Central Ribeira Belt Research Articles

New U-Pb and Hf data of the contact region between the Oriental and Cabo Frio terranes, Central Ribeira Belt, Brazil: Implications for the closure of the Ediacaran-Cambrian oceanic space

The Ribeira Belt represents a deeply eroded orogen which exposes rocks from the different stages of magmatic arcs and associated metasedimentary basins. This contribution focus on the last collisional episode between the Outer Magmatic Arc System (OMAS) and the Cabo Frio Terrane (Angola craton). The OMAS represents a long-lived magmatic arc that collided against the Inner Magmatic System (IMAS) at ca. 590 Ma (M1 metamorphism), and against the Cabo Frio Terrane at ca. 530 Ma (M2 metamorphism). The nature of the magmatism recorded in the first collisional episode, and the subduction polarity of the second is a current topic of debate. New U-Pb age and Lu-Hf analyses in zircon grains from units close to the contact are presented to propose a tectonic model for the closure of the oceanic space between the OMAS and the Cabo Frio Terrane. The results in combination with published data suggest that the metasedimentary rocks of the São Fidélis Group are in a back-arc position of the OMAS and were affected by both the M1 and M2 metamorphic episodes, reflected by the presence of metamorphic zircons with ages between ca. 592–553 Ma and 543–533 Ma. The orthogneiss of the Tinguí Complex intrusive in the back-arc metasedimentary units yielded a crystallization age of 578 ± 9 Ma, coeval with the syn-collisional stage M1. A younger metamorphic overprint at 529 ± 7 Ma was also detected (syn-M2). The orthogneiss of the Região dos Lagos Complex (Cabo Frio Terrane) rendered a crystallization age of 1968 ± 26 and a M2 metamorphic overprint at 522 Ma. Zircon analyses from folded leucosomes in the orthogneiss also indicated a crystallization age of 523 ± 10 Ma, which constrain the time of folding after thrusting. Undeformed post-collisional granitoids and pegmatites yielded crystallization ages between 496 and 499 Ma, closing the tectonic evolution of the belt with the orogen collapse. Our results corroborate with terrane accretion model for the Central Segment of the Ribeira Belt.

Investigating assembly timing of Ediacaran Serra dos Órgãos batholith – Hints on prolonged magmatism at Ribeira belt, SE Brazil

The Pan-African/Brasiliano orogeny, result of the West-Gondwana assembly from the Neoproterozoic to Ordovician, had produced the Ribeira and Araçuaí belts at the edges of the São Francisco and Congo cratonic margins. In the central Ribeira belt, the docking of accretionary systems to the reworked continental margin of the São Francisco paleoplate culminated in a complex Ediacaran granite production event. Syn-to late-collisional I- and S-type granites and hybrid granitoids intruded the reworked margins and accreted continental masses, as well as the Tonian to Ediacaran juvenile magmatic arcs represented by the Serra da Prata and Rio Negro Complexes. The Serra dos Órgãos batholith is the main intrusion emplaced at the juvenile crust, forming an elongated NE-SW trending tabular body that crops out in the highland region of Rio de Janeiro State, Southeast Brazil. Geochemical and geochronological data in addition to field and petrological evidence was gathered to explore the assembly timing of the SOB granitic end-member. Zircon U–Pb SHRIMP and LA-ICP-MS geochronological data yield a prolonged emplacement timelapse for the studied granites and, along with published U–Pb ages of the SOB granodioritic end-member, enable the characterization of the Serra dos Órgãos as a composite batholith. The wide time interval of heat input inside intermediate crust is consistent with a prolonged, slow-cooling hot orogenic belt model for the Araçuaí-Ribeira Orogenic System.

