Pb/ Pb-ages of zircons from syn-collisional I-type porphyritic granites ...

Gondwana Research 9 (2006) 326 – 336www.elsevier.com/locate/gr207 Pb/ 206 Pb-ages of zircons from syn-collisional I-type porphyriticgranites of the central Ribeira belt, SE BrazilJulio Cezar Mendes a, * ,1 , Ciro Alexandre Ávila b , Ronaldo Mello Pereira c ,Mônica P.L. Heilbron c,1 , Candido A.V. Moura d,1a Departamento de Geologia, IGEO-Universidade Federal do Rio de Janeiro (UFRJ), Cidade Universitária, 21949-900, Rio de Janeiro, Brasilb Departamento de Geologia e Paleontologia, Museu Nacional, Universidade Federal do Rio de Janeiro (UFRJ),Quinta da Boa Vista, 20940-040, Rio de Janeiro, Brasilc Faculdade de Geologia, Universidade do Estado do Rio de Janeiro (UERJ), Rua São Francisco Xavier, 524, 20550-013, Rio de Janeiro, Brasild Departamento de Geoquímica e Petrologia, Universidade Federal do Pará, Rua Augusto Correa 1, 66075-110, Belém, BrasilReceived 15 February 2005; accepted 9 November 2005Available online 10 January 2006AbstractIn the central segment of the Ribeira belt, southeast Brazil, several foliated porphyritic granitic bodies intrude high-grade migmatitic gneissesof the Andrelândia and Juiz de Fora domains and Embú Complex. Results of geological, geochemical and geochronological investigations of theMaromba, Pedra Selada, Serra do Lagarto and Funil porphyritic I-type granites provide profound similarities, except for the distinct geochemicalbehavior of the Funil Granite, perhaps related to a different crustal source. These granitoids show similar structural, textural and mineralogicalfeatures. Pb-evaporation of single zircons provided ages of 586T6, 579.6T6.3, 586.3T4.8 and 584T5 Ma for the granites, respectively, coincidentwith the syn-collision I episode of the central Ribeira belt. The intrusion of I-type porphyritic granitoids coeval with the main collisional event hasnot often been reported in the geological literature. The most common syn-collisional granitic magmatism has normally an S-type signature oreven a slightly peraluminous I-type character. However, the occurrence of coeval I- and S-type syn-tectonic granites along the central Ribeira belt,as observed in the investigated area and discussed in this paper is noteworthy.D 2005 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.Keywords: Central Ribeira belt; Neoproterozoic granites; Western Gondwana; 207 Pb/ 206 Pb geochronology1. Introduction and tectonic settingThe Neoproterozoic Ribeira orogenic belt extends overmore than 1000 km along the southeastern coast of Brazil (Fig.1). It developed as the result of the amalgamation of WesternGondwana during the Brasiliano/PanAfrican orogenic episodes.Reported geochronological data indicate that orogenicactivity was diachronic along the supercontinent, ranging intime from ca. 900 to 480 Ma.Four major orogenic episodes have been recognized in theRibeira belt (Machado et al., 1996; Heilbron et al., 1995, 2000,* Corresponding author.E-mail addresses: julio@geologia.ufrj.br (J.C. Mendes),avila@mn.ufrj.br (C.A. Ávila), rmello@uerj.br (R.M. Pereira),heilbron@uerj.br (M.P.L. Heilbron), candido@ufpa.br (C.A.V. Moura).1 CNPq (National Research Council).2004; Trouw et al., 2000; Campos Neto, 2000; Heilbron andMachado, 2003; Schmitt et al., 2004): a) 790 to 600 Masubduction and magmatic arc generation; b) 600 to 560 Macollision I episode; c) 530 to 510 Ma collision II episode; andd) 510 to 480 Ma orogenic collapse.Collision I episode (600 to 560 Ma) is the most importantorogenic event along the Ribeira belt and its northerncounterpart, the Araçuaí belt. This tectonic episode ischaracterized by widespread generation of granitoids. Threecommon types of granitoids, related to this episode, areleucogranites, porphyritic (hornblende)-biotite granites andhornblende granites.Between the towns of Itatiaia, Visconde de Mauá and SantaRita de Jacutinga, in the Minas Gerais/Rio de Janeiro/SãoPaulo state boundaries, voluminous and important porphyriticgranitoid magmatism was recognised and studied by Heilbron(1993), Junho et al. (1999) and Pereira et al. (2001a). Ona1342-937X/$ - see front matter D 2005 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.doi:10.1016/j.gr.2005.11.004

