Deformation and geochronology of syntectonic granitoids emplaced in the Major Gercino Shear Zone,...

Gondwana Research 17 (2010) 688–703

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Deformation and geochronology of syntectonic granitoids emplaced in the MajorGercino Shear Zone, southeastern South America

Cláudia Regina Passarelli ⁎, Miguel A.S. Basei, Oswaldo Siga Jr., Ian Mc Reath, Mário da Costa Campos NetoInstituto de Geociências, Universidade de São Paulo, Rua do Lago, 562, CEP 05508-080, São Paulo — SP, Brazil

⁎ Corresponding author. Rua Dr. Augusto de Miranda,São Paulo — SP, Brazil. Fax: +55 11 30913993.

E-mail address: [email protected] (C.R. Passarelli).

1342-937X/$ – see front matter © 2009 International Adoi:10.1016/j.gr.2009.09.013

a b s t r a c t
a r t i c l e i n f o
Article history:Received 1 December 2008Received in revised form 3 September 2009Accepted 19 September 2009Available online 25 October 2009

Keywords:Dom Feliciano BeltMajor Gercino Shear ZoneMicrostructuresU–Pb zircon ageK–Ar age

The Major Gercino Shear Zone is one of the NE–SW lineaments that separate the Neoproterozoic DomFeliciano Belt, of Brazil and Uruguay, into two different domains: a northwestern supracrustal domain from asoutheastern granitoid domain. The shear zone, striking NE, is composed of protomylonites to ultramyloniteswith mainly dextral kinematic indicators.In Santa Catarina State, southern Brazil, the shear zone is composed of two mylonite belts. The myloniteshave mineral orientations produced under greenschist facies conditions at a high strain rate. Strongflattening and coaxial deformation indicate the transpressive character, while the role of pure shear isemphasized by the orientation of the mylonite belts in relation to the inferred stress field component. Thequartz microstructures point out that different dynamic recrystallization regimes and crystal plasticity werethe dominant mechanisms of deformation during the mylonitization process. Additionally, the fabricssuggest that the glide systems are activated for deformation conditions compatible with the metamorphismin the middle greenschist facies.Elongated granitoid intrusions belonging to two petrographically, geochemically and isotopically distinctrock associations occur between the two mylonite belts. The structures observed in the granites result from adeformation range from magmatic to solid-state conditions points to a continuum of magma straining duringand just after its crystallization.Conventional U–Pb analysis of multi-crystal zircon fractions yielded essentially identical ages of 609±16 Maand 614±2 Ma for the two granitic associations, and constrain the transpressive phase of the shear zone. K–Ar ages of biotites between 585 and 560 Ma record the slow cooling and uplift of the intrusions. Some K–Arages of micas in regional mylonites are similar, suggesting that thermo-tectonic activity was intense up tothis time, probably related to the agglutination of the granite belt to the supracrustal belt NW of the MGSZ.

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1. Introduction

The Mantiqueira Province extends from eastern Brazil to southernUruguay and includes the Araçuaí, Ribeira and Dom Feliciano foldbelts, bordering the São Francisco, Paranapanema and Rio de la Platacratons and surrounding the Luís Alves craton (Almeida et al., 1981;Brito Neves and Cordani, 1991; Cordani and Sato, 1999; Mantovaniand Brito Neves, 2005, Silva et al., 2008, among others).

The Neoproterozoic Brasiliano/Pan-African orogenic cycle recordsGondwana amalgamation and is characterized, in southern andsoutheastern Brazil, by arc and collisional settings, with significantgranitic magmatism (e.g. Schmitt et al., 2008; Siga Junior et al., 2009).The collisional events led also to a system of steep dipping dextraltranspressive shear zones that cut earlier consolidated orogens and

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the post collisional period is recorded by extensive magmatism alongmajor transcurrent shear zones (Heilbron et al., 2004).

The evolution of the Dom Feliciano Belt (DFB), southern part of theMantiqueira Province, is associated with transpressive tectonicsduring the Neoproterozoic and Early Paleozoic West Gondwanacollage. The shear zones, at present defining the distinct boundary ofthe orogen, also control the emplacement of syntectonic plutons.Examples for the emplacement of granite plutons (Archanjo et al.,1999; Neves et al., 2003; Souza et al., 2006) and pegmatite dykes(Araújo et al., 2001) in a similar fashion in Brazil are found in theMantiqueira Province, such as the late orogenic plutons in theAraçuaí-Ribeira Fold Belt controlled by ductile shear zones (Wiede-mann et al., 2002), in some of the suites of the Pelotas Batholith in theDFB (Fernandes et al., 1992; Tommasi et al., 1994; Philipp andMachado, 2005), and in the Borborema Province. The ascent andemplacement of granitoids magmas, related to crustal-scale shearing,has beenwidely studied in the ancient orogens (e.g. Hutton and Reavy1992; Vigneresse 1995; Spanner and Kruhl 2002; Festa et al. 2006). Itis envisaged that crustal thickening during transpressive orogenesis

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has a high potential for generating granitic melts due to the presenceof fluids that support potential melting (Hutton and Reavy, 1992;Spanner and Kruhl, 2002). It has also been claimed that granites canmigrate from their source to the mid-crust through relatively narrowshear zones and that deformation is necessary for magma ascent(Vigneresse and Tikoff 1999; and Vigneresse and Clemens 2000).Shear zones serve as the path for magma ascent and emplacement atthe upper crustal levels and deformation controls the transport andlocation of plutons (Vigneresse, 1995). According to Vigneresse andTikoff (1999), strain partitioning causes horizontal migration of meltinto shear bands while the shear bands act as vertical conduitsallowing melt migration to higher structural levels. Deformation-assisted pumping of melt is believed to be the mechanism by whichthis upward migration can take place. Additionally, triple pointsdevelop into an extensional and low-mean-pressure zone, producedby changes in the rheological properties of the surrounding rocks ofthe shear zones, act as attractors for ascending magmas, giving rise toplutons emplacement (Weinberg et al., 2004). Accordingly, shearzones may act as magma pumps, producing and destroying magmatraps, leading to increasing magma addition to upper crustal plutons(Weinberg et al., 2009).

In southern Brazil, the Neoproterozoic Dom Feliciano Belt (DFB) ispart of a major tectonic province of the Brasiliano-Pan-African Cycleand consists of supracrustal rocks and granitic batholiths withcontacts defined by high-angle ductile shear zones with main NE–SW strike. The Major Gercino Shear Zone (MGSZ) in Santa Catarinastate (Fig. 1) is part of a lithospheric-scale discontinuity and one of themajor NE–SW lineaments that affect all southern Brazilian andUruguayan Precambrian terrains. The shear zone is characterized bya regional NE trend and an oblique resultant dextral movement sensewhere ductile–brittle structures predominate.

In order to determine the timing of emplacement of graniticassociations relative to shearing process and characterize thedeformation mechanisms we carried out structural, petrographicand geochronological studies of two contrasting granite typesemplaced between the two mylonitic belts of the MGSZ, in theCanelinha-Garcia region eastern part of the state of Santa Catarina.

2. Geological setting

In the southeastern South American Neoproterozoic terrains, theDom Feliciano Belt (DFB) represents a major geotectonic unit in thesouthern part of the Mantiqueira Province (Almeida et al., 1981; daSilva et al., 2005a). It forms a strip with roughly N–S orientation, andoccupies the entire eastern segment of southern Brazil and Uruguay(Fig. 1a).

The Major Gercino Shear Zone (MGSZ) in Santa Catarina state(Fig. 1) is part of a lithospheric-scale discontinuity in the DFB, and is aprominent feature of the Proterozoic terrains in southern Brazil andUruguay (Bitencourt and Nardi, 1993; Hallinan et al., 1993; Basei et al.,2000). Thismajor crustal-scale suture, defined by Basei et al. (2005) asthe Sierra Ballena–Major Gercino Lineament (SBMGL), forms a∼1400 km-long shear system, and is marked by strong linear negativegravity anomalies (Mantovani et al., 1989). From its southern limit inUruguay to its termination in Santa Catarina state in Brazil, the DFB iscomposed of three crustal sectors separated by tectonic contacts: A)the Granite Belt (Florianópolis and Pelotas Batholiths in Brazil and theAiguá Batholith in Uruguay, composed of deformed I-type medium tohigh calc-alkaline granites and alkaline granitoid rocks); B) the SchistBelt (Brusque and PorongosMetamorphic Complexes in Brazil and theLavalleja Group in Uruguay, constituted by volcanosedimentary rocksmetamorphosed in greenschist to amphibolite facies with associatedgranitoids; and C) the foreland basin deposits, composed ofanchimetamorphic sedimentary and volcanic rocks, are situatedbetween the Schist belt and the Archean–Paleoproterozoic foreland

to the West and comprise the Itajaí and Camaquã Basins in Brazil andthe Arroyo del Soldado and Piriápolis Basins in Uruguay.