Siderian to Rhyacian evolution of the Juiz de Fora Complex: Arc fingerprints and correlations within the Minas-Bahia Orogen and the Western Central Africa Belt

The Juiz de Fora Complex (JFC) is one of the basement units of the Occidental Terrane, Central Ribeira belt, in the Neoproterozoic framework of Western Gondwana, regarded as part of the reworked border of the São Francisco Paleocontinent. The JFC comprises Paleoproterozoic granitoid rocks with TTG, calc-alkaline, sanukitoid, and tholeiite-type geochemical compositions. They exhibit complex deformational fabrics associated with two episodes of granulite-facies metamorphism, occuring in the early Orosirian and the Ediacaran. The new U-Pb (SHRIMP) isotopic ages and Lu-Hf (LA-MC-ICPMS) data combined with available lithogeochemical, whole-rock Sm-Nd and Rb-Sr isotopic data constrain the tectonic evolution of the JFC to a Siderian-Rhyacian evolved arc system. Five samples were selected for U-Pb geochronology and Lu-Hf analysis on zircon grains: a low-K tholeiite, one TTG, two sanukitoids, and a mafic tholeiite. Combined whole-rock Sm-Nd, Rb-Sr isotopes, and zircon Lu-Hf isotopic data suggest the following evolution: an initial moderately juvenile (2445 Ma) low-K tholeiitic magmatism (εNd(t) of − 0.1; εHf(t) from − 8 to + 3), followed by a moderately juvenile to evolved (2200–2182 Ma) coeval TTG-sanukitoid pulse (εNd(t) from − 0.6 to − 4.8; εHf(t) from − 11.7 to + 2.5), and eventually by a moderately juvenile (2130 Ma) tholeiitic magmatism (εNd(t) = − 0.05). The integrated information provides preliminary petrogenetic insights for these magmatic episodes, such as interaction between mantle sources and supracrustal subducted material, melting of the oceanic slab, and crustal assimilation. A ca. 2035 Ma collisional granulite event recorded in metamorphic rims and neoformed zircon grains in leucosomes, possibly marks the amalgamation of the JFC onto the Mantiqueira Complex. Geochronological and isotopic correlations with coeval terranes of the Minas-Bahia Orogen in the Brazilian and African counterparts are addressed for tectonic implications at a broader scale.

Tectonic Geoheritage as Records of Western Gondwana History: a Study Based on a Geosite’s Potential in the Central Ribeira Belt, Southeastern Brazil

The geological characterization of geosites is essential in the process of valorization of the geoheritage, as it is the first step to reveal its scientific relevance and to promote geoscience dissemination. The structural and petrological analysis of the geosite “Boudins of the Camburizinho Islet”, in the northern coast of Sao Paulo state, allowed the characterization of important geological features that record the complex regional evolutionary history from the Neoproterozoic to the present in the southeastern Brazilian margin. These structures are related to the amalgamation and breakup of the Western Gondwana, represented by ductile structures from the Brasiliano Cycle in the central Ribeira Belt and the Mesozoic brittle deformation. Field techniques associated with geospatial data acquired by UAV were used for geological mapping, structural interpretation, and development of dissemination products, including a virtual geological tour. These products proved to be highly effective in both scientific and outreach approaches, justifying its selection as a geosite and opening perspectives on its sustainable use on geotourism and education.

Provenance of Neogene deposits of Barreiras Formation in the southeastern Brazilian continental margin

The Barreiras Formation records continental-to-shallow–water marine sequences deposited during the Paleogene–Neogene periods in the Brazilian continental margin. In northern Rio de Janeiro and southern Espirito Santo states, the Barreiras Formation preserve alluvial depositional system. The sediments were derived from a source located in the hinterland of the southeastern Brazilian continental margin, where rocks formed or reworked notably during the Gondwana supercontinent assembly are exposed. Detrital zircon U–Pb ages are mostly late Neoproterozoic, dominantly close to 608 Ma. Source rocks of this age occurred within the Ribeira and Aracuai belts and were formed during their main magmatic activity. U–Pb zircon age distribution is similar to the southern and central Ribeira belt. Zircon fission-track ages occur in four main groups, between 429 and 358 Ma, 351 and 274 Ma, 270 and 171 Ma, and 167 and 127 Ma. Complex variation in the detrital zircon fission-track ages is related to the continental crust thermal evolution in the source areas. Older ages also occur between 534 and 433 Ma (Cambrian-to-Silurian periods) and are chrono-correlated to post-orogenic processes after the Gondwana supercontinent agglutination. Zircon fission-track ages between 429 and 274 Ma (Silurian to Permian periods) are related to the formation of the Pangea supercontinent, whereas the predominant zircon fission-track age group, between 270 and 171 Ma (Permian to Jurassic periods), is chrono-correlated to both orogeneses in the Gondwana supercontinent west margin and the Pangea supercontinent breakup. Zircon fission-track ages from 167 to 127 Ma are in the same period as the opening of the North and South Atlantic oceans. Data indicate that the thermal evolution of the source region either during the Gondwana supercontinent and South America Platform stages is complex.

