Ribeira Orogen Research Articles

Microstructural and metamorphic evolution of the Carrancas Klippe, interference zone of the Neoproterozoic southern Brasilia and Ribeira orogens, SE Brazil

The Carrancas Klippe is the basal structure of a nappe system, at the southwestern edge of the São Francisco Craton, in the southern part of the Southern Brasília Orogen. The klippe comprises three metasedimentary units: from base to top, 1) a micaceous quartzite (São Tomé das Letras Unit), 2) chloritoid phyllite grading to staurolite schist with quartzite and gray phyllite intercalations (Campestre Unit), and 3) biotite schist (Santo Antônio Unit), all belonging to the Neoproterozoic Andrelândia Group. This paper describes the results of detailed mapping, structural and metamorphic analyses, including microtectonics, mineral chemistry and related geothermobarometry, and Sm–Nd dating of garnet. Deformational structures were grouped into three sets, D1, D2 and D3. D1 generated thrusting to the east-southeast with associated stretching lineation and tight recumbent folds. D2 produced tight to close asymmetrical folds with axial planes dipping to the SSW, S and SE, and subhorizontal axes. Steep north-south trending axial planes and shallow fold axes characterize open D3 folds and crenulations. In the northwestern part of the klippe garnet chloritoid phyllites formed during upper greenschist facies metamorphism of D1 age with syn-D1 snowball garnet. Towards the southeast the phyllites grade into amphibolite facies schists with syn-D2 garnet and staurolite. Geothermobarometric calculations based on mineral chemical compositions resulted in the following average values for D1 equilibria: 470 ± 45 °C and 12.3 ± 1 kbar. For D2 the following average values were calculated: 580 ± 35 °C and 8.4 ± 1.4 kbar. An age of 562 ± 24 Ma was obtained by Sm–Nd dating of D2 garnet growth. In conclusion, D1 is characterized by east to southeastward thrusting, including the emplacement of the klippe, related to the Southern Brasília Orogen tectonics, at upper greenschist facies conditions with relatively high pressures. During D2, the direction of tectonic transport changed to north, under greenschist facies conditions in the northwest and medium pressure amphibolite facies conditions in the southeast, producing staurolite and kyanite isograds that cut the klippe structure. This phase is attributed to the NNW-SSE shortening in the Ribeira Orogen. D3 reflects late E-W shortening also recorded in major NE-SW dextral shear zones under upper greenschist facies conditions with lower pressures.

Provenance shift through time in superposed basins: From Early Cryogenian glaciomarine to Late Ediacaran orogenic sedimentations (Araçuaí Orogen, SE Brazil)

Records of Precambrian glaciation have been reported from southeastern Brazil for over a century. We present sedimentological, stratigraphic and isotopic (U-Pb and Lu-Hf on detrital zircons, C and O on carbonates) studies on diamictite-rich to diamictite-free successions of the Araçuaí Orogen, the Brazilian counterpart of the Araçuaí – West Congo Orogenic System (AWCO). From base to top, the Chapada Acauã Formation (Macaúbas Group) includes a diamictite-rich unit, with lenses of graded sandstone and clast-supported conglomerate, that gradually passes to graded sandstone, pelite with sparse oversized clasts and rare carbonate lenses on top, representing mass flow and turbidity current deposits of submarine fan, followed by finer-grained turbiditic sedimentation with iceberg discharges along fan fringes, from glaciomarine to post-glacial scenarios. The Salinas Formation comprises a deep-sea sand-mud sequence composed of pelites and pelitic wackes. The Chapada Acauã Formation was deposited in-between 750 and 667 Ma. It shows wide spectra of zircon ages (typical of continental rift to passive margin settings) that start around 3.2 Ga and display main age peaks indicating sediment provenance from the Rhyacian-Orosirian basement (εHf(t) = +14.6 to −18.5) and Early Tonian anorogenic rocks (950–880 Ma; εHf(t) = −3.2 to −23.2) for the diamictite-rich lower unit and, for the upper unit, also in Early Cryogenian anorogenic rocks (ca. 715 Ma, εHf(t) = −9.6). In contrast, the Salinas Formation shows most ages in-between 676 and 620 Ma and a maximum sedimentation age around 551 Ma, with εHf(t) from +6.9 to −18.2, unravelling an important shifting of sediment provenance to sources located in magmatic arcs and collisional granites of the Araçuaí and Ribeira orogens. The Early Cryogenian age and negative Hf signature for the glaciomarine Chapada Acauã Formation point to sediment sources in the 720–670 Ma anorogenic igneous rocks found in AWCO and adjacent Congo – São Francisco Craton, and suggest correlation with the Sturtian global glaciation.

