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Journal of South American Earth Sciences 32 (2011) 369e378

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Journal of South American Earth Sciences

journal homepage: www.elsevier .com/locate/ jsames

Heterogeneity in syntectonic granitoids emplaced in a major shear zone,southern Brazil

Cláudia Regina Passarelli a,*, Ian McReath b, Miguel Ângelo Stipp Basei b, Oswaldo Siga Jr. b,Mário da Costa Campos Neto b

a Federal University of São Paulo, Department of Exact and Earth Sciences, Rua Prof. Artur Riedel, 275 Jd. Eldorado, CEP 09972-270, Diadema, SP, Brazilb Institute of Geosciences, University of São Paulo, Rua do Lago 562, Cidade Universitária, CEP 05508-080, São Paulo, SP, Brazil

a r t i c l e i n f o

Article history:Received 10 September 2010Accepted 3 March 2011

Keywords:Major Gercino shear zoneSyntectonic granitoid associationsSr isotopic heterogeneity

* Corresponding author. Tel.: þ55 11 38621346; faxE-mail address: crpass@usp.br (C.R. Passarelli).

0895-9811/$ e see front matter � 2011 Published bydoi:10.1016/j.jsames.2011.03.002

a b s t r a c t

Two granitoid associations were emplaced during the development of the Major Gercino Shear Zone ineastern Santa Catarina, southern Brazil. The 614 Ma Fernandes Granitoid Association is composed mainlyof hornblende syenogranites including A-type, and alkali-calcic and calc-alkaline types. Initial Sr isotoperatios are heterogeneous, but systematic isotopic and element variations suggest that different sourcerocks produced magmas which underwent mixing and fractional crystallization. The Rolador GranitoidAssociation of biotite-bearing granodiorite is less isotopically heterogeneous, but systematic isotopic andelement variations are not obvious.

� 2011 Published by Elsevier Ltd.

1. Introduction and brief geological setting

Over the last ten years structural geology and more refinedgeochronological methods have been applied to demonstrate theintimate relationship between shear zones activity and theemplacement of granite masses in many orogenic belts (e.g.Vigneresse,1995; Paterson andMiller,1998; Brownand Solar,1998a,1998b; Liotta et al., 2008).

Shear zones have a high potential to generate granitic melts(Hutton and Reavy, 1992), and the action of differential stresses canpromote the segregation of melt formed in feldspathic rocks(Kohlstedt et al., 2010).

The Dom Feliciano Belt (DFB), a geotectonic unit in the southernpart of the Mantiqueira Province (Almeida et al., 1981; Silva et al.,2005), occupies the entire eastern part of Southern Brazil andUruguay (Fig. 1). It is composed, from its northern limit in SantaCatarina to its termination in Uruguay, of three crustal portionsseparated by tectonic contacts. From southeast to northwest, thesecrustal portions are: a granitoid belt containing calc-alkaline toalkaline rocks, deformed to different degrees; a meta-volcanose-dimentary belt of greenschist to amphibolite facies rocks; anda foreland basin with sedimentary and anchimetamorphic volcanicrocks, which abuts onto the older terrains to the west.

The Major Gercino Shear Zone (MGSZ) is part of the SouthernBrazilian Shear Belt, a lithospheric-scale discontinuity, active from

: þ55 11 3091 3993.

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650 to 580Ma (Bitencourt andNardi, 2000, 2004). This shear zone isoneof themajorNEeSWlineaments that affect all southernBrazilianand Uruguayan Precambrian terrains (Fig. 1) and is characterized bya regionalNE trendandanoblique resultantdextralmovement sensewhere ductile-brittle structures predominate (Passarelli et al., 2010).

In the eastern part of the state of Santa Catarina, the MGSZseparates two different domains of the DFB: the schist belt (Brus-que Group), and the granitic belt (Florianópilos Batholith) (Baseiet al., 2000). In the studied area (Fig. 2), the MGSZ presenta 1e3.8 km wide well-defined northwestern mylonite belt (NWB)and a 500 m to 2 km wide southeastern mylonite belt (SEB). Themylonitic belts form the limits of two petrographically, geochemi-cally and isotopically different granitoid associations.

The granitoid intrusions, which represent the object of thisresearch study, are elongated in the NE-SW direction (Fig. 2) andhave sheared borders composed of mylonitic rocks with mineralorientations produced under high strain, greenschist facies condi-tions. The intrusions are only weakly deformed in the inner parts,except for local highly deformed zones. Sub-magmatic and high tomoderate temperature solid state textures are found in the innerregions as well. Contacts between the granitoids and the mylonitebelts are usually transitional. Granite emplacement occurredduring dextral transpressive shearing (Passarelli et al., 2010).

