/

, ) 't "/ 85

by M. C. Geraldcsl , W. Teixeira? and M . Heilbron!

Lithospheric versus asthenospheric source of theSW Amazonian craton A-types granites: the roleof the Paleo- and Mesoproterozoic accretionarybelts for their coeval continental suitesI Fac uldade de Geo logia - Univers idade do Estado do Rio de Janei ro. Rua Sao Francisco Xav ier 524, 20559-900 Rio de Janeiro (RJ). Brazil.

E-mails:geraldes@ uerj.brand he ilbron@uerj .br2 Instituto de Geoc iencias, Univ ersidade de Sao Paulo - Rua do Lago 562 , Cidade Universiuiria, 05508-900 Sao Paul o (SP) Brazil.

E-mai l: wteixeir@ usp.br

The tectonic setting and source ofthe Paleo- and Meso ­proterozoic magmatic suites in the SW Amazonian cra­ton revealed by integrated isotopic and geochemicaldata allow correlation between the accretionary mobilebelts and the contemporary continental magmatism(e.g. rapakivi complexes ) within the f oreland. The conti­nental magmatism may represent the synorogenicresponse to high heat flow in the asthenosphere result­ing from oceanic crust subduction, which led to thedevelopment of the successive Proterozoic magmaticarcs.

Introduction

The or igin of Proterozoic rapaki vi granites is controve rsial. Mostmodels for this peculi ar continental magmati sm envision heat ormagma transfer from the asthenosphere to the base of the lithosphereleading to part ial melting of the lower crust.

Ramo and Haap ala, ( 1995 and references therein) based onocc urrence of rapa kivi rocks along Gul f of Finland proposed thattheir orig in was caused by thermal doming or hot spo t activitybenea th Baltica. Co rrobo rating this model , Hoffman ( 199 1) postu­lated that such mid i-Prot erozoic gra nites were generated by theaction of a major mantle upwell ing beneath the stationary Rodini asupercontinent.

Alternatively, lithosp here extension and thinning have beenlinked to the genera tion, asce nt and emplace ment of rapak ivi mag­mas (Co llins et al., 1982; Anderson et al ., 1999; Windl ey. 1991; Eby,1992). As sugges ted by Windley (199 1), the ea rly Proterozoic Keti­lidian rapakiv i gran ites of south Gree nland are post-orogenic rocksgenerated by crus tal meltin g deep within a thrust-thi ckened crustthat had begun to undergo ex tensional co llapse . In this case, thegranites were formed during the late stages of the Ketilidianorogeny, synchronously with a period of exte nsio nal tectonics andlow-p ressure granulite facies metamorphi sm (Dempster et al., 1991).According to Co llins et al. ( 1982) , all granitic rocks with A-typeaffinities are intruded late in the mag matic cycle. genera ted by lowercrus t part ial melt ing. These granites are com mon ly associated withextensional regim es in continental blocks, but also occ ur in areasthat do not appear to be related to orogeny tectonic selling. Thi sauthor arg ued aga inst product ion of the A-type melts by frac tionalcrys tallization of I-type melts for two reasons: firstly, the A- type

Episodes , 1'01.27, 110.3 , .;:. ,'-\ <,

melt s arc initially dry or almos t anhydro us, as ev idenced by precipi ­tation of bio tite and amphibole only as intersti tial crystals; conse­quen tly, any further fractiona tion from a felsic I-type melt wouldlead to an anhydrous melt . Secondly: the low Rb content and fairlyhigh Sr content are inconsistent with their production by ex tensivefractio nat ion invo lving feldspars. A chemical argu ment aga inst thepost-orogenic origin model for anorogenic su ites was also pro posedby Cre ase r et al. ( 1991) and Eby ( 1992) : if the A-type gra nitoidswere highly fractionated l-types, then the obse rved enr ichment intrace cleme nts would be a function of the degree of frac tiona l crys ­talli zation. In contras t, A-type gra nites exh ibit chemica l analysescharacterized by high Si02, Na+K 20, Fe/Mg, F, Zr, Nb, Ga , Sn, Yand REE (exce pt Eu) contents and low Ca and Ba,

Other interest ing exa mple on intra-pl ate magmatism is reportedfrom Austra lia, where the major granitic intrus ions (coveri ng 5000km2) in the Monte lsa inlier are rocks with uniform A-type geo ­chemical pattern , dated between 1870 Ma and 1840 Ma (Wy born etal., 1988). Their emplacement in the upper crust is related to atec tonothermal eve nt that produ ced the heat requ ired to cause largesca le melt ing for the generation of the A-type gra nites .

