Ž .Lithos 46 1999 463–483

Granites and the geodynamic history of the neoproterozoicBrasılia belt, Central Brazil: a review´

Marcio M. Pimentel a,), Reinhardt A. Fuck, Nilson F. Botelho´a Instituto de Geociencias, UniÕersidade de Brasılia, Brasılia DF, 70910-900, Brazilˆ ´ ´

Received 19 December 1997; accepted 24 June 1998

Abstract

Recent field and geochronological studies have demonstrated the importance of granitic magmatism in the evolution ofthe Neoproterozoic Brasılia Belt, in Central Brazil. This is an orogenic belt developed in response to the convergence´between the Amazon, Sao Francisco–Congo and Parana continental blocks. The presence of Neoproterozoic juvenile arc˜ ´rocks and syn-collisional peraluminous granites challenged previous intracontinental evolution models for the belt. Thegranitoid intrusions reviewed in this paper record the different stages of evolution of the orogen and their field and isotopiccharacteristics can be used to reconstruct the tectonic history of the belt. The main field and isotopic characteristics of four

Ž . Ž .granite suites associated with the Brasılia Belt are reviewed: i 1.77–1.58 Ga old rift related A-type granite intrusions, ii´Ž . Ž . Ž .ca. 0.8–0.7 syn-collisional granitoids, iii arc metatonalites and metagranodiorites ca. 0.9 to 0.63 Ga , and iv bimodal

post-orogenic suite ranging in age from ca. 0.59 to 0.48 Ga. These rocks suggest that during most of the Neoproterozoic thewestern margin of the Sao Francisco continent faced a large oceanic basin, where subduction and oceanic lithosphere˜consumption started at ca. 0.9 Ga, roughly coeval with the initial stages of the break up of Rodinia. Final ocean closurehappened at ca. 0.63–0.60 Ga with crustal thickening, uplift and erosion. Post-orogenic extension-related magmatism tookplace between ca. 0.6 and 0.5 Ga and was partially contemporaneous with the deposition of the Paraguay and Tucavacasedimentary successions, resulting from the rifting event related to the break up of Laurentia from southwestern Gondwana.q 1999 Elsevier Science B.V. All rights reserved.

Keywords: Neoproterozoic; Brazil; Granite; Geodynamics; Geochronology

1. Introduction

The Tocantins Province, in Central Brazil, is alarge Neoproterozoic orogen developed in responseto the convergence and collision between three major

) Corresponding author. Fax: q55-61-3474062; E-mail:marcio@guarany.unb.br

continental blocks: the Sao Francisco–Congo conti-˜nent in the east, the Amazon continent in the west,and the Parana Block, in the south, most of which is´presently covered by the Phanerozoic sedimentary

Ž .rocks of the Parana Basin Fig. 1 . The Brasılia Belt´ ´constitutes the eastern part of the Tocantins Provinceand represents a N–S belt developed along the west-ern margin of the Sao Francisco–Congo plate.˜

This orogen provides important geological infor-mation to understand the processes which led to the

0024-4937r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0024-4937 98 00078-4

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Ž .Fig. 1. Sketch map of the southern portion of the Tocantins Province a ; geological sketch map of the Brasılia Belt showing the four areas investigated.´

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assembly of West Gondwana at the end of theŽ .Proterozoic Neves and Cordani, 1991 . Much debate

on the evolution of the Tocantins Province, and morespecifically of the Brasılia Belt, has taken place´during recent years. The discussion has been focusedmainly on whether or not it involved the closure of alarge oceanic basin. This specific question has beenaddressed in several studies on field relations,geochronology and petrogenesis of granitoids as wellas associated volcanic rocks in the Brasılia Belt´ŽPimentel et al., 1991a,b; Pimentel and Fuck, 1992;Fuck et al., 1994; Pimentel and Fuck, 1994; Pimentelet al., 1996a,b; Viana et al., 1996; Pimentel et al.,

.1997b . Although many issues are still controversialand several problems remain unsolved, the additionalgeochronological and petrological data provided newinformation for some of the controversial issues andhelped to clarify and prove a number of points such

Ž .as: i the existence of Neoproterozoic juvenile is-Ž .land arc terranes accreted to the orogen, ii the age

and nature of the older sialic basement underlyingŽ .the supracrustal sequences, iii the timing of meta-

Ž .morphic and deformational events, iv the tectonicŽ .significance of post-orogenic -590 Ma alkali-rich

granites intruded into some areas of the province,Ž .and v the role of this orogenic belt in relation to the

amalgamation of West Gondwana and to the modelsof continental reconstruction for the Neoproterozoic.

This paper reviews some of the published data aswell as presents and discusses the tectonic signifi-cance of new unpublished isotopic results for granite

Žsuites in four selected areas of the Brasılia Belt Fig.´.1 .