In this paper we present petrographic, geochemical and isotopicdata of granitoid rocks which occur between two mylonite belts(NWB and SEB) within the MGSZ. Two main magmatic associationswere distinguished, at first based on modal compositions, and bydifferent Sr isotopic and geochemical characteristics. They are

Fig. 1. General outline of the Dom Feliciano Belt (Brazil-Uruguay), simplified from Basei et al. (2000). Dom Feliciano Belt (DFB): 1. Foreland basins; 2. Granite belt: FlorianópolisBatholith (F), Pelotas Batholith (P), Aiguá Batholith (A); 3. Schist belts and intrusive granitoids: Brusque Metamorphic Complex (SC), Porongos Metamorphic Complex (RS), Lavallejametamorphic Complex (UY); 4. Basement Inliers; 5. Foreland: Luis Alves Microplate (LA) and Rio de La Plata Craton (Taquarembó - T, Rivera e R, Nico Perez Terrane- NPT and PiedrasAltas Terrane- PAT); 6. Intrusive granites; 7. Metamorphic rocks; PET: 8. Granitoids; 9. Metasedimentary cover (Rocha Group), 10. High grade gneisses. SGB: São Gabriel Block; CDT:Cuchilla Dionisio Terrane; PET: Punta del Este Terrane. SBSB: Southern Brazilian Shear Belt.

C.R. Passarelli et al. / Journal of South American Earth Sciences 32 (2011) 369e378370

represented by the Rolador Granitoid Association (RGA) and the Fer-nandes Granitoid Association (FGA) (Fig. 2). The new data combinedwith literature geochronological and structural data (Passarelli et al.,2010, and references therein) provide new constraints for the role ofthe major shear zones as controlling the emplacement of coevalgranite plutons from isotopically-distinct sources.

2. Petrography

On the basis of modal compositions, in the RGA biotite mon-zogranite predominates, while the FGA is mainly composed ofamphibole syenogranite (Fig. 3). Contacts between the two associ-ations are not directly seen in the field, but where the two

associations are in close proximity they must be sharp. The outcroppatterns of the two associations are discontinuous, andmany of thesamples of RGA apparently fill the space between portions of theFGA. Two outcrops of isotropic biotite monzogranite belonging tothe RGAwere found inside the mylonite belts, one (outcrop n. 8) atthe border in the northwestern belt, the other (outcrop n. 1) at theborder of the southeastern belt (see Fig. 2 for the location of theoutcrops).

The rocks of the FGA are characterized by two facies: the por-phyroid, and the inequigranular (Fig. 3). Whit regard to the syenog-ranite, the porphyroid facies is pink, coarse-grained to porphyroidand hololeucocratic to leucocratic (Fig. 3). The phenocrysts areeuhedral to anhedral microcline with dimensions of about 3 � 1 cm,

Fig. 2. Top left: location of studied area in Santa Catarina State. Centre: location of the studied area in eastern Santa Catarina (SC). Main map: geological map of the granites withinMajor Gercino Shear Zone (MGSZ). 1) Alluvium; MGSZ: 2) North Mylonitic Belt (NWB) and South Mylonitic Belt (SEB); Major Gercino Magmatic Suite: 3) Fernandes GranitoidAssociation; 4) Rolador Granitoid Association; North Terrains of MGSZ: 5) Brusque Group; 6) Intrusive Granitoids; South Terrains of MGSZ: 7) GraniteeMigmatitic Complex(Camboriú Complex) and intrusive granites; 8) principal faults; 9) outcrops with geochemical data (full triangles represent samples of the Fernandes Granitoid Association, and fullsquares represent samples of the Rolador Granitoid Association - the numbers correspond to the samples listed in Table 1); 10) mylonitic foliation; 11) main foliation; 12) minerallineation; 13) magmatic lineation; 14) dextral movement along the mylonitic belts.

Fig. 3. Modal compositons of FGA and RGA rocks in the QAP diagram (Streckeisen,1976). Inequigranular and porphyroid facies are separated in each case. No mylonitesfrom either association were analyzed.

C.R. Passarelli et al. / Journal of South American Earth Sciences 32 (2011) 369e378 371

set in a medium- to coarse-grained granodioritic matrix containinggroups of hornblende and subordinate biotite grains. A variety withrapakivi texture is sometimes encountered (Fig. 4a). The inequi-granular facies (Fig. 3) is pink to greyish pink, medium- to coarse-grained and equigranular to inequigranular containing biotite andhornblende (Fig. 4b). Fine- to medium-grained and coarse-grainedvarieties also occur, the latter being richer in mafic minerals, andsometimes having rapakivi texture. In both facies, plagioclase isusually oligoclase, and the accessory minerals are allanite, zircon,titanite and apatite. A characteristic feature of this association is thepresence of euhedral hornblende and allanite.

Subordinate compositional variations occur within the FGA,including gray to pinkish gray, hololeucocratic to leucocratic, equi-to inequigranular, medium-grained monzogranites (Fig. 3), whichusually have rapakivi texture. Interstitial biotite and amphibole arepresent between the major rock-forming minerals.

