Oriental Terrane Research Articles

Long-term magmatism in the Ribeira Orogen, Western Gondwana: SHRIMP U-Pb and O isotope data for two Ediacaran-Cambrian magmatic events in the Rio de Janeiro area

In the southeastern Brazilian margin, a Neoproterozoic to the Early Paleozoic long-term magmatic interval is recorded within the Oriental Terrane, part of the Ribeira Orogen, characterizing it as a hot orogenic belt. Metaigneous and igneous rock units of ca. 570–540 Ma and 510–480 Ma are coeval with two main collisional orogenies related to the Western Gondwana assembly: the Ribeira Orogeny (590–565 Ma) and the Búzios Orogeny (540–490 Ma). The Oriental Terrane, interpreted as a continental microplate amalgamated during the convergence between the São Francisco and Congo cratons, comprises Cryogenian to Ediacaran metaigneous rocks and Cambro-Ordovician plutons intrusive in metasedimentary sequences. Zircons from an orthogneiss and a porphyritic leucogneiss sample gave SHRIMP U-Pb crystallization ages of 558 ± 2 Ma and 560 ± 2 Ma, respectively, with high δ18O values from 7 ‰ to 10 ‰. Zircons from a tonalite pluton and tonalitic dikes, gave SHRIMP U-Pb crystallization ages of 483 ± 2 Ma and 492 ± 2 Ma, with lower δ18O values, between 6 ‰ to 8 ‰. These two magmatic pulses, integrated with a compiled database from other plutons in this terrane, corroborate with the hypothesis that they represent crustal scale magmatic provinces here interpreted as pre- and post-collisional in relation with the Búzios Orogeny. Their distribution within the tectonic map of the Oriental terrane reflect a paleogeography linked to their tectonic origin. The 570–540 Ma plutons are aligned ENE -WSW, parallel and close to the suture zone with the Cabo Frio Tectonic Domain, representing a crustal melt related to a NW-subduction. A subsequent magmatic hiatus of 30 ma is interpreted as the collision of the Cabo Frio Tectonic Domain. The post collisional period is marked the intrusion of non– linear 510–480 Ma pluton, widespread in the Oriental terrane.

Contrasting P-T-t paths of basement and cover within the Búzios Orogen, SE Brazil – Tracking Ediacaran-Cambrian subduction zones

Deeply eroded collisional orogens show complex structural and inverted stratigraphic relations with juxtaposition of rock stacks from distinct crustal levels, origins and ages, hence with contrasting P-T-t paths during convergent tectonics. This paper presents petrochronological data on Paleoproterozoic (basement) and Ediacaran (cover) gneisses tectonically interleaved during the Ediacaran-Cambrian Búzios Orogeny, in southeastern Brazil. U-Pb in zircon and EPMA U-Th-Pb in monazite data, coupled with geothermobarometric data, plus Zr-in-rutile, reveal that at a first orogenic stage (ca. 530–520 Ma), these units were at distinct crustal levels. Samples within the Paleoproterozoic basement show metamorphic near-peak conditions of ∼800 °C and 10 kbar, at a depth of c.37 km. Contrastingly, Ediacaran kyanite-orthoclase-garnet-biotite granulite with retrometamorphic sillimanite (cover) reached near-peak conditions of 15 kbar and 818 °C–785 °C at depths of c. 55 km, in high-pressure granulite facies. This deep burial of Ediacaran sediments in less than 20 m.y. would be compatible with a low angle subduction zone active from ca. 550 to 530 Ma. Intrusion of ca. 550 Ma tholeiitic dykes in the Paleoproterozoic gneiss indicates a high geothermal gradient for this subduction setting, which is consistent with a low subduction rate. In a second orogenic stage (ca. 520–500 Ma), these distinct stratigraphic units were placed tectonically side by side during a fast exhumation, preserving an inverted metamorphic stack. The cover underwent retrometamorphic conditions of 800 °C and 10 kbar on a clockwise return path due to decompression. It is proposed here that the contact between reworked units within a Paleoproterozoic continental crust and Ediacaran magmatic and sedimentary units represent the suture of an Ediacaran NW-subduction of the Angola continental paleomargin below the Oriental Terrane of the Ribeira belt. This paper reports the highest pressure recorded in Ediacaran-Cambrian metamorphic rocks from the Brasiliano belts along the actual South Atlantic continental margins. The suture we propose here is aligned along strike, with a medium to high-pressure Ediacaran metamorphic occurrence 700 km to the SW, in the Curitiba Terrane.

