Closing the Clymene ocean and bending a Brasiliano belt: Evidence for the Cambrian formation of Gondwana, southeast Amazon craton

We report new paleomagnetic and geochronological data from Ediacaran rift-drift carbonates in the Paraguai belt at the southern end of the suture zone between the Amazon craton and the Sao Francisco and Rio de Plata cratons, South America. Early thrusting resulted in remagnetization ca. 528 +/- 36 Ma or later; the mean age is established by (40)Ar/(39)Ar encapsulation dating of mixed authigenic and detrital illite from remagnetized carbonates from the unmetamorphosed fold-thrust belt. This remagnetization overlaps with a 525 Ma Gondwana reference pole. Metamorphic illite from the slate belt yields (40)Ar/(39)Ar ages of 496-484 Ma, the timing of peak regional metamorphism. Oroclinal bending of the Paraguai belt was caused by a 90 degrees clockwise rotation of the east-west limb after ca. 528 Ma, probably reflecting the irregular margin of the southeast Amazon craton. The age of the Paraguai belt overlaps with that of the Pampean orogeny farther south along the western margin of the Rio de Plata craton, suggesting a coeval closure for the Clymene ocean separating the Amazon craton from the Sao Francisco and Rio de Plata cratons.

Geochronology of the lpanema layered mafic-ultramafic complex, Minas gerais, Brazil: Evidence of extension at the meso-neoproterozoic time boundary

The mafic/ultramafic Ipanema Layered Complex (ILC), Minas Gerais Brazil, consists of seven individual bodies. These units crosscut polyphase orthogneisses and interlayered paragneisses of the Paleoproterozoic Juiz de Fora Complex. Intrusive granitoids tectonically related to [lie Neoproterozoie Aracuai orogen are also present in the study area.A Sm-Nd whole-rock linear array for seven samples metapyoxenites, metaperidotiles, metagabbro. and meta-anorthosite) from the Santa Cruz massif, the largest body of the ILC. suggest that it was emplaced at 1104 +/- 78 Ma the original magma was derived from a depleted mantle source (epsilon(Ndt)= +3.8). U-Pb single-grain zircon stud of a meta-anorthosite yields all upper intercept age of 1719 +/- 4 Ma, which is interpreted to represent inheritance. The lower intercept at 630+/-3 Ma indicates (hat a Neoproterozoic tectonothermal episode overprinted the ILC, this event occurred under upper-amphiolite-, to granulite-facies conditions. The 630 Ma episode is consistent with the timing of regional metamorphism and deformation of the adjacent Aracuai orogen (Brasiliano collage). Emplacement of the ILC and other coeval metamafies and meta-ultramafics (of alkaline affinity) in the re, oil is attributed to early extension tectonics, accompanying accretion of the Rodinia super- continent during the Mesoproterozoic-Neoproterozoic time boundary.

Isotopic evidence for the late Brasiliano (500-550 Ma) ore-forming mineralization of the Araes gold deposit, Brazil

