UNIVERSIDADE DE BRASÍLIA - INSTITUTO DE GEOCIÊNCIAS

ABSTRACTS
DE CONTRIBUIÇÕES CIENTÍFICAS DO IG/UnB
ANO 2002

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The nature of orthomagmatic, carbonatitic fluids precipitating REE,Sr-rich fluorite: Fluid inclusion evidence from the Okorusu fluorite deposit, Namibia 

B. Bühn ^a,*
A.H. Rankin ^b
J. Schneider ^a
P. Dulski ^c

^a Institut für Geowissenschaften und Lithosphärenforschung der Universität,  Germany
* Present address: Instituto de Geociências, Universidade de Brasília, Brazil,
^b School of Earth Sciences and Geography, Kingston University, U.K. 
^c GeoForschungsZentrum Potsdam, Germany

 Chemical Geology (2002), 186:75-98

Efeitos dos minerais acessórios na geoquímica de elementos terras-raras em granitóides do Complexo Jequitinhonha no extremo Sul da Bahia

Celino,J.J.
Botelho,N.F.

Revista da Escola de Minas(2002),55(2):131-137

Palavras chave: Química Mineral, Granitos

RESUMO

Objetiva-se comparar e discutir padrões de ETR em rochas e minerais acessórios (monazita, apatita e zircão) de granitos crustais, de idade Neoproterozóica, nas proximidades dos povoados de São Paulinho (SPL), Santa Luzia do Norte (SLN) São João do Sul (SJS) e Santo Antônio do Jacinto (SAJ), denominados de Suíte São Paulinho (SSP) no extremo sul do Estado da Bahia. Apenas nas rochas muito diferenciadas desta suíte, pobres em minerais acessórios, ocorrem mudanças significativas nos formatos dos padrões de ETR, sugerindo que nos estágios iniciais da evolução magmática as variações de formatos, devido ao fracionamento de minerais essenciais, são em boa parte mascaradas pelos minerais acessórios.

Campos & Dardenne,2002

Pavimentos estriados do Grupo Santa Fé - Neopaleozóico da Bacia Sanfranciscana, MG

José Eloi Guimarães Campos
Marcel Auguste Dardenne

In: Schobbenhaus,C.; Campos,D.A.; Queiroz,E.T.; Winge,M.; Berbert-Born,M.L.C. (edts) 2002. Sítios Geológicos e Paleontológicos do Brasil. DNPM/CPRM-Comissão Brasileira de Sítios Geológicos e Paleobiológicos(SIGEP); pp 161-164

RESUMO

Os pavimentos estriados da região de Santa Fé de Minas, no noroeste do Estado de Minas Gerais são pouco conhecidos da comunidade geológica. Contudo estas estruturas são importantes no contexto geológico regional da Bacia Sanfranciscana, auxiliando na identificação dos ambientes deposicionais da unidade basal (permo-carbonífera) da cobertura Fanerozóica do Cráton do São Francisco. Trata-se de ocorrências limitadas, mas muito bem expostas que devem ser preservadas e integrar o patrimônio geológico do Brasil, como um sítio do tipo estratigráfico.

THE STRIATED PAVEMENTS OF THE SANTA FÉ GROUP – LATE PALEOZOIC OF THE SANFRANCISCANA BASIN

ABSTRACT

The striated pavements observed in the Santa Fé de Minas region, in the northwestern of the Minas Gerais State, are not well known by the Brazilian geologists. However these structures are important in the regional geologic context of the Sanfranciscana Basin, and were fundamental to the identification of the depositional environment of the basal units of the São Francisco Craton Phanerozoic covers. The structures are limited in area, but are very well preserved and must be integrated to the geological inheritance of Brazil as one stratigraphic site.

Coimbra & Do Carmo,2002

TAXONOMY AND ECOLOGY OF CORNUCOQUIMBA RAMOSAE SP.NOV. (OSTRACODA, CRUSTACEA) IN THE BRAZILIAN EQUATORIAL SHELF

COIMBRA, J. C.
DO CARMO, D. A.

 Journal of Micropalaeontology( 2002), 21(1): 23 - 28.

Keywords: Cornucoquimba, Brazilian Equatorial Margin, Ostracods, Ecology, Depth, Sediment Type

ABSTRACT

Three species of Cornucoquimba Ohmert, 1968 are known from the Brazilian shelf: C. decorata, and C. nana in the north and northeast, and C. conulata in the south. An analysis of 339 samples collected between the international boundary at the Oiapoque river (Amapá State) and Cape São Roque (Rio Grande do Norte State), a distance of almost 1400 km, revealed the new species for this genus, C. ramosae sp. nov. It is restricted to the equatorial shelf and prefers biodetritic sand, and ranges from 15 to 67 m in depth.