The Neoproterozoic Andrelândia group: Evolution from an intraplate continental margin to an early collisional basin south of the São francisco craton, Brazil

New U–Pb and Lu–Hf zircon data for the Andrelândia Group are presented and discussed with the previously published data to propose an evolutionary model. The Andrelândia Group consists of two depositional sequences that record the geotectonic evolution from Tonian to Ediacaran times. Both sequences are part of the Andrelândia Nappe System, a fold-and-thrust system, generated in the 630-600 Ma interval, during the evolution of the Neoproterozoic convergence in the interference zone between the Southern Brasília and the Ribeira orogens, SE Brazil. The Andrelândia Group rocks record greenschist to granulite facies metamorphism related to this convergence. The lower sequence consists of paragneiss, metapelite, quartzite, metachert, calc-silicate and mafic rocks distributed in five lithofacies associations. They record shallow to deep water sedimentation coeval with mafic magmatism at the southern border of the São Francisco Craton. U–Pb data obtained from detrital zircon grains of deep water quartzites indicate a maximum depositional age of ca. 970 Ma. The U–Pb age peaks can be correlated to major events in the adjacent São Francisco paleocontinent. εHft isotopic signature obtained in this study shows a predominance of juvenile sources for the Mesoproterozoic and crustal reworking and juvenile additions for the Paleoproterozoic. This can be related to source rocks from the Espinhaço Supergroup and from the Mineiro Belt, respectively. We interpret our data together with previous published detrital zircon data and whole-rock εNdt signatures to suggest that the lower sequence of the Andrelândia Group is related to a passive margin of the São Francisco paleocontinent with a platformal and a deep water succession with roughly similar sources, indicating likely a resedimentation process. The upper sequence of the Andrelândia Group consists of a monotonous package of biotite schist interpreted as metamorphosed plagioclase sandstones and siltstones. The upper sequence covers the lower sequence units unconformably. Deposition occurred in a relatively deep and fast subsiding basin during the early collisional stage of the southern Brasília Orogen. Previously published U–Pb, Sm–Nd and Lu–Hf data for the upper sequence constrain likely sources in the southern São Francisco Craton and possibly in the early stages of the Neoproterozoic Socorro-Guaxupé arc, established at the eastern margin of the Paranapanema paleocontinent, which collided with the São Francisco paleocontinent during West Gondwana assembly.

Magmatic activity period and estimation of P-T metamorphic conditions of pre-collisional opx-metatonalite from Araçuaí-Ribeira orogens boundary, SE Brazil

The southern region of the Espírito Santo State is located in the Araçuaí-Ribeira Orogen System (AROS) limit, where neoproterozoic orthoderived rocks undergone metamorphism at high-grade condition exhibiting partial melting and deformation features. This study focuses on the Pre-collisional Opx-granitoids Unit that shows varied proportions of orthopyroxene and garnet. Petrographic, geochemical, geothermobarometric and U-Pb in zircon studies were carried out. We distinguished two mineral assemblages: (i) consisting of plagioclase, quartz, biotite, amphibole, orthopyroxene and ilmenite; and (ii) with plagioclase, quartz, orthoclase, biotite, amphibole, garnet and ilmenite. The former is related to the opx-metatonalite protolith and the latter to metamorphic paragenesis. The geochemical analyses of the Pre-collisional Opx-granitoids Unit reveal an arc-related environment, similar to the Rio Doce and Rio Negro - Serra da Prata Arcs defined in the Araçuaí and Ribeira orogens, respectively. The temperature conditions obtained for assemblage (ii) were 670–860 °C (granulite facies) and pressure from 5.24 to 10.2 kbar. Four different intervals of concordant ages were yielded in the studied samples. The oldest age of 716 Ma is interpreted as related to the pluton emplacement (E1). The ages of 604 Ma and 595 Ma are associated with the same magmatic event (E2), and the magmatic age of 586 Ma (E3) indicates another magma injection event. The age of 574 Ma (E4) is related to a high-grade metamorphic event whose P-T conditions were obtained in assemblage (ii) from geothermobarometric calculations. These results confirm the extension of Rio Doce magmatic arc toward the south and establish a connection between Araçuaí and Ribeira orogens.

Proterozoic to Ordovician geology and tectonic evolution of Rio de Janeiro State, SE-Brazil: insights on the central Ribeira Orogen from the new 1:400,000 scale geologic map

Abstract A comprehensive tectonic evolution of Proterozoic to Cambrian rocks that integrate the crystalline basement of Rio de Janeiro State, based on the new geological map, is presented. The map combines detailed geological data integrated to a 1:400.000 scale and new geochemical and geochronology data to characterize the distinct terranes amalgamated during three pulses of the Brasiliano orogeny. The Occidental terrane comprises the Tonian to Late Cryogenean passive margin of the Sao Francisco paleocontinent, including reworked cratonic basement. At the same period, two magmatic arc systems were developed outboard: The Continental Inner arc system represented by the dismembered terranes (Paraiba do Sul, Embu, and Cambuci domain) integrated now in the Central Superterrane: and the Intra-oceanic Outer Arc System, the Oriental Terrane, developed from 860 to 620 Ma. Related active basins were detected in association with the two arc systems that collided at ca. 620 - 595 and 605 - 550 Ma against the Sao Francisco passive margin. A later collision episode at ca. 535 to 510 Ma brought the Cabo Frio Terrane (Angola tip) and closed the Buzios back-arc basin. Finally, the collapse of the orogenic belt, probably related to slab detachment and asthenosphere upwelling marks transition to Gondwana stabilization.

