granites of the central Ribeira belt, SE Brazil

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1 Gondwana Research 9 (2006) Pb/ 206 Pb-ages of zircons from syn-collisional I-type porphyritic granites of the central Ribeira belt, SE Brazil Julio Cezar Mendes a, *,1, Ciro Alexandre Ávila b, Ronaldo Mello Pereira c, Mônica P.L. Heilbron c,1, Candido A.V. Moura d,1 a Departamento de Geologia, IGEO-Universidade Federal do Rio de Janeiro (UFRJ), Cidade Universitária, , Rio de Janeiro, Brasil b Departamento de Geologia e Paleontologia, Museu Nacional, Universidade Federal do Rio de Janeiro (UFRJ), Quinta da Boa Vista, , Rio de Janeiro, Brasil c Faculdade de Geologia, Universidade do Estado do Rio de Janeiro (UERJ), Rua São Francisco Xavier, 524, , Rio de Janeiro, Brasil d Departamento de Geoquímica e Petrologia, Universidade Federal do Pará, Rua Augusto Correa 1, , Belém, Brasil Received 15 February 2005; accepted 9 November 2005 Available online 10 January 2006 Abstract In the central segment of the Ribeira belt, southeast Brazil, several foliated porphyritic granitic bodies intrude high-grade migmatitic gneisses of the Andrelândia and Juiz de Fora domains and Embú Complex. Results of geological, geochemical and geochronological investigations of the Maromba, Pedra Selada, Serra do Lagarto and Funil porphyritic I-type granites provide profound similarities, except for the distinct geochemical behavior of the Funil Granite, perhaps related to a different crustal source. These granitoids show similar structural, textural and mineralogical features. Pb-evaporation of single zircons provided ages of 586T6, 579.6T6.3, 586.3T4.8 and 584T5 Ma for the granites, respectively, coincident with the syn-collision I episode of the central Ribeira belt. The intrusion of I-type porphyritic granitoids coeval with the main collisional event has not often been reported in the geological literature. The most common syn-collisional granitic magmatism has normally an S-type signature or even 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 geochronology 1. Introduction and tectonic setting The Neoproterozoic Ribeira orogenic belt extends over more than 1000 km along the southeastern coast of Brazil (Fig. 1). It developed as the result of the amalgamation of Western Gondwana during the Brasiliano/PanAfrican orogenic episodes. Reported geochronological data indicate that orogenic activity was diachronic along the supercontinent, ranging in time from ca. 900 to 480 Ma. Four major orogenic episodes have been recognized in the Ribeira belt (Machado et al., 1996; Heilbron et al., 1995, 2000, * Corresponding author. 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 and Machado, 2003; Schmitt et al., 2004): a) 790 to 600 Ma subduction and magmatic arc generation; b) 600 to 560 Ma collision I episode; c) 530 to 510 Ma collision II episode; and d) 510 to 480 Ma orogenic collapse. Collision I episode (600 to 560 Ma) is the most important orogenic event along the Ribeira belt and its northern counterpart, the Araçuaí belt. This tectonic episode is characterized by widespread generation of granitoids. Three common types of granitoids, related to this episode, are leucogranites, porphyritic (hornblende)-biotite granites and hornblende granites. Between the towns of Itatiaia, Visconde de Mauá and Santa Rita de Jacutinga, in the Minas Gerais/Rio de Janeiro/São Paulo state boundaries, voluminous and important porphyritic granitoid magmatism was recognised and studied by Heilbron (1993), Junho et al. (1999) and Pereira et al. (2001a). Ona X/$ - see front matter D 2005 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. doi: /j.gr

2 J.C. Mendes et al. / Gondwana Research 9 (2006) Fig. 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 coeval syn-collisional, I-type porphyritic and S-type granites in the Ribeira belt. The purpose of this paper is the presentation of new zircon Pb-evaporation ages of four major plutons representative of the porphyritic I-type syn-collision I magmatism of the belt (Maromba, Pedra Selada, Serra do Lagarto and Funil) and the discussion of their geological and geochemical data in the context of the Gondwana amalgamation. 