Transamazonian Event Research Articles

Defining Lithospheric Limits with Deep Sourced Geophysics: The Case of São Francisco Craton and Borborema Province

The East Brazilian Shield is a product of the Neoproterozoic convergence and collage of rigid Archean lithospheric blocks, which stabilized at the end of the Paleoproterozoic Trans-Amazonian event (2.0 Ga). The areal extent of the São Francisco Craton is well understood by regional gravimetric surveys, but its northern limit with the Borborema Province is not so clear. The integration of gravity, magnetics and seismic velocity anomaly maps allows mapping this northern limit to the Borborema Province. Applying the vertical integration low-pass filter to satellite magnetics yields a remarkable match to the probable lithospheric suture zone. This magnetic map identifies the deep response from thinner/thicker lithospheres of Neoproterozoic mobile belts or stable cratons. One potential microcraton is the NE extension of the Borborema Province, one site of the oldest rocks in South America, a residue of the Benino-Nigerian Craton, separated from the main occurrence by the opening of the Atlantic. The Sergipana, Riacho do Pontal and Rio Preto Neoproterozoic fold-and-thrust belts mark areas of the foreland tectonism, hundreds of kilometers inside the São Francisco Craton lithospheric block. The presence of younger rocks inside the deep geophysical limits may be evidence of a large detachment of younger continental crust over the old cratonic lithosphere.

Metallogenetic Mn-model of the Rhyacian-aged Buritirama Formation, Carajás domain (Amazon Craton)

The Mn-bearing Buritirama Formation is composed of clastic and chemical metasedimentary rocks representing a Rhyacian platformal basin (ca. 2.18–2.06 Ga) that limits the northern border of the Archean-Paleoproterozoic Carajás domain with the Paleoproterozoic Bacajá domain in the southern Amazon Craton, northern Brazil. The Buritirama Formation is divided into three main stratigraphic units. The Lower and Upper units consist of a basal quartzite followed by carbonate–silicate rocks (marls), dolomitic marbles (dolostone) and minor biotite schist while the Intermediate Unit consists of Mn-carbonate rocks topped by a supergene Mn-oxide deposit (Cenozoic age). These rocks were deformed and metamorphosed during the Transamazonian event (ca. 2.06 Ga).Geochemical data from dolomitic marbles (Lower Unit) exhibit negative Ce anomalies, high Y/Ho ratios (ca. 48) and seawater-like REE + Y patterns that reinforce an open platform setting for the Buritirama basin. Positive δ13C values (+3.2‰ to +5.0‰ V-PDB) for dolomitic marbles are considered to be an expression of the Lomagundi Carbon Isotopic Excursion. The metallogenesis of Mn-carbonate rocks is evaluated based on redox-sensitive trace elements (e.g. Zn and Ni), PAAS-normalized REE + Y patterns and δ13C values (−2.56‰ to +0.15‰ V-PDB) and point out to a classical multi-stage redox-controlled Mn-model involving: i) a primary hydrothermal fluid source related to ocean vents and/or normal faults; ii) microbially-mediated reduction of Mn-oxides particles at the ocean bottom; and iii) reactions between Mn2+ and CO32− (organic matter and seawater related), giving rise to diagenetic Mn-carbonates. The Rhyacian age, Mn deposits, and presence of Lomagundi-like δ13C allows to correlate the Buritirama Formation with the Águas Claras Formation within the Carajás domain further south, thus establishing an important metalliferous event that was up to now largely overlooked within the southern Amazon Craton.

Metamorphism and exhumation of basement gneiss domes in the Quadrilátero Ferrífero: Two stage dome-and-keel evolution?

