138-121 Ma asthenospheric magmatism prior to continental break-up in the North Atlantic and geodynamic implications

Along the Galicia and Gorringe banks and in the Iberia Abyssal Plain of the North Atlantic, unroofed subcontinental mantle fills the gap between 'true ' oceanic crust and the continental crust margin. These lithospheric perido tites are intruded by gabbros and dolerites, and locally covered by basalts . Primary magmatic zircons extracted from gabbros and meta-gabbros of the t wo banks were dated by the U-Pb chronometer, and initial hafnium isotope si gnatures (epsilon Hf-i) were determined on the same grains. For Mt. Gettysb urg at Gorringe, gabbro emplacement ages of 137.5 +/- 0.5 (2 sigma) Ma and 135.7 +/- 0.8 Ma are obtained, and corresponding epsilon Hf-i lie at +20.5 +/- 0.3 (2 sigma) and +19.5 +/- 0.4, substantiating magma formation from se verely LILE-depleted mantle domains. Gabbro zircons from Mt. Ormonde at Gor ringe yield a much younger age of 77.1 +/- 0.4 Ma and the Hf isotopes docum ent an intermediately LILE-depleted mantle source having a epsilon Hf-i of +7.6 +/- 0.4. Given its age and Hf signature, emplacement of this rock can be ascribed to the alkaline magmatic event that also affected the Iberian C ontinent in Upper Cretaceous time. Concerning the Galicia section, zircons from a meta-gabbro yield an emplacement age of 121.7 +/- 0.4 Ma and a epsil on Hf-i of +14.0 +/- 0.2, and a epsilon Hf-i of +14.6 +/-0.2 is obtained fo r zircons from a previously dated meta-gabbro of identical age. These resul ts indicate magma extraction from mantle reservoirs that are slightly less LILE-depleted than those sampled by the about 20 Myr older Gorringe gabbros . The data demonstrate that magmatism occurring prior to complete separatio n of Europe from America was essentially of asthenospheric origin. Both the 138-135 Ma ages for the Gorringe gabbros and 122 Ma ages for the Galicia g abbros are at least 5 Myr older than the oldest sediments on Gorringe, and the break-up unconformity at the Galicia Bank, respectively. Magma source s ignatures of the syn-rift gabbros are in agreement with values expected for differently depleted Cretaceous MORE-type mantle reservoirs, and the age-d ifference for magmatism between the Gorringe and Galicia banks suggests a r ate of 4.4 +/- 0.3 cm/yr for northward progression of continental rifting. Based on the new results, a structural model for Iberia-America rifting is discussed putting forward the idea that magma emplacement produces a level of weakness and decollement between the rifting crust and its underlying li thospheric mantle. (C) 2000 Elsevier Science B.V. All rights reserved.