High prec ision V-Pb data from the Go thian oroge n of the south­west Sweden (Ahlill et al., 2000) indicated that westward grow th ofBaltica Shie ld occ urred in a j uve nile crus tal acc ret ionary belt formedon or amalgamated to the evolving continental margin , and the step­wise growth was approx imately synchronous with inboard , episodicrapakivi magmatism within the cra ton.

Thi s paper addresses the tectonic setting and sources of thePaleo- and Mesoproterozoic magmati c suites in the SW Amazo niancraton, Mato Grosso State, Brazil. The imp roved chronologica l res­olutio n ob tained using V-Pb and Sm-Nd methods, adde d to inte­grated geoc hemical data, allows a correlatio n between the crustalgrow th of the SW Amazo nian crat on margin and distal coevalrapakivi magmati sm within the foreland. A synoroge nic model isproposed for the origi n of the rapakivi com plexes where the highheat flow in the asthenos phere resulted from the ocea nic crust sub­duct ion that led to the developm ent of successive accre tionary arcsto the evo lving co ntinent.

The SW Amazonian Cratonconfiguration

During the last five years sys tematic high resoluti on isotop ic map­ping using V-Pb and Sm-Nd methods in the SW Amazo nian cratonhas led to beuer understanding about the framewo rk of the Protero-

The plutonic actrvity in Cachoeirinha oroge n comprises severaltonalite to gran ite plutons intruded into the Alto Ja uru oroge n rocks(Gera ldes et al., 200 1). These rocks yielded V-Pb ages ranging from1587 ± 4 Ma to 1522 ± 12 Ma with TOMfrom 2.05 Ga to 1.75 Ga andENd values from -0.8 to + 1.0. These data indica te that the ir magmaswere probably mantle-derived but with important contribution fromthe Alto Jauru basement. Geoc hemica l data are most consistent witha calc -alkaline magm atic suite, and the Cachoeirinh a oroge nic rocksprobably represe nt continental margin magmatic arc activi ty.

The rapak ivi rocks in Rondonia State reported at this time( t .58-1.52 Ga) are represented by the Serra da Providencia IntrusiveSuite. This unit occurs as a large (140 X 40 km) elongated oval­shaped batholith (Bettencourt et al., 1999) consis ting of gabbro­charnoc kite- mangeri te-granite as well as satellite stocks described inthe southeas tern part of the Rondonia T in Province. Th e V-Pb agerepor ted by Bettencourt et al. ( 1999) range from 1606 ±24 Ma to1566 ± 5 Ma. These results are in agreeme nt with the zircon V-PbSHR IMP age of 1588 ± 16 Ma obtained in a granitic rock (A-type )in the same region reported by Tassinari and Maca mbira ( t999) .

1.58-1.52 events : The Cachoeirinhaorogen and the Serra da ProvidenciaSuite

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1.79-1.74 Ga period: The Alto Jauruorogen and the Aripua na bimodal suite

The Proterozoic basement in SW Mato Grosso consists of igneousand metamorphic rocks interpreted as the southeaste rn extension ofthe Rio Negro/Juruena Province (Tassi nari and Macamb ira, 1999).Precambrian base ment in this region includes volcano-sedi mentarybelts, felsic or thogneisses and intrusive granitoids. Van Schmus etal. ( 1999) reported V-Pb zircon dates on volca noclastic rocks(metatuffs) yie lding 1.767 ± 24 Ma and Sm-Nd TOl\\ model ages of1.87 Ga (ENd(l) = +2.4) indicate that volcanism was genera ted mostlyfrom mantle-derived magma. This assumption is sim ilarly supportedby U-Pb zircon ages of 1747 ± 13 Ma and 1746 ±20 Ma (TDM= 1.93Ga and 1.77 Ga and ENd(l) values of 2.2 and 2.4, respec tively) ofortogneissic rocks (Gera ldes, 2000). In addition, Pinho (1996)reported V-Pb SHRIMP result s for volcanic rocks of Alto Jauru arcwhich yielded two age groups: the older one of 1769 ± 29 Ma and theyounges t of 1724 ± 30 Ma . Th e Roosevelt and Jamari terranes (bothyielding V-Pb ages about 1.75 Ga) are probably related to the RioNegro Jur uena geochro nological province (Scandolara et al., 1999)

Geoc hemica l and petrological studies on plutonic rocks indi­cate calc-alkaline affinity, which may be indica tive of an arc rela tedenvironment. Pinho et al. (1997) identified chemica l results of gran­itoids with ITG affini ty, whils t Geraldes (2000) found calc-alkalinetrend. The settings of mafic and ultramafic rocks ascr ibed to the AltoJauru Gree nstone belt (Monteiro et al., 1986) have been interpretedby Pinho et al. (1997), who proposed an arc-re lated setting for thebasic to intermediate rocks, and an ocea n-floor setti ng for the ultra­mafic rocks .