The presence of dome-and-keel provinces in Archean cratons has been connected with the initiation of plate tectonics on Earth as these features are most commonly observed in Archean rocks. The Quadrilátero Ferrífero in Brazil has been identified as a Paleoproterozoic dome-and-keel province for more than three decades. The prevailing model suggests that it formed during the Rhyacian Transamazonian orogeny, making it unique among dome-and-keel provinces. However, a lack of appropriate lithologies, datable minerals and the metamorphic overprint of later orogenesis has resulted in a cryptic metamorphic record for the formation of this dome-and-keel province. A clinopyroxene-bearing migmatite from the core of the Bação dome has peak P–T conditions of 5–7 kbar and 700–750 °C and a published age of ca. 2730 Ma based on U–Pb ages of zircon from leucosomes, suggesting that this age represents the migmatisation event. A fine-grained epidote-albite-titanite assemblage overprints the coarse-grained clinopyroxene and amphibole, giving P–T conditions of 8–9 kbar and 550 °C with an associated titanite age of ca. 2050 Ma. A garnet-bearing amphibolite sample also from the core of the dome has peak P–T conditions of 7–8 kbar and 650–700 °C, and texturally late titanite from this sample produces an age of ca. 2060 Ma. Three additional samples were collected from the edges of the dome. A garnet-gedrite bearing felsic schist produces peak P–T conditions of 8–9 kbar and 650–700 °C on a clockwise P–T evolution. This sample has a U–Pb zircon age of ca. 2775 Ma, which could date metamorphism or be the age of its volcaniclastic protolith. Texturally unconstrained titanite from the sample gives an age of ca. 2040 Ma. A garnet-bearing amphibolite that occurs as a boudin within the felsic schist gives both zircon and titanite ages of ca. 2050 Ma and has peak P–T conditions of 5–6 kbar and 650–700 °C on a near isobaric P–T path. An amphibolite dike, observed to cross-cut the felsic schist produces a zircon U–Pb age of ca. 2760 Ma. Altogether this data suggests that the samples were metamorphosed in the Archean (ca. 2775–2730 Ma) and again during the Transamazonian event. The most plausible explanation for this data is that dome-and-keel formation occurred in the Archean with migmatisation and high-temperature metamorphism occurring at this time. The Paleoproterozoic event is interpreted as a reactivation of the dome-and-keel formation structures, with Paleoproterozoic keels crosscutting Archean keels and producing metamorphic aureoles. The high radiogenic heat production and the presence of dense sedimentary successions in Archean terranes make dome-and-keel provinces a uniquely Archean feature, but they are susceptible to reworking, resulting in an enigmatic record of formation.

Stratigraphy, petrography and tectonics of the manganese-bearing Buritirama Formation, Northern Carajás Domain, Amazon Craton

The Buritirama Formation (BF) occurs at the extreme north of the Carajas Province, close to the contact with the Bacaja domain, in the southeastern portion of the Amazon Craton (Brazil). The BF consists of a 40 km long, ca. 3 km wide NW-SE trending structure arranged in four imbricated thrusts that individualize three main stratigraphic units. The lower unit is composed of orthoquartzite followed by carbonate-silicate rocks. The intermediate unit hosts a supergene manganese ore deposit formed by weathering of kutnohorite-rich marble. Quartzite/ mica- quartz schist followed by carbonate-silicate rocks make up the upper unit. The local basement is constituted by orthogneiss-migmatite (Xingu Complex) and the Buritirama metagranite. Mineral chemistry data and metamorphic textures record high consumption of carbonate and quartz to produce clinopyroxenes. The structural assemblage of the BF records mass transport from NE to SW and the following deformational phases: D1 (compressional ductile), D2 (compressional brittle) and D3 (extensional brittle). The BF is interpreted as part of a platformal depositional system positioned at the border of the Carajas domain, which was probably inverted, deformed and metamorphosed during the Transamazonian event (ca. 2.1 Ga), in a deformation belt related to the amalgamation between the Carajas and Bacaja domains.

Metamorphic imprints on units of the southern Araçuaí belt, SE Brazil: The history of superimposed Transamazonian and Brasiliano orogenesis

The extensive Neoproterozoic Brasiliano belts that surround the São Francisco craton in southern Brazil are a consequence of tectonic processes that resulted in the formation of the supercontinent Gondwana in the Ediacaran to Cambrian. One of these Brasiliano-age orogens, the Araçuaí belt, overprints an earlier orogen developed during the so-called Transamazonian event at the transition between the Rhyacian and Orosirian periods. Archean and Paleoproterozoic basement units in the southern Araçuaí belt have been shown to have experienced both the Transamazonian and Brasiliano orogenies and have the potential to have preserved the P-T conditions of both events.This study investigates a series of granulite- and amphibolite-facies samples from the southern Araçuaí belt. Granulite-facies samples from the Acaiaca Complex and Pedra Dourada Granulite give monazite U-Pb ages in the range: 2015–1960Ma. Both the granulite samples preserve two metamorphic textures. Coarse-grained garnet+biotite+orthopyroxene+K-feldspar+plagioclase+quartz+ilmenite+inferred melt and later, fine-grained garnet+biotite+hornblende+quartz that grows at the expense of orthopyroxene. Peak conditions for the earlier event were c. 750–800°C and 6kbar, whereas those of the later event are poorly constrained. A garnet-bearing amphibolite-facies sample from the Mantiqueira Complex contains monazite included within garnet, which gives U-Pb ages of c. 1940Ma, whereas matrix monazite grains give ages of c. 590–540Ma. Garnet core compositional zonation indicates P-T conditions of 6kbar and 700°C for the c. 1940Ma metamorphism, while the matrix assemblage records a P-T path culminating in peak conditions of 8–9kbar at 650°C. A second amphibolite-facies sample was obtained from the Dom Silvério Group, whose depositional age post-dates the Transamazonian event. This sample gives two monazite U-Pb age populations, 597±3Ma and 547±5Ma. Garnet core zonation indicates a prograde P-T evolution with peak conditions of c. 8–9kbar and 650°C, although it is uncertain at this time what age population this event corresponds to. The results of this study provide better constraints on the chronology and metamorphic regimes of both the Transamazonian and Brasiliano events in the southern Araçuaí belt.