Contrasting P-T conditions of Oriental Terrane and Central Superterrane (Ribeira Belt), NW of Rio de Janeiro state, Brazil

The study area comprises the northwest region of Rio de Janeiro state where ortho- and paraderived rocks of Neoproterozoic-Cambrian age of the Oriental Terrane and the Central Superterrane of the Ribeira Belt occur. From field and petrography studies, this paper presents new data on mineral chemistry and conventional geothermobarometry, fundamental to the understanding the geology of the region. The metabasic rocks of the Cambuci Domain, associated with the retroarc basin of the Central Superterrane, recorded two metamorphic events: M0 (before the formation of the main foliation, under conditions of 788 °C) and M1 (contemporary to the main foliation, with peak at 718-752 ºC and 6.62 kbar). The rocks of the Costeiro Domain, interpreted as representatives of the Rio Doce arc (Angelim Suite) and retroarc basin (São Fidélis Goup) of the Oriental Terrane, recorded a metamorphic peak in granulite facies yielding the highest pressure and temperature among the studied domains (752-784oC and 8.2-9 kbar). For the Italva Domain, representative of the retroarc basin of the Oriental Terrane, the temperature and pressure values obtained for the metamorphic peak were 731 ºC and 6.78 kbar, in the transition zone between the conditions of upper amphibolite and granulite facies conditions. This work presents the newest record of metamorphic evolution in the Oriental Terrane of the Ribeira Belt, suggesting a anticlockwise P-T-t path and near-isobaric cooling (IBC) to the rocks of the Cambuci domain. Therefore, this result contributes to the understanding of the evolution of orogenic belts during the amalgamation of the Gondwana in the Neoproterozoic.

Unravelling source and tectonic environment of an Ediacaran magmatic province from southeast Brazil: Insights from geochemistry and isotopic investigation

This work focuses on the 575–540 Ma meta-igneous units that crop out near Rio de Janeiro city, southeast Brazil. Tectonically, they are located in the southern part of the Oriental Terrane known as Costeiro Domain, near to the suture with the Cabo Frio Tectonic Domain. We present new zircon U-Pb and oxygen isotope SHRIMP along with geochemical data to investigate the possible source(s) and tectonic setting of this magmatic event related to the final stages of Western Gondwana consolidation. Five units comprise this Late Ediacaran magmatic province: Facoidal orthogneiss, Cassorotiba garnet-biotite orthogneiss, Maricá porphyritic orthogneiss, Tinguí hornblende-biotite orthogneiss, and Itacoatiara porphyritic meta-granite. These acid to intermediate plutons show similar whole-rock geochemistry, Sm-Nd data, and petrographic characteristics. The original magmas are calc-alkaline that evolved to a high-K calc-alkaline, and a shoshonitic term. Geochronological data indicate three intervals of magmatic activity: 575–565 Ma medium to high-K calc-alkaline intrusions; 555–545 Ma high-K and shoshonitic magmas; and the 545–540 Ma calc-alkaline medium-K Itacoatiara meta-granite, with Nd isotopic ratios presenting the highest εNd (3.7–5.1), and TDM ages (1.3–1.8 Ga). The TDM Nd model ages of all units range from 1.3 Ga to 1.8 Ga and the εNd(t) values vary mostly from −3.7 to −5.7 corroborating with the crustal affinity of these rocks. The oxygen isotope (δ18O) values average is 8‰ with slight differences among the units. These high δ18O ratios suggest crustal values, typical for continental crust in a collisional environment. Based on the geochemical data allied with melting experiments from literature, we envisage that the Late Ediacaran magmatic province could be formed by partial melt from a lower crust amphibole-biotite rich source. All the presented data point to magmatic activity in a transitional environment from subduction to continental collision in the Cambrian.