In the DFB, the Southern Brazilian Shear Belt (SBSB), part ofSBMGL, is a crustal discontinuity that encompasses anastomosingnetwork of NNE and NE trending shear zones (Fig. 1a) with dominanttranscurrent kinematics which controlled the intrusion of calc-alkaline granites, and in which syntectonic peraluminous and alkalinegranites occurred (Bitencourt and Nardi, 2000; 2004). The GranitoidBelt in the DFB in Santa Catarina is represented by the FlorianópolisBatholith (Fig. 1b) which is composed of three main suites: the ÁguasMornas (deformed tonalites to granodiorites associated with migma-tites), São Pedro de Alcântara (quartz–diorites to quartz–monzonites)and Pedras Grandes Suite (isotropic leucogranites with alkalineaffinity).

Paleoproterozoic migmatitic gneisses occur in the Schist Belt andin the Granite Belt. In the former they are correlated with theCamboriú Complex (da Silva et al., 2000; Bitencourt and Nardi, 2004),and in the latter with the Águas Mornas Complex (da Silva et al.,2000). They were interpreted by Basei et al. (2000) as preservedremnants of Dom Feliciano Belt nuclei (DFB).

The Schist Belt in the DFB is represented by the BrusqueMetamorphic Complex (Silva, 1991; Philipp et al., 2004) in SantaCatarina, composed of meta-volcanosedimentary rocks. These rocksunderwent polyphase deformation, resulting in NW-verging nappesformed during the main metamorphic episode in the Neoproterozoicwhich reached upper greenschist–lower amphibolite facies (Basei etal., 2000), preceding the sub-vertical transpressional period of MGSZin a continuumdeformational process. The Schist Belt was intruded bylate-tectonic granites: the two-mica leucogranites of the São JoãoBatista suite, the porphyritic biotite granites of the Valsungana suite,and the biotite granites of the Nova Trento suite. Similar features arealso documented in other parts of the Schist Belt in Rio Grande do Sul,Porongos Metamorphic Complex, and Uruguay, Lavalleja Metamor-phic Complex (Basei et al., 2006). Both Schist and Granitoid Belts hadindependent origins and evolutions, and achieved their presentconfiguration only after about 540 Ma, when they were transportednorthwestwards during the upper Neoproterozoic and lower Phan-erozoic, to the border of the craton which today underlies the Paranábasin (Basei et al., 2000). The MGSZ separates these two domainswhich have different model Nd TDM ages of 1290 to 1690 Ma in theSchist Belt, and around 2000 Ma in the Granite Belt (Mantovani et al.,1989; Basei, 1990).

3. The Major Gercino Shear Zone

In the eastern part of the state of Santa Catarina, the MGSZ presenta 1 to 3.8 km wide well-defined northwestern mylonite belt (NWB)and a 500 m to 2 km wide southeastern mylonite belt (SEB) strikingNE, composed of protomylonites to ultramylonites with mainlydextral kinematics (Passarelli et al., 1993, 1997). The mylonitic beltsform the limits of two petrographically, geochemically and isotopi-cally different granitoid associations, usually referred to as the CentralGranitoids: the Fernandes Granitoid Association (FGA), and theRolador Granitoid Association (RGA) (Fig. 2).

A gradation from protomylonite to ultramylonites and phyllonite,passing through mylonite, characterizes the fault-related rocks ofthese two belts. The dip of NWB mylonitic foliation (Fig. 2) is mainlysub-vertical and shows a systematic strike variation: in the NE sector(Diagram 1, Fig. 2) strikes N65E preferentially, N54E in the middle(Diagram 2, Fig. 2), and then strikes N10E in the SW part (Diagram 3,Fig. 2). The stretching lineation on the foliation is defined byelongated feldspar porphyroclasts, quartz ribbons and recrystallizedquartz-feldspathic trail. In addition, it is generally parallel to thepreferential alignment of biotite flakes. The lineation exhibits dipdirections of N55E with shallow plunges in the NE sector, N45E andS57W trends in the middle with intermediate to steep plunges, and

690 C.R. Passarelli et al. / Gondwana Research 17 (2010) 688–703

Fig. 2. Geological map of Major Gercino Shear Zone (MGSZ). 1) Cenozoic deposits; MGSZ: 2) Northwestern mylonite belt (NWB), Southeastern mylonite belt (SEB); CentralGranitoids of MGSZ: 3) Fernandes granitoid association; 4) Rolador granitoid association; terrains north of MGSZ: 5) Brusquemetamorphic complex; 6) intrusive granitoids; terrainssouth ofMGSZ: 7) granite–migmatitic complex (Camboriú complex); 8) principal faults; 9)mylonitic foliation; 10)main foliation; 11) cataclastic foliation; 12)mineral lineation; 13)magmatic lineation; 14) outcrops with microstructural and petrographic analyses cited in the text; 15) outcrops with geochronological U–Pb data; 16) outcrops withgeochronological K–Ar data; 17) outcrops with geochronological K–Ar fine fractions data. Stereograms of a lower-hemisphere Schmidt projection. Poles to mylonitic foliations (Sm)andmineral stretching lineations (Lm) inMGSZ: 1)61poles to Smand10 Lm—NE sector of NWB;2) 95poles to Smand46 Lm— central sector ofNWB; 3)69poles to Smand13 Lm— SWsector of NWB; 4) 59 poles to Sm and 21 Lm — SEB. Poles to foliation in central granites: 5) 43 poles to magmatic and submagmatic foliation (S) and 4 magmatic lineations (L).


plunge at intermediate to steep angles towards S20W in the SW sector(Fig. 2). In a few areas, the mylonitic foliation is gently folded, withaxial-plane foliation striking ca. N40E dipping at moderate to steepangles to SE, and gently to moderately NE or SW-plunging fold-axessub-parallel to the stretching lineations.

The strike of the main mylonitic foliation of the SEB (Diagram 4,Fig. 2) is preferentially N50E, with high-angle, NW dip. The stretchinglineation is defined by quartz ribbons, elongated feldspar porphyr-oclasts and it is parallel to the alignment of biotite and muscoviteflakes. The lineation dips S50W, with intermediate to low plunge(max. 20) (Diagram 4, Fig. 2).

Fig. 1. a: General outline of the Dom Feliciano Belt (Brazil–Uruguay), simplified from Basei eBatholith (F), Pelotas Batholith (P), Aiguá Batholith (A); 3. schist belts and intrusive granitoidmetamorphic Complex (UY); 4. basement inliers; 5. foreland: Luis Alves Microplate (LA) anPiedras Altas Terrane — PAT); 6. intrusive granites; 7. metamorphic rocks; PET: 8. granitoidBlock; CDT: Cuchilla Dionisio Terrane; PET: Punta del Este Terrane. SBSB: Southern Braziliandeposits; 2. Paleozoic toMesozoic sediments of Paraná Basin; 3. Late Neoproterozoic–Early PaGranitoids (Major Gercino Shear Zone); Neoproterozoic granitoid belt (Florianópolis Batholi9. Neoproterozoic volcanosedimentary units of Brusque Complex; 10. Neoproterozoic gneiCatarina Granulitic Complex. PXZ: Perimbó Shear Zone; MGSZ: Major Gercino Shear Zone;craton; C: Congo craton; DFB: Dom Feliciano Belt.

The variations in plunge in stretching lineation within the MGSZmay reflect along-strike variations in finite strain or strain partition-ing, revealing, in a no continuous manner, the record of a progressivetransition from early stages of thrust to transpressive tectonics.

3.1. Mylonite belts

3.1.1. Petrography and microstructuresThe mylonites of NWB and SEB derived from granitoids which

undergone dominant processes of grain-size reduction by dynamicrecrystallization of quartz, fracturing of plagioclase and K-feldspar

t al. (2000). Dom Feliciano Belt (DFB): 1. foreland basins; 2. granite belt: Florianópoliss: BrusqueMetamorphic Complex (SC), Porongos Metamorphic Complex (RS), Lavallejad Rio de La Plata Craton (Taquarembó — T, Rivera — R, Nico Perez Terrane — NPT ands; 9. metasedimentary cover (Rocha Group), 10. high grade gneisses. SGB: São GabrielShear Belt. b: The southern tip of Dom Feliciano Belt, Santa Catarina, Brazil. 1. Cenozoicleozoic Itajaí volcanosedimentary basin; 4. Neoproterozoic intrusive granites; 5. Centralth): 6. Pedras Grandes Suite, 7. São Pedro de Alcântara Suite, 8. Águas Mornas Complex;sses and intrusive granites of Camboriú Complex; 11. Archean/Paleoproterozoic SantaA: Amazonas craton; RP: Rio de la Plata craton; SF: São Francisco craton; K: Kalahari

Fig. 3. Photographs of XZ sections: a) hand specimen of protomylonite from FGA from NWB (sample MG-31) with centimetric mylonitic and phyllonitic bands. Most of the σ-typeporphyroclasts are K-feldspars, and the plagioclase forms smaller porphyroclasts; b) hand specimen of S–Cmylonite (sampleMG-78) fromNWB. σ-type porphyroclasts of K-feldsparand plagioclase in amesocratic matrix. Mafic mineral are preferentially concentrated on C planes. S/C fabric indicates dextral shear sense; c) hand specimen of bandedmylonite fromNWB (sample MG-256) with milimetric quartz-feldspathic intercalated with mafic-rich layers.


during deformation. The mylonitization in greenschist facies condi-tions promoted the neoformation of sericite, chlorite, biotite, albite,clinozoisite/zoisite and epidote.