J.C. Mendes et al. / Gondwana Research 9 (2006) 326–336 327Fig. 1. Tectonic map of Brazil modified after Schobbenhaus et al. (1984). Legend: 1—Phanerozoic basins; 2—Bambuí group; 3—Neoproterozoic orogens; 4—Cratons (A—Amazon, B—São Luis, C—São Francisco, D—Luis Alves, E—Rio de la Plata).regional scale, it is important to stress the occurrence of coevalsyn-collisional, I-type porphyritic and S-type granites in theRibeira belt.The purpose of this paper is the presentation of new zirconPb-evaporation ages of four major plutons representative of theporphyritic I-type syn-collision I magmatism of the belt(Maromba, Pedra Selada, Serra do Lagarto and Funil) andthe discussion of their geological and geochemical data in thecontext of the Gondwana amalgamation.2. Tectonic organization and regional geologyThe central segment of the Ribeira belt is subdivided in fivedistinct tectonic terranes, in the sense of Howell (1989). Theseare: Occidental, Paraíba do Sul, Embú, Oriental and Cabo FrioTerranes (Heilbron et al., 2000, 2004). The investigated plutonsare located in the Occidental and Embú Terranes (Fig. 2).The Occidental Terrane is subdivided in two structuraldomains named Andrelândia and Juiz de Fora (Fig. 2). Bothcontain three units: a) an older Paleoproterozoic basement (theMantiqueira Complex in the Andrelândia Domain and the Juizthe Fora Complex in the Juiz de Fora Domain); b) a highlydeformed and metamorphic cover association (Andrelândiamegasequence) and c) Neoproterozoic granitoids. The Paleoproterozoicbasement association includes hornblende andbiotite migmatitic orthogneisses (Mantiqueira complex) andvaried orthogranulites (Juiz de Fora complex). The Andrelândiamegasequence includes banded biotite gneisses withquartzites, amphibolites and pelitic gneisses, and sillimanite–garnet–bitotite gneisses with several layers of calcsilicaterocks, gondites, amphibolites and quartzites. As pointed outby Junho et al. (1999), in the studied area, the extensivemigmatization transformed the paragneisses in stromaticmetatexites, crosscut by multiple leucosomatic veins, whichprogressively grade into (garnet)–biotite–muscovite graniticleucogneiss. In the northwestern part of the investigated area,in the Carvalhos Klippe, high-pressure granulites weredescribed from the Andrelândia megasequence (Trouw et al.,2000; Campos Neto and Caby, 2000).The Paraíba do Sul and Embú Terranes also containbasement and supracrustal associations (Eirado et al., in press).In the former terrane, basement is represented by the QuirinoComplex and other orthogneisses, while the Neoproterozoiccover, named Paraíba do Sul Group or Complex is composedof pelitic gneisses, schists, calcsilicate rocks and marbles. Inthe Embú Terrane, cover consists of tourmaline rich peliticgneisses and schists with quartzitic layers, while basementassociations are represented by hornblende orthogneisses(Fernandes et al., 1990; Almeida et al., 1993; Pereira, 2001;Heilbron et al., 2004).Local structures are subdivided in two deformational pulses,correlated to regional deformations D 1 +D 2 and D 3 of Ribeiroet al. (1995) and Heilbron et al. (1995, 2000). The oldest one,D 1 +D 2 , created the main foliation, locally mylonitic, and tightto isoclinal inclined to subhortizontal folds that repeat andthicken the units. In the Juiz de Fora domain, an important