The Cabo Frio Thrust: A folded suture zone, Ribeira belt, SE Brazil

Kinematic evolution of thrust systems in which vergence changes along strike result in divergent interpretations regarding the subduction polarity. The Ribeira belt, located in southeastern Brazil, resulted from progressive amalgamation of several terranes against the southeastern margin of the São Francisco Craton (SFC) between ca. 620 and 580 Ma. This study brings new structural analysis of the Cabo Frio Thrust (CFT) located in the Ribeira belt, that displays strike variations and apparently double thrusting vergence, generating current debate on subduction vergence for the region. Four major stages of deformation (Dn-1, D1cf, D2cf and D3cf) have been identified in the southwestern segment of the CFT. The Dn-1 affected only the metasedimentary rocks of the Oriental Terrane previously the collision. The D1 phase is associated with the thrusting of orthogneisses of the Cabo Frio Terrane onto the metasedimentary rocks of the Oriental Terrane, with the development of a penetrative S1 foliation and L1 stretching lineation. The D2 deformation phase produced a recumbent synform in the early/syn-D2 stages that overturned the CFT. This resulted in F2 isoclinal folds and S2 axial plane schistosity with F2 axis plunges towards W-SW, parallel to the L1 stretching and L2 intersection lineations. During the late-D2 stage, the recumbent F2 fold was sheared and refolded, leading to horizontal rotation of linear and planar elements, changing the CFT strike from W-SW to N–S. A series of irregular upright folds were developed in the D3 deformation phase. The double vergence and strike variations of the CFT are a result of the major F2 fold geometry, influenced by the granitic batholith (Tinguí Complex) that acted as an indenter during continuous crustal shortening towards northwest.

Petrografia e termobarometria de granitoides diatexíticos portadores de anfibólio da Região de São José de Ubá e São João do Paraíso (RJ)

Na região norte do Rio de Janeiro afloram rochas orto e paraderivadas neoproterozoicas de alto grau metamórfico, englobadas no Terreno Oriental da Faixa Ribeira. Dado a carência de estudos de detalhe na região, o presente trabalho contribui com novos aspectos de campo, petrografia, química mineral e geotermobarometria dos granitoides diatexíticos aflorantes na região entre São José de Ubá e São João do Paraíso, no noroeste fluminense. O litotipo é caracterizado por biotita-hornblenda gnaisses migmatíticos e protomiloníticos que são correlacionáveis a ortognaisses e granulitos do Complexo Serra da Bolívia (Domínio Cambuci da Faixa Ribeira). Dois litotipos neoformados são observados: um mesocrático, de caráter residual; e outro leucocrático, que correspondente ao leucossoma. O enriquecimento de K-feldspato no leucossoma em relação ao neossoma residual sugere baixos volumes de fusão por meio de reações de fusão envolvendo água. Aplicando-se diferentes geotermobarômetros, foram obtidas como condições de equilíbrio para o pico metamórfico 6,8 Kbar e 672–680°C (fácies anfibolito superior). Tais valores destoam dos encontrados na literatura para ouras rochas da região, reforçando a ideia de que os domínios do Terreno Oriental da Faixa Ribeira apresentam evoluções metamórficas distintas, em diferentes níveis crustais.

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.

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.

550–490 Ma pre-to post-collisional shoshonitic rocks in the Ribeira Belt (SE Brazil) and their tectonic significance