The foliation of the protomylonites is defined by preferredalignment of flattened and rotated K-feldspar porphyroclasts, elon-gated quartz grains and aligned biotite. Smaller clasts up to 1 cm longof plagioclase are present in a leucocratic fine- to medium-grainedgroundmass that contains also K-feldspar, quartz and mafic minerals.Several types of microstructures develop with increasing mylonitiza-tion. The protomylonites grade intomylonites inwhich porphyroclastsare fewer and highly stretched.Mylonitic or phyllonitic bands (Fig. 3a)and well-developed banded mylonites are often completely epido-tized. The mylonitic foliation is defined by stretched minerals, withrare 2–3 mm long porphyroclasts of K-feldspar and plagioclase, trailsof frequently blue quartz and orientation of fine-grained biotite. Themylonites grade into, or are intercalatedwith schistose ultramylonitesand phyllonites. S–C fabric developed in mylonites (Fig. 3b), thatgenerally consist of a mm-scale, fine-grained mesocratic matrix andmm-size K-feldspar and σ-type plagioclase porphyroclasts.

Banded mylonites are not rare in the studied area (Fig. 3c), andtheir presence probably reflects increasing strain causing a decreasingof the angle between S and C foliations. The banded structure iscomposed of stretched quartz-feldspathic layers up to1 mm thick, andmafic minerals (biotite, amphibole and opaque) bearing layers.

In the protomylonites, σ-type feldspar porphyroclasts (Fig. 4a),indicate mainly dextral movement on a N50E direction, although bothsymmetric and σ-type porphyroclasts indicative of sinistral shearsense may occur (Fig. 4b). In the S–C mylonites, the matrix consists ofquartz preferentially recrystallized along the S plane (Fig. 4c).

Fig. 4. XZ thin-section photographs under crossed polarizers: a) protomylonite of NWB (MGamphibole. Secondary minerals include chlorite, sericite and epidote; b) mylonite of NWB (suggesting sinistral shear sense in a matrix of fine-grained quartz, K-feldspar, plagioclaseconcentrated together with feldspars on the C plane, and quartz along the S plane. The minzircon and apatite recognized as accessory minerals, and chlorite, sericite and epidote occu

Morphologically oriented biotite and amphibole are concentratedtogether with feldspars on the C plane, and muscovite occurs alongboth planes.

In the investigated mylonites and protomylonites the deformationproduced a strong modification of the original quartz grains withdevelopment of intra-crystalline deformation substructures such asundulatory extinction, subgrains and small dynamically recrystallizedgrains. In the mylonites of the SEB complete segregation of quartz isvery common, with quartz ribbons in large monocrystalline domains(Fig. 5a, b). The low-angle lattice divergence would be significant forgrain-size enlargement of quartz grains (Fig. 5a, b) as postulated byHongn and Hippertt (2001). The similar c-axis orientation betweenthe large grains, the adjacent recrystallized grains and the size of theselarge grains, not rarely crossing the width of the thin section, suggestthe grain coalescence interpretation. The grain coalescence can beinterpreted as generated mainly by subgrain rotation recrystallization(SRR), as explained by Hongn and Hippertt (2001). Subsequently, thepredominance of rotation recrystallization and the increase of the rateof dislocation climb may have led to formation of newly recrystallizedgrains (Fig. 5a, arrows; Fig. 5b black arrows). Therefore, the largegrains are surrounded by a mantle of recrystallized grains, which wasformed by SRR (Stipp and Kunze, 2008) in dislocation creep regime 2as defined by Hirth and Tullis (1992).

In protomylonites of the SEB elongated quartz porphyroclasts withundulose extinction, serrated and irregular grain borders (Fig. 5c) arevery common features, and most probably originated by local grainboundary migration recrystallization. The new grains with the finelyserrated grain boundaries (Fig. 5c, arrows) are indicative of slowmigration and have been described as products of bulging

-124). σ-type porphyroclasts of K-feldspar in a matrix of quartz, K-feldspar, plagioclase,sample MG-198) with symmetric and σ-type porphyroclast of saussuritized plagioclase, biotite and sericite; c) S–C mylonite of NWB (sample MG-198). Mafic minerals areeral constituents are quartz, K-feldspar, oligoclase, amphibole, biotite; sphene, allanite,r as secondary minerals. S/C fabric indicates dextral shear sense.

Fig. 5. XZ thin-section photographs under crossed polarizers: a) protomylonite of SEB (sample MG-172) with mylonitic bands. Fractured feldspar porphyroclasts surrounded byquartz ribbons comprised of elongate recrystallized grain aggregates and large monocrystalline domains (arrows), normally with undulatory extinction; b) protomylonite of SEB(sample MG-219). Undulose extinction on elongate quartz and asymmetric muscovite porphyroclast (mica-fish) in a matrix mainly composed of quartz, K-feldspar, plagioclase andbiotite. Secondary minerals include chlorite, sericite and epidote; c) elongate quartz grains with undulose extinction and serrate grain boundaries in protomylonite of SEB (sampleMG-219); d) protomylonite of NWB (sample MG-186). Monocrystalline domain of quartz ribbon with subgrains and undulatory extinction. The irregular borders are probablyoriginated by slow grain boundary migration; e) mylonite of quartz vein of SEB (sample MG-88). Recrystallized grains forming foam texture with sutured shape of the grainboundaries and similar crystallographic orientation.


recrystallization, dominant between 280 and 400 °C by Stipp et al.(2002a). Some microstructures of several mylonites, as subgrainboundary formation, suggest that the dynamic recrystallization occursby progressive subgrain rotation (Fig. 5d), and the serrated or suturedboundaries (Fig. 5d, arrows) may indicate that the slow grainboundary migration operated as well.

In a highly strained quartz vein in the SEB the sutured shape of thegrain boundaries and the aggregates of recrystallized grains (Fig. 5e)with similar crystallographic orientation also indicate that both slowgrain boundarymigration and rotation recrystallizationwere effectiveduring deformation (cf. Fig. 4g, Stöckhert et al. 1999; cf. Figs. 3a, b,Mancktelow and Pennacchioni 2004).

The dominant recrystallization mechanisms determined from themicrostructures can be comparedwith the experimentally establisheddislocation creep regimes (Tullis et al. 2000; Stipp et al., 2002b). Asproposed by Hirth and Tullis (1992), regime 1 is dominated by bulgingrecrystallization involving slow grain boundary migration. With an

Fig. 6. XZ thin-section photographs under crossed polarizers: a) protomylonite of NWB (samgrained quartz, K-feldspar, plagioclase and biotite. Accessory minerals are sphene, allab) protomylonite of SEB (sample MG-357) with dark green amphibole partly altered to biobiotite. Accessory minerals are zircon apatite, sphene and allanite; secondary minerals are emica-fish (arrows) suggests dextral shear sense, in a matrix of fine-grained quartz, K-feldsp

increase in temperature or decrease in strain rate, the rate ofdislocation climb becomes sufficiently rapid to accommodate recov-ery and progressive rotation is the dominant process of dynamicrecrystallization (dislocation creep regime 2), that is, diffusion-controlled dislocation climb is fast enough to allow subgrainnucleation and rotation, and in regime 3, grain boundary migrationdominates. The gradual transition between these regimes has usuallybeen considered temperature- and/or strain rate- dependent (e.g.Stipp et al., 2002a).

The subgrain rotation mechanism is expected to occur at a higherstrain rate and lower temperature than grain boundary migration(Jessell and Lister, 1990). As discussed by Stipp et al. (2002a),subgrain rotation recrystallization can be dominant within the ca.400 °C–500 °C interval, and the transition to dominant grain bound-ary migration recrystallization occurs at ∼500 °C. For some authorsthe coexistence of these different recrystallization processes in quartzis in fact also common under low-grade metamorphic conditions

ple MG-198). Feldspar porphyroclasts with fairly symmetrical wings in a matrix of fine-nite, zircon and apatite; secondary minerals are epidote, sericite and clinozoisite;tite and chlorite. Rock is composed of quartz, K-feldspar, plagioclase, hornblende andpidote, sericite and clinozoisite; c) protomylonite of SEB (sample MG-172). Muscovitear, plagioclase and biotite.