São Bento do Sapucaí Shear Zone: Constraining age and P-T conditions of a collisional Neoproterozoic oblique shear zone, Ribeira Orogen, Brazil

The São Bento do Sapucaí Shear Zone is a subvertical NE-SW anastomosing shear zone of at least 100 km length and few km wide. It affects orthogneisses, paragneisses and igneous rocks of the Socorro-Guaxupé Nappe, the uppermost nappe of a nappe stack, which constitutes the Southern Brasília Orogen, part of a Neoproterozoic orogenic system surrounding the São Francisco Craton. The Socorro-Guaxupé Nappe is interpreted as the root of a magmatic arc. The nappe stack is partly affected by superposed deformation related to the slightly younger Ribeira Orogen. The mylonitic foliation of the shear zone dips steeply to both SE and NW, and contains a stretching lineation with shallow plunge predominantly to NE and locally to SW. Kinematic indicators including sigma and delta objects, asymmetric mica fish and synthetic shear bands point to oblique dextral movement with a SE-up component. Evidence of quartz and feldspar recrystallization-mechanisms was applied to estimate temperature during mylonitization as between 450 °C and 650 °C. The geothermobarometry calculated from the composition of mineral rims, partially or fully recrystallized during mylonitization, yielded a temperature of 479 ± 66 °C and a pressure of 2.94 ± 1.56 kbar. Geochronological investigation by LA-ICP-MS of U–Pb in zircon and monazite was applied in order to constrain the age of rocks which were affected by, or cross-cut the shear zone. Metamorphic rims on detrital zircon grains collected from a paragneiss indicate a main metamorphic episode at ca. 613 Ma. Analysis of a gabbronorite affected by the shear zone yielded a crystallization age of ca. 610 Ma; a pegmatitic leucogranite which cross-cuts the shear zone, produced various zircon ages between 656 and 611 Ma, interpreted as inheritance. Monazite crystals of the same leucogranite yielded an age of 552 Ma, interpreted as the age of crystallization. Hence the age of the shear zone is constrained between 610 and 552 Ma. The data produced in this study fit well the proposed evolutionary history of the Socorro-Guaxupé Nappe, where a magmatic arc developed between 680 and 630 Ma, followed by continental collision and associated metamorphism ranging from 630 to 600 Ma. The younger shear zone is related to the tectonics of the superimposed Ribeira Orogen in the time span 600-560 Ma.

Coeval high Ba-Sr arc-related and OIB Neoproterozoic rocks linking pre-collisional magmatism of the Ribeira and Araçuaí orogenic belts, SE-Brazil

Two arc systems have been described within the Neoproterozoic Ribeira Belt: the inner continental magmatic arc and the outer intra-ocean magmatic arc. New geochemical and U-Pb and Lu-Hf (LA-ICPMS) data from zircons of the Marceleza-Leopoldina stocks, belonging to the inner magmatic arc system, in the boundary region between the Ribeira and Araçuaí orogenic belts are presented. The intermediate to felsic magmatic rocks have high Ba, Sr, K/Rb, LILE and LREE contents. Low Rb, Th, U, Nb, and very low Y and HREE contents point to their generation in a supra-subduction tectonic scenario, from melting of the asthenospheric wedge above a subducting oceanic plate. Zircon U-Pb (LA-ICPMS) data indicates crystallization ages between ca. 650 and 600 Ma, with high-grade metamorphic overprint at ca. 582 Ma. Tonian inherited zircons were found in the older rocks, suggesting that the development of the continental magmatic arc might have begun earlier. Associated basic (gabbros and monzogabbros) to intermediate (diorites and monzodiorites) rocks are geochemically heterogeneous. Some represent less evolved calc-alkaline basaltic magmas, while relatively more enriched varieties are intraplate basalts with OIB components. Together with lithogeochemical data, Lu-Hf signatures showing TDM model ages between ca. 1.7 to 1.3 Ga, with initial ɛHf values ranging from slightly negative values (−0.9 to −3.75) to strongly negative (up to −10.4) indicate a magmatic arc developed on continental margin environment (i.e., continental-type arc). The studied rocks represent the Rio Doce arc in the studied region and are correlated with other arc segments located from the north, in the Araçuaí Orogen, to the southernmost Ribeira Orogen, defining a c. 1000 km long continental-type magmatic arc. This inner arc system first collided against the São Francisco craton during the Neoproterozoic Brasiliano orogeny around 580 Ma. The geochemical diversity of coeval intermediate to felsic rocks and the variability of associated basic rocks are suggestive of a complex evolution of the subduction zone system. Possible mechanisms for the enrichment in incompatible elements of magmatic products may have involved slab roll-back and subduction of fore-arc rocks, and the influx of ascending asthenosphere associated with slab windows (slab break off, ridge subduction) prior to collision.

