Central Ribeira Belt Research Articles

Boron isotope composition of tourmalinite and vein tourmalines associated with gold mineralization, Serra do Itaberaba Group, central Ribeira Belt, SE Brazil

Important concentrations of tourmaline occur as gold-bearing stratiform tourmalinites and in mineralized quartz–tourmaline veins at the Tapera Grande and Quartzito gold prospects in the Mesoproterozoic Serra do Itaberaba Group, central Ribeira Belt (São Paulo State, SE Brazil). The main rock types in both prospects constitute the volcanic–sedimentary Morro da Pedra Preta Formation, which formed in a submarine back-arc setting. At Tapera Grande, the volcanic–sedimentary sequence is composed of metabasic and metavolcaniclastic rocks, graphitic and sulfur-rich metapelites, banded iron formation, metandesite, metarhyolite, calc-silicates, tourmalinites and metahydrothermalites derived from mafic and felsic rocks. The Mesoproterozoic rocks at Quartzito prospect are lithologically similar but they have been affected by Neoproterozoic faulting and shearing and by the emplacement of granitic rocks, resulting in the formation of tourmaline-rich quartz–carbonate veins with gold and base metal mineralization. We conducted a chemical and B-isotope study of tourmalines in order to better understand the origin of the stratiform tourmalinites in the Morro da Pedra Preta Formation and their relationship with gold mineralization. The overall range of δ 11B values obtained for the tourmalinite and vein tourmalines is between − 15‰ and − 5‰, with the tourmalinites falling at the low end of this range (− 15 to − 8‰). Such values are typical for continental crust and inconsistent with a primary marine boron signature as expected from the submarine-exhalative model for the gold prospects. We conclude from this that tourmaline formed or recrystallized from crustal fluids related to the amphibolite-grade metamorphism which affected the Serra do Itaberaba Group and that gold deposition occurred syn- to post-peak metamorphism by phase immiscibility, as attested by fluid inclusions in Tapera Grande tourmalinite tourmaline and quartz. The vein-hosted tourmalines at Quartzito have isotopically variable boron signatures, with heavier δ 11B values of − 5‰ to − 8‰ for acicular green tourmalines and lighter values (− 15‰ to − 7‰ for light blue, Ti-free tourmaline from quartz–carbonate veins). We attribute the heavier boron to fluids derived from the volcano-sedimentary rocks of marine affinity whereas the lighter boron was contributed by crustal fluids related to the granitoids or metasediments in the continental crust.

Sedimentary provenance in the central Ribeira belt based on laser-ablation ICPMS 207Pb/ 206Pb zircon ages

207Pb/ 206Pb ages from 420 zircons from 9 upper amphibolite and granulite facies quartzites of the post-1.8 Ga metasedimentary cover from the Occidental and Oriental terranes from the Neoproterozoic central Ribeira belt, in eastern Brazil, are discussed. Ages from the Occidental terrane show that Proterozoic ages predominate with a modal class at 2.1–2.2 Ga and that Archean ages are less common and lie within the 2.5–3.6 Ga range. A wider dispersion on Proterozoic ages down to 1.1 Ga and youngest Neoproterozoic ages (550–700 Ma) were also observed and could be related to the Brasiliano/PanAfrican metamorphic events. The data show that detrital sediments of the Occidental terrane were derived predominantly from erosion of a 1.8–2.2 Ga Paleoproterozoic terrane with a small proportion of a 2.6–3.6 Ga Archean crust. Ages from the Oriental terrane are greatly dispersed, but show a significant proportion of ages between 1.7 and 2.1 Ga, although the great majority are between 0.5 and 1.6 Ga. Only 3 out of 144 grains yield Archean ages, between 2.5 and 2.9 Ga. Based on the greatest errors observed on the data from the Oriental Terrane (100–300 Ma), and a continuum of obtained ages between 2072 Ma (Paleoproterozoic) and 517 Ma (Cambrian), the more plausible interpretation is that the dispersion of ages results from partial resetting of the U–Pb system by Brasiliano metamorphism in Proterozoic–Archean zircons, although a Mesoproterozoic source could not be discarded.