Reply to Comment by Heilbron and Valeriano on “Tectono Metamorphic Evolution of the Central Ribeira Belt, Brazil: A Case of Late Neoproterozoic Intracontinental Orogeny and Flow of Partially Molten Deep Crust During the Assembly of West Gondwana”

AbstractThe exercise of paleogeographic/tectonic reconstruction of past orogenic belts is a complex task that includes the interpretation and integration of multi‐technique approaches such as basin tectonics, structural geology, petrology, geochemistry, geochronology, and geodynamics. Proterozoic geologic records are fragmented and incomplete, which make efforts to reconstruct paleogeography and orogenic processes even more challenging. Therefore, for our understanding of the tectonic evolution of past orogenic systems to advance, it is essential that any existing model, including the ones proposed by Heilbron and Valeriano (2020, https://doi.org/10.1029/2019tc005897), are repeatedly exposed to debate and testing. In this reply we address the points raised by Heilbron and Valeriano in order to clarify the scientific foundations of our tectonic interpretation for one of the Neoproterozoic Brasiliano/Pan‐African orogens in South America (the Central Ribeira Belt), presented in Meira, Garcia‐Casco, Hyppolito, et al. (2019, https://doi.org/10.1029/2018TC004959). We also evaluate the proposed paleogeographic reconstructions for this part of West Gondwana and conclude that an intracontinental model better explains the currently available data and observations, including the space problem pointed out in recent publications, and that existing geochemical and geochronologic data by themselves are not conclusive with respect to an unequivocal tectonic environment.

Comment on “Tectono Metamorphic Evolution of the Central Ribeira Belt, Brazil: A Case of Late Neoproterozoic Intracontinental Orogeny and Flow of Partially Molten Deep Crust During the Assembly of West by Meira et al.

AbstractThe commented article contributes with new metamorphic petrology and U‐Pb geochronology data concerning the sin‐collisional stage of two terranes (Embu and Costeiro) belonging to the southern Ribeira orogen (not Central Ribeira belt, as the title suggests), in order to propose an intracontinental evolution for the Ribeira‐Araçuaí orogenic system. Several issues regarding their review of the literature and interpretation of data should be considered in face of the proposition for an intracontinental evolution of an orogenic system that has long been considered as having developed through a Wilsonian sequence of events, involving continental rifting and ocean development, subduction and building of magmatic arc, terrane accretion, and continental collision in the context of amalgamation of Gondwana. In order to contribute to a comprehensive discussion of the tectonic evolution of the Ribeira‐Araçuaí orogenic system, we bring forward the following points that have not been properly acknowledged by the authors in their proposed tectonic scheme. Without substantial new data set and without integrating all the available age, petrologic and deformational data, there seems not to be commented enough evidence for the proposal of an intracontinental evolution for the Ribeira‐Araçuaí orogenic system. In contrary, an accretionary to collisional model is preferred because it in fact integrates all the documented tectonic features: subduction of passive São Francisco paleocontinental margin, generation of magmatic arcs with typical signatures of modern ones elsewhere in the world, three accretionary‐collisional episodes, and an episode of orogenic collapse in the Cambro‐Ordovician transition, associated to slab breakoff and upwelling of asthenosphere.