2. Regional geological setting

2.1. The main geological features of the TocantinsProÕince

The Tocantins Province belongs to the Brasil-ianorPan-African orogen, formed mostly by theBrasılia and Araguaia metasedimentary fold-and-´

Žthrust belts, as well as by the Paraguay fold belt Fig..1 . The Araguaia Belt is a ca. 1000 km long N–S

belt flanking the eastern margin of the Amazon

Ž .Craton Abreu et al., 1994; Hasui et al., 1977 .Sedimentary sequences metamorphosed up to amphi-

Ž .bolite facies Tocantins and Estrondo Groups arethe main constituents of the belt. Like the Brasılia´Belt, the supracrustal rock units of the Araguaia Beltare characterized by regional low-angle foliation andthrust sheets showing tectonic transport to the west,

Ž .towards the Amazon Craton Abreu et al., 1994 .Large mafic–ultramafic bodies which form a longbelt parallel to regional N–S lineaments, have been

Žinterpreted as ophiolite remnants Kotschoubey and.Hieronymus, 1996; Teixeira, 1996 and represent the

main evidence for the closure of a Neoproterozoicoceanic basin.

The sedimentaryrmetasedimentary rocks of theParaguay Belt and their corresponding cratonic coverdeposited on the southern margin of the AmazonCraton, display a somewhat different deformationalstyle compared to the Brasılia and Araguaia belts.´Instead of thrusts and low angle foliations, the defor-mational pattern displayed by the Paraguay Belt rockunits is marked by open folds with verticalrsubverti-

Ž .cal axial planes Alvarenga and Trompette, 1992 .Additionally, the sedimentary rocks seem to be con-

Ž .siderably younger -610 Ma as indicated by thepresence of Ediacara-like fauna in their basal forma-tions. No evidence for oceanic lithosphere consump-tion, such as calc–alkaline magmatism or ophiolites,have been found and, therefore, this basin has beeninterpreted as formed in an intracontinental setting,possibly representing an aulacogen or foreland mo-

Ž .lassic basin Trompette, 1994; Trompette, 1997 .According to the tectonic scenario of Trompette

Ž .1997 the Araguaia and Brasılia belts had a long-´lived tectonic history, with sedimentation starting ataround 1.1–1.0 Ga and final closure at ca. 0.6 Ga.The Paraguay Belt, on the other hand, is younger andevolved over a much shorter tectonic cycle, whichstarted at ca. 0.6 Ga and deformed and metamor-phosed in the Cambrian, at ca. 520 Ma.

2.2. The Brasılia belt´

This is a complex fold-and-thrust belt extendingca. 1100 km along the western margin of the Sao˜Francisco craton. It displays tectonic vergence to-

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Žwards that cratonic area Fig. 1; for a review see theŽ ..work of Fuck et al. 1994 . Metamorphic P and T

conditions increase progressively towards the west.Sedimentary rock units of varied nature and age

Ž .constitute most of the Brasılia Belt Fig. 1 . At the´base of the sedimentary succession is the Araı Group´which rests unconformably on the sialic basement.This is a ca. 1.77 Ga old rift sequence with basalcoarse-grained terrigenous sediments and felsic–mafic volcanic rocks. The Araı succession is inter-´preted as an early rift attempt which did not seem tohave evolved into the opening of a large oceanicbasin in the Mesoproterozoic. This rift sequence isunconformably overlain by the detritic sequence ofthe ca. 1.0–1.1 Paranoa Group which, together with´the Canastra Group in the south, are the main com-ponents of the external part of the Brasılia belt. They´are interpreted as the shelf sequence of a typical

Ž .passive margin Fig. 1; Fuck et al., 1994 formed atthe western margin of the Sao Francisco–Congo˜continent. The internal part of the supracrustal se-quence of the Brasılia Belt is formed mainly by the´Araxa Group metasediments which include mostly´pelitic micaschists and quartzites, representing deeperwater sediments formed probably on the continentalslope of the passive margin. The overall structural

Ž .pattern of the Araxa rock units is dominated by: i´shallow-angle foliations associated with thrust sheetswith tectonic transport towards the Sao Francisco˜

Ž .Craton, ii long, nearly E–W, lineaments whichprobably correspond to lateral ramps or wrench faultsoriginated by differential displacement of the thrust

Ž .wedges Strieder and Nilson, 1992 . The deformationbecomes progressively less intense towards the cra-tonic area. A distinctive feature of the Araxa Group´is the presence of a number of small serpentinitebodies forming a ca. 200 km long ophiolitic melange´Ž .Drake, 1980; Strieder and Nilson, 1992 . Thismelange and associated stack of thrust sheets suggest´that the Araxa sequence corresponds to an accre-´tionary wedge thrust over the platformal sequence ofthe passive margin.