Geoquímica e geocronologia do granitóide Barra Alegre, faixa móvel Ribeira, Rio de Janeiro

Orthoderived rocks of the Rio Negro Complex and Serra dos Órgãos batholith are the dominant rocks of the mountain region of Rio de Janeiro State. They are locally associated with paraderived gneisses and belong to the Neoproterozoic Central Ribeira Belt. Between the cities of Bom Jardim and Trajano de Moraes, the NE-SW elongated Barra Alegre granitoid body occurs enveloped by migmatitic gneisses of the Rio Negro Complex and paraderived Imbé Unit. The granitoid is monzogranitic to quartz monzonitic in composition and shows a porphyritic texture. The size and shape of the microcline phenocrysts made possible the definition of two lithofacies in the granitoid body: 1. the Barra Alegre facies presents euhedral to subhedral megacrysts and weak schistossity; 2. the Carijó facies is characterized by augen texture and well developed schistossity. Biotite is the dominant mafic phase, while opaque minerals, zircon, apatite, and allanite are the main accessories minerals. Stretching quartz grains, fold biotite and plagioclase lamellae, and phenocrysts as well matrix recrystalization are the main deformation features. Based on field and petrographic evidences, the Carijó lithofacie is interpreted as a result of heterogeneous deformation from the Barra Alegre lithofacie. The geochemical characteristics along with petrographic and field data, point to a high-K calc-alkaline I-type, mildly peraluminous, character for these rocks. The restrict SiO2 variation indicates a moderately differentiated sequence. The rare earth element patterns of different rocks are similar, displaying LREE enrichment and a smooth Eu negative anomaly. 207Pb/206Pb single zircon evaporation dating yielded a minimum crystallization age of 579 ± 3 Ma for the Barra Alegre granitoid. Such age is concordant with the syn-collisional episode established for the central segment of the Ribeira Belt.

Geoquímica e geocronologia do granitóide Barra Alegre, faixa móvel Ribeira, Rio de Janeiro

Orthoderived rocks of the Rio Negro Complex and Serra dos Orgaos batholith are the dominant rocks of the mountain region of Rio de Janeiro State. They are locally associated with paraderived gneisses and belong to the Neoproterozoic Central Ribeira Belt. Between the cities of Bom Jardim and Trajano de Moraes, the NE-SW elongated Barra Alegre granitoid body occurs enveloped by migmatitic gneisses of the Rio Negro Complex and paraderived Imbe Unit. The granitoid is monzogranitic to quartz monzonitic in composition and shows a porphyritic texture. The size and shape of the microcline phenocrysts made possible the definition of two lithofacies in the granitoid body: 1. the Barra Alegre facies presents euhedral to subhedral megacrysts and weak schistossity; 2. the Carijo facies is characterized by augen texture and well developed schistossity. Biotite is the dominant mafic phase, while opaque minerals, zircon, apatite, and allanite are the main accessories minerals. Stretching quartz grains, fold biotite and plagioclase lamellae, and phenocrysts as well matrix recrystalization are the main deformation features. Based on field and petrographic evidences, the Carijo lithofacie is interpreted as a result of heterogeneous deformation from the Barra Alegre lithofacie. The geochemical characteristics along with petrographic and field data, point to a high-K calc-alkaline I-type, mildly peraluminous, character for these rocks. The restrict SiO2 variation indicates a moderately differentiated sequence. The rare earth element patterns of different rocks are similar, displaying LREE enrichment and a smooth Eu negative anomaly. 207Pb/206Pb single zircon evaporation dating yielded a minimum crystallization age of 579 ± 3 Ma for the Barra Alegre granitoid. Such age is concordant with the syn-collisional episode established for the central segment of the Ribeira Belt.

207Pb / 206Pb-ages of zircons from syn-collisional I-type porphyritic granites of the central Ribeira belt, SE Brazil

In the central segment of the Ribeira belt, southeast Brazil, several foliated porphyritic granitic bodies intrude high-grade migmatitic gneisses of the Andrelândia and Juiz de Fora domains and Embú Complex. Results of geological, geochemical and geochronological investigations of the Maromba, Pedra Selada, Serra do Lagarto and Funil porphyritic I-type granites provide profound similarities, except for the distinct geochemical behavior of the Funil Granite, perhaps related to a different crustal source. These granitoids show similar structural, textural and mineralogical features. Pb-evaporation of single zircons provided ages of 586 ± 6, 579.6 ± 6.3, 586.3 ± 4.8 and 584 ± 5 Ma for the granites, respectively, coincident with the syn-collision I episode of the central Ribeira belt. The intrusion of I-type porphyritic granitoids coeval with the main collisional event has not often been reported in the geological literature. The most common syn-collisional granitic magmatism has normally an S-type signature or even a slightly peraluminous I-type character. However, the occurrence of coeval I- and S-type syn-tectonic granites along the central Ribeira belt, as observed in the investigated area and discussed in this paper is noteworthy.

