Luiz Carlos da Silva

Juvenile accretion at 750–700 Ma in southern Brazil

A revista Economic Geology nao autoriza a publicacao de seus artigos em repositorios institucionais.

U-Pb and Sm-Nd geochronology of the neoproterozoic granitic-gneissic Dom Feliciano belt, Southern Brazil

The Brasiliano Cycle in southern Brazil and Uruguay is represented by three major NE-SW trending geotectonic units: the Vila Nova belt, Tijucas belt and Dom Feliciano belt. The Vila Nova belt is located in western part of Rio Grande do Sul State; its evolution took place between 900 and 700 Ma and it corresponds to one of the few areas with juvenile accretion during the Neoproterozoic in Brazil. The Tijucas belt, situated between the Vila Nova and Dom Feliciano belts, consists of a rift-related Mesoproterozoic (?) volcano-sedimentary sequence which was strongly deformed during the Brasiliano cycle. The Dom Feliciano belt is located along the eastern coast of southern Brazil and Uruguay and is a typical granite-gneiss-migmatite terrane. This belt is a key area for understanding West Gondwana assembly during Neoproterozoic and Early Paleozoic times, because of its direct connection to the Gariep and Damara belts in southern Africa. The present study defines the main tectonic phases of the ca. 600 Ma Dom Feliciano event in the Sao Feliciano belt. U-Pb zircon data for flat-lying gneisses yield ages between 610 ± 5 Ma and 616 ± 2 Ma, which we believe correspond to the approximate age of thrusting. The strike-slip deformation (main transcurrent phase) is well dated by U-Pb zircon ages for the syn-transcurrent granites (Arroio Moinho Granite, 595 ± 1 Ma; Encruzilhada do Sul Granite, 594 ± 5 Ma). These results indicate a relatively rapid evolution, from about 620 Ma (upper limit for the age of the gneiss) to 594 Ma (syn-trancurrent granites), for the known thrust related and strike-slip related tectonic phases of the Dom Feliciano belt. Sm-Nd results can be considered in three major groups. The first group (I) includes Brasiliano gneisses, granitoids, and one anorthosite with TDM ages of ca. 2.0 Ga and very negative eeNd(600) values. They may represent either direct melting of Transamazonian (Paleoproterozoic) basement or extensive contamination with older material of Paleoproterozoic to Archean age. The second group (II) includes granitoids and gneisses with TDM model ages from 1.31 to 1.41 Ga. The third group (III) comprises samples with TDM ages between 1.58 to 1.75 Ga. For groups II and III it is clear these rocks or their protoliths represent pre-Brasiliano continental crust. Unlike Group I rocks, groups II and III granites and gneisses may also contain a small fraction of a juvenile Brasiliano material. However, we have not yet found any sample from the Dom Feliciano belt with a Neoproterozoic TDM age and positive eNd value at 600 Ma that could be considered largely juvenile. Based on results from the Vila Nova belt, in which the main orogenic process developed between 753 and 704 Ma, we conclude that the Vila Nova belt was stable for over 100 Ma before the Dom Feliciano event reached its peak. It is probable that the collage of terranes in the Dom Feliciano belt and the region comprised by the Tijucas and Vila Nova belts were assembled during the Dom Feliciano event (ca. 600 Ma).

Late Neoproterozoic–Cambrian granitic magmatism in the Araçuaí orogen (Brazil), the eastern brazilian pegmatite province and related mineral resources.

Abstract The Araçuaí orogen extends from the eastern edge of the São Francisco craton to the Atlantic margin, in southeastern Brazil. Orogenic igneous rocks, formed from c. 630 to c. 480 Ma, cover one third of this huge area, building up the Eastern Brazilian Pegmatite Province and the most important dimension stone province of Brazil. G1 supersuite (630–585 Ma) mainly consists of tonalite to granodiorite, with mafic to dioritic facies and enclaves, representing a continental calc-alkaline magmatic arc. G2 supersuite mostly includes S-type granites formed during the syn-collisional stage (585–560 Ma), from relatively shallow two-mica granites and related gem-rich pegmatites to deep garnet-biotite granites that are the site of yellow dimension stone deposits. The typical G3 rocks (545–525 Ma) are non-foliated garnet-cordierite leucogranites, making up autochthonous patches and veins. At the post-collisional stage (530–480 Ma), G4 and G5 supersuites were generated. The S-type G4 supersuite mostly consists of garnet-bearing two-mica leucogranites that are the source of many pegmatites mined for tourmalines and many other gems, lithium (spodumene) ore and industrial feldspar. G5 supersuite, consisting of high-K–Fe calc-alkaline to alkaline granitic and/or charnockitic to dioritic/noritic intrusions, is the source of aquamarine-topaz-rich pegmatites but mainly of a large dimension stone production.

