Brasiliano Orogenic Cycle Research Articles

The Nico Pérez terrane (Uruguay) and its archean and paleoproterozoic inheritance

A U-Pb SHRIMP zircon geochronological study was carried out in the Nico Pérez Terrane in the central-eastern portion of Uruguay with the aim of constraining its geological evolution and its cratonic affinity. Nico Pérez Terrane is made up by a mosaic of tectonic blocks with diferente lithologies and sizes. This terrane is limited to the west by the Piedra Alta Terrane through the Sarandí del Yí shear zone, and to the east, is in tectonic contact through the Retamosa thrust with the Dom Feliciano Belt (Brasiliano orogenic cycle). Lithologically, the Nico Pérez Terrane is composed by medium to high grade metamorphic rock contained in three tectonic blocks (Pavas, Valentines and Rivera blocks) represented mainly by granitoids, as well as ortho and parametamorphic rocks, such as amphibolites, metapyroxenites, BIFs, schists, quartzites with fuchsite, among others. Archean inheritance, Rhyacian, Statherian, and Neoproterozoic magmatic ages are reported here. Similar ages, which show Archaean inheritance and Neoproterozoic imprint, reported in units of the Piedra Alta Terrane modify the idea that the Río de La Plata Craton only corresponds to a juvenile Paleoproterozoic tectonic unit. This, together with recently published geophysical information, supports the cratonic affinity of the Nico Pérez Terrane with the Río de La Plata Craton.

Zircon U-Pb geochronology and Hf isotopes of the Luís Alves Terrane: Archean to Paleoproterozoic evolution and Neoproterozoic overprint

The Luís Alves Terrane in southern Brazil is one of the largest expositions of the Archean to Paleoproterozoic units that acted as basement for the development of the Neoproterozoic Pan-African/Brasiliano orogenic belts in the Mantiqueira Province, in South America. Combined field observations, petrography, Hf and U-Pb LA-ICP-MS zircon and titanite isotopic data are presented for the basement of this terrane. The zircon U-Pb dataset spans a large time range with concordant ages from 3.2-1.8 Ga. Several magmatic and metamorphic events are recorded by the U-Pb concordant ages reflecting the complex evolution of this crustal block. The oldest inherited zircon grains have Mesoarchean ages (3.2–3.1 Ga), while two other magmatic populations yield ages of 2683 ± 70 Ma and 2498 ± 11 Ma. Most zircon grains crystallized during two high-grade metamorphic events at 2352 ± 23 Ma and 2183 ± 17 Ma. Titanite from two samples crystallized at 2.02–1.99 Ga, probably during a retrograde metamorphic trajectory that re-equilibrated the studied samples in amphibolite-facies conditions. A late pegmatite dyke was emplaced between ca. 1.96 and 1.79 Ga. Cathodoluminescence images show that the analyzed zircon samples, including those in the pegmatite, had their internal structure modified by hydrothermal processes. The localization of the outcrop close to the rim of the Campo Alegre Basin affected by an important Neoproterozoic hydrothermal event as well as normal and reverse discordant U-Pb ages in our dataset indicate that this event took place during the Brasiliano orogenic cycle at the end of the Neoproterozoic. The Hf isotope data show predominantly crustal signatures with εHf(t) values ranging from 1.3 to −16.8. Hf TDM model ages cluster in two groups (4.32–3.48 and 3.38–2.27 Ga) indicating two pulses of magma differentiation from the mantle with subsequent mixing of the material during the Paleoproterozoic high grade metamorphic events.

The architecture of the Eopaleozoic Cococi Basin, northeastern Brazil: 2D geological modelling from magnetic and gravimetric data

Cococi Basin is a small Eopaleozoic syneclise that is overlying basement rocks of the Neoproterozoic Central Ceará Domain of the Borborema Province, northeastern Brazil. Its origin is associated with the final stages of the Brasiliano orogenic cycle (600 My). The Basin classified as rift type has its basement constituted by rocks of the Canindé Unit and the crosshead complex and delimited by Senador Pompeu and Tauá shear zones. Three geophysical cross-sections were created using aero-magnetometric and field gravimetric data, aiming to outline the litho-structural underground features of the Basin. We have used the following methods for processing and interpreting geophysical data: minimum curvature, analytic signal amplitude, Euler deconvolution, and 2D modeling. The obtained depth solutions were interpreted and combined with geological data to generate geological models featuring the limits and depth of the Basin, its internal main structures, and its lithological variations. The developed geophysical-based geological modeling unravels the yet unknown geological features of the Cococi Basin, which corroborates with geological field observations, surface mapping and the results of previous investigations.

