Campo Belo Metamorphic Complex: Tectonic Evolution of an Archean sialic crust of the southern São Francisco Craton in ()

ARILDO H. OLIVEIRA and MAURÍCIO A. CARNEIRO

Departamento de Geologia da Escola de Minas da Universidade Federal de (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

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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) Granite; 6) Rio das Velhas Supergroup; 7) Divinópolis Metamorphic Complex, and 8) 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, , 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 per- formed in the Campo Belo, Oliveira, Itapecerica, GEOLOGIC CONTEXT Cláudio, Carmópolis de Minas regions has charac- terized 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 com- plexes (Fig. 1). Locally, remains of the supracrustal

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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 – 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 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

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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).

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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 gran- odioritic 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. Finally, the bilizates. The texture of the Cláudio gneisses is Candeias Unit rocks are green and their composi- predominantly granoblastic to granolepidoblastic. tion is granitic to granodioritic. The geologic con- The crystals vary from subidioblastic to idioblastic, tacts between the gneissic units are masked by the medium- to fine-grained, being more rarely coarse- thick weathering mantle and by the superposition grained. Intergrowth textures are common (e.g. of tectonothermal processes taking place since the perthite, myrmekite and sometimes antiperthite). In Mesoarchean. However, the gneissic units have dis- general, plagioclase is the main constituent of these tinct outcrop domains. As an example, the Itapecer- rocks (30-40%). The crystals are predominantly ica Gneissic Unit, with rocks cropping out in the medium-grained and those showing polysynthetic surroundings of the city it borrows the name from, twinning vary from subhedral to euhedral. Quartz is characterized by intense migmatization, which is in general the second constituent in volume (20- imprinted an average mineralogic composition of 30%). The microcline percentage is also variable, granitic nature. This migmatization is more pro- sometimes being comparable to the other two con- nounced in the western portion of its geographic stituents. However, it is more common to be sub- occurrence. Three pegmatite vein families are as- ordinated (15-30%). As varietal mineral, biotite (5- sociated with the gneissic units. The first family is 10%) shows light to dark or greenish brown formed by mobilizates concordant with the gneissic pleochroism, altering to chlorite. It has zircon in- foliation and presents centimetric thicknesses and clusions and sometimes it is associated with opaque variable (centimetric to metric) lengths. These mo- minerals. Amphibole (hornblende) is rare and shows bilizates are essentially composed of feldspars and greenish pleochroism and can alter to biotite. small quantities of mica. The mobilizates are usu- Opaque minerals, zircon and apatite are accessories, ally light pink and sometimes white. The second as well as rutile, tourmaline and allanite, which are pegmatite family is discordant from the gneissic fo- rarer. As secondary minerals chlorite, sericite, car- liation and is centimetre- to metre-thick, with veins bonate and epidote occur.

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Itapecerica Gneissic Unit the main constituent, quartz (20-25%) and the mi- crocline content (15-20%) varies from sample to The rocks from the Itapecerica Gneissic Unit sample but can reach up to 40%. Hyperstene, when crop out in the NW portion of the area and present present, is medium-grained, has low birefringence granitic composition, are light pink, migmatized and and pleochroism varying from light pink to pale show inequigranular to equigranular textures and are green. It alters to amphibole, biotite and chlorite. medium- to fine-grained. In some outcrops closer to Biotite, sometimes showing symplectitic inter- the contact with the Candeias Gneissic Unit (Fig. 2), growth with quartz, presents light brown, sometimes the rocks are greenish. In other localities the gneiss greenish, pleochroism. In general, this mineral de- is grayish, as the outcrops located east of Itapecerica fines the banding of the rock. Locally, biotite alters city. Pegmatitic or amphibolitic dikes constituting to chlorite. Opaque minerals, zircon and apatite are boudins are common features. The texture of the the commonest accessories of the rock. Chlorite is Itapecerica Gneissic Unit rocks is granoblastic to the commonest secondary mineral, overgrowing bi- granolepidoblastic with subidioblastic to idioblas- otite and pyroxene. tic, fine- to medium-grained crystals. Microcline is the predominating mineral (30-40%), plagioclase Amphibolitic Unit second in abundance (15-30%) and quartz is sub- ordinated to feldspar (±20%). Biotite (<10%) is a The rocks of this unit are fine-grained, melanocratic, varietal mineral and presents dark brown to greenish dense, phaneritic, black and strongly foliated. In pleochroism, locally altering to pale-green chlorite. most places, the amphibolites are found in the form Amphibole is rarer, fine-grained and alters to biotite. of disrupted thin boudins, representing disrupted Common accessories are zircon, apatite and opaque and metamorphosed dikes emplaced in the rocks minerals. Sericite, chlorite, carbonate and epidote of the gneissic units. The texture of the amphi- are secondary minerals. bolites varies from nematoblastic to granolepido- nematoblastic. The crystals are fine- to medium- grained, strongly oriented and altered. Green horn- Candeias Gneissic Unit blende (±50%), sometimes altered to biotite, is a The gneisses of this unit crop out in the SW por- main mineral. Locally, biotite schist (or biotitite) tion of the area (Fig. 2). In the most deformed do- results from retrograde metamorphism and shear- mains the rocks present granodioritic composition ing. Plagioclase is the second constituent in volume (opdalites). However, in the more homogeneous do- (20-30%). Quartz appears subordinated to the other mains the composition is granitic (charnockites). In minerals and its content varies from 5 to 10% of general, these rocks are green, slightly migmatized, the rock volume. Biotite, sometimes chloritized, is deformed and medium- to coarse-grained. The min- the product of amphibole transformation. Pale yel- eralogic banding, in some places, is difficult to be low clinopyroxene, present in some rocks, can reach observed due to the great homogeneity of the bodies, 10% of the volume and alters to amphibole. Acces- except for the hyperstene-biotite-gneiss and biotite- sory minerals are zircon, opaque minerals and ap- gneiss where the biotite planar orientation is more atite. Besides biotite, secondary minerals are chlo- enhanced. Close to the contact with the Itapecerica rite and amphibole, epidote and sericite. Gneissic Unit, light pink pegmatitic mobilizates are Supracrustal Unit found. The texture of the Candeias-type gneisses varies from granolepidoblastic to granoblastic and Ultramafic rocks (metaperidotite, chlorite-amphi- they are subdioblastic to idioblastic, predominating bole-schist and hornblendite), amphibolite, garnet- granoblastic. Antiperthitic plagioclase (30-45%) is sillimanite-schist, garnet-sillimanite-quartzite and