This work presents a detailed study in a new occurrence of shoshonite rocks in the central Ribeira Belt, SE Brazil. It is the Tinguí Unit, a (hornblende) biotite orthogneiss with shoshonitic affinity, and undeformed dykes of similar composition. They crop out along the contact between Costeiro (part of Oriental Terrane, Ribeira Belt) and Cabo Frio Tectonic Domains. The Tinguí Unit is represented by mesocratic orthogneiss, ranging from quartz-diorite to granodiorite in composition. Plagioclase, biotite, quartz, hornblende, microcline, titanite, apatite, allanite, zircon, and opaque minerals are the main minerals. Tonalitic to granodioritic dykes, with similar composition, cross cut the Tinguí orthogneiss. Besides mineralogical similarities, analyzed samples from both the orthogneiss and the dykes have shoshonitic signature with crustal and mantle Nd-Sr isotope nature. They are basic to intermediate rocks with low Fe contents and high alkalis, Ba, Rb, Sr, Zr, and LREE values. High K2O/Na2O (up to 2.06) and LREE/HREE ratios along with others geochemical parameters reinforce the shoshonitic signature. Sm-Nd data point to εNd values of −13.2 to −4.38, and 1.7 to 1.2Ga TDM Nd model ages. Although the 143Nd/144Nd ratios indicate a predominance of lower continental crust heritage, the 87Sr/86Sr ratios ranging from ca. 0.705 to 0.707 suggest an enriched mantle contribution. Nevertheless, geochronological data from Tingui orthogneiss and tonalitic dykes reveal a difference of ca. 50m.y. in their crystallization ages: concordia ages of 551±3Ma, for the orthogneiss; 495±4Ma and 488±4Ma for the mesocratic and leucocratic tonalitic dykes, respectively. Comparing our new data and the tectonomagmatic events recorded in the Oriental Terrane, we propose a tectonic model emphasizing the role of this shoshonitic magma, interpreted as pre- and post-collisional intrusions regarding to the Buzios Orogeny, and possibly derived from the similar source. This magmatic activity is attributed to a long-term maintenance of high temperatures and low cooling rates in the Ribeira Belt, considered as a hot orogen. The 50m.y. crystallization age gap between the studied rocks could be assigned to two periods of melting of a similar or even same source at different tectonic stages of the orogen, pre- and post-collisional.

Provenance of the Neoproterozoic high-grade metasedimentary rocks of the arc-related Oriental Terrane of the Ribeira belt: Implications for Gondwana amalgamation

The Costeiro domain integrates the Oriental terrane of the Ribeira belt, which encompasses arc-related orthogneisses of the Rio Negro complex (ca.790–605 Ma), with a well-documented subduction signature, and the high-grade metasedimentary rocks of the São Fidélis group. The arc-related rocks intruded the lower unit of the São Fidélis group, while both units are crosscut by syn-to late collision granitoids related to the development of different stages of the Brasiliano Orogeny (ca. 605–480 Ma). New U–Pb (LA-ICP-MS) data of detrital zircon grains from quartzites of the top unit of the São Fidélis group yielded a large spectrum of Mesoproterozoic and Paleoproterozoic ages, with subordinated Archean and Neoproterozoic ones. The largest age peaks occur at ca. 1.2 Ga and 2.2 Ga. The youngest detrital zircon of ca. 613 Ma, and metamorphic overprints, with ages varying from ca. 602 to 570 Ma bracket the age of sedimentation of the top unit. Two orthogneisses of the Rio Negro complex intruded within the basal unit of the São Fidélis Group rendered similar ages of ca.620 Ma. These orthogneisses and the basal unit of the São Fidélis group are interpreted as possible sources of the upper unit. The provenance pattern of the São Fidélis Group is similar to that of the Kaoko Belt, suggesting that the Angolan basement, where Mesoproterozoic ages are common, constitutes another important source area.

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).

The Serra da Bolívia complex: The record of a new Neoproterozoic arc-related unit at Ribeira belt