Locally quartz-rich grey, hololeucocratic syenogranite occur,with on average 55modal% quartz, 30 modal % K-feldspar, 10 modal% plagioclase, and less than 5 modal% mafic minerals (Fig. 3). Theserocks are porphyritic, with sparse K-feldspar megacrysts. Thematrix is also quartz-rich, fine-grained, equigranular, with feld-spars, biotite, amphibole, and sometimes rare muscovite.

Alkali feldspar quartz syenites (Fig. 3) are hololeucocratic,inequigranular, coarse- to very coarse-grained, containing morethan 80% xenomorphic K-feldspar crystals together with interstitialxenomorphic quartz and amphibole. Alkali feldspar granites arealso present.

Microgranular mafic enclaves are rare and were found in onlyone outcrop.

The primary mineral assemblage of the FGA was partially re-equilibrated in greenschist facies. Amphibole altered to biotite andchlorite e albite - clinozoisite/zoisite - epidote e calcite. Chlorite issometimes the only mafic mineral present. Biotite is also altered to

Fig. 4. Photographs of different granite facies. a) Inequigranular to porphyritic syenogranite from the Fernandes Granitoid Association (FGA) with local rapakivi texture; b)Inequigranular syenogranite from the Fernandes Granitoid Association (FGA); c) Inequigranular to porphyritic facies of monzogranite of Rolador Granitoid Association (RGA); d)Equigranular to inequigranular facies of monzogranite from the Rolador Granitoid Association (RGA).

C.R. Passarelli et al. / Journal of South American Earth Sciences 32 (2011) 369e378372

chlorite, and is sometimes crenulated or intergrown with musco-vite. Oligoclase is often heavily saussuritized. The grains frequentlyhave deformed twins and later microfaults.

Rocks of the RGA are mainly monzogranite (Fig. 3) which hasbetween 5 and 15 vol.% mafic minerals. The main facies is medium-grey, porphyritic to porphyroid (Fig. 4c), and contains megacrystals ofK-feldsparandplagioclaseusuallyset inamedium-grained, tonalitic togranodioritic matrix. Titaniferous biotite is the predominant maficmineral, which sometimes occurs concentrated in 3 to 4 cm-widebands.Greenbiotite is lesscommon.Apatite, euhedral titanite, allanite,zircon, muscovite and rare tourmaline are the accessory minerals. Avariety of this facies is mafic-poor with a fine-grained matrix.

The second important facies of the RGA is grey to greyish rosemonzogranite, equigranular to inequigranular (Fig. 4d), usuallymedium-grained but sometimes coarser. Finer-grained equigranulargreymonzogranite with amphibole and biotite forms aminor part ofthis facies.

Other minor components of the RGA include grey biotitegranodiorite with rare K-feldspar megacrysts, and dark grey tona-lite with rare plagioclase and anhedral quartz megacrysts. Theserocks belong to a different granitic series (Lameyre and Bowden,1982).

A few outcrops of RGA contain biotite-rich microgranular maficenclaves containing small K-feldspar phenocrysts. Some of theenclaves are flattened ellipsoids (pancakes)with long axes parallel totheorientationofK-feldsparmegacrysts in the surroundinggranitoid.It was not possible to obtain fresh samples of these enclaves.

The primary mineral assemblage of the RGA re-equilibratedunder greenschist facies conditions, with formation of chlorite,

albite, clinozoisite/zoisite and epidote. Twins in plagioclase areslightly deformed, K-feldspar is sometimes recrystallized, andquartz is mostly present as sub-grains showing a tendency towardshomogeneous extinction, and with polygonal microtextures. Thelargerquartz grains haveundulatingextinction, and commonlyhavemicrogranular to interpenetrating contacts with other minerals.

A single isolated outcrop of hololeucocratic quartz-monzonitecontaining fibrous riebeckite associated with hornblende wasencountered at the contact between the RGA domain and thesoutheastern mylonite belt. It contains K-feldspar and smallerplagioclase megacrystals in a fine-grained matrix, hornblende andgreen biotite occur as mafic minerals. Magmatic accessory mineralsare titanite, zircon and apatite. This rock seems to have no obviousrelationship with either association, but may be associated witha later magmatic event (see later).

3. Geochemistry

The modal QAP analyses shown in Fig. 3 were obtained using allhand samples including deformed examples. Analyses of 16samples of less-deformed fresh granitoids from the FGA and RGAwere obtained fromACTLABS Ltd., Canada. The sample localities areshown in Fig. 2, and analyses, including all samples with calculatedRðtÞo values, are given in Table 1. The RðtÞ

o values are also indicated onsome of the variation diagrams presented. Some mylonitic sampleswere analyzed as well, mainly to check element mobility duringdeformation. No radical differences were found which would affectcritical points such as rock classification, and this aspect is notfurther discussed here.

Table 1Geochemical data for granitoid rocks of Major Gercino Shear Zone. a: Analyses of rocks from the Fernandes Granitoid Association. b: Analyses of rocks from the RoladorGranitoid Association.