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banded iron formation were characterized in the nematoblastic to granonematoblastic. The crys- study area. Regionally, the Supracrustal Unit (Fig. tals are fine- to medium-grained. Green hornblende 2) can be correlated with the Rio das Velhas Super- (±40%), sometimes alters to biotite and chlorite. group, which crops out in the Quadrilátero Ferrífero Plagioclase (±35%) is the second constituent in vol- in Minas Gerais. In some places, they are strongly ume and alters to sericite and epidote, and quartz deformed, metamorphosed and sometimes cataclas- is subordinated (±10%). Biotite and clinopyrox- tic. In the field, the domain of the ultramafic rocks ene occur occasionally. The main accessories are is marked by positive magnetic anomalies as can zircon, opaque minerals and apatite. The garnet- be observed in aeromagnetometric maps (Oliveira sillimanite-schist is a fine- to medium-grained, dark 1999). The metaperidotite is a holocrystalline rock gray, strongly mylonitized magnetic rock. The rock with inequigranular, anhedral to subhedral crystals texture varies from granolepidoblastic to granone- and grain-size varying from fine to coarse. It shows matoblastic. The crystals vary from subidioblas- cumulatic and/or mesh texture. Enstatite makes up tic to granoblastic. Quartz is the main constituent 50% to 60% of the total volume of the rock. An- (±40%). The crystals are elongated (ribbons) and hedral clinopyroxene and olivine crystals can reach follow the rock foliation. Plagioclase (±15%) is up to 20% of the rock volume. The hornblende per- the second main constituent. Sillimanite in vari- centage varies from 20% to 50%. Spinel can reach able proportions (up to 70%) presents yellow po- 10% of the rock volume and appears as granular larization color, is fine- to medium-grained and the crystals of high relief and color varying from green crystals are fibrous (more common) or prismatic. It to greenish-brown. Serpentine, rare in the major- alters to white mica like biotite (±10%). Garnet ity of the thin sections, is found as fibro-lamelar (±5%) is a light brown mineral with dark-gray po- aggregates, resultant from the alteration of olivine larization color, almost isotropic with opaque min- and pyroxenes. The opaque minerals can reach 5 erals, quartz and white mica inclusions. Opaque to 10% of the rock. They are isotropic and gen- minerals, zircon and rutile are the commonest ac- erally result from spinel transformation. Chlorite cessories in this rock. Rutile is translucent brown is sometimes present as secondary mineral. The and can be associated with biotite. White mica, epi- chlorite-amphibole-schist is a greenish rock with dote and carbonate are secondary minerals, alter- porphyroblastic texture. Chlorite sums up 30% of ation products after plagioclase. White mica is also volume and appears as poikilitic porphyroblasts em- formed from sillimanite and biotite. The garnet- bedded in a fine hornblende mass (±70%). A small sillimanite-quartzite is a fine-grained rock, pale amount of opaque minerals represents the acces- greenish yellow to white, mylonitized and with sub- sory minerals. The hornblendite is composed of vertical foliation. The texture varies from granolepi- predominantly medium-grained granonematoblas- doblastic to granonematoblastic and the grain-size tic hornblende crystals (±95%) and opaque min- varies from fine to medium. Quartz (±70%) is the erals. The amphibolite is a fine- to medium-grained main rock constituent. The crystals are elongated rock, melanocratic, dense, phaneritic, black and (ribbons) accompanying the foliation. Sillimanite strongly foliated. The amphibolite is, in general, (±20%) presents yellow polarization color, grain- found in contact with ultramafic rocks, mainly in size varying from fine to medium with fibrous crys- the surroundings of Cláudio, in the supracrustal unit tals (fibrolite) oriented according to the foliation. It domain (Fig. 2). The main foliation trend is NE/ alters to white mica. Garnet (±3%) is a light brown SW, accompanying the direction of the magnetic mineral and has dark gray polarization color. Rare anomaly observed in the aeromagnetometric maps biotite shows light to dark brown pleochroism, is (Oliveira 1999). The rock texture varies from fine-grained, with zircon inclusions, and alters to