Contribuição dos processos magmáticos e tectono-metamórficos na gênese dos minérios sulfetados de Ni-Cu de Mangabal I e Mangabal II, Goiás, Brasil

Os complexos estratiformes Mangabal I e Mangabal II fazem parte de um conjunto de intrusões máfico-ultramáficas sinorogênicas, brasilianas, que ocorrem na região central do Brasil. Neste trabalho, são apresentados novos dados petrográficos e de microscopia eletrônica de varredura dos sulfetos de Ni-Cu de ambos os complexos. Em um litotipo com textura cumulática preservada, ocorrem as associações pirrotita-pentlandita e pirrotita-pentlandita-calcopirita na forma de glóbulos de sulfetos intercumulus. A pirrotita apresenta conteúdos de ferro entre 64,9 e 65,5%, a pentlandita apresenta conteúdos de níquel entre 14,6 e 25,3% e a calcopirita apresenta conteúdos de cobre entre 29,1 e 31,5%. Nos litotipos intensamente transformados pelo metamorfismo e que não preservam textura ígnea, ocorrem as associações pirrotita-pentlandita, pirrotita-pentlandita-calcopirita e pirrotita-pentlandita-calcopirita-pirita, todas associadas a concentrações variadas de rutilo. Essas associações apresentam pirrotita com conteúdos de ferro entre 59,1 e 62,1%, pentlandita com conteúdos de níquel entre 15 e 40% e calcopirita com conteúdos de cobre entre 32,9 e 36,6%. A associação pentlandita-pirita, bem como vênulas compostas de pirita ou calcopirita, constituem fases de substituição e remobilização dos sulfetos preexistentes. Baseando-se na mineralogia, na textura e nos conteúdos de metais base, observa-se que as associações de sulfetos encontradas nos litotipos intensamente transformados pelo metamorfismo são semelhantes em ambos os complexos e diferem das associações encontradas no litotipo que preserva textura ígnea. Dessa forma, sugere-se que os processos tectono-metamórficos atribuídos ao Ciclo Brasiliano ocasionaram remobilizações e mudanças químicas nos sulfetos de Ni-Cu dos complexos Mangabal I e Mangabal II.

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.

Vista Alegre Meta-Ultramaphic body: A key towards tectono-metamorphic evolution of the Araçuaí orogen high-grade core

The Vista Alegre Meta-Ultramafic body is part of a cluster of meta-ultramafic occurrences located in the internal tectonic domain of the Araçuaí-West Congo orogen in southeast Brazil. They consist of several small northeast-trending bodies hosted by an orthogneiss basement and metasedimentary package tectonized and intruded by granites during the Brasiliano orogenic cycle. To improve the existent models of the tectono-metamorphic evolution of the Araçuaí orogen high-grade metamorphic core, we performed a geophysical and detailed mineralogical study of the meta-ultramafic body and its host rocks. The magnetic response indicates three relatively shallow portions instead of one deep continue meta-ultramafic body. These portions are interpreted as mega boudins aligned parallel to the regional foliation. The petrographic and mineral chemistry studies indicate a high-grade progressive metamorphic path followed by a retrograde process. The metamorphic peak must have reached granulite facies conditions related to the collisional stage of the Araçuaí-West Congo orogen. The posterior stabilization, in greenschist facies conditions, represents a metamorphic retrogression related to the thermal event during the post-collisional stage.