Sample n� MG300 MG318 MG304 MG93 MG299 MG333 MG281 MG73 MG264

Map n� :1 :2 :3 :4 :5 :6 :7 :8 :9

Rock typea q-afes sg sg sg sg sg sg mg sg

SiO2 62.72 67.64 68.04 68.71 71.31 72.75 72.81 73.95 74.79TiO2 0.53 0.60 0.57 0.58 0.40 0.37 0.45 0.19 0.33Al2O3 17.79 15.42 14.56 14.12 13.38 13.21 13.76 12.31 12.44Fe2O3tb 3.40 4.41 3.98 4.09 3.00 2.96 3.38 1.79 3.20MnO 0.04 0.06 0.07 0.06 0.04 0.05 0.06 0.03 0.06MgO 0.58 0.69 1.46 0.64 0.43 0.38 0.49 0.16 0.18CaO 2.07 1.73 2.45 2.04 1.51 1.37 1.82 0.85 1.07Na2O 4.32 3.38 3.06 3.30 2.84 3.19 3.00 3.09 3.54K2O 6.41 5.90 4.50 4.70 5.71 5.15 4.59 5.10 5.06P2O5 0.17 0.17 0.21 0.18 0.13 0.11 0.12 0.05 0.08Total 98.05 100.00 98.90 98.42 98.75 99.55 100.28 97.52 98.69Ni 2 4 9 2 2 2 2 2 2Cr 1 7 34 1 1 3 4 1 4Rb 276 255 210 181 260 266 211 267 255Cs 2.8 8.2 7.6 3.8 5.4 2.9 4.7 5.4 12.7Sr 222 181 427 162 133 114 187 45 81Ba 1012 1156 1105 811 882 527 824 125 751Sc 8 9 6 15 7 6 7 3 8Y 53 42 23 72 40 56 34 52 58La 112.0 68.1 78.8 74.3 85.3 108.0 56.8 73.8 83.9Ce 223.0 162.0 164.0 173.0 179.0 241.0 121.0 139.0 165.0Nd 88.0 74.0 62.0 87.0 62.0 100.0 46.0 51.0 68.0Sm 13.1 12.5 8.93 15.4 10.9 14.6 8.57 9.47 12.9Eu 1.83 2.25 1.62 1.91 1.27 1.33 1.49 0.56 1.17Tb 1.80 0.70 0.90 2.20 1.50 1.80 12.7 1.6 1.6Yb 4.30 4.73 2.10 6.22 3.47 5.45 2.67 5.09 4.56Lu 0.66 0.67 0.30 0.79 0.48 0.73 0.37 0.70 0.69Th 34.1 15.7 27.0 17.9 24.4 49.0 12.7 53.5 20.7U 5.9 15.6 4.9 3.5 2.9 5.0 1.6 9.6 3.7Zr 393 379 203 492 345 301 496 189 415Hf 10.8 10.0 6.0 10.0 8.3 10.0 10.0 5.9 10.7Nb 25.0 23.0 15.0 28.0 17.0 21.0 22.0 23.0 31.0Ta 10.8 3.9 3.2 3.7 1.6 2.7 2.2 5.9 4.9

Sample n� MG343 MG296 MG277 MG220 MG315 MG245 MG353 MG249

Map n� -1 - 2 - 3 - 4 - 5 - 6 - 7 - 8

Rock typec ms ms ms ms ms ms ms ms

SiO2 68.49 69.12 69.45 69.72 70.34 70.67 72.19 73.08TiO2 0.46 0.58 0.46 0.42 0.51 0.44 0.40 0.19Al2O3 15.62 14.92 15.36 14.29 14.77 14.46 13.39 13.64Fe2O3tb 3.41 3.23 3.05 2.84 3.00 2.77 3.75 1.78MnO 0.06 0.06 0.05 0.04 0.06 0.06 0.06 0.04MgO 1.10 1.23 1.14 1.02 1.10 1.05 0.28 0.21CaO 2.44 2.49 2.22 2.35 2.31 2.29 1.37 1.07Na2O 3.52 3.31 3.35 3.09 3.41 3.27 2.88 3.54K2O 4.18 4.77 4.62 4.81 4.60 4.68 5.74 4.68P2O5 0.22 0.18 0.19 0.16 0.16 0.14 0.11 0.14Total 99.50 99.89 99.89 98.74 100.26 99.83 100.17 99.83Ni 6 7 9 7 8 9 2 2Cr 34 22 18 15 25 17 15 2Rb 257 187 242 194 274 208 272 349Cs 9.7 7.8 14.1 6.2 9.2 8.6 7.0 14.1Sr 373 556 403 441 377 431 80 109Ba 963 1154 932 1005 826 808 544 470Sc 7 6 7 3 6 5 10 3Y 35 25 14 12 40 19 58 32La 54.4 51.2 39.6 27.1 86.4 32.0 133.0 54.0Ce 108.0 106.0 79.0 49.0 180.0 71.0 302.0 105.0Nd 44.0 42.0 31.0 17.0 79.0 24.0 125.0 30.0Sm 8.68 7.06 5.27 3.41 11.7 4.75 21.4 6.18Eu 1.2 1.51 1.01 0.69 2.13 1.03 1.88 0.81Tb 0.1 0.5 0.7 0.5 1.6 0.7 2.8 1.1Yb 2.25 1.95 1.0 1.03 3.44 1.91 5.53 3.01Lu 0.36 0.27 0.16 0.14 0.48 0.27 0.87 0.42Th 21.2 16.5 14.2 13.4 21.0 17.4 24.4 28.9U 4.2 7.5 6.4 3.8 6.3 7.0 2.8 8.5Zr 185 178 137 191 147 122 408 153Hf 6.7 5.1 3.6 4.4 4.3 3.9 11.9 4.9Nb 21.0 17.0 13.0 13.0 15.0 12.0 28.0 30.0Ta 3.9 2.8 1.8 4.5 4.8 3.0 3.8 5.9