The Bambuı Group overlies the passive margin´sequence and also covers large areas of the Sao˜Francisco Craton, where it is flat-lying and com-pletely free of deformation. It constitutes a detriticand carbonatic sequence with a basal diamictite. Thetectonic significance of the Bambuı basin has been´

debated and recent ideas have interpreted the se-Ž .quence or at least its upper formations as a foreland

basin formed in response to the development of theŽAraxa thrust sheets in the west Pimentel et al.,´

.1992; Thomaz Filho et al., 1996; Guimaraes, 1997 .˜There are no reliable or conclusive geochronolog-

ical data for the deposition of the sedimentary se-Žquences above the only exception being the Araı rift´

sequence, with a U–Pb zircon age of 1.77 Ga,.Pimentel et al., 1991a . However, well developed

stromatolitic structures observed throughout thestratigraphic columns of the Paranoa and Bambuı´ ´Groups indicate deposition between 1.2 and 0.9 Gafor the former and between 0.9 and 0.6 Ga for the

Žlatter Dardenne et al., 1972; Cloud and Dardenne,.1973; Marchese, 1974 .

Due primarily to the lack of geochronologicaldata, until recent years the evolution of the Brasılia´Belt was generally believed to be intracontinental,without involving substantial contemporaneous ig-neous activity. However, recent geochronologicalstudies, combined with field and structural data insome key areas have revealed the importance ofNeoproterozoic igneous activity in Central Braziland that the Brasılia Belt displays many features of´

Ž .modern collisional belts, such as: i juvenile mag-Žmatic arc terranes e.g., the Arenopolis and Mara´

. Ž .Rosa arcs in the western part of the belt , ii colli-sion-related granitic magmatism associated with the

Ž .Araxa Group, and iii high grade metamorphism of´Brasiliano age.

U–Pb and Rb–Sr geochronological data, as wellas Sm–Nd garnet ages, suggest that the tectono-metamorphic history of the Brasılia Belt was fin-´ished by ca. 0.6 Ga and was marked by two meta-

Žmorphic peaks: an early one at ca. 790 Ma Ferreira.Filho et al., 1994; Suita et al., 1994 and a late one

with a typical Brasiliano age of ca. 0.63–0.61 GaŽ .Pimentel et al., 1991b, 1997a .

Most of what is known of the tectonic history ofthe Brasılia Belt has been assessed through field,´geochronological and petrological studies of grani-toid intrusions and associated volcanic rocks formedat different stages of the evolution of the belt. In thispaper, we review the main characteristics of grani-toid suites from four different areas of the Brasılia´Belt, and together with new unpublished Sm–Ndisotopic data, we discuss their tectonic significance.

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Fig. 2. Geological sketch map of the Goias Tin Province. RTS—Rio Tocantins Sub-Province, RPS—Rio Parana Sub-Province.´ ˜

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Ž .The four suites discussed are: i A-type granites ofŽ .the Goias Tin Province, ii syn-tectonic peralumi-´

nous granites and sub-volcanic rocks of southernŽ . ŽGoias, iii arc granitoids mainly metatonalites to´

.metagranodiorites of the Neoproterozoic arcs ofŽ .Arenopolis and Mara Rosa, and iv alkali-rich late-´

to post-orogenic granites and gabbros intruding theŽ .arc terranes Fig. 1 .

3. A-type granites of the Goias Tin province´

These granite intrusions are exposed in northernŽ .Goias Fig. 1 and can be divided into two sub-pro-´

Ž .vinces: the Rio Tocantins Sub-Province RTS in theŽ .west, and the Rio Parana Sub-Province RPS in the˜

Ž .east Fig. 2 . In the former, the main tin-bearinggranites form large ellipsoidal bodies and occur asN–S elongate domes exposed within the terrigenoussediments of the Araı and Serra da Mesa groups. In´the Rio Parana Sub-Province, the granite intrusions˜are small and intruded into the ArchaeanrPaleopro-

terozoic basement gneisses overlain by the nearlyflat-lying Araı Group supracrustals. Although the´basal conglomerates of this group contains fragmentsof the granites, U–Pb zircon ages of ca. 1.77 Ga forboth Araı rhyolites and subvolcanic Sucuri and´

ŽSoledade granites show that they are coeval Pimen-.tel et al., 1991a; Fig. 2 .

These granites are, in general, alkali-rich andhave high F, Sn, Rb, Th, Y, Nb, Ga, and REE

Ž .contents Botelho, 1997 . In the Rio Parana Sub-Pro-˜vince, two distinct families of granite intrusions havebeen identified and are referred to as the g1 and g2

Žgranites. Granites of the g1 family are older 1.77.Ga, U–Pb zircon age , coeval with the Araı vol-´

canics, more potassic and display clear alkalic affin-ity with high Nb, Zr, Th, Y and REE contents. The

Žg2 granites are younger 1.6–1.5 Ga, U–Pb zirconand Rb–Sr isochrons; Pimentel et al., 1991a; Botelho

.and Pimentel, 1993 show metaluminous to peralu-minous character, with lower KrNa ratios and higher

Ž .Li, Rb, Sn and Ta Fig. 3 . Tin and indium depositsand occurrences are always associated with g2 gran-ites.

