ABSTRACT: Sr-Nd-Pb isotopic signatures were determined for silica-undersaturated hypabyssal alkaline basic rocks of the Los Baldecitos Formation in the Cerro Morado area, situated in the Ischigualasto - Vale de la Luna Provincial Park (northernmost San Juan Province, Argentina). The basic rocks show slight variations of the Sr-Nd-Pb isotopes and similar behavior of whole-rock geochemistry, which suggest a single source for the Cerro Morado volcanic rocks. The present-day Pb isotopic data show moderately radiogenic Pb compositions (206Pb/204Pb = 18.31 - 18.35), in addition to low values of 87Sr/86Sr initial ratios of 0.70314 - 0.70386 and highly radiogenic Nd initial isotopic compositions (+7.1 < εNd (228 Ma) < +9.3), which point to a PREMA mantle reservoir signature for the alkaline basalts with little evidence of crustal contamination. The involvement of a HIMU component in the source, previously suggested for the Cerro Morado volcanics, should be disregarded in the light of the Pb isotopic signature. The Sr-Nd signature together with Nd-TDM model ages not older than 295 Ma does not support the involvement of an ancient crust.

Os procedimentos analíticos para análises U-Pb por diluição isotópica no Laboratório de Geologia Isotópica (Pará-Iso) da Universidade Federal do Pará (UFPA) são descritos detalhadamente. Esses procedimentos são aplicados para análises de zircão, titanita, rutilo, apatita, columbita-tantalita e rocha total. A purificação dos reagentes e os procedimentos químicos são feitos em salas limpas. As amostras são dissolvidas em microcápsulas de Teflon em bombas do tipo Parr InstrumentTM. U e Pb são separados em colunas com resina de troca iônica AG 1x8. Os brancos de procedimento para amostra típica(0,01-1 mg) são menores que 1 pg de U e 20-30 pg de Pb. As análises isotópicas de Pb e de U são feitas em um único filamento de Re em um espectrômetro de massa Finnigan MAT 262 nos modos estático e dinâmico. O nível analítico atual é comprovado pelas análises do padrão internacional de zircão 91500, usando três diferentes traçadores de U-Pb e nas análises de padrões de rocha total. Resultados geocronológicos obtidos em cristais de zircão de duas amostras de rochas arqueana e paleozóica são também apresentados e comentados.

Analytical procedures for U-Pb isotope dilution analyses at the Pará-Iso isotope geology laboratory of the Federal University of Pará (UFPA) are described in detail. The procedures are applied to zircon, titanite, rutile, apatite, columbite-tantalite and whole rock. Reagent preparation and chemical processing are done in clean-room conditions. Samples are dissolved using TeflonTM microcapsules in steel jacket TeflonTM Parr InstrumentTM bomb or TeflonTM screw cap containers. U and Pb are separated using anion exchange AG 1x8 resin columns. Typical blanks for mineral sample amounts of 0.01 to 1.0 mg are less than 1 pg U and 20-30 pg Pb. Isotope analysis of the U and Pb from the same filament are carried out using a Finnigan MAT 262 mass-spectrometer in static and dynamic modes. The current analytical level is demonstrated on analyses of international standard zircon 91500 with three different 235U-205Pb and 235U-208Pb isotope tracers and whole rock standards. Results of analyses of two zircon samples are also presented.