2. Tectonic organization and regional geology The central segment of the Ribeira belt is subdivided in five distinct tectonic terranes, in the sense of Howell (1989). These are: Occidental, Paraíba do Sul, Embú, Oriental and Cabo Frio Terranes (Heilbron et al., 2000, 2004). The investigated plutons are located in the Occidental and Embú Terranes (Fig. 2). The Occidental Terrane is subdivided in two structural domains named Andrelândia and Juiz de Fora (Fig. 2). Both contain three units: a) an older Paleoproterozoic basement (the Mantiqueira Complex in the Andrelândia Domain and the Juiz the Fora Complex in the Juiz de Fora Domain); b) a highly deformed and metamorphic cover association (Andrelândia megasequence) and c) Neoproterozoic granitoids. The Paleoproterozoic basement association includes hornblende and biotite migmatitic orthogneisses (Mantiqueira complex) and varied orthogranulites (Juiz de Fora complex). The Andrelândia megasequence includes banded biotite gneisses with quartzites, amphibolites and pelitic gneisses, and sillimanite garnet bitotite gneisses with several layers of calcsilicate rocks, gondites, amphibolites and quartzites. As pointed out by Junho et al. (1999), in the studied area, the extensive migmatization transformed the paragneisses in stromatic metatexites, crosscut by multiple leucosomatic veins, which progressively grade into (garnet) biotite muscovite granitic leucogneiss. In the northwestern part of the investigated area, in the Carvalhos Klippe, high-pressure granulites were described from the Andrelândia megasequence (Trouw et al., 2000; Campos Neto and Caby, 2000). The Paraíba do Sul and Embú Terranes also contain basement and supracrustal associations (Eirado et al., in press). In the former terrane, basement is represented by the Quirino Complex and other orthogneisses, while the Neoproterozoic cover, named Paraíba do Sul Group or Complex is composed of pelitic gneisses, schists, calcsilicate rocks and marbles. In the Embú Terrane, cover consists of tourmaline rich pelitic gneisses and schists with quartzitic layers, while basement associations 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 Ribeiro et al. (1995) and Heilbron et al. (1995, 2000). The oldest one, D 1 +D 2, created the main foliation, locally mylonitic, and tight to isoclinal inclined to subhortizontal folds that repeat and thicken the units. In the Juiz de Fora domain, an important

3 328 J.C. Mendes et al. / Gondwana Research 9 (2006) Fig. 2. Simplified tectonic map of Santa Rita do Jacutinga Resende Bananal region, modified after Heilbron et al. (2004). Geological data were compiled from Silva 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. 580 Ma); 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 Main foliation; 16 Major thrusts; 17 Synform; 18 Antiform; 19 Geochronological sampling, with abbreviation of the name of the investigated plutons represented by 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 several thrust slices. The D 3 phase refolded the previous structures, generating open to tight folds with steep axial surfaces and SW plunging axes. In the Andrelândia domain, in the northern sector of the investigated area (Fig. 2), the interference between D 1 +D 2 and D 3 phases resulted in dome-and-basin and refolded-fold patterns shown on the map. In the central and southern sector of the area, D 3 developed important subvertical shear zones, such as the Além Paraíba, Alto da Fartura and Cubatão shear zones. The rocks of the region underwent syn-d 2 barrovian metamorphism, with increasing temperatures and medium pressure (Heilbron, 1993, Trouw et al., 2000). The regional metamorphism culminated with the formation of migmatites and granites in the basal feldspathic metasedimentary rocks of the Andrelândia megasequence (Ribeiro et al., 1995). As mentioned above, the migmatitic rocks were subdivided into two units grading into each other: a stromatic biotite gneiss with metatexitic structures and a (garnet) muscovite biotite diatexitic leucogneiss. Both are intruded by post-d 2 finegrained equigranular leucogranite containing garnet, tourmaline, biotite and muscovite. It crops out as minor concordant lenses or as dykes and sills. Both the diatexite and the intrusive leucogranite result from progressive partial melting of the

4 J.C. Mendes et al. / Gondwana Research 9 (2006) metatexite and show typical peraluminous S-type characteristics (Junho, 1995), in contrast with the I-type foliated granites studied in this work and described below. 