Marly Babinski

Juvenile accretion at 750–700 Ma in southern Brazil

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

Proterozoic geologic evolution of the SW part of the Amazonian Craton in Mato Grosso state, Brazil

Abstract This paper presents new geochronologic, isotopic, and geochemical data bearing on the evolution of Proterozoic crust in SW Mato Grosso state, Brazil, which is at the southern end of the ≈1.6–1.8 Ga Rio Negro–Juruena orogenic belt of the Amazonian Craton (Amazonia). Our data define three major crustal events: (i) the Alto Jauru terrane occurs in the eastern part of the region and is comprised of island arc-related rocks with U/Pb ages from 1.79 to 1.74 Ga. These rocks have e Nd ( t ) values range from +2.8 to +2.0 with crustal residence ages ( T DM ) from 1.93 to 1.78 Ga, indicating a mainly juvenile signature at 1.8 Ga; (ii) the Cachoeirinha suite occurs in the central part of the region and consists of calc-alkaline plutons emplaced into Alto Jauru terrane host rocks. These plutons have U/Pb ages from 1.56 to 1.54 Ga and e Nd ( t ) values ranging from +1.0 to −0.8, with crustal residence ages ( T DM ) from 1.88 to 1.75 Ga. We interpret this orogen as the roots of a continental margin arc built upon basement comprised of the Alto Jauru terrane; (iii) the Santa Helena batholith occurs in the western part of the region and is a large, elongate body of calc-alkaline rocks ranging from granodiorite to highly evolved granite. These units yield U/Pb ages from 1.45 Ma to 1.42 Ga, with e Nd ( t ) values ranging from +4.1 to +2.6 and crustal residence ages ( T DM ) from 1.70 to 1.50 Ga. The Rio Alegre domain occurs west of the Santa Helena batholith and includes juvenile 1.52 to 1.47 Ga volcanic and mafic plutonic rocks. Regional geologic relationships suggest that these rocks are part of the crust into which the Santa Helena batholith was emplaced. We interpret this batholith as the magmatic core of a juvenile arc accreted to the edge of the Alto Jauru terrane, with incorporation of some older crust (Alto Jauru terrane) in the east and derivation from mainly juvenile crust (Rio Alegre domain) in the west. The Rio Branco suite occurs to the east of exposed Alto Jauru terrane rocks as large hills protruding through younger Aquapei Group sedimentary rocks. It consists of gabbro and granophyric rocks with U/Pb ages of 1.47 Ga (gabbro) and 1.43 Ga (granophyre) and inherited older Nd, suggesting hinterland derivation from Alto Jauru terrane basement during development of the Santa Helena batholith. Several undeformed 1.5–1.4 Ga granitic plutons occur within the Alto Jauru terrane; these are also regarded as inboard manifestations of subduction related magmatism associated with accretion of the Rio Alegre domain and formation of the Santa Helena batholith. The siliciclastic Aguapei Group was deposited sometime between 1.0 and 1.4 Ga; it overlies Jauru terrane basement, rocks of the Santa. Helena batholith, and the Rio Alegre domain. In the east, it is flat-lying and undeformed, but in the west it is deformed and metamorphosed in the NNW trending Aguapei thrust belt. Farther west the Aguapei Group is horizontal, undeformed, and overlies the Paleoproterozoic Paragua block in Bolivia, where it has been correlated with the Sunsas Group. The Aguapei thrust belt has K/Ar cooling ages of about 930 Ma and is apparently a foreland fold and thrust belt formed by reactivation of an older rift basin during the 1.0 Ga Sunsas orogeny, which occurs west of the Paragua block in Bolivia.

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).

Pb–Pb dating and Pb isotope geochemistry of Neoproterozoic carbonate rocks from the São Francisco basin, Brazil: implications for the mobility of Pb isotopes during tectonism and metamorphism