Geochemistry and U–Pb SHRIMP zircon geochronology of microgranular enclaves and host granitoids from the South Khasi Hills of the Meghalaya Plateau, NE India: evidence of synchronous mafic–felsic magma mixing–fractionation and diffusion in a post-collision tectonic environment during the Pan-African orogenic cycle

Abstract Felsic magmatism in the South Khasi Hills of the Meghalaya Plateau, NE India, referred herein as the South Khasi granitoids (SKG: 519.5±9.7 Ma), invariably contains rounded to elongate, fine- to medium-grained, mafic to porphyritic microgranular enclaves (ME: 515±13 Ma) showing sharp to crenulate contacts with the host SKG. Compositions of plagioclase, amphibole and biotite in the ME are slightly distinct or similar to those of the host SKG, which appear re-equilibrated through diffusion mechanisms during partial liquid (semi-solid) conditions prior to the complete solidification of the mafic–felsic interacting system maximum at shallow continental crustal depths of approximately 9.5 km ( c. 250 MPa) under oxidizing conditions. Although the ME are chemically modified, both the ME and SKG exhibit a wide chemical variation as high-K 2 O metaluminous (I-type) granitoids. Linear to near-linear variations of chemical elements against SiO 2 may suggest the origin of the ME as the result of the mixing of crystal-charged mafic and felsic magmas in various proportions. However, the data scatter and ill-defined chemical variations can be attributed to chaotic chemical mixing, diffusion and, to some extent, mechanical sorting of the crystals. The identical trace element patterns of the ME and the respective SKG have strengthened the idea of chemical re-equilibration at varying levels between them through diffusion during synchronous mixing–fractionation and mingling. Mean zircon 207 Pb/ 206 Pb ages from the ME (515±13 Ma) and SKG (519.5±9.7 Ma) underline the co-existence of Cambrian mafic and felsic magmas formed during the later stages of the assembly of East Gondwanaland as an integral part of the Pan-Indian–African–Brasiliano orogenic cycle. The ME in SKG thus represent mingled, undercooled, heterogeneous hybrid magma globules formed by linear to chaotic mixing that was synchronous with fractionation of coeval crystal-charged mafic (enclave) and felsic (SKG) magmas, which experienced differential degrees of chemical exchange through diffusion with the surrounding felsic host in an open magma system.

The Bossoroca Complex, São Gabriel Terrane, Dom Feliciano Belt, southernmost Brazil: U[sbnd]Pb geochronology and tectonic implications for the neoproterozoic São Gabriel Arc

UPb LA-ICPMS geochronological analyses were carried out on zircon grains from metavolcanic rocks of the Bossoroca Complex and for one ash tuff of the Acampamento Velho Formation of the Camaquã Basin, in order to understand the evolution of the Neoproterozoic São Gabriel magmatic arc. A total of 42 analyses of igneous zircon grains were performed in three samples. The results yielded UPb ages of 767.2 ± 2.9 Ma for the metavolcanic agglomerate (BOS-02); 765 ± 10 Ma for the metacrystal tuff (BOS-03) and 565.8 ± 4.8 Ma for the ash tuff (BOS-04). The Orogenic Cycle in Brazil is characterized by a set of orogenic belts consisting of petrotectonic associations juxtaposed by two collisional events that occurred at the end of the Neoproterozoic. In southern Brazil this orogeny formed the Dom Feliciano Belt, a unit composed of associations of rocks developed during two major orogenic events called São Gabriel (900–680 Ma) and Dom Feliciano (650–540 Ma). The main São Gabriel associations are tectonically juxtaposed as elongated strips according to the N20–30°E direction, bounded by ductile shear zones. The Bossoroca Complex comprises predominantly metavolcano-sedimentary rocks, characterized by medium-K calc-alkaline association generated in a cordillera-type magmatic arc. The volcanism occurred in sub-aerial environment, developing deposits generated by flow, resurgence and fall, sporadically interrupted by subaqueous epiclastic deposits, suggesting an arc related basin. The São Gabriel Terrane contains the petrotectonic units that represent the closure of the Charrua Ocean associated to the subduction period of the Brasiliano Orogenic Cycle in the Sul-rio-grandense Shield.