Dardenne & Campos,2002

Parque Nacional da Chapada dos Veadeiros - GO

Marcel Auguste Dardenne
José Eloi Guimarães Campos

In: Schobbenhaus,C.; Campos,D.A.; Queiroz,E.T.; Winge,M.; Berbert-Born,M.L.C. (edts) 2002. Sítios Geológicos e Paleontológicos do Brasil. DNPM/CPRM-Comissão Brasileira de Sítios Geológicos e Paleobiológicos(SIGEP); pp 323-333.

RESUMO

O presente texto pretende mostrar os principais sítios geológicos e geomorfológicos preservados no Parque Nacional da Chapada dos Veadeiros e adjacências. Entre os sítios geológicos de interesse geológico destacam-se os aspectos litológicos, sedimentológios e estruturais enquanto as feições geomorfológicas mais importantes estão relacionadas aos planaltos e serras de grande beleza natural. A divulgação destes dados é importante, uma vez que, a região apresenta grande potencial para o turismo e a preservação dos sítios está associada ao conhecimento de sua importância, fragilidade ou raridade. Em geral os turistas não tem informação sobre a gênese, importância, ou particularidades dos locais e estruturas visitadas, o que traz riscos a preservação dos sítios naturais da região.

CHAPADA DOS VEADEIROS NATIONAL PARK, GOIÁS STATE, BRAZIL

ABSTRACT

The present paper intend to show the main geological and geomorphological sites preserved in the Chapada dos Veadeiros National Park and neighbourhod. Among the sites of geologic interrest the lithologic, sedimentological and structural aspects are evidenced, while the most important geomorphological features are associated to the highlands, ridges and hills of great natural beauty. The publication of these data set is important because the region shows ample touristic potential, and the sites preservation is associated to the knowledge of their importance, fragility or rarity. In general the tourists do not have any information about the origin, importance or particularity of the places and structures visited, what brings risks to the preservation of the natural sites found in the region.

D'el-Rey Silva & Barros Neto,2002

The Santa Terezinha-Campos Verdes emerald district, central Brazil: structural and Sm–Nd data to constrain the tectonic evolution of the Neoproterozoic Brasília belt 

Luiz José Homem D'el-Rey Silva
Leonel de Souza Barros Neto

Journal of South American Earth Sciences 15(6), 693-708.
Full Text + Links

Keywords: Structural analysis; Sm–Nd data; Brasília belt; Brasiliano cycle; Neoproterozoic; Emeralds 

ABSTRACT

Structural analysis coupled with Sm–Nd isotope data and a detailed description of the geology of the Santa Terezinha-Campos Verdes emerald district (Goiás State, Central Brazil) constrain the evolution of the Neoproterozoic Brasília belt. The area is composed of tectonic slices of Archean–Paleoproterozoic gneiss, a Meso-Neoproterozoic metavolcanic sedimentary sequence called the Santa Terezinha sequence, and crustal-derived intrusive rocks such as mylonitic (ortho)gneiss and a syntectonic porphyry granite. It underwent a Neoproterozoic greenschist facies polyphase ductile deformation (D1–D3). Structures indicate an event of rotational deformation along a typical frontal ramp dipping gently to the west (i.e. an event of simple shear with top to ESE relative regional movement due to a subhorizontal WNW–ESE compression). A Sm–Nd whole-rock isochron age of 577±77 Ma for the intrusive rocks constrains the timing of at least part of the deformation/metamorphism in the area. Primary and metamorphic planar structures (mainly D1–D2) strike SW–NE and dip at low to moderate angles to the NW in the northern part of the area. However, they gradually rotate to SSE in the central SE part, where the Peixe River synclinorium is developed. This synclinorium is also the nest of the D2 sheath folds that control emerald ore shoots. The Santa Cruz dome is a basement-cored, major elliptic structure in the SW of the area. The Santa Terezinha sequence represents a back-arc basin that received input from the Neoproterozoic Goiás magmatic arc to the west and the São Francisco ancient continental margin to the east. The basal and upper sections of this sequence correlate, respectively, with other passive margin and back-arc sequences of the Brasília belt.