Deformation regime variations in an arcuate transpressional orogen (Ribeira belt, SE Brazil) imaged by anisotropy of magnetic susceptibility in granulites

The Ribeira belt of southeastern Brazil displays an arcuate shape, with a structural trend that varies from ∼NS in the northern domain to ENE–WSW in the southern domain. This curvature is accompanied by a transition from contraction-dominated to transcurrent-dominated tectonics. The transition in deformation regime is accommodated in the central domain of the belt where granulitic rocks dominate and mineral-stretching lineations are commonly concealed by metamorphic recrystallization. We present anisotropy of magnetic susceptibility (AMS) data from 664 samples from 62 sites in high-temperature gneisses, granulites and migmatites of the transitional, central domain of the belt, with the aim of investigating: (1) how well AMS allows one to map the mineral-stretching lineation and foliations in domains displaying a complex kinematic framework and (2) to investigate the kinematic pattern at the transition between the thrusting dominated and a wrench-faulting dominated orogenic segments. The mean magnetic susceptibility is 7.54×10 −3 SI. The degree of anisotropy varies from 1.32 up to 4.31, with an average value of 1.53. The shape parameter T is generally >0 meaning that the AMS ellipsoid is dominantly oblate. Magnetic lineations and foliations form a consistent pattern correlated with the modification of the structural characteristics observed along the Ribeira belt. In the southern wrench-fault-dominated domain, the magnetic lineation is subhorizontal, parallel to the trend of the steeply dipping magnetic foliation. This correlation with the fabric observed in mylonites suggests that the magnetic fabric is a valid proxy of the tectonic fabric in granulites. Results from the northern domain show that it comprises two sub-domains both displaying a ∼NS-trending magnetic foliation. Eastward, over a broad area, the magnetic foliation is consistently steeply dipping and bears a shallowly to moderately plunging magnetic lineation. Westward, the dip of the foliation decreases progressively and the lineation rotates from nearly parallel to nearly orthogonal to the trend of the foliation, suggesting a progressive transition from transcurrent to contractional tectonics. At the junction between the northern and southern domains, the magnetic fabric is composite: the magnetic foliation displays a constant NNE–SSW orientation, but the magnetic lineation spreads within the foliation plane from parallel to normal to the foliation direction. The AMS pattern mapped in the Central Ribeira belt supports a scenario in which the transcurrent and contractional deformation regimes are coeval, with a transition between the two regimes accommodated progressively over a large transpressional area where both strain regimes were active.

Barrovian and Buchan type metamorphism in the Pan-African Kaoko belt, Namibia: implications for its geotectonic position within the framework of Western Gondwana

The polyphase deformed Pan-African Kaoko belt, Namibia, which is part of the late Neoproterozoic mobile belt system of western Gondwana, consists of pre-Pan-African basement gneisses, Pan-African volcano-sedimentary cover sequences and large granitoid intrusions. The eastern part of the Kaoko belt comprises a nearly complete Barrovian sequence of medium to high temperature/medium pressure metapelitic rocks, which range in grade from greenschist- to upper amphibolite-facies conditions. From east to west, north-northwest to northwest trending subparallel garnet-, staurolite-, kyanite-, and kyanite-sillimanite-muscovite metamorphic zones can be distinguished, whereas the western Kaoko belt is dominated by a high temperature/low pressure Buchan-type metamorphism with sillimanite-K-feldspar- and garnet-cordierite-sillimanite-K-feldspar metamorphic zones. The two metamorphic terranes are tectonically separated by the Puros shear zone. The pressure-temperature path for each metamorphic zone was inferred by using observed mineral assemblages and reaction textures, a combination of various geothermobarometers and, in particular, pseudosections. Peak metamorphic conditions of the Barrovian and Buchan zones indicate peak pressure-temperature ratios of ~20°C/km and 50°C/km, respectively. The Barrovian-type terrane is probably linked to a fold-and-thrust belt, the Buchan-type terrane may be linked to a magmatic arc setting. Similarities between the Kaoko belt in northwest Namibia and the central Ribeira belt in South America might point to a common geological history.