The anorogenic magmatism coe val with the Alto Jauru oroge nis represe nted by the Arip uan? bimodal volcanic-plutonic sequenceintruded into the Ventuari/Ta paj6s foreland province. The studiedarea (Neder et al., 2002) is located in the NE corner of Figure 1, closeto the Ventu ari/Tapaj6s and Rio Negro Ju ruen a provi nces boundary .The region is dominated by granitoid rocks intrud ed into a seq uenceof acid volcanic rocks with subordinate interca lated basa lts. V-Pb(SHRIMP) data yielded crystallizat ion ages between 1762 Ma and1755 Ma and chemica l, isotopic and geo logica l evidence indicatethat this ensialic magmatic event may represent the tectonic stabi­lization process of the Ventuari-Ta pajos Province.

o Proterozoic sedimentarycovers

_ Suns sand Aguapegroups

~ Phanerozoic covers

Roosevelt terrane

Jamari terrane

Alto Jauru orogen

Cachoc irinha orogen

Suns s orogenNova Brasil ndia orogenAguape orogen

Rio Alegre orogen

Santa Helena orogen

San Ignacio orogen

Provinc e

SunsasCycl e( 1.24- 0.93 Ga)

Rio NegrolJuru en a(1.8 0-1.52 Ga )

San Ign ac ioRond on lano(1.4 5-1.34 Gal

F igure 1 Th e accretionary events ofS lY A mazonian craton : eachProvince ha s been divided into orogenic events.

zoic terra nes. The craton has been divided (Figure I) into two majordomains (Teixeira et al., 1989; Tassi nari et al., 2000) : the Archeannuclei in which is included the Cent ral Amazo nian Prov ince ; and theProterozoic Provinces, represe nted by the Maroni-Itaca itinas (ca . 2.2Ga), the Vent uari-Tapaj6s (1.95-1.80 Ga), the Rio Negro-Juruena1.79-1.52 Ga), the Rondonian-San Ignacio (1.5 1-1.34 Ga) , and theyoungest Sunsas -Aguapef (1.24- 1.00 Ga) .

Each of these prov inces has been divided into oroge nic belts. Inthis way, the Rio Negro/Juruena Province is represented by the 1.79­1.74 Ga Alto Jauru and the 1.58-1.52 Ga Cachoeirinha magmaticarcs (Van Schmu s et al., 1999; Geraldes et al., 200 1, respectively).The Rondon ian/S an Ignacio Province is marked by important even tsinvolving magmatic arc settings and continental collisio n processesbetween 1.51 Ga and 1.34 Ga. These comprise the 1.51-1.48 Ga RioAlegre, the 1.45- 1.42 Ga Santa Helena and the 1.42- l.3 2 Ga SanIgnacio arcs (Matos et al., 200 I ; Gera ldes et al., 2000 ; Tassinari etal., 200 1, respec tively) . Finally, the youngest Sunsas /Ag uapefProvince comprises sequences deposited during bas in tecton ic (1.1Ga Nova Brasilfindia and 1.0 Ga Aguapei; Rizzotto et al., 1999; Ger­aIdes et aI., 1997, respective ly) and magmatic products (1.0 Sunsas:Litherland et al., 1986;) .

We summarize below the characteristics of the Proterozoic evo­lution and major mag matic suites in SW Amazo nian craton to sup­port the contempora nei ty of the juvenile accre tions and the inboardmagmatism .

66 588

12

1.45-1.40 Ga events: The Santa Helenaorogen and the Rio Branco and theSanto Antonio Intrusive Suites.

The Santa Helena bathol ith is a large (30 X 75 km) NW-elongatedbody comprised granitoids who se V-Pb zircon ages cluster within arelatively narrow range of 1444 ± 2 1 to 1424 ± II Ga, indicating thatthe diverse phases of the batholith were emplaced as part of a majormagmat ic episode (Gera ldes et al., 2001). Th e Sm-Nd TOM resultsrange from 1.48 Ga to 1.63 Ga, with ENd(l) values ranging from +2.6to +4.0 . The strongly positive EN d( l) values added to chemical result s(Bell et al., 1999) indicate that the magm as for the Santa Helenarocks were derived largely from ju venile or nearly ju venile source .

