40Ar/39Ar analyses were performed on two bulk samples of plagioclase extracted from flaser gabbros that constitute the entire basement core at Site 900. The two age spectra are disturbed by alteration processes and possibly excess argon. One of the two plagioclases, despite a strong contamination by K-rich phases, displays a flat section of apparent ages on its age spectrum with a weighted mean of 136.4 ± 0.3 m.y., which may represent a reasonable estimate of the closure time of the plagioclase. Considering the petro-structural constraints on the evolution of these gabbros (Cornen et al., chapter 26, this volume), this age is coherent with the final stage of the metamorphic event that affected these gabbros at the end of the Mesozoic continental rifting. When compared to the previous 40Ar/39Ar plateau-ages obtained on magmatic rocks of the Galicia Margin to the north and Gorringe Bank to the south, this age is consistent with a northward propagation of the Atlantic ocean opening.
Date of initial receipt: 27 February 1995
Date of acceptance: 19 July 1995
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