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white mica. Opaque minerals, zircon and rutile are 5% of the rock. Quartz appears in small quantities, common accessories. White mica is the secondary with clean, anhedral, fine-grained crystals. Biotite mineral in the rock and it is the alteration product is rare and reddish, pleochroic. Zircon and opaque after sillimanite and biotite. Finally, the banded minerals are accessory minerals. White mica, epi- iron formation is a magnetic rock, of predominantly dote, carbonate and chlorite are secondary miner- fine grain-size, composed of magnetic minerals and als. The gabbro presents ophitic, subophitic or in- quartz. This rock presents granoblastic texture and tergranular textures. The medium- to fine-grained the contact between grains is elongated due to de- crystals vary from subhedral to euhedral. Plagio- formation. Quartz (±60%) is the main constituent clase is the most abundant mineral in volume, vary- of the rock, with elongated crystals in the form of ing from 30-50%. Augite contents, second compo- ribbons; it is fine- to medium-grained and strongly nent in volume, vary from 20 to 30%. The crystals recrystallized. Magnetite contents vary from 30 to are poikilitic and alter to amphibole. Hornblende 40% in terms of modal composition. is the uralitization product after pyroxene, being its content in volume variable between 5 and 15%. Fissure Mafic Unit When present, microcline is rare, showing cross- hatch twinning and is associated with myrmekite. This unit is represented by NW/SE-trending dikes Quartz appears in small quantities. Biotite is also (Fig. 2). These dikes have varied thickness, from rare, reddish and pleochroic. Zircon, opaque miner- metric to dozens of meters and their length can reach als and apatite are accessory minerals. White mica, dozens of kilometers. The dikes are subvertical epidote and chlorite are secondary minerals ob- and commonly develop ramifications involving the served in some of the rocks. gneissic massifs. Compositionally, the dikes are composed of gabbronorite, gabbro and diabase. The gabbronorite and the gabbro are medium- and METAMORPHISM coarse-grained, possibly representing variations of Three distinct and anachronic metamorphic peaks crustal emplacement during magmatic crystalliza- are registered in the rocks of the study area. The tion. These rocks are in general dark-colored to first, of highest metamorphic grade related to the greenish, and present massive structure and phaner- granulite facies, is present in the rocks of the Gneis- itic texture. However, when very fine-grained, they sic Units and of the Amphibolitic Unit. The second, can be aphanitic. Along the same lineament small predominantly of the amphibolite facies, is observed diabase dikes can be observed as a dark, dense and in the mafic-ultramafic and clastic-pelitic rocks of fine-grained rock. The gabbronorite shows ophitic, the Supracrustal Unit. The third is characterized subophitic to intergranular textures, with subhedral by regional retrograde metamorphic processes that to euhedral crystals, and is medium- to fine-grained. reached the greenschist facies and cover the whole Plagioclase is the most abundant mineral in vol- region. The main paragenesis of the Cláudio, Ita- ume (40-60%). (Clino- and ortho-) pyroxene con- pecerica and Candeias Gneissic Units is constituted tents vary from 20 to 35% in volume. In general, by plagioclase ± quartz ± (antiperthitic) microcline augite predominates over hyperstene and is predom- ± amphibole ± hyperstene ± biotite. As secondary inantly medium-grained, presents variable polariza- paragenesis sericite, carbonate, chlorite and epidote tion color (from high to low), the crystals are occur. Antiperthite and hyperstene indicate that poikilitic, altering to amphibole. Hyperstene is light these rocks underwent metamorphic conditions of pink, pleochroic, from medium- to fine-grained. the granulite facies. On the other hand, the sec- Hornblende with green pleochroism is the alteration ondary paragenesis indicates greenschist-facies ret- product of pyroxenes (uralitization), reaching up to rograde metamorphism. TheAmphibolitic Unit, en-