LATE CAMBRIAN Au-Pd MINERALIZATION AND Fe ENRICHMENT IN THE ITABIRA DISTRICT, MINAS GERAIS, BRAZIL, AT 496 Ma: CONSTRAINTS FROM U-Pb MONAZITE DATING OF A JACUTINGA LODE

Bonanza-style mineralization of palladiferous gold in specular hematite-rich lodes engendered the most important gold rush in Brazilian history, which took place at the end of the seventeenth century in the province that became known as Minas Gerais. The timing of this itabirite-hosted vein mineralization, locally referred to as jacutinga, is controversial; different views have ascribed jacutinga to one of the two orogenic events recorded in the Quadrilatero Ferrifero of Minas Gerais, the ~2.0 Ga Transamazonian event and the ~0.6 Ga Brasiliano event. This controversy is due mainly to the absence of reliable age data for hydrothermal minerals. Here we report the first U-Pb age for a jacutinga lode, obtained by laser ablation-sector field-inductively coupled plasma-mass spectrometry (LA-SF-ICP-MS) on monazite grains recovered from the Conceicao Fe-ore deposit within the Itabira district. The monazite analyses yield a precise Late Cambrian age of 495.6 ± 2.2 Ma, which is well within the time period between ca. 524 and 444 Ma, suggested by recently published geochronologic data on regionally distributed similar hydrothermal systems. The age of 496 Ma for the jacutinga monazite corroborates an orogen-scale hydrothermal overprint at later stages of the Brasiliano event. This hydrothermal overprint also affected surrounding Neoarchean rocks of the Minas Supergroup, as well as younger rocks of the southern Serra do Espinhaco, along the platiniferous Au-Pd belt of Minas Gerais.

Composição Química da Turmalina de Turmalinitos Estratiformes da Mina de Passagem de Mariana, Sudeste do Quadrilátero Ferrífero (MG)

The Passagem de Mariana Mine in the southeastern part of the Quadrilátero Ferrífero, Minas Gerais, Brazil, is one of the oldest Brazilian gold mines. Gold is associated with stratiform tourmalinites and tourmaline-bearing quartz and carbonate veins and breccias. The highest gold contents (up to 200 g/t) have been recorded in the stratiform tourmalinites, which belong to the Batatal Formation (Caraça Group, Minas Supergroup). Three petrographic types of tourmaline have been defined and analyzed by electron microprobe and ICP-MS. Type 1 corresponds to the very fine-grained tourmaline that composes homogeneous tourmalinites. They occur as continuous levels or boudins, and as fragments in quartz-carbonate veins and breccias. Type 2 zoned tourmaline is coarser-grained and composes both homogeneous and banded tourmalinites. Type 3 coarse-grained, zoned tourmaline grows perpendicularly to the contact between the tourmalinite or its fragments and the quartz-carbonate veins and breccias. The main chemical variations observed in the three tourmaline types are decrease of SiO2 and Al2O3 and increase of MgO, Na2O and F contents with increasing CaO and TiO2 contents. Additionally, types 2 and 3 tourmalines also contain higher concentrations of LREE, Rb, Sr, Y, Cs, Th and U in relation to type 1. These chemical variations are coherent with the geochemical conditions that prevail during diagenesis and metamorphism. No evidence was found of the participation of external (e.g., magmatic) fluids in the generation of the stratiform tourmalinites, for which the main source for boron could have been the Batatal Formation carbonaceous metapelites. Therefore, the syngenetic origin of the tourmalinites cannot be completely ruled out. The tourmalinites could have evolved during the prograde metamorphism of the beginning of the Transamazonian Event. Type-3 tourmaline, grown in the contact between the tourmalinite and quartz-carbonate veins and breccias, could have been generated after the metamorphic peak and during the extensional phase of the Transamazonian Event, with gold remobilization in veins and breccias

40Ar– 39Ar geochronology across Archean and Paleoproterozoic terranes from southeastern Guiana Shield (north of Amazonian Craton, Brazil): Evidence for contrasting cooling histories