The contemporary inboard magmati c activities related to devel­opment of the Santa Helen a orogen are character ized in Mato GrossoState by the Rio Branco Intrusive Suit e (Geraldes et al., 1999). Thi sunit has been considered as part of a bimodal igneous suite, com­prising coeval mafic and felsic rocks which is confi ned within thevolcanic-plutonic rocks of the 1.79-1.52 Rio Negro Juruen aProvince. V-Pb iso topic analyses of zircons from the granitesyielded an upper intercept age of 1423 ± 2 Ma while those from anassociated gabbro yielded an age of 1471 ± 31 Ma. Sm-Nd TOM agesvary from 1.73 to 1.80 Ga for the mafic rocks (ENd(l) values from+ 1.24 to + 1.91) and from 1.81 Ga to 1.89 Ga for felsic rocks (ENd(l)

values from +0.16 to -0.96), which sugges t that these rocks had anolder continenta l lithosphere com ponen t in their magma. Similarmodel ages (1.93- 1.77 Ga) were recorded in the surrounding AltoJauru basement.

The anoroge nic mag matic units ascribed to the Santa Helenaorogen in northern Rondonia, particularly in the Rondonia TinProvince (Bettencourt et al., 1999), comprise dist inctly bimod alintrapl ate rapakivi suites, which intruded the ca. 1.75- 1.53 Ga RioNegro/Juruena crust. Thi s magmatism is represented by the SantoAntonio and Teoton io Intrusive Suites (V-Pb ages of 1406 Ma and1387 Ma, respectively).

1.41-1.42 Ga events: The San Ignacioorogen and the Teotonio and AltoCandeias Intrusive Suites

The San Ignacio orogen comprises a significan t syn- to post-tectonicgranitoid magmati sm (Litherland et al., 1986) represented by a calc­alkaline complex (Rb/Sr ages about 1.32 to 1.28 Ga) and by the EITigre alkaline Complex (1286 ± 46 Ma). Darbyshire (2000) reportedSm-Nd result s from the same samples studied by Litherland et al.( 1986), with TOMbetween 2.09 to 1.51 and (Nd values from -0.9 to+3.9. Geraldes et ai. (2002) sugges ted a possible extension of thisunit in Brazil , for which SHRIMP and conventional V-Pb datingyielded ages from 1.42 to 1.36 Ga. These results obtained in tonalitesand granod iorites were interpreted as magmatic event ascr ibed to theSan Ignacio orogen.

The San Ignacio arc acc retion in Bolivi a and Braz il was fol­lowed in north ern Rond onia by a prolon ged period of voluminousanorogenic rapakivi plutoni sm (Bettencourt et al., 1999) and meta­morphism (Tass inar i et al., 1999) prob ably the thermal effects ofrapakivi complex intrusions. Two distinct intraplate rapakivi agegroups which intruded the 1.75-1.53 Ga Rio Negro/Juru ena crust arerepresent ed by the Alto Candeias (1338- 1346 Ma) and SaoLourenco-Caripunas (1314-1309 Ma) suites. Despite the interpreta­tion considering the age of 1.34 Ga as the time of regional metamor­phism (Tassinari et al., 1999) assigned to the Rondonian/San IgnacioOrogeny , we can also consider that the V-Pb age of 1.34 Ga couldrepresent the Alto Candeias therm al effects leading to granulitiza tionof the older Rio Negro-Juru ena gneisses . Recent V-Pb and Sm-Nd

Episodes, \10/.27 , 1/0. 3

187

data of gneiss ic rocks of the northern Rondonia, reported by Payollaet , al. (2002), sugges t a high-grade metamorphi sm at 1.33-1.31 Ga,which corroborate with the Tassinari et al , ( 1999) result s and sug­ges ts a thermal eve nt linked to anorogenic magmatism.

1.1-0.99 Ga events: The Sunsas orogenand the Santa Clara Intrusive Suite andthe Younger Granites

The Sunsas cycle (Litherland et al., 1986) began with an importantcont inental distension event, which involved alkaline plutoni sm anddeposition of the Sunsas Group rocks. Th e geochronological data,based mainly on Rb/Sr and K/Ar ages , sugges ted that metamor­phism , deform ation and plutonism of this belt ex tended from 1280Ma to 950 Ma. The Sunsas oroge n (ca. 1000 Ma) which occurs in thesouthem part of the San Ignacio orogen ic rocks is represented byreacti vated basement , sin- and post-tectonic granitoids, and sparseoutcrop s of metasedimentary rocks. Th e San Diablo gra nitoids,descr ibed in the southernmost part of the Sun sas Orogen , are inter­preted (Saes and Fragoso-Cesar, 1996) as juvenile magmatic prod­ucts formed durin g the S-SW dipping subduction zone responsiblefor Sunsas magmatic arc.

