Oswaldo Siga Júnior

Microssonda Iônica de Alta Resolução e de Alta Sensibilidade (SHRIMP IIe/MC) do Instituto de Geociências da Universidade de São Paulo, Brasil: método analítico e primeiros resultados

This paper presents the characteristics of the high resolution secondary ion mass spectrometer coupled with an Ion Microprobe (SHRIMP IIe/MC), installed at the Institute of Geosciences of the University of Sao Paulo (IGc-USP), as well as the respective analytical procedures and the first results obtained with standard samples, making possible to conduct routine analysis of zircon samples. The standard of the Temora 2 zircon, with age of 416.78 Ma, was analyzed by the SHRIMP IIe/MC at the Institute of Geosciences of the University of Sao Paulo and yielded the age of 416.8 ± 3.8 Ma. Samples of the Archean zircon OG1 and the Neoproterozoic zircon Z6266, which yielded U-Pb ages of 3465.4 ± 0.6 Ma and of 559 ± 0.2 Ma, respectively, using Thermal Ionization Mass Spectrometry (TIMS), were analyzed by SHRIMP IIe/MC at the IG-USP and yielded U-Pb ages of 3462.6 ± 5.1 Ma and 561.0 ± 0.92 Ma, respectively, in Concordia diagrams. Therefore, these values and the literature values are very close. Both OG1 and Z6266 samples, due to the homogeneity in their composition and isotopic relationship, can be used as SHRIMP standard. These results indicate that data obtained using SHRIMP IIe at the IGc-USP are reliable and compatible with international standards. Dating of samples with ages ranging from Cenozoic to Archean, obtained using SHRIMP IIe/MC U-Pb geochronology on zircon crystals, is presented here to illustrate this point.

Rapid Growth of Continental Crust Between 2.2 to 1.8 Ga in the South American Platform: Integrated Australian, European, North American and SW USA Crustal Evolution Study

Abstract The production of juvenile continental crust in the South American Platform (SAP) and other continents was larger in the Proterozoic than in the Archean. In the SAP the juvenile crust production was about 65% during the Proterozoic, while during the Archean it was around 34% of the total volume. Similar proportions are found in the Australian continent. The largest accretion from mantle to crust happened between 2.2 to 1.8 Ga and corresponded to 35% of the total volume of the present SAP continental crust. During the Archean an intense recycling between mantle and continental crust took place, while in the Paleoproterozoic the rate of mantle accretion to continental crust was larger than the assimilation. For the other continents (Europe and North America) the Paleoproterozoic was also the main accretion event. In the SW USA, rapid continental growth (about 45% of actual volume) occurred in a short time interval between 1.9-1.7 Ga. During Meso and Neoproterozoic little accretion of juvenile material occurred in the SAP, where crustal reworking predominated, but in Eurasia much juvenile continental crust was generated during the Phanerozoic in Central Asia.

A Formação Água Clara na Região de Araçaíba - SP: registro U-Pb de uma bacia mesoproterozóica

O objetivo deste trabalho fundamenta-se no estudo isotopico dos corpos metabasiticos associados a Formacao Agua Clara e as suas relacoes com as encaixantes. A Formacao Agua Clara e constituida predominantemente por uma sequencia de rochas carbonaticas (marmores puros a impuros). Rochas calcio-silicaticas, calcio xistos, mica xistos, anfibolio xistos, quartzitos, granada-clorita-biotita xistos, metacherts, metatufos basicos e intermediarios, metabasitos, anfibolitos e cornubianitos. As rochas metabasicas estudadas normalmente tem dimensoes longitudinais expressivas chegando a quilometricas, com dimensoes transversais de ate centenas de metros. Sao de coloracao cinza - esverdeada, apresentam granulometria fina a media e textura normalmente nematoblastica. Ocorrem termos mais isotropos que exibem textura granoblastica. Sao compostos por piroxenios (diopsidio ou augita), anfibolios (actinolita e hornblenda) e plagioclasios (andesina/oligoclasio). Os acessorios mais comuns sao apatita, magnetita, epidoto, titanita e raramente zircao. Em lâmina observa-se texturas ofiticas e subofiticas preservadas o que indica uma provavel origem ignea para esses corpos. Os dados geoquimicos sugerem composicoes semelhantes a basaltos enriquecidos de cadeias mesoceânicas (E-MORB) com tendencias a basaltos de ilhas oceânicas (OIB). As caracteristicas de basaltos toleiticos, subalcalinos semelhantes a basaltos enriquecidos de cadeias mesoceânicas com tendencia a basaltos de ilhas oceânicas (OIB), permitem sugerir como ambiente geotectonico gerador deste magmatismo basico, am-bientes distensivos ou em bacias de retroarco. Os dados analiticos U-Pb (convencional, EMF e SHRIMP) obtidos para os litotipos metabasicos indicam epocas de cristalizacao dos zircoes e consequente formacao dessas rochas durante o mesoproterozoico, com idades do intervalo 1590 - 1470 Ma. Os valores neoproterozoicos obtidos (~600 Ma) provavelmente referem-se a processos de recristalizacao e neoformacao de zircoes nas rochas metabasicas, colocacao de rochas de natureza granitica e resfriamento regional da Formacao Agua Clara.