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closed in these gneissic units, also presents high- evidence supports the first hypothesis (Carneiro et al. grade paragenesis, with the presence of hyperstene, 1998b, Teixeira et al. 1996b, Teixeira et al. 1998). and retrograde metamorphic features of the green- These authors, respectively, defend a metamorphic schist facies, characterized by the presence of chlo- event of the granulite facies in the Mesoarchean and rite. However, Supracrustal Unit rocks show slightly another of amphibolite facies in the Neoarchean (Rio different parageneses. Except for the Supracrustal das Velhas Tectonothermal Event). Therefore, it is

Unit ultramafic rocks, where green Al2O3-richer believed that the first possibility is more realistic to spinels would indicate metamorphic conditions explain the tectonic evolution of the region, accord- compatible with the amphibolite-granulite fa- ing to the reasons exposed in the following items. cies (Pactunç 1984), the mineral assemblages of the schists (quartz ± plagioclase ± sillimanite ± garnet) STRUCTURAL ANALYSIS and quartzites (quartz ± sillimanite ± garnet ± bi- The rocks of the study region present distinct struc- otite) indicate that the metamorphism in these rocks tural features (Fig. 3, Fig. 4, Table II). For in- reached at most the lower granulite facies. After- stance, the following features were observed in the wards, as show the processes of sillimanite alteration supracrustal unit rocks: 1) a mylonitic foliation (Sm) to white mica and plagioclase sericitization, these of anastomosing character; 2) structures S/C; 3) S assemblages were re-equilibrated to greenschist fa- and Z intrafolial folds, tight and/or isoclinal sheath cies conditions. However, if in the Supracrustal mesofolds; 4) foliation sigmoids; 5) rotated quartz Unit rocks metamorphism reached the amphibolite veins, and 6) tension gashes. The gneissic unit rocks or even granulite facies, this was not the case in present, besides the structural features described the Fissure Mafic Unit rocks. These rocks have above, the following ones: 7) a gneissic foliation preserved igneous textures, showing only metamor- (Sg) of (local) mylonitic character and anastomos- phic re-equilibration in the greenschist facies that, in ing pattern; 8) shear bands (Sb1 and Sb2);9)peg- fact, is common to all units described in this paper. matitic vein folds and rotated amphibolite boudins. So, two significant considerations can be expressed: Local normal and reverse faults and a regional mega- a) the chronostratigraphic character of the Fissure structure here named Cláudio Shear Zone were char- Mafic Unit that, due to its igneous textures and meta- acterized. The area also shows intense NW-SE- morphic parageneses at most of the greenschist fa- trending crustal fracturing along which mafic dike cies, indicates their crustal emplacement after the swarms were emplaced (Fig. 2). tectonothermal events of higher metamorphic facies Foliations – The gneissic foliation (of mylonitic that affected the older units; b) the metamorphic fa- character) presents steep to low-angle dips. This fo- cies variation between rocks of the Gneissic,Amphi- liation is characterized by a fine compositional band- bolitic and the Supracrustal Units. Two hypotheses ing represented by the orientation of biotite, sericite can be put forward to explain this variation. The and amphibole. In general the foliation is anasto- first hypothesis would imply in the existence of two mosing. The (mylonitic) foliation Sm observed in tectonothermal events. The first one, of the gran- the supracrustal unit rocks presents steep to low- ulite facies, would be previous to the generation of angle dips and is characterized by the orientation of the Supracrustal Unit rocks and the second one, lo- the following minerals: pyroxene (in the ultramafic cally reaching the granulite facies, would follow the rocks); amphibole (in the amphibolites); elongated generation of the Supracrustal Unit rocks. The sec- quartz (ribbons) and sillimanite (in the quartzites); ond hypothesis would favor the existence of a single biotite, sericite and sillimanite (in the schists). The event of the granulite facies that occurred after the structural style and strain of the foliation deforma- generation of the Supracrustal Unit rocks. Isotopic tion indicate that they could have originated from

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AABC 73 3 t1 406 ARILDO H. OLIVEIRA and MAURÍCIO A. CARNEIRO