Structural analysis combined with new geothermobarometric and geochronological results of the Além Paraíba shear zone, between Três Rios and Bananal, Ribeira Orogen, SE Brazil

The Além Paraíba Shear Zone (APSZ) is composed of subvertical mylonites trending ENE/WSW with subhorizontal stretching lineation indicative of dextral movement. In the western part of the area mapped in this paper the shear zone, defined as an elongated body of mylonites, dies out. This constrains the possible displacement along the zone. The mylonites are of relatively high metamorphic grade, product of deformed orthogneisses, granulites, paragneisses and associated syn-tectonic granitoids. These rocks reached granulite facies conditions at temperatures and pressures in excess of 850 °C and 8 kbar. Geothermobarometric results indicate that the progressive ductile deformation related to the shear zone occurred under amphibolite facies conditions at temperatures of 714 ± 23 °C to 562 ± 38 °C and pressures between 7.1 ± 0.8 kbar and 4.3 ± 1.1 kbar. Zircon U-Pb dates by LA-ICPMS were obtained from three samples, one from a mylonitic orthogneiss with age 583.6 ± 7.1 Ma, one from a mylonitic paragneiss with age 591.5 ± 6.1 Ma, and one from a pegmatitic vein that cuts the mylonites obliquely and yields an age of 597.7 ± 9.4 Ma. Monazite crystals from the mylonitic orthogneiss were also analyzed by LA-ICPMS U-Pb, resulting in a spread of concordant dates ranging from 611 to 553 Ma. Taking these data together, we estimate the age of peak metamorphism to be between 595 and 590 Ma. The age of the monazite crystals suggests continuous crystallization and/or re-equilibration after the metamorphic peak, probably catalyzed by the activity of the shear zone and/or fluid migration under amphibolite facies conditions. Since the APSZ affects a 535 Ma granite, we argue that the shear zone was probably active in the time span between 580 and 530 Ma, after metamorphic peak conditions but still at relatively high temperatures. The temporal coincidence with the main collision in the Ribeira Orogen supports the idea that the shear zone is the result of oblique collision.

The Rio Doce Group revisited: An Ediacaran arc-related volcano-sedimentary basin, Araçuaí orogen (SE Brazil)

The Araçuaí orogen developed between the São Francisco and Congo cratons, during the Western Gondwana amalgamation in Late Neoproterozoic time. Located in the eastern Araçuaí orogen, the Ediacaran Rio Doce magmatic arc includes a large amount of plutonic rocks (the G1 supersuite, ca. 630-580 Ma) spatially associated with a supracrustal succession, the Rio Doce Group (RDG). This group, poorly studied until recently, encompasses four formations (from base to top): Palmital do Sul, Tumiritinga, São Tomé, and João Pinto. The Palmital do Sul and Tumiritinga formations mostly include pelitic schists to paragneisses, interbedded with metavolcanic and meta-volcaniclastic rocks, representing proximal deposits in relation to the volcanic arc front. Pyroclastic deposits represented by lapilli-rich meta-tuffs with scattered volcanic bombs denote explosive volcanism of dacitic to rhyolitic composition. Zircon U-Pb data provide magmatic crystallization ages of 595 ± 13 Ma and ca. 584 ± 5 Ma for the volcanic protoliths. The São Tomé Formation mainly includes psammitic to pelitic meta-turbidites with lenses of calcsilicate rocks, indicating relatively distal marine deposits with a maximum depositional age of ca. 594 Ma and a striking arc-related lithochemical signature. The João Pinto Formation mostly includes quartz sandstones and micaceous to feldspathic sandstones, with a maximum sedimentation age of ca. 620 Ma, representing fluvial to continental shelf environments related to the late stages of the Rio Doce arc development. The Rio Doce Group can be correlated with other orogenic supracrustal successions (i.e., parts of the Andrelândia Group) found in the region covered by the Rio Doce arc, connecting the Araçuaí and Ribeira orogens.