The Araes gold deposit, located in eastern Mato Grosso State, central Brazil, is hosted in Neoproterozoic volcanosedimentary rocks of the Paraguay belt, which formed during collision of the Amazonian craton and the Rio Apa block. Ar-40/Ar-39 geochronology and Pb and S isotopic analyses constrain the timing and sources of mineralization. Three biotite flakes from two samples of metavolcanic host rock yield Ar-40/Ar-39 plateau ages between 5941 and 531 Ma, interpreted as cooling ages following regional metamorphism. Clay minerals from a hydrothermal alteration zone yield an Ar-40/Ar-39 integrated age of 503 +/- 3 Ma. Galena grains from ore-bearing veins yield values of Pb-206/(204)pb from 17.952 to 18.383, Pb-207/Pb-204 from 15.156 to 15.811, and Pb-208/Pb-204 from 38.072 to 39.681. Pyrite grains from ore-bearing veins yield values of Pb-206/Pb-204 from 18.037 to 18.202, Pb-207/Pb-204 from 15.744 to 15.901., and Pb-208/(204)pb from 38.338 to 38.800. Pb isotope variations may be explained in terms of mixing a less radiogenic lead component (mu similar to 8.4) from mafic and ultramafic basement host-rocks (Nova Xavantina metavolcanosedimentary rocks) and a more radiogenic lead component (mu similar to 9.2) probably derived from supracrustal rocks (Cuiaba sedimentary groups). Sulfur isotope compositions are homogeneous, with delta S-34 values ranging from -1.1 parts per thousand to 0.9 parts per thousand (galena) and -0.7 parts per thousand to 0.9 parts per thousand (pyrite), suggesting a mantle-derived reservoir for the mineralizing solutions. Based on the Ar, Pb, and S isotope data, we suggest that the precious metals were remobilized from metavolcanic host rocks by hydrothermal solutions during Brasilide-Panafrican regional metamorphism. The Arabs gold deposit probably formed during a late stage of the orogeny, coeval with other mineralization events in the Paraguay Belt.

O grupo natividade e sua correlacao com o grupo bambui

The Natividade Group is a metasedimentary sequence discontinuously exposed in the southeastern region of the Tocantins State. It rests unconformably on the Archean gneissic-granitoid complex and its associated supracrustals, as well as on granite intrusives of the Lajeado Suite (1.870 Ma). It is unconformably covered by the Monte do Carmo Formation and the Serra Grande Formation. The sequence is preserved on tilted blocks and grabens. The western portion is constituted of only detritic metasediments. The intermediate outcrops presents detritic and some carbonatic metasediments. A carbonatic sequence, with some detritic levels, is recognized at the eastern area. The sections of these different domains are interpreted as constituted of fining-up sequences due to three transgressive episodes into an ensialic paleobasin, with uplifted border to the western side and a carbonate platform to the east, which represents the western extension of the Mambui Group. The Natividade Group presents folds with variable styles and no defined vergence, which are synchronous to the regional metamorphism (lower to upper greenschist facies). Two groups of faults cut the sequence. -from English summary

Caracterização geotermobarométrica dos metabasitos de Cajamar (SP), Grupo São Roque, Cinturão Ribeira

Metamafic orthoderivate rocks, characterized by MORB signatures and paleoproterozoic ages occur in Cajamar region central east part of São Paulo State, Brazil. They are associated to philites, metasandstones and metalimestones of São Roque Group volcanosedimentary sequence, Southeastern portion of the Ribeira Belt. For the metamafic rocks the assemblage of metamorphic peak is represented by actinolitealbite- epidote-chlorite typical of regional metamorphism, greenschist facies. For a good characterization of P and T parameters of metamorphism microprobe analyses of main minerals were performed. Metamorphism did not destroy completely the primary mineralogy of original basalt and microgabbro, and augite crystals are registered in many samples, preserved together with metamorphic assemblage, and possibly indicate that the igneous temperatures of crystallization are between 900 and 1.100°C. Textures of mineral equilibrium for the peak paragenesis are not always present in the studied samples. It may be partially caused by the superposition of retrometamorphic events that is a consequence of regional shearing process. Where the equilibrium is reached, chiefly in the occurrences in the south of the area temperature and pressure determinations indicate values close to 487 ± 42°C and 8,2 ± 2.0 kbar, respectively. Other values for P and T lower than that of the peak were also obtained in central and north portion of the area and they are: 450 ± 68°C for T e 4.7 kbar for P and 315 ± 26°C for T and 3.0 kbar for P and may indicate that the mineral equilibrium was not reached in these samples or that they represent the retrometamorphic event. As a concluding remark considering the metamorphic P-T path now obtained one can say that the exhumation process in this area occurred by isothermal decompression.