The Neoproterozoic arc related rocks of the central segment of the Ribeira belt crop out at the Oriental Terrane that is subdivided into three thrust sheets verging northwestern ward, each of them with subduction related rocks. The recently discovered Serra da Bolívia complex crops out at the Cambuci domain that represents the lowermost thrust sheet of the Oriental terrane. The unit comprises orthogneisses of varied compositions, ranging from basic to acid rocks. Hornblende bearing coarse grained orthogneisses, with granodiorite to quartz-diorite compositions, is the most common association in the complex; followed by quartz poor rocks such as monzo-gabbros, monzodiorites, monzonites and syenites. Coarse grained gabbros (norites) and leucogabbros occur as lenses inside the orthogneisses.New geological, geochemical and geochronological data for recently described arc-related orthogneisses of the Serra da Bolívia complex is presented. The connection with other arc-related rocks of the Araçuaí and southern Ribeira belt in the context of West Gondwana reconstructions is also explored.Chemical data indicates at least three groups of sub-alkaline rocks and one group of alkaline to transitional character (SH). Among the sub-alkaline groups three calc-alkaline groups (CA1, CA2 and CA3) with progressive enrichment in Alkalis are discriminated. Tectonic discriminate diagrams suggest that the calc-alkaline groups display compositions compatible with arc settings, while the shoshonite group, with more evolved compositions indicates progressive maturity during arc development. Few basic rocks indicate CAB or IAT filiations.Obtained U–Pb ages for tonalites and gabbros yielded values between ca. 596 and 591Ma, but an older TIMS age pointed to ca. 620Ma. The youngest monzonite orthogneiss yielded ca. 574Ma, similar to the metamorphic ages previously described for the Ribeira belt. Sm-Nd data resulted on Tdm model ages of ca. 1.72Ga and 2.04Ga, except for three samples, which display Mesoproterozoic model ages. The initial ɛNd, calculated for 605Ma, which is the average age for the complex are between −8 and −12.Regional correlations suggest that the Serra da Bolivia Complex represents the inner cordilleran magmatic arc of the Ribeira belt that continues northward to the Galiléia arc (G1 association of the Araçuaí belt), and southwards to the Socorro arc at the interference zone of the Brasília belt. Combined U–Pb data indicate the development of a cordilleran arc between ca. 650 and 590Ma. Youngest syenitic to monzonitic bodies are coeval with the metamorphic episode and intrude the older rocks of the arc. The other two previously described arc related complexes, Rio Negro and Serra da Prata arcs are located easternmost at the belt and display contrasting more juvenile signatures and older ages reaching ca. 840M.Looking to the West Gondwana scenario, during Tonian to Cryogenian time, subduction all around older and stables plates resulted on the generation of large volumes of arc-related rocks. In most of the belts, only the plutonic zones of the magmatic arcs are exposed. Based on time span of rock crystallization of all the arc related rocks of the Ribeira belt, the subduction period lasted at least 240 million years. Even though subduction was breaking down into three individual zones, the tectonic scenario suggests oceanic plate consumption in a protracted evolution. The initial phases of most magmatic arcs are represented by more primitive and juvenile rocks, but at the end of the subduction period, at the Ediacaran, a more cordilleran setting was achieved. By the end of the Ediacaran period and the beginning of Cambrian, all the pieces were attached together in several diachronic collisions leading to the final amalgamation of West Gondwana.

Evolução Tectônica e Magmática da Faixa Ribeira entre o Neoproterozoico e o Paleozoico Inferior na Região Serrana do Estado do Rio de Janeiro, Brasil

The Highlands of the Rio de Janeiro (RSF) are composed of granitic gneiss, paragneiss and granite that are part of the Ribeira Belt. Their geological features were studied for over 50 years by authors who described orthogneisses and granites arranged in large overlapping tectonic sheaths. Tectonic domains and terranes were proposed for the Ribeira Belt, and the rocks of the RSF were included in the Oriental Terrane. The tectonic and magmatic evolution of the RSF begins in the Cryogenian with the installation of the magmatic arc of the Rio Negro Complex and sedimentation of the Italva, St. Fidélis and Bom Jesus do Itabapoana groups. During the Ediacaran collision of this arc with a passive margin the migmatitic leucogneisses of the Cordeiro Suíte were generated. At the collisional ending the calc-alkaline magmatic Serra dos Órgãos Suíte was produced. At the end of evolution, tectonic exhumation and tectonic collapse was coeval with the intrusion of Cambro-Ordovician granites of the Nova Friburgo Suíte.

Padrão de fraturas da região da Serra dos Órgãos (RJ) e sua relação com a tectônica mesozóico-cenozóica do sudeste do Brasil