Fig. 3. Ta=Nb diagram showing the compositional difference between the g1 and g2 granites of the Goias Tin Province.´

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Nd isotopic characteristics of g1 and g2 granitesare illustrated in Fig. 4. There are no evident differ-ences in Nd isotopic compositions between g1 andg2 granites suggesting that their crustal sources andevolution of the original granitic magmas are similar.Initial ´ values are quite variable ranging from ca.Nd

q3.4 to y14.0 with T model ages ranging fromDM

2.0 to 2.7 Ga. The large range in initial ´ valuesNdŽ .can be preliminarily interpreted in two ways: i it

indicates that the original magmas are products ofdifferent degrees of mixing between 1.7 and 1.8 Ga

Žmafic magmas basalt flows are known at the lower.part of the Araı Group and felsic liquids derived´

Ž .from melting of the older sialic basement, or iitheir sialic sources are heterogeneous in terms of ageand isotopic characteristics. Nd isotopic compositionof one diorite associated with the granites has values

Ž .of ´ 1.77 sq3.9 and T of ca. 1.8 Ga suggest-Nd DM

ing that it represents the mantle end-member in aŽ .mafic–felsic mixture Fig. 4 . Two basalt samples of

the Araı Group showed intense contamination with´older sialic material, indicating the interaction be-tween mafic and felsic liquids. They display low147 144 Ž .Smr Nd ratios ;0.12 , T model ages ofDM

Ž .2.5–2.6 Ga and ´ 1.77 of ca. y5.0. This igneousNd

event is, therefore, of bimodal character and theproduction of granite liquids might have been aresult of crustal remelting, induced by the heat inputprovided by the emplacement or underplating ofmafic magmas.

Both g1 and g2 granites have features similar toA-type granites and the 1.77 and 1.5–1.6 Ga intru-sive events are considered to be anorogenic andassociated with a continental rift system. This riftsystem extended much further to the east, being

Ž .represented in Bahia State ca. 600 km to the east

Fig. 4. Nd isotopic characteristics of 1.77 Ga g1 and ca. 1.6 Ga g2 granites of the Goias Tin Province.´

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by the 1.72–1.78 Ga Paramirim tin-bearing rhyolitesŽat the base of the Espinhaco Supergroup de Brito

.Neves et al., 1979; Turpin et al., 1988 , which is arift association bearing many similarities with theAraı Group. The evolution of this rift system is still´the subject of debate. At the present level of knowl-edge it is still not clear whether the rift evolved andled to ocean opening and closure during Mesopro-terozoic times or whether it aborted without theformation of oceanic lithosphere. In the Brasılia Belt´there is little information on the evolution betweenthe establishment of the Araı rift at 1.77 Ga and the´rifting event marked by the deposition of the Paranoa

Ž .Group ca. 1.1 Ga passive margin sequence, and,therefore, no conclusive evidence for a Mesoprotero-zoic ocean exists at this stage.

4. Arc granitoids of Mara Rosa and Arenopolis´

In recent years, two main areas of Neoproterozoicjuvenile crust have been identified in western andnorthern Goias, here referred to as the Arenopolis´ ´

Ž .and Mara Rosa areas, respectively Fig. 1 . Most ofthis arc magmatism is represented, at the presentlevel of erosion, by metaplutonic rocks ranging incomposition from tonalitic to granodioritic. They areexposed between narrow NNW to NNE belts ofNeoproterozoic arc volcano-sedimentary sequencesŽ .see maps of Figs. 5 and 6 . These rocks wereformerly interpreted as exposures of ArchaeanrPaleoproterozoic basement units. Geochronologicaland isotopic data revealed that the oldest representa-tives of the arc magmatism are the so-called

Fig. 5. Geological sketch map of the Mara Rosa area.

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Fig. 6. Geological sketch map of the Arenopolis area.´

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Fig. 7. General geochemical characteristics of metatonalitic–metagranodioritic orthogneisses of the Mara Rosa and Arenopolis arcs.´

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Arenopolis, Matrinxa, Sanclerlandia and Mara Rosa´ ˜ ˆtonalitic to granodioritc orthogneisses dated between

Žca. 0.93 and 0.86 Ga Rb–Sr isochrons and U–Pbzircon ages; Pimentel et al., 1991b; Pimentel and

.Fuck, 1994; Pimentel et al., 1997a . These rocks aregeochemically very primitive, being similar to man-tle derived granitoids of young intraoceanic island