Late Tonian within-plate mafic magmatism and Ediacaran partial melting and magmatism in the Costeiro Domain, Central Ribeira Belt, Brazil

The tectonic evolution of the Ribeira Belt comprises a protracted history involving several magmatic, deformational and metamorphic events mainly associated with the Neoproterozoic Brasiliano Orogenic Cycle. Most tectonic models relate the evolution of the Ribeira Belt to a protracted orogenic history with diachronous accretion of different terranes against larger cratonic blocks. However, recent proposals challenge models of multi-stage terrane accretions. In this paper we revisit the current tectonic models proposed for the Central Ribeira Belt, based essentially on a large data compilation and new geochemical and geochronological data from samples of the Costeiro Complex. The critical evaluation of these data indicates a long-lasting shared evolution for two geological domains in the Central Ribeira Belt (Embu and Costeiro Domains), usually thought to be different terranes accreted during the late Ediacaran. The first report of late Tonian within-plate tholeiitic to alkaline mafic magmatism and the late Cryogenian-early Ediacaran metamorphism in the Costeiro Complex support the interpretation of shared evolution on both Embu and Costeiro Domains, challenging the current models of diachronous accretion of terranes during the late Ediacaran in the Central Ribeira Belt.

Tectono‐Metamorphic Evolution of the Central Ribeira Belt, Brazil: A Case of Late Neoproterozoic Intracontinental Orogeny and Flow of Partially Molten Deep Crust During the Assembly of West Gondwana

AbstractRecent advances in understanding the plate tectonics, intracontinental deformation, and flow of partially molten crust have significantly improved our knowledge of collisional tectonics and the way in which we understand complex ancient orogens. The Central Ribeira Belt represents a Neoproterozoic fold‐and‐thrust belt formed in the Brasiliano Orogenic Cycle associated with the assembly of West Gondwana. This fold‐and‐thrust belt is currently interpreted as a result of recurrent collisions and amalgamation of terranes against large cratons. Based on an integrated structural, petrological, and geochronological study in two metamorphic complexes of the Central Ribeira Belt (Embu and Costeiro complexes), we challenge the current model that involves multiple terrane collisions. Our data show for the first time metamorphic ages older than 600 Ma for samples from Costeiro and Embu complexes and suggest that both geological units experienced an intermediate‐P metamorphism (M1) at circa 620 Ma and a low‐P metamorphism (M2) at circa 575 Ma. Our proposed tectonic model is consistent with an M1 event related to an intracontinental orogeny, formed in response to the collision between the São Francisco Craton and the Paranapanema Block. On the other hand, the later M2 metamorphism records extensional and wrench tectonics associated with orogenic collapse, constrained by the decompression paths of the metasedimentary sequences and M2‐related S3 mylonitic foliation. The M2 metamorphism is associated with wide, right‐lateral strike‐slip shear zones and voluminous peraluminous magmatism in the Embu Domain and widespread partial melting of the middle crust forming migmatitic rocks and peraluminous leucogranites in the Costeiro Domain.

The Barreiro suite in the central Ribeira Belt (SE-Brazil): a late Tonian tholeiitic intraplate magmatic event in the distal passive margin of the São Francisco Paleocontinent

New geochemical, U-Pb, Lu-Hf and Sr-Nd data from the Barreiro Suite metabasites in comparison with enclaves within the distal Andrelândia Group and the orthogranulites of the Juiz de Fora Complex are presented. Geochemical data suggest intraplate setting, with high and low- TiO 2 , TDM Nd ages between 1.80 and 1.41 Ga, negative ENd t and ( 87 Sr/ 86 Sr) i between 0.714 and 0.747. Results contrast with part of the Juiz de Fora Complex enclaves, with island arc tholeiites- calcalkaline basalts (IAT-CAB) geochemical signatures, TDM Nd ages between 2.58 and 2.16 Ga, positive ENd t values and ( 87 Sr/ 86 Sr) i between 0.700 and 0.712. U-Pb data for the Barreiro Suite yielded a crystallization age of 766 ± 13 Ma and a metamorphic overprint of 619 ± 6 Ma. The results indicate three episodes of mafic magmatism in the Occidental terrane of the Ribeira Belt. The two older episodes are related to Rhyacian arc evolution (ca. 2.2 to 2.1 Ga) and to the Statherian (ca. 1.7 Ga) tectonics, and occur only within the Juiz de Fora Complex, while the younger ca. 766 Ma episode constrains the timing of distal passive margin evolution. An important implication is that these late Tonian metabasic rocks could have been a source of detrital zircons for the sedimentation along the distal Andrelândia basin.