328J.C. Mendes et al. / Gondwana Research 9 (2006) 326–336Fig. 2. Simplified tectonic map of Santa Rita do Jacutinga–Resende–Bananal region, modified after Heilbron et al. (2004). Geological data were compiled fromSilva et al. (1992), Heilbron (1993), Ribeiro et al. (1995), Almeida et al. (1993), Pereira (2001), Eirado et al. (in press). Legend: 1—Alkaline Phanerozoic plutons;2—Phanerozoic basins; 3—Syn-collision II granitoids (ca. 530 Ma); 4—S-type syn-collision I granitoids (ca. 580 Ma); 5—I-type syn-collision I granitoids (ca. 580Ma); 6—Hornblende granitoids of uncertain age; 7—Embú Terrane; 8—Paraíba do Sul Terrane; 9–11 Structural domains of the Occidental Terrane: 9—Juiz de Fora(JFD), 10—Andrelândia (ANDD) and 11—Carvalhos klippe of the Occidental Terrane; 12—Faults; 13—Normal faults; 14—Lateral shear zones; 15—Mainfoliation; 16—Major thrusts; 17—Synform; 18—Antiform; 19—Geochronological sampling, with abbreviation of the name of the investigated plutons representedby capital letters: M=Maromba; PS=Pedra Selada; SL=Serra do Lagarto; FU=Funil. Major towns are Bom Jardim de Minas (BM), Resende (RE), Volta Redonda(VR), and Santa Rita de Jacutinga (SRJ).tectonic imbrication mixed cover and basement rocks in severalthrust slices. The D 3 phase refolded the previous structures,generating open to tight folds with steep axial surfaces and SWplunging axes. In the Andrelândia domain, in the northernsector of the investigated area (Fig. 2), the interference betweenD 1 +D 2 and D 3 phases resulted in dome-and-basin andrefolded-fold patterns shown on the map. In the central andsouthern sector of the area, D 3 developed important subverticalshear zones, such as the Além Paraíba, Alto da Fartura andCubatão shear zones.The rocks of the region underwent syn-D 2 barrovianmetamorphism, with increasing temperatures and mediumpressure (Heilbron, 1993, Trouw et al., 2000). The regionalmetamorphism culminated with the formation of migmatitesand granites in the basal feldspathic metasedimentary rocks ofthe Andrelândia megasequence (Ribeiro et al., 1995). Asmentioned above, the migmatitic rocks were subdivided intotwo units grading into each other: a stromatic biotite gneisswith metatexitic structures and a (garnet)–muscovite–biotitediatexitic leucogneiss. Both are intruded by post-D 2 finegrainedequigranular leucogranite containing garnet, tourmaline,biotite and muscovite. It crops out as minor concordantlenses or as dykes and sills. Both the diatexite and the intrusiveleucogranite result from progressive partial melting of the

330J.C. Mendes et al. / Gondwana Research 9 (2006) 326–336Fig. 4. Shand’s diagram for the porphyritic I-type granites. Symbols as in Fig.3.In this paper, geochemical data of porphyritic granites fromHeilbron (1993), Junho et al. (1999) and Pereira et al. (2001a)are ploted in selected diagrams, as discussed below. The AFMand Shand’s diagrams (Figs. 3 and 4) reinforce the calc-alkalineand metaluminous to slightly peraluminous signature of thegranites. It must be emphasized that towards the most evolvedmagmas of an acid I-type igneous sequence there is a tendencyto crystallize slightly peraluminous rocks (Clarke, 1981).The Harker diagrams (Fig. 5) show the SiO 2 range of thegranites. The more evolved character of the samples fromMaromba and Funil granites contrasts with the less evolvedfrom the Pedra Selada granite. From field and petrographicobservations, as well as linear trends observed in theSiO 2 Fe 2 O 3 , MgO, CaO, P 2 O 5 , Ba and Sr variation diagrams,a genetic link of Maromba, Pedra Selada and Serra do Lagartogranites may be inferred. The Funil granite represents the moreFig. 5. Harker diagrams for the porphyritic I-type granites. Symbols as in Fig. 3.