Eeckhout et al.,2002

Spectroscopic studies of spessartine from Brazilian pegmatites

SIGRID G. EECKHOUT
CRISTIANE CASTAÑEDA
ANA CLÁUDIA M. FERREIRA
ANTÔNIO C.S. SABIONI
EDDY DE GRAVE
DANIELA C.L. VASCONCELOS

American Mineralogist, Volume 87, pages 1297–1306, 2002

ABSTRACT

Mössbauer spectra (MS) at room temperature (RT) and Fourier-transform infrared (FTIR) spectra in the OH stretching region were acquired for natural spessartine-almandine garnet samples from different Brazilian pegmatites, including the complex, zoned Alto Mirador pegmatite, the simple, zoned Escondido pegmatite, and the simple Poaiá pegmatite. From MS, it is obvious that in the samples from the Alto Mirador pegmatite, ferrous iron is present at the dodecahedral site and ferric iron at the octahedral site of the garnet structure. The Fe2+/Fe3+ ratio is comparable for all samples from this pegmatite, i.e., ~10%, implying the same geological history, namely similar oxygen fugacities at the moment of garnet formation. In the simple pegmatites, however, almost no ferric iron was detected. On the basis of the multi-band FTIR spectra the nature of the point defects involved in the process of hydrogen uptake in spessartine-almandine garnet has been elucidated, if not completely clarified. The behavior of the absorption bands provides no evidence for multiple incorporation mechanisms, hence the hydrogarnet substitution is proposed to be the only mechanism for the incorporation of hydrogen in the samples studied. As geological setting is difficult to separate from chemical composition, FTIR spectra have the potential to provide useful information on some aspects of the geological history of the samples.

Ferreira et al.,2002

Origin and Intensive Parameters in the Crystallization of Ultrapotassic Syenites: 
The Serra do Man Pluton, Northeastern Brazil

Marco A.F. Ferreira *1
Valderez P. Ferreira *
Alcides N. Sial *
Marcio M. Pimentel **

* NEG-LABISE, UFPE
** IG/UnB, Brasilia
1 Departmento de Biologia, UFPI, Teresina, PI

Gondwana Research(2002), 5(2): 275-285.

Key words: Ultrapotassic, syenite, intensive parameters, petrogenesis.

ABSTRACT

The late Neoproterozoic ultrapotassic Serra do Man pluton intruded supracrustal schists and igneous rocks of the Alto Pajeu tectonostratigraphic terrane, in the Borborema province of northeastern Brazil. This quartz-alkali feldspar syenite is composed of microcline, albite, normally zoned aegirine-augite, winchite (formed after pyroxene), titanite and apatite, with the major-element geochemistry in consonance with early fractionation of pyroxene-titanite-apatite assemblage. Chondrite-normalized REE-patterns are LREE-enriched (La/Lu ~ 20) with discrete negative Eu anomaly, and primitive mantle-normalized elemental concentration diagrams display enhanced negative Nb, P and Ti anomalies that are also observed in patterns of similar rocks in NE Brazil. Whole-rock δ18O values are rather high (avg. +8.7‰SMOW) and in the +7.8 to +8.6‰SMOW range when clinopyroxene-corrected. A five-point whole-rock Rb–Sr isochron yields an age of 580± 28 Ma with high Sro (0.7108), which together with εNd (0.58 Ga) value of –17 and tchur of 2.3 Ga allow hypothesizing a Paleoproterozoic enriched-mantle source for the syenites in the ultrapotassic province of the Alto Pajeu terrane. The aegirine-augite, titanite, and quartz assemblage attests to a high oxygen fugacity prevailing during magmatic crystallization. The almost complete absence of primary hydrated phases assures that the magma was rather anhydrous, and its viscosity, at the liquidus temperature, was relatively high. Density is slightly higher than expected for syenitic magmas (2.8 g/cm3), being explained by a relatively high iron content of the magma. Geothermobarometry suggests shallow emplacement and temperatures of 700–750oC and 850–900oC, respectively, for the solidus and liquidus of the magma.

Junges et al.,2002

Nd Isotopic study of the Neoproterozoic Mara Rosa Arc, central Brazil: implications for the evolution of the Brasilia Belt

Sérgio Luiz Junges
Márcio Martins Pimentel
Renato de Moraes

Precambrian Research(2002),117: 101 - 118.

