Campo Belo Metamorphic Complex: Tectonic Evolution of an Archean Sialic Crust of the Southern São Francisco Craton in Minas Gerais (Brazil)

Campo Belo Metamorphic Complex: Tectonic Evolution of an Archean sialic crust of the southern São Francisco Craton in Minas Gerais (Brazil) ARILDO H. OLIVEIRA and MAURÍCIO A. CARNEIRO Departamento de Geologia da Escola de Minas da Universidade Federal de Ouro Preto (DEGEO/EM/UFOP), Morro do Cruzeiro / Campus Universitário Manuscript received on August 30, 2000; accepted for publication on April 17, 2001; presented by Wilson Teixeira ABSTRACT Systematic geological studies performed in the study area allowed the characterization of six lithodemic units: three gneissic, one amphibolitic, one supracrustal and one fissure mafic. The mineral assemblage and the structural record of these lithodemic units indicate that the study area was affected by five tectonothermal events. The structural pattern of the first and oldest event occurred under granulite facies conditions and reveals essentially a sinistral kinematic pattern. The second event, showing dominant extensional characteristics, is related to the generation of an ensialic basin filled by the volcano-sedimentary sequence of the supracrustal lithodemic unit. The third event, which is the most expressive in the study region, is characterized by a vigorous regional migmatization process and by the generation of the Claudio Shear Zone, presenting dextral kinematic movement. The fourth event is represented by a fissure mafic magmatism (probably two different mafic dike swarms) and finally, the fifth event is a regional metamorphic re-equilibration that reached the greenschist facies, closing the main processes of the tectonic evolution of the Campo Belo Metamorphic Complex. Key words: Craton, Archean, tectonic evolution, lithodemic units, metamorphic complex. INTRODUCTION differentiation stages associated with crustal re- The São Francisco Craton is a vast Precambrian plat- working processes (Carneiro et al. 1996a, Carneiro form (Alkmim et al. 1993, Almeida 1977), which et al. 1997a,b, Carneiro et al. 1998a,b, Noce 1995, encompasses part of Minas Gerais and Bahia States Pinese 1997, Teixeira 1993, Teixeira and Silva 1993, (Fig. 1). Its southern portion (Fig. 1) presents sig- Teixeira and Canzian 1994, Teixeira et al. 1996a,b, nificant expositions of Neoarchean granite-green- Teixeira et al. 1997, Teixeira et al. 1999). stone terranes; its tectonic evolution started in the The apex of these processes took place in the Mesoarchean (Carneiro et al. 1998a, Teixeira et Neoarchean, during the Rio das Velhas Tectonother- al. 1996a, Teixeira et al. 1998). From that period mal Event (Carneiro et al. 1998a). Other events are on the first sialic crusts and supracrustal sequences also represented in this crustal segment. They are: were generated by means of successive accretion/ two events of tectonothermal nature in the Meso- archean [3.2 and 2.9 Ga, (Teixeira et al. 1996a, Correspondence to: Maurício Antônio Carneiro E-mail: [email protected] / [email protected] Teixeira et al. 1998)]; a migmatization event [2.86 An. Acad. Bras. Cienc., (2001) 73 (3) AABC 73 3 t1 398 ARILDO H. OLIVEIRA and MAURÍCIO A. CARNEIRO Fig. 1 – Geologic map of southern São Francisco Craton (modified from Machado Filho et al. 1983). 1) Undivided marginal belts; 2) Porto dos Mendes Granite; 3) São João del Rey Supergroup; 4) Minas Supergroup; 5) Formiga Granite; 6) Rio das Velhas Supergroup; 7) Divinópolis Metamorphic Complex, and 8) Barbacena Metamorphic Complex. Ga, (Noce 1995)]; a fissure mafic magmatism event Corrêa da Costa et al. 1998, Corrêa da Costa 1999, [2.658 Ga, (Pinese 1997)] and a felsic magmatism Fernandes et al. 1997, Fernandes et al. 1998, Fer- event [2.65 Ga, (Noce 1995)]. Due to this succes- nandes and Carneiro 2001, Oliveira et al. 1998a,b, sion of events, the tectonic relationships between Oliveira 1999, Oliveira and Carneiro 1999, Oliveira the generated (or reworked) units were progressively et al 1999). In this work the results of one of these masked. Therefore, a complex structural pattern studies (Oliveira 1999) are presented, carried out was imprinted in the lithodemic units that consti- in the Cláudio, Itapecerica, São Francisco de Paula, tute the metamorphic complex, their supracrustal Oliveira and Carmópolis de Minas region, involving sequences and intrusive bodies that crop out in the lithostructural mapping at the 1:200,000 scale (Figs. study area. Recent research, involving regional 1 and 2). (1:200,000) and detailed (1:10,000) mapping performed in the Campo Belo, Oliveira, Itapecerica, GEOLOGIC CONTEXT Cláudio, Carmópolis de Minas regions has characterized new lithodemic units and presented new facts The sialic crust of the southern São Francisco Craton to the tectonic evolution of the southern São Fran- is constituted by gneisses, granitoids, amphibolites, cisco Craton (Carneiro et al. 1996a,b, Carneiro et mafic and ultramafic rocks, schists and quartzites al. 1997a,b,c, Carneiro et al. 1998a,b, Carvalho that were grouped by Machado Filho et al. (1983), in Júnior et al. 1997, Carvalho Júnior et al. 1998, the Divinópolis and Barbacena metamorphic complexes (Fig. 