Geochemistry, Sm-Nd isotopes and SHRIMP U-Pb geochronology of the Morro do Coco Granite (RJ, Brazil): another piece of the post-collisional magmatism of the Ribeira Belt

Abstract The Morro do Coco granite (MC) is a post-collisional intrusive body that was emplaced at Ribeira Belt, in the Cambro-Ordovician, succeeding the final stages of the Gondwana supercontinent amalgamation. This unit requires more studies, contrasting with similar best investigated occurrences of the central Ribeira Belt. This work presents a detailed investigation of such unit using petrography, Inductively Coupled Plasma (ICP), X-Ray Fluorescence (XRF) whole-rock geochemistry, U-Pb (SHRIMP) geochronology and Sm-Nd isotopes. The rock is a nearly undeformed syenogranite, geochemically classified as a high-K calc-alkaline magnesian, with metaluminous to slightly peraluminous affinity. Strongly light rare earth elements (LREE)-enriched patterns with significant negative Eu anomalies, typical of post-collisional granites, are identified. Similarly to other post-collisional granites of the central Ribeira Belt, U-Pb zircon data provide crystallization age of 496 ± 3 Ma . Sm-Nd isotopic data suggest crustal magma source as indicated by eNd(496) of -9, 143Nd/144Nd ratios between 0.511747 and 0.511752 and Mesoproterozoic TDM ages of 1.3 Ga. Based on its geochemical characteristics, morphology and geochronology, this unit is considered here as a member of Nova Friburgo Suite. Geochronological data defined three pulses of magmatism in this part of the orogen during the Cambro-Ordovician (515, 490 and 460 Ma).

Geologia e geocronologia do Charnockito São Francisco Xavier e do Gabronorito Ribeirão do Paiol (MG/SP) e suas possíveis relações com processos crustais extensionais nos orógenos Brasília Meridional e Ribeira Central

Na região entre as cidades de São Francisco Xavier, Monteiro Lobato, Campos do Jordão e São Bento do Sapucaí, estão expostas rochas metamórficas de fácies granulito de média pressão a anfibolito, associadas a Nappe Socorro e ao Terreno Embu. O Gabronorito Ribeirão do Paiol e o Charnockito São Francisco Xavier constituem corpos magmáticos encaixados em metagranitoides do Batólito Serra da Água Limpa. O Gabronorito Ribeirão do Paiol é isotrópico, tem textura inequigranular seriada hipidiomórfica a xenomórfica e idade de cristalização U-Pb LA-ICPMS de 534 ± 6 Ma. O Charnockito São Francisco Xavier possui textura isotrópica, equigranular a inequigranular seriada hipidiomórfica a xenomórfica e idade de cristalização entre 579 ± 5 Ma (núcleo dos grãos) e 562 ± 8 Ma (borda dos grãos), compatível com a evolução da Faixa Ribeira Central. O geotermômetro baseado em saturação de zircônio em rocha total aponta temperatura de cristalização de aproximadamente 880ºC para o charnockito. Os dois corpos exibem valores de εNd (t) negativos (-7,32 para o gabronorito e -12,41 para o charnockito), indicando contaminação e /ou origem crustal do magma gerador de ambos.

The Serra das Araras Batholith: An example of Ediacaran syntectonic peraluminous granitic magmatism in the southwestern Paraíba do Sul Domain

The Serra das Araras Batholith (SAB), located at the southwest portion of the Paraíba do Sul Domain, Central Ribeira Belt, is composed of orthogneisses with peraluminous character. Combined LA-ICP-MS U-Pb and Lu-Hf isotopic analyses have been performed in zircon grains for a representative sample (SA-3H) of SAB. Data point to Rhyacian to Orosirian (ca. 2.1–1.8 Ga) inherited zircon cores with positive ɛHf(t) values and Ediacaran U-Pb crystallization age (595 ± 8 Ma). A leucogranite dyke (SA-3D), 15–25 cm thick, which crosscuts SAB and a biotite-hornblende quartz diorite enclave (SA-06) have been analyzed. Crystallization ages for the three samples (ca. 600-590 Ma) and ɛHf(t) values, ranging from −5.6 to −29.4 for Neoproterozoic zircon grains, indicate an Ediacaran crustal magmatism involving mid to lower crust anatexis and the reworking of Paleoproterozoic juvenile and crustal rocks.