a rock types: mg ¼ monzogranite; sg ¼ syenogranite; q-af-s ¼ quartz-alkali feldspar syenite.b Total iron reported as Fe2O3.c rock types: ms ¼ monzogranite.

Fig. 5. Alumina saturation in Maniar and Piccoli’s (1989) diagram.

Fig. 7. Fe number (Frost et al., 2001) versus SiO2. FGA e filled triangles, undeformedgranitoids; open triangles, mylonites. RGA e filled squares, undeformed granitoids;open squares, mylonites.

C.R. Passarelli et al. / Journal of South American Earth Sciences 32 (2011) 369e378374

Most of the less-deformed rocks are metaluminous to weaklyperaluminous (Fig. 5).

The less-differentiated rocks of the RGA are calc-alkalic andmagnesian according to the modified alkali-lime index (MALI;Fig. 6) and Fe number [Fe#¼ FeOt/(FeOtþMgO)] (Fig. 7) defined byFrost et al. (2001). In these diagrams there is no systematicgeochemical variation accompanying the variation of the Srisotopic composition. A few of the FGA rocks are also calc-alkalicand magnesian, but most of them, together with the more differ-entiated RGA rocks, are alkali-calcic and ferroan. The least siliceousFGA sample is similar to many A-type granites. The FGA therefore isdifferent from the standard associations used by Frost et al. (2001).There is some systematic variation of chemical typewith Sr isotopiccomposition for some of the samples. All samples of both associa-tions are high-K granitoids (Fig. 8).

Chondrite (Evensen et al., 1978) normalised GdN values wereestimated by extrapolation from NdN through Smn. In most cases,the inferred GdN values so obtained agreed with those found byextrapolating from Ybn through TbN. In general terms, the patternsof RGA and FGA rocks are very similar, and typical of calc-alkaline toalkali-calcic granitoids everywhere. FGA rocks have higher totalREE contents. Their larger negative Eu anomalies (Fig. 9a), suggesteither greater fractionation of plagioclase, or more retention ofplagioclase in the source. The HREE are less fractionated in the FGA(TbN/LuN ¼ 0.7 to 2.3) than in the RGA samples (TbN/LuN ¼ 0.2 to

Fig. 6. Modified alkali-lime index (MALI; Frost et al., 2001) versus SiO2 for FGA (a) with Ri(t)MG93 does not fit a systematic trend involving the other samples. The key to symbols is in

3.0), suggesting that the roles of amphibole and/or garnet in theirpetrogenesis were different (Fig. 9a and b).

Ocean Ridge Granite (ORG; Pearce et al., 1984) normalized spi-dergrams resemble those of syncollisional granites. The negative Nbanomalies, relative to Ta and Ce, are typical of subduction-relatedgranites (Fig. 10), but are also encountered in anorogenic granites.

3.1. RbeSr and Sr isotope geochemistry

The Sr isotope geochemistry is a feature which distinguishes thetwo petrographic associations, but is also important in the attemptto unravel the geochemistry of the rocks. Significant differences ofSr and Rb contents of the two associations are showed in Table 2.

Previous RbeSr data (Passarelli et al., 1997) and UePb zirconages (FGA - 614 � 16 Ma, RGA - 609 � 2 Ma, Passarelli et al. (2010))together with some new analyses obtained at the Centre ofGeochronological Research of the Institute of Geosciences of theUniversity of São Paulo (Table 2) were utilized to calculate modelð87Sr=86SrÞðtÞo ¼ RðtÞ

o values.The isotopic compositions of both associations are heteroge-

neous. The RðtÞo values of the RGA range from 0.7091 to 0.7100

whereas the FGA samples are more heterogeneous, with valuesfrom 0.7069 to 0.7103.