(FitzGerald and Stünitz, 1993; Hongn and Hippertt, 2001), specificallyunder high greenschist facies conditions (Lloyd and Freeman, 1994;Stöckhert et al., 1999; Nishikawa et al., 2004). In addition, thetransition from rotation to migration recrystallization regime can beinterpreted as a local decrease of strain rate (Takeshita and Hara,1998).

These quartz microstructures suggest that different dynamicrecrystallization regimes and crystal plasticity were the dominantmechanisms of deformation during the mylonitization process. Theyindicate that both subgrain rotation and grain boundary migrationpervasively operated as mechanisms of recrystallization.

K-feldspar and plagioclase crystals were destroyed almost exclu-sively by microfracturing and reaction-assisted mechanisms (Fig. 6a).K-feldspar and plagioclase porphyroclasts have deformed mainlyby fracturing (Fig. 6a). Occasionally the fractures are filled withmatrix material (Fig. 5a), especially quartz, chlorite and epidote. Thegrain-size reduction and formation of sericite as a product ofsoftening reactions (Fig. 6a) are common. Mortar textures are rare,and when they occur are only weakly developed in microclines.Symmetrical porphyroclasts are very common (Figs. 4b and 6a),suggesting that a pure shear component operated, and σ-type K-feldspars and less frequently δ-type K-feldspar porphyroclastsindicate mainly a dextral movement on a NE/SW direction. Asym-metric and symmetric pressure shadows of K-feldspar porphyroclastsare usually composed of an aggregate of recrystallized quartz andsericite (Figs. 4b, 5a, and 6a) and, more rarely, recrystallizedmicrocline. Plagioclase develops smaller porphyroclasts than K-feldspar, and may be completely saussuritized. Products resultingfrom crystal plasticity are restricted to sporadically bent twin lamellaein plagioclase, and plagioclase breakdown is observed in the form ofmica-producing softening reactions that were more intense at themargins.

Amphibole is usually in part altered to biotite and chlorite (Fig. 6b),and was deformed only by fracturing. Biotite with light to red–brownpleochroism is either well-developed with homogeneous opticalproperties in the case of syn-mylonitic grains, or has very strong

Fig. 7. a) Hand specimen of monzogranite from outer parts of SEB showing cataclastic featurepolarizers of a thin section of a cataclastic syenogranite of FGA (sample MG-162): b) brokengrainedmatrix mainly composed of quartz, plagioclase, K-feldspar epidote and sericite; d) mwith epidote and recrystallized quartz indicative of cataclastic features.

undulatory extinction. Both magmatic and syn-mylonitic crystals areusually well-oriented and are altered to chlorite or green biotite.

Asymmetric porphyroclasts of muscovite forming mica-fishes arecommon (Fig. 6c), and cleavage traces are usually oriented parallel tothe S foliation in S–C mylonites. These structures indicate thatdeformation involved a non-coaxial component, and suggest a dextralsense-of-shear which is confirmed by other indicators. Deformedgrains with strong undulatory extinction are also found (Fig. 5b, whitearrow).

In the outer parts of the shear zones, the mylonitic foliation seemsto grade structurally upwards into a cataclastic foliation with planfractures commonly fulfilled by epidote (Fig. 7a). In the NWB, thegranites with cataclastic features are medium-grained with brokenfeldspar porphyroclasts and quartz (Fig. 7b) and usually a very fine-grained matrix is developed (Fig. 7c). In the SEB the cataclasticgranites are medium to coarse-grained in which feldspars and allanitecrystals are intensely fractured (Fig. 7d).

The occurrence of late brittle deformation is documented by thepresence of fractures or faults which cutting the mylonitic foliation,affecting feldspar and less commonly quartz porphyroclasts. This latedeformation is also observed in the megascopically isotropic graniteswhich are cut by sub-vertical epidotized bands, moderately-dippingfractures with fine-grained quartz-feldspathic in-fillings.

3.1.2. Quartz c-axis fabrics and strain ellipsoidsQuartz bcN axes were measured by universal stage in thin sections

of protomylonites and mylonites, cut perpendicular to the foliationand parallel to the stretching lineation, and represent normallytransitional patterns between the main types of quartz fabrics (Tulliset al., 1973; Tullis, 1977; Lister 1977; Schmid and Casey 1986;Passchier and Trouw 1996). None of the analyzed samples confirms aclear quartz c-axis lattice preferred orientation (LPO), Fig. 8, but theyare statistically significantly different from random. The studiedsamples discussed in this text are indicated on themap (Fig. 2). QuartzbcN axis fabrics indicate that total strain of the granitoids involved twomain components: coaxial strain ranging between pure shear and

s, with fractures filled with epidote (sample MG-347); b–d) Photographs under crossedfeldspar porphyroclasts and quartz; c) K-feldspar and quartz porphyroclasts in a fine-

icrofaults in allanite and plagioclase crystals and fractures in microcline and quartz filled

Fig. 8. Representative examples of quartz LPO fabrics from Major Gercino Shear Zone reported on a Schmidt diagram (lower hemisphere). Projection plane is XZ plane of strainellipsoid (mylonitic foliation) and the stretching lineation (parallel to X axis of strain ellipsoid) is horizontal in this plane. The number of measurements and the density contours areindicated.

Table 1Flinn Diagram parameters — mylonitic rocks of the Major Gercino Shear Zone.

Sample Rxz Ryz Rxy K D Elipsoid Flinn Diagram

MG-31 2.09 1.47 1.42 0.89 0.63 Oblate Plane strainMG-32 2.92 2.35 1.24 0.18 1.37 Oblate General apparent flatteningMG-38 2.56 2.30 1.11 0.08 1.30 Oblate Uniaxial apparent flatteningMG-114 2.33 2.25 1.04 0.03 1.25 Oblate Uniaxial apparent flatteningMG-171 2.38 1.57 1.52 0.91 0.77 Oblate Plane strainMG-172 2.37 1.90 1.25 0.28 0.93 Oblate General apparent flatteningMG-186 3.08 2.11 1.46 0.41 1.20 Oblate General apparent flatteningMG-187 2.44 2.00 1.22 0.22 1.02 Oblate General apparent flatteningMG-198 2.44 1.83 1.33 0.40 0.89 Oblate General apparent flatteningMG-218 2.71 1.89 1.43 0.48 0.98 Oblate General apparent flatteningMG-340 3.31 2.11 1.57 0.51 1.24 Oblate General apparent flattening


general flattening and simple shear. Most of the diagrams displayalmost symmetric dispersal typical of pure shear, whereas only someof them show patterns representative of strain close to simple shear.

Quartz bcN axis diagrams display a spectrum of transitionalpatterns from (I) type of crossed girdles (Lister, 1977; Schmid andCasey, 1986) to a tendency to scatter along two small circles aroundthe poles to foliation (Fig. 8a, b). In some diagrams (e.g. Fig. 8c, d) thescatter along two small circles around the poles to foliation is moreevidenced. The maxima asymmetrically distributed in relation to theZ-axis indicate a non-coaxial component of deformation. The externalasymmetry of the LPO used as a kinematic indicator suggests bothdextral (Fig. 8a) and sinistral (Fig. 8b) shear sense, confirmed by themeso- and microscopic kinematic indicators.

In Fig. 8, the maxima localized at the outer rim, near Z-axis of thefinite strain ellipsoid indicate that basal baN glide system wasactivated. The c-axis maxima at intermediate positions, between theY- and Z-axes, indicate that rhombohedral baN slips becameincreasingly more important (Fig. 8a, b). As the activation of slipsystems of quartz is temperature dependent, at lower temperaturesbasal baN slips induce the c-axis point maxima to concentrate near theZ-axis, and with increasing temperature, rhombohedral baN slipsbecome gradually more important, forcing the c-axis maxima to

migrate to intermediate positions between the Y- and Z-axes (Tulliset al. 1973; Bouchez and Pêcher, 1981; Schmid and Casey, 1986). In afew situations the prism baN slip system begins to be operative(Fig. 8a, b), resulting in point maxima near the Y-axis. These systemsare activated for deformation conditions which are compatible withthe metamorphism in the middle greenschist facies between 400 and

Fig. 9. The Flinn Diagram. The shape of the strain ellipsoid is expressed by the shapefactor (k). In the investigated mylonitic rocks of MGSZ the shape of the strain ellipsoidrange from pancake-shaped (oblate; kb1) to a plane strain with k=1.