Ž .arcs Fig. 7 . Some of their geochemical features

such as low K OrNa O ratios, high Ni, Cr and Sr2 2

contents, and strongly fractionated REE patterns withhigh LarYb and SrrYb ratios are similar to modernadakites, which formed in subduction environments

Ž .where the subducted slab is young -20 Ma andhot and therefore capable of undergoing partial melt-

Žing Drummond and Defant, 1990; Schiano et al.,.1995; Martin, 1997 . The ‘adakitic’ composition of

Ž .Fig. 8. Initial Nd and Sr isotopic characteristics of the arc metavolcanics and orthogneisses and of the late- to post-orogenic -600 Maintrusive granites of the Arenopolis and Mara Rosa areas.´

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the Neoproterozoic arc granitoids of Goias could be,´therefore, explained using a model in which theoriginal magmas were formed in response to thesubduction of young and hot oceanic lithosphereconstituting small oceanic basins such as back arcsettings, in between island arc systems.

Nd and Sr isotopes also indicate the very primi-tive nature of the parental magmas of the Arenopolis´and Mara Rosa metatonalites and metagranodiorites.Nd T model ages are mostly between 0.8 and 1.1DM

Ž .Ga, ´ T values are in the range q2.0 to q6.5,Nd

and initial 87Srr86Sr ratios in the interval betweenŽ .0.7025 and 0.7042 Fig. 8 .

In the Mara Rosa arc, detrital metasediments rep-resented by feldspathic garnet micaschists and fine-grained biotite gneisses are abundant in the

Ž .supracrustal belts see map of Fig. 5 . Sm–Nd iso-topes for these metasediments indicate T valuesDM

dominantly in the range 0.9–1.2 Ga, with only oneanalysis yielding a significantly older model age of1.6 Ga. This indicates that they are the products oferosion of the arc rocks, with little contribution fromolder sources. The deposition of the original sedi-ments must have taken place, therefore, far from anyold continental source area, probably in an intrao-ceanic setting.

In the Ipora–Jaupaci area, east of Arenopolis´ ´Ž .Fig. 6 , the arc magmatism is significantly younger.Arc-type tonalites, granodiorites and granites have

Žbeen dated between ca. 0.64 and 0.73 Ga Rb–Srisochrons with initial 87Srr86Sr ratios between 0.703

.and 0.705 and arc metarhyolites have U–Pb zirconŽages between 0.76 and 0.64 Ga Pimentel et al.,

. Ž .1991b; Rodrigues et al., 1998 . Their ´ T are alsoNdŽ .positive q0.5 to q2.0 . Geochemically these

younger rocks are slightly different from the ca. 0.9to 0.85 Ga tonalites of Arenopolis and Mara Rosa,´being more evolved, with K OrNa O)1.0 and2 2

less fractionated REE patterns.The data above suggest that between ca. 0.9 and

Ž .0.63 Ga an arc or multi-arc system developed in anintraoceanic setting to the west of the Sao Fran-˜cisco–Congo continent, following the break-up of

Ž .Rodinia supercontinent Hoffman, 1991 . The olderarc successions are typically of intraoceanic charac-

Ž .ter, whereas the younger ones -0.76 Ga such asthe arc rocks in the Ipora–Jaupaci area, have a´slightly more evolved nature, indicating progressive

amalgamation and growth of island arcs or develop-ment of an active continental margin. This scenariois compatible with recent continental reconstructions,according to which the Brasılia Belt represents a´peripheral orogen and, therefore, subduction couldhave started immediately after initial break up of

ŽRodinia Hoffman, 1991; Neves and Cordani, 1991;.Dalziel, 1995; Dalziel, 1997 .

In the Neoproterozoic magmatic arc setting, aRb–Sr isochron on mylonitic rocks, U–Pb spheneages and U–Pb zircon data for syn-tectonic intru-sions are all in the interval between 0.63 and 0.59 GaŽPimentel et al., 1991a; Viana et al., 1996; Pimentel

.et al., 1997a . They register the main deformationalrmetamorphic event of the Brasiliano Cycle, possiblyrelated to the collision between the Amazon and Sao˜Francisco continents.

5. Neoproterozoic syn- and late-tectonic granitesof southern Goias´

In recent years several mapping projects in thesouthern part of the Brasılia Belt have demonstrated´the existence of a number of granite plutons em-placed into the supracrustal rocks of the Araxa Group´Ž .Fig. 1 . These intrusions normally display zones ofhigh strain with the development of mylonitic tex-tures, especially along their margins. In some casesthey form narrow elongate bodies, conformable withthe low-angle regional foliation of the metapelites ofthe Araxa Group. They have been interpreted as syn-´and late-tectonic intrusions in relation to the maindeformation of the Araxa Group. The granitoid rocks´vary in composition from granodioritic to granitic,with slight to strong peraluminous character. Somehave enclaves of metapelitic rocks and quartzites.