A 40Ar/ 39Ar geochronological study was performed on amphibole and biotite from some representative units of distinct tectonic domains of the southeastern Guiana Shield, north of the Amazonian Craton, the Amapá Block and the Carecuru Domain. In the Amapá Block, an Archean continental block involved in the Transamazonian orogenesis (2.26–1.95 Ga), the investigated minerals, from rocks of the Archean high-grade basement assemblage, give only Paleoproterozoic ages, indicating their complete resetting during the Transamazonian orogenic event. Amphibole ages vary from 2087 ± 3 to 2047 ± 20 Ma, and biotite ages spread mainly between 2079 ± 18 and 2033 ± 13 Ma. In the Carecuru Domain, in which the geodynamic evolution is related to Paleoproterozoic magmatic arc setting during the Transamazonian event, calc-alkaline granitoids yield amphibole age of 2074 ± 17 Ma, and biotite ages of 1928 ± 19 Ma and 1833 ± 13 Ma. These data reinforce the importance of the Transamazonian orogenic cycle in the investigated area, and indicate that the rocks were not significantly affected by post-Transamazonian events. When coupled with available U–Th–Pb monazite and Pb–Pb zircon geochronological records and petro-structural observations, the new 40Ar/ 39Ar data delineate contrasting cooling and exhumation histories for the tectonic domains. In the Amapá Block, the data suggest nearly vertical T– t paths that reflect fast cooling rates, which indicate tectonically controlled exhumation, related to collisional stages of the Transamazonian event, between 2.10 and 2.08 Ga. Conversely, in the Carecuru Domain, low cooling rates suggest that the arc-related granitoids underwent slow and monotonous cooling since their emplacement until reaching the biotite isotopic closure temperature.

Paleoproterozoic (2155–1970 Ma) evolution of the Guiana Shield (Transamazonian event) in the light of new paleomagnetic data from French Guiana

We present a comprehensive paleomagnetic study on Paleoproterozoic (2173–2060 Ma) plutonic and metamorphic rocks from French Guiana, representative of the full range of the main Transamazonian tectonothermal steps. Twenty-seven groups of directions and poles were obtained from combination of 102 sites (613 samples) based on age constraint, similar lithology and/or geographical proximity. Paleomagnetic results show variations between rocks of different ages which are supposed to be characteristic of magnetizations acquired during uplift and cooling of successive plutonic pulses and metamorphic phases. This is also reinforced by positive field tests (baked contact and reversal tests). Recent U/Pb and Pb/Pb on zircon and complementary 40Ar/ 39Ar on amphibole and biotite allow questioning the problem of magnetic ages relative to rock formation ages. Estimated magnetic ages, based on amphibole dating as a proxy, enable us to construct a Guiana Shield apparent polar wander path for the 2155–1970 Ma period. It is also possible to present paleolatidudinal evolution and continental drift rates related to specific Transamazonian tectonic regimes. French Guiana and probably the Guiana Shield were located at the Equator from ca. 2155 to 2130 Ma during the Meso-Rhyacian D1 magmatic accretion phase, related to subduction of Eorhyacian oceanic crust. After closure of the Eorhyacian Ocean and collision of West African and Amazonian plates, the Guiana Shield moved. The first evolution towards 60° latitude, occurs after 2080 Ma, during the Neorhyacian D2a post collisional sinistral transcurrent phase. During the Late Rhyacian D2b phase, up to 2050 Ma, the Guiana Shield reaches the pole and starts to move to lower latitudes on an opposite meridian. By the Orosirian D2c phase, from ca. 2050 to 1970 Ma, the Guiana Shield reaches the Equator. Based on the amphibole 40Ar/ 39Ar dates, we estimate the continental drift between 12 and 16 cm/y for the Meso to Late Rhyacian period followed by a lower rate between 9 and 14 cm/y up to Orosirian time. This study highlights rock ages and magnetic ages are prerequisite to any continental reconstruction especially when it is shown continental drift is important for a 100–200 Ma time period. Our results confirm the possibility of APWP construction on Paleoproterozoic plutonic rocks but suggest improvement will rely on the combination with multidisciplinary approaches such as structural geology and multi-method radiometric dating.

Determination of the physico-chemical characteristics of hydrothermal fluids from the post-metamorphic Omai gold deposit, Guiana Shield, using analysis of ionic species by crush-leach technique and capillary electrophoresis

Capillary electrophoresis (CE) or capillary ion analysis (CIATM) represents a relatively new, simple and fast analytical technique suited in particular to the analysis of aqueous fluids and leach solutions. The mechanisms responsible for separation in CE are based on the ion charge/size ratios. The application of capillary electrophoresis to a number of key vein-forming minerals is described for the first time for a Precambrian orogenic gold deposit. The Omai deposit is situated in the north-central part of the Barama-Mazaruni Supergroup, Guiana Shield, a Paleoproterozoic granitoid-greenstone belt metamorphosed during the Trans-Amazonian tectonothermal event at ca. 2.1 Ga.