Evolução geológica do sudoeste do cráton amazônico região limítrofe Brasil-Bolívia - Mato Grosso

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Basaltos almofadados da Suíte Ofiolítica Morro do Agostinho: registros de fundo oceânico na porção centro-oeste do Cinturão Araguaia

Ao longo do domínio de baixo grau metamórfico (porção centro-oeste) do Cinturão Araguaia, afloram dezenas de corpos máficos e/ou ultramáficos de natureza ofiolítica. Cita-se como exemplo a Suíte Ofiolítica Morro do Agostinho nos arredores da cidade de Araguacema (TO) que configura um pequeno corpo isolado que sustenta o Morro do Agostinho e encontra-se encaixado tectonicamente em metarenitos, ardósias e filitos da Formação Couto Magalhães (Grupo Tocantins). A Suíte Ofiolítica Morro do Agostinho é constituída por peridotitos serpentinizados, basaltos e cherts ferríferos todos afetados por incipiente metamorfismo. A associação de basaltos é caracterizada por um expressivo derrame submarino com estruturas em lavas almofadadas, sobrepostas aos peridotitos serpentinizados. Os basaltos foram classificados em tipos maciços e hipovítreos com esferulitos. Os basaltos maciços são homogêneos, com textura intersertal definida, essencialmente, por finas ripas de plagioclásio, clinopiroxênio e raramente olivina, calcocita e calcopirita. Os basaltos hipovítreos apresentam feições texturais formadas por ultrarresfriamento de lavas apresentando esferulitos de plagioclásio, feixes de cristais aciculares e esqueletais de clinopiroxênio e plagioclásio, e cristais com terminações tipo rabo-de-andorinha. Geoquimicamente, os basaltos revelaram natureza subalcalina toleítica, compatíveis com o tipo MORB. As razões (La/Yb)_n < 1 e (La/Sm)_n < 1 apontam, mais especificamente, para magmas do tipo N-MORB na evolução dessas rochas relacionadas ao ambiente de fundo oceânico. Estas rochas revelaram que nos estágios iniciais da evolução do Cinturão Araguaia houve uma fase importante de oceanização da Bacia Araguaia, com exposição de peridotitos do manto litosférico seguido de extravasamento de lavas e sedimentação de cherts e formações ferríferas bandadas em ambiente oceânico profundo. Após o preenchimento sedimentar da Formação Couto Magalhães (Grupo Tocantins), e o descolamento da litosfera oceânica, a fase tectônica principal propiciou a inversão tectônica que levou à exumação dos corpos ofiolíticos, principalmente ao longo de superfícies de cavalgamento, fragmentando-os e misturando-os tectonicamente às rochas supracrustais, acompanhado de metamorfismo regional em condições da fácies xisto verde baixo. A Suíte Ofiolítica Morro do Agostinho representa, assim, um pequeno fragmento alóctone de um segmento litosférico manto/crosta oceânica, bem preservado, do início da evolução da Bacia Araguaia, similar a outros no Cinturão Araguaia, que é um importante registro da fase de oceanização do Cinturão Araguaia, durante o Neoproterozoico.

Mapeamento geológico-estrutural da Sequência Canto, porção sul do Greenstone Belt do Rio Itapirucu, Bahia

The study area is located in the southern portion of Rio Itapicuru Greenstone Belt, Bahia, where were found rocks that belongs to the Canto Sequence, that comprises felsics and intermediary rocks, beyond metasediments. The studied maps and drill holes help to recognized stratigraphy column that was divided in three mains domains, from the base to the top: andesitics to felsics lavas domain, carbonaceous and metasedimentary domains. The data obtained in mapping and petrography analysis allows classify the area in three deformational phases, Dn, Dn+1, Dn+2. The metamorphism recognized according to the mineralogical associations permitted to classify a progressive regional metamorphism (lower to medium greenchist facies) till biotite zone, falling until chlorite zone due to retrometamorphism. The auriferous mineralizations are mainly related to hydrothermal veins included in the different lithologies, mainly in carbonaceous schists