(Ver texto integral)

Keywords: Neoproterozoic; Brasília Belt; Mara Rosa Arc; Nd isotopes; Intraoceanic arc

ABSTRACT

The Goiás Magmatic Arc is a 0.9-0.64 Ga island arc underlying large areas of the western part of the Brasília Belt, in central Brazil. It is exposed in two separate sections, known as the Arenópolis Arc in the south and the Mara Rosa Arc in the north. It is made of island arc volcanic-sedimentary sequences forming narrow NNE to NNW belts, separated from each other by tonalitic-granodioritic-granitic orthogneisses. All rocks are metamorphosed under upper greenschist to amphibolite facies conditions, but have kept major geochemical and isotopic features indicating the juvenile nature of the igneous protoliths. The meta-igneous and metasedimentary rocks are intruded by a number of late- to post-orogenic granites (<600 Ma), associated with gabbro-dioritic bodies, constituting a typically post-orogenic bimodal association. Tonalitic and minor granitic orthogneisses dated at 850 Ma are the oldest rocks in the Mara Rosa Arc and have _Nd(850) ranging from +1.5 to +4.6, with T_DM values between 1.0 and 1.1 Ga. Metasedimentary rocks in the Mara Rosa supracrustal belts display geochemical features suggesting that they represent first cycle, immature sediments which preserve chemical characteristics of their igneous sources. Their Nd isotopic characteristics support this interpretation. T_DM model ages for detrital metasediments are mostly in the interval between 0.9 and 1.2 Ga, indicating that they are a product of erosion of the arc rocks. The absence of older contributions to these sediments seems to indicate an intraoceanic setting. This is also suggested by the Nd isotopic composition of the 0.8 Ga old tholeiitic amphibolites, which display positive initial _Nd (+4.9), indicating derivation from a depleted mantle source. Syn-orogenic deformed granites forming small intrusions within the supracrustal belts have T_DM ages ranging from 0.77 to 2.54 Ga. The older model age values indicate the presence of hidden fragments of ancient crust, which were probably tectonically interleaved with younger rocks during accretion of the arc. The post-orogenic granites are moderately peraluminous and the Nd model ages between 1.0 and 1.3 Ga indicate that their parental magmas were predominantly re-melts of the arc rocks. Their peraluminous nature reflects the incorporation of substantial amounts of arc sediments in the original magma. Garnet-whole rock Sm-Nd ages for the metasediments may be interpreted to refer to two metamorphic events: an older event at 0.76-0.73 Ga and a younger event at 604-610 Ma. Mineral chemistry indicate that the older episode was characterized by higher P/T and the younger by lower P/T. The older metamorphic event is interpreted preliminarily as the result of the accretion of island arc terranes to the western edge of the orogen and the younger event, as a result of final ocean closure and continental collision between the Amazon and São Francisco continents. The Goiás Arc formation and accretion is partially coeval with Rodinia break up, suggesting that the western margin of the São Francisco-Congo was peripheral in that supercontinent and faced a wide oceanic basin during most of the Neoproterozoic.

Lenharo et al.,2002

Petrogenetic and mineralization processes in Paleo- to Mesoproterozoic rapakivi granites: examples from Pitinga and Goiás, Brazil

Sara L. R. Lenharo
Márcia A. Moura
Nilson F. Botelho 

Precambrian Research, Holanda (2002), 119: 277-299.

Full Text

Keywords: Brazil; Proterozoic; Within-plate magmatism; Rapakivi granite; Sn; In; Rare metals 

