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Short Papers - IV South Americall Symposium all Isotope Geology J ()o.o 3

ISOTOPIC AND GEOCHRO NOLOGICAL CONSTRAINTS FOR THE ORIGIN OF SYN- TOPOST-BRASILIANO MAFIC MAGMATI SM AND CRUSTAL RE-MELTING

IN THE BRASiLIA BELT (CENTRAL BRAZIL)

Hollanda, M.H.B.M.I ; Pimentel, M.lVI? and Ar mstrong, R,3

I. Centro de Pesquisas Gcocronologicns, Instituto de Geociencias - Univcrsidadc de Siio Paulo, Run do Lago 562,Cidade Univcrsitaria, 05508-080, Sao Paulo-SP, Brasil

2. lnstituto de Gcociencias - Universidade de Brasilia, Campus Universitario Darcy Ribeiro, 70910-900, Brasilia-OF, Brasil3. Research School of Earth Sciences,Australian National University, Canberra,ACT0200, Australia

Keywords: Brasilia Belt, mafic magmatism, U-Pb SHRIMP, Sr-Nd isotopes, Brasiliano orogeny

INTR ODUCTIO NThe syn- to post-oroge nic stages of the Brasiliano

orogeny in central Brazil were characterized by theemplacement of numerous mafic intrusions that occureither associated with larger volumes of K-rich graniticintrusions, within the Neoproterozoic Goias MagmaticArc or as individual mafic-ultramafic layered intrusionswithin the arc and in the so-called Anapolis-Itaucu high­grade Complex, to the east. Until recently, very fewgeochronological constraints existed for the emplacementages of these mafic intrusions, and they were mainlyindirect ages based on Rb-Sr data for the associatedgranites (Pimentel et al., 1996). Recent U-Pb zircon andSm-Nd isotopic data, allied with previous regionalgeological knowledge, indicat e that the Neoproterozoicmafic magmatism played an important role in theBrasiliano tectonic evolution in Central Brazil, and wasprobably close ly related to the heating, crustal re-meltingand high temperature metamorphism in both GoiasMagmatic Arc and Anapolis-Itaucu Complex.

REGIO NAL GEOLOGYThe Brasi lia Belt is a large Neoproterozoic orogenic

belt developed along the western margin of the SaoFrancisco Craton, Central Brazil. It is composed of fourmain tectonic segments: (i) a thick Meso-Neoproterozoicmetasedimentary/sedimentary pile comprising typicalpassive margin (Paranoa and Canas tra groups) and back­arc (e.g., Araxa and lb ia groups) sucessions, and ayounger unit (Bambui Group) probably representing apost-inversion, foreland sequence; (ii) a largeNeoproterozo ic juvenile arc in its western portion (G oiasMagmatic Arc); (iii) a micro-continent (or allochthonoussialic terrain) formed mainly by Archaean rock units (theCrixas -Go ias granite-greenstones) and Paleo- toMesoproterozo ic mafic-ultramafic high-grade complexes(Barro Alto, Niquelfindia and Ca nabrava complexes), and(iv) a metamorphic core complex comprising felsicgranulites and sillimanite-garnet and hypersthene-bearinggneisses and intrusive peraluminous granite, known asthe Anapolis-ltaucu Complex, in the central part of thebelt.

Two distinct high-grade metamorphic events areconstrained in the southern (Anapolis-Itaucu Complex)and northern parts of the Brasilia Belt. While high-grademetamorphism in the northern complexes occurred

between ca. 740-780 Ma (Ferreira Filho et al., 1994;Suita et al., 1994; Correa et al., 1997), granulites in theAnapolis-Itaucu Complex are younger and metamorphicrecrystallization took place at ca. 650 Ma (Fischel et al.,1998; Piuzana et al., 2002) . Sm-Nd isotopic signature andSHRIMP U-Pb metamorphic ages of the Araxametasediments and the Anapolis-Itaucu granulites and itsintrusive granites (650-640 Ma) are all very similar(Pimentel et al., 1999a; Fischel et al., 1999a, b; Seer,1999), suggesting that the granulites might representhigh-grade equivalents of the Ara xa metasedimentaryrocks and that at least some of the intrusive peraluminousgranites are partial melt products of this metasedimentaryprotolith (Pimentel et al., 1999a; Fischel et al., 1999a).Mineral assemblage containing sapphirine and quartz ingarnet-orthopyroxene-sillimanite granulites of thatcomplex indicates metamorphism at ultra-hightemperatures (ca. 1050 °C at 10 Kbar) (Moraes et al.,200 I ), compatible with extensive re-melting of the crust.These new geochronological and isotopic data, therefore,suggest that the Anapolis-Itaucu complex may be nowconsidered to be the metamo rphic core complex of theBrasilia Belt and not the sialic basement of the AraxaGroup (Piuzana et al., 2002). Mafic-ultramafic bodiesremained, however, undated, and their tectonicsignificance, uncertain.