One of the main representative examples of thisgroup of granitic rocks is the Marata sequence, a´strongly deformed conformable rock unit consisting

Ž .of a sub-volcanic intrusion or intrusions crystal-Žlized at ca. 794 Ma U–Pb zircon age; Pimentel et

. Ž .al., 1992 Fig. 9 . Pb inheritance indicates that theoriginal magma was derived from older, possiblyPaleoproterozoic, continental crust. Rb–Sr isochronages for other small granite intrusions in this region

Žare in the interval between 0.70 and 0.78 Ga e.g.,.the Sesmaria granite in Fig. 10 , with initial Sr ratios

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Ž . Ž .Fig. 9. Geological sketch map of the southern part of the Brasılia Belt in Goias a , and of the Araxa Group in the Pires do Rio area b showing the Marata Sequence. U–Pb data´ ´ ´ ´Ž .for the Marata Sequence are from the work of Pimentel et al. 1992 .´

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Ž . Ž .Fig. 10. Geological sketch map of the southern part of the Brasılia Belt in Goias a , and of the Araxa Group in the Ipameria area b , showing the Sesmaria and Encruzilhada´ ´ ´granites.

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between 0.705 and 0.710. The preliminary isotopicand geochemical data for these granitic rocks revealtwo distinct groups of granites: one formed by the

Ž .Sesmaria and Tambu granites Fig. 11 , which are´only slightly peraluminous with Sm–Nd T modelDM

ages of 1.1–1.0 Ga, and another formed by theMarata rocks and the Encruzilhada granite, which are´strongly peraluminous and have distinctly older Sm–

Ž .Nd model ages 1.73 to 2.5 Ga . This latter graniticrocks have Nd isotopic compositions which are verysimilar to that of metasedimentary rocks of the AraxaGroup country-rocks. The original magmas of theMarata Sequence and Encruzilhada granite are inter-´preted, therefore, as products of remelting of theAraxa metasediments. On the other hand, the Nd´isotopic characteristics of the Sesmaria and Tambugranites cannot be explained in that way. They havean obvious juvenile component, with much youngerT model ages suggesting the presence of a Neo-DM

proterozoic juvenile crustal component in the source

region of the parental magmas. Neoproterozoic juve-nile crust is still unknown, however, in that part ofthe Brasılia Belt.´

The main characteristics of these 0.7–0.8 granitesare not much different from those of S-type, syn-col-lisional granites associated with younger orogenicbelts. Their presence is indicative, therefore, of an‘early Brasiliano’ collisional event at ca. 0.79 Ga, inthe southern part of the Brasılia Belt. Similar peralu-´minous granites of Neoproterozoic age are unknownin the northern part of the belt. ‘Early Brasiliano’metamorphism is also registered by U–Pb ages ofmetamorphic zircons of the granulitic Niquelandiaˆand Barro Alto mafic–ultramafic complexes, further

Žto the north Ferreira Filho et al., 1994; Suita et al.,.1994; Correia et al., 1997 .

The existing data suggest that the Araxa Group´rocks can be interpreted as an accretionary wedgethat was thrust over the passive margin, with conse-quent thickening of the crust and production of

Ž .Fig. 11. Alumina saturation diagram for granitoid rocks Marata, Tambu, Sesmaria and Encruzilhada associated with the Araxa Group, in´ ´Ž . Ž .the southern part of the Brasılia Belt a , and initial Nd isotopic composition of the same rock units b .´

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granitic melts at ca. 0.79 Ga. This event may beinterpreted as resulting from the collision betweenthe southwestern edge of the Sao Francisco Plate and˜the continental block covered by the PhanerozoicParana Basin.´

6. Late- to post-orogenic bimodal magmatism

The last tectono-metamorphic event in the Brasılia´Belt happened between 0.63–0.59 Ga. This was

Ž .responsible for the development of: i W–NW dip-ping foliation showing kinematic indicators revealing

Žtectonic transport to the east e.g., in the Mara Rosa. Ž .arc area , and ii wide and long subvertical, domi-

nantly NE-trending mylonite zones, such as theŽ .Transbrasiliano Lineament Fig. 1 .

During the waning stages or immediately afterthis last deformation, a number of small mafic–ultra-mafic layered complexes, gabbro–dioritic intrusions,and large granite plutons were emplaced into thewestern part of the Brasılia Belt, especially into the´Mara Rosa and Arenopolis arc areas.´

The gabbro–dioritic intrusions and mafic–ultra-Žmafic layered complexes e.g., Lajeado intrusion and

.Americano do Brasil Complex, in western Goias are´either only slightly deformed or completely free ofany pervasive deformational fabric. The gabbro–di-orites commonly display magma mixing features withgabbro ‘globules’ enclosed in dioritic rocks. Theserocks display typical arc geochemical signaturesŽ .Rodrigues et al., 1998 . The Americano do BrasilComplex is a small layered intrusion differentiatedfrom peridotites to gabbros and the original tholeiitic