Similarities and differences between the Brazilian and African counterparts of the Neoproterozoic Araçuaí-West Congo orogen

Abstract The Araçuaí–West Congo orogen encompasses orogenic domains located to the SE of the São Francisco Craton in Brazil, and to the SW of the Congo Craton in Africa. From the opening of the precursor basin to the last orogenic processes, the evolution of the orogen lasted from the very beginning of the Neoproterozoic up to the Cambrian–Ordovician boundary. After the spreading of the South Atlantic Ocean in Cretaceous time, the Araçuaí–West Congo orogen was split into two quite different but complementary counterparts. The Brazilian side (Araçuaí orogen) inherited two thirds of the whole orogenic edifice, including all the Neoproterozoic ophiolite slivers, the entire magmatic arc and syn-collisional to post-collisional magmatism, and the suture zone. The African counterpart (West Congo Belt), a fold–thrust belt free of Neoproterozoic ophiolite and Pan-African orogenic magmatism, inherited the thick pile of bimodal volcanic rocks of the Early Tonian rift stage, implying that the precursor basin was an asymmetrical rift with the thermal–magmatic axis located in the West Congo Belt. Both counterparts of the Araçuaí–West Congo orogen include Neoproterozoic glaciogenic deposits, allowing tentative lithostratigraphic correlations, but identification of the ice ages remains uncertain because the lack of sufficient well-constrained geochronological data.

SHRIMP U/Pb Zircon Dating of Neoproterozoic Granitic Magmatism and Collision in the Pelotas Batholith, Southernmost Brazil

The introduction of robust geochronological methods for age determinations of the southernmost segment of the Neoproterozoic terranes of Brazil, namely the Dom Feliciano Belt, provides important clues for unraveling the complex evolution of the Brasiliano/Pan-African orogeny in this southwestern portion of the Gondwana supercontinent. Except for associated small schist belts and post-orogenic foreland basins, the belt is represented in this region of southeastern South America by the Pelotas Batholith. Precise SHRIMP U/Pb zircon geochronological techniques based on the study of 95 individual spots on 74 zircon crystals (three samples) and on Nd-isotopic determinations (three samples) are used to assess the late Neoproterozoic history of the belt, especially the orthogneisses interleaved with the batholithic plutons. Three petrotectonic associations were selected for detailed isotopic investigations—the Pinheiro Machado syncollisional monzogranites, the widespread Piratini gneiss tonalitic xenoliths, and t...

The hot back-arc zone of the Araçuai Orogen, Eastern Brazil: From sedimentation to granite generation

This article presents new lithochemical and geochronological data obtained from gneisses and granites occurring in the region located to the east of the Rio Doce calc-alkaline arc (630 - 580 Ma), which corresponds to the back-arc basin of the Aracuai orogen. The Nova Venecia Complex, represents the most fertile source of peraluminous granitic melts in the studied back-arc zone. It mostly consists of migmatitic Al-rich paragneisses, ranging from biotite-rich gneisses to biotite-free cordierite-rich granulites, whose main protoliths were graywacky sediments. An EW-oriented section across the northern back-arc region reveals a zone rich in cordierite granulites of the Nova Venecia Complex at the base, followed by migmatites that gradually pass to the Ataleia foliated granites rich in metasedimentary enclaves, which in turn lay beneath the Carlos Chagas batholith. To the south of the Carlos Chagas batholith, orthopyroxene-bearing rocks often occur in both the Nova Venecia Complex and the Ataleia Suite, suggesting a deeper crustal level. Our U-Pb data suggest that melting processes started on the Nova Venecia Complex during the late development of the Rio Doce arc, around 590 Ma, forming autochthonous peraluminous melts related to the Ataleia Suite. Progressive anatexis and melt accumulation attained the climax around 575 Ma, leading to the development of the syn-collisional Carlos Chagas batholith. Around 545 - 530 Ma, a late to post-collisional anatectic episode formed garnet-cordierite leucogranites, mostly from the re-melting of the Ataleia and Carlos Chagas granites. A remarkable post-collisional plutonism caused widesperead re-heating of the back-arc domain from ca. 520 Ma to 480 Ma. This long lasting history (ca. 110 Ma) of granite generation in the back-arc zone requires distinct heat sources, such as asthenosphere ascent under the back-arc region in the pre-collisional stage, thrust stacking of the hot arc onto the back-arc, radiogenic heat release from the collisional thickened crust and, finally, asthenosphere uprising during the gravitational collapse of the Aracuai orogen.