INCLUSÕES FLUIDAS EM GRANULITOS E CAMINHOS P-T-t RETROMETAMÓRFICOS PARA O COMPLEXO JUIZ DE FORA

Fluid inclusion analyses in granulitic rocks are presented as an important source for relevant petrogenetic data for the study of the metamorphic evolution of the Juiz de Fora Tectonic Domain (DTJF) - Ribeira Belt, in the southwest Juiz de Fora (MG) region. The area is characterized by an intense “mixing” between basement units, composed by hornblende gneisses and orthogranulites of the Juiz de Fora Complex, and metasedimentary units, correlated to the Andrelândia Group. The fluid inclusions are typical of granulite facies, as they consist of 81 to 93% CO 2 with moderate densities, varying between 0.79 and 0.94 g/cm 3 , for the basement rocks; and 92 to 100% CO 2 , and moderate to high densities varying from 0.916 to 1.068 g/cm 3 for the metasediments of DTJF. The fluids associated to basement rocks have shown that the latest formed inclusions (trails) present systematically higher densities than the earlier ones (isolated), pointing to a near isobaric cooling retrometamorphic path, during the Transamazonian Event. The likely P-T-t path for the Andrelândia Group supracrustal rocks, defined by a pattern of progressive lowering in inclusion densities (from D 2 -phase to D 4 -phase associated samples), indicates a more significant decrease in pressure than temperature after the metamorphic peak, suggesting an isothermal decompression situation, with a clockwise P-T-t path for the Brasiliano Event. The available data for structural domains where basement rocks prevail indicate that the granulite facies metamorphism can be related to a Transamazonian extensional tectonic event, associated to magmatic intra and underplating, providing CO 2 -rich fluids derived from mafic rocks and the necessary excess heat to the older granulite metamorphism (4-6kb/800-850 o C). During the Brasiliano, the collisional event that generated the Ribeira Belt caused intense anatexis that possibly removed large amounts of H 2 O from the system, leading the metasediments to reach granulite facies metamorphism during the Brasiliano Event, with temperatures not higher than 800 o C and pressures between 6-7 kb.

Ages of Detrital Zircon from Siliciclastic Successions South of the São Francisco Craton, Brazil: Implications for the Evolution of Proterozoic Basins

In this work we report 207Pb/ 206Pb LA-ICPMS ages of 152 detrital zircons from lower greenschist facies quartzites from Proterozoic basin successions of the southern border of the São Francisco Craton, southern Minas Gerais State, Brazil. These are the intracratonic São João del Rei basin, the intraplate continental margin Andrelândia basin, and the Serra do Ouro Grosso sequence, developed on a crystalline basement older than 1.8 Ga, and deformed and metamorphosed during the Brasiliano Orogeny, ca. 0.59–0.50 Ga. The data constrain both the ages of the sources and the interval of sedimentation. The detrital zircons of the Serra do Ouro Grosso sequence were derived predominantly from the erosion of a Neoarchean crust, 2.5–2.8 Ga old, with only one grain showing a Paleoproterozoic age (2, 245±83 Ma) older than the Transamazonian event. Zircons extracted from a shelf quartzite of the lowermost sequence of the São João del Rei basin indicate derivation from the 1.8–2.2 Ga Transamazonian crust, with subordinate contribution from the 2.5–2.9 Ga Archean crust. The 1, 809±41 Ma age is interpreted as the maximum limit for sedimentation in this basin. The results confirm the regional correlation with the Espinhaço Rift successions. The zircons extracted from an autochthonous quartzite of the Andrelândia sequence yielded ages in the 1.0–2.2 Ga range, with a modal class at 1.2–1.3 Ga. Only two of the forty analyzed zircons yield Archean ages. The youngest zircon yields 1, 086±85 Ma. The zircons from the allochthonous quartzite yield ages between 1.0–2.7 Ga, with a modal class at 2.1–2.2 Ga. Only five of 45 analyzed grains yield Archean ages. The youngest zircon has an age of 1, 047±77 Ma. The results indicate that the detrital sediments deposited during the second marine flooding event of the Andrelândia sedimentation were mainly derived from the erosion of Mesoproterozoic and Paleoproterozic rocks. The 1, 047±77 Ma age is interpreted as the maximum depositional age for the described association.

U–Pb geochronology of the southern Brası́lia belt (SE-Brazil): sedimentary provenance, Neoproterozoic orogeny and assembly of West Gondwana

