activ-ities, from 1.4 up to 0.95 Ga, which involved manyof the lithogenetic products generated by all previoustaphrogenetic processes. By the end of this collage, alarge continental landmass, of global character, had

Ž .thus been performed Rodinia . The Eo-Neoprotero-zoic fission of this large landmass generated, at thesame time, different Neoproterozoic crustal blocks

Žand paleogeographic sites continental and oceanic

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.basins , whose arrangement defined the elements ofinteraction for the evolution of the Brasiliano Cycle.The collage of the Gondwana supercontinent oc-curred due to interactions of such blocks and basinclosures along at least three main phases: ca. 750, ca.600 and 550 Ma. Such collage promoted the forma-tion of the Gondwana supercontinent, with the base-ment of the South America platform located in itswestern side. The established structural frameworkcontrolled the most important pre-drift features ofthe whole continent.Important records of a tectonic heritage from the

Brasiliano collage trends can be identified within allthe generation stages of the platform cover.The Phanerozoic evolution of the platform was

developed during six tectono-sedimentary phases.The older one, from Neoproterozoic III up to Eo-Ordovician, includes many taphrogenetic basins asan epilogue to the Brasiliano Cycle, filled up by avaried immature sediments and volcanics and cut bysome anorogenic granites. This is the transition stageto the following Paleozoic phases of stability.Large intracratonic syneclises and main sedimen-

tary basins were developed in the wide platformŽ .from Ordovician Silurian mostly to Permian peri-

ods. There are widespread records of many seaingressions forming typical successions alternatedwith deposits of fluvial, deltaic and glacial origins.Three major cratonic sequences were then devel-oped. The Triassic period is marked by the beginningof a geocratic stage represented by continental sedi-ments and culminating with desert conditions inUpper Jurassic and Lower Cretaceous. Probably

Ž .younger than this Upper Permian , but reaching thesame time span, an important record of basalticmagmatism is found in the northern part of theplatform. This is part of the group of tectonic phe-nomena that indicates a new evolutionary stage —the actiÕation — of the platform, which culminatedwith the Atlantic opening during the Aptian-Albian.

Ž .The actiÕation stage Upper Jurassic onwardshas formed — in the whole platform — faults, basicand subsaturated alkaline magmatic intrusions as wellas extensive flood basalts. Thousands of Mesozoicdiabase dikes are known. The basaltic and associatedmagmatic activities were accompanied and suc-ceeded in many provinces by a diversified alkalinemagmatism until Miocene. The Atlantic continental-

margin taphrogenetic basins exihibit records — intheir volcano-sedimentary piles — of all these tec-tonic and magmatic phases. The last stage of plat-

Ž .form evolution restabilization , post-continental driftis characterized by the evolution of the present phys-iography and its connected sedimentary features, bothof the continental interior and of the continentalshelf.


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Fernando Flavio Marques de Almeida´has been a Full-Professor at the MiningDepartment of the Polytechnics Schoolof the University of Sao Paulo. Al-˜though he graduated as a Civil EngineerŽ .1938 at the University of Sao Paulo he˜did his doctorate in Geology and dedi-cated his career specifically to the Geol-ogy and Geotectonics of Brazil. He be-gan his professional career on mineralprospection at the Geology and Mineral-

Žogy Division of DNPM National De-.partment of Mineral Production , after which he took up teaching

at the Escola Politecnica, until he retired in 1974. Between 1974´and 1978 he teached Geotectonics in the Geosciences Institute ofthe same university, when he moved to a research position in the

Ž .Technological Research Institute of the Sao Paulo State IPT ,˜where he remained till 1995. From 1985 and 1990 Prof. Fernandoalso taught at the Geosciences Institute of the State University of

Ž .Campinas Unicamp . His scientific production includes morethan two hundred papers, books and reports on Brazilian Geology.He has received awards as: the Jose Bonifacio Gold Medal´ ´Ž . Ž .1964 , from the Brazilian Geological Society SBG ; Patriarca

Ž .Medal from Santos Municipality 1963 ; Emeritus Professor ofŽ .the Escola Politecnica USP 1976 ; Merit Medal from the Engi-´

Ž .neering and Architecture Federal Council 1995 ; AlmiranteAlÕaro Alberto Prize from the Science and Technology Ministry

Ž .of the Brazilian Government 1986 ; Honoris Causa Doctor fromŽ .the State University of Campinas 1991 ; CaÕaleiro da Gra-Cruz˜

Ž .Medal of the Nacional Scientific Order of Merit 1995 . He wasŽ .elected Vice-President of the Societe Geologique de France 1971´ ´ ´

and is a Member of the Brazilian Academy of Sciences and of theSao Paulo State Academy of Sciences. He was Vice-President of˜the Comission for the Geological Map of the World co-sponsoredby the IUGS.

Benjamim Bley de Brito Neves got hisPh.D. at the University of Sao Paulo˜Ž .USP in 1975. During the last 40 years,he worked on geological aspects of theBorborema Province, including geologi-cal mapping, hydrogeology, tectonicsand isotope geology. Since 1984 he hasworked as Full Professor at the Geo-sciences Institute of the University ofSao Paulo. Since 1996 he has been˜member of the Brazilian Academy ofSciences. Presently he is a Brazilian rep-

resentative at the Subcommission on Precambrian Stratigraphy ofthe IUGS.

Celso Dal Re Carneiro got his Ph.D. at´Ž .the University of Sao Paulo USP in˜

1984. During the last 28 years, heworked on geological aspects of the Pre-cambrian of the Mantiqueira Province,including geological mapping, structuralgeology, tectonics, geomorphology andapplied geology. Since 1986 he hasworked as Professor at the GeosciencesInstitute of the State University of

Ž .Campinas Unicamp . Since 1996 he hasbeen an associate member of the Brazil-

ian Academy of Sciences. Currently he is the coordinator of theundergraduate course on Earth Sciences of Unicamp and a re-search project on the evolution of a Precambrian zone of Sao˜Paulo State that has been subjected to environmental stressescaused by an intensive human occupation.