Superposition of structures in the interference zone between the southern Brasília belt and the central Ribeira belt in the region SW of Itajubá (MG), SE Brazil

ABSTRACT: The study area is localized in the Socorro nappe, part of the southern Brasília belt, with a minor part in the Embu terrane, part of the central Ribeira belt. Three phases of deformation were detected, Dn-1, Dn and Dn+1. Sn-1 seems to be generally transposed into Sn, but in the northwestern part it is well preserved, dipping about 60º to W and SW, with a stretching and/or mineral lineation plunging down dip. The main foliation in most of the area is Sn, dipping about 70º to SSE. Dn folds are tight to isoclinal, with axes that plunge about 40º to SW. Quartz-feldspathic segregation veins are folded by Dn. The structures related to Dn-1 and Dn are cut and modified by four important shear zones ascribed to deformation phase Dn+1. Two samples of a granite that is elongated along the Caxambu shear zone, and also cut by it, were dated. One yielded a crystallization age of 575 ± 5 Ma, and the other one, from the shear zone, an age of 567 ± 8 Ma, interpreted either as representing the age of movement along the Caxambu shear zone, or as metamorphic growth.

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.

Monazite ID-TIMS U-Pb geochronology in the LAGIR laboratory, Rio de Janeiro State University: protocols and first applications to the assembly of Gondwana supercontinent in SE-Brazil

The chemical and spectrometric procedures of the U-Pb geochronology method on monazites, recently installed in the LAGIR laboratory, are described in detail. In addition, preliminary results on monazite samples from the Brasília and Ribeira belts are reported and discussed in the context of the regional geology. Several experiments for calibration of ion exchange chromatographic columns with the AG-1x8 resin, were performed with HCl, using dissolved natural monazite samples. The Pb blanks of reagents are ∼0.5 pg/g in acids and ∼1 pg/g in H2O. The total Pb blanks in chemical procedures were below 22 pg. Preliminary results are presented from three case studies related to Brasiliano orogenic belts of SE-Brazil, which correlate very well with previous age determinations from literature: two sub-concordant grains from an Araxá Group quartzite (southern Brasília belt) define a concordia age of 602.6 ±1.4 Ma; a -0.8% discordant grain from a quartzite of the São Fidelis Group (Costeiro Domain, central Ribeira belt) yielded a concordia age of 535.3 ± 2.4 Ma; two 0.4 % and 1.3 % discordant monazite grains from the post-collisional Itaoca Granite (Costeiro Domain, central Ribeira belt) define a concordia age of 476.4 ± 1.8 Ma.

A new interpretation for the interference zone between the southern Brasília belt and the central Ribeira belt, SE Brazil