The Pico do Itapeva Formation: A record of gravitational flow deposits in an Ediacaran intracontinental basin, southern Brasília Orogen, SE Brazil

The Pico do Itapeva Formation is a key metasedimentary unit to the understanding of the extensional events that occurred between the late stage of the southern Brasília Orogen collision and the main collision in the central Ribeira Orogen. The formation crops out in a 20 km long NE-trending narrow belt in the Mantiqueira mountain range in eastern São Paulo State, Brazil. It is located in the interference zone of the southern Brasília and the central Ribeira orogens and records deformation and greenschist facies metamorphism (biotite zone) related to the Brasiliano orogeny. The Pico do Itapeva Formation rests unconformably on a metaigneous substratum of the Socorro-Guaxupé Nappe/Embu Terrane and, on the southern side, is truncated by a steep SE-dipping dextral reverse shear zone. It consists of a coarsening- and thickening-upward succession, with minimum thickness of 800 m, composed of lutite, arkose and conglomerate. These rocks constitute three distinct lithofacies associations: LAI- arkose, arkose-lutite composite beds, lutite and fine conglomerate beds; LAII- arkose, pebbly arkose and scarce lutite and; LAIII- conglomerate and pebbly arkose. Most of the beds are massive; graded beds, dish and convolute structures occur locally. Bed thickness varies from thin to very thick and amalgamated bodies constitute up to 30m thick strata. Three mappable units at scale 1:20,000 were recognized based on different proportions of the three lithofacies associations. The deposits are interpreted as the record of mass flows and associated processes in a fan delta setting developed in an intermontane rift basin. U-Pb LA-ICP-MS detrital zircon ages suggest the maximum depositional age at ca. 611 Ma and the basin evolution is interpreted in the range between 611 and 580 Ma during an inter-orogenic stage between the Brasília and Ribeira orogenies.

Evidence of a Neoproterozoic active continental margin - Geochemistry and isotope geology of high-grade paragneiss from the Ribeira Orogen, SE Brazil

Ediacaran paragneisses from the Palmital Unit are located in a key region, between two major tectonic domains of the Ribeira Orogen (in Rio de Janeiro, SE Brazil): the Cabo Frio Tectonic Domain and the Oriental Terrane. We present here petrographic, geochemical and isotopic data in order to partially unravel the origin and tectonic nature of the protoliths from these metamorphic rocks. Litharenites interpreted as immature sediments, mostly derived from the erosion of felsic rocks (granites/rhyolites and diorites/andesites) are here described. Multi-elements patterns and trace elements ratios reinforce an upper continental crust nature for the composition of the protoliths. These were probably located close to the source area and accommodated in semi-arid climate and high topography conditions. Tectonic discrimination diagrams indicate that the Palmital basin developed in an active continental margin, corroborated by the zircon detrital spectra. The main population of detrital zircon (ca. 750-550 Ma) is partially coeval with the age of the Rio Negro continental magmatic arc, resident in the Oriental Terrane. The Palmital basin could represent a forearc environment with no oceanic crust material, but only a continuous sedimentation of turbidites derived from the arc, with gradational bedding signifying a subaqueous environment, without outside tectonic disturbances. On the other hand, TDM ages of 1.6–1.8 Ga suggest that these sediments are not juvenile, indicating also a contribution from an ancient crust. This recycled continental crust could come either from the basement of the Oriental Terrane (which was not identified yet) or from the basement of the Cabo Frio Tectonic Domain. In the last assumption the Palmital deposition would be concomitant with the initiation of continental collision and the subduction of the passive margin of the Cabo Frio Tectonic Domain towards west. This unit was subsequently metamorphosed/deformed during the ca. 540 Ma collision between both terranes.

Constraining timing and P-T conditions of continental collision and late overprinting in the Southern Brasília Orogen (SE-Brazil): U-Pb zircon ages and geothermobarometry of the Andrelândia Nappe System

The Andrelândia Nappe System (ANS) is an important component of the Neoproterozoic Southern Brasília Orogen (SBO) in SE Brazil and its evolution is related to the convergence and subsequent collision between the passive margin of the São Francisco paleocontinent and Paranapanema block during the Brasiliano Orogeny. The ANS is mainly composed of a pile of deformed meta-sedimentary rocks that record an inverted metamorphic pattern, ranging respectively from amphibolite facies at the bottom to high-P granulite conditions at the top. The ANS is here simplified into the lower, middle and upper nappes. Four samples from different tectonic settings within the ANS were collected for zircon isotopic investigation. In addition, geothermobarometry modelling were performed on seven samples from the middle nappe. Retro-eclogite slices associated to basement gneissic rocks occur along the contact between the upper and middle nappes and have zircon grains with metamorphic rims grown in high-P conditions of 12–16kbar with temperatures of ∼700–800°C, during the continental subduction dated here by U-Pb SHRIMP in zircon at 625±6Ma. A period of nappe emplacement accompanied by melting in the middle nappe is evidenced by a garnet-muscovite-staurolite-kyanite schist with leucosome parallel to the main foliation dated by U-Pb SHRIMP in zircon at 618±5Ma. Re-equilibrium at pressures of 4–7kbar with temperatures of ∼500–700°C is ascribed to a second heat pulse under lower pressure conditions. The period of melting associated to this event in the middle nappe occurred at 586±9Ma and is marked by an anatectic tourmaline-garnet-muscovite bearing S-type granite. This granite body is part of a NE-SW belt of granites, with ages between 605 and 563Ma and covering both the Central Ribeira Orogen (CRO) and the SBO. The age and the alignment of this granite belt justify the interpretation that it is genetically related to the CRO. Thus, the new data provide evidence that tectonothermal events related to the collisional front of the CRO extended up to ∼200km overlapping the consolidated SBO at ca. 580Ma.