The Serra dos Órgãos range, between Petrópolis and Teresópolis cities, Rio de Janeiro State, is one of the most remarkable physiographic features of southeastern Brazilian coast. This feature, part of the Serra do Mar coastal range, with summits over 2.200 m.a.s.l, follow Precambrian basement structures constituted of metamorphic rocks intruded by several generations of granitoid rocks, included in the Oriental Terrane of the central segment of the Ribeira folded belt. After a long period of tectonic quiescence, the present-day southeastern Brazilian coastal region was subjected to an important process of tectonic reactivation, manifested since Jurassic times, which culminated with the opening of South Atlantic Ocean. Analysis of the fracture pattern of the Serra dos Órgãos range in the region of the Serra dos Órgãos National Park (Parque Nacional da Serra dos Órgãos - PARNASO) indicated the existence of five tectonic deformation phases, active from Neoproterozoic-Cambrian until the Recent times, in part correlatable with previously recognized deformations in southeastern Brazil: (1) Neoproterozoic-Cambrian left-lateral E-W-trending strike-slip; (2) and (3) Early Cretaceous and Late Cretaceous to Paleocene left-lateral E-W-trending strike-slip; (4) Eocene NW-SE-trending extension; and (5) Holocene E-W-trending compression.

Padrão de fraturas da região da Serra dos Órgãos (RJ) e sua relação com a tectônica mesozóico-cenozóica do sudeste do Brasil

The Serra dos Orgaos range, between Petropolis and Teresopolis cities, Rio de Janeiro State, is one of the most remarkable physiographic features of southeastern Brazilian coast. This feature, part of the Serra do Mar coastal range, with summits over 2.200 m.a.s.l, follow Precambrian basement structures constituted of metamorphic rocks intruded by several generations of granitoid rocks, included in the Oriental Terrane of the central segment of the Ribeira folded belt. After a long period of tectonic quiescence, the present-day southeastern Brazilian coastal region was subjected to an important process of tectonic reactivation, manifested since Jurassic times, which culminated with the opening of South Atlantic Ocean. Analysis of the fracture pattern of the Serra dos Orgaos range in the region of the Serra dos Orgaos National Park (Parque Nacional da Serra dos Orgaos - PARNASO) indicated the existence of five tectonic deformation phases, active from Neoproterozoic-Cambrian until the Recent times, in part correlatable with previously recognized deformations in southeastern Brazil: (1) Neoproterozoic-Cambrian left-lateral E-W-trending strike-slip; (2) and (3) Early Cretaceous and Late Cretaceous to Paleocene left-lateral E-W-trending strike-slip; (4) Eocene NW-SE-trending extension; and (5) Holocene E-W-trending compression.

Cambrian orogeny in the Ribeira Belt (SE Brazil) and correlations within West Gondwana: ties that bind underwater

Abstract A 530–490 Ma tectono-metamorphic event, the Búzios orogeny, is recognized within the Ribeira Belt, along the coast of SE Brazil. Tectonic evolution started with a Late Neoproterozoic marine basin and volcanic activity at c . 610 Ma. The rocks in this basin were affected by high-grade metamorphism at c . 530 Ma, coeval with deformational phases D 1 –D 2 , which generated compressive low-angle tectonic structures with top-to-NW tectonic transport. Large recumbent folds with NW–SE axes parallel to the main stretching lineation formed during D 3 as the Cabo Frio tectonic domain, the focus of this study, collided with the Oriental terrane to the NW. D 4 sub-vertical shear zones are limited in extent. A new U–Pb age of 501±6 Ma is reported for zircon from an amphibolite-facies shear zone related to either D 3 or D 4 . Post-tectonic 440 Ma pegmatites mark the final stage of tectono-magmatic activity. The Cabo Frio tectonic domain has African affinities and is exotic to the Ribeira Belt. Middle Cambrian deformational and metamorphic ages are also reported from the ‘Angolan’ Pan-African belt, the southern Kaoko and Damara belts in Namibia, and the Cuchilla Dionisio–Punta Del Este terrane in Uruguay. The occurrence of Cambrian metamorphic rocks along the present African and South American coastlines shows that Mesozoic rifting closely follows Palaeozoic sutures of West Gondwana.