The Archean BIF-hosted Lamego gold deposit, Rio das Velhas greenstone belt, Quadrilátero Ferrífero: Evidence for Cambrian structural modification of an Archean orogenic gold deposit

ABSTRACT The Lamego orogenic gold deposit (1.3 Mt measured resources at 7.26 g/t Au) is located at the south-western end of the 5 km-long Cuiaba–Lamego trend, Quadrilatero Ferrifero region, Brazil. Both Archean orogenic gold deposits are situated in the Rio das Velhas greenstone belt with the lithological succession at Lamego consisting of metamorphosed (greenschist facies) mafic volcanic rock, chert and banded iron formation (BIF), and carbonaceous and micaceous pelites. The Lamego fold, which controls the Lamego deposit, is the most visible structure related to the D1–D2 event, with a perimeter of about 4.8 km and a maximum width of 450 m. The fold is defined by the layering of the mafic unit with minor BIF and large exposures of carbonaceous and micaceous pelites. It is a rootless, reclined, isoclinal, cylindrical fold with an axial trace striking northwest–southeast dipping 20° to 30°. The hinge zone is thickened and the limbs are thinned, with the limbs dipping 20° to 30° to the SE. Orebodies consist of the Lamego BIF, where gold-mineralized zones are related to iron-rich bands, and associated silicification zones. Replacement-style mineralization is associated with sulfide bands; mainly pyrite, As-rich pyrite, and arsenopyrite. Two structural generations, G1 and G2, are recognized and encompass a set of structural elements. The G1 structural generation developed in a progressive deformation event and resulted in structures oriented from NE to SW and dipping to the SE. Structures pertaining to the G2 structural generation are oriented N–S dipping to the W. The four major high-grade gold orebodies are Carruagem, Queimada, Arco da Velha, and Cabeca de Pedra. Their gold grade shows a spheroidal pattern and a distribution that varies along the S1–2 foliation. These lenses represent the hinge zone of F2 reclined folds with the plunge of the orebodies controlled by the F2 fold axes. The lower-grade gold lenses are controlled by pinch and swell, and locally expressed quartz boudins developed during D1–D2. They have two orthogonal directions, one to the NW–SE and the other to the NE–SW, thereby defining chocolate-tablet style boudinage. Hydrothermal monazite grains in a mineralized mafic volcanic rock indicate that mineralization formed at 2730 ± 42 Ma (U–Pb SHRIMP). Younger monazite dated at 2387 ± 46 Ma, and xenotime dated at 518.5 ± 9 Ma suggest Siderian and Cambrian imprint in the Lamego deposit area. The Cambrian age reflects the late stages of the Brasiliano orogenic cycle, which is expressed in the Lamego deposit by the S3 crenulation cleavage (trend NS and dip steeply to the E). Importantly, the Cambrian structural modification is responsible for the present geometry of the Lamego orebodies, but is not associated with hydrothermal alteration; it has not introduced any new gold and also has not caused remobilization of the orebodies or ore minerals.

Dating low-grade metamorphism and deformation of the Espinhaço Supergroup in the Chapada Diamantina (Bahia, NE Brazil): a K/Ar fine-fraction study

This study focuses on the northernmost part of the Mesoproterozoic Espinhaço Supergroup that crops out in the Chapada Diamantina. The fine-fraction K/Ar dating obtained on slightly metamorphosed sediments of the siliciclastic Espinhaço Supergroup shows a polyphase deformation history that corresponds to the Brasiliano (Pan-African) orogenic cycle. The isotopic results are interpreted to indicate three age domains coincident with three structurally different domains. Constrained by the Kübler Index ('illite crystallinity') and illite polytypism, the thermal conditions generated during the tectonic activity show a gradual trend from the craton margins to the interior from epizonal to diagenetic. The northern Chapada Diamantina is situated in the foreland of the Riacho do Pontal belt and comprises the sediments of the Espinhaço Supergroup northeast of the Irecê basin. The K/Ar ages for < 2 µm illite fractions range between 645 and 621 Ma [mean 637±9 Ma (2s)] and for < 0.2 µm fraction range between 625 and 603 Ma [mean 614±9 Ma (2s)]. Samples from the central Chapada Diamantina east of the Irecê basin are not affected by a Brasiliano deformation event and therefore, the N-S-trending structures are assumed to be older. The deformation of the southern Chapada Diamantina was established in conjunction with the formation of the Araçuai orogenesis and the inversion and reactivation of the Paramirim impactogen. The last stage of deformation in this area is recorded by the K/Ar fine-fraction dating between 470 and 460 Ma.