3. Porphyritic granites The Maromba, Pedra Selada, and Serra do Lagarto porphyritic I-type granites occur, respectively, adjacent to the Visconde de Mauá (Rio de Janeiro), Bocaina de Minas and Santa Rita de Jacutinga (Minas Gerais) towns (Fig. 2) These granitic bodies have a lenticular shape concordant with the main southeast dipping regional S 2 foliation (Heilbron et al., 1995). The Serra do Lagarto granite is situated in the Juiz the Fora domain, while the Maromba and Pedra Selada granites are located in the Andrelândia domain. The Funil granite is located in the Embú Terrane and extends from south of Areias to the town of Itatiaia, where it disappears under sediments of the Resende Basin (Fig. 2). The predominant structure in the Maromba, Pedra Selada, and Serra do Lagarto porphyritic I-type granites is a foliated and lineated porphyritic fabric with a superimposed protomylonitic texture. The first structure is probably derived from magmatic flow grading to solid state flow of a partially crystallised magma (Heilbron, 1993; Junho et al., 1999). The porphyritic texture is characterized by microcline megacrysts, up to 3 cm long, within a medium grained granodioritic matrix consisting of quartz, plagioclase, microcline, biotite, hornblende, sphene, allanite, zircon and apatite. The abundance of polygonal quartz in the groundmass and in fractures of microcline megacrysts indicates the more ductile behavior of quartz in relation to feldspars. The Serra do Lagarto granite shows the less deformed textures of the three bodies. The Maromba granite presents clearly intrusive contacts with the surrounding biotite and muscovite bearing fine grained equigranular S-type leucogranite (Almeida, 1996). It is limited to the southwest by the Cretaceous alkaline Itatiaia massif and to the northeast by steep fault contact with the diatexitic leucogneiss. The contact with the metatexitic gneiss is concordant. The Pedra Selada granite is a large sill-like body, that extends from south of Visconde de Mauá to the neighborhood of Bocaina de Minas and Passa Vinte (Fig. 2). The contact with the metatexitic paragneiss is sharp and concordant with the regional foliation. Metric lenses of dark fine-grained quartzdiorite are found throughout but mainly along its borders. South of Bocaina de Minas, this pluton contains small irregular lenses of leucogranite gneiss with diffuse contacts. East of the locality of Carlos Euler, the Serra do Lagarto granite forms the Serra do Lagarto range (Heilbron, 1993). It is also characterized by the presence of dark and fine-grained quartzdioritic lenses, mainly along the borders, and displays sharp and subconcordant contacts with the paragneisses and the hornblende migmatitic gneiss of the surroundings. At the eastern end several apophyses can be observed in the country rocks, resulting in a lit par lit mesostructure (Junho et al., 1999). The Funil granite is a non-to weakly deformed body. It is monzogranitic in composition and the porphyritic texture is given by microcline megacrysts up to 7 cm long surrounded by a fine to medium-grained groundmass composed of quartz, zoned plagioclase, microcline and biotite. Apatite, sphene, zircon, allanite and opaque minerals (magnetite, pyrite, molibdenite and rare ilmenite) are accessory phases. The orientation of microcline megacrysts and the aligned quartz and biotite aggregates are considered to be markers of the primary igneous foliation of the rock (Pereira et al., 2001a). Maromba and Funil granites show the lightest matrix colour index and are devoid of dark quartzdioritic enclaves. A close relationship with leucogranite and hornblende migmatitic gneiss of the basement association, as observed in the Pedra Selada and Serra do Lagarto granites, is missing. Pereira et al. (2001b) obtained geochronological data for the Mendanha and Funil porphyritic I-type granites, the former situated northwest of the granites focused on this paper. They achieved a 207 Pb / 206 Pb age of 592T5 Ma for the Mendanha granite and 584T5 Ma for the Funil granite. 4. Geochemical data Geochemical data of the porphyritic I-type granites were reported by Heilbron (1993), Junho et al. (1999) and Pereira et al. (2001a). Junho et al. (1999) recognised a clear geochemical relation between the Maromba, Pedra Selada and Serra do Lagarto I-type granites, characterized by high-k, calc-alkaline and metaluminous to slightly peraluminous signature. They interpreted the trends of the studied diagrams as a problabe combination of crystal fractionation and/or magma mixing process. The Funil granite was classified as an I-type as well, indicating a slightly peraluminous, high-k calc-alkaline composition (Pereira et al., 2001a). Fig. 3. AFM diagram for the porphyritic granites. Calc-alkaline boundary from Irvine and Baragar (1971). Symbols: cross: Funil porphyritic I-type granite; filled circle: Maromba porphyritic I-type granite; filled square: Pedra Selada porphyritic I-type granite; filled triangle: Serra do Lagarto porphyritic I-type granite. Source of geochemical data: Maromba, Pedra Selada and Serra do Lagarto granites Junho et al. (1999); Funil granite Pereira et al. (2001a).