ABSTRACT

The 1.8 Ga Pitinga granites––Água Boa and Madeira massifs––in Amazonas (northern Brazil) are within-plate, shallow-level rapakivi granites associated with an extensional fracture system. They comprise an early facies of pyterlitic to wiborgitic rapakivi granite, a fine- to medium-grained biotite granite, as well as topaz granite (Água Boa massif) and albite granite (Madeira massif). The granites are usually metaluminous to peraluminous, the albite granite, however, is peralkaline. They are enriched in SiO2, K2O, Na2O, F, Rb, Th, Nb, Y, Zr and the rare-earth element (REE) and impoverished in MgO, TiO2, P2O5 and Sr, as the majority of Sn-mineralized granites. Sn contents range from ~1 ppm in the rapakivi facies to ~3400 ppm in the albite granite. Nd (at 1.8 Ga) values vary from -2.2 to +0.4 and Nd model ages lie between 2.4 and 2.1 Ga. Mineralization in the Madeira massif includes disseminated magmatic cryolite, zircon, cassiterite, pyrochlore, columbite-tantalite and xenotime and massive cryolite bodies in the F-rich peralkaline albite granite. In the Água Boa massif, Sn mineralization is associated with greisens and episyenite. The 1.77 Ga (g1) and 1.58–1.57 Ga (g2) rapakivi granites of Goiás (central Brazil) are coeval with the Araí rift basin. Granites of the g1 suite are metaluminous and alkaline, while the g2 suite is metaluminous to peraluminous. Both are enriched in F, Sn, Rb, Y, Th, Nb, Ga and the REE. Primary micas range from Fe-rich biotite to zinnwaldite. The micas of the more evolved granites and the metasomatic micas are strongly enriched in F, with F/Li between 2 and 10. The initial Nd values of both suites show a considerable range (~-14 to 0) and indicate substantial compositional variation in source. Sn deposits in Goiás are hosted mainly by greisens. Indium is concentrated in quartz–topaz rock and albitized g2d granite of the Mangabeira massif and is always related to a cassiterite-sulfide association. This quartz–topaz rock is of metasomatic origin, probably generated by a hydrothermal fluid derived from the topaz–albite granite. Mineralization in the studied deposits was essentially associated with F enrichment. In the peralkaline Madeira albite granite, extremely high F contents favored disseminated mineralization, while in the Goiás Tin Province (GTP) and Água Boa massif greisenization was related to early fluid saturation. The GTP and Pitinga granites display tectonic, petrogenetic, geochemical, isotopic and metallogenic similarities that can be applied in search of Sn and rare-metal deposits.

Moraes et al.,2002

Characterization and P–T Evolution of Melt-bearing Ultrahigh-temperature Granulites: an Example from the Anápolis–Itauçu Complex of the Brasília Fold Belt, Brazil

R. MORAES
M. BROWN
R. A. FUCK
M. A. CAMARGO
T. M. LIMA

Journal of Petrology. Oxford, 42(9): 1673 - 1705.

(Ver texto integral)

Keywords: Brasília Blt, granulite, melting, ultrahigh-temperature metamorphism

ABSTRACT

Sapphirine–quartz occurs in orthopyroxene–garnet granulites in two areas ~22 km apart within ‘common’ granulites of the Anápolis–Itauçu Complex, recording extreme P–T conditions: a minimum of 1030–1050°C at ~10 kbar. In one area, the post-peak evolution is constrained to begin at P > 10 kbar by down-temperature stability of garnet–orthopyroxene–sillimanite–quartz and the absence of cordierite. In a second area, the post-peak evolution is constrained by a succession of melt-present reactions that occur at P < 10 kbar, inferred from microstructural relations between garnet, orthopyroxene and sillimanite, and coronae or symplectites involving corundum, sapphirine, spinel, cordierite, plagioclase, sillimanite, orthopyroxene, biotite and ilmenite. The retrograde segment of the P–T path was further constrained by biotite-producing reactions. A composite P–T path was constructed from samples that exhibit different amounts of retrograde reaction, reflecting different amounts of melt retention. Using back-calculated compositions for garnet and orthopyroxene, thermobarometry yields 1012–960°C and 9·7–8·1 kbar; these P–T results underestimate ‘peak’ conditions, in part as a result of modification of garnet compositions in rocks in which some melt was retained. In samples from both areas, orthopyroxene porphyroblasts have high Al₂O₃ (12·9–9·7 wt %) in cores, which suggests maximum T > 1150°C in both areas. After decompression at the metamorphic peak, the P–T path for both areas followed a near-isobaric cooling stage to <900°C, but these two paths are separated by ~3kbar. This ultrahigh-temperature metamorphism occurred in a collisional tectonic setting, in which the extreme thermal perturbation probably was a result of asthenosphere replacement of mantle lithosphere.

Oliveira et al.,2002

Infrared and Mössbauer study of Brazilian tourmalines from different geological environments

Ester Figueiredo de Oliveira
Cristiane Castañeda
Sigrid Griet Eeckhout
Messias Gilmar de Menezes
Rogério Ribeiro Kwitko
Eddy De Grave
Nilson Francisquini Botelho 

 American Mineralogist(2002), 87(8-9):1154-1163.

(Ver texto integral - pdf)

ABSTRACT

Infrared (IR) spectra in the OH stretching region and Mössbauer spectra (MS) have been acquired for natural tourmalines from magmatic and hydrothermal deposits in the state of Minas Gerais, Brazil. In the magmatic environment, samples of tourmaline were collected from granitic pegmatites and granitoids. In the former, the elbaite-schorl series is prevalent, while in the latter the schorl species is dominant. Tourmalines from hydrothermal gold deposits are typically intermediate members of the dravite-schorl series, whereas the uvite and Fe-bearing uvite species are present in tourmaline samples from a magnesite deposit. Different behaviors are discussed in terms of local lattice environment, geological history, fluid/rock interaction or magma evolution, and degree of crystallinity.