The effects of the Sunsas oroge n in the northern Rondoniaregi on occurred between 1.07 Ga and 0.97 Ga (Bettencourt et al.,1999). The magmatism in Rondonia is co mposed of rapakivi gra n­ites and associated mafic rocks, including the Santa Clara Intrusivesuite (1.07 Ga ) and Younger Granites of Rond onia (1.0-0.9 7 Ga).The Santa Clara Intrusive suite encompasses the granites from thefollowin g massifs : Santa Cla ra, Oriente Velho , Oriente Novo , andManteiga. The older rock association is composed of porphyriticquartz-mon zonite, monzonite and syenog ranite with subordinatedamounts of quartz-monzonit e and less pyterlit e. Biotite and minorhornblend are the main mafic minerals and zircon, apatite, ilmenite,magnetite, allanite, fluorite and sphene are esse ntial minerals. Ayoun ger association includes syenite, trachyt e and peraluminous andperalk aline gra nites

The Youn ger Granit es of Rond onia are subdivided by Leite Jr,( 1996) and Bettencourt et al., ( 1999) into two distinct suites. The firstone is composed of metalum inous to marginally peraluminous sub­solvus and subalkaline featu res with minor associ ated qua rtz-syen­ite, qu artz-monzonite, monzonite and the second, restricted in area,shows an hypersolvus charac ter and as well alkaline affinity. Thefield relation ship s sugges t that the alkaline rock s arc younger thanthe subalkaline types.

Discussion and conclusion

Th e western part of the Amazonian craton is a multi- orogenic regionformed betw een 1.8 and 1.0 Ga (Figure 2) where success ive magma­tism , metamorphism , and deformation took place. Therefore, juv e­nile accretionary events progressively amalgamated to the older con­tinent al margin durin g the Paleo- and Mesoproterozoic times givesthe framework of the evolution of the SW Ama zonian craton, likeBalti ca and Laurenti a shields.

The definition of dist inct accretionary stages on the older crus tand their temp oral correlation of these stages with the rapakiviepisodes (Figure 3) sugges t that the latter were probably related toprocesses occurring at the evolving continental margin. Therefore,the temporal link between oroge nic belts and the peculi ar continen­tal magmatism in SW Amazonian craton may be explained by reor­ganization of intracratonic lithosphere stresses when eac h episode ofsubduction stepped westward (actual). We co ncur with ex isting pet­rogenetic models for the origin of the rapakivi granites, which theirgenera tion in lower lithosphere sources may be explained by decom­pression melt ing, with asce nt probably guided by features of the pre-

September 2004

Referen ces

(!1'lJArc-relal ed mogmohsm ~

Figure 3 Sketch summanzm g the synonorogenic model hereproposed f or th e origin of the Proterozoic granitic magmatism.Integrated isotopic and geoc he mical data allow a temporalcorrelation between the crustal growth of the SH' A ma zoniancraton western (actual) ma rgin and contempouany emplaceme ntof bim odal magm atism withill the fo reland. Length of ballonsrepresents th e tim e ran ge of the magmatic activity.

Ahiill, K-I, Connelly, J .N., and Brewe r, T .S., 2000 . Episodic rapak ivi mag­matism due to distal oroge ncs is?: co rrelation of 1.69 - 1.50 Ga orogenicand inboard, "anoroge nic" events in the Baltic Shield, Geology, 28:823­826.

Anderson, J. So derlung . V., Cornell, D., Johansson , L., Mollcr , C. 1999. Sve­conorweg ian (Grenvillian) deforma tion , metamorphism and leucosomeformation is SW Sweden, SW Baltic Shield: constra ints from a Mesop ro­terozoic granite intrusion . Precamb rian Research, 98:151-171.

Bell, S., Condie, K. and Geraldes, M.C. , 1999. Origin of arc-related gra ni­toids from the SW Amazo n cra ton: Juvenile crustal additions at 1550­1450 Ma. Th e Geo logical Society of Amer ica 1999 Annual Meetin g.Denver-CO . Abstrac t with Program s, 205.

Bettencourt, J.S., Tosdal , R.M., Le ite JR., W.B., and Payolla , B.L. , 1999.Mesoprotero zoic rapakivi granites of the Rondonia Ti n Province, south­westem border of the Amazo nian craton, Brazil - I. Reconn aissance V-Pbgeoc hro nology and regional implications. Precamb rian Research 95:41­67.