Abstract The Brasilia belt borders the western margin of the Sao Francisco Craton and records the history of ocean opening and closing related to the formation of West Gondwana. This study reports new U–Pb data from the southern sector of the belt in order to provide temporal limits for the deposition and ages of provenance of sediments accumulated in passive margin successions around the south and southwestern margins of the Sao Francisco Craton, and date the orogenic events leading to the amalgamation of West Gondwana. Ages of detrital zircons (by ID–TIMS and LA-MC-ICPMS) were obtained from metasedimentary units of the passive margin of the Sao Francisco Craton from the main tectonic domains of the belt: the internal allochthons (Araxa Group in the Araxa and Passos Nappes), the external allochthons (Canastra Group, Serra da Boa Esperanca Metasedimentary Sequence and Andrelândia Group) and the autochthonous or Cratonic Domain (Andrelândia Group). The patterns of provenance ages for these units are uniform and are characterised as follows: Archean–Paleoproterozoic ages (3.4–3.3, 3.1–2.7, and 2.5–2.4 Ga); Paleoproterozoic ages attributed to the Transamazonian event (2.3–1.9 Ga, with a peak at ca. 2.15 Ga) and to the ca. 1.75 Ga Espinhaco rifting of the Sao Francisco Craton; ages between 1.6 and 1.2 Ga, with a peak at 1.3 Ga, revealing an unexpected variety of Mesoproterozoic sources, still undetected in the Sao Francisco Craton; and ages between 0.9 and 1.0 Ga related to the rifting event that led to the individualisation of the Sao Francisco paleo-continent and formation of its passive margins. An amphibolite intercalation in the Araxa Group yields a rutile age of ca. 0.9 Ga and documents the occurrence of mafic magmatism coeval with sedimentation in the marginal basin. Detrital zircons from the autochthonous and parautochthonous Andrelândia Group, deposited on the southern margin of the Sao Francisco Craton, yielded a provenance pattern similar to that of the allochthonous units. This result implies that 1.6–1.2 Ga source rocks must be present in the Sao Francisco Craton. They could be located either in the cratonic area, which is mostly covered by the Neoproterozoic epicontinental deposits of the Bambui Group, or in the outer paleo-continental margin, buried under the allochthonous units of the Brasilia belt. Crustal melting and generation of syntectonic crustal granites and migmatisation at ca. 630 Ma mark the orogenic event that started with westward subduction of the Sao Francisco plate and ended with continental collision against the Parana block (and Goias terrane). Continuing collision led to the exhumation and cooling of the Araxa and Passos metamorphic nappes, as indicated by monazite ages of ca. 605 Ma and mark the final stages of tectonometamorphic activity in the southern Brasilia belt. Whilst continent–continent collision was proceeding on the western margin of the Sao Francisco Craton along the southern Brasilia belt, eastward subduction in the East was generating the 634–599 Ma Rio Negro magmatic arc which collided with the eastern Sao Francisco margin at 595–560 Ma, much later than in the Brasilia belt. Thus, the tectonic effects of the Ribeira belt reached the southernmost sector of the Brasilia belt creating a zone of superposition. The thermal front of this event affected the proximal Andrelândia Group at ca. 588 Ma, as indicated by monazite age. The participation of the Amazonian craton in the assembly of western Gondwana occurred at 545–500 Ma in the Paraguay belt and ca. 500 Ma in the Araguaia belt. This, together with the results presented in this work lead to the conclusion that the collision between the Parana block and Goias terrane with the Sao Francisco Craton along the Brasilia belt preceded the accretion of the Amazonian craton by 50–100 million years.

Early Proterozoic Calc-Alkaline and Middle Proterozoic Tholeiitic Dyke Swarms from Central–Eastern Argentina: Petrology, Geochemistry, Sr–Nd Isotopes and Tectonic Implications

The Rio de La Plata craton in Argentina (Azul and Tandil Nb spikes. They have variable (Nd) values (−0·5 to 12·1), regions) is characterized by Early Proterozoic (2·0 Ga) calcwhich mainly reflect derivation from a depleted source mantle. alkaline and Middle Proterozoic (1·6 Ga) tholeiitic dyke swarms High-TiO2 tholeiitic dykes have more enriched IE-PM patterns intruding the crystalline basement involved in the Transamazonian and are characterized by (Nd) values (−1·4 to −7·5) typical Orogeny (2·2–1·9 Ga). The calc-alkaline dykes have andesitic of an enriched source mantle. Chemical and isotopic data and and rhyolitic compositions and trend east–west, whereas the melting modelling indicate that both calc-alkaline and tholeiitic tholeiitic dykes mainly trend N30°W and are represented by dykes originated by different melting degrees of a heterogeneous basalts with low (0·9–1·7 wt %) and high TiO2 (up to source mantle, the variable IE enrichment of which may have 3·7 wt %). The calc-alkaline dykes have primitive mantle occurred in Late Archaean to Early Proterozoic times. The emplacement of the calc-alkaline dykes is associated with the (PM)-normalized trace element patterns enriched in Rb, Ba, K, La, Ce and Nd, and significant negative Nb and Ti transtensional stage of the Transamazonian Orogeny, whereas the tholeiitic dykes reflect extensional tectonics succeeding the anomalies. These dykes are characterized by (Nd) values of −3 to −4, similar to those of the EMI mantle component. Transamazonian event. The calc-alkaline and tholeiitic dykes are similar in emplacement age and characteristics to metaLow-TiO2 tholeiitic dykes have low incompatible-element (IE) contents and PM-IE patterns with slightly positive or negative morphosed granites and volcanic rocks outcropping within the

REGISTROS DA TECTÔNICA BRASILIANA NA REGIÃO DE GUANHÃES (SE, BRASIL): DEFORMAÇÃO E METAMORFISMO DAS ROCHAS DE IDADE PÓS-TRANSAMAZÔNICA