Ordoñez & Pimentel,2002

Rb_¯Sr and Sm_¯Nd isotopic study of the Puquí complex, Colombian Andes

Oswaldo Ordóñez Carmona ^{a, b} 
Márcio M. Pimentel ^b

^a Departamento de Recursos Minerales, Universidad Nacional de Colombia, A.A. 1027 Medellín, Colombia
^b Instituto de Geociências, Universidade de Brasília, CEP: 70910-900 Brasilia, DF, Brazil
Journal of South American Earth Sciences(2002), 15(2): 173-182.

Keywords: Colombian Andes; Central cordillera; Puquí complex
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Abstract

The Puquí Complex is exposed in the northwestern part of the Colombian Central Cordillera and consists of a complex association of metamorphic rocks (amphibolite facies gneisses, migmatites, granulites, and amphibolites) intruded by the Puquí Tonalite, a large and mostly undeformed intrusion. The Puquí Tonalite is calc-alkaline and displays a mild peraluminous character, whereas the gneissic country rocks are strongly peraluminous (and interpreted as derived from a sedimentary protolith). A peculiar feature of this tonalitic body is the common presence of garnet (up to 7% modal proportion in some localities). The Rb_¯Sr data for the Puquí Gneiss yielded an isochron age of 306±11 Ma with a high initial ⁸⁷Sr/⁸⁶Sr ratio (ca. 0.70980). This is interpreted as the result of Late Paleozoic metamorphic resetting of the Rb_¯Sr isotopic system of the sedimentary/metasedimentary protolith. This metamorphic event is also recognized in other parts of the Colombian Andes. Sm_¯Nd garnet_¯whole rock ages, however, indicate that these rocks have also been involved in a much younger metamorphic event, between ca. 140 and 120 Ma. Sm_¯Nd model ages reveal a Precambrian age for the source of the original sediments (T_DM model ages are in the range 1.33_¯1.53 Ga). The Puquí Tonalite was emplaced at ca. 248±17 Ma (whole-rock Rb_¯Sr isochron) with an initial ⁸⁷Sr/⁸⁶Sr ratio of ca. 0.70748, which suggests some contribution from older sialic material in the original magma. This is confirmed by the Sm_¯Nd isotopic data, which show _Nd(248) between -5 and -6.15 and T_DM model ages between 1.17 and 2.13 Ga. These results suggest that the original tonalitic magma is strongly contaminated by the Puquí Gneiss. These preliminary Nd_¯Sr isotopic data suggest the presence of Precambrian crustal material in this part of the Colombian Central Cordillera. We also postulate that the Puquí Gneisses derived from Precambrian peraluminous crustal sources served as the contaminant of the tonalitic magma.

Resumen

El Complejo de Puquí se encuentra en la parte noroeste de la Cordillera Central de los Andes Colombianos y esta formado por el Gneis de Puquí, una compleja asociación de rocas metamórficas (gneises de facies anfibolita, migmatitas, granulitas y anfibolitas), intruidas por la Tonalita de Puquí, una grande e indeformada intrusión. La Tonalita de Puquí es calco-alcalina, mostrando sin embargo un claro carácter peraluminoso. Las rocas metamórficas encajantes son fuertemente peraluminosas e interpretadas como derivadas de un protolito sedimentario. La característica mas peculiar de la tonalita es la presencia común de granate (hasta 7% en proporción modal). Los datos Rb_¯Sr preliminares para el Gneis de Puquí arrojaron una edad de 306±11 Ma y una relación ⁸⁷Sr/⁸⁶Sr inicial alta (0.70980). Esta edad es interpretada como resultado de un reajuste isotópico por metamorfismo en el Paleozoico Superior del protolito sedimentario/metasedimentario. En otras regiones de los Andes Colombianos se han encontrado edades semejantes. Adicionalmente, las edades isocrónicas Sm_¯Nd con el par Granate-Roca total, indican que estas rocas fueron afectadas por un evento metamórfico posterior, el cual fué datado entre 140_¯120 Ma. Las edades modelo Sm_¯Nd revelaron edades precámbricas para las fuentes de los metasedimentos (edades modelo T_DM entre 1.33 y 1.53 Ga). La intrusión de la Tonalita de Puquí ocurrió hace ca. 248±17 Ma (isócrona Rb_¯Sr en roca total) y la relación ⁸⁷Sr/⁸⁶Sr inicial de 0.70748 sugiere contaminación del magma tonalítico original con material siálico más antiguo. Isótopos de Sm_¯Nd muestran _Nd(248 Ma) de -5 y -6.15, y edades modelo T_DM de 1.17 y 2.13 Ga. Estos datos sugieren que el magma tonalítico original fue ampliamente contaminado por las rocas del Gneis de Puquí. Preliminarmente, los datos isotópicos Nd_¯Sr indican la presencia de material cortical de edad Precámbrica en esta parte de la Cordillera Central colombiana. Adicionalmente, sugerimos que el Gneis de Puquí, derivado de fuentes corticales peraluminosas precámbricas, sirvió como contaminante del magma tonalítico.