Creaser, R.A.; Price, R.C.; and Wormald, R.1. 1991 A-type gra nites revis­ited:assessment of a residual-source model. Geology 19: 163-166.

Collins, W.1. Beans, S.D., White, A.1.R. and Chappell, B.W. 1982 nature andorigi n of A-type gra nites with particular references to southeastern Aus­tralia. Contribuitions to Mineralogy and Petro logy 80: 189-200.

Demp ster , T.1., Hutton, D.H.W., Harri son, T .N., Brown , P.E. and Jenkin,G.R.T. 1991 Tex tural evolution of the rapak ivi gra nites, south Green­land- Sr, a and II isotopic investigations. Contribuitions to Mineralogyand Petrology, 107:459-471.

Darbyshire, D.P .F. 2000 The Precambri an of Eastern Bolivia - a Sm-Nd iso­tope study. Ill: 3 1 International Geo logic Congress, Rio de Janeiro,Brazil. Abstract Volume (CD-Roo m).

Eby, G.N. 1992 The A-type gra nitoids : a review of their occ urence andchemical charac teris tics and spec ulations on their petrogenesis. Lithos26: 115-134.

Geraldes, M.e. , 2000 . Geo qufmica e Gcocronolog ia dos plutons gra nfticosMesoproteroz 6icos do SW do Estado de Malo Gro sso (SW do Cnit oAmaz?nico). Universidade de Sao Paulo, Sao Paulo-Sf' , Braz il, unpub­lished Docto ral dissertation, 185 p.

V",,:uarllopq s RioNegrO/.k.ruero P10vince R<Jo:;or;on'San Ign cia stovrce Slns sProv.ncoAge

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1.790 Me to1740 Me

1420 Me to1380 Me

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Fig ure 2 Hypoth etical ske tch for the evolution ofSW Amazoniancraton from 1.8 Ga to 1.0 Ga. Ages are refered to V -Pb(co nve ntional and SHRIMP ) data. Th e juvenile arc-relatedmagmatism s have their coeval bimodal magm atism within thecontinent.

cursor accretionary event. In add ition, the genetic link of accre­tionary events and rapakivi magmatism hypotheses may take intoacco unt (i) an overthickened lithosphere which atte mpted to regainisostatic equilibrium, result ing in extensional collapse, and (ii) themantle conditions beneath the large and immature crustal terrane ofthe arc-accretionary SW sector of the Amazonian cra ton had quiteconstant co nditions from 1.8 to 1.0 Ga . This means that the newaccre tionary setting always found a sialic crust formed recently,what may have important consequences for the high heat flowbeneath it.

Up to now, the Rio Alegre accretionary eve nt does not haverespective inboard magmatism reported. The 1.51-1.50 Ga Rio Ale­gre oroge n is the only ju venile magmatic event with lack of coevalrapakivi complexes which may be explained probably due to the ori­gin of this magm atism. It had taken place away (as an exotic terrane)from the continenta l margin of the Amazo nian craton at that time, asspeculated in Figure 2.

The hypothesis here presented also has consequences for thepaleogeograp hic reconstruction of a Mesoproterozoic ( 1.6- 1.5 Ga)supercontinent. If the correlation between accretionary belts andcontinental magm atism is correc t, and if there are tempo ral correla­tions between accre tionary belts and rapakivi granites in Baltica,Laurenti a and Amazonia, then the paleogeographic configurat ions ofthese conti nental masses should show the jux taposition of a continu­ously evolving margin with contiguous landmasses in which intra­continental magmatism was taking place.

1000 Ma

Geraldes, r-,'I.C.• Figueiredo. B.R ., Tassin ari , e.e.G., and Ebe rt, H.D., 1997 .Mid dle Proterozoic vei n-hosted go ld dep osits in the Pont es e Laccrd aregion, southwestern Amazonian craton, Bra zil. Internat ional GeologyReview 39:438-448 ,

Gc raldes, M.e., Betten court . J.S. , Van Sc hmus, Te ixe ira, W., c Matos , J.B.1999. The Mcsop rotcrozoic Rio Branco suite , SW Am azon Craton,Braz il: A reconn aissance isotope study on a bimoda l magmatic co mplex .III The or igin of Granites and Related Rock s. B. Barbarin (ed .). Abstractsof For th Hutton Symp osium : p. 149.

Geraldes, M.e. , Teixe ira, W. , and Van Sc hmus , W. R., 2000. Isotopi c andchem ica l evi de nce for thr cc acc retioa ry magm atic arcs ( 1.79- 1.42 Ga) inSW Amazon craton, Mato Gro sso sta te , Brazi l. Rcvi sta Brasileira deGeociencia s 30:99-10 I.

