Lithosphere/asthenosphere interaction during continental breakup: preliminary isotopic data on the passive Galicia Margin (north-Atlantic)

The Galicia Margin ultramafic ridge has been cross-cut by diorites, pyroxenites and gabbros before the end of the rifting stage, and then by dolerites, after the continental breakup; it has been further overlaid by basaltic lava flows. The younger the rocks, the higher the initial epsilon(Nd) (2.2-8.8). This evolution would be the result of the contamination of liquids; extracted from the asthenosphere, by the enriched (epsilon(Ndi) = 4.0) and partially melted previous continental lithosphere. Time-decreasing contamination is related to progressive lithospheric thinning from the end of rifting to the beginning of oceanic spreading. (C) Academie des sciences/Elsevier, Paris.