Abstract A U–Pb study was carried out on carbonate rocks from the Neoproterozoic Bambui Group in the southern part of the Sao Francisco basin, Brazil. Pb isotopic compositions and U and Pb concentrations were determined on more than 90 samples from different parts of the basin. These samples were found to contain four distinct types of Pb, here called Types I, II, III and IV. Type I Pb was found in samples with low Pb concentrations and relatively high U concentrations; it represents in situ growth of radiogenic Pb and has a large enough variation to define Pb/Pb isochron ages. Type II Pb is present in samples with relatively high Pb concentrations and low U concentrations; it is nonradiogenic crustal Pb that could either represent average crustal Pb at the time of deposition or, as interpreted here, at the time of deformation of these rocks. Type III Pb is also found in samples with high Pb concentrations and low U concentrations, but it apparently is radiogenic crustal Pb from the Archean or Paleoproterozoic basement that was incorporated into the carbonates during the 600 Ma Brasiliano orogeny. Type IV Pb is intermediate in composition between Type III and Type I Pb; it represents a mixture of these two types. Pb/Pb isochron ages obtained from mesoscopically undeformed carbonates containing Type I Pb range from 686±69 to 520±53 Ma; the older age is the minimum depositional age for carbonate rocks from the Sete Lagoas Formation. During the interval from 690 to 500 Ma, the Pb isotope system of carbonates from the Sao Francisco basin was disturbed, and in some areas it was totally reset. Type III Pb compositions define a straight line; Type II Pb falls on the lower end of this line, which intercepts the Stacey and Kramers (S&K) Pb growth curve at about 520 and 2100 Ma. This line is interpreted as representing a stage of crustal Pb evolution beginning about 2100 Ma and ending about 550 to 500 Ma, when variably radiogenic Pb from the basement was incorporated into these carbonates. The ages determined in this study are in agreement with most published ages for Brasiliano fold belts marginal to underlying Sao Francisco craton, showing that isotopic systems of Sao Francisco basin rocks were largely affected by Brasiliano tectonism.

Tectonic setting and U/Pb zircon dating of the plutonic Socorro Complex in the Transpressive Rio Paraíba do Sul Shear Belt, SE Brazil

The Precambrian Rio Paraiba do Sul Shear Belt comprises a 200-km-wide anastomosing network of NE-SW trending ductile shear zones extending over 1000 km of the southeastern coast of Brazil. Granulitic, gneissic-migmatitic, and granitoid terrains as well as low- to medium-grade metavolcano-sedimentary sequences are included within it. These rocks were affected by strong contractional, tangential tectonics, due to west-northwestward oblique convergence of continental blocks. Subsequent transpressional tectonics accomodated large dextral, orogen-parallel movements and shortening. The plutonic Socorro Complex is one of many deformed granites with a foliation subparallel to that of the shear belt and exposes crosscutting relationships between its tectonic, magmatic, and metamorphic structures. These relationships point to a continuous magmatic evolution related to regional thrusts and strike slip, ductile shear zones. The tectonic and magmatic structural features of the Serra do Lopo Granite provide a model of emplacement by sheeting along shear zones during coeval strike-slip and cross shortening of country rocks. Geochronological data indicate that the main igneous activity of Socorro Complex spanned at least 55 million years, from the late stage of the northwestward ductile thrusting (650 Ma), through right-lateral strike slip (595 Ma) deformation. The country rocks yield discordant age data, which reflect a strong imprint of the Transamazonian tectono-metamorphic event (1.9 to 2.0 Ma). We propose a model for the origin of calc-alkaline granites of the Ribeira Belt by partial melting of the lower crust with small contributions of the lithospheric mantle during transpressional thickening of plate margins, which were bounded by deep shear zones. The transpressional regime also seems to have focused granite migration from deeper into higher crustal levels along these shear zones.

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.

Accurate and precise zinc isotope ratio measurements in urban aerosols.

We developed an analytical method and constrained procedural boundary conditions that enable accurate and precise Zn isotope ratio measurements in urban aerosols. We also demonstrate the potential of this new isotope system for air pollutant source tracing. The procedural blank is around 5 ng and significantly lower than published methods due to a tailored ion chromatographic separation. Accurate mass bias correction using external correction with Cu is limited to Zn sample content of approximately 50 ng due to the combined effect of blank contribution of Cu and Zn from the ion exchange procedure and the need to maintain a Cu/Zn ratio of approximately 1. Mass bias is corrected for by applying the common analyte internal standardization method approach. Comparison with other mass bias correction methods demonstrates the accuracy of the method. The average precision of delta(66)Zn determinations in aerosols is around 0.05 per thousand per atomic mass unit. The method was tested on aerosols collected in Sao Paulo City, Brazil. The measurements reveal significant variations in delta(66)Zn(Imperial) ranging between -0.96 and -0.37 per thousand in coarse and between -1.04 and 0.02 per thousand in fine particular matter. This variability suggests that Zn isotopic compositions distinguish atmospheric sources. The isotopic light signature suggests traffic as the main source. We present further delta(66)Zn(Imperial) data for the standard reference material NIST SRM 2783 (delta(66)Zn(Imperial) = 0.26 +/- 0.10 per thousand).