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Sgarbi & Gaspar,2002

^a Departamento de Geologia, Instituto de Geociências, UFMG
^b IG, Universidade de Brasília

Journal of South American Earth Sciences(2002), 14(8): 889-901

Keywords: Kamafugite; Mafic alkaline volcanism; Cretaceous; Santo Antônio da Barra; Brazil

ABSTRACT

This work focuses on the kamafugites from Santo Antônio da Barra, Minas-Goiás Alkaline Province. These rocks contain olivine, clinopyroxene, titanomagnetite, perovskite, leucite (pseudomorphs), kalsilite, nepheline, and phlogopite.
The rocks investigated are ultrabasic, with high contents of CaO, FeO, and TiO₂, high to moderate contents of Al₂O₃, alkalis, and P₂O₅, and low contents of MgO. The alkaline characteristic of the rocks is reflected in TiO₂, K₂O, and Na₂O contents and in the frequent presence of normative nepheline and leucite. K₂O contents are not primary since most of the leucite was replaced by analcime.
The negative K anomaly verified in the extended incompatible element distribution diagram for kamafugites seems to be mainly related to alteration. Kamafugites are characterized by a marked enrichment in incompatible and large ion lithophile elements together with other typical compatible elements.
The Santo Antônio da Barra kamafugites are less enriched in titanium, niobium, zirconium, and REE than the Mata da Corda and most of the Toro-Ankole ones. San Venanzo-Cupaello rocks have much lower titanium contents.

Tazava & Oliveira,2002

The Igarapé Bahia Au-Cu-(REE-U) Deposit, Carajás Mineral Province, Northern Brazil

Edison Tazava
Claudinei Gouveia de Oliveira

In: Porter,T.M. (Edit) 2002. Hydrothermal Iron Oxide Copper-Gold & Related Deposits: A Global Perspective. PGC Publishing, Adelaide, volume 1: 203-212.

ABSTRACT

The Igarapé Bahia Au-Cu-(REE-U) deposit is located in the Carajás Mineral Province - Northern Brazil - and hosted by an Archaean low-grade metamorphosed volcanosedimentary sequence characterized by metavolcanic rocks at the footwall and metavolcanoclastic/metasedimentary rocks at the hangwall. An intense hydrothermal alteration occurred in this sequence, promoting intense chloritization, Fe-metasomatism, Cu-sulphidation (chalcopyrite and bornite), carbonatization, silicification, tourmalinization and biotitization.
The Cu-Au mineralization is hosted in the contact between metavolcanic and metavolcanoclastic/metasedimentary rocks and is divided in, magnetitic/sideritic heterolithic breccias and hydrothermalized metavolcanic rock, which are enriched in REE (monazite, alanite, xenotime, bastnäsite and parisite), Mo (molybdenite), U (uraninite), F (fluorite), Cl (ferropyrosmalite) and P (apatite). We believe that saline and F-Cl-rich fluids at high temperatures may have been responsible for carrying the REE, according to fluid inclusion and paragenetic studies.
C and O isotopic data from hydrothermal carbonates and fluid inclusion analyses from carbonate and quartz suggest the mixing of two fluids in both alteration and mineralization processes. Negative values of d13C (-9,3 to -5,8‰) indicate the presence of magmatic fluid with high salinity; moreover, the large variation of d12O (0,7 to 9,4‰) suggests a mixture between magmatic fluids of high T (higher isotopic values) and meteoric fluids (lower values).
On the basis of chemical and mineralogical composition, isotopic and fluid inclusion data, we propose in this paper a genetic model for the Igarapé Bahia deposit, that may be compared to the other intrusion-related hydrothermal Fe-oxide-(Cu-Au-U- REE) deposits.