In southeastern Brazil, the Neoproterozoic NNW–SSE trending southern Brasília belt is apparently truncated by the ENE–WSW central Ribeira belt. Different interpretations in the literature of the transition between these two belts motivated detailed mapping and additional age dating along the contact zone. The result is a new interpretation presented in this paper. The southern Brasília belt resulted from E-W collision between the active margin of the Paranapanema paleocontinent, on the western side, now forming the Socorro-Guaxupé Nappe, with the passive margin of the São Francisco paleocontinent on the eastern side. The collision produced an east vergent nappe stack, the Andrelândia Nappe System, along the suture. At its southern extreme the Brasília belt was thought to be cut off by a shear zone, the “Rio Jaguari mylonites”, at the contact with the Embu terrane, pertaining to the Central Ribeira belt. Our detailed mapping revealed that the transition between the Socorro-Guaxupé Nappe (Brasília belt) and the Embu terrane (Ribeira belt) is not a fault but rather a gradational transition that does not strictly coincide with the Rio Jaguari mylonites. A typical Cordilleran type magmatic arc batholith of the Socorro-Guaxupé Nappe with an age of ca. 640 Ma intrudes biotite schists of the Embu terrane and the age of zircon grains from three samples of metasedimentary rocks, one to the south, one to the north and one along the mylonite zone, show a similar pattern of derivation from a Rhyacian source area with rims of 670–600 Ma interpreted as metamorphic overgrowth. We dated by LA-MC-ICPMS laser ablation (U–Pb) zircon grains from a calc-alkaline granite, the Serra do Quebra-Cangalha Batholith, located within the Embu terrane at a distance of about 40 km south of the contact with the Socorro Nappe, yielding an age of 680 ± 13 Ma. This age indicates that the Embu terrane was part of the upper plate (Socorro-Guaxupé Nappe) by this time. Detailed mapping indicates that the mylonite zone is not a plate boundary because motion along it is maximum a few tens of kilometres and the same litho-stratigraphic units are present on either side. Based on these arguments, the new interpretation is that the Embu terrane is the continuation of the Socorro-Guaxupé Nappe and therefore also part of the active margin of the Paranapanema paleocontinent. The Brasília belt is preserved even further within the central Ribeira belt than previously envisaged.

U–Pb geochronology of the granite magmatism in the Embu Terrane: Implications for the evolution of the Central Ribeira Belt, SE Brazil

The Embu Domain represents the central part of the Ribeira Fold Belt in São Paulo, Brazil. It hosts several granitic occurrences of varied composition ranging from small granitic bodies that outcrop in a domain bounded by the intersection of two major sutures (Taxaquara and Guararema Faults) and batholitic masses outcropping to the east. Understanding the evolution of such granites is of vital importance to better constrain evolutionary models for the Ribeira Belt. In the present study, a set of samples from eleven main granite occurrences from the east of São Paulo state was selected for geochronological investigation using laser ablation-multicollector-inductively coupled plasma spectrometry (LA-MC-ICP-MS) and thermal ionization mass spectrometry (TIMS) U–Pb dating of zircon and monazite crystals, respectively. The results indicate a remarkable cluster of ages around 590Ma with older events of granite magmatism between 660 and 600Ma registered for four plutons, indicating a long history of crustal reworking and magma generation.The ages of reworked sources were evaluated from inherited zircon cores. Although highly discordant these point to the predominance of Paleoproterozoic (2.4–2.1Ga) sources, with minor contributions from Mesoproterozoic (1.1–0.9Ga) and Archean sources (∼3.1Ga). The new data bring important insights into the role played by the Embu Domain on the paleogeography and evolution of the Ribeira Belt.

U-Pb LA-MC-ICPMS geochronology of Cambro-Ordovician post-collisional granites of the Ribeira belt, southeast Brazil: Terminal Brasiliano magmatism in central Gondwana supercontinent

Abstract New U-Pb ages for zircon and titanite obtained by LA-MC-ICPMS are reported for post-collisional granites from the central Ribeira belt (Rio de Janeiro State, southeast Brazil). These post-collisional, I-type, megaporphyritic and equigranular leucogranite plutons and dykes intrude high-grade metasedimentary units, orthogneisses, and migmatites within the root zone of the deeply eroded Neoproterozoic-Cambrian Ribeira belt. The ages obtained are: 511.2 ± 6.9 Ma (zircon) for the Surui Granite; 490.3 ± 8.7 Ma (zircon) for a cross-cutting dyke of the pegmatitic facies of the Andorinha Granite from the same outcrop; 480.7 ± 6.1 Ma (zircon) for the Frades Granite; 488.7 ± 4.2 Ma (titanite) for the Nova Friburgo Granite; and 490.9 ± 9.8 Ma (zircon) for the Sana Granite. These new U-Pb ages and those compiled from the literature for post-orogenic intrusions distributed ∼400 km along the strike of the orogen (Rio de Janeiro and Espirito Santo States), define two separate intervals for magmatic activity, which are consistent with mineralogical and structural signatures. The magmatic intervals consist of an older Cambrian magmatic pulse occurred at ca.512 Ma (Pedra Branca, Surui and Buarama plutons), and a younger Ordovician event at ca. 486 Ma (Mangaratiba, Favela, Andorinha, Frades, Nova Friburgo and Sana granites). The Cambrian pulse post-dates the end of the first and main collisional phase by ca. 35 m.y. It also post-dates the onset of the second collisional episode by ca. 20 m.y. The late-Ordovician magmatic pulse post-dates the end of the second collisional episode by ca. 25 m.y. In map view, the alignment of the post-collisional plutons and stocks depicts a sinuous belt running along the eroded roots of central/northern Ribeira and Aracuai belts. This granite belt probably marks the zone where preferential heating and melting of lower continental crust took place, either caused by breaking off of subducted slab, or by the extensional collapse of hot, overthickened continental crust.