Paleoproterozoic granitoids (~1,7Ga) intrude gneisses and migmatites of the tonalitic-trondjemiticgranodioriticbasement of the Neoproterozoic Araçuaí fold-thrust belt in the Guanhães region (MinasGerais).Previous studies attributed deformation and metamorphism of the country rocks to thePaleoproterozoic Transamazonian Event. The role of the younger Brasiliano Event is not well understoodand different interpretations have been postulated. Post-Transamazonian rocks, recently identified in theregion, are deformed and metamorphosed. The deformation is complex, with a ductil character increasingto east. The associated metamorphism reach the amphibolite facies, with migmatitic remobilization, andgranitogenesis at about 540 Ma. The available data suggest that after the Transamazonian event the Guanhãesregion underwent granitogenesis, deformation, metamorphose in the course of the Neoproterozoic BrasilianoEvent.

Timing of granitic magmatism in the northern Borborema Province, Brazil: a U–Pb study of granitoids from the Alto Pajeú Terrain

The Alto Pajeú Terrain (APT) is a SW–NE trending fold belt in the Neoproterozoic Borborema Province (NE Brazil), formed by metasedimentary sequences interlayered with metavolcanosedimentary units, both intruded by granitic rocks of different compositions and ages. The tectonic evolution of the APT involves Mesoproterozoic and Neoproterozoic transtensional and contractional tectonic events. In this study, we present U–Pb zircon ages of five granitoids selected according to a generally accepted relative chronology: (a) the Tuparetama granite and Tuparetama migmatitic granite with low angle gneissic foliation represent syn-tangential rocks presumably related to a Transamazonian event; (b) the Amparo granite with high angle gneissic foliation related to the Brazilian/Pan-African event; and (c) the Tabira and Jabitacá post-strike-slip granites. The results obtained do not confirm the generally accepted chronology of events. Granites previously classified in different groups (Tuparetama, Amparo, and Jabitacá) belong to the same orogenic cycle (Brazilian). Large batholiths, previously suspected to have been emplaced after the strike-slip deformation, represent older crustal remnants within them. In each case they consist of particular rock types — an alkaline high-temperature granite (Tabira) and a ‘granulite’ (Tuparetama migmatitic granite). These older ages imply that pre-Brazilian crust-forming events such as the Transamazonian Orogeny (Tuparetama migmatitic granite estimated at ca. 2050Ma) and Cariris Velhos event (Tabira granite at ca. 972Ma) occurred in the APT, and their records survived the intense recycling which characterizes the Brazilian/Pan-African orogeny.

PB/PB SINGLE-ZIRCON DATING OF PALEOPROTEROZOIC CALC-ALKALINE/ ALKALINE MAGMATISM IN THE SOUTHEASTERN SÃO FRANCISCO CRATON REGION, BRAZIL

Two chemically and petrographically distinct intrusive bodies (a calc-alkaline trondhjemite and an alkaline quartz-syenite cut by granitic aplites) occur in the vicinity of Piranga (Minas Gerais State), near the southeastern border of the Sao Francisco craton, Brazil. They intrude the Rio das Velhas greenstone belt and the Mantiqueira complex, both of Archean age. Both intrusive bodies are variably deformed and mylonitized. Age determinations using the Pb/Pb single-zircon evaporation method yielded Paleoproterozoic ages for the emplacement/ crystallization of the trondhjemite (2058 ± lOMa), quartz-syenite (2036 ± 4Ma), as well as the younger aplites (2012 ± 8Ma). These ages allow us to interpret the calc-alkaline as well as the alkaline magmatism as manifestations of the Transamazonian event in the southeastern Sao Francisco craton region. Furthermore, the calc-alkaline trondhjemite is interpreted to have intruded during compressional deformation, while the alkaline quartz-syenite and its aplitic differentiates are post-orogenic, possibly extension-related. In terms of the Rb vs. (Y +Nb) contents, the trondhjemite plots in the field of volcanic arc granites, whereas the syenite and the aplites plot in the field of post-collision granites. The mylonitic overprinting of the syenite and the trondhjemite is also Paleoproterozoic, as deduced from the 2012 ± 8Ma age of the undeformed younger aplitic dykes which cut the syenite. It is likely that this alkaline magmatism correlates with a Paleoproterozoic post-collisional magmatic event well documented in the Transamazonian mobile belts of the northern Sao Francisco craton.