J.C. Mendes et al. / Gondwana Research 9 (2006) 326–336 331Fig. 6. Chondrite-normalized REE diagrams for the porphyritic I-type granites. Symbols as in Fig. 3. Normalization values from Boynton (1984). (A) Funil Granite,(B) Serra do Lagarto Granite, (C) Maromba Granite and (D) Pedra Selada Granite.acid part of the trend, whereas the Pedra Selada granite formsthe more basic one. On the other hand, in some diagrams theFunil granite samples tend to be clustered and do not fit verywell in the assumed trends (e.g. SiO 2 Fe 2 O 3 , CaO, Ba andSr). This behavior is also apparent in Figs. 3, 6 and 7).The preferential occurrence of quartzdioritic microgranularenclaves in the Pedra Selada and Serra do Lagarto porphyriticgranites could indicate prior assimilation and/or contaminationprocess.The REE patterns of Fig. 6 highlight the probable cogeneticlink between the Maromba, Pedra Selada and Serra do Lagartoporphyritic I-type granites. They have quite similar REEdistribution pattern, showing to be highly fractionated with aclear negative Eu anomaly. The Pedra Selada granite shows thehighest total REE content. REE concentrations of the Marombagranite are lower than that of the Pedra Selada granite despiteits highest SiO 2 content.The well developed negative Eu anomaly in some patternsof the Funil granite is possibly caused by feldspar extractionduring fractional crystallization processes. However, its lessfractionated REE patterns and the strong negative Eu anomalycontrast with all others. The HREE enriched flat pattern may beassociated with sphene and zircon modal abundance, buthybridization processes could also provoke changes in the REEconcentrations of the melt, as pointed out by Dini et al. (2004).The different REE distribution of the Funil granitereinforces its possible more evolved character, maybe relatedto a different crustal source associated with the Embu Complexhost rocks, while the Maromba, Pedra Selada and Serra doLagarto granites are surrounded by the Andrelândia and Juiz deFora domains. Further achievement of isotopic Sr and Ndanalyses possibly will provide adequate elements to elucidatethis assumption.In the R 1 R 2 diagram (Fig. 7), after Batchelor and Bowden(1985), two trends can be envisaged: one from field 6 (syncollisionfield) to field 4 (late-orogenic field) and the otherfrom field 6 to field 7 (post-orogenic field). The former may beassociated with the formation of I-type granites during syn-tolate-orogenic processes. The alignment of granite samplesseems to reflect the evolution from less evolved metaluminousto more evolved, slightly peraluminous granitic magma, in asyn-tectonic environment.5. 207 Pb/ 206 Pb geochronology5.1. SamplingFig. 7. R 1 R 2 diagram for the porphyritic I-type granites. Symbols as in Fig. 3.Fields from Batchelor and Bowden, 1985.Zircon crystals were extracted from samples of Maromba,Pedra Selada and Serra do Lagarto porphyritic I-type granites(Fig. 2), after crushing, sieving, panning and concentrationusing Frantz magnetic separation and heavy liquid (bromo-

332J.C. Mendes et al. / Gondwana Research 9 (2006) 326–336Table 1207 Pb/ 206 Pb data for single zircons of the Maromba GraniteZircon Description Evaporation T (C-) Measured ratios204 Pb/ 206 Pb207 Pb/ 206 Pb207 Pb/ 206 Pb* Age (Ma) Zircon age (Ma)1 euh, stpr, tr, py 1450 32 0.000050T39 0.06081T29 0.05984T41 598T151500 36 0.000038T05 0.06029T22 0.05975T24 595T091550 38 0.000108T20 0.06050T324 0.05901T21 568T08 581.7T19.62 euh, lgpr, tr, py 1450 40 0.000047T03 0.06033T08 0.05970T09 593T031500 34 0.000057T11 0.06006T11 0.05925T10 576T04 585.5T16.43 ‘ euh, lgpr, tr, py 1500 38 0.000015T02 0.05993T24 0.05972T27 594T101550 40 0.000038T04 0.06239T15 0.06183T09 669T034 euh, stpr, tr, py 1500 40 0.000162T13 0.06174T28 0.05935T35 580T13 580T135 euh, lgpr, tr, py 1450 16 0.000202T06 0.06247T13 0.05957T21 588T08 588T086 euh, stpr, tr, py 1450 # 0 0.000312T28 0.06198T25 0.05744T48 509T181500 34 0.000104T09 0.06143T15 0.05958T54 589T20 589T208 sbh, stpr, tr, py 1500 36 0.000012T06 0.05990T13 0.05975T19 595T07 595T0714 euh, lgpr, tr, py 1500 40 0.000031T07 0.06019T26 0.05963T35 591T13 591T13Average age586T6Uncertainties are given at 2r.