Proveniência dos arenitos da Bacia de Campos (Andar Alagoas) por meio da composição da granada

Neste trabalho, realizou-se um estudo da composição química da granada para a identificação das principais rochas-fonte envolvidas na proveniência dos arenitos da Bacia de Campos, Andar Alagoas. Em análises obtidas por microssonda eletrônica, dos 17 poços do segmento offshore do estudo, verificou-se a contribuição de diferentes rochas-fonte portadoras de granadas, identificadas como dos tipos A, B e C. Entre elas, predominaram as granadas do tipo B. Um refinamento dessa classificação permitiu categorizar tais granadas em granadas do tipo Bi, derivadas de rochas ígneas ácidas e intermediárias, notadamente granitoides. A ausência ou pequena incidência de zonações composicionais e de inclusões, embora não excluam totalmente as outras categorias, apontam para essa classificação. Granadas do tipo D e E não aparecem em nenhuma das amostras analisadas. A interpretação dos resultados de aproximadamente 4.000 análises das 65 amostras indicou duas contribuições principais para a proveniência desses arenitos: no setor sul da bacia, material derivado do terreno Cabo Frio (rochas metamórficas da fácies anfibolito superior a granulitos, granitos, metapelitos aluminosos e secundariamente metabasitos) e, na porção norte da bacia, derivação do terreno oriental do Orógeno Ribeira (terreno de alto grau e granitoides associados). A direção de suplemento sedimentar mais importante é de sudoeste para nordeste, interpretada como resultado da identificação dessa maior contribuição do terreno-fonte. Os arenitos localizados no setor norte, com granadas derivadas do tipo B e Bi, sobretudo, indicam a presença dominante de rochas granitoides na área-fonte. Essas rochas-fonte geram areias de composição mais quartzosas e/ou quartzo-feldspáticas, com maior potencial para produzirem arenitos-reservatórios de melhor qualidade.

The tectonic significance of the Cabo Frio Tectonic Domain in the SE Brazilian margin: a Paleoproterozoic through Cretaceous saga of a reworked continental margin

ABSTRACT: The Cabo Frio Tectonic Domain is composed of a Paleoproterozoic basement tectonically interleaved with Neoproterozoic supracrustal rocks (Buzios-Palmital successions). It is in contact with the Neoproterozoic-Cambrian Ribeira Orogen along the SE Brazilian coast. The basement was part of at least three continental margins: (a) 1.97 Ga; (b) 0.59 - 0.53 Ga; (c) 0.14 Ga to today. It consists of continental magmatic arc rocks of 1.99 to 1.94 Ga. Zircon cores show a 2.5 - 2.6 Ga inheritance from the ancient margin of the Congo Craton. During the Ediacaran, this domain was thinned and intruded by tholeiitic mafic dykes during the development of an oceanic basin at ca. 0.59 Ma. After the tectonic inversion, these basin deposits reached high P-T metamorphic conditions, by subduction of the oceanic lithosphere, and were later exhumed as nappes over the basement. The Cabo Frio Tectonic Domain collided with the arc domain of the Ribeira Orogen at ca. 0.54 Ga. It is not an exotic block, but the eastern transition between this orogen and the Congo Craton. Almost 400 m.y. later, the South Atlantic rift zone followed roughly this suture, not coincidently. It shows how the Cabo Frio Tectonic Domain was reactivated as a continental margin in successive extensional and convergent events through geological time.

Unraveling the tectonic evolution of a Neoproterozoic–Cambrian active margin in the Ribeira Orogen (Se Brazil): U–Pb and Lu–Hf provenance data