The Golas Magmatic Arc, in the westernmost part ofthe Brasilia Belt, consists of a large Neoproterozoicju venile terrane formed by arc-type volcano-sedimentaryrocks and tonalite/granodiorite gneisses. It comprises twomain magmatic terranes - the Areno po lis and Mara Rosaarcs, located in western and northern Goias, respectively(Pimentel & Fuck, 1992; Fuck et al., 1994; Pimentel etal., 1997, 2000a, b). In both areas, geological evolutionstarted at ca. 900-860 Ma with the crystallization of veryprimitive calc-alkaline volcanics and associa tedtonalites/granodiori tes (ENd values between ca. +3 and +6,and T D~ I values mostly between ca. 0.8 and 1.1 Ga;Pimentel et al., 1991, 1997, 2000; Pimentel & Fuck,1992). Geochemical and isotopic data from s uggest thatthe original magmas were formed in an intraocean icmulti-arc system (Pimentel, 1991, Pimentel et al., 1997).Calc-alkaline igneous activity lasted until ca. 640 Ma, andthe main deformational-metamorphic episode occurred atca. 620 Ma, as indicated by U-Pb titanite and Sm-Ndgarnet ages (for a review, see Pimentel et al., 2000).

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During the waning stages or immediately after the lastBrasi liano de format ion, a num ber of small mafic­ultramafic layered co mplexes, gabbro -dior itic intrusions,and large granite plutons were emplaced into the Go iasMag matic Arc (Pimentel et al., 1996). In the Are nopo lisregio n, Rb-Sr and U-P b data suggest that the Brasi lianogra nite magmatism took place in two distin ct episodes : (i)an older eve nt be twee n ca . 590 and 560 Ma, and (ii) ayoung eve nt dated be twee n ca. 508 and 485 Ma (Pimen telet al., 1996 ).

T HE MAFIC-ULTRAMAFIC INTRUS IONS IN THEANA pO Ll S-ITA U<;: U CO MPLEX

Layered , mafi c-ultramafic intru sions in the Anapo lis­ltaucu Co mplex form NW-SE elongated bod ies, parallelto the region al deformational fabri cs o f the gra nuliticco untry-rocks. Exa mples of mafic-ult ramafic bodi es are(i) the Goianira-Trindade Co mp lex, which is made up ofpyroxenit e, ga rne t-hypersthene pyroxenit e, metagabb ro,amphibole schist, and is associated with suprac rustalca lc-si licate rocks (N ilso n & Mo tta, 196 9) and the (ii)Ta quara l zo ned intrusio n comprising gab bros andperidotit es marginal to main pyroxeniti c and gabbro icunit s, position ed in tec tonic co ntact with granulitegneiss ic rocks (Si lva , 1997) . They frequently dis play welldevelop ed foli ation and mylon itic fabri cs, however, relictfeatures such as cumulate textu res and igneous layeringare fou nd in most of the intrusions, attest ing to theirorig ina l layered charac ter (Silva, 1991 , 1997).

SYN- TO POST- OROGENI C MAFIC AN DULTRAMAF IC INT RUSI ONS IN T HE GOlASMAGMATIC ARC

In a broad regional sca le, the syn- to post-orogenicigneous activity in the Go ias Magmatic Arc is typicallybimodal , including large bodies of K-rich ca lc-alka linegranites (e .g., Serra Negra, Serra do Iran , Ca iapo, Ipora,Sanclerlftndia and Se rra do Impertinente gra nites)assoc iated with minor gabbo -dioritic end-members. Smallmafic-ul tram afic layered complexes (e .g., Americano doBrasi l and Corrego Lageado co mplexes) occ ur asindi vidual bodi es. The gabbro-di oriti c intru sion s andmafic-ult ramafic layered complexes are either onlysligh tly deformed or completely free of any pervasivedefo rmational fabr ic. Th e gabbro-diori tes have typical arcgeochemica l signa ture and commonly displ ay magmamixing features with gabbro "g lobules" enclosed in

gran itic rocks.