A B

0.5 % 1.2 % 1.0 % 2.4 % 1.5 % 2.1 % 3.7 % 2.6 % 4.9 % 3.1 % 3.6 % 6.1 % 4.1 %

C D

5.9 % 11.1 % 11.8 % 17.6 % 22.2 % 23.5 % 33.3 % 29.4 % 35.3 % 44.4 % 41.2 %

E F

4.3 % 4.5 % 9.1 % 8.7 % 13.6 % 13.0% 18.2 % 17.4 % 22.7 % 27.3 % 21.7 % 31.8 %

Fig. 3 – Stereographic diagrams representing the polar projections of the foliations of the study area: A) General foliation poles of the area (Sg and Sm), of NE/SW NS and NW/SE directions. Number of foliation measurements: 388; Maximum: 310/40, 156/40 and 200/37. B) Foliation poles Sg and Sm and mineral lineation (filled squares) of the Cláudio Shear Zone domain. Number of foliation measurements: 246; Maximum: 310/40 and 155/40. Number of lineation measurements: 21. C) Poles to foliation of dextral Sb1 (Shear bands) of NW/SE direction. Number of foliation measurements: 27; Maximum: 040/82; the rosette represents the foliation direction. D) Poles to foliation of sinistral Sb1 (Shear bands) of E-W direction. Number of foliation measurements: 22; Maximum: 130/82 and 344/83; the rosette represents the plane direction. E) Poles to foliation of sinistral Sb2 (Shear bands) of ENE/WSW direction. Number of foliation measurements:17 ; Maximum: 218/88 and 040/85. F) Poles to foliation of dextral Sb2 (Shear bands) of NW/SE direction. Number of foliation measurements: 9; Maximum: 170/83 and 350/85.

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AABC 73 3 t1 TECTONIC EVOLUTION OF THE CAMPO BELO COMPLEX, MG 407

C S 150/70 C (130/75) S PH PH

Sg=150/63 A 0 10cm B 0 10cm

Anf.

PH PH

PH

''Sb2 ''=140/75 C D Sg=140/70 Peg. 0 20cm 0 20cm

280 100 60 250 ''Sb2 '' PV Peg. Diab. PH F E 05m 0 20cm

N R'

X Sb2 Anf. T 110 Sb2 G 290 PV H 04m

Fig. 4 – Kinematic indicators (A-G) and schematic models explaining one of the events active in the study area (H). A) Structure S/C in gneiss of Cláudio Unit, with Event 1 sinistral movement; B) Sigmoidal foliations of the Supracrustal Unit schists with dextral movement; C)Amphibolite boudin rotated clockwise and broken by shear zones (shear bands Sb2, X); D) Asymmetric folded pegmatite vein, Z-shaped, with dextral movement in the gneisses; E) (Brecciated) diabase dike in Cláudio Unit gneisses, displacing pegmatite vein and the gneissic foliation, denoting Event 3 Phase 2 normal movements; F) Shear bands (Sb2, R’); G) Rotated amphibolite boudins, vertical plane view, with reverse movements; H) Schematic model representing Event 4, sinistral, indicating shear bands (Sb2), reverse faults and the intrusion of the Fissure Mafic Unit. Symbology: PH – Horizontal plane; PV – Vertical plane; Peg. – Pegmatite; Anf. – Amphibolite; Diab. – Diabase, and R’ and X (anti-Riedel and secondary). the same deformation phase. Sg and Sm foliations NS to NW-SE). The NW-SE direction predominates present inflections along the study area so that the over the NS one (Fig. 3A). The lateral relation be- main trend is NE-SW (Figs. 3A and B). Locally, a tween these directions can be seen in some key out- change in the foliation direction is observed (from crops (points 9A, 9D, 9G, 9I; Fig. 2). At these points

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AABC 73 3 t1 408 ARILDO H. OLIVEIRA and MAURÍCIO A. CARNEIRO

TABLE II

Simplified table of the structural elements associated with the tectonothermal events.

Tectono- Foliation Kinematic Faults and Metamorphism Associated thermal indicators shear zones (M) rocks E5 M3 greenschist facies E4 Foliation Reverse fault Fissure Mafic sigmoids, Unit ‘‘shear bands’’ (Sb2, R’ and X) E3 Gneissic foliation ‘‘Z’’ folds, Cláudio Shear M2 Migmatization (of mylonitic foliation, sigmoids, Zone (ZCC) and Amphibolite and deformation (character or not) amphibolite and normal fault facies of the previous and Sm foliation rotated and units (of the Supra- displaced pegmatite crustal unit) veins, ‘‘shear bands’’ (Sb1) E2 (?) (?) (?) (?) Mafic-ultramafic magmatism and sedimentation of the Supracrustal Unit E1 Gneissic foliation ‘‘S’’ and sheath (?) M1 Formation of the (?), fold with folds, S/C Granulite protolith of the double vergence structures facies Gneissic and and S/C Amphibolitic Units

the predominant foliation (NE-SW) is folded and its Shear bands – Shear bands Sb1 and Sb2 are subver- trajectory in folds inflects to NS and NW-SE. The tical and displace the foliation of the gneissic units, last trend represents one of flanks of folds of 2 to 3 as seen in horizontal and vertical planes of the quar- meters of amplitude. Thus the several trends char- ries of the region. The foliation Sb1 is oriented ac- acterized in the region as a whole reflect the prin- cording to NW/SE and E-W trends (Figs. 3C and cipal foliation inflection (NE/SW). In general the D) and pegmatitic mobilizates intrude along the fo- foliation dip is steep (70◦ to 80◦), mainly along the liation planes. The NW/SE trend presents a dex- Cláudio Shear Zone (Fig. 2). However, outside the tral kinematic component whereas the E-W trend Cláudio Shear Zone domain medium to low-angle has a sinistral component (seen on the horizontal ◦ ◦ dips (50 to 20 ) are observed. The mineral lin- plane). Foliation Sb2 is oriented along ENE/WSW eation (lm) also presents a certain variation: from and NW/SE trends (Figs. 3E and F). Sb2 does not strike (Kinawa quarry, point 9C, Fig. 2) to oblique present pegmatitic mobilizates along its planes. The (Corumbá quarry, point 9B; Fig 2) and dip (in the kinematics of the ENE/WSW foliation has a sinistral quartzites, point 9D). In these three cases, the folia- component and that of the NW/SE plane a dextral tion planes present NE-SW trends. component (seen on the horizontal plane). These