Caracterização geológica da terminação norte do Domo de Ambrósio - Bahia

The Ambrósio dome is a granodioritic batholiths of elliptical geometry, 40km length in the N-S direction and variable width of up to 8 km, has a weakly deformed nucleus with intensely deformed margins, in its northern portion is intruded in orthogneiss that belong to the Archean basement, and its southern part comes in direct contact with the volcano-sedimentary sequence of Paleoproterozoic Rio Itapicuru Greenstone Belt (RIGB), Bahia. From geological mapping on 1:25:000 scale were recognized two structural domains, termed West Domain and East Domain. From investigation of these domains was identified a major shear zone, which puts in contact two distinct stratigraphic sequences, one west, consisting primarily of metavulcanic and metapyroclastic rocks with records of low-grade regional metamorphism, and east discontinuity a metassedimentary domain, with record of gradational contact metamorphism, deformation and compression generated from the rise of Ambrosio Pluton. Such records put into question the structural and stratigraphic models in the literature so far

Geologia do depósito de ouro do alvo guarda-mor, na sequência Jaupaci (Israelândia - GO)

The study area of the Guarda-Mor target (Israelândia-GO) contextualizes the terrains of the estaern portion of the Arenópolis Magmatic Arc, more accurately, the Neoproterozoic associations inserted in the field of the Jaupaci volcano-sedimentary sequence. The mapped area is located in the central-western of Goiás state. The presente paper has as main objective to characterize the structural, petrographic and litogeochemistry of the target rocks beyond to compare these factors with the Mina Bacilândia rocks (Fazenda Nova-GO) in order to assess the genetic similarity of volcano-sedimentary units and deposits associated. The Guarda-Mor target is represented by the Jaupaci metavolcanic rocks sequence presenting bimodal volcanism. This sequence consists in metafelsic rocks like sericite/muscovite quartz schist, phyllite and metarriolites and/or metariodacites with calc-alkaline geochemical signature and metamafic rocks with tholeiitic character formed by actinolite - chlorite schist and chlorite - quartz schist. Besides the package of supracrustal rocks also can be observed the occurrence of a local intrusion syn- to late - tectonic named Granito Subvulcânico. In the study area beyond the marked volcanism also seen an event of crustal melting granitogênese evidenced by the presence of 2 granites at the east and the west of the map, Granito Israelândia and Granito Iporá respectively. Structural analysis both at the macro and micro have identified 3 deformation phases. The types of rock on the region record features in the metamorphic facies top and down. These features were subdivided into 3 metamorphic areas: the east and West areas show thermal metamorphism due to intrusion of adjacente granites while the central domain displays features of regional metamorphism. Gold mineralization of the Guarda –Mor deposit target suggests a possible structural control beyond pronounced the hydrothermal alteration. The mineralization may also be...

Análise estrutural e microtectônica da inflexão do jordino e depósitos de ouro associados no greenstone belt de Pilar de Goiás

The study area is included in the geological context of the Goias Median Massif, a region where there are associations of Archean granite-gneiss complex (Block Moquém) and a Paleoproterozoic metavolcano-sedimentary sequence ( Pilar greenstone belt ). At the south of area, the greenstone sequence is partially overlain by Neopreoterozoic metasediments of the Araxá Group. The lithostratigraphic units of the Pilar greenstone belt define a shift from about N30W direction (north of the deposit) to N60 and 70W in the region south of the Jordino deposit, where are truncated by the Araxá Group rocks. Mineralogical associations described in this paper allow to indicate that the regional metamorphism that affected the rocks of the greenstone belt and Araxá Group, in the mapped area, reached the upper greenschist facies (garnet zone). Data obtained during mapping and by microtectonics analysis allow to indicate the existence of at least four deformational events that acted on the rocks of the Guarinos greenstone belt and Araxá Group, represented by the phases called Dn-1, Dn, Dn + 1 and Dn+2. It was observed that the pattern of sulphide porfitoblasts in mineralized levels is similar to garnet, biotite and muscovite porfiroblasts (tardi to post Dn) that marks the metamorphic peak of the area