The Limoeiro Deposit: Ni-Cu-PGE Sulfide Mineralization Hosted Within an Ultramafic Tubular Magma Conduit in the Borborema Province, Northeastern Brazil

The Limoeiro deposit, located in a high-grade mobile belt of the Brasiliano orogenic cycle (650–500 Ma), represents one of the few magmatic Ni-Cu(-PGE) sulfide discoveries in recent years. The deposit is hosted by an ultramafic chonolith consisting in a partly deformed and metamorphosed tube-like body affected by brittle faults within high-grade gneiss and schist. The absolute age of the ultramafic intrusion that hosts the Limoeiro deposit is not yet known, but regional geologic constraints suggest that both intrusion and host rocks belong to a terrane older than the orogenic cycle. Peak metamorphic parageneses of the ultramafic intrusion and country rocks indicate granulite facies of metamorphism, interpreted as the result of the Neoproterozoic Brasiliano orogenic cycle. The Limoeiro deposit is the first significant discovery of Ni-Cu sulfide in the Borborema province, opening a new window for exploration in this large orogenic belt. Despite the high-grade metamorphism and the tectonic deformation, the primary magmatic structure of the intrusion hosting the Limoeiro deposit is remarkably well preserved. This intrusion forms a tube-like conduit (chonolith) with cross sections a few hundreds of meters in diameter and with a considerable length of more than 4 km. The conduit consists of two distinct sequences of ultramafic rocks, denominated Upper and Lower sequences, each one consisting of a core of harzburgite enveloped by orthopyroxenite, with an irregular and discontinuous outer shell of amphibolite. These ultramafic sequences, characterized by similar composition and structure, have distinctively different S contents resulting from the presence of disseminated Ni-Cu sulfide mineralization only in the Upper sequence. The magmatic system of the Limoeiro deposit is interpreted to result from the input of two major pulses of magma with similar composition, but sulfide saturation and segregation is restricted to the second magma pulse (Upper sequence). Nickel-Cu(-PGE) sulfide mineralization is broadly concordant with the Upper sequence and different orebodies resulted from tectonic slicing of the originally continuous tube-like structure. The bulk of the mineralization in all orebodies consists of thick (up to 150 m) and elongated (up to 1 km) masses of rock containing disseminated sulfides broadly concordant with the chonolithic structure. Thin layers of massive ore occur in the lower portions of some of the orebodies, and represent about 3 vol % of the Limoeiro deposit. The intrusion hosting the Limoeiro deposit is probably the best preserved chonolith that has been overprinted by granulite facies metamorphism within an orogenic terrane. This study indicates that reconstruction of small magma conduits, an important tool for exploration for Ni-Cu sulfide deposits, may be successfully achieved within high-grade orogenic terranes.

The tectonic evolution of the Neoproterozoic Brasília Belt, central Brazil, based on SHRIMP and LA-ICPMS U–Pb sedimentary provenance data: A review

Abstract The Brasilia Belt is a Neoproterozoic orogenic belt in central Brazil, developed between the Amazon, Sao Francisco-Congo and Paranapanema cratons. It consists of a thick sedimentary pile, made up of several stratigraphic units, which have been deformed and metamorphosed along the western margin of the Sao Francisco Craton during the Brasiliano orogenic cycle. In the western part of the belt, a large, juvenile magmatic arc is exposed (the Goias Magmatic Arc), consisting of calc-alkaline plutonic suites as well as volcano-sedimentary sequences, ranging in age between ca. 860 and 650 Ma. Regional-scale, west-dipping thrusts and reverse faults normally mark the limits between the main stratigraphic units, and clearly indicate tectonic transport towards the east. The age of deposition and tectonic significance of the sedimentary units comprising the Brasilia Belt have been a matter of continuous debate over the last three decades. In the present paper, recent provenance data based on LA-ICPMS U–Pb ages of detrital zircon grains from several of these units, are reviewed and their significance for the age of deposition of the original sediments and tectonic evolution of the Brasilia Belt are discussed. The Paranoa, Canastra and the Vazante groups, in the central part of the Belt, have detrital zircon grains with ages older than ca. 900 Ma and are interpreted as representative of the passive margin sequence deposited on the western margin of the Sao Francisco Craton. On the other hand, samples from the Araxa and Ibia groups have a much younger population of Neoproterozoic zircon grains, as young as 650 Ma, and have been interpreted as syn-orogenic (fore-arc?) deposits. The Bambui Group, exposed in the easternmost part of the belt and covering large areas of the Sao Francisco Craton also has young zircon grains and is interpreted, at least in part, as the foreland basin of the Brasilia Belt.