500 °C (Bouchez and Pêcher, 1981; Schmid and Casey, 1986;Mainprice et al., 1986).

The LPO of (I) type crossed girdles suggests a plane deformation(Lister, 1981; Price, 1985) and a LPO characterized by a small circlearound Z suggests a flattened strain ellipsoid (Tullis et al., 1973; Listerand Hobbs, 1980; Law, 1986; Schmid and Casey, 1986; Passchier andTrouw, 1996). These data are corroborated by the distribution of thedeformation ellipsoids in the Flinn Diagram and the shape of theellipsoids determinate from measured sectional ellipses by the FryMethod (Fry, 1979).

The Fry technique was applied to eleven samples in order toquantify finite strain, and the central points of more than 100 feldspargrains per section were used to calculate strain (Passarelli et al.,1993). The initial distribution of the porphyroclasts is assumed to behomogeneous because of the observation of the undeformed granitetextures. The results obtained by this method were corroborated byfield observations. Two-dimensional strain measurements were madeon XZ and YZ sections (X≥Y≥Z, finite strain axes) in order to estimatethe three-dimensional strain geometry, and define the Flinn Diagramparameters (Rxy, Ryz, K, D), Table 1.

The shape of the strain ellipsoids range from pancake-shaped(oblate; kb1), corresponding to the flattening strain field, to the planestrain shape (k=1). The Flinn Diagram of Fig. 9 shows the field thatincludes all of the analyzed samples.

The transpressive nature of the MGSZ is revealed by the flatteningof the deformation ellipsoids, leading to significant coaxial deforma-

Fig. 10. a) Hand specimen of monzogranite of Rolador Granitoid Association (RGA). Scale barto 4 cm long and tabular crystals of plagioclase in a dark fine matrix mainly composed ofAssociation (FGA) with local rapakivi texture. Scale bar=1 cm. Rock with porphyroid texturmainly composed of hipidiomorphic to idiomorphic plagioclase, quartz and amphibole.

tion. The orientation of the shear zone, relative to the principalcompressional stress direction of N65W, assured a significantcomponent of pure shear deformation, and led to local sinistralmovements (Passarelli et al., 1993, 1997).

4. The central granitoids of the MGSZ

The granitoid bodies that occur between the NWB and the SEBare elongated (Fig. 2) and their borders are sheared. In the coresthey are either moderately oriented or megascopically isotropic. Twomainmagmatic associations were characterized: the Rolador GranitoidAssociation (RGA), where biotite monzogranite predominates(Fig. 10a), and the Fernandes Granitoid Association (FGA), predomi-nantly composed by amphibole syenogranite (Fig. 10b) with localrapakivi-type texture. Apart from the petrographic differences men-tioned, the two associations also differ geochemically and isotopically(Passarelli et al., 1997). Sub-volcanic and volcanic acid rocks, apliteveins, two-mica granites, and riebeckite–quartz–monzonite subordi-nately occur in both granitoid associations.

Usually there is a transition between the non deformed granitesand the mylonite belts, and the shear deformation increases untilmylonites predominate on both sheared borders. Furthermore,towards the main granitoid bodies highly deformed internal zonesare locally present.

4.1. Structures and microstructures of the granites

A study of the microstructures of the granitic rocks was made inorder to determine if the deformation took place in the magmaticstate or in the solid state. In accordance with several studiesconcerning the microstructural analysis in granitic rocks, thefollowing types of microstructures are defined: 1) magmatic and/orsubmagmatic; 2) moderate to high-temperature solid state and 3)low-temperature microstructures. The foliated syenogranites of theFGA present undulating, anastomosed planeswith aligned, sometimesstretched feldspar and stretched quartz megacrysts. Monzogranites ofthe RGA have incipient foliation with alignment of felsic megacrysts(Fig. 11a) and of the mafic minerals. Stretching of quartz and feldsparis rarely seen.

The magmatic origin of most of the observed foliations in the RGAand the FGA is suggested by the alignment of euhedral to subhedralcrystals which are set in a usually undeformed quartz-feldspathicmatrix (Fig. 11b), as described by Paterson et al., 1989. The presence ofquartz-filled fractures in feldspars, of megacrystals rotated into thefoliation, of deformed twins in feldspars, of myrmekites and

=1 cm. Rock with porphyritic texture, with hipidiomorphic K-feldspar megacrystals upquartz and mafic minerals; b) Hand specimen of syenogranite of Fernandes Granitoide with hipidiomorphic to xenomorphic K-feldspar up to 3,4 cm long in a mediummatrix

Fig. 11. a) Photograph of monzogranite of RGA with magmatic foliation defined essentially by the alignment of tabular idiomorphic to hipidiomorphic megacrystals of feldspar. b–f)thin-section photographs under crossed polarizers: b) magmatic foliation defined by the alignment of idiomorphic to hipidiomorphic megacrystals of plagioclase and K-feldspar(monzogranite of RGA); c) submagmatic microfractures on microcline phenocryst sealed mainly with quartz (monzogranite of FGA); d) high-temperature solid state withchessboard-like subgrain boundaries in quartz grains (monzogranite of RGA); e) high-temperature solid state with bent plagioclases (syenogranite of FGA); f) kinked biotite(syenogranite of FGA). Scale bar=1 mm.


plagioclase-quartz aggregates at contacts between K-feldspar grainsall suggest that submagmatic flow also occurred (Paterson et al.,1989). Intragranular fractures in microcline phenocrysts are sealedwith quartz and partly with microcline, and the quartz usually crossesthe phenocryst (Fig. 11c). These features were described by Büttner(1999) in the submagmatic stage of the Variscan granites, and pointout the presence of residual melt during plastic deformation offeldspar. Similar fabrics have been described by Bouchez et al. (1992)as “submagmatic microfractures” indicating the presence of meltduring fracturing.

The strike of magmatic/submagmatic foliations ranges from NW toENE, and tend to grade towards NE along the granites borders. Dipsare variable and subhorizontal attitudes predominate (Diagram 5,Fig. 2). The flow lineations, whose measurements are scarce, aredefined by the orientation of feldspar prisms and are sub-parallel tothe mylonite zones.

The high–moderate temperature subsolidus deformation isobserved in both granite associations, mainly where they comprisean incipient foliation, in the inner parts of the intrusions. Themicrostructures are characterized usually by chessboard-like sub-grains in quartz, explained by a combination by basal baN andprism bcN slip (Mainprice et al., 1986; Kruhl, 1996), (Fig. 11d), bent

plagioclases (Fig. 11e) and kinked biotites are less frequently present(Fig. 11f).

At the microscope scale, the low-temperature fabric observed ingranites outside the shear zones include: (1) deformed quartz withundulatory extinctionwhich grades into slightlymisoriented subgrainboundaries; (2) recrystallized quartz to fine-grained aggregates; (3)microfaults in feldspar. The primary mineral assemblage of the RGAand RGA granites was partially re-equilibrated under greenschistfacies conditions. Amphibole changed to biotite and chlorite–albite–clinozoisite/zoisite–epidote–calcite, and biotite is also changed tochlorite, and is sometimes crenulated or intergrown with muscovite.The plagioclase is often saussuritized and the twins are usuallyslightly deformed. K-feldspar is occasionally recrystallized, and quartzis mostly present as subgrains showing a tendency toward homoge-neous extinction, and polygonal microtextures. The larger quartzgrains have undulatory extinction, and commonly havemicrogranularto interpenetrating contacts with other minerals.

Mylonitic bands up to 3 m wide may also occur in the granitedomain, with foliation striking in the same direction as in themylonite belts (NWB and SEB), ca. N65E, with mainly high-angle NWdips. The mineral stretching lineation, whose measurements arescarce, dips at intermediate angles to N35E and S50W and at high


angles to NEE, SEE and S20E. The FGA has mylonitic and proto-mylonitic features, characterized by a mylonitic foliation definedby stretched feldspar porphyroclasts and quartz. Protomyloniteswith potassic feldspar porphyroclasts reaching 2 cm in size andstretched quartz occur in the RGA. Quartz and mafic minerals areabundant in the fine/medium-grained matrix. In these myloniticbands the primary quartz grains usually show strong undulatoryextinction, and have conjugate fractures now filled by microgranularquartz. Albite is deformed andmicro-faulted, sphene is deformed alongslip planes, allanite is cut by micro-faults, and porphyroclasts havesigmoidal forms.