Walker et al.,2002

1 Isotope Geochemistry Laboratory, Department of Geology, University of Maryland, USA
2 Department of Earth Sciences, University of Cardiff, Wales, UK
3 Department of Earth Sciences, Faculty of Science, Kanazawa University, Japan
4 Instituto de Geociências, Universidade de Brasília, Brasília-DF, Brazil

Geochimica et Cosmochimica Acta(2002), 66 (2): 329-345

Texto integral

Abstract

Chromites separated from the upper mantle or lower crustal portions of 18 ophiolites ranging in age from 900 Ma to 50 Ma are examined for Re-Os isotopic systematics. The ophiolites include both MORB and back arc types, although most are from supra-subduction zone (SSZ) settings. The chromites are robust indicators of the initial Os isotopic compositions of the systems sampled. There is very limited range in calculated initial _Os values, with the entire group averaging +1.31. Least squares linear regression of the age of chromite formation (in Ga) versus initial ¹⁸⁷Os/¹⁸⁸Os of a filtered suite yields a slope of 0.0058±0.0019 (2) and a present day intercept of 0.12809±0.00085 (2), equivalent to a _Os value of +0.9±0.6. Of the suite of 51 samples analyzed, 68% lie within ±1% of this evolution trajectory.
Although most of the samples formed in SSZ environments, there is little evidence to suggest modification of the mantle Os isotopic composition via radiogenic melts or fluids derived from subducting slabs. The ophiolite data are interpreted as representative of the convecting upper mantle and suggest that the present isotopic composition of the convecting upper mantle averages approximately 1.2% less radiogenic than the estimated minimum composition of the primitive upper mantle of 0.1296±8 (Meisel et al., 2001). The most likely explanation for the difference is the formation, subduction and isolation of some portion of the mafic oceanic crust. Using models based on the assumption that the convecting upper mantle comprises 50% of the total mass of the mantle, and that the average isolation period for subducted oceanic crust is 1.5 to 2.0 Ga, it is estimated that approximately 2 to 3% of the total mass of the mantle is composed of subducted mafic oceanic crust that remains isolated from the convecting upper mantle. Because the isotopic compositions of the DMM and PUM overlap within uncertainties, however, the results do not require any isolated slab component.

Wiedemann et al.,2002

Architecture of Late Orogenic Plutons in the Araçuaí-Ribeira Fold Belt, Southeast Brazil

C.M. Wiedemann^1,2
S.R. de Medeiros³
I.P. Ludka²
J.C. Mendes²
J. Costa-de-Moura²

ABSTRACT

Post-collisional to late orogenic magmatism (580 to 480 Ma) in the Araçuaí-Ribeira Fold Belt, SE Brazil, is characterized by the predominance of high-K metaluminous, allanite-titanite-bearing granitoids. Small lenses of coronitic gabbro, anorthosite, pyroxenite and phlogopite-peridotite are also common in deeper exposed areas of this fold belt. In the region of southern Espírito Santo State, the deep erosional level associated with a steep topography reveals the internal architecture of the intrusions: a tendency to funnel-shaped bodies, with sub-vertical hemi-ellipsoidal/conical roots changing upwards to shallow-dipping tops. Associated stocks, sills and dykes of basic and acid rocks generally intrude the enclosing gneisses along the foliation planes, local ductile shear zones and parallel to fold hinges. The contact between intrusions and the enclosing rocks is sharp in deeply eroded plutons e.g., Santa Angélica, Venda Nova, Mimoso do Sul and Várzea Alegre, but at shallow levels e.g., Castelo, Pedra Azul and Conceição de Muqui agmatic stoping zones occur along the borders. A magmatic foliation within the granitoids is usually well marked, but the schistosity in the surrounding gneisses wraps around the plutons. The intrusions have a bimodal chemical distribution and generally are reversely zoned with mafic cores (gabbro, diorite to tonalite) surrounded by a mingled (marble cake) zone where basic and acid rocks are interfingered, and an external zone of syenomonzonite and granite. Widespread evidence of mingling and mixing between contrasting magmas of gabbroic and granitic and/or syenomonzonitic compositions is characteristic for all intrusive complexes. We suggest that replacement of lithospheric mantle by hot asthenospheric mantle induced partial melting of the crust. The mantle exchange was due to lithospheric mantle delamination and slab breakoff following collisional orogenesis. The bimodal plutons result from interaction among the contrasting magmas. Ascent and emplacement followed older regional structures, such as regional fold hinges and ductile shear zones.