U–Pb SHRIMP ages for the Cerro Bori Orthogneisses, Dom Feliciano Belt in Uruguay: Evidences of a ∼800 Ma magmatic and ∼650 Ma metamorphic event

Neoproterozoic ages of magmatic and metamorphic events were obtained from in situ SHRIMP analysis of zircons from the Cerro Bori Orthogneisses, eastern domain of the Dom Feliciano Belt in Uruguay. Detailed textural analysis of zircons and their ages revealed a much more complex evolutionary history for these rocks than previously thought. Twelve samples were studied and revealed crystallization ages between 802 and 767Ma, determined from the typical magmatic oscillatory zoning domains from the zircons. These magmatic domains are cut by recrystallization fronts and mantled by metamorphic rims. The recrystallization fronts and rims are interpreted to be related to a high grade metamorphic event with a maximum age of ∼676Ma, whereas the rims considered to be related to partial melting are 654±3Ma old. The new magmatic ages demand a reinterpretation of the evolutionary history of this crustal segment, which is one of the few occurrences of the early Brasiliano Orogenic Cycle rocks in South Brazil and Uruguay. The metamorphic/partial melting event is inferred to be related to crustal thickening as a consequence of collision of the Rio de la Plata with the Congo and Kalahari cratons, during the amalgamation of West Gondwana.

Geotectonic setting and metallogeny of the northern São Francisco craton, Bahia, Brazil

Abstract This paper aims at establishing a tectonic and temporal framework to characterize the metallogenic processes that contributed to the origin of the mineral provinces in the northern Sao Francisco Craton. Many Archean mineralizations (eg. massive sulfide zinc, lead, zinc and copper, besides magnesite–talc, iron–titanium–vanadium, iron, chromite and manganese) were generated before the assembly of the Craton. Deposits of chromite, nickel, gold and emerald were produced during the Paleoproterozoic orogenic cycle, when the Craton was amalgamated into the Atlantica paleocontinent. An extension event is recorded in the Neoproterozoic, during the breakup of Rodinia, associated with deposits of phosphorite and uranium. Kimberlite diamond and gold mineralization were generated during the Brasiliano orogenic cycle, coeval with the amalgamation of West Gondwana. A long-lasting and rather uniform crustal stress is recorded in the area during the Cambrian period. Resetting of the isotopic and magnetic systems that affected the Neoproterozoic sediments of the Irece Basin at about 520 Ma was attributed to the regional-scale fluid migration and mineralization in the aftermath of the Brasiliano orogenic cycle.

Neoproterozoic tectonic synthesis of Uruguay

Neoproterozoic–lower Palaeozoic successions in the Brasiliano fold belts are described and a brief synthesis of these terranes is presented in order to erect a tectonic framework for this region. Tectonic events that occurred around the Río de La Plata craton were diachronous and reflected successive stages of the Brasiliano orogenic cycle. They took place in mobile belts that constituted part of the Gondwana supercontinent. The most thoroughly investigated Neoproterozoic sections are located in the eastern and southeastern regions of Uruguay. The Dom Feliciano Belt shows a tectonic evolution from back-arc to foreland basin characterized by fold-and-thrust, thick-skinned belts developed during the Brasiliano/Pan-African orogenic cycle. The most conspicuous features were late-tectonic high-K calc-alkaline granitoids, HT-LP metamorphism, significant displacements along shear zones, and post-tectonic granitoids. The final stage was characterized by post-collisional basins (molassic sequences) and extensional magmatism related to a phase of crustal stretching. Several lithotectonic units are present as basement inliers in the Dom Feliciano Belt: these include a low-to-medium metamorphic grade sequence (the Zanja del Tigre Formation), granitoids and gneisses (the Campanero Unit), high-grade basement of the Cerro Olivo Complex (Palaeoproterozoic or Neoproterozoic), and a low-metamorphic grade orogenic belt (the Rocha Formation). This paper provides a simplified tectonic map of eastern Uruguay, which we use to describe tectonic evolution from Precambrian to early Palaeozoic time.