# Heating steps not used in the age calculation; * 207 Pb/ 206 Pb ratio corrected for common Pb contamination; ‘ inherited zircon. Description of the zircon crystals:euh=euhedral; sbh=subhedral; stpr=short prism (l /w between 1 and 3); lgpr=long prism (l /w >3); tr=transparent; py=pale yellowish.form). Zircons from the less magnetic fractions were selectedbecause they tend to be more concordant (Krogh, 1982).The analyzed non-magnetic zircons are transparent tooccasionally translucent, colorless, pale yellowish or pink.Bipyramidal prism length varies from long (width vslength=15) to short (width vs length=12) and the grainssometimes display subhedral morphology (Tables 1 2 and 3).5.2. Analytical techniquesGeochronological analyses by Pb-evaporation technique inzircons (Kober, 1986, 1987) were carried out at theLaboratory of Isotope Geology of the Federal University ofPará (Pará-Iso).Zircon is a heavy mineral with crystalline structure veryresistant to alteration and it tends to preserve the isotopicinformation since its crystallization. Kober (1986) has shownthat the Pb components with the highest activation energynormally reside in the intact crystalline zircon phase that showsno post-crystallization Pb-loss and consequently yields concordant207 Pb/ 206 Pb ages. The progressive heating of a zirconcrystal during the Pb-evaporation techniques gradually releasesPb isotopes. The theoretical basis for this technique is that theactivation energy necessary to release Pb from damaged ormetamict zones of zircon are much less than from crystallinedomains (Kober, 1986). Thus, radiogenic Pb from metamictzones can be purged at relatively lower evaporation temperatures,while radiogenic Pb from crystalline domains in zircon isreleased only at higher temperatures (Ansdell and Kyser,1993).Isotope analyses on porphyritic I-type granites wereperformed on a Finnigan MAT 262 thermo-ionization massspectrometer (TIMS) using the stepwise heating process(Kober, 1986, 1987) with a double-filament arrangement(evaporation and ionization). In this procedure, the zirconwas mounted on the canoe-shape rhenium evaporation filament,positioned in front of the rhenium ionization filament.Initially, the ionization filament was heated, in order to becleaned. Afterward, the evaporation filament was heated(evaporation step) to allow structurally bound radiogenic Pbto diffuse out of the crystal. The Pb was deposited on the coldionization filament that was, subsequently, heated releasing thedeposited Pb for isotope determinations. The isotope data wereacquired dynamically using the ion counting system of theinstrument, with the intensity of the 207 Pb beam between30,000 and 100,000 counts per second (cps). The intensity ofdifferent Pb isotopes were measured in the mass sequence206 Pb, 207 Pb, 208 Pb, 206 Pb, 207 Pb, 204 Pb. Ten (10) mass scanTable 2207 Pb/ 206 Pb data for single zircons of the Pedra Selada GraniteZircon Description Evaporation T(C-) Measured ratios204 Pb/ 206 Pb207 Pb/ 206 Pb207 Pb/ 206 Pb (*) Age (Ma) Zircon age (Ma)3 sbh, stpr, tl, py 1500 34 0.000169T04 0.06188T13 0.05946T18 584T07 584T075 euh, lgpr, tl, py 1500 28 0.000106T16 0.06077T28 0.05905T18 569T07 569T077 euh, stpr, tl, py 1500 30 0.000108T12 0.06057T18 0.05886T37 562T14 562T1413 euh, lgpr, tl, py 1500 30 0.000048T05 0.06005T19 0.05929T18 578T07 578T0714 euh, lgpr, tl, py 1500 32 0.000378T02 0.06491T18 0.05948T25 585T091550 8 0.000395T28 0.06504T54 0.05931T68 579T25 584.2T8.615 euh, lgpr, tl, py 1450 32 0.000197T49 0.06217T36 0.05965T42 591T151500 16 0.000040T04 0.06024T24 0.05966T25 591T09 .3T7.7Average age579.6T6.3Uncertainties are given at 2r.* 207 Pb/ 206 Pb ratio corrected for common Pb contamination. Description of the zircon crystals: euh=euhedral; sbh=subhedral; stpr=short prism (l /w between 1 and3); lgpr=long prism (l /w >3); tl=translucent; py=pale yellowish.