Petrogenesis of the Orthogneisses of the Mantiqueira Complex, Central Ribeira Belt, SE Brazil: An Archaean to Palaeoproterozoic Basement Unit Reworked During the Pan-African Orogeny

The Occidental terrane of the central segment of the Brasiliano-Pan-African Ribeira belt comprises two crustal scale thrust sheets (Andrelândia and Juiz de Fora domains) taken as reworked Neoproterozoic products of the São Francisco cratonic margins. Pre-1.8 Ga orthogneisses and associated rocks of the Mantiqueira Complex comprise the basement for rocks of the Andrelândia Depositional Cycle within the Andrelândia tectonic domain. Geochemical data indicate that the Mantiqueira Complex comprises rocks that can be grouped as follows: intermediate to acid calc-alkaline rocks and a transitional basaltic series. On the basis of quantitative analysis of the lithogeochemical data, these lithotypes cannot be related. Statistical and/or petrological criteria made it possible to define suites and/or groups within each one of those units and to constrain petrogenetic models based mostly on their REE data. Simple least-square regression analysis indicates that the basic rocks are unlikely to constitute a single suite themselves. The results of the geochemical modelling presented in this work suggest that crustal partial melting rather than fractional crystallisation is the most likely petrogenetic process associated with the rocks of the Mantiqueira Complex. The partial melting processes might have taken place under oxidising conditions, typical of tectonic settings associated with the generation of calc-alkaline rocks.

Evolution of the Central Ribeira Belt, Brazil: Implications for the Assembly of West Gondwana

Evolution of the Central Ribeira Belt, Brazil: Implications for the Assembly of West Gondwana

The Mauá granitic massif, Central Ribeira Belt, São Paulo: petrography geochemistry and U-Pb dating

The Maua granitic massif is an elongated body dominated by a porphyritic biotite monzogranite which grades, in its southwestern extremity, to lighter equigranular granite and greisenized (tourmaline)-biotite-muscovite leucogranite. Abundant enclaves can be divided in three types: grey micro granular enclaves, with rounded shapes and igneous textures, are compositionally similar to the enclosing porphyritic granites; dark, rounded, micaceous enclaves have high K/Na, and may correspond to highly assimilated metasedimentary xenoliths; and angulous gneiss xenoliths seem to be fragments of an unexposed type of country rock. The primitive magmas that formed the massif were Zr, P and LREE-saturated, and became progressively enriched in U, Cs, Y, HREE, F and possibly Ta. Geochemical data show that most of the observed compositional variation can be a reflection of crystal fractionation at the level of emplacement. However, other processes such as magma mixing, contamination and post-magmatic alteration seem to respond for local chemical variations. U-Pb monazite dating point to a crystallization age of 588 ± 2 Ma which is ca. 20 myr. younger than those of nearby crust-derived syn-orogenic granites.

U–Pb chronostratigraphy of the granitic magmatism in the Agudos Grandes Batholith (west of São Paulo, Brazil) — implications for the evolution of the Ribeira Belt

U–Pb zircon and monazite dating was used to investigate the emplacement ages of granitic bodies in the Agudos Grandes Batholith, Central Ribeira Belt, southeast Brazil. The most voluminous granite type in the batholith, a syn-orogenic, isotropic to strongly foliated porphyritic hornblende-biotite quartz monzonite to monzogranite (Ibiúna type), was dated at 610±2 Ma. The less abundant Turvo type, a foliated peraluminous (muscovite+biotite±garnet±tourmaline) leucogranite appearing as scarce bodies intruding the Ibiúna type, has an identical U–Pb monazite age (610±1 Ma), suggesting a short duration for the syn-orogenic period in the batholith. The late-orogenic Piedade massif, a zoned elliptical body cropping out in the NW border of the batholith, was also dated and yielded three similar ages in the range 600–605 Ma. A more precise age was obtained for a porphyritic muscovite-bearing facies ( 601±2 Ma in monazite); other two ages, less precise owing to inheritance and Pb loss, were obtained: a porphyritic titanite-bearing biotite granite yielded 604±8 Ma (zircon), and an inequigranular, more fractionated pink biotite granite yielded 605±7 Ma (zircon and monazite). The post-orogenic Serra da Bateia massif, made up of biotite syenogranites with a strong magmatic foliation and A-type geochemical affinities, was dated at 564±8 Ma (zircon). The U–Pb ages are consistent with field relations previously established and show that the evolution of the Agudos Grandes Batholith spanned at least 45 million years. The ages of the syn-orogenic granites (Ibiúna and Turvo types, ca. 610 Ma) are older than those of similar rocks in the northern part of the Mantiqueira Orogenic System (590–565 Ma in the north Ribeira and Araçuaı́ Fold Belt in Rio de Janeiro and Espı́rito Santo States), demonstrating the diachronous character of the orogenies in the belt.