Short Papers - IV SOli/IIAmerican Symposium 0 11 Isot ope Geo logy

determin ed relative to those measured in the RSESsta ndard SLl3 .

Sm-N d isotopic analyses followed the methoddescrib ed by G ioia & Pimentel (2000) and were car riedout at the Geochrono logy Laboratory of the University ofBras ilia. Sm and Nd samples were loaded on Reevapo ration filaments of doubl e filament assemblies andthe isotopic meas urements were carried out on a mul ti­co llec tor Finnigan MAT 262 mass spec tro mete r in stat icmode. Uncertainties for SmlNd and I ~ J Nd/I ~~ Nd ratios are

better than ± 0.4 % ( 10") and ± 0.005% ( 10") resp ectively,based on repeated analyses of internat ion al rock standardsBHVO-I and BCR-l . 14JNd/ 144Nd ratios were normalized

to 146Nd/144Nd of 0.72 19 and the decay constant (A) usedwas 6.54 x 10. 12

• Isochron ages and parameters werecalculated using Isopl ot/Ex 2.4 7 (Ludwig, 200 Ib).

SH R IM P AN D C ON VENT IONA L U-pb R ESULTSSa mples from four mafic intru sion s - Rio Ca iapo

gabbro and Se rra do Iran diorit e, and the Co n-egoLageado and Americano do Brasil co mplexes werese lected for D-Pb analyses .

RIO CAIApO GAB BROZircon grains from this gabbro are pink , euhedral to

subhedral and show strong concentric compositionalzoning. Cathodoluminescence imagens reveal oscilatoryzoning and cores of possibly inherited zirco n. Only thezoned rims and crys tals were analysed . S ix analysis haveprodu ced a discordi a indic at ing the upp er intercept age o f

589± 19 Ma (M SWD =0.9) .

SE RRA DO IRAN DIORI T EZirco n form pink , prismatic stubby crysta ls.

Cathodo luminescence imaging sho ws thick magmaticconce ntr ic comp ositional zoning . Core might be present ,but these were not analysed. Ten spot analys is defined adiscordia with an upp er intercept age of 623 ± 16 Ma,which ag rees with previous Rb-Sr data indicatin g theisoc hro n age o f ca. 622 Ma (Pimente l et al., 1996).

CO RREG O LAGE ADO COMPLEXZirco n grai ns are co lourless to light pink forming very

clear, frac ture- and inclusion -free crys tals.Ca thodo luminescence imaging shows well-developedsector and subord inate conce ntric co mpos itional zo ning.Fourteen analysis yie lded conco rdant res ults which

indicate a concordia age of 672 ±6 Ma.

ANALYTICAL PROC ED URESU- Pb ana lysis were carried out using SHRIMP I and

II at the Research Schoo l of Earth Sc iences, Austra lianNational Univers ity, Ca nberra . Data were co llected andreduced as described by W illiams & Claesson ( 1987) andCo mpsto n et al. ( 1992). Unce rta inties are given at I 0"level, and final age was quoted at 95% confidence level.Reductio n of raw data was carried out using Sq uid 1.02(Ludwig, 200 Ia). l.l/Pb ratios were referenced to theRSES sta ndard zirco n FC I ( 1099 Ma,·2116pb/m U=0.1859). U and Th conce ntra tions were

AMERICANO DO BRASIL COMPLEXMos t of the gra ins are long and tabul ar displ aying

thick zoning typical of crystals formed in mafic magma s.Cathodo luminescence images show no obvio us inheritedcores. Mag matic cores are surro unded and embayed bythin and very bright rim , which certainly have low U andTh, and might represe nt late stage crystallizat ion from themafic magma.

Eight spot analysis yiel ded a concordia age of 63 1±6Ma. Sm/Nd analys is of mafic and ultramafic rocks fromthis complex de fine an isochron (MSW D = 0.9)

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indicating the age of 6 16±8 1 Ma (2a), with ENd (T) of+2.4. The large uncertainty in Sm-Nd isochron age iscaused by the limited spread in Sm/Nd ratio of thesamples used .