J.C. Mendes et al. / Gondwana Research 9 (2006) 326–336 333Table 3207 Pb/ 206 Pb data for single zircons of the Serra do Lagarto GraniteZircon Description Evaporation T(C-) Measured ratios204 Pb/ 206 Pb207 Pb/ 206 Pb207 Pb/ 206 Pb (*) Age (Ma) Zircon age (Ma)1 euh, lgpr, tr, cl 1450 30 0.000036T05 0.05998T21 0.05945T16 584T061500 28 0.000038T04 0.06029T17 0.05948T22 585T08 584.3T4.72 euh, stpr, tr, cl 1450 # 0 0.000448T36 0.06634T69 0.05985T87 598T321500 08 0.000136T08 0.06176T35 0.05979T37 596T13 596.3T13.43 euh, lgpr, tr, cl 1450 40 0.000170T03 0.06183T20 0.05932T24 579T091500 T38 0.000150T09 0.06130T13 0.05962T29 590T11 583.6T10.64 euh, lgpr, tr, cl 1450 36 0.000102T06 0.06079T22 0.05939T22 582T081500 32 0.000156T06 0.06122T27 0.05920T16 575T06 577.2T6.85 euh, stpr, tr, cl 1450 # 0 0.001416T34 0.07991T64 0.05950T94 586T341500 36 0.00131T04 0.06174T18 0.05980T19 596T07 596.5T7.06 euh, lgpr, tr, cl 1450 16 0.000368T11 0.06496T38 0.05980T13 597T051500 38 0.000114T05 0.06155T18 0.05981T15 597T05 596.7T3.58 ‘ euh, stpr, tr, cl 1450 34 0.000337T30 0.09677T77 0.09209T88 1470T181500 28 0.000030T07 0.12076T14 0.12033T19 1961T031550 34 0.000055T04 0.12506T36 0.12429T37 2019T059 euh, lgpr, tr, cl 1450 36 0.000084T06 0.06062T17 0.05942T19 583T071500 38 0.000107T02 0.06077T14 0.05924T12 576T041550 14 0.000118T21 0.06084T31 0.05879T95 560T35 577.9T4.810 euh, stpr, tr, cl 1450 30 0.000138T04 0.06146T25 0.05949T25 589T091500 30 0.000027T02 0.06017T21 0.05977T22 596T08 591.2T17.0Average age586.3T4.8Uncertainties are given at 2r.* 207 Pb/ 206 Pb ratio corrected for common Pb contamination; ‘ inherited zircon. Description of the zircon crystals: euh=euhedral; stpr=short prism (l /w between 1 and3); lgpr=long prism (l /w >3); tr=transparent; cl=colorless.define one block of data with eighteen (18) 207 Pb/ 206 Pb ratios.Discrepant isotopic values were eliminated using Dixon’s test.The 207 Pb/ 206 Pb ratios were measured in three steps ofevaporation at temperatures of 1450, 1500 and 1550 -C. Theaverage 207 Pb/ 206 Pb ratio of each step was determined basedon five blocks of data. In general, the average 207 Pb/ 206 Pbratios obtained in the highest evaporation temperature wereconsidered for age calculation of each zircon grain. However,the ages of the lower temperature steps were also integratedwhen they overlapped within error with those ages obtained atthe highest evaporation temperature. The uncertainties aregiven in 2r errors. Common Pb corrections were madeaccording to the Stacey and Kramer (1975) two-stage model.Only blocks with 204 Pb / 206 Pb ratios lower than 0.0004 wereused for apparent age determination of each evaporation step.The data treatment followed the procedure described inGaudette et al. (1998).In order to control the accuracy of the single zircon Pbevaporation ages carried out in the Para-Iso, a number of207 Pb/ 206 Pb apparent ages of the international standard zircon95,100 were measured. The ages obtained varied from1061.6T5.9 to 1077.2T5.9 Ma, giving an average age ofFig. 8. Age (Ma)heating step diagram for zircon crystals of the Maromba porphyritic I-type granite. Filled circles: accepted blocks for age calculation; squares:rejected blocks due to increasing or decreasing values of the 207 Pb/ 206 Pb ratio. Uncertainties are given at 2r.