The Neoproterozoic–Ordovician Central Ribeira Orogen (southeastern portion), in SE Brazil, presents two contrasting tectonic domains in its southern portion: (a) the arc domain constituted of Neoproterozoic to Paleozoic magmatic rocks and low P-high T metamorphic para (São Fidelis Group)- and ortho-derived units (in Oriental Terrane); and (b) the basement domain, constituted of a Paleoproterozoic and Neoproterozoic medium P-high T metamorphic para (Palmital–Búzios succession)- and ortho-derived units (in Cabo Frio Tectonic Domain). Our work focuses on paraderived rocks sequences from both domains. The provenance analysis using U–Pb and Lu–Hf in zircon grains is presented here as an effective tool to unravel the paleogeography and nature of the pre-collisional sedimentary basins. We performed 505 analyses (U–Pb) on detrital zircon grains and some metamorphic overgrowths from six paragneiss samples. Besides, 141 analyses (Lu–Hf) in six samples only on the detrital zircon grains domains. All samples present a main peak from Neoproterozoic sources (750–570Ma) and the other minor peak in the Stenian/Tonian periods (1200–850Ma), this indicate an orogenic contribution for this basin. Scarce register from the Mesoproterozoic and two peaks in the Archean/Paleoproterozoic (2.6 and 1.9Ga) are recognized as a contribution from an ancient continent. The Lu–Hf data reveals a juvenile source for the detrital zircon grains from Buzios succession while Palmital and São Fidelis Group units show a main crustal signature for their detrital zircon population. Based on the U–Pb and Lu–Hf data presented here, plus petrological data, geological correlations, and compilation of data from literature, we propose a tectonic model for the origin of para-derived rocks from the eastern part of the Ribeira Orogen. In at least two stages, starting with an extensional environment of ca. 600Ma in a back-arc basin (Buzios succession deposition) and continuing as an active margin between 570 and 550Ma in the fore-arc and prism basin (São Fidelis Group and Palmital succession).

Rochas charnockíticas do sudeste de Minas Gerais: a raiz granulítica do arco magmático do Orógeno Araçuaí

The Aracuai and Ribeira orogens are a Cambrian-Neoproterozoic belt that extends from the eastern edge of the San Francisco craton to the Atlantic Ocean, south of parallel 15 °. The connection between these orogens is roughly situated along the 21 ° S parallel. High-grade metamorphic rocks are exposed in the southern Aracuai Belt, SE Minas Gerais, reflecting a deep crustal level. The Divino Suite comprises mafic- to felsisc charnockites, metamorphosed in granulite facies, interpreted as an expanded calc-alkaline suite metaluminous to slightly peraluminous. Chemical and Nd isotopic signatures suggest the Divino Suite evolved in a continental magmatic-arc setting, and also that Paleoproterozoic basement was a major source for parental magmas. U-Pb dating of charnockite samples (592±7, and 603±4 Ma) indicates the suite is coeval with the pre-collisional stage of Aracuai Orogen evolution, and probably represents the magmatic arc granulitic roots. The southward extension of this arc is of great importance in order to unveil the link between Aracuai and Ribeira orogens.

Rochas charnockíticas do sudeste de Minas Gerais: a raiz granulítica do arco magmático do Orógeno Araçuaí

The Araçuaí and Ribeira orogens are a Cambrian-Neoproterozoic belt that extends from the eastern edge of the San Francisco craton to the Atlantic Ocean, south of parallel 15 º. The connection between these orogens is roughly situated along the 21 º S parallel. High-grade metamorphic rocks are exposed in the southern Araçuaí Belt, SE Minas Gerais, reflecting a deep crustal level. The Divino Suite comprises mafic- to felsisc charnockites, metamorphosed in granulite facies, interpreted as an expanded calc-alkaline suite metaluminous to slightly peraluminous. Chemical and Nd isotopic signatures suggest the Divino Suite evolved in a continental magmatic-arc setting, and also that Paleoproterozoic basement was a major source for parental magmas. U-Pb dating of charnockite samples (592±7, and 603±4 Ma) indicates the suite is coeval with the pre-collisional stage of Araçuaí Orogen evolution, and probably represents the magmatic arc granulitic roots. The southward extent of this arc is of great importance in order to unveil the link between Araçuaí and Ribeira orogens.

Provenance of deep-water reservoir sandstones from the Jubarte oil field, Campos Basin, Eastern Brazilian Margin

The provenance of the Maastrichtian deep-water reservoir sandstones from the Jubarte oil field (Campos Basin, eastern Brazil), was studied using an integrated approach that included quantitative petrography, conventional heavy mineral analysis, garnet mineral chemistry and zircon geochronology. The reservoirs are predominantly coarse, poorly-sorted sandstones with feldspathic composition derived from uplifted basement terrains. The fourth- and fifth-order depositional sequences analyzed show no major variations in composition or in provenance through time. However, variations in apatite:tourmaline presents potential to be used as a parameter for sandstone correlation within the field. The composition of heavy minerals indicates derivation from high temperature and low-to-medium pressure aluminous metapelitic rocks, from granites and subordinate mafic rocks, derived from the Cabo Frio Tectonic Domain and the Oriental terrain of Ribeira orogen, characterizing a supply route from SW to NE. The low ZTR index, as well as the absence of low-grade stable heavy minerals and of metasedimentary rock fragments, suggest that by the end of Cretaceous all supracrustal, low-grade terrains had already been totally eroded, and that plutonic, infracrustal rocks were exposed, similarly to the present situation. The integration of the compositional and textural aspects suggests a tectonically active source area, located relatively close to the basin. The active tectonic uplift of the basement terrains provided a large volume of coarse, poorly-sorted sediments carried directly from mountain streams and alluvial fans to deep water. The high frequency shifts in apatite:tourmaline values support a direct derivation of the deep-water sandstones from adjacent alluvial systems with short but differential exposure to weathering. The voluminous deposition of such coarse-grained and immature turbidites under the general transgressive conditions of the Maastrichtian indicates a tectonic, rather than eustatic control on their deposition, with important implications for the exploration of similar reservoirs in Campos Basin and in similar basins of eastern Brazilian margin.

