Individualization of textural and reactional microdomains in eclogites from the Bergen Arcs (Norway): Consequences for Rb/Sr and Ar/Ar radiochronometer behavior during polymetamorphism

Geochemistry, Geophysics, Geosystems, v. 9, p. Q12001, 2008. http://dx.doi.org/10.1029/2008GC002098International audienceRb/Sr, 40Ar/39Ar, and Sm/Nd isotopic data are reported in Caledonian eclogites from the Lindås Nappe, Bergen Arcs, Norway, in order to investigate processes controlling isotopic equilibrium at mineral scale in polymetamorphic rocks. The Bergen Arcs exposes Sveconorwegian ∼950–930 Ma granulites, partially overprinted by Caledonian eclogite-facies metamorphism at ∼425 Ma and amphibolite-facies metamorphism at ∼410 Ma. Geochemical and Rb-Sr data from more than 10 phengite fractions separated from one sample reflect the composition of the microdomain (a few hundred microns in size) in which phengite crystallized. Phengite crystallized after garnet or plagioclase by dissolution-precipitation processes yield apparent age between 700 and 600 Ma. At the time of their crystallization, these phengites inherited the isotopic composition of their precursor minerals, at a microdomain scale. Phengite from quartz veins, which crystallized from elements mobilized by the circulating fluid, yield an age closer to the eclogite-facies metamorphic age. The closed system evolution of the eclogitizing fluid, the segregation of textural and reactional microdomains, the high Sr content of the studied phengite, and the short duration of the recrystallization processes (