PB/PB SINGLE-ZIRCON DATING OF PALEOPROTEROZOIC CALC-ALKALINE/ ALKALINE MAGMATISM IN THE SOUTHEASTERN SÃO FRANCISCO CRATON REGION, BRAZIL

Two chemically and petrographically distinct intrusive bodies (a calc-alkaline trondhjemite and an alkaline quartz-syenite cut by granitic aplites) occur in the vicinity of Piranga (Minas Gerais State), near the southeastern border of the São Francisco craton, Brazil. They intrude the Rio das Velhas greenstone belt and the Mantiqueira complex, both of Archean age. Both intrusive bodies are variably deformed and mylonitized. Age determinations using the Pb/Pb single-zircon evaporation method yielded Paleoproterozoic ages for the emplacement/ crystallization of the trondhjemite (2058 ± 10Ma), quartz-syenite (2036 ± 4Ma), as well as the younger aplites (2012 ± 8Ma). These ages allow us to interpret the calc-alkaline as well as the alkaline magmatism as manifestations of the Transamazonian event in the southeastern São Francisco craton region. Furthermore, the calc-alkaline trondhjemite is interpreted to have intruded during compressional deformation, while the alkaline quartz-syenite and its aplitic differentiates are post-orogenic, possibly extension-related. In terms of the Rb vs. (Y +Nb) contents, the trondhjemite plots in the field of volcanic arc granites, whereas the syenite and the aplites plot in the field of post-collision granites. The mylonitic overprinting of the syenite and the trondhjemite is also Paleoproterozoic, as deduced from the 2012 ± 8Ma age of the undeformed younger aplitic dykes which cut the syenite. It is likely that this alkaline magmatism correlates with a Paleoproterozoic post-collisional magmatic event well documented in the Transamazonian mobile belts of the northern São Francisco craton.

Geochemistry and geochronology of paleoproterozoic gneissic rocks of the Paraíba do Sul Complex (Quirino Unit), Barra Mansa region, Rio de Janeiro, Brazil

This paper presents and discusses new geochemical data from orthogneisses rocks of Paraiba do Sul Complex (PSC) from the NW of the Rio de Janeiro State, denominated Quirino Unit. U-Pb geochronological data (carried out on GEOTOP-UQAM, Canada), displayed in Valladares (1996) and Machado et al. (1996), are also discuss. This Unit, as well the Metasedimentary Unit of PSC and intrusive brazilian granitoids, comprise the Paraiba do Sul Tectonic Domain (PSTD) or Upper Tectonic Domain, at the central segment of the Ribeira belt. The Quirino Unit occurs as extensive bodies of homogeneous gneisses of upper amphibolite facies metamorphism, with hornblende and or biotite comprising approximately 50% of the studied area and ca. 70% of the PSTD. This unit yields U-Pb zircon ages of 2185±3 Ma and 2169±3 Ma, temporally related to the Transamazonian event. Minimum ages of 2846 Ma and 2981 Ma (zircon U-Pb data) reveal the pre-existence of Archean crust as Pb source of part of the investigated gneisses. Geochemical studies of orthogneisses Quirino discriminate two calc-alkaline suites: one of medium to high-K and other of high-K, with enrichment in LILE. The chemical characteristics and the ages U-Pb allow to perform a tectonic correlation of the Quirino Unit with arc magmatic calc-alkaline granitoids, which were generated during the Transamazonian orogeny (2.2 - 1.9 Ga). Ages between 605-503 Ma (zircon and sphene U-Pb data) indicate reworking of Quirino Unit during Brazilian Orogeny (ca. 700-450 Ma).

Eventos magmáticos arqlieanos de natureza cálcio-alcalina e tholeiítica no Quadrilátero Ferrífero e suas implicações tectônicas.

Mafic amphibolitic dikes cutting tonalitic rocks with little evidence of metamorphism are one of the singularities in the crusta, evolution of the Quadrilatero Ferrifero, in Minas Gerais. Petrographic, geochemical and geochronogical studies show that in the Bonfim Metamorphic Complex these rock types constitute two distinct magmatic series, one tholeiitic and the other calcalkaline. They were partially metamorphosed under the conditions of higher greenschist to lower amphibolite facies. In the calcalkaline tonalites the metamorphic reactions, which required the introduction of water, were difficulted by their massive structure. Therefore primary textures and mineralogy were mostly preserved. In the tholeiitic rocks of the dikes the metamorphism was more homogeneous because the primary igneous mineralogy, rich in pyroxenes, is more sensible to hydration reactions than the mineralogy of the tonalites. The access of the needed aqueous fluids certainly was more effective to the mafic rocks because fluids tend to be concentrated in zones of crustal weakness such as dikes. We conclude that the non-penetrative metamorphic recrystallization of the two rocks series under the conditions of the higher greenschist to lower amphibolite facies, and also the preservation of early Archean U/Pb ages in titanites of the tonalites, indicate that the studied region was not submitted to any penetrative tectonometamorphic event of higher metamorphic conditions than the lower amphibolite facies during the Proterozoic. Therefore we also conclude that the tectonometamorphic overprinting of the Transamazonian Event (2.2 My) in the Bonfim Metamorphic Complex and probably also in the rest of the Quadrilatero Ferrifero was heterogeneous, not affecting all areas with the same intensity.