Granites with cataclastic fabrics developed normally next to themylonitic belts. The cataclastic FGA granites is characterized byrepetitive planes defined by minerals intensely fractured, especiallyfeldspars, or more rarely by shape preferred orientation of biotite oraggregates of quartz. No rarely the fractures are filled with quartz,feldspar and epidote. The foliation surface is usuallymarked by an oftenundulating cleavage with spacing between 0.5 cm and 1 cm andoccasionally is well-developed parallel to the narrow epidotizedmylonitic belts (2 m thick) in the granites domain. RGA monzogranitesrarely have cataclastic bands. Brittle deformation is recognized bystrongly fractured and brecciated zones usually filled by quartz, epidote,chlorite and iron hydroxides. Such zones are found not only close to themain mylonite belts, but also towards the interior of the intrusion.Feldspar and quartz grains are typically crushed, and some microclinecrystals are cut bymicrofaults. In these rocks, plagioclase and biotite areboth folded and broken.

The cataclastic granites close to the NWB, in the NE sector, has afoliation striking concentrated between N42E and N30W, with dips of82SE and 53NE, respectively, and in the central and SW sectors, thefoliation strikes NE and dips steeply SE, or strikes NS and dips steeplyto the E and W. The cataclastic foliation in the granites close to theSEB, in the central sector, preferentially strikes NE and dips SE andNW, and in the SW sector, a foliation striking NEE and NW dips NWand NE respectively, characterizing the field-observed folds, withN40E/80NW axial plane. The N–S, NW and NNE striking cataclasticfoliations possibly reflect later brittle deformation associated withfaults which cross-cut the shear belts (Fig. 2).

Despite the apparent gradual passage from ductile–brittle struc-tures to brittle structures, we were not able to unequivocally confirmthat the cataclastic surfaces were formed during the late stages oftranscurrent movement along the shear zone, under ductile to fragileconditions. The possibility of a subsequent brittle deformation of theMGSZ cannot be discarded; moreover the K-Ar fine fractions resultsindicate latter brittle reactivation of the shear zone.

Table 2Major Gercino Shear Zone Granitoid Associations analytical data.

SPU Magneticfraction

207Pb/235Ua Error(%)

206Pb/238Ua Error(%)

207Pb/206Pb Error(%)

MG 220 (biotite monzogranite) — Rolador Granitoid Associationsp. 208♦ M(3) AA 0.476940 1.130 0.057220 1.120 0.060453 0.167sp. 208♦ M(4) AA 0.458622 1.070 0.055268 1.050 0.060184 0.146922 M(1) 0.792591 1.090 0.094719 0.869 0.060689 0.6491623 M(2) 0.738821 0.545 0.089500 0.525 0.059871 0.146

MG 362 (amphibole syenogranite) — Fernandes Granitoid Association1574 M (−1) 0.684801 0.705 0.083802 0.659 0.059267 0.246sp. 208♦ M(−1)AA 0.598703 0.944 0.071972 0.923 0.060332 0.189sp. 208♦ M(2)AA 0.696066 0.954 0.083812 0.928 0.060234 0.209

U–Pb method.Sample location coordinates as in Fig. 2.SPU: laboratory number (spike Pb205); ♦ data from Passarelli et al. (1997).Magnetic fractions: numbers in parentheses indicated the tilt used on Frantz separator at 1Total U and Pb concentrations corrected for analytical blank.Ages: given in Ma using Isoplot/Ex program Ludwig (2002). Decay constants recommended

a Radiogenic Pb corrected for blank and initial Pb; U corrected for blank.b Not corrected for blank or non-radiogenic Pb.

5. Geochronology

Zircon U/Pb analytical data and K–Ar mineral analyses werecarried out at the Geochronological Research Center (CPGeo) of theInstitute of Geoscience, University of São Paulo. The analytical detailsof the U–Pb analyses are given in Passarelli et al. (2009) and theconventional K–Ar analytical techniques were similar to thosedescribed by McDougall (1985). The K–Ar fine fractions analyseswere undertaken at the laboratories of the Institut für Geologie undDynamik der Lithosphäre, Universität Göttingen, Germany. Thelocalization of the dated samples is indicated on the map of Fig. 2.

5.1. U–Pb multigrain zircon ages

U–Pb dating was carried out on zircons from non deformed rocksof both granitic associations. The results of the U–Pb analyses aregiven in Table 2, which also includes previous data obtained byPassarelli et al. (1997), and new results using a 205Pb spike. TheConcordia diagrams are presented in Fig. 12a and b, and the isochronswere calculated using the program ISOPLOT V. 2.49 (Ludwig, 2002).

Zircon concentrates prepared from sample MG-220 of the RGAbiotite monzogranite (Fig. 2) contained idiomorphic, colorless,inclusion-free and un-fractured crystals with length/width ratiosaround 4. The concentrates were separated into un-abraded fractionsM(1) and M(2), and abraded (AA) M(3) and M(4) magnetic fractions.The former weighed ca. 0.05 mg, and to which a spike 205Pb wasadded, while the latter weighed ca. 0.5 mg, to which 208Pb spike wasadded (Table 2). The analyzed fractions yielded discordant, ratherwell-aligned isotopic compositions which define an upper interceptage of 609±16 Ma (Fig. 12a), interpreted as the best estimate for thetime of formation of the granitic rocks of this association. The moremagnetic fractions M(3) and M(4), despite abrasion, are much morediscordant than the un-abraded ones.

An amphibole biotite syenogranite, sample MG-362 (Fig. 2), from theFGA yielded zircon concentrates with clear, transparent, well-formedfracture-free crystals with length/width ratios around 2. They containedrare inclusions. The concentrates were separated into abraded M(2) andM(−1), and un-abradedM(−1)magnetic fractions, the formerweighingca. 0.8 mg (208Pb spike) and the latter weighing ca. 0.06 mg (205Pb spike)(Table 2). The analyzed fractions also gave well-aligned discordantisotopic compositions defining an upper intercept age of 614±2Ma(Fig. 12b), interpreted as the time of formation of the FGA rocks.

The analyzed zircon fractions of both granites are discordant andshowamarkedly skeweddistribution indicative of significant Pb loss. Noevidence for the presence of much older inherited nuclei is recognized.

206Pb /204Pbb Pb(ppm)

U(ppm)

Weight(mg)

206Pb /238UAge (Ma)

207Pb /235UAge (Ma)

207Pb /206PbAge (Ma)

330.0 93.0 1344.7 0.632 358 396 619479.7 79.3 1251.0 0.438 346 383 610944.5 22.8 230.7 0.061 583 593 628

3101.2 23.4 251.4 0.039 552 561 598

632.7 19.5 211.8 0.061 518 529 576152.5 53.2 496.0 0.817 448 476 615257.4 37.5 346.7 0.791 518 536 612

.5 A. Current; AA: abraded fractions.

by Steiger and Jager (1977).

Fig. 12. 206Pb/238U vs. 207Pb/235U Concordia diagram (zircon) of MGSZ granitoids. a) Rolador Granitoid Association; b) Fernandes Granitoid Association.


5.2. K/Ar minerals and fine fractions dating

A total of 7 muscovite and biotite concentrates (60–150 um), andfive b2 um fractions from non deformed granites, phyllites andmylonites were separated by conventional separation techniquesfrom roughly 2 kg samples and dated by K–Ar technique. K/Ar ages onbiotites were determined for undeformed biotite syenogranites ofFGA and biotite monzogranite of RGA. In order to examine theinfluence of deformation on the K–Ar system, mica separates of themylonitic rocks of the MGSZ were also dated.

K–Ar biotite ages from the undeformed granites range from 561±18 to 584±25 Ma (Table 3), and may represent the time of coolingthrough the 300–350 °C interval (McDougall and Harrison, 1999)after the regional thermal peak. Muscovite and biotite from themylonitic rocks of the NWB yielded 569±14 Ma and 569±10 Ma(Table 4) which are indistinguishable from the granites cooling age.One muscovite sample from a mylonite in the SEB is most likelyyounger (539±13 Ma, Table 4), suggesting that the last movementsalong the two mylonitic belts occurred at different times. Althoughthis conclusion is based on a single determination for the SEB, webelieve that the total data point in this direction. The K–Ar agesobtained reflect cooling of MGSZ structures through closure tem-peratures below 300 °C (biotite) and in the range 350–410 °C(muscovite) (McDougall and Harrison, 1999; Willigers et al., 2001).

In order to date the very low-grade metamorphism, in the chloritezone of the greenschist facies or even at lower temperatures, andassociated main phase deformation or reactivation of the MGSZ, K/Aranalyses on illite/sericite fine fractions (b2 µm and b0.2 µm) fromrocks within and adjacent to the shear zone (NWB) were carried out.The data revised from Basei et al. (1995) are presented in Table 5. Theoldest age of 532±13 Ma (sample BR-7-93) obtained in phyllite ofthe Brusque Group outside the shear zone is younger than theregional cooling age and is equivalent to the youngest age obtained formylonites of the SEB (sample MG-219, 539±13 Ma). Intermediateages of 359 Ma and 377 Ma were found both in phyllite (sample BR-14-93) from outside the shear zone as in phyllonite (sample BR-9-93)in the mylonite belt. One sample from the central part of the shear

Table 3Analytical data for the RGA and FGA — Major Gercino Shear Zone.

