Mathematics – Logic
Scientific paper
Apr 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995gecoa..59.1611p&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 59, Issue 8, pp.1611-1622
Mathematics
Logic
8
Scientific paper
Three different geochronological techniques (U-Pb conventional multigrain, single zircon step-wise Pb-evaporation, and Sm-Nd on minerals) were applied to rocks from a norite-eclogite transition observed in the West European Variscan belt and combined with major and trace element (including REE) geochemistry. The studied rocks belong to a cogenetic tholeiitic suite which originated in an immature back-arc basin or within-plate magmatic environment. The initial T =480 Ma Nd values around +1.2 imply that the noritic protoliths were emplaced in a geotectonic setting in close association with the continental crust. U-Pb multigrain and the Sm-Nd dating of a norite sample indicate an age of 481 Ma for the basic magmatism. Sm-Nd dating on two whole-rock-garnet pairs yields an age of 408 Ma for the eclogite-facies metamorphism. These two ages are consistent with the present understanding of the European Variscan belt. Finally, the single zircon Pb-evaporation study of an eclogite sample demonstrates the co-existence of several types of zircons. Each zircon population yields a particular type of geochronological information, related to the contrasting behaviours of the U-Pb isotopic system during the eclogite-facies tectonometamorphic event. These analytical results suggest that complex and heterogeneous behaviour within the U-Pb zircon system may also be recorded in numerous other samples. This last point raises the question of the significance of results obtained by U-Pb conventional multigrain dating in metabasic rocks.
Couturier Marc
Louis Paquette Jean
Monchoux Pierre
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