The Juiz De Fora Granulite Complex of the Central Ribeira Belt, SE Brazil: A Paleoproterozoic Crustal Segment Thrust During the Pan-African Orogeny

The granulites of the Juiz de Fora complex occur within thick basement thrust slices associated with the Pan-African shortening process in the central segment of the Ribeira belt. Five lithological units of the Intermediate tectonic domain of the belt can be identified on the basis of detailed geological mapping: a) orthogranulites, b) orthogneisses; c) kinzigite; d) intrusive garnet charnockite and e) amphibolite facies metasediments of probable Meso to Neoproterozoic age, correlated to the cover of the belt. Petrological data indicate high temperatures and intermediate to low lithostatic pressure conditions for the Paleoproterozoic granulite facies metamorphism. Textures and CO 2-rich fluid inclusions are probably related to an IBC path. Geochemical data do not show relevant compositional change as a result of the granulite metamorphism. Two calc-alkaline suites and tholeiitic to alkaline basic rocks can be related to compressional and extensional settings, respectively. The overall composition of the granulites, the lack of substantial LILE depletion as well as the composition of the fluid inclusion points to granulitization process driven by CO 2-rich fluids. Orthogranulites gave rise to banded gneisses as a result of the Pan-African retrograde metamorphism and intense deformation. The U and Th depletion detected in few rocks is possibly related with the hydrated conditions of the retrograde reactions.

Determination of 207Pb/206Pb ages on zircon and monazite by laser-ablation ICPMS and application to a study of sedimentary provenance and metamorphism in southeastern Brazil

The recently developed method of laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) allows the determination of 207Pb/206Pb ages of single zircon grains. The main advantages of the method are minimal sample preparation, low cost, and high throughput. In this work we present an analytical routine for geochronological analyses of zircon and monazite by LA-ICPMS and its application to the Ribeira Belt of the Brazilian Orogen in southeastern Brazil. The 207Pb/206Pb ages of one hundred and thirty-seven detrital zircons from amphibolite facies quartzites from three lithotectonic domains in the central Ribeira Belt indicate that they are derived mainly from Paleoproterozoic crust of Transamazonian age (2.0−2.3 Ga). A small number of zircons originated in 2.6−2.9 Ga Archean crust. These results are coherent with 2.1−2.2 Ga and 2.6−3.0 Ga UPb ages previously obtained for basement gneisses. The viability of the method to date monazite is also assessed. Monazites from the same quartzite samples yield ages between 2.1 Ga and 0.57 Ga. indicating variable resetting of the UPb system during amphibolite facies metamorphism. In contrast, monazite from a basement migmatite and syn-metamorphic granitoids yields ages in the 500–700 Ma range, in general agreement with U-Pb ages of 590-565 Ma for the main metamorphic event.The LA-ICPMS 207Pb/206Pb ages are coherent and agree with expected results based on previous UPb geochronology, and show that the method has immediate applicability. At present, the most significant limitations of the method are the inability to yield reliable U/Pb values, analytical precision in the 1–10% range, and the requirement of grains larger than 80 gmm The method may be advantageous for provenance studies of Precambrian detrital sequences.

UPb geochronology of the central Ribeira belt (Brazil) and implications for the evolution of the Brazilian Orogeny

The Ribeira belt in southeastern Brazil was generated during the Brazilian Orogeny, the South American equivalent of the Pan-African Orogeny (∼ 700-450 Ma). The central sector of the belt is characterised by a series of northwest-vergent crustal-scale thrust stacks emplaced under amphibolite-grade conditions. Major shear zones separate four lithotectonic domains: Andrelândia to the northwest, Juiz de Fora, Paraíba and Coastal domains. Analyses of zircon, monazite and titanite from a variety of rock types from the four domains lead to the following conclusions. All domains record an important tectono-thermal event at 590-565 Ma, represented by partial melting, granitoid intrusion and remobilisation of older basement gneisses, and corresponding to thrusting and development of dextral shear zones. Also recorded are older metamorphic ages (611-604 Ma) of unknown significance. Only the Paraíba and Coastal domains, in the south, record a younger metamorphic event (M2) at 535-520 Ma most likely associated with another episode of thrusting and shear zone development. The absence of M2 metamorphism in the Andrelândia and Juiz de Fora domains can be explained by post-535-520 Ma emplacement of the Paraíba domain. Late- to post-tectonic activity is represented by 503-492 Ma metamorphism. Precursors of the basement gneisses are 2185-2134 Ma old rocks generated during the Transamazonian Orogeny some of which contain a significant Archean component with minimum ages of 2981-2846 Ma. The ages presented above define specific metamorphic events for the Brazilian Orogeny for the first time in southeastern Brazil. In addition, together with previously published data they indicate that the basement rocks found in the allochthonous domains of the central Ribeira belt represent an extension of the units present in the southern São Francisco craton and indicate that the foreland was more extensive than the presently defined ‘craton’.