Caracterização estrutural da mineralização da mina de Fazenda Nova - GO

The study area is includes in the geological context of Arenópolis Magmatic Arc, a region where there are neoproterozoic associations of granodioritic and tonalitic composiotion. (Ortogneisses of the western Goiás) and sequences metavolcanic-sedimentary (Jaupaci Metavolcanic-sedimentary Sequence ). In the mapped area, both units are covered by a cover-laterite. The Ortogneisses from Goiás West consist of a source granodioritic gneisses, corresponding to the Biotite granodiorite gneisse, and also by tonalitic gneiss composition corresponding to Metatonalit. The Jaupaci Metavolcanic-sedimentary Sequence is formed by Chlorite Schist (Metabasalt), Biotite Schist (Metadacite) and Sericite Schist (Metarhyolite), and even intrusions Sin/Tardi e Post Tectonic, granite to diorite composition (Diorites), and alson tonalitic (Bacilandia Tonalite). Post tectonic intrusions are observed, wich were Hornblend Diorite Porphyry and Lamprophyres, Structural analysis allowed the identification of three deformational events, Dn-1, Dn and Post-Dn. The first event is associated with a bygone foliation, lineation which generates an intersectional event, generating the foliation Sn, this being the most important structure in the study area, generating even the type mineral lineation and stretch. The last deformational event is characterized by folds on different scales, affecting the Sn foliation. The rocks of the region have features s active hydrothermal and regional metamorphism, and are composed os assembly indicative of mineralogical facies metamorphism Green Schist, in chlotite zone, with evidence of retro metamorphism. Locally there are sulfides as pyrite, arsenopyrit and pyrhotite, and te mineralization is associated with the arsenopyrite

Rochas calciossilicáticas e feldspáticas na região de Castro Alves e Rafael Jambeiro (BA)

The towns of Castro Alves and Rafael Jambeiro, central-east of Bahia state, are located in the east of São Francisco Craton, in granulite terrains of Salvador-Curaçá Belt, formed in Paleoproterozoic. The region of study contains ortognaisses of Caraíba Complex, metamafic and metaultramafic rocks of São José do Jacuípe Suite, metasedimentary rocks of Tanque Novo-Ipirá Complex, granitoids, pegmatites and alkaline rocks. The study carried out regional and detailed geological mapping in addition to petrographical and geochemical characterization of six areas in the search for targets of feldspar and white diopside, minerals used in ceramic industry. The areas consist of granitic ortognaisses interspersed with lenses of mafic granulite rocks, calc-silicate rock, banded iron formations, paragnaisses, quartzites, and bodies of quartz-feldspar or feldspar pegmatites and alkaline rocks that fill discontinuities. The region of study contains four deformations phases, with a predominance of ductile structures. The foliation Sn has N30E to N70W direction, high angle of dip and is characterized by compositional banding of granoblastic and felsic bands interspersed with nematoblastic or lepidoblastic mafic bands. A mineral or stretching lineation Ln is associated with Sn and has trend of S55E to S72E. The rocks have been suffered a regional metamorphism with granulite facies peak and partial retrogression to greenschist facies. Geochemical studies indicate that the green coloring calc-silicate rocks have lower SiO2, MgO and higher Fe2O3 content compared with white calcssilicate rocks. The alkaline rocks of the studied area have higher Na2O, SiO2 and lower K2O, Fe2O3 content compared with others Paleoproterozoic alkaline rocks of Bahia state. The targets of diopside are associated with white calc-silicate rocks, while the targets of feldspar are associated with paragnaisses, pegmatites and alkaline rocks

