Lu-Hf ANALYSIS BY LA-MC-ICP-MS: METHODOLOGY AND APPLICATION IN CAPIVARITA ANORTHOSITE ZIRCONS

A metodologia de Lu-Hf em zircão é importante para a geocronologia, pois complementa estudos de proveniência e de crescimento crustal. Nas últimas décadas, tornou-se um dos métodos mais utilizados neste campo fornecendo dados com qualidade e rapidez devido à eficiência dos MC-ICP-MS. Em vista disso, o presente artigo apresenta um estudo da metodologia Lu-Hf aplicada a zircões magmáticos e metamórficos do Anortosito de Capivarita, localizado no sul do Brasil. As análises in situ de Lu-Hf nos zircões foram feitas primeiramente no Laboratório de Geologia Isotópica (LGI) da Universidade Federal do Rio Grande do Sul, Brasil, e depois no Microanalysis Facilities Inco Innovation Centre da Memorial University of Newfoundland, Canada, com objetivo de testar a reprodutibilidade da metodologia desenvolvida no LGI. Ambos os laboratórios utilizaram o mesmo modelo de MC-ICP-MS, o Neptune da Thermo Finnigan, mas diferentes lasers assim como diferentes metodologias de trabalho. As principais diferenças além de diferentes lasers residem no fato de ter sido utilizado durante as análises no LGI um diâmetro maior para o furo do laser e nitrogênio. Os resultados dos dois laboratórios mostram boa reprodutibilidade nas análises de Hf. As idades TDM e os valores de epsilon Hf obtidos são concordantes dentro dos erros experimentais

Mid amphibolite facies metamorphism of harzburgites in the Neoproterozoic Cerro Mantiqueiras Ophiolite, southernmost Brazil

Valuable information is retrieved from the integrated investigation of the field relationships, microstructure and mineral compositions of harzburgites from the Neoproterozoic Cerro Mantiqueiras Ophiolite. This important tectonic marker of the geological evolution of southernmost Brazilian Shield was thoroughly serpentinized during progressive metamorphism, because the oldest mineral assemblage is: olivine + orthopyroxene + tremolite + chlorite + chromite. This M1 was stabilized in mid amphibolite facies - 550-600ºC as calculated from mineral equilibria. No microstructural (e.g. ductile deformation of olivine or chromite) or compositional (e.g. mantle spinel) remnant of mantle history was identified. A metamorphic event M2 occurred in the low amphibolite facies along 100 m-wide shear zones, followed by intense serpentinization (M3) and narrow 1-3 m-wide shear zones (M4) containing asbestos

Geologia estrutural folha Paulo Saldanha, SE de Rondônia

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Geologia estrutural folha Paulo Saldanha, SE de Rondônia

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Geologia estrutural folha Paulo Saldanha, SE de Rondônia

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Unravelling a Proterozoic basin history through detrital zircon geochronology : the case of the Espinhaço Supergroup, Minas Gerais, Brazil.

New U–Pb in situ zircon dating provides a new approach for the study of the Espinhaço Supergroup units exposed in the São Francisco Craton and Araçuaí Orogen. Located in Southern Espinhaço, Minas Gerais, the following two basins were formed in intraplate conditions: (i) the Lower Espinhaço Basin is marked by a volcano-sedimentary sequence with alluvial, fluvial and eolian deposits that were formed from 1.68 Ga to 1.80 Ga; (ii) the overlying sequence, called the Upper Espinhaço Basin, is a rift-sag basin that presents basal diamond-bearing continental deposits (the Sopa–Brumadinho Formation) with a younger zircon peak at 1192 Ma, which is the maximum age for deposition of the upper units. The obtained U–Pb detrital zircon data of the studied units show age groupings related to the orogenic cycles of Jequié (Neoarchean) and Transamazonian (Paleoproterozoic), which are associated with the stability period from 1.8 Ga to 0.91 Ga in the São Franscico Craton, and record deposition during at least the three basinal cycles. The Stenian–Tonian ages of the Upper Espinhaço Basin units (906 Ma to 1192 Ma) suggest a direct link with the evolution of the Rodinia Supercontinent and represent a marker for Mesoproterozoic to Early Neoproterozoic evolution in the São Franscico Craton and adjacent areas