Zircon U-Pb Shrimp Dating of the Serra dos Órgãos and Rio de Janeiro Gneissic Granitic Suites: implications for the (560 Ma) Brasiliano/Pan-African Collage

* Tables 2, 3 and 4 referred in the text are available by request from the Senior Author by e–mail. 1 CPRM-Coordenação Nacional de Geocronologia /Dep. Gelogia Geral e Aplicada UnB (Pesquisador Associado), Pesquisador do CnP q, Brasília-DF 2 Centre for Global Metallogeny, School of Earth and Geographical Sciences, The University of Western Australia, Nedlands 600 9, Au tralia 3 Universidade Federal do Rio Grande do Sul, Porto Alegre, RS. 4 Corresponding author e-mail: luizcarlos@df.cprm.gov.br Abstract Recent systematic geological mapping at a scale of 1:400,00 scale revealed that the State of Rio de Janeiro State is an important field-laboratory to test the connections of the Costeiro Domain of the Ribeira Belt (former Rio Doce Orogen) and the Araçuaí Orogen. The regional geological mapping was supported by zircon U-Pb SHRIMP geochronological data of three synorogenic, thrust-related key plutons, namely the preto syn-collisional Serra dos Órgãos Batholith and the syn-collisional Cor covado and Pão de Açúcar gneissic granites (Rio de Janeiro Suite). These plutons have a chemical and tectonic signatures akin to the A ustralian I-and S-type continental arc granitoids. The Serra dos Órgãos Batholith yielded a crystallization age of 569 ± 6 Ma, whereas the syncollisional Pão de Açúcar and Corcovado gneissic granites ages of 559 ± 4 Ma and 560 ± 7 Ma, respectively. The new data, supported by the integration of the geochronological data on the Araçuaí Orogen magmatism in Espírito Santo and Minas Gerais states, cast new lights on the regional Neoproterozoic (Brasiliano) evolution. Similar late Precambrian ages, spanning from ca. 580-560 Ma, recently obtained on similar syn-collisional plutons from the northern segment of the Araçuaí Orogen, highlights the synchronous develop ment of the collisional event in Costeiro Domain of Rio de Janeiro State and in the northern domain of the Araçuaí Orogen and, accor dingly, the synchronicity of the Neoproterozoic orogenic collage in both domains. As far as the Pan-African connections are related, th age of the collisional peak in the western African orogens (West Congo, Kaoko, Damara, Gariep and Saldania) are also synchronous wi th the ca. 560 Ma Brasiliano collisional event. However, the data do not confirm previous correlation of this segment with the adjacent southwestern Paranapiacaba Orogen (Ribeira Belt), owing to the much older collisional climax ( ca. 630 Ma) dated on the latter.

Idade, proveniência e ambiente tectônico do Complexo Jequitinhonha de alto grau, Orógeno Araçuaí

The Jequitinhonha Complex of the northeastern Aracuai orogen is an extensive sedimentary unit metamorphosed in the amphibolite-granulite facies transition around 580-545 Ma. The unit consists of Al-rich (kinzigitic) paragneisses with decametric intercalations of graphite gneisses and quartzites, and centimetric to metric lenses of calcsilicate rocks. A new detrital zircon U-Pb age spectrum is reported for a sample of quartzite, and whole-rock geochemical (major and trace elements, 9 samples) and Sm-Nd isotope data (10 samples) for Jequitinhonha Complex paragneiss. Together with published data these show that: (1) the geochemistry of paragneiss samples of the Jequitinhonha Complex are similar to those of passive margin sedimentary protoliths; (2) detrital zircon data yield U-Pb age populations between ca. 0.9 and 2.5 Ga; and (3) Sm-Nd TDM model ages range from 1.6 to 1.8 Ga and eNd(575 Ma) around -7.5. The data reveal a mixture of Cryogenian to Mesoproterozoic rift-related igneous rocks with the Palaeoproterozoic-Archaean basement rocks of the Sao Francisco-Congo palaeocontinent as the main source areas, and also support the correlation between the Jequitinhonha Complex and the passive margin units of the upper Macaubas Group, constituting the precursor basin of the orogen. Our results, with the absence of ophiolites in the Jequitinhonha Complex, reinforce the interpretation that the Sao Francisco-Congo palaeocontinent was not divided to the north of the focused region, suggesting an ensialic termination of a gulf during the Neoproterozoic.2Département des Sciences de la Terre et de l’Atmosphère, GEOTOP, Université du Québec à Montréal, Montréal, Québec, Canada. E-mail: stevenson.ross@uqam.ca 3Geological Survey of Brazil, CPRM-SUREG-BH, Belo Horizonte (MG), Brazil. E-mail: luiz.silva@cprm.gov.br 4Universidade Federal de Ouro Preto, Departamento de Geologia, Morro do Cruzeiro, Ouro Preto (MG), Brazil. E-mail: alkmim@degeo.ufop.br 5IG-Laboratório de Geocronologia, Universidade de Brasília, Asa Norte, Brasília (DF), Brazil. E-mail: marcio@unb.br, marcio.pimentel@ufrgs.br