5 330 J.C. Mendes et al. / Gondwana Research 9 (2006) Fig. 4. Shand s diagram for the porphyritic I-type granites. Symbols as in Fig. 3. In this paper, geochemical data of porphyritic granites from Heilbron (1993), Junho et al. (1999) and Pereira et al. (2001a) are ploted in selected diagrams, as discussed below. The AFM and Shand s diagrams (Figs. 3 and 4) reinforce the calc-alkaline and metaluminous to slightly peraluminous signature of the granites. It must be emphasized that towards the most evolved magmas of an acid I-type igneous sequence there is a tendency to crystallize slightly peraluminous rocks (Clarke, 1981). The Harker diagrams (Fig. 5) show the SiO 2 range of the granites. The more evolved character of the samples from Maromba and Funil granites contrasts with the less evolved from the Pedra Selada granite. From field and petrographic observations, as well as linear trends observed in the SiO 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 Lagarto granites may be inferred. The Funil granite represents the more Fig. 5. Harker diagrams for the porphyritic I-type granites. Symbols as in Fig. 3.

6 J.C. Mendes et al. / Gondwana Research 9 (2006) Fig. 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 forms the more basic one. On the other hand, in some diagrams the Funil granite samples tend to be clustered and do not fit very well in the assumed trends (e.g. SiO 2 Fe 2 O 3, CaO, Ba and Sr). This behavior is also apparent in Figs. 3, 6 and 7). The preferential occurrence of quartzdioritic microgranular enclaves in the Pedra Selada and Serra do Lagarto porphyritic granites could indicate prior assimilation and/or contamination process. The REE patterns of Fig. 6 highlight the probable cogenetic link between the Maromba, Pedra Selada and Serra do Lagarto porphyritic I-type granites. They have quite similar REE distribution pattern, showing to be highly fractionated with a clear negative Eu anomaly. The Pedra Selada granite shows the highest total REE content. REE concentrations of the Maromba granite are lower than that of the Pedra Selada granite despite its highest SiO 2 content. The well developed negative Eu anomaly in some patterns of the Funil granite is possibly caused by feldspar extraction during fractional crystallization processes. However, its less fractionated REE patterns and the strong negative Eu anomaly contrast with all others. The HREE enriched flat pattern may be associated with sphene and zircon modal abundance, but hybridization processes could also provoke changes in the REE concentrations of the melt, as pointed out by Dini et al. (2004). The different REE distribution of the Funil granite reinforces its possible more evolved character, maybe related to a different crustal source associated with the Embu Complex host rocks, while the Maromba, Pedra Selada and Serra do Lagarto granites are surrounded by the Andrelândia and Juiz de Fora domains. Further achievement of isotopic Sr and Nd analyses possibly will provide adequate elements to elucidate this assumption. In the R 1 R 2 diagram (Fig. 7), after Batchelor and Bowden (1985), two trends can be envisaged: one from field 6 (syncollision field) to field 4 (late-orogenic field) and the other from field 6 to field 7 (post-orogenic field). The former may be associated with the formation of I-type granites during syn-to late-orogenic processes. The alignment of granite samples seems to reflect the evolution from less evolved metaluminous to more evolved, slightly peraluminous granitic magma, in a syn-tectonic environment Pb/ 206 Pb geochronology 5.1. Sampling Fig. 7. R 1 R 2 diagram for the porphyritic I-type granites. Symbols as in Fig. 3. Fields from Batchelor and Bowden, 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 concentration using Frantz magnetic separation and heavy liquid (bromo-