1). Locally, remains of the supracrustal An. Acad. Bras. Cienc., (2001) 73 (3) AABC 73 3 t1 TECTONIC EVOLUTION OF THE CAMPO BELO COMPLEX, MG 399 Fig. 2 – Geologic map of the study area (modified from Oliveira 1999), showing the different lithodemic units: 1) Fissure Mafic Unit; 2) Supracrustal Unit; 3) Candeias Gneissic Unit; 4) Itapecerica Gneissic Unit; 5) Cláudio Gneissic Unit; 6) Inferred Contact; 7) Foliation; 8) Cláudio Shear Zone, and 9) Key Outcrops: A – Fernão Dias road; B – Corumbá dimension stone quarry; C – Kinawa dimension stone quarry; D – Morro da antena; E – Vista Bela farm; F – Carmo da Mata dimension stones quarry; G – Marilan dimension stones quarry; H – Lila dimension stones quarry; I – Alemão dimension stones quarry; J – Oliveira dimension stones quarry. sequences are found, correlated with the Rio das lithotypes listed above presents three principal de- Velhas or Minas Supergroups. The geographic dis- formation and/or fracturing directions: NS; NW/SE tribution of the Divinópolis Metamorphic Complex, and NE/SW. The tectonic meaning of these direc- according to (Machado Filho et al. 1983), occurs tions according to Machado Filho et al. (1983) in the vicinity of Divinópolis, Itaúna, Formiga and is the following: 1) NS direction, of more plastic Passa Tempo cities (Fig. 1). In the remaining re- character, would be responsible for the compres- gion, rocks from the Barbacena Metamorphic Com- sion of supracrustal sequences in a tight syncline; plex would predominate. However, Teixeira et al. 2) the NW/SE structures displace the N/S structures (1996b) consider these two complexes in the Campo and slightly metamorphosed basic dikes were em- Belo region (Fig. 1) as a single unit, naming it placed along this trend; 3) the last direction is re- Campo Belo Metamorphic Complex. According to lated to the generation of the Atlântico Mobile Belt, Machado Filho et al. (1983), the majority of the at the southeastern-eastern margin of the Southern An. Acad. Bras. Cienc., (2001) 73 (3) AABC 73 3 t1 400 ARILDO H. OLIVEIRA and MAURÍCIO A. CARNEIRO TABLE I General characteristics of the rocks types of the study area and their probable ages and tectonothermal events. Unit Lithology Rock-forming Ages Tectono- minerals thermal events Fissure Mafic gabbronorite plagioclase, ortho- and Ages for the E4 and E5 clinopyroxenes intrusions: gabbro Plagioclase, 2.658 Ga3 clinopyroxene and 1.875 Ga3. Supracrustal Rio Banded iron Quartz and Probable ages E3, (E4)? das Velhas formation opaque minerals for the mafic- and E5 Supergroup quartzite Quartz, sillimanite ultramafic and garnet magmatism, schist Sillimanite, quartz, followed by mica and garnet sedimentation (2.8 Ga2). amphibolite Clinoamphibole, plagioclase and quartz ultramafic Orthopyroxene, olivine, clinoamphibole Amphibolitic amphibolite Hornblende, Probable age: E1, (E2?), plagioclase and 3,38-3,0 Ga1 E3, (E4?) quartz (?) and E5. Candeias Gneiss of grano- Plagioclase, Probable age E1, (E2?), Gneiss dioritic to granitic microcline, quartz for the E3, E4 composition and hypersthene protolith and E5 Itapecerica Gneiss of granitic Microcline, (3.38-3.0 Ga1) Gneiss composition plagioclase and and probable quartz age for the migmatization (2.75-2.72Ga2). Cláudio Gneiss of grano- Plagioclase, Gneiss dioritic composition microcline and quartz 1(Teixeira et al. 1996b), 1(Teixeira et al. 1998), 2 (Carneiro et al. 1998b), 3(Pinese 1997). An. Acad. Bras. Cienc., (2001) 73 (3) AABC 73 3 t1 TECTONIC EVOLUTION OF THE CAMPO BELO COMPLEX, MG 401 São Francisco Craton. reaching some meters in length. It is in general coarse-grained and light pink, formed by feldspar GEOLOGY OF THE AREA phenocrysts and less abundant centimetric mica Using satellite imagery and aeromagnetometric map flakes. The third family is represented by pegmatite interpretation and by means of regional geologic veins that crosscut either the gneissic foliation or the sections (Oliveira 1999) it was possible to charac- other two families. These are centimetre-thick and terize six lithodemic units (Table I and Fig. 2). The metre-long veins. They are fine- to medium-grained older three of these units are gneissic, the fourth is and fill NS-trending fracture planes. amphibolitic (cannot be mapped at the scale adopted), the fifth is a supracrustal sequence (of Cláudio Gneissic Unit greenstone belt type), and the youngest is a mafic The rocks of this unit crop out in the eastern por- dike swarm. tion of the study area (Fig. 2) and present granodioritic to dioritic, locally granitic, compositions. Gneissic Units They are gray, fine- to medium-grained, banded and The petrographic distinction of the gneissic units migmatized. In some outcrops light pink pegmatitic (Fig. 2) was mainly based on their color and com- mobilizates occur, giving a rosy tint to the rock. Re- position. The Cláudio Unit rocks are gray and their gionally, the color of these gneisses can vary, so composition is predominantly granodioritic. Those that eastwards (Fig. 2), it is predominantly gray, from the Itapecerica Unit are light pink and their diminishing the percentage of light pink felsic mo- composition is predominantly granitic.

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