Candido Augusto Veloso Moura

Soft-sediment deformation at the base of the Neoproterozoic Puga cap carbonate (southwestern Amazon craton, Brazil): Confirmation of rapid icehouse to greenhouse transition in snowball Earth

Stratigraphic and isotopic data identify the lower 45 m of the Araras Group, on the southwest margin of the Amazon craton, as a Neoproterozoic platform cap carbonate deposited below wave base upon Varanger glacial diamictites of the Puga Formation. The basal beds consist of moderately deep water pinkish dolomudstone with stratiform to wavy fenestral microbialites locally cut by tube-like structures and fenestral nonmicrobial planar laminites with tepee-like features. Above the basal carbonates are deep-water bituminous lime mudstones with alternating thin calcite crusts and lime mudstone laminae commonly disrupted by calcite crystal fans (pseudomorphs after aragonite). The basal contact of the Puga cap exhibits soft-sediment deformational structures (principally load casts) that are here attributed to rebound-induced seismicity acting upon both recently deposited carbonate sediments and underlying unconsolidated diamictite. These features constitute the first clearly recognized sedimentological evidence for the rapid change from icehouse to greenhouse conditions as postulated in the snowball Earth model of Neoproterozoic glaciation.

Comparison of single filament Pb evaporation/ionization zircon ages with conventional U-Pb results: examples from the Precambrian of Brazil

Abstract This paper documents a systematic and detailed study comparing the 207 Pb/ 206 Pb apparent ages obtained by single zircon Pb evaporation/ionization method, in rock samples previously dated by conventional U-Pb zircon dissolution procedures. The technique applied here uses a single rhenium center filament to evaporate, ionize and measure the Pb isotopic ratios, in a thermal ionization mass spectrometer, by direct evaporation/ionization of a single zircon grain. The single zircon Pb evaporation/ionization method was applied in igneous and metamorphic rocks of Archean to Neoproterozoic age. The 207 Pb/ 206 Pb obtained for the igneous rocks with little or no evidence of deformation are, within the error, similar to the published U-Pb ages and may be considered as good indicator of the crystallization age of these rocks. However, uncertainties on the 207 Pb/ 206 Pb ratios measured on zircons may constitute a limitation of the Pb evaporation/ionization method, particularly for the precise determination of the age of crystallization of Neoproterozoic rocks or younger. For granulite facies rocks, where a Pb loss discordancy has been imprinted upon the zircons, the 207 Pb/ 206 Pb ages obtained by Pb evaporation/ionization method must be considered as minimum ages. The results obtained here along with the relative simplicity of the analytical procedures used in the single zircon Pb evaporation/ionization method show great potential for the application of this technique in the reconnaissance evaluation of the ages of rocks units of Precambrian crust where a geochronological framework is not well established.

Archean crustal sources for Paleoproterozoic tin-mineralized granites in the Carajas Province, SSE Para ´, Brazil: Pb /Pb geochronology and Nd isotope geochemistry

Whole-rock and K-feldspar Pb, single zircon Pb-evaporation, and Nd whole-rock isotopic data are presented for granites of the Velho Guilherme intrusive suite and the volcanic rocks of the UatumaGroup in the Carajas Mineral Province, southern Para ´, Brazil. Pb � /Pb zircon ages of 18679 /4 Ma, 18629 /16 Ma, and 18669 /3 Ma for the Antonio Vicente, Mocambo, and Rio Xingu granite massifs, respectively, show that the granites were emplaced at � /1870 Ma. Pb � /Pb whole-rock reference age of the Uatumavolcanic rocks, 18759 /158 Ma, is comparable within the limits of the error. 1870 Ma thus represents the time of an important magmatic event in the eastern part of the Amazonian craton. Nd model ages of the granites (3.0 � /3.2 Ga) and volcanic rocks (2.9 � /3.1 Ga) and their strongly negative initial oNd values indicate that the magmas were derived from Mesoarchean rocks with a long crustal residence time. A mantle contribution cannot be completely ruled out, however. # 2002 Elsevier Science B.V. All rights reserved.