The RðtÞo vs. 1/Sr diagram (Fig. 11) using Sr concentrations

determined during the isotope study, indicates that no singlesimple petrogenetic process is capable of generating the observed

values and sample numbers as above, and RGA (b) with Ri(t) values. In the FGA samplethe Fig. 5.

Fig. 8. K2O versus SiO2 diagram (Peccerillo and Taylor, 1976). Symbols and Ri(t) valuesas above. As before, in FGA sample MG93 does not fit a systematic trend. The key tosymbols is in the Fig. 5.

Fig. 10. ORG-normalized (Pearce et al., 1984) multi-element diagrams. (a) FGA; (b)RGA.

C.R. Passarelli et al. / Journal of South American Earth Sciences 32 (2011) 369e378 375

isotopic heterogeneity in the FGA. But a combination of fractionalcrystallization (FC) and magma mixing or assimilation e fractionalcrystallization (MM/AFC) might generate the isotopic heteroge-neity. TwoMM/AFC paths are apparent. The heterogeneity may alsobe partly due to differences in the source rocks involved.

3.2. Sm/Nd data

Model Nd TDM ages of 1543 and 1341 Ma were found for biotitemonzogranite MG-220 from the RGA, and amphibole syenograniteMG-333 from the FGA, respectively with corresponding 3ðtÞNd valuesofw�6.2 and �3.6 (Table 3). Both Nd TDM ages and the moderatelynegative 3ðtÞNd values are comparable to those found for rocks fromthe Florianópolis Batholith (Basei et al., 2000, 2001), particularlysimilar to some granites associated to MGSZ in the region of PortoBelo (Chemale et al., 2003). The available geochronological data for

Fig. 9. Chondrite-normalized (Evensen et al., 1978) rare earth element patterns. A fewGd(N) values were obtained by extrapolation, see text for details. (a) FGA; (b) RGA.

the Pinheiro Machado, Viamão and Dom Feliciano suites of thePelotas Batholith (Babinski et al., 1997; Phillip and Machado, 2005),indicate similar TDMmodel ages for the granites here studied. In theDom Feliciano Belt, the Mesoproterozoic is also recorded in thedetrital zircons from the Porongos Metamorphic Complex (Gruberet al., 2008). Model Nd TDM ages for the Granite Belt are system-atically younger (between 1.2e1.7 Ga, clustering in the 1.4e1.6 Gainterval) than those for the Schist Belt units (granitic and meta-sedimentary rocks with values around 1.9 Ga) and much youngerthan those found in the Luis Alves-or even Rio de La Plata-typebasement units, around 2.5 Ga (Cordani and Sato, 1999; Basei et al.,2001; Chemale et al., 2003).

The slightly negative 3ðtÞðNdÞ values (Fig. 12) suggest that thecrustal residence times of the source rocks was short. On the otherhand, the wide range of initial (87Sr/86Sr) ratios suggest thata variety of source rocks was involved in the formation of thisFlorianópolis Batholith, and the source rocks for the analyzed RGAand FGA samples were amongst the least radiogenic.

4. Petrogenesis

Simple mixing can be geochemically treated as a simple massbalance problem. Using the concentrations of a component i in oneend member A, in the second end member B, and in the hybrid Hwe have:

FA$CA þ ð1� FAÞ$CB ¼ CH

where FA is the weight fraction of component A. Rearranging:

FAðCA � CBÞ ¼ CH � CB

and a graph of (CA � CB) versus (CH � CB) will have a slope of FA.Sr concentrations canbeused to establish anFA for a selectedpath,

and fromthis ahybrid composition for anyelement canbe calculated:

Table 2Rb/Sr analytical data for the Fernandes Granitoid Association and Rolador Granitoid Association.

Sample no. Rock type Rb ppm Sr ppm 87Rb/86Sr 87Sr/86Sr Error (2s) t Ma Ri(t) Ɛ(t)

Fernandes Granitoid AssociationMG93 sg 214.2 156.9 3.966 0.74393 0.00009 614 0.70920 76.8MG264 sg 260.1 74.3 10.22 0.79804 0.00013 614 0.70850 66.9MG299 sg 250.8 130.3 5.598 0.75677 0.00010 614 0.70774 56.1MG300 q-af-sg 297.8 221.3 3.908 0.74453 0.00009 614 0.71029 92.4MG318 sg 225.3 182.0 3.594 0.73992 0.00008 614 0.70844 66.1MG333 sg 265.4 112.2 6.890 0.76726 0.00010 614 0.70694 44.8Rolador Granitoid AssociationMG220 mg 212.8 488.9 1.261 0.72003 0.00009 609 0.70908 75.1MG277 mg 260.3 426.1 1.771 0.72543 0.00010 609 0.71005 88.9MG296 mg 201.9 471.5 1.241 0.72040 0.00014 609 0.70962 82.9MG304 mg 195.5 441.5 1.283 0.72104 0.00009 609 0.70990 86.7MG315 mg 246.3 358.3 1.993 0.72721 0.00010 609 0.70990 86.8

rock types: sg ¼ syenogranite; q-af-s ¼ quartz-alkali feldspar syenite; mg ¼ monzogranite.