7 332 J.C. Mendes et al. / Gondwana Research 9 (2006) Table Pb/ 206 Pb data for single zircons of the Maromba Granite Zircon Description Evaporation T (C-) Measured ratios 204 Pb/ 206 Pb 207 Pb/ 206 Pb 207 Pb/ 206 Pb* Age (Ma) Zircon age (Ma) 1 euh, stpr, tr, py T T T41 598T T T T24 595T T T T21 568T T euh, lgpr, tr, py T T T09 593T T T T10 576T T euh, lgpr, tr, py T T T27 594T T T T09 669T03 4 euh, stpr, tr, py T T T35 580T13 580T13 5 euh, lgpr, tr, py T T T21 588T08 588T08 6 euh, stpr, tr, py 1450 # T T T48 509T T T T54 589T20 589T20 8 sbh, stpr, tr, py T T T19 595T07 595T07 14 euh, lgpr, tr, py T T T35 591T13 591T13 Average age 586T6 Uncertainties 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 selected because they tend to be more concordant (Krogh, 1982). The analyzed non-magnetic zircons are transparent to occasionally translucent, colorless, pale yellowish or pink. Bipyramidal prism length varies from long (width vs length=15) to short (width vs length=12) and the grains sometimes display subhedral morphology (Tables 1 2 and 3) Analytical techniques Geochronological analyses by Pb-evaporation technique in zircons (Kober, 1986, 1987) were carried out at the Laboratory of Isotope Geology of the Federal University of Pará (Pará-Iso). Zircon is a heavy mineral with crystalline structure very resistant to alteration and it tends to preserve the isotopic information since its crystallization. Kober (1986) has shown that the Pb components with the highest activation energy normally reside in the intact crystalline zircon phase that shows no post-crystallization Pb-loss and consequently yields concordant 207 Pb/ 206 Pb ages. The progressive heating of a zircon crystal during the Pb-evaporation techniques gradually releases Pb isotopes. The theoretical basis for this technique is that the activation energy necessary to release Pb from damaged or metamict zones of zircon are much less than from crystalline domains (Kober, 1986). Thus, radiogenic Pb from metamict zones can be purged at relatively lower evaporation temperatures, while radiogenic Pb from crystalline domains in zircon is released only at higher temperatures (Ansdell and Kyser, 1993). Isotope analyses on porphyritic I-type granites were performed on a Finnigan MAT 262 thermo-ionization mass spectrometer (TIMS) using the stepwise heating process (Kober, 1986, 1987) with a double-filament arrangement (evaporation and ionization). In this procedure, the zircon was 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 be cleaned. Afterward, the evaporation filament was heated (evaporation step) to allow structurally bound radiogenic Pb to diffuse out of the crystal. The Pb was deposited on the cold ionization filament that was, subsequently, heated releasing the deposited Pb for isotope determinations. The isotope data were acquired dynamically using the ion counting system of the instrument, with the intensity of the 207 Pb beam between 30,000 and 100,000 counts per second (cps). The intensity of different Pb isotopes were measured in the mass sequence 206 Pb, 207 Pb, 208 Pb, 206 Pb, 207 Pb, 204 Pb. Ten (10) mass scan Table Pb/ 206 Pb data for single zircons of the Pedra Selada Granite Zircon Description Evaporation T(C-) Measured ratios 204 Pb/ 206 Pb 207 Pb/ 206 Pb 207 Pb/ 206 Pb (*) Age (Ma) Zircon age (Ma) 3 sbh, stpr, tl, py T T T18 584T07 584T07 5 euh, lgpr, tl, py T T T18 569T07 569T07 7 euh, stpr, tl, py T T T37 562T14 562T14 13 euh, lgpr, tl, py T T T18 578T07 578T07 14 euh, lgpr, tl, py T T T25 585T T T T68 579T T euh, lgpr, tl, py T T T42 591T T T T25 591T09.3T7.7 Average age 579.6T6.3 Uncertainties 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 and 3); lgpr=long prism (l / w >3); tl=translucent; py=pale yellowish.