Paleoproterozoic Crustal Evolution of the São Luís Craton, Brazil: Evidence from Zircon Geochronology and Sm-Nd Isotopes

Abstract The Sao Luis Craton, northern Brazil, is composed of a few granitoid suites and a metavolcano-sedimentary succession. New single zircon Pb evaporation ages and Nd isotope data, combined with other available information, show that the metavolcano-sedimentary succession developed from 2240 Ma to approximately 2200-2180 Ma from juvenile protoliths. The subduction-related calc-alkaline suites of granitoids, spatially associated with the metavolcano-sedimentary sequence, formed in an oceanic island arc setting between 2168-2147 Ma. Most of these granitoids are tonalitic and formed from juvenile, mantle- or oceanic plate-derived protoliths, whereas minor true granites are the product of the reworking of the juvenile island arc material. These arc-related successions represent an accretionary event around 2.20±0.05 Ga, which is coincident with one of the main periods of crustal growth in the South American Platform. This accretionary orogen has subsequently been involved in a collision episode, at ca. 2100-2080 Ma, which is mainly recorded in the nearby Gurupi Belt. The rock associations, inferred geological settings, and the crustal evolution detected in the Sao Luis Craton are similar to what is described in Paleoproterozoic domains of major geotectonic units of the South American Platform, such as part of the Sao Francisco Craton, southeastern Guyana Shield, and of the West African Craton.

GEOLOGIA E GEOCRONOLOGIA Pb-Pb DE ROCHAS GRANÍTICAS E VULCÂNICAS ÁCIDAS A INTERMEDIÁRIAS PALEOPROTEROZÓICAS DA PROVÍNCIA PITINGA, CRATON AMAZÔNICO

The Pitinga Province is responsible for 60% of the Brazilian tin product ion, and about 3% of the world production of this not ferrous metal. Associates to the Sn occurs important reserves of Nb,Ta and F (cryolite), beyond anomalous grades of Zr, Y, Rb, U, Th and HREE. The geological selling of the Pitinga Province consists in Paleoproterozoic, volcanic and granitic units or Uatuma Supergroup, younger granites related to the Madeira Suite and by the vulcano-sedimentary Urupi Formation. The granitic units of the Mapuera and Madeira Suite arc represented by eight plutons that are intrusive in to acid and intermediate volcanic rocks of the Iricoume Group. Only the granites of the Madeira Suite have previous geochronologic information. This paper shows 207 Pb/ 206 Pb zircon ages of four granitic units of the Mapuera Suite and 207 Pb/ 206 Pb zircon ages of the volcanic rocks that enclose these plutons. Petrographic, geochemical and field data, allow us to distinguish two magmatic pulses in the region. The first one is responsible for the generation of volcanic and granitic rocks of ages between 1,890 and 1,876 My. The second magmatic pulse generated A type granites with ages ranging from 1,829 and 1,798 My. Based on field and remote sensor data, the volcanic rocks of the Iricoume Group were divided in three formations, for which are proposed the names Divisor Formation, Paraiso Formation and Ouro Preto Formation. The ages of these units are around 1,89 and xa01,88 Ga. The field data indicate s that the Urupi Formation shows intercalations of pyroclastic rocks. These pyroclastic rocks are interpreted as part of the Paraiso Formation; consequently it locates the Urupi Formation in the Paleoproterozoic, suggesting its correlation with Roraima Supergroup. All these lithologic units are associated with a regional, extensional, arc-shaped structure, which encloses the whole Pitinga Province. Granitic plutons of the Mapuera Suite occupy the peripheral parts of the structure, which is filled by effusive and pyroclastic rocks of the Iricoume Group, and sedimentary rocks of the Urupi Formation. The tectonic setting suggests that the emplacement of volcanic and pyroclastic rocks of the Iricoume Group, in the Pitinga Province, was controlled by extensional processes of caldera collapse. Subsequent reactivation of the extensional system allowed the emplacement of the Madeira Suite, in the central portion of this wide structure.

EVIDENCE OF BRASILIANO/PANAFRICAN DEFORMATION IN THE ARAGUAIA BELT: IMPLICATION FOR GONDWANA EVOLUTION

RESUMO EVIDENCIA DE DEFORMACAO BRASILIANA/PANAFRICANA NO CINTURA ARAGUAIA: IMPLICACOES PARA A EVOLUCAO DO GONDWANA. Datacoes de monocristais de zircao em granitos e veios graniticos sin a tardi-tectonicos, associados as rochas supracrustais do Cinturao Araguaia, por evaporacao gradativa de Pb forneceram idades plato para a razao 207 Pb/ 206 Pb de 635 ± 53 Ma, 513 ± 17 Ma e de 655 ± 24 Ma. As idades destes cristais calculadas atraves da media ponderada dos diversos blocos de leitura da razao 207 Pb/ 206 Pb foram de 542 ± 72 Ma, 498 ± 19 Ma and 583 ± 39 Ma. Considerando o caracter sin-tectonico, ou no maximo tardi-tectonico destas rochas graniticas, esses dados sugerem que o Ciclo Brasiliano foi responsavel pela estruturacao do Cinturao Araguaia. Consequentemente, o desenvolvimento estrutural do Cinturao Araguaia pode ser colocado no contexto da amalgamacao do Gondwana durante o Proterozoico Superior. Palavras-chaves: Geocronologia, datacao de monocristais de zircao, Cinturao Araguaia, Brasiliano/Pan-Africano, Gondwana.