The Mavuradonha Layered Complex

Der Mavuradonha Layered Complex repräsentiert einen 862 ? 4 Ma alten Komplex, der in einem tiefkrustalen Milieu intrudierte. Eine mehrphasige magmatische Differentiation ist in macro-rhythmischen Einheiten und kleinmaßstäblichen Lagenbau erkennbar, aus denen die Kristallisationssequenzen Pyroxenite, Gabbros/Norite, Leuko-Gabbros oder Ferro-Gabbro und Anorthosite resultieren. ?Nd-Werte zwischen + 0.3 und + 6.6 zeigen krustale Kontamination eines aus dem verarmten Mantel stammenden, tholeiitischen Ursprungsmagma an. ?Nd-Werte (+ 2.4 bis - 3.5) anderer tholeiitischer Gabbros in unmittelbarer Nähe des Komplexes deuten ebenfalls auf Krustenkontamination hin, jedoch in stärkerem Maße.Der Komplex wurde um 554 ? 13 Ma unter granulitfaziellen Bedingungen von 13 ? 2 kbar und 840 ? 30° C überprägt. Die anschließende retrograde, amphibolitfazielle Metamorphose mit Bedingungen von 11 ? 2 kbar und 680 ? 20° C ereignete sich um 546 ? 9 Ma. Abkühlung bis zur Grünschieferfazies erfolgte spätestens um 501 ? 6 Ma.Die vorgestellten Daten zeigen, dass sich der Sambesi-Gürtel im NE Simbabwes als fehlgeschlagenes Rift oder intrakratonisches Becken während einer frühen Pan-Afrikanischen Extensionsphase entwickelte, während die granulitfazielle Metamorphose um 300 Ma später erfolgte. Somit deutet die Intrusion des Mavuradonha Layered Complex rift-bedingten Magmatismus in einer frühen Riftphase an, während das Becken oder Rift während der Pan-Afrikanischen Orogenese geschlossen wurde.

Detrital Zircon Evidence for Mixing of Mazatzal Province Age Detritus With Yavapai Age (ca. 1700-1740 Ma) and Older Detritus in the ca. 1650 Ma Mazatzal Province of Central New Mexico, USA

Preliminary detrital zircon age distributions from Mazatzal crustal province quartzite and schist exposed in the Manzano Mountains and Pedernal Hills of central New Mexico are consistent with a mixture of detritus from Mazatzal age (ca. 1650 Ma), Yavapai age (ca. 1720 Ma.), and older sources. A quartzite sample from the Blue Springs Formation in the Manzano Mountains yielding 67 concordant grain analyses shows two dominant age peaks of 1737 Ma and 1791 Ma with a minimum peak age of 1652 Ma. Quartzite and micaceous quartzite samples from near Pedernal Peak give unimodal peak ages of ca. 1695 Ma and 1738 Ma with minimum detrital zircon ages of ca. 1625 Ma and 1680 Ma, respectively. A schist sample from the southern exposures of the Pedernal Hills area gives a unimodal peak age of 1680 Ma with a minimum age of ca. 1635 Ma. Minor amounts of older detritus (>1800 Ma) possibly reflect Trans-Hudson, Wyoming, Mojave Province, and older Archean sources and aid in locating potential source terrains for these detrital zircon. The Blue Springs Formation metarhyolite from near the top of the Proterozoic section in the Manzano Mountains yields 71 concordant grains that show a preliminary U-Pb zircon crystallization age of 1621 ¿ 5 Ma, which provides a minimum age constraint for deposition in the Manzano Mountains. Normalized probability plots from this study are similar to previously reported age distributions in the Burro and San Andres Mountains in southern New Mexico and suggest that Yavapai Province age detritus was deposited and intermingled with Mazatzal Province age detritus across much of the Mazatzal crustal province in New Mexico. This data shows that the tectonic evolution of southwestern Laurentia is associated with multiple orogenic events. Regional metamorphism and deformation in the area must postdate the Mazatzal Orogeny and ca. 1610 Ma ¿ 1620 Ma rhyolite crystallization and is attributed to the Mesoproterozoic ca. 1400 ¿ 1480 Ma Picuris Orogeny.