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AABC 73 3 t1 TECTONIC EVOLUTION OF THE CAMPO BELO COMPLEX, MG 409

two foliations were interpreted as a pair R’ and X cally there are boudins indicating sinistral move- (anti-Riedel and secondary, Fig. 4H, Sanderson and ment. Marchini 1984). Shear bands Sb1 and Sb2 are differ- Faults – Faulting systems can be observed in sev- ent in the following aspects: the first group presents eral key outcrops (points 9A, 9C, 9D, 9G, 9H and 9J; pegmatitic mobilizates along its planes, lacking in Fig. 2). At the sides of the Fernão Dias highway, a the second group. Besides, the kinematic trends of NW/SE fault plane is intruded by volcanic material both groups are opposite (Figs. 3C; 3D; 3E and 3F). that precedes or is coeval to the fault generation, ap- Structures S/C – Structures S/C [S150/70 C(130/ pearing as a breccia. The fault plane cuts the gneis- 75), seen in the horizontal planeAC] can be observed sic banding, is subvertical and displaces pegmatite at restricted points in the area (points 9C and 9B; veins, indicating normal movement (Fig. 4E). In Fig 2). Their dimensions are centimetric and the the Montueira Ridge, close to Carmópolis de Minas, kinematics indicate sinistral movements (Figs 4A). a fault with the same trend is observed, yielding a steep slope. The reverse faulting of local nature does Folds – Folds are observed locally and can be de- not present regional correlation of major magnitude. scribed as: intrafolial and tight, with S and Z asym- The indicators for this movement are the (sinistral) metries; sheath mesofolds (rare) and tight and/or iso- S foliation, seen in the vertical plane, and the fo- clinal symmetric (with flanks inverted or not). The b2 liation sigmoids seen in the gneisses, schists and most common folds have S and Z asymmetries (the quartzites. latter being more prominent). The S and Z intrafo- Finally, the mega-structure that characterizes lial folds are important in the characterization of two the Cláudio Shear Zone, firstly revealed by aero- of the phases of tectonic evolution of the study area. magnetometric images (Oliveira 1999), describes Foliation Sigmoids – Foliation Sigmoids are a roughly NE-SW alignment (Fig. 2). Along the observed both in schists and gneisses and charac- Cláudio Shear Zone are observed migmatized terize movements with dextral directional compo- gneisses, ultramafites, amphibolites, quartzites and nents (seen on the AC plane), where the foliation schists showing high strain. The foliation inside dip is steep and of high strain [e.g. 130/80 (lm = the mega-structure shows medium- to high-angle 225/40)]. Locally, sigmoids indicative of normal dips and mylonitic character. Its fabric describes movements with an oblique component are observed NE/SW (Fig. 3B) and subordinately NS and NW/SE (seen on vertical AC plane). trajectories (Fig. 3A), characterizing regional fold- Pegmatites – In the gneissic unit domain Z-folded ing. The general fabric is represented by ‘‘S’’ tec- pegmatitic mobilizates are observed indicating dex- tonites composed of pegmatitic mobilizates concor- tral movement seen on theAC plane. Locally appear dant with the foliation. Other features presented by veins denoting normal movements. this mega-structure are: Z intrafolial folds, folia- tion sigmoids, amphibolite boudins, pegmatite and Quartz Veins – Both in the supracrustal unit rocks rotated quartz veins with dextral movement. The and the gneissic unit domains, rotated centimetric meaning of the variation of the different (strike, quartz veins are observed. These veins indicate dex- oblique or dip) mineral lineation trends, contained tral movement, seen on the AC plane. Locally, veins in the foliation planes is still an open issue in the indicating reverse movement occur. regional context. Amphibolite Boudins – In the domains of the three gneissic units centimetric to metric amphibolite TECTONIC EVOLUTION boudins occur disrupted, strongly deformed and ro- It is known that the period between 3.6 and 2.5 tated, in general showing dextral movement. Lo- Ga was responsible for the worldwide formation of