Žparental magma also had arc characteristics Nilson,.1981 . Gabbros and diorites also occur as small

bodies inside the larger granite intrusions. Some ofthese gabbro–dioritic intrusions have been dated by

Ž .the Rb–Sr isochron method see Table 1 , givingpoorly constrained ages of ca. 610–630 Ma, withinitial 87Srr86Sr ratios of ca. 0.703–0.704, suggest-ing little contamination with much older sialic crustŽ .Rodrigues et al., 1998 . The Americano do Brasilcomplex was recently dated by the Sm–Nd methodat 612"66 Ma, with an initial ´ value of q3.1Nd

also indicating little or no contamination with an-Ž .cient crust Nilson et al., 1997 .

The granite intrusions form large bodies of K-richcalc–alkaline granites. Most are completely unde-formed and made exclusively of equigranular biotite

Table 1Geochronological data for late-rpost-orogenic intrusions in western Goias´

Ž .Rock unit Age Ma Initial isotopic ratio Referencea Ž .Amercano do Brasil mafic–ultramafic complex 612"66 ´ T sq31 1ND

bGongome gabbro–dioritic complex 615"108 0.7154 2´bSerra do iran diorite ;622 0.7039 3bLajeado gabbro–dioritic complex ;651 0.7029 4

bSerra do Iran granite 588"19 0.7044 3bŽ .Serra do Impertenente granite 1 576"18 0.7032 3bŽ .Serra do Impertenente granite 2 485"18 0.7067 3bIsraelandia 554"20 0.7045 3ˆ

c579"3 – 5bMicrogranite dyke 564"15 0.7094 3bSerra Negra granite 508"24 0.7057 3bIpora granite 490"24 0.7057 3´

aSm–Nd whole-rock isochron.bRb–Sr whole-rock isochron.c U–Pb sphene age.

Ž .1—Nilson et al. 1997 .Ž .2—Winge 1995 .

Ž .3—Pimentel and Fuck 1994 .Ž .4—Rodrigues et al. 1998 .

Ž .5—Pimentel et al. 1996a,b .

()

M.M

.Pim

enteletal.r

Lithos

461999

463–

483479

Ž . Ž .Fig. 12. General geochemical characteristics of the post-orogenic granites of western Goias Arenopolis area data from the works of Pimentel et al., 1996a,b .´ ´

( )M.M. Pimentel et al.rLithos 46 1999 463–483480

granites, although some have hornblende-bearing andŽ .porphyritic facies Pimentel et al., 1996a . K-rich,

high crustal level, microgranite dykes also occur insmall swarms in some localities in the Arenopolis´area. This igneous event is, therefore, typically bi-modal and, except for some localities where magmamixing between mafic and felsic end members tookplace, intermediate rocks are not found.

In the Arenopolis region, the post-Brasiliano gran-´ite magmatism took place in two distinct episodesŽ . Ž .Table 1 : i an older event between ca. 590 and 560

Ž .Ma, and ii a young event dated between ca. 508Ž .and 485 Ma Pimentel et al., 1996b . Both groups

are predominantly metaluminous, have K OrNa O2 2

)1 and display general characteristics that are tran-sitional between the high-K calc–alkaline and theshoshonitic series. The younger group has a morealkalic character, being similar to A-type granitesŽ .Fig. 12 , whereas the older group has characteristicssimilar to the Caledonian I-type granites. This rela-tionship is clearly displayed by the Serra do Imperti-

Ž .nente intrusion. The early 575 Ma porphyritic fa-cies of this intrusion has Caledonian I-type character-istics with lower Ga, ZrqNbqCeqYb, and higher

Ž .LarYb ratios, whereas the late ca. 485 Maequigranular facies has a typical A-type signatureŽ .Fig. 12 . Both groups have Sr and Nd isotopiccharacteristics which are compatible with remeltingof the Neoproterozoic arc granitoids and metavol-

Žcanics initial Sr ratios between 0.703–0.710 andŽ . .Ž .´ T between y4.6 and q3.0 Fig. 8 . Involve-Nd

ment of important amounts of much older continen-tal crust can be ruled out.

In the Mara Rosa–Porangatu area, the post-Brasi-liano granite and gabbro–diorite intrusions are onlyknown superficially. They have not yet been dated,but their mostly post-deformation character indicatethat they are younger than ca. 0.63 Ga. They displaya more peraluminous character when compared withtheir correlatives in the Arenopolis area discussed´above. Initial Nd isotopic compositions of thesegranites are similar to those of the metasediments ofthe Mara Rosa volcano-sedimentary sequence indi-cating that this supracrustal material was involved inthe origin of the granite parental magmas, producingtheir slightly peraluminous character. They do not

Ž .indicate the presence of much older )1.1 Ga sialicmaterial in the source areas of the original magmas.