8 J.C. Mendes et al. / Gondwana Research 9 (2006) Table Pb/ 206 Pb data for single zircons of the Serra do Lagarto Granite Zircon Description Evaporation T(C-) Measured ratios 204 Pb/ 206 Pb 207 Pb/ 206 Pb 207 Pb/ 206 Pb (*) Age (Ma) Zircon age (Ma) 1 euh, lgpr, tr, cl T T T16 584T T T T22 585T T4.7 2 euh, stpr, tr, cl 1450 # T T T87 598T T T T37 596T T euh, lgpr, tr, cl T T T24 579T T T T T29 590T T euh, lgpr, tr, cl T T T22 582T T T T16 575T T6.8 5 euh, stpr, tr, cl 1450 # T T T94 586T T T T19 596T T7.0 6 euh, lgpr, tr, cl T T T13 597T T T T15 597T T3.5 8 euh, stpr, tr, cl T T T T T T T T T T T T05 9 euh, lgpr, tr, cl T T T19 583T T T T12 576T T T T95 560T T euh, stpr, tr, cl T T T25 589T T T T22 596T T17.0 Average age 586.3T4.8 Uncertainties 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 and 3); 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 of evaporation at temperatures of 1450, 1500 and C. The average 207 Pb/ 206 Pb ratio of each step was determined based on five blocks of data. In general, the average 207 Pb/ 206 Pb ratios obtained in the highest evaporation temperature were considered for age calculation of each zircon grain. However, the ages of the lower temperature steps were also integrated when they overlapped within error with those ages obtained at the highest evaporation temperature. The uncertainties are given in 2r errors. Common Pb corrections were made according to the Stacey and Kramer (1975) two-stage model. Only blocks with 204 Pb / 206 Pb ratios lower than were used for apparent age determination of each evaporation step. The data treatment followed the procedure described in Gaudette et al. (1998). In order to control the accuracy of the single zircon Pb evaporation ages carried out in the Para-Iso, a number of 207 Pb/ 206 Pb apparent ages of the international standard zircon 95,100 were measured. The ages obtained varied from T 5.9 to T 5.9 Ma, giving an average age of Fig. 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.

9 334 J.C. Mendes et al. / Gondwana Research 9 (2006) Fig. 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> Uncertainties are given at 2r T2.3 Ma. The accepted age for this 95,100 zircon is T0.3 Ma (Wiedenbeck et al., 1995) New isotopic results The zircon crystals of the Maromba porphyritic I-type granite gave very consistent 207 Pb/ 206 Pb apparent ages varying between 589T20 and 580T13 Ma (Table 1 and Fig. 8). The average 207 Pb / 206 Pb age, calculated based on Pb/ 206 Pb ratios of seven zircons grains, is 586T6 Ma. In this body, one crystal (zircon 3) yielded an age of 669 T 3 Ma that was interpreted as an inherited crystal or a mixture of lead from an old inherited core and a magmatic overgrowth. The Pedra Selada porphyritic I-type granite yielded an apparent 207 Pb / 206 Pb age of 579.6T6.3 Ma (Table 2 and Fig. 9). This age was calculated based on Pb / 206 Pb ratios of six zircons, whose ages vary from 562T14 to T7.7 Ma. In the Serra do Lagarto porphyritic I-type granite, eight zircon crystals gave 207 Pb/ 206 Pb apparent ages varying between T 6.8 and T 13.4 Ma (Table 3 and Fig. 10). The weighted-mean 207 Pb/ 206 Pb apparent age of these crystals, calculated based on Pbs / 206 Pb ratios, is T 4.8 Ma. One crystal (zircon 8) provided an age of 2019T5 Ma and is considered an inherited crystal or a mixture of lead from an old inherited core and a magmatic overgrowth. The lack of significant change in the 207 Pb/ 206 Pb ratios on progressive heating of analyzed grain 15 of the Pedra Selada porphyritic I-type granite and grains 1, 2, 4, 6 and 10 of the Serra do Lagarto porphyritic I-type granite suggests the existence 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> Uncertainties are given at 2r.