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AABC 73 3 t1 410 ARILDO H. OLIVEIRA and MAURÍCIO A. CARNEIRO

granite-greenstone terranes, with their apex around Event 2, of essentially extensional characteristics, 3.0-2.6 Ga (e.g. Byerly et al. 1996, Collerson and the ensialic basin would be installed for sedimen- Macdonald 1998, Friend et al. 1996, Goodwin 1991, tation and magmatism of the Supracrustal Unit (or Hamilton et al. 1979, Hamilton 1998, Kröner et Rio das Velhas Supergroup). al. 1996, Myers 1993, Sarkar et al. 1993, Tankard Event 3 – As said before, it was between 2.78 and et al. 1982, Windley 1976, Worden et al. 1995). 2.70 Ga that in the Southern São Francisco Craton In the case of the southern São Francisco Craton, the main accretion and crustal reworking phase took it was in the Neoarchean (from 2.78 to 2.70 Ga) place (e.g. Carneiro et al. 1998a,b, Machado et al. that this crustal segment had its main accretion stage 1996, Noce 1995). This evolution was related to the and crustal reworking (e.g. Carneiro et al. 1998a, Rio das Velhas Tectonothermal Event. The peak of Machado et al. 1996, Noce 1995). Considering that this event occurred around 2.78-2.77 Ga (Carneiro in the southwestern portion of the study area in the et al. 1998b, Machado and Carneiro 1992), when Campo Belo region the gneissic units present zir- the zircons of the Alberto Flores Gneiss from the con U-Pb ages of the order of 3.2 Ga (Teixeira et Bonfim Metamorphic Complex suffered overgrow- al. 1996a, Teixeira et al. 1998), it is possible to in- ing, indicating isotopic re-equilibration under high fer that the beginning of the crustal evolution of the amphibolite metamorphic facies conditions. These studied segment started in the Mesoarchean. Thus, metamorphic conditions have correspondence in the on a primordial sialic crust formed by the protoliths study region, where an ample and vigorous migma- of the gneissic and amphibolitic units, a tectonother- tization process is observed, imposed on the rock mal event took place, preserving some kinematic fabric generated during Event 1. In structural terms, and petrologic records that characterize Event 1. the migmatitic rocks originated during this process Event 1 – It is the oldest event or the event that present the same kinematic elements of the Supra- was not totally obliterated by the successive crustal Unit rocks. Two important considerations events imprinted in the rocks of the study region. Its derived from this fact: a) migmatization follows the deformational structures are foliations of S/C-type, Supracrustal Unit sedimentation and magmatism; b) sinistral-rotated amphibolitic boudins and centimet- due to the tectonic intensity of this process, the pres- ric intrafolial S folds that are observed locally (Fig. ence of high amphibolite or granulite facies par- 4). Systematically, the rocks that were affected by ageneses, common in the Supracrustal Unit rocks this event have relic parageneses of the granulite fa- of the study region but lacking in the Quadrilátero cies. Later migmatization processes related to Event Ferrífero Archean supracrustal rocks, would be ex- 3 developed on this fabric. plained. In hierarchic terms, Event 3 can be di- vided in two phases. To Phase 1 would correspond Event 2 – Similarly to the Quadrilátero Ferrífero, the migmatization process and the structural ele- where the Rio das Velhas Supergroup crops out, the ments mainly found in the Cláudio Shear Zone (Fig. study area also presents remmants of the greenstone- 2), which are: asymmetric Z folds, foliation sig- belt type supracrustal sequences, with ultramafic moids, pegmatitic vein folds and rotated amphibo- and clastic-pelitic rocks (Fig. 2). In the case of lite boudins indicating dextral movement. Associ- the Quadrilátero Ferrífero, Carneiro et al. (1998b) ated with the final phase of this dextral directional consider that the sedimentation and magmatism re- flow brittle-ductile shear zones are observed, here sponsible for the formation of the Rio das Velhas represented by Sb1 foliations (shear bands, Figs. Supergroup rocks would be in operation around 2.8 3C and 3D), associated with pegmatitic mobilizates.

Ga, in an ensialic-type basin. This would be, in prin- NW/SE – trending Sb1 presents a dextral component ciple, the situation of the study area where, during and the less conspicuous E/W – trending Sb1 has a