Geocronologia e geologia isotópica dos terrenos pré-cambrianos da porção sul-oriental do Estado de São Paulo

In the southeastern region of Sao Paulo State four major tectonic domains were defined. The Embu Domain, north of the Cubatao Shear Zone (CSZ), is composed of metasedimentary rocks, and peraluminous granites (ca. 600 Ma), whose intrusion was controlled by E-NE shear zones, and which have model Nd Tdm age around 2.0 Ga. Gneiss-migmatite rocks (612 Ma) and related granites (580 Ma) predominate in the Mongagua Domain, which is limited by the Cubatao and Itariri shear zones. These rocks have different model Nd Tdm ages between 1.7 and 2.2 Ga. The Registro Domain, between Cubatao - Itariri Shear System (CISS) and the Serrinha Shear Zone (SSZ), is formed by metasediments and granitic rocks with migmatitic features, and represents a paleoproterozoic domain (1.9 - 2.2 Ga) intensely affected in Neoproterozoic times (750 - 580 Ma). Rocks of the Iguape Domain, limited to the north by the SSZ, include granites (ca. 600 Ma) and low grade metasediments. It is probable that all these tectonic blocks were juxtaposed during a short time interval at the end of Neoproterozoic. The joining of the Registro to the Embu Domain occurred at about 596 Ma along an E-W shear zone. The 598 Ma peraluminous granites of the Embu Domain may register the arrival of the Mongagua Domain against the newly formed Registro-Embu Domain. The welding of the Iguape and Registro domains probably occurred at 575 Ma, as suggested by the U-Pb (monazite) age of the protomylonitic granites of the SSZ.

Extensional and colisional magmatic records in the Apiai Terrane, south-southeastern Brazil: integration of geochronological U-Pb zircon ages

O principal objetivo deste trabalho e apresentar uma sintese dos dados geocronologicos disponiveis para as sequencias metavulcano-sedimentares e nucleos de embasamento que ocorrem na porcao sul do Terreno Apiai, sul-sudeste do Brasil. Os dados obtidos, principalmente na ultima decada, tem modificado substancialmente o cenario tectonico do sul-sudeste brasileiro, identificando a presenca de bacias extensionais (rifts continentais) com magmatismo e sedimentacao associada do final do Paleoproterozoico (1790 - 1750 Ma) e do Mesoproterozoico (1600 - 1450 Ma). O desenvolvimento desses processos parecem ter se iniciado no final do Paleoproterozoico (Nucleos Betara, Perau e Apiai Mirim), evoluindo para bacias mais amplas durante o Mesoproterozoico com a deposicao das sequencias metavulcanossedimentares Betara, Perau, Votuverava e Agua Clara. Padrao distinto e observado para a Sequencia Itaiacoca, que ocupa a porcao setentrional do Terreno Apiai. O estudo geologico-geocronologico caracterizou a existencia de dois conjuntos litologicos temporalmente distintos, o primeiro representado por uma associacao plataformal metacarbonatica, que inclui rochas metabasicas de natureza toleitica/subalcalina, com idades minimas de deposicao relacionadas ao final do Mesoproterozoico/inicio do Neoproterozoico (Sequencia Itaiacoca: 1.030-900 Ma). O segundo conjunto e representado principalmente por rochas metapsamiticas com metavulcânicas associadas, incluindo traquitos ultrapotassicos, depositados na transicao Criogeniano/Ediacarano (Sequencia Abapa: 645-628 Ma). O presente cenario sugere historias tectono-sedimentares distintas para o Terreno Apiai, refletindo uma evolucao policiclica. Durante o Mesoproterozoico predominaram condicoes relativamente estaveis com a deposicao de grande parte das sequencias metavulcanossedimentares (Lajeado, Agua Clara, Betara, Perau, Votuverava), num contexto de margem passiva, sucedidos pela deposicao da Sequencia Itaiacoca, associada a regimes extensionais do Toniano. No Ediacarano prevaleceram condicoes instaveis, de margem ativa, caracterizadas por restrita sedimentacao (Sequencias Abapa, Antinha e Iporanga) e colocacao dos batolitos graniticos Tres Corregos, Cunhaporanga e Agudos Grandes, representantes de arcos magmaticos.

O maciço alcalino de Tunas, PR: novos dados geocronológicos

The NW-SE elongated Tunas intrusion, occupying an area of 22 km², crops out about 80 km from the city of Curitiba, and it is emplaced into Precambrian metavolcanic-sedimentary rocks of the Acungui Group. Five sub-circular volcanic structures are recognized. Petrographically, the massif mainly consists of syenites and alkali syenites with subordinate alkali gabbros, essexites, syenogabbros and syenodiorites; small late syenitic dykes and magmatic breccias are also found. Previous K-Ar data give a Late Cretaceous mean age of 82.2 Ma for Tunas rocks. In this article we present new U-Pb age determinations on zircon crystals from syenitic rocks (ID-TIMS, 82.7 ± 0.7 Ma; SHRIMP, 84.7 ± 1.2 Ma) which confirm that age, placing the massif into the Late Cretaceous group of alkaline intrusions tectonically associated with the Sao Jeronimo-Curiuva Lineament, a NW-SE-trending branch of the Ponta Grossa Arch in southern Brazil.

Novas técnicas aplicadas ao método U-Pb no CPGeo - IGc/USP: avanços na digestão química, espectrometria de massa (TIMS) e exemplos de aplicação integrada com SHRIMP

The present work describes new techniques of U-Pb ID-TIMS developed at the CPGeo-IGc/USP. Techniques of partial and total chemical digestions, U and Pb concentration using anion exchange in a micro column, and the utilization of a 205Pb spike are discussed. Main geochronological equations used are also discussed. Examples of the application of U-Pb data, obtained by leaching technique using a microwave oven in zircon with isotopic inheritances in the core and border, are presented. The genesis of zircon based on the initial 176Hf/177Hf ratio is discussed. Finally complete listings of the mathematical formulations used in the PBDAT software are given in the appendix.