10 J.C. Mendes et al. / Gondwana Research 9 (2006) Discussion and conclusions As suggested before by previous workers (Junho et al., 1999; Heilbron et al., 1995, 2000; Trouw et al., 2000; Pereira et al., 2001a), the geochemical as well as the new geochronological data confirm that the Maromba, Pedra Selada and Serra do Lagarto porphyritic I-type granites are syn-tectonic and crystallized and intruded during the D 1 +D 2 Ribeira main deformation phase. The common high-k calc-alkaline, metaluminous to slightly peraluminous chemistry of the melt of Maromba, Pedra Selada and Serra do Lagarto porphyritic granites is probably related to a unique reservoir. Despite the lack of Sr and Nd isotopic data, the granites geochemical behavior and minimum crystallization age suggest such a relationship. The different REE pattern of the Funil Granite is possibly associated with fusion of a different protolith (perhaps related to the metasedimentary rocks of the Embú Complex) and/or with the contribution of accessory minerals (e.g. sphene, apatite and zircon) and feldspar extraction during the fractional crystallization process. The occurrence of I-type porphyritic granitic rocks yielding ages contemporaneous with the main continental collisional event has not often been reported in the geological literature. The most common syn-collisional granitic magmatism has normally an S-type signature, sometimes with a typical aluminous paragenesis (Chappell and White, 1974), or even a slightly peraluminous I-type character. Janasi et al. (2001) discuss the relationship between granites of the Agudos Grandes batholith, in the southern portion of the Ribeira Belt in São Paulo State. They propose a scenario in which I-type syn-collisional porphyritic granite (Ibiuna type, with an age of 610 T 2 Ma) is closely associated with the contemporary S-type Turvo granite (610 T 1 Ma). As described before (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) is coeval with diatexites and associated leucogranites. This is also demonstrated by coeval S-type granitoids of the Rio Turvo batholith, cropping out in the investigated area, dated at about 580 Ma (Machado et al., 1996). Another example is the Cantagalo leucogranite with an age of ca. 590 Ma (Heilbron and Machado, 2003) that intrudes pre-collisional tonalitic gneisses (about 630 Ma) of the Oriental Terrane of the central Ribeira belt in Rio de Janeiro State. Other examples of coeval S- and I-type syn-tectonic porphyritic granites are reported from the Archaean basement of southern India (Moyen et al., 2003) and from northern Brazil (Barros et al., 2001). Toteu (1990) described a PanAfrican metadioritic syn-collisional suite followed by late-collisional porphyritic granite. Also focusing on plutonic rocks formed during the PanAfrican Orogeny, Van de Flierdt et al. (2003) investigated the syn-orogenic quartz diorite porphyritic granite/granodiorite leucogranite association from the Bandombaai Complex, Namibia. The porphyritic granite/granodiorite is slightly peraluminous and contrasts with the strongly peraluminous character of the leucogranite. Describing a geological setting similar to the area focused in this paper, the authors emphasize the production of these different magmas during the metamorphic peak. A close relationship between syn-orogenic I-type granodiorites and tonalites and S-type granites is described by Pankhurst et al. (2001) in the Famatinian orogenic belt from NW Argentina, where the granitoids present an age of about 495 to 465 Ma. The geochronological results of 584T5 Ma for the Funil porphyritic granite and 592T5 Ma for the Mendanha porphyritic granite (Pereira et al., 2001b) are very close to the new geochronological data presented here. This age interval, synchronous with the central Ribeira belt deformational and metamorphic peak, should be considered as an important period of crustal magma generation, when the production of granites showing contrasting mineralogy, textures and geochemical (I- and S-type signatures) characteristics was quite significant. In conclusion, the contemporary intrusion of S- and I-type syn-collisional granitoids in the investigated area is probably related to the structural style of the central Ribeira belt, characterized by interfingering of cover and basement rocks on a crustal scale (Heilbron and Machado, 2003, Trouw et al., 2000, Schmitt et al., 2004, Heilbron et al., 2004). The collisional crustal thickening may have induced widespread melting of different sources, resulting in magmas with various geochemical signatures. Acknowledgements The authors are grateful to GR referees Frank Söllner and Randall Van Schmus for detailed reviews of the manuscript. Special thanks to Maria C. B. Junho for her great support on field trip activities. We are grateful to Dr. Claudio M. Valeriano for critically reading the manuscript. Special acknowledgement to Fundação Carlos Chagas Filho de Amparo à Pesquisa do Rio de Janeiro (FAPERJ) for the financial support of the research. 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