The bimodal nature of this magmatism suggeststhat the input of heat required to promote large scalemelting of the crust was most probably provided bythe emplacement andror underplating of maficmagma into the continental crust. This abundantpost-Brasiliano granitic magmatism is spatially con-fined to the regions underlain by the juvenile arcrocks. This may reflect that these were preferablesites of uplift and melting of the mantle and conti-nental crust or, alternatively, that the juvenile crustbehaved as a more ‘fertile’ material for granite pro-duction. Field and geochronological evidence sug-gest that the emplacement of these bodies occurredunder extension which accompanied rapid uplift andunroofing of the region. Amphibolite facies metavol-canic rocks with U–Pb sphene ages of ca. 590 Maare intruded by ca. 560 Ma sub-volcanic microgran-ite dykes. Therefore, an uplift rate of 0.4 to 0.6mmryear is reported for the Arenopolis region in the´period between 0.59–0.56 Ga, in the same order ofmagnitude as rates estimated for modern mountain

Ž .belts Pimentel et al., 1996b .

7. Conclusions

Field, isotopic and geochronologic data gatheredover the last few years have demonstrated the re-gional importance of Neoproterozoic granite magma-tism associated with the evolution of the Brasılia´Belt. The main stages of the tectonic evolution ofthis orogen, from an early rift stage at ca. 1.77–1.60Ga, through arc magmatism, peraluminous syn-colli-sional intrusions, to a post-orogenic bimodal event,are registered by granitoid suites.

The 1.77–1.6-Ga rift event was important region-ally, but its subsequent history, between 1.6 and 1.2Ga is obscure. We have not yet documented rift-re-lated igneous activity at ca. 1.1 Ga, the approximateage of initial deposition of the passive margin se-quence at the western margin of the Sao Francisco˜continent. Most of the Neoproterozoic magmatismdocumented in the Brasılia Belt is related to conver-´gence and closure of an oceanic basin or to post-col-lisional uplift.

The ca. 0.85–0.90 Ga primitive metatonalites ofArenopolis and Mara Rosa represent the earliest arc´products formed during convergence between the

( )M.M. Pimentel et al.rLithos 46 1999 463–483 481

Amazon and Sao Francisco continents. The long˜history of arc magmatism, between ca. 0.9 and 0.63

Ga, suggests that during most of this period of time,the western margin of the Sao Francisco continent˜

Fig. 13. Schematical reconstruction model of the Neoproterozoic tectonic history of the Brasılia Belt.´

( )M.M. Pimentel et al.rLithos 46 1999 463–483482

faced a large oceanic basin where intraoceanic islandarcs formed and were accreted to each other or to the

Ž .continental margin Fig. 13-1 . This is compatiblewith a peripheric position of the Brasılia Belt in´relation to the Rodinia supercontinent. Final closurebetween the Amazon and Sao Francisco continents˜occurred at ca. 0.6 Ga, with the development ofextensive N–NE trending shear zones as well aseast-verging thrust faults.

The ca. 0.79 Ga old collisional magmatism asso-ciated with the accretionary wedge of the Brasılia´Belt is documented only in the southern part of thebelt, south of Brasılia. This points to distinct tectonic´evolution models for the northern and southern partof the belt. Although there is no tectonic evidencedocumented for it, at the present level of knowledge,this ‘early Brasiliano’ collisional event is interpretedas representing the collision between the southwest-ern part of the Sao Francisco continent and the˜Parana Block, most of which is now covered by the´

ŽPaleo-Mesozoic rocks of the Parana Basin Fig. 13-´.2 .

Ž .The typically Brasiliano 0.63 Ga metamorphicages recorded in several areas of the province areinterpreted as resulting from the collision betweenthe Amazon continent and the Sao Francisco conti-˜nent, at that time already welded to the Parana Block´Ž .Fig. 13-3 .

Following the closure of the ocean and crustalthickening, some areas of the orogen were invadedby mafic magmas which promoted dehydration melt-ing of the crustal rocks, generating large volumes ofpost-orogenic K-rich calc–alkaline granites. TheirNd–Sr isotopic composition indicate that the originalmagmas were mostly derived from juvenile Neopro-terozoic arc rocks. This early phase of the ca. 0.59–0.56 Ga extension-related bimodal magmatism isroughly coeval with the deposition of the Paraguaybelt rocks and its correlatives of the Tucavaca Belt inBolıvia. The latter is interpreted in recent reconstruc-´tion models as being formed during rifting of Lau-rentia from the western margin of Gondwana.

Acknowledgements

ŽThe authors would like to thank CNPq Brazilian.Research Council and PADCT for the financial

support for most of the field and analytical workreported in this paper. We also thank Dr. A. Vauchezand Dr. U.G. Cordani for their critical reviews.

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