C.R. Passarelli et al. / Journal of South American Earth Sciences 32 (2011) 369e378376

CHðcalcÞ ¼ FACAðobservedÞ þ ð1� FAÞCjBðobservedÞThe difference D ¼ CH(observed) e CH(calculated) can then becompared to probable analytical error to check whether it issignificant.

Visual inspection of the analyses of FGA rocks used in thedetermination of the Ri(t) values shows that no satisfactorycombination of samples leads to a viable mass balance solution.Using Sr concentrations to evaluate the path A¼MG93, H¼MG299and B ¼ MG333 a value of FA ¼ 0.40 is found. Satisfactory massbalance is found for SiO2, Al2O3, MgO, CaO and Zr (Table 4), while nogood fits are found for the major components Fe2O3(t), Na2O andK2O, the minor components TiO2, P2O5 together with probablyassociated trace elements such as Rb, Cs, Ba, Nb and Ta. Thedifferences may be due to selective accumulation of alkali feldsparand of accessory minerals such as titanite and apatite in sampleMG299.

The more limited Sr isotopic heterogeneity of the RGA is notaccompanied by systematic geochemical changes. Perhaps moreefficient magma mixing occurred during ascent and emplacementof the magmas, but different source rocks were probably involvedin their genesis.

Fig. 11. Initial (87Sr/86Sr) ratios [Ri(t)] calculated for model multi-crystal zircon UePbages of 614 Ma for FGA, and 609 Ma for RGA, versus 1/Sr. Symbols as in Fig. 2, samplenumbers for FGA as in Table 1A. See text for discussion. Possible arrays formed bymagma mixing or assimilation e fractional crystallization, and by fractional crystalli-zation are indicated.

5. Discussion

5.1. The tectonic environment of the Major Gercino shear zone

The Adamastor Ocean separating Neoproterozoic South Americaand southern Africa would be located to the west of the Flo-rianópolis magmatic arc and all of the oceanic crust there wassubducted towards the east and continentecontinent collisionresulted in a major crustal-scale suture represented by the SierraBallena e Major Gercino Lineament, which separates the terranesof African and South-American affinities (Basei et al., 2000, 2005).

A change from collisional to post-collisional or extensionaltectonics occurred over a long belt from SE and S Brazil to Uruguayat around 610Ma. Magmatic rocks of this age are mainly felsicwith calc-alkalic or alkali-calcic and high-K compositions. Latermagmatic rocks included both volcanic and intrusive types, with animportant peralkaline component e.g., some granitic intrusions anddykes of Pelotas and Florianópolis Batholiths (Bitencourt and Nardi,2000; Phillip and Machado, 2001). An extensional environment ispostulated for this phase. The formation of plutonic and volcaniccalc-alkalic and alkali-calcic rocks continued down to around550e540 Ma.

The magmatic rocks found in the Major Gercino region followthis pattern of evolution of types. Bitencourt and Nardi (1993)studied granites from the area of Porto Belo. They identified theEstaleiro Complex whose intrusion was related to the first phase ofsyn-transcurrent magmatism, and whose rocks are very similar tothose studied here. Our FGA is very similar to the Zimbros instru-sive suite of metaluminous alkaline rocks described by Bitencourtet al. (1995), which they related to the late transcurrent phase.

5.2. SmeNd TDM ages

The Mesoproterozoic model ages of the RGA and FGA mayrepresent the time of differentiation of the source rocks frommantle. For the DFB Schist Belt, there is a concentration of valuesaround 1.9 Gawhereas for the Granite Belt the average age lies closeto 1.6 Ga. In the context of the Damara Belt (Africa) the average alsofalls close to 1.6 Ga. The isotopic differences between the GraniteBelt and those located west corroborate to the suggestion that thelithosphere on both sides of the Major Gercino shear zone shouldhave different isotopic characteristics (Basei et al., 2001, 2005).

Therefore, if on the South-American side model Nd agesbetween 1.2 and 1.6 Ga are characteristics of the Granite Belt of DFB,values in the same interval are common in several regions ofthe South-African portion. Mesoproterozoic ages predominate forthe rocks of western Damara with calc-alkaline granitoids andintra-plate syenites to granites presenting model ages between

Table 3Sm/Nd analytical data for the Fernandes Granitoid Association and Rolador Granitoid Association.

