Núcleo de Ciências da Terra Seminários

1. Núcleo de Ciências da Terra Seminários 13 Março 2006 14.30 h Laser ablation ICP-MS – a fast, attractive and highly sensitive in situ microanalytical technique – application to mineral trace element analysis and isotopic U-Pb zircon dating François Bussy Institute of Mineralogy and Geochemistry University of Lausanne, Switzerland Laser-ablation inductively coupled plasma mass spectrometry, or LA-ICPMS, was first used in 1985 and has been an increasingly important analytical tool in the earth sciences since the early 1990s. This microanalytical technique closes the gap between electron microprobe (EMP) and thermal ionisation mass spectrometry (TIMS) in the same way as the SIMS (secondary ion mass spectrometry) technique does, but in a much faster (less than a minute) and cheaper way. Indeed, LA-ICPMS allows in situ trace-element quantification at the sub-ppm level with spot size of 10 to 100 microns in diameter, as well as measurement of some isotopic ratios like U and Pb. This talk will present the principles of the LA-ICPMS technique, its analytical potential and limitations, as well as applications like zircon U-Pb isotopic dating or the modelling of a layered mafic intrusion based on mineral trace-element quantification. 13 Março 2006 16.00 h Geodynamic reconstruction of the Anti-Atlas (Morocco)during Pan-African times with the emphasis on inversion tectonics and metallogenic activity at the Precambrian-Cambrian transition Dominique Gasquet CISM-EDYTEM, CNRS – UMR Université de Savoie, France New geochemical and geochronological (U-Pb SIMS zircon ages) data have been used to produce a new reconstruction of the complex orogenic front that developed at the northern edge of the Eburnian West African craton during Pan-African times. Three Neoproterozoic magmatic series can be distinguished in the Anti-Atlas belt, i.e., high-K calc-alkaline granites, high-K calc-alkaline to shoshonitic rhyolites and andesites, and alkaline-shoshonitic trachytes and syenites, which have been dated at 595-570 Ma, 570-545 Ma and 530 Ma respectively. The accretion of the Pan-African Anti-Atlas belt to the West African super continent (WAC) was a four-stage event, involving extension, subduction, moderate collision and extension. The calc-alkaline magmatism of the subduction stage was associated with large-scale base metal and gold mineralization. Metallogenic activity was greatest during the final extensional stage, at the Precambrian-Cambrian boundary. It is characterised by world-class precious metal deposits, base-metal porphyry and SEDEX-type occurrences. Anfiteatro B2-CPII, Universidade do Minho, Campus de Gualtar, Braga Contactos: Graciete Dias Tel: 253 604300 graciete@dct.uminho.pt Organização: Núcleo de Ciências da Terra da Universidade do Minho Apoio: Entrada Livre