334J.C. Mendes et al. / Gondwana Research 9 (2006) 326–336Fig. 9. Age (Ma)Heating step diagram for zircon crystals of the Pedra Selada porphyritic I-type granite. Filled circles: accepted blocks for age calculation; squares:rejected blocks due to increasing or decreasing values of the 207 Pb/ 206 Pb ratio; crosses: rejected blocks due to 204 Pb/ 206 Pb>0.0004. Uncertainties are given at 2r.1069.8T2.3 Ma. The accepted age for this 95,100 zircon is1065.4T0.3 Ma (Wiedenbeck et al., 1995).5.3. New isotopic resultsThe zircon crystals of the Maromba porphyritic I-typegranite gave very consistent 207 Pb/ 206 Pb apparent ages varyingbetween 589T20 and 580T13 Ma (Table 1 and Fig. 8). Theaverage 207 Pb / 206 Pb age, calculated based on 424 207 Pb/ 206 Pbratios of seven zircons grains, is 586T6 Ma. In this body, onecrystal (zircon 3) yielded an age of 669T3 Ma that wasinterpreted as an inherited crystal or a mixture of lead from anold inherited core and a magmatic overgrowth.The Pedra Selada porphyritic I-type granite yielded anapparent 207 Pb / 206 Pb age of 579.6T6.3 Ma (Table 2 and Fig.9). This age was calculated based on 210 207 Pb / 206 Pb ratios ofsix zircons, whose ages vary from 562T14 to 591.3 T7.7 Ma.In the Serra do Lagarto porphyritic I-type granite, eightzircon crystals gave 207 Pb/ 206 Pb apparent ages varyingbetween 577.2T6.8 and 596.3T13.4 Ma (Table 3 and Fig.10). The weighted-mean 207 Pb/ 206 Pb apparent age of thesecrystals, calculated based on 466 207 Pbs / 206 Pb ratios, is586.3T4.8 Ma. One crystal (zircon 8) provided an age of2019T5 Ma and is considered an inherited crystal or a mixtureof lead from an old inherited core and a magmatic overgrowth.The lack of significant change in the 207 Pb/ 206 Pb ratios onprogressive heating of analyzed grain 15 of the Pedra Seladaporphyritic I-type granite and grains 1, 2, 4, 6 and 10 of theSerra do Lagarto porphyritic I-type granite suggests theexistence of only one stable radiogenic Pb phase.Fig. 10. Age (Ma)heating step diagram for zircon crystals of the Serra do Lagarto porphyritic I-type granite. Filled circles: accepted blocks for age calculation;squares: rejected blocks due to increasing or decreasing values of the 207 Pb/ 206 Pb ratio; crosses: rejected blocks due to 204 Pb/ 206 Pb>0.0004. Uncertainties aregiven at 2r.