Association Sample Rock Material Ar40/Ar38 Ar38/A

RGA MG-296 Monzogranite Biotite 3.7075 2663.1FGA MG-331 Syenogranite Biotite 3.0954 1587.7FGA MG-281 Monzogranite Biotite 3.0030 1392.3FGA MG-318 Syenogranite Biotite 3.5445 5494.7

K–Ar method. Sample location coordinates as in Fig. 2).

zone NWB (sample BR-10-93, Fig. 2) yields K/Ar Triassic ages of 230±Ma for b2 µm and 206±5 Ma for b0.2 µm.

The closure temperature of sericite in the K–Ar system of ca. 270 °C(Wemmer, 1991; Wemmer and Ahrendt, 1997) is comparable to thatof zircon fission track dating (Hurford and Green, 1983). Devonianages are also found for fission tracks in zircons and K–Ar fine fractionsonmylonites of the Cubatão Itariri Shear System of the São Paulo State(Passarelli et al., 2008) and were interpreted as the result of astabilization phase with low exhumation of tectonic blocks of theMantiqueira Province, correlated to the evolution of the Paraná Basinevolution (Hackspacher and Tello, 2006). In addition, the K–Ar finefractions Triassic ages of the NWB mylonite are probably associatedwith the Mid Triassic ages that reflect the initial removal of heat frombeneath Pangea (Veevers, 2006) and the late Triassic ages, that recordthe beginning of fragmentation of Gondwana.

6. Discussion

6.1. The tectonic control on the emplacement

The MGSZ is steeply dipping, with strikes ranging from N45E toN65E. It has a mainly dextral sense of displacement, developed undergreenschist facies conditions. The major deformation processesobserved in the main phase of the MGSZ mylonitization were themicrofraturing of feldspars, the crystal–plastic deformation indicatedby deformation bands and subgrains microstructures, the softeningreactions and recrystallization. The recrystallization mechanismsinvolved both subgrains rotation, which principally involve theformation of new grain boundaries, and migration mechanisms,which principally involve grain boundary migration.

In the Canelinha region of Santa Catarina syntectonic, meta- to per-aluminous calc-alkaline granites form elongated circumscribed intru-sions within the two mylonitic belts of the shear zone. Two graniticassociations related to the development of shear zone werediscriminated: the Rolador (RGA) and Fernandes (FGA) GranitoidAssociations, which encompasses mainly monzogranites and syeno-granites respectively. Intense solid-state foliation under greenschist

r36 % K error (%) 40Ar % atm. contam. Age (Ma) (1σ)

6.15 0.8 161.7 2.73 575±125.41 3.4 144.9 5.76 584±255.02 1.9 128.4 6.83 561±186.27 0.6 163.9 1.22 572±10

Table 4Analytical data for the Major Gercino Shear Zone mylonitic rocks.

Mylonitic Belt Sample Rock Material Ar40/Ar38 Ar38/Ar36 % K error (%) 40Ar % atm. contam. Age (Ma) (1σ)

NWB MG-257 Mylonite Muscovite 4.2294 4477.2 8.34 1.4 216.8 1.31 569±14NWB MG-198 Protomylonite Biotite 3.1011 5095.7 7.17 0.5 186.3 1.53 569±10SEB MG-219 Protomylonite Muscovite 3.5745 3732.6 8.39 1.2 204.3 1.94 539±13

K–Ar method. For sample location coordinates, see Fig. 2.


facies conditions is developed in their border zones, overprintingearly magmatic and submagmatic fabrics, which are preserved mainlyon the inner parts of the intrusion. The magmatic foliation grades tosubmagmatic flow and to mylonitic textures near the mylonitic belts.Not rarely, some magmatic/submagmatic foliations are undulating,subhorizontal and oblique to the foliations of the mylonitic belts and tothe border of the shear zone. The presence of flat-lying structures(Diagram 5, Fig. 2) of magmatic/submagmatic origin and high-temperature solid-state microstructures may be interpreted as a resultof the early control of the intrusionby the initial stagesof anoblique shearzone that evolved to a transpressive one, as observed in several shearzones of the Southern Brazilian Shear Belt onRioGrandedo Sul and SantaCatarinastates (Fernandeset al., 1992;Bitencourt andNardi, 1993, 2000).Moreover, flat-lying low-temperature deformation microstructuresare locally preserved on the SW sector of the NWB (Diagram 3, Fig. 2).

In the present study, the observation of a range betweenmagmaticand high-temperature solid-state (subsolidus) microstructures, andfinally to low-temperature textures, along with continuity in theassociated fabrics, points to a continuum of magma straining duringand just after its crystallization (Paterson et al. 1989; Miller andPaterson 1994; Gleizes et al., 1998; Schofield and D'Lemos 1998; Ferréand Améglio, 2000; Pawley and Collins 2002; Mattioli et al., 2003;Duguet and Faure 2004), or a syn-kinematic magmatism. Therefore,the oblique magmatic and solid-state foliations develop successivelyduring the same deformation event throughout the crystallization ofthe granites. In addition, the tectonic control over the internalmagmatic fabric through the rotation of the magmatic structuresinto the direction of the mylonitic belts, the circumscription of theintrusions within the two mylonitic belts of the shear zone and theelongated shape of the bodies also calls for an emplacement of theboth granitic associations during the dextral transpressive shearingevent. This dextral sense of shear is also confirmed by microscopicobservations of subhorizontal XZ sections.

6.2. Age of deformation

It is thus suggested that the intrusion of the Fernandes and Roladormagmas was controlled by a dextral transpressive shear zone,considered as a major lithospheric-scale discontinuity, which hasinfluenced its ascent and emplacement. Therefore, the ages between609 and 615 Ma (crystallization ages) indicate the timing of thetranspressive process of the MGSZ in the Canelinha region, SantaCatarina state. Differently in the SBSZ, Uruguay, the granites emplacedof an interval of 590 to 540 Ma, during the transpressional phase ofthe shear zone (Oyhantçabal, 2005).

Table 5K/Ar analyses of fine fractions (b2 µm and b0.2 µm).

Sample Unit Rock

BR-7-93 Brusque G. near NWB phylliteBR-9-93 NWB biotite–chlorite–phylloniteBR-10-93 NWB myloniteBR-10-93 NWB myloniteBR-14-93 Brusque Group phyllite

Laboratories of the Institut für Geologie und Dynamok der Lithosphäre, Universiteit Gotting

Along major transcurrent shear zones of the Southern BrazilianShear Belt, the zircon ages for the early-transcurrent magmatism,mainly controlled by flat-lying shear zones, as observed in SantaCatarina (Porto-Belo region), range from 650 to 630 Ma (Bitencourtand Nardi, 1993, 2000) and in Rio Grande do Sul from 610 to 630 Ma(Philipp and Machado, 2005). The syn-transcurrent magmatism tookplace from 630 to 617 Ma, followed by late-magmatism and youngeralkaline magmatism around 600 Ma (Bitencourt and Nardi, 1993,2000; Chemale et al., 2003).

Older ages of 658 to 617 Ma (Gastal et al., 2005) or 658 to 634 Ma(Frantz et al., 2003) of intrusions are attributed to the syn-transcurrent and/or transpressive period suites related to the DCSZ.The emplacement of the late- and post tectonic granite suitesoccurred later between 615 and 580 Ma, with a peak at approximately600 Ma (Babinski et al., 1997). The transition from transpressive totranstractive period in the DCSZ evolution is defined by theemplacement of granites between 625 and 617 Ma (Frantz et al.,2003). The emplacement of syntectonic granites marks, in Uruguay,the development of the Sierra Ballena Shear Zone (SBSZ) and the firsttranspressional deformation phase at 658–600 Ma, and, the secondtranspressional event occurred at about 586–560 Ma is associatedwith the emplacement of porphyry dikes and granites (Oyhantçabalet al. 2009).

K–Ar ages of biotite and muscovite from the MGSZ mylonites ofaround570 Mawere recorded for theNWB, similar to thoseobtained forcentral granitoids, and one age of muscovite of around 540 Ma for thesouthern mylonitic belt. These cooling ages are significantly youngerthan those U–Pb ages related to the syn-transpressive granitesformation, and reflect intense thermo-tectonic activity in the areaafter a major phase of movement of theMGSZ. Ages around 570 Ma canbe related to the late stages of the Florianópolis Batholith magmatism(Basei et al. 2000; da Silva et al., 2005b). Similar Rb–Sr whole-rock agesare also found for parts of the Pelotas Batholith (Philipp and Machado,2005) and, despite their poor precision, analogous K–Ar ages (biotite)were obtained in syn-trancurrent granitoids of the DCSZ on Rio Grandedo Sul state (Koester et al., 1997). Additionally, late-tectonic granitebodies within the Aigua Batholith (Uruguay) yielded U–Pb zircon agesup to 570 Ma (Preciozzi et al., 2001).