Ge raldes , M.e., Tassinari, e.e.G., Teixe ira, W. , and Van Schmus, W.R .,2002. Gcocrono log ia U- Pb con venciona l e SHRIMP c Sm-Nd de rochasgra nit6idcs na Se rra Santa Barba ra (S W do Esrado do Mato Grosso):Uma possivcl ex tensiio do oroge no Sa n Ignac io da Bol ivia (?) Con ­tribuicoes a Geo logia da Amazo nia, vo l. III: 143-152

Geraldes M.C. Van Sc hmus , W.R. , Cond ie, K.C.; Bell S.; Tei xe ira, W.Babinski, M. 200 I. Proterozoic Geo log ic Evo lutio n of the SW Part of theAm azoni an Crato n in Mal o G rosso State, Brazi l. Precambrian Resear ch,111:91-128.

Hoffman, P.F., 199 1. Did the breakout of Laurentia turn Go ndwa nalandinside out? Science 252: 140 9- 1412.

Litherlan d, M.; Annels, R.N .; App leton, J.D.; Ber range, J .P.; B;oomfie ld, K.;Burton , C.CJ .; Darbyshir e. D.P .F. ; Fletcher. CJ.N.; Hawkin s, M.P.;Klink , B.A .; Llanos, A.; Mitchel, W.I.; OConnors , E.A.; Pit field , P.EJ .;POW ER, G c we rb, b.c . 1986 tHE Geo logy and Minera l Reso urces of theBolivian Precambrian Shie ld, British geolog ical Survey. Ove rseas Mem­o ir 9. Lond on, Her Majesty°o/"cs Stationery O ffice. 140 p.

Matos, J.B. , Sc horschc r, J. H. D. , Gcraldes . M. C. and Souza, M. Z. A. 200 1The Rio A legre Volc anosed imcntar Se quence (SW Amazonian Craton,Brazil ): Chemica l and ISOIOpc (U/ Pb and Sm/N d) co nstra in and tectonicimplica tions Geo logy of the SW Amazonian Craton: State-of-the-Art".Extende d Abstract Vo lume, p. 56-59.

Mo nteiro, H. , Maced o, P.M ., S ilva , M.D., Mo raes, A.A ., and Marcheto,e.M.L., 1986. 0 gree nstone belt do Alto Jaunl. XXX IV Co ngressoBrasil ciro de Geo logia 2 , 630-646.

Nedcr, R.D .N., Leit e , J. A. D., Figueiredo, B.R., McNaught on , NJ . 2002 1.76Ga volcano-plutonism in the southwes tern Amazonian craton, Arip uanii­MT , Brazi l: Tcc tono-s tratigraphic implications from SHRIMP U-Pb zir­co n da ta and roc k geoc he mist ry. Preca mbr ian Research 119:17 1- 187.

Payoll a, B. L , Bettenco urt, J .S. Kozuch , M., Le ite , Jr , W. B., Fett er, A.H. andVan Schmu s, W.R. 2002 Geologica l evo lution of thc basement rocks inthe eas t-ce ntral part o f thc Rondoni a T in Provin ce, SW Amazonia n cra ­ton , Brazil: U-Pb and Srn-Nd isoto pic constra ints , Precam brian Research119:141 -169.

Pinh o, F.E.C., 1996 Th e origin of the Cabacal Cu-A u deposit , Alto Jaurugree nstone bel t, Brazil. Unive rsi ty of Western On tario, Lond on- ON,Canada, Ph.D. thesis, 2 1I p.

Pinh o, FooE.e., Fy fc, W.S., and Pinho, M.A.S .B., 1997. Early Proterozoicevolution of the Alto Jau ru gree nstone be lt, sou thern Amazonian craton ,Bra zil. Intern at ional Geo logy Review 39 :220-2 29.

Ram o, T. and Haapall a, I., 1995 One hun dred yea rs of Rap akivi gra nite.Me ni r. Petrol. 52 : 129-1 8 5.

Rizzotto, G .L. , Chcmale Jr. ,F., Lima. E.F., Van Sc hmuss , R., & Fe tte r, A. ,1999. Dados isotopi cos Sm-Nd e U-Pb das rochas da Sequej ncia meta­plut ono-vulc anossed iment ar Nova Brailan dia (SMNB) -RO (1999) - VISimposi o de Geo log ia da Amazo nia, Ext. Abstrac t Bul l., p.490 -493.