Extrusão tectônica e transporte lateral de massa na porção central do cinturão Paraíba do Sul, seção Três Rios - Matias Barbosa (RJ/MG)

This paper deals with the geometry and kinematics of structures, described along a section cross-cutting the northern limb of a NE-trending regional fan-like geometry. The structure occurs on the core of the Ribeira orogen in SE Brazil. Three structural domains were described, from south to north. In the domain I, the mylonitic foliation runs towards NE with sub-vertical to vertical dip. Stretching lineation is horizontal, and assymmetric structures suggest dextral transpressive motion in the core of the divergence (domain I). Tablet-of-chocolate boudinage suggests secondary vertical stretching and shortening orthogonal to the flow plane occurs. Northwards occurs a crustal slice composed mainly by orthogneiss, and minor metassediments and deformed granitoids, called Paraíba do Sul domain – or domain II. This slice tectonically overlies the granulitic rocks of the Juiz de Fora domain (domain III). Foliation in domain II delineates a guirdle on the stereoplot, with slowly ENE- plunging axes. Stretching lineation plunges slowly to NE, parallel to the girdle axes. Tight to isoclinal folds with axis parallel to the stretching lineation occur within the mylonitic foliation. Slowly southward dipping mylonitic foliation (and associated folds) are deformed by open to tight, nearly upright extension-parallel folds with axial surfaces sub-parallel to the regional transpression zones. L-tectonites in deformed granitoids, and mullion structures parallel to the axes of both families of folds also were described. Shear-sense indicators suggest top-to-SWW shearing, sub-parallel and oblique to the orogenic trend, coeval with a NW-SE contractional component. Later structures are ductile-ruptile top-toNW thrusts and sinistral transtrative shear zones. The structural evolution of the area is interpreted in terms of regional dextral unconfined transpression. Our observations suggest that strain path partitioning during oblique extrusion and convergence may explain coeval parallel orogenic motion either in flat lying or steeply dipping shear zones, and WNW-directed dextral thrusts described in the deeper portions of the belt. Tectonic extrusion may also explain the presence of L-tectonites and the absence of vertical stretching lineation in the highly strained zones in the Paraíba do Sul Belt.

Extrusão tectônica e transporte lateral de massa na porção central do cinturão Paraíba do Sul, seção Três Rios - Matias Barbosa (RJ/MG)

This paper deals with the geometry and kinematics of structures, described along a section cross-cutting the northern limb of a NE-trending regional fan-likegeometry. The structure occurs on the core of the Ribeira orogen in SE Brazil. Three structural domains were described, from south to north. In the domain I, the mylonitic foliation runs towards NE with sub-vertical to vertical dip. Stretching lineation is horizontal, and assymmetric structures suggest dextral transpressive motion in the core of the divergence (domain I). Tablet-of-chocolate boudinage suggests secondary vertical stretching and shortening orthogonal to the flow plane occurs. Northwards occurs a crustal slice composed mainly by orthogneiss, and minor metassediments and deformed granitoids, called Paraiba do Sul domain – or domain II. This slice tectonically overlies the granulitic rocks of the Juiz de Fora domain (domain III). Foliation in domainII delineates a guirdle on the stereoplot, with slowly ENE- plunging axes. Stretching lineation plunges slowly to NE, parallel to the girdle axes. Tight to isoclinal folds with axis parallel to the stretching lineation occur within the mylonitic foliation. Slowly southward dipping mylonitic foliation (and associated folds) are deformed by open to tight, nearly upright extension-parallel folds with axial surfaces sub-parallel to the regional transpression zones. L-tectonites in deformed granitoids, and mullion structures parallel to the axes of both families of folds also were described. Shear-sense indicators suggest top-to-SWW shearing, sub-parallel and oblique to the orogenic trend, coeval with a NW-SE contractional component. Later structures are ductile-ruptile top-to- NW thrusts and sinistral transtrative shear zones. The structural evolution of the area is interpreted in terms of regional dextral unconfined transpression. Our observations suggest that strain path partitioning during oblique extrusion and convergence may explain coeval parallel orogenic motion either in flat lying or steeply dipping shear zones, and WNW-directed dextral thrusts described in the deeper portions of the belt. Tectonic extrusion may also explain the presence of L-tectonites and the absence of vertical stretching lineation in the highly strained zones in the Paraiba do Sul Belt.