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AABC 73 3 t1 TECTONIC EVOLUTION OF THE CAMPO BELO COMPLEX, MG 411

sinistral component. To Event 3, Phase 2 would lithodemic units and the amphibolitic unit were em- correspond a crustal relaxation phase with the gen- placed in the crust in the Mesoarchean and reworked eration of faults with normal movement, observed in in this period and mainly in the Neoarchean; 2) the the Supracrustal Unit quartzites that crop out south volcano-sedimentary sequence of the supracrustal of Cláudio and in the gneissic rocks at the sides of the unit was deposited before Event 3, which is the main Fernão Dias highway, north of Carmópolis de Mi- migmatization event in the area; 3) the main defor- nas. The lineation contained by the foliation planes mational events in the area essentially occurred in is steep (parallel to the dip to slightly oblique) and the Neoarchean. This conclusion is based on Sm-Nd the kinematic indicators associated with this phase geochronologic data (Pinese 1997), obtained from are rotated pegmatite veins and fault planes contain- dikes of gabbronoritic composition that crystallized ing volcanic material breccia (Fig. 4G). This phase 2,658 ± 44 Ma ago. These dikes are not deformed is not expressive, does not obliterate the previous and only present the original magmatic paragenesis one and its kinematics do not have regional signifi- re-equilibrated to the greenschist facies. Thus, as cance. in the study area, similar dikes are found intrusive in the gneissic and supracrustal units that have relic Event 4 – The initial phase of this event had a brittle- high-grade metamorphic parageneses, showing that ductile flow character, and produced S foliations b2 the fissure mafic unit was a placed in the crust af- (shear bands, Figs. 3E and 3F), forming a pair R’ ter the main Archean tectonothermal events of the and X (R’ anti-riedel and secondary, Fig. 4H) of Campo Belo Metamorphic Complex. NE/SW and NW/SE directions and high-angle dips. These foliations crosscut the mylonitic gneissic foli- ACKNOWLEDGEMENTS ation (points 9B, 9C, 9F, 9G and 9H; Fig 2). Seen on the XZ plane, these foliations indicate reverse move- The authors wish to thank FONTEX/SA – Orna- ments. With the final phase the emplacement of the mental Rocks Mining Company and FAPEMIG 2.658 Ga old, NW/SE-trending fissure mafic mag- (CRA-827/97; 2220/96) for the financial support matism could be correlated (e.g. Pinese et al. 1995, during field work; Drs. Issamu Endo, Hermínio Pinese 1997) and the 2.612 Ga granitic magmatism, A. Nalini Júnior, Mônica Heilbron and Caroline J. interpreted as the final stage of the Archean plat- Sousa for careful reading and suggestions to this form consolidation (Noce 1995). Regionally this work; CAPES for the M.Sc. scholarship and two event can be correlated to the Rio das Velhas Event anonymous reviewers for their valuable comments 2, with sinistral component (Endo 1997). and suggestions.

Event 5 – The fifth event is a regional metamorphic RESUMO re-equilibration to the greenschist facies, later than Estudos geológicos sistemáticos permitiram a caracteri- the fissure mafic magmatism. Lacking more precise zação de seis unidades litodêmicas na área estudada: três chronostratigraphic indicators to limit its minimum gnáissicas, uma anfibolítica, uma supracrustal e uma má- age, it is suggested that Event 5 be of Transamazo- fica fissural. A assembléia mineral e os registros estru- nian age or younger. turais dessas unidades litodêmicas mostraram que a área estudada foi afetada por cinco eventos tectonotermais. O CONCLUSIONS padrão estrutural do primeiro e último evento ocorreu em From the geologic results discussed above and condições de fácies granulito e revelaram uma cinemática geochronologic data of the southern São Francis- essencialmente sinistral. O segundo evento mostrou uma co Craton, the following conclusions can be formu- tectônica extensional relacionado à abertura da bacia en- lated: 1) the protoliths of the gneisses from the three siálica onde se alojou a seqüência vulcanosedimentar da

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AABC 73 3 t1 412 ARILDO H. OLIVEIRA and MAURÍCIO A. CARNEIRO

unidade supracrustal. O terceiro evento, que é o mais ex- Carneiro MA, Teixeira W, Carvalho Júnior IM pressivo na região estudada, é o responsável por um in- de, Oliveira AH de and Fernandes RA. 1997a. tenso processo de migmatização regional e pela geração Archean Sm/Nd isochron age from the Ribeirão dos da Zona de Cisalhamento Cláudio (de cinemática dex- Motas layered rocks sequence, southern São Fran- cisco Craton, Brazil. In: South-American Sym- tral). O quarto evento é representado por um magmatismo posium on Isotope Geology, Campos do Jordão. máfico fissural (provavelmente dois tipos diferentes de Anais..., Campos do Jordão, 1997. p. 63-64. enxames de diques máficos) e, finalmente o quinto evento é um retrometamorfismo regional que atingiu a fácies xisto Carneiro MA, Teixeira W, Nalini Júnior HA, Car- verde, finalizando os principais processos de evolução tec- valho Júnior IM de, Oliveira AH de, Fernan- tônica do Complexo Metamórfico Campo Belo. des RA and Moutte J. 1997b. A seqüência aca- madada de Ribeirão dos Motas. In: Simpósio Sul- Palavras-chave: Cráton, Arqueano, evolução tectônica, Americano de Geologia Isotópica, Campos do unidades litodêmicas, Complexo Metamórfico. Jordão, Guia de excursões..., Campos do Jordão, 1997. p. 83-86.

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