Sacs , G .S.; & Fragoso Ce sar, A. R. S. 1996. Ac rescao de terrenos mesopro­terozoicos no SW da Amazonia. In: 39 Co ngre sso Brasileiro de Geo logia,Abstra cts p.348.

Sca ndo lara, J.E., Rizzo tto, G J ., de Amo rim, J .L., Bahia, R.B.C., Quadros ,M.L., & C.R. da Si lva, 1999. Geo logical map of Rond onia I: 1,000,000.Companhia de Pesquisa de Recur sos Min crai s.

Tassinari, e.e.G. & Macamb ira, MJ .B, 1999. Geochro nolog ica l Prov incesof the Amazonian Craton. Episode s, 22(3): 174-182.

Tass inari, e.G., Bet tencourt , J.S., Ge raldes, M.C., Macambi ra, MJ.B., andLafon , J.M., 2000. Th e Amazon cra ton. III Co rdani, U., Mil ani, EJ .,Th om az Fi lho, A., and Ca mpos, D.A. , cds., Tcc tonic evolutio n of So uthAme rica , 3 1st Intern ation al Gcolog ical Congrcss , Rio de Janeiro, Brazil ,4 1- 95 .

Teixei ra, W., Ta ss inari, C.e.G., Cordani, U.G., and Kawashi ta, K., 1989. Arcvi ew of the gcoc hrono logy of thc Am azonian craton: tcctonic impli ca­tions. Prccambrian Rcscarch 42 :2 13-227 .

Van Schmus , W.R., Gcrald cs , M.C., Fe tte r, A.H.. Ru iz, A., Matos, J. , Tassi­nari , e.e.G., and Tci xci ra, W., 1999. Jauru Tcrr anc: A latc Palcoprotcro-

Episodes , Vol. 27. no .3

189

zo ic orogcn in SW Amazon craton, Mato Grosso State, Brazil. Euro peanUnion of Geosc iences Mee ting (E UG - IO), Journal of Confere nceAbstrac ts 4, 129-130.

Wind ley , B.F. 1991 Ear ly Proterozoic co llision tectoni cs , and rapakivi gra n­ites as intrus ions in an exte nsio nal thru st -th ickened crust: the Ketilid ianorogen, south Greenland. Tectonophysics, 195 :1-10 .

Wyborn , L.A .I., Page, R.W. and McCu lloch , M.T. 1988 Petrology, Geo­chronology and iso tope geoc hemistry of thc post- 1820 Ma granites of theMount lsa inl ier : mechanisms for thc ge nera tion o f Proterozoic anoro ­genic gra nites. Preca mb rian Research, 40/4 1:509-54 1.

Mauro Cesar Gera ldes is professorof Geochro no logy at Faculty ofGeology/Rio de Jan eiro State Uni­versity! Brazil. He worked in the min ­era i industry in Amazonia reg ionfr om 199810 1994 and gal his masterdegree in State University of Camp­inas (Brazil) in economic geo logy .He received his PhD in geoc hemistryand geochronology from Universityof Sao Paulo (Brazihltlniversity ofKansas (EUA) in 2000. He earnedpost-doctorates at University of SaoPaulo on metallogeny, His currentresearch is fo cused on crustal evo lu-lion and its temp oral corre lationswith min eral deposits.

l l'ilson Teixeira is a Full Professorof the Geosciences Institute of theUnivers ity of Sao Paulo (US P). andm ember of the Geochronolog ica lResearch Center of this institution.Director of the Estaciio Ciencias ofUSP . Me mber of the Bra zilianAcademy ofScie nces . and researcher1A of the Bra zil ian NationalResearch Council, CNPq. Co- leaderof IGCP-204 "Evo lution of theAmazonian Craton (UNESCO/ lUGS1983-1987). Main areas of interest:geochronology and crustal evolutionof Archean and Proterozoic terranesand tectonics of Precamb rian maficdikes.

MOllica He ilbro n is professor ofGeotectonic at Faculty of Geology/Rio d e Jan eiro S ta te Un iversitylBrazil. She is also the head of theTEKTOS Research Group that works011 se veral themes related withgeological mapping, tectonics, geo­chro nology , petrology and theirapplications 011 economic geology andenvironment problems. She gal herPhD degree at Sao Paulo University in1993. Her major interest is related 10the tectonic evolution of the Neo ­proterozoic Ribeira bell, SE Brazil,and its re lati onsh ip wilh Iheamalgamalion ofWeslem GOlldwana.More recenlly she is workillg wilh lhebasemenl reactil'Olioll dllring Ihebreak-lip of Go nd walla and thelectonic inherilance al Campos andSmllos basin.