Age and geotectonic setting of Late Neoproterozoic juvenile mafic gneisses and associated paragneisses from the Ribeira belt (SE Brazil) based on geochemistry and Sm–Nd data — Implications on Gondwana assembly

Mafic gneisses and associated paragneisses from the Cabo Frio Tectonic Domain in the southeastern part of the Ribeira Belt, along the coast of Rio de Janeiro State in southeast Brazil, were subjected to a geochemical and Sm–Nd isotope study. Four lithotypes are distinguished: aluminous paragneisses (mainly sillimanite–kyanite–garnet–biotite gneiss), calcsilicate lenses, quartzo–feldspathic metasedimentary gneisses and mafic–ultramafic lenses. The whole-rock major and trace, including rare earth element distributions in the mafic–ultramafic intercalations indicate that derivation from subalkaline basalt/gabbro of tholeiitic affinity with E-MORB signature from a non-subduction environment. These mafic rocks have positive ε Nd(t) and T DM of 1.1 Ga. The metasedimentary rocks have negative ε Nd(t) and T DM of 1.7 Ga. A Sm–Nd whole rock isochron of mafic rocks yielded an age of 604 ± 38 Ma for the crystallization. This matches with the age of some detrital zircon grains from the paragneisses. The depositional basin, named Buzios–Palmital, was active at least until 620 Ma (age of the youngest detrital zircon) and was subsequently deformed and metamorphosed at ca. 525 Ma (age of metamorphic zircons) during the Buzios Orogeny. It is interpreted as a back arc basin with relation to the 630 Ma magmatic arc of the Oriental Terrane in the Ribeira Belt to the NW. However, after 600 Ma, the Buzios–Palmital basin changed to an active margin setting because the arc collided with the continental margin and the subduction shifted to the back arc environment. By 610 Ma, most of the Brasiliano belts registered collisional events related to multiple convergent blocks. The stress fields and paleocontinent shapes would have allowed the occurrence of extensional areas with not only sedimentary deposition but also ocean floor spreading. Its remnants are preserved in this Brazilian coastal region as an ancient suture, reworked intensively during the Mesozoic rifting events. The reconnaissance of Late Neoproterozoic basins in the Brasiliano–Pan-African belts is of major importance to partially unravel the final amalgamation events of SW Gondwana. Considering that the Buzios–Palmital basin rock units are mostly covered by the marginal Atlantic basins, it is possible that other evidence could be preserved in the coastal regions of SW-Africa and SE-South America.

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.

Os terrenos tectônicos da Faixa Ribeira na Serra da Bocaina e na Baía da Ilha Grande, sudeste do Brasil

The central segment of the Neoproterozoic Ribeira belt in the studied area is composed of four tectonic units: the Occidental, Oriental, Paraiba do Sul and Embu terranes. The metasedimentary sequence of the Occidental Terrane represents the reworked margin of the Sao Francisco Craton. The Oriental Terrane comprises the cordilleran magmatic arc that collided with the Sao Francisco Plate at ca. 580 Ma. The Paraiba do Sul Terrane is composed of Paleoproterozoic orthogneisses and metasedimentary rocks, and the Embu Terrane comprises supracrustal succession and undated orthogneisses. Granitoid rocks intrude all terranes. These terranes were thrust-stacked during the main deformational phase related to the collisional stage of the Brasiliano Orogeny, when the Occidental Terrane was covered by the other ones. The nature and the paleogeography of the Paraiba do Sul and Embu terranes are unclear, but a relation to a magmatic arc environment can be suggested.

OS TERRENOS TECTÔNICOS DA FAIXA RIBEIRA NA SERRA DA BOCAINA E NA BAÍA DA ILHA GRANDE, SUDESTE DO BRASIL

The central segment of the Neoproterozoic Ribeira belt in the studied area is composed of four tectonic units: the Occidental, Oriental, Paraíba do Sul and Embu terranes. The metasedimentary sequence of the Occidental Terrane represents the reworked margin of the São Francisco Craton. The Oriental Terrane comprises the cordilleran magmatic arc that collided with the São Francisco Plate at ca. 580 Ma. The Paraíba do Sul Terrane is composed of Paleoproterozoic orthogneisses and metasedimentary rocks, and the Embu Terrane comprises supracrustal succession and undated orthogneisses. Granitoid rocks intrude all terranes. These terranes were thrust-stacked during the main deformational phase related to the collisional stage of the Brasiliano Orogeny, when the Occidental Terrane was covered by the other ones. The nature and the paleogeography of the Paraíba do Sul and Embu terranes are unclear, but a relation to a magmatic arc environment can be suggested.