Tourmaline and rutile as indicators of a magmatic-hydrothermal origin for tourmalinite layers in the São José do Barreiro area, NE Ribeira Belt, southern Brazil

Tourmalines from tourmaline-rich layers intercalated with schists of the Rio Una Unit of the Embu Complex and from coarse-grained tourmalinite layers associated with quartzite from Sao Jose do Barreiro and Formoso (Central Ribeira Belt, Sao Paulo State, Brazil) were analyzed for major, trace- and rare-earth elements and boron stable isotopes. Two main phases of tourmaline formation were identified by mineral chemistry. The tourmalines from the schist-hosted tourmaline layers are characterized by relatively low MgO (from 4.7 to 6.5%), Na2O (1.5 to 2.1%) and CaO (from 0.2 to 1.1%) contents and high Al2O3 (from 32 to 35%) and FeO (from 6.7 to 9.0%) contents, and also by two (REE)CN patterns, one represented by (La/Yb)CN from 2.7 to 4.3 and positive Eu anomalies, and the other by (La/Yb)CN from 0.2 to 0.3 and practically no Eu anomaly. The variations in major-element contents reflect the composition of the rock being metamorphosed and in which tourmaline is crystallizing, whereas the (REE)CN patterns indicate the evolution of the metamorphic fluid in face of changing metamorphic conditions. The tourmaline of the tourmalinite layers intercalated in quartzite is characterized by relatively high Al2O3 (from 32.3 to 33.92%), FeO (from 6.54 to 7.3%) and Na2O (from 1.8 to 2.1%) contents and very low total REE (3.5 ppm) contents, in particular of HREE. The (REE)CN pattern for this tourmaline is characterized by a positive Eu anomaly, indicating a high fluid/rock ratio. The δ11B values for this tourmaline fall in the -12.3 and -13.9‰ interval. On the other hand, the tourmaline of a massive tourmalinite also associated with quartzite has the highest MgO (from 7.3 to 9.7%), CaO (from 0.8 to 2.5%), F, Th, U, Hf, Zr, Y, Sr and total REE (305 ppm) contents and the lowest Al2O3 (from 28.6 to 31.8%) and FeO (from 5.4 to 8.3%) contents, when compared to the other tourmalines analyzed. Differing from all other (REE)CN patterns, the one that characterizes this tourmaline is LREE-enriched [(La/Yb)CN = 5.8] and shows a slightly negative Eu anomaly. Negative δ11B values in the -13.9 and -15.8‰ interval indicate, as the other interval, filiation with S-type granite magmatism-hydrothermalism. The massive tourmalinite also contains zircon, scheelite, and monazite as accessory minerals and relatively abundant (green and red) rutile. The green rutile is enriched in Nb, Al, Fe and Sn when compared to the red rutile, which is Cr rich. The differences in composition observed in the tourmaline and rutile of the massive tourmalinite indicate the involvement of magmatic-hydrothermal fluids with metamorphic fluids from which tourmaline and other minerals crystallized. The syn-collisional, S-type Sao Jose do Barreiro Granite is the possible source for the magmatic-hydrothermal fluids.