J.C. Mendes et al. / Gondwana Research 9 (2006) 326–336 3356. Discussion and conclusionsAs suggested before by previous workers (Junho et al.,1999; Heilbron et al., 1995, 2000; Trouw et al., 2000; Pereira etal., 2001a), the geochemical as well as the new geochronologicaldata confirm that the Maromba, Pedra Selada and Serrado Lagarto porphyritic I-type granites are syn-tectonic andcrystallized and intruded during the D 1 +D 2 Ribeira maindeformation phase.The common high-K calc-alkaline, metaluminous to slightlyperaluminous chemistry of the melt of Maromba, Pedra Seladaand Serra do Lagarto porphyritic granites is probably related toa unique reservoir. Despite the lack of Sr and Nd isotopic data,the granites geochemical behavior and minimum crystallizationage suggest such a relationship. The different REE pattern ofthe Funil Granite is possibly associated with fusion of adifferent protolith (perhaps related to the metasedimentaryrocks of the Embú Complex) and/or with the contribution ofaccessory minerals (e.g. sphene, apatite and zircon) andfeldspar extraction during the fractional crystallization process.The occurrence of I-type porphyritic granitic rocks yieldingages contemporaneous with the main continental collisionalevent has not often been reported in the geological literature.The most common syn-collisional granitic magmatism hasnormally an S-type signature, sometimes with a typicalaluminous paragenesis (Chappell and White, 1974), or even aslightly peraluminous I-type character.Janasi et al. (2001) discuss the relationship between granitesof the Agudos Grandes batholith, in the southern portion of theRibeira Belt in São Paulo State. They propose a scenario inwhich I-type syn-collisional porphyritic granite (Ibiuna type,with an age of 610T2 Ma) is closely associated with thecontemporary S-type Turvo granite (610T1 Ma). As describedbefore (Junho, 1995; Junho et al., 1999; Junho and Mendes,2000), this situation is also observed in the central Ribeira belt,where the I-type syn-collisional magmatism (Funil, Maromba,Pedra Selada and Serra do Lagarto porphyritic granites) iscoeval with diatexites and associated leucogranites. This is alsodemonstrated by coeval S-type granitoids of the Rio Turvobatholith, cropping out in the investigated area, dated at about580 Ma (Machado et al., 1996). Another example is theCantagalo leucogranite with an age of ca. 590 Ma (Heilbronand Machado, 2003) that intrudes pre-collisional tonaliticgneisses (about 630 Ma) of the Oriental Terrane of the centralRibeira belt in Rio de Janeiro State.Other examples of coeval S- and I-type syn-tectonicporphyritic granites are reported from the Archaean basementof southern India (Moyen et al., 2003) and from northern Brazil(Barros et al., 2001). Toteu (1990) described a PanAfricanmetadioritic syn-collisional suite followed by late-collisionalporphyritic granite.Also focusing on plutonic rocks formed during thePanAfrican Orogeny, Van de Flierdt et al. (2003) investigatedthe syn-orogenic quartz diorite–porphyritic granite/granodiorite–leucograniteassociation from the Bandombaai Complex,Namibia. The porphyritic granite/granodiorite is slightlyperaluminous and contrasts with the strongly peraluminouscharacter of the leucogranite. Describing a geological settingsimilar to the area focused in this paper, the authors emphasizethe production of these different magmas during the metamorphicpeak.A close relationship between syn-orogenic I-type granodioritesand tonalites and S-type granites is described by Pankhurstet al. (2001) in the Famatinian orogenic belt from NWArgentina, where the granitoids present an age of about 495to 465 Ma.The geochronological results of 584T5 Ma for the Funilporphyritic granite and 592T5 Ma for the Mendanha porphyriticgranite (Pereira et al., 2001b) are very close to the newgeochronological data presented here. This age interval,synchronous with the central Ribeira belt deformational andmetamorphic peak, should be considered as an importantperiod of crustal magma generation, when the production ofgranites showing contrasting mineralogy, textures and geochemical(I- and S-type signatures) characteristics was quitesignificant.In conclusion, the contemporary intrusion of S- and I-typesyn-collisional granitoids in the investigated area is probablyrelated to the structural style of the central Ribeira belt,characterized by interfingering of cover and basement rocks ona crustal scale (Heilbron and Machado, 2003, Trouw et al.,2000, Schmitt et al., 2004, Heilbron et al., 2004). Thecollisional crustal thickening may have induced widespreadmelting of different sources, resulting in magmas with variousgeochemical signatures.AcknowledgementsThe authors are grateful to GR referees Frank Söllner andRandall Van Schmus for detailed reviews of the manuscript.Special thanks to Maria C. B. 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