Avaliação de fontes magmáticas de séries shoshoníticas pós-colisionais com base na normalização pela Associação Shoshonítica de Lavras do Sul - aplicação de sliding normalization

The Lavras do Sul Shoshonitic Association is related to the early stages of post-collisional alkaline magmatism in the Brasiliano Orogenic Cycle in southern Brazil. Their magmas were produced through melting of a litospheric mantle metasomatized during a previous subduction. The LSSA is composed of co-genetic rocks derived from magmas evolved by fractional crystallization which show a large silica range. These features are quite adequate for interpretations based on the sliding normalization technique, which allows comparing magmas with different degrees of differentiation in order to evaluate and discuss their potential sources, petrogenetic mechanisms and geotectonic settings. The LSSA composition was used as the pattern for normalizing data available in bibliography for the Late Precambrian post-collisional Tismana pluton, the 1.8 Ga Svecofennian post-collisional shoshonitic magmatism in the Fennoscandian shield, the monzonitic series from the Variscan Tormes Dome, the post-collisional potassic and ultrapotassic magmatism in northern Tibet, and the Baranadag pluton in Central Anatolia, Turkey. This procedure allowed the identification of two distinct geochemical trends: (i) Zr-Nb-Ti-Y-REE enrichment, and (ii) increasing of Rb-K2O with concomitant Nb decrease. The former probably reflect the participation of an astenospheric component, whilst the second trend is related to crustal assimilation. The Nb-Rb-K2O diagram, used for values normalized by this technique, is proposed for discriminating post-collisional shoshonitic associations with different contributions of astenospheric and crustal materials.

Avaliação de fontes magmáticas de séries shoshoníticas pós-colisionais com base na normalização pela Associação Shoshonítica de Lavras do Sul - aplicação de sliding normalization

The Lavras do Sul Shoshonitic Association is related to the early stages of post-collisional alkaline magmatism in the Brasiliano Orogenic Cycle in southern Brazil. Their magmas were produced through melting of a litospheric mantle metasomatized during a previous subduction. The LSSA is composed of co-genetic rocks derived from magmas evolved by fractional crystallization which show a large silica range. These features are quite adequate for interpretations based on the sliding normalization technique, which allows comparing magmas with different degrees of differentiation in order to evaluate and discuss their potential sources, petrogenetic mechanisms and geotectonic settings. The LSSA composition was used as the pattern for normalizing data available in bibliography for the Late Precambrian post-collisional Tismana pluton, the 1.8 Ga Svecofennian post-collisional shoshonitic magmatism in the Fennoscandian shield, the monzonitic series from the Variscan Tormes Dome, the post-collisional potassic and ultrapotassic magmatism in northern Tibet, and the Baranadag pluton in Central Anatolia, Turkey. This procedure allowed the identification of two distinct geochemical trends: (i) Zr-Nb-Ti-Y-REE enrichment, and (ii) increasing of Rb-K2O with concomitant Nb decrease. The former probably reflect the participation of an astenospheric component, whilst the second trend is related to crustal assimilation. The Nb-Rb-K2O diagram, used for values normalized by this technique, is proposed for discriminating post-collisional shoshonitic associations with different contributions of astenospheric and crustal materials.