Ar–Ar dating of micas from the Canguçu and Pinheiro Machadoregions of Rio Grande do Sul yielded ages of 534 and 536 Ma, believedto represent the age of reactivation of faults in the eastern PelotasBatholith (Philipp and Machado, 2005). K–Ar ages on biotite rangingfrom 540 to 530 Ma have also been reported on mylonites of ductileshear zones in Rio Grande do Sul by these authors who interpretedthem as the result of a late thermo-tectonic event probably related tothe installation of the Camaquã Basin.

Material % K2O % Ar40nl/g STP Age (Ma) (2σ)

f.fb2 µm 3.74 74.58 532±13f.fb2 µm 1.90 26.70 377±9f.fb2 µm 5.31 42.03 230±6f.fb0.2 µm 4.46 31.29 206±5f.fb2 µm 0.46 5.89 359±9

en, Germany (Basei et al., 1995). For sample location coordinates, see Fig. 2.


The K–Ar ages of fine fractions extracted from phyllites of theBrusque Group and from phyllonite and mylonite from the mylonitesof the NWB provided three main intervals. The geological significanceof the Cambrian ages (∼532 Ma) obtained in phyllitic rock of theBrusque Group, similar to muscovite K–Ar age of mylonite of SEB hasto be verified. The Devonian ages (359 and 377 Ma) of the phylliticrocks may be associated to an exhumation event related to blocktectonics of the Mantiqueira Province involved in the Paraná Basinevolution (Hackspacher and Tello, 2006). The Triassic ages (206 and230 Ma) from mylonites of the NWB is associated to a thermal pulseconnected to an early phase of the opening of the South Atlantic Oceanwith a possible reactivation of the MGSZ, responsible for the resettingof the K/Ar fine fractions isotope system.

6.3. Regional correlations

The tectonic evolution of the DFB is related with a Neoproterozoicoblique collision (Nardi and Frantz, 1995; Basei et al., 2000; Frantz andBotelho, 2000; Philipp and Machado, 2005) that evolved to atranspression regime on its late stages. N–S sinistral (Rio Grande doSul state— RS) and NE dextral (Santa Catarina state— SC) transcurrentshearing developed under low-grade metamorphic conditions (Basei,1990; Fernandes et al., 1992; Bitencourt and Nardi, 1993) and areexplained as local movements related to the major collision.

Correlations between the major southern Brazilian and Uruguaianshear zones have already been discussed by a number of authors (Basei,1990; Passarelli et al., 1993; Fernandes and Koester 1999; Bitencourtand Nardi, 2000; Bossi and Gaucher, 2004; Oyhantçabal et al., 2009)based on the robust suggestion of geometrical and geophysicalcontinuities (Mantovani et al., 1989). Together with the Cordilheira(RS) and Sierra Ballena (UY) shear zones, theMGSZ is considered part ofa lithospheric-scale discontinuity that separates geochemical andisotopically distinct terranes (Basei et al., 2008). The crystallinebasement in Uruguay can be separated into three large geotectonicunits: the Piedra Alta Terrane (PAT) and Nico Perez Terrane (NPT)representing the Rio de la Plata Craton and the Cuchilla Dionisio Terrane(CDT) encompassing Neoproterozoic granitoids and the Punta del EsteTerrane (Bossi and Campal, 1992; Sanchez-Bettucci et al., 2003; BossiandGaucher, 2004;Gaucher et al., 2004;Mallmannet al., 2007). ThePATis separated from the NPT by the N- to NNW-trending dextral Sarandidel Yí Shear Zone, and the latter separated from the CDT, by the N–NEtrending sinistral Sierra Ballena Shear Zone.

The eastern limit of the Granite Belt can be observed only inUruguay. The Alferez-Cordillera Shear Zone (ACSZ) forms the tectonicboundary between the Aiguá granites to the west and Punta del EsteTerrane (PET) to the east (Preciozzi et al., 1999; Basei et al., 2000). Allthese shear zones were grouped in the Southern Brazilian Shear Beltby Bitencourt and Nardi (2000) including anastomosed, N–S andN60E dominantly transcurrent kinematics, with both dextral andsinistral movements.

7. Conclusions

The Major Gercino Shear Zone, striking NE, limits two differentdomains in the Dom Feliciano Belt: a northwestern supracrustal from asoutheastern granitoid domain. In detail, this shear zone is composed oftwomylonite belts developedmainly during dextral lateral and obliquemovement under compression, controlling syntectonic granitic bodies,during which the Granite Belt was uplifted. The dextral transpressionalnature, as demonstrated by several previous structural studies,waswellcharacterized by a shortening component nearly orthogonal to thedeformation zone, and the influence of thepure shear, revealed by somesymmetry of the preferred crystallographic orientation and theflattened strain ellipsoid.

The MGSZ is associated with the N–NE/S–SW-oriented shear zonesystem which affects the Dom Feliciano Belt as an important suture

which separates different and unrelated terrains of magmatic arcgranites to the East and a folded supracrustal belt to the West. Thesetwo belts had independent origins and evolutions, and achieved theirpresent configuration when they were transported northwestwardsand accreted to the border of a craton underlying the Paraná basin inthe upper Neoproterozoic and lower Phanerozoic.

The investigated mylonites were derived from granitic protolithsof two distinct associations, which were deformed under greenschistfacies conditions, as suggested by the absence of crystal–plasticdeformation in feldspars, quartz recrystallization and formation ofsericite as a product of softening reactions. The detailed investigationof the structural patterns and the U–Pb zircon dating of granitesconstrained the timing of granite emplacement relative to shear zonemovement. Several deformation mechanisms that operate simulta-neously during the main phase of mylonitization were recognizedthrough the petrographic and microstructural analysis of myloniticrocks.

The deformation in the protomylonite and mylonite was accom-modated mainly by microfracturing in K-feldspar and incipientrecrystallization, softening retrograde reactions (such as saussuritiza-tion) that affected plagioclase, kinking and orientation of biotiteaggregates, and deformation of quartz grains via crystal–plasticprocesses, with the coexistence of different recrystallization processesas slow grain boundary migration and subgrain rotation.

The quartz c-axis fabrics indicate that basal baN glide system wasactivated, several times the rhombohedral baN slips became increas-inglymore important and in a few situations the prism baN slip systembegins to be operative. These systems are activated for deformationconditions which are compatible with the metamorphism in themiddle greenschist facies (400–500 °C) and the LPO suggests both aplane deformation and a flattened strain ellipsoid corroborated by thedistribution of the deformation ellipsoids in the Flinn Diagram.

The area provides an example of strain partitioning that occurredduring the emplacement of the granites associations coeval with themain compressional event, around 610–614 Ma, most likely related tothe agglutination of the granite belt to the supracrustal belt NW of theMGSZ.

The presence of a range between magmatic and high-temperaturesolid-state microstructures, where additionally submagmatic micro-structures were characterized and the apparent rotation of themagmatic structures into the direction of the mylonitic belts indicatethat during and after crystallization of the plutons, the shear zone con-trolled the ascent and emplacement ofmagma in a dextral transpressivetectonic regime.

The preserved flat-lying magmatic/submagmatic and high-tem-perature solid-state microstructures are interpreted as a record of anearly control of the intrusion by the initial stages of an oblique/thrustshear zone, which placed the Granite Belt over the metamorphicrocks, that evolved to a transpressive one, only during the later stages,after the peak of dynamic metamorphism as observed in several shearzones of the Southern Brazilian Shear Belt.

A slow cooling and uplift of the intrusions is suggested by thebiotite K–Ar ages. In addition, the results on K–Ar fine fractionsindicate latter brittle reactivation of the shear zone in Devonian times,possibly associated with the stabilization phase with low exhumationof tectonic blocks of the Mantiqueira Province, and in the Triassictimes, reflecting the initial removal of heat from beneath Pangea andthe beginning of fragmentation of Gondwana.

Acknowledgments

We thank FAPESP (grant 92/3729-3) for financial support given tofieldwork, litogeochemical and isotopic analyses. To Ahrendt, H. (inhonor) and Wemmer, K. of Institut für Geologie und Dynamik derLithosphäre, Univ. Gottingen, Germany, for the K/Ar analyses of finefractions.


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Deformation and geochronology of syntectonic granitoids emplaced in the Major Gercino Shear Zone,...

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