Isotopic Evidence for the Late Brasiliano (500-550 Ma) Ore-Forming Mineralization of the Araés Gold Deposit, Brazil

The Araés 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. 40Ar/39Ar 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 40Ar/39Ar plateau ages between 541 and 531 Ma, interpreted as cooling ages following regional metamorphism. Clay minerals from a hydrothermal alteration zone yield an 40Ar/39Ar integrated age of 503 ± 3 Ma. Galena grains from ore-bearing veins yield values of 206Pb/204Pb from 17.952 to 18.383, 207Pb/204Pb from 15.156 to 15.811, and 208Pb/204Pb from 38.072 to 39.681. Pyrite grains from ore-bearing veins yield values of 206Pb/204Pb from 18.037 to 18.202, 207Pb/204Pb from 15.744 to 15.901, and 208Pb/204Pb from 38.338 to 38.800. Pb isotope variations may be explained in terms of mixing a less radiogenic lead component (μ ~ 8.4) from mafic and ultramafic basement host-rocks (Nova Xavantina metavolcanosedimentary rocks) and a more radiogenic lead component (μ ~ 9.2) probably derived from supracrustal rocks (Cuiabá sedimentary groups). Sulfur isotope compositions are homogeneous, with δ34S values ranging from -1.1‰ to 0.9‰ (galena) and -0.7‰ to 0.9‰ (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 Araés gold deposit probably formed during a late stage of the orogeny, coeval with other mineralization events in the Paraguay Belt.

Distribuição e assinatura isotópica de Pb em sedimentos de fundo da Foz do Rio Guamá e da Baía do Guajará (Belém - Pará)

Lead analyses in bottom sediments from the hydrographic system of Belem (Para) indicated low contents of this metal for the sediments from the Guama river, with no significant anthropogenic contribution. A concentration of 18.1 ± 1.5 mg kg-1 and 206Pb/207Pb isotopic signature of 1.196 ± 0.002 are assigned for Pb from natural sources. On the other hand, the significant increase of Pb contents in the sediments from the Guajara bay, together with the decrease of 206Pb/207Pb ratios (1.172 < 206Pb/207Pb < 1.188) point to an anthropogenic lead contribution, originated by the industrial and urban activities of the city of Belem.

Quimioestratigrafia da porção basal do Grupo Bambuí no sudoeste do Cráton do São Francisco: implicações para os paleoambientes de Gondwana