Lithofacies associations and structural evolution of the Archean Rio das Velhas greenstone belt, Quadrilátero Ferrífero, Brazil: A review of the setting of gold deposits

The Rio das Velhas greenstone belt is located in the Quadrilátero Ferrífero region, in the southern extremity of the São Francisco Craton, central-southern part of the State of Minas Gerais, SE Brazil. The metavolcano–sedimentary rocks of the Rio das Velhas Supergroup in this region are subdivided into the Nova Lima and Maquiné Groups. The former occurs at the base of the sequence, and contains the major Au deposits of the region. New geochronological data, along with a review of geochemical data for volcanic and sedimentary rocks, suggest at least two generations of greenstone belts, dated at 2900 and 2780 Ma. Seven lithofacies associations are identified, from bottom to top, encompassing (1) mafic–ultramafic volcanic; (2) volcano–chemical–sedimentary; (3) clastic–chemical–sedimentary, (4) volcaniclastic association with four lithofacies: monomictic and polymictic breccias, conglomerate–graywacke, graywacke–sandstone, graywacke–argillite; (5) resedimented association, including three sequences of graywacke–argillite, in the north and eastern, at greenschist facies and in the south, at amphibolite metamorphic facies; (6) coastal association with four lithofacies: sandstone with medium- to large-scale cross-bedding, sandstone with ripple marks, sandstone with herringbone cross-bedding, sandstone–siltstone; (7) non-marine association with the lithofacies: conglomerate–sandstone, coarse-grained sandstone, fine- to medium-grained sandstone. Four generations of structures are recognized: the first and second are Archean and compressional, driven from NNE to SSW; the third is extensional and attributed to the Paleoproterozoic Transamazonian Orogenic Cycle; and the fourth is compressional, driven from E to W, is related to the Neoproterozoic Brasiliano Orogenic Cycle. Gold deposits in the Rio das Velhas greenstone belt are structurally controlled and occur associated with hydrothermal alterations along Archean thrust shear zones of the second generation of structures. Sedimentation occurred during four episodes. Cycle 1 is interpreted to have occurred between 2800 and 2780 Ma, based on the ages of the mafic and felsic volcanism, and comprises predominantly chemical sedimentary rocks intercalated with mafic–ultramafic volcanic flows. It includes the volcano–chemical–sedimentary lithofacies association and part of the mafic–ultramafic volcanic association. The cycle is related to the initial extensional stage of the greenstone belt formation, with the deposition of sediments contemporaneous with volcanic flows that formed the submarine mafic plains. Cycle 2 encompasses the clastic–chemical–sedimentary association and distal turbidites of the resedimented association, in the eastern sector of the Quadrilátero Ferrífero. It was deposited in the initial stages of the felsic volcanism. Cycle 2 includes the coastal and resedimented associations in the southern sector, in advanced stages of subduction. In this southern sedimentary cycle it is also possible to recognize a stable shelf environment. Following the felsic volcanism, Cycle 3 comprises sedimentary rocks of the volcaniclastic and resedimented lithofacies associations, largely in the northern sector of the area. The characteristics of both associations indicate a submarine fan environment transitional to non-marine successions related to felsic volcanic edifices and related to the formation of island arcs. Cycle 4 is made up of clastic sedimentary rocks belonging to the non-marine lithofacies association. They are interpreted as braided plain and alluvial fan deposits in a retroarc foreland basin with the supply of debris from the previous cycles.

Tectonic and fluid inclusion constraints on the origin of quartz veins with giant crystals in the Tocantins structural province (Cristalândia, central Brazil)

Large, lens-shaped quartz veins between the Cristalândia and Pium settlements (Tocantins, Brazil) occur in a N–S-oriented, up to 3 km wide, dextral, orogen-parallel strike-slip shear zone. Formation PT conditions recorded by multiphase brine inclusions in quartz crystals from drusy cavities correspond to 280–400 °C and up to 300 MPa, as indicated by high-density CO 2 bubbles (up to 0.96 g/cm 3) observed in some brine inclusions. The estimated crystallization pressures correspond to approximately 10–12 km of overburden, assuming a lithostatic load. The high salinity brines (up to 48 wt%) cannot be correlated with halite-fractionated seawater due to the low Br concentrations. Similarly, the high salt concentrations cannot be attributed to magmatic fluid released during crystallization of a silicic magma at such great pressures. The brines likely were liberated from a hydrated oceanic crust and cover sediments of the Goiás Ocean, subducting beneath the Amazonian plate during the Neoproterozoic Brasiliano orogenic cycle. Fluids derived from the hydrated crust have been trapped in en échélon-arranged tension gashes during a superimposed transtensional/extensional orogenic stage.