CRETACEOUS SEAMOUNTS ALONG THE CONTINENT-OCEAN TRANSITION OF THE IBERIAN MARGIN: U-PB AGES AND PB-SR-HF ISOTOPES

Abstract

To elucidate the age and origin of seamounts in the eastern North Atlantic, 54 titanite and 10 zircon fractions were dated by the U-Pb chronometer, and initial Pb, Sr, and Hf isotope ratios were measured in feldspars and zircon, respectively. Rocks analyzed are essentially trachy-andesites and trachytes dredged during the "Tore Madeira" cruise of the Atalante in 2001. The ages reveal different pulses of alkaline magmatism occurring at 104.4 ? 1.4 (2?) Ma and 102.8 ? 0.7 Ma on the Sponge Bob seamount, at 96.3 ? 1.0 Ma on Ashton seamount, at 92.3 ? 3.8 Ma on the Gago Coutinho seamount, at 89.3 ? 2.3 Ma and 86.5 ? 3.4 Ma on the Jo Sister volcanic complex, and at 88.3 ? 3.3 Ma, 88.2 ? 3.9, and 80.5 ? 0.9 Ma on the Tore locality. No space-time correlation is observed for alkaline volcanism in the northern section of the Tore-Madeira Rise, which occurred 20-30 m.y. after opening of the eastern North Atlantic. Initial isotope signatures are: 19.139-19.620 for 206Pb/204Pb, 15.544-15.828 for 207Pb/204Pb, 38.750-39.936 for 208Pb/204Pb, 0.70231-0.70340 for 87Sr/86Sr, and +6.9 to +12.9 for initial epsilon Hf. These signatures are different from Atlantic MORB, the Madeira Archipelago and the Azores, but they lie in the field of worldwide OIB. The Cretaceous seamounts therefore seem to be generated by melts from a OIB-type source that interact with continental lithospheric mantle lying formerly beneath Iberia and presently within the ocean-continent transition zone. Inheritance in zircon and high 207Pb of initial Pb substantiate the presence of very minor amounts of continental material in the lithospheric mantle. A long-lived thermal anomaly is the most plausible explanation for alkaline magmatism since 104 Ma and it could well be that the same anomaly is still the driving force for tertiary and quaternary alkaline magmatism in the eastern North Atlantic region. This hypothesis is agreement with the plate-tectonic position of the region since Cretaceous time, including an about 30? anti-clockwise rotation of Iberia. ? 2006 Elsevier Inc. All rights reserved.