The Bambui Group, the most extensive carbonate-siliciclastic cover on the Sao Francisco craton, has been a matter of debate because of its potential correlations to global glacial events. Unfortunately, most available chemostratigraphic data came from samples collected on surface rock exposures, ever susceptible to the aggressive chemical weathering that characterizes the southeastern Brazil. On the other hand, we present here high-resolution chemostratigraphic studies based on C, O and Sr isotopic data from 53 samples collected along a weathering-free, continuous, 175 m thick sedimentary succession. This succession was recovered by borehole drilling in the southwestern Sao Francisco craton, where occur the Carrancas and Sete Lagoas formations, the lowermost units of the Bambui Group. The drill cores reveal extremely irregular contacts between the basal diamictite and its basement, an Archaean foliated granodiorite. Geochronological and sedimentological data strongly suggest that the diamictite represents a lodgement till. This glaciogenic deposit is covered by a limestone succession which starts with impure carbonates showing aragonite pseudomorph fans and thin bands of black shale. The limestone pile grades to a marl-mudstone interval, which turns to a carbonate with biological components, succeeded by stromatolitic dolomite at the top. C and O isotopic signatures (referred to V-PDB) allow to the subdivision of the lower carbonate-pelite section into three intervals. The first isotopic interval corresponds to a cap carbonate, and displays negative values of δ13C (c . -4‰), and a large oscillation of the δ18O (-6 to -15‰). The Interval II shows a striking homogeneity in δ13C and δ18O, around 1‰ and -7‰, respectively. At the top, Interval III shows a large positive excursion of the δ13C (up to 8‰) and δ18O (-8 to -3‰) values. Unaltered 86Sr/87Sr ratios range from 0.7075 to 0.7077, mainly at the top of the section. The geochemistry of the carbonates is controlled by their terrigenous content (mostly quartz and clay minerals) which is concentrated in the lower units. Samples free of terrigenous contamination show Y/Ho ratios ranging from 25 to 50, suggesting a freshwater input during carbonate deposition. It is concluded that the diamictite has a glaciogenic origin and is covered by a cap carbonate. This pair has been identified along the basin and is related to one of the main Neoproterozoic glaciations. Discrepancy between the 86Sr/87Sr values and the global variation curves can be related to freshwater input during the carbonate deposition. Based on the regional tectonic context, the Bambui Basin may have been a restricted marine basin, totally or partially surrounded by mountain ranges within Gondwana, in the Neoproterozoic/Paleozoic boundary. In its early stages, the sedimentation was influenced by a global glacial event, whose melting phase was responsible by freshwater input in the basin. The gradual rise of the temperature was followed by an increase of the biological activity. Finally, a sudden increase in the biological activity could have been driven by paleogeographic changes caused by the active tectonic.Manuscript ID: 30285: Received in: 04/21/2015. Approved in: 02/12/2016. ABSTRACT: The Bambuí Group, the most extensive carbonate-siliciclastic cover on the São Francisco craton, has been a matter of debate because of its potential correlations to global glacial events. Unfortunately, most available chemostratigraphic data came from samples collected on surface rock exposures, ever susceptible to the aggressive chemical weathering that characterizes the southeastern Brazil. On the other hand, we present here high-resolution chemostratigraphic studies based on C, O and Sr isotopic data from 53 samples collected along a weathering-free, continuous, 175 m thick sedimentary succession. This succession was recovered by borehole drilling in the southwestern São Francisco craton, where occur the Carrancas and Sete Lagoas formations, the lowermost units of the Bambuí Group. The drill cores reveal extremely irregular contacts between the basal diamictite and its basement, an Archaean foliated granodiorite. Geochronological and sedimentological data strongly suggest that the diamictite represents a lodgement till. This glaciogenic deposit is covered by a limestone succession which starts with impure carbonates showing aragonite pseudomorph fans and thin bands of black shale. The limestone pile grades to a marl-mudstone interval, which turns to a carbonate with biological components, succeeded by stromatolitic dolomite at the top. C and O isotopic signatures (referred to V-PDB) allow to the subdivision of the lower carbonate-pelite section into three intervals. The first isotopic interval corresponds to a cap carbonate, and displays negative values of δ13C (c. -4‰), and a large oscillation of the δ18O (-6 to -15‰). The Interval II shows a striking homogeRESUMO: O Grupo Bambuí, mais importante unidade de cobertura do Cráton do São Francisco, tem sido alvo de intensos estudos e debates, entre outros motivos, pela possibilidade de correlação com eventos glaciais globais. A maior parte dos dados quimioestratigráficos disponíveis, no entanto, provém de amostras coletadas em afloramentos, sujeitos a expressivo intemperismo químico. Neste trabalho, é apresentado um levantamento quimioestratigráfico de alta resolução, baseado em análises de C, O e Sr realizadas em 53 amostras coletadas em 175 m de sequência sedimentar contínua, livre de intemperismo. Tal sequência foi obtida a partir de testemunhos de sondagem realizada na porção sul do Cráton do São Francisco, onde ocorrem rochas das formações Carrancas e Sete Lagoas, as mais basais do Grupo Bambuí. Os testemunhos revelaram contato extremamente irregular entre uma camada de diamictito e seu embasamento, um granodiorito foliado de idade arqueana. Dados sedimentológicos e geocronológicos indicam que o diamictito representa um tilito de alojamento, que apresenta contato brusco com a sequência carbonática sobrejacente. Essa sequência se inicia com calcário impuro, que exibe leques de cristais pseudomorfos de aragonita e delgadas camadas de folhelho negro. O calcário passa gradacionalmente para um intervalo argiloso, que por sua vez volta a gradar para uma espessa sequência de calcário com laminação microbiana, sucedido por dolomito estromatolítico no topo da coluna. As assinaturas isotópicas de C e O permitem a identificação de três intervalos distintos. O Intervalo I, basal, corresponde a um carbonato de capa, exibindo valores negativos de δ13C (c. -4‰), e grande oscilação nos valores de δ18O (-6 a -15‰). O Intervalo II exibe marcante homogeneidade nos valores de δ13C e δ18O, que se situam em torno de 1‰ e -7‰, respectiChemostratigraphy of the lower Bambuí Group, southwestern São Francisco Craton, Brazil: insights on Gondwana paleoenvironments