Síntese geológica e geocronológica do Cráton São Luís e do Cinturão Gurupi na região do Rio Gurupi (NE-Pará / NW-Maranhão)

The Gurupi region in northern Brazil has been divided into Paleoproterozoic (Sao Luis Craton) and Neoproterozoic (Gurupi Belt) geochronological domains based on Rb-Sr and K-Ar. Recent zircon (Pb evaporation) dating of most of the lithostratigraphic and lithodemic units, in addition to limited whole-rock Sm-Nd determinations, shows virtually all rock units (juvenile or reworked) formed between ~2.0 Ga and 2.2 Ga. A single granitoid shows a Neoproterozoic crystallization age (~0,55 Ga), and the Archean has been recorded only in inherited zircons and in protolith Sm-Nd model ages. The compositional, metamorphic, structural, geophysical and geochronological characteristics of the geological units supports the previous subdivision into two domains. However, they also reveal a common Paleoproterozoic evolution of both the Gurupi and Sao Luis domains. The Gurupi domain is characterized as a collisional orogen, whereas the Sao Luis domain shows accretionary characteristics, with both participating in the widespread Paleoproterozoic collage at 2.2 - 2.0 Ga. During the Neoproterozoic, the southern border of this region was affected by strong strike-slip shearing and very limited lithogenesis, reflecting the Brasiliano orogenic collage elsewhere and defining the presently observed relationship between the Sao Luis Craton and the Gurupi Belt.

Geology and geochemistry of the Caxias gold deposit, and geochronology of the gold-hosting Caxias Microtonalite, São Luı́s Craton, northern Brazil

Abstract The Caxias gold deposit, located in the Sao Luis Craton, is hosted by a steeply dipping strike-slip shear zone crosscutting schists and a fine-grained, hydrothermally altered tonalite (Caxias Microtonalite). Petrography and whole-rock geochemistry have characterized both pelitic and mafic protoliths for the hosting schists. The Caxias Microtonalite shows major and trace element behavior compatible with modern calc-alkaline, metaluminous, subduction-related granitoids. Geochronological studies on the Caxias Microtonalite have defined a minimum crystallization age of 1985±4xa0Ma, obtained by single-zircon Pb evaporation, and Sm–Nd crustal residence age (TDM) of 2.17xa0Ga, with eNd(T) +0.74, suggesting a juvenile protolith. The exact origin and role of the Caxias Microtonalite remain uncertain. It may be interpreted as representing either a late manifestation of the regionally dominant Tromai magmatism, or a juvenile episode unrelated to this major magmatism. Rock, quartz veins, and saprolite geochemistry have shown that As, Sb, Ba, Rb, V, Cr, Co, and Ni, as well as Au, are useful elements that can be used in exploration for similar deposits in the region.

O MAGMATISMO PALEOPROTEROZÓICO DA REGIÃO DE SÃO FÉLIX DO XINGU, PROVÍNCIA ESTANÍFERA DO SUL DO PARÁ: PETROGRAFIA E GEOCRONOLOGIA

New petrographical and geochronological data are presented for three granitics bodies of the Velho Guilherme Intrusive Suite and for the Uatuma Group volcanic rocks, which occur around Sao Felix do Xingu town, Southern Para Tin Province. The faciological characterization of these rocks reveals remarkable similarities among the Serra da Queimada, Santa Rosa, and Vila Santa Rosa porphyritic massifs, strengthening the results or other studies on other bodies of this suite. The volcanic rocks of the Uatuma Group represent a bimodal suite closely associated with these massifs. Pb-Pb zircon ages of approximately 1880 Ma for these rocks point to an exclusively Paleoproterozoic volcano-plutonic magmatism.

Determinação da composição isotópica de estrôncio em águas naturais: exemplos de sua aplicação em águas subsuperficiais da zona costeira na Região Bragantina-PA

Analytical procedures used for determining the concentrations and isotope composition of strontium in subsurface waters, by mass spectrometry, are described. Sampling was performed in coastal plateaus, salt marsh and mangrove environments in the coastal region of Para. Coastal plateau waters have d87Sr between 1.51 and 6.26x89 and Sr concentration bellow 58 ppb. Salt marsh waters show d87Sr between 0.55 and 0.90x89 and Sr concentration between 93 and 114 ppm, while mangrove waters have d87Srx89 around zero and Sr concentration above 15 ppm. Differences in the 87Sr/86Sr ratio in these subsurface waters are detected, as well as seasonal variations in the coastal pleteau waters.