GO IAN IRA-TR INDADE COM PLEXZirco n gra ins separated from one leucogabbro sample

of this complex were analysed by ID-TIM S. Threeconcordant analysis yielded the concordia age of 626 ±2Ma which is interpreted here as the best estimate for thecrystallisation age of the complex.

SIIl-Nd ISOTOPI C RESULTSSr and Nd isotopic analyses were carried out on

thirteen representative samples of the mafic magmatismin Goias Magmatic Arc, seven of them from the syn- tolate-orogenic gabbros and diorites (Rio Ca iapo, Serra doIran, lpora, Israelfindia), and six from the CorregoLageado and Americano do Brasil mafic-ultramaficcomplexes. These samples exhibit a relatively narrowrange of initial 87Sr/86Sr ratios from 0.70262 to 0.70363,excluding one samp le from the Serra do Iran gabb ro with87 Sr/86Sr ratios of 0.7056. ENd values calculated for theirrespective U-Pb crystallization ages vary from slightlynegative (-0.75 to -0.32) to positive (+2 .1 to +5.5) . Thelpora diorite presents moderately negative ENd value of ­2.14, calculated for ca. 500 Ma crysta llization agedetermined for a contemporaneous and spat iallyassoc iated granite. 1~7Sm/l~4Nd ratios ranging from 0.11to 0.153 combined with higher than 0.511 8 1 43 Nd/ 1 4~Nd

ratios provide T D~I ages betwee n ca. 1.1 and 0.88 Ga,closely coeva l with the rocks from the Neoprote rozo icjuveni le Goias Magmatic Arc. T DM ages higher than 1.1Ga for the lpora, Serra do Iran and lsraelandia dioritesseem to indicate some incipient contamination with oldercrustal rocks.

Sm-Nd isotopic analysis were also carried out insamp les from the Goianira-Trindade mafic-ultramaficComp lex. These are metamorphosed into amphibo lite togranulite facies, however samples used in this study arefrom outcrops in which igneous textures and structuressuch as composi tional layering and cumulate textures arepreserve d. Seve n samples define an "e rrorchro n"

(MSWD = 6.1) with age of ca. 621 Ma. The ENd (T) valueis close to zero (-OJ), suggesting some degree of crustalcontamination of the original magma.

CONCLUS IONSThe SHR IMP U-Pb zirco n ages for mafic rocks

within the Goias Magmatic Arc and the Anapo lis-lta ucugranulite Complex prese nted in this study represe nt thefirst geoc hronological constraint for the emplacement ageof these rocks. The data suggest that mafic magmatismwas important between ca. 670 and 590 Ma, and probablyplayed an impor tant role during the final stages of theNeopro terozoic evolution of this oroge n.

Within the Go ias Magmatic Arc, mafic rocksassoc iated with the Serra do Iran, Rio Caiapo and Iporagranite intrusions and both Corrego Lagead o andAmericano do Brasil complexes indicate that mafic

SIIor/ Papers - IV So/IIII American Symposium a ll Isotope Geology

magmatism was important mainly dur ing late- to post­orogenic stages. Nd isotopic data for these rocks revealmostly positive ENd(T) values between ca. +5.5 and +2. 1,indicating the depleted nature of the mantle source andlittle contaminat ion with older continental crust.

All ages presented in this study overlap with U-Pbtitanite and Sm-Nd garnet (between ca. 600 and 630 Ma)ages for metamorphic rocks of the Goias Magmatic Arc(Pimentel et al., 2000) and with SHRI MP U-Pb ages forgran ite emplacement and high-grade metamorph ism inthe Anapo lis area (Piuzana et al., 2002) . This s uggeststhat final metamorphism and mafic magmatism tookplace roughly within the same time interval, and werecoeva l or shortly followed by uplift, extension anderos ion of the orogen (Pimentel et al., 1996, 2003). Upliftwas probably more pronounced in the Anapolis-ItaucuComp lex allowing expos ure of high gracle rocks, and alsoof a larger amount of mafic intrusions, when comparedwith the Goias Magmatic Arc.

H is likely that the late Neo proterozo ic maficmagmatism have underplated importa nt fractions of thecentral and western parts of the Brasilia Belt continentalcrust, ancl may have been responsible for the deve lopmentof ultra-high temperature metamorphic mineralassemblages, observed in the Anapolis-ltaucu Complex.

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Short Papers - IV Sou/II Americall Symposium Oil Isotope Geology

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