Sample Material Sm (ppm) Nd (ppm) 147Sm/144Nd 143Nd/144Nd 3 (0) TDM (Ma) 3 (TDM) T1 (Ma) 3 (T1)

MG333 Syenogranite - FGA 14.08 80.83 0.1053 0.51209 �10.71 1341.1 4.92 610.0 �3.60MG 220 Monzogranite- RGA 7.02 39.70 0.1069 0.51196 �13.21 1542.7 4.46 610.0 �6.22

Fig. 12. 3ðtÞNd vs. 3ðtÞSr for Major Gercino granites. Dashed fields represent other graniticrocks from Florianópolis Batolith (FB), in Santa Catarina State (unpublished data) andPelotas Batolith (PB), in Rio Grande do Sul State (Babinski et al., 1997; Phillip andMachado, 2005). The former was calculated using CHUR, the latter using the WholeEarth as standards. The key to symbols is in the Fig. 5.

C.R. Passarelli et al. / Journal of South American Earth Sciences 32 (2011) 369e378 377

1.1e1.5 Ga (McDermott and Hawkesworth, 1990). A very similarpattern was also found for metaluminous A-type granitoids ofcentral Damara (Jung et al., 1998).

5.3. Initial Sr isotope heterogeneity in granites

Initial Sr isotope heterogeneity in granites has been detected inmany cases (e.g. Schärer et al., 1984; Tsuboi and Suzuki, 2003;Phillip et al., 2002; Phillip and Machado, 2005). A number of Bra-zilian examples of granite intrusion related to activity along shearzones have been studied. In the Borborema Province of north-eastern Brazil a group of Brasiliano/Pan African-age intrusions wereintruded during a short time interval of about 25 Ma during theevolution of a major shear zone in western Rio Grande do Norte(Galindo et al., 1995; McReath et al., 2002). The petrographic andgeochemical affinities range between low- to medium-K calc-

Table 4Mass balance for selected elements in a possible magma mixing path with endmembers MG93 and MG333, and the hybrid MG299 (FGA).

Component C (obs) C (calc) D jDj,% Error,%

SiO2 71.3 71.1 0.2 0.3 2TiO2 0.40 0.45 �0.05 12.5 5Al2O3 13.38 13.57 �0.19 1.4 2Fe2O3(t) 3.00 3.41 �0.41 13.7 10MgO 0.43 0.48 �0.05 11.6 10CaO 1.51 1.64 �0.13 8.6 10Na2O 2.84 3.23 �0.39 13.7 10K2O 5.71 4.57 1.14 20 5P205 0.13 0.20 �0.07 54 20Zr 345 377 �32 9.3 10

Concentrations for oxides in wt. % rounded to one decimal place, for Zr in p.p.m.;C (obs) ¼ observed concentration in hybrid; C(calc) ¼ calculated concentration inhybrid; D ¼ C(obs) e C(calc); D,% relative to C(obs); Error,% ¼ probable analyticalerror (2s).at the measured concentration level.

alkaline suites, in part associated with K-diorites, through high-Kalkali-calcic and shoshonitic suites to alkaline or A-type suites. Ri(t)values for 3 different plutons are heterogeneous, with an extremerange from 0.706 to 0.713 in one case. The Brasiliano-age CoronelJoão Sá intrusion in the Sergipana fold belt in the southern part ofthe Borborema Province is deformed along a bifurcation of a majorshear zone in the region. Ri(i) values cover a range from about0.7078 to 0.7104 (McReath et al., 1998; Long et al., 2004). Differentsource rocks were involved, and complicated igneous plumbingwas probably responsible for the distribution of rock types withinthe intrusion.

6. Conclusions

In the studied MGSZ granites, the FGA shows clear evidence forparticipation of source rocks with both more and less radiogenic Sr.The extreme scarcity of microgranular mafic enclaves is unusual,and may be related to the present exposure level. The less-differ-entiated part of the RGA is isotopically more homogeneous. Thisassociation contains biotite-rich microgranular mafic enclaveswhich could be restites from the source rock (s). The reasons for thedifferent efficiencies of magma mixing in the two associationsrequire further studies.

The available isotopic and geochemical data suggest that thestudied granitoids correspond to a meta- to peraluminous calc-alkaline to alkaline affinity, high-potassic plutonism, originated bymagma mixing or assimilation-fractional crystallization with anemplacement-controlled by the shear zone.

The Mesoproterozoic model ages of the RGA and FGA, compa-rable to those of the Granite Belt and distinct from the otherdomains of the DFB, also corroborate to the lithosphere-scale of theMGSZ that has juxtaposed terrenes of different make-up.

While Sr isotopic compositions point to a possible influence ofmagma mixing in the genesis of the FGA syenogranites and relatedrocks, trace element geochemistry indicates that is not the onlypetrogenetic process involved.

Deep-reaching shear zones allow the escape of any magmawhich was present at depth, and compressive forces promote theliberation of small volumes of liquid. Different crustal rocks withdifferent residence times were involved in magma genesis, andmixing of some of the magmas from different sources occurredwith differing degree of efficiency during magma rise.

Acknowledgements

We thank FAPESP (grant 92/3729-3) for financial support givento fieldwork, lithogeochemical and isotopic analyses. I. McReath, M.Basei and M. Campos Neto acknowledge National Research CouncilScholarships.

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