| 1. |
- Merle, Renaud, et al.
(author)
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Cretaceous seamounts along the continent–ocean transition of the Iberian margin : U–Pb ages and Pb–Sr–Hf isotopes
- 2006
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In: Geochimica et Cosmochimica Acta. - : Elsevier BV. - 0016-7037 .- 1872-9533. ; 70:19, s. 4950-4976
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Journal article (peer-reviewed)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 sigma) Ma and 102.8 +/- 0.7 Ma on the Sponge Bob seamount, at 96.3 +/- 1.0 Ma on Ashton seamount, at 92.3 +/- 13.8 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 Pb-206/Pb-204, 15.544-15.828 for Pb-207/Pb-204, 38.750-39.936 for Pb-208/Pb-204, 0.70231-0.70340 for Sr-87/Sr-16, 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 Pb-207 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 degrees anti-clockwise rotation of Iberia.
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| 2. |
- Merle, Renaud E., 1976-, et al.
(author)
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Cretaceous seamounts along the continent–ocean transition of the Iberian margin : U–Pb ages and Pb–Sr–Hf isotopes
- 2006
-
In: Geochimica et Cosmochimica Acta. - 0016-7037 .- 1872-9533. ; 70:19, s. 4950-4976
-
Journal article (peer-reviewed)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 (2σ) 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.
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| 3. |
- Merle, Renaud E., 1976-, et al.
(author)
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Occurrence of inherited supra-subduction zone mantle in the oceanic lithosphere as inferred from mantle xenoliths from Dragon Seamount (southern Tore–Madeira Rise)
- 2012
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In: Journal of the Geological Society. - : Geological Society of London. - 0016-7649 .- 2041-479X. ; 169:3
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Journal article (peer-reviewed)abstract
- Spinel-bearing peridotite and pyroxenite xenoliths dredged from the Dragon Seamount (southern Tore–Madeira Rise, West Iberia and Morocco margin) give an insight into the composition of the underlying lithosphere. These xenoliths are devoid of evidence of strong host lava–peridotite interaction and re-equilibration or late impregnation in the plagioclase facies. The spinels and pyroxenes from the Dragon peridotites have compositions distinct from those of both lherzolites and harzburgites from the Iberia margin and the Mid-Atlantic Ridge. They display a highly depleted composition, in particular, high Cr-number, up to 0.63 in the spinels, consistent with a melting degree between 12 and 19%. Because of the strong chemical similarities between the Tore–Madeira Rise, Newfoundland peridotites, and peridotites from supra-subduction zones, we propose that the Dragon peridotites formed in a similar context. The pyroxenites display a cumulate texture and are probably a high-temperature–high-pressure cumulate formed by fractional crystallization from a melt. The Tore–Madeira Rise peridotites may represent a former mantle wedge in an oceanic arc, later included into the continental lithosphere and finally tectonically disseminated within the lithosphere during the rifting of the Newfoundland–Iberia continental lithosphere. As a consequence, rifting processes may produce heterogeneities in the oceanic lithosphere and influence isotopic compositions of ocean island basalt-type lavas during plume–lithosphere interactions, as inferred for the southern Tore–Madeira Rise.Supplementary material: Further information on the analytical results is available at http://www.geolsoc.org.uk/SUP18512.
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| 4. |
- Merle, Renaud E., 1976-, et al.
(author)
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Segregation vesicles, cylinders, and sheets in vapor-differentiated pillow lavas : examples from Tore-Madeira Rise and Chile Triple Junction
- 2005
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In: Journal of Volcanology and Geothermal Research. - 0377-0273 .- 1872-6097. ; 141:1, s. 109-122
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Journal article (peer-reviewed)abstract
- We conducted a detailed field and laboratory study of internal segregation structures of two hand-size pillow lavas samples. They were dredged, respectively, on the Josephine seamount, Tore-Madeira Rise (TMR), and on a small quaternary volcanic edifice located on the continental edge of the trench close to the Chile Triple Junction (CTJ). Both pillows display a combination of four types of segregation structures (spherical vesicles, pipe vesicles, vesicle cylinders, and vesicle sheets) observed so far only within subaerial basalt flows typically 2–10 m thick. In particular, the samples offer a remarkable exposure of the transition between pipe vesicles and cylinders. We show that the vesicle sheets are not generated by the same mechanism in both occurrences; they do not seem to be connected to cylinders in the CTJ pillow as they are in the TMR pillow. The two pillows are geochemically distinct, the TMR being alkaline and the CTJ calc–alkaline. Two types of internal differentiation are proposed. The first one implies the extraction of the residual liquid from the host lava and transport towards the segregation structures, whereas the other one results from in situ crystallization within one given structure. In the latter case, glass composition is highly dependant on the nature of the neighbouring crystallizing minerals. The degree of crystallization required to produce a crystal framework strong enough for generating the segregation structures seems to be lower in pillows (ca. 25% crystallization) than in vapor-differentiated basaltic lava flows (35% crystallization).
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| 5. |
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| 6. |
- Merle, Renaud, et al.
(author)
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Evidence of multi-phase Cretaceous to Quaternary alkaline magmatism on Tore–Madeira Rise and neighbouring seamounts from 40Ar/39Ar ages
- 2009
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In: Journal of the Geological Society. - : Geological Society of London. - 0016-7649 .- 2041-479X. ; 166:5, s. 879-894
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Journal article (peer-reviewed)abstract
- The Tore-Madeira Rise is a seamount chain located 300 kin off the Portugal and Morocco coasts attributed to hotspot activity. U-Pb ages of lavas from the northern and central Tore-Madeira Rise range between 103 and 80.5 Ma whereas Ar-40/Ar-39 ages from the central and southern Tore-Madeira Rise yield ages ranging from 94.5 to 0.5 Ma. We performed new Ar-40/Ar-39 measurements to better understand the geodynamic history of the Tore-Madeira Rise. Plagioclase ages from the Bikini Bottom and Torillon seamounts suggest ages of >90 Ma and >= 60 Ma, respectively. Amphiboles from the Seine seamount yield an age of 24.0 +/- 0.8 Ma. Biotites from lavas of the Ashton seamount give ages of 97.4 +/- 1.1 Ma and 97.8 +/- 1.1 Ma. The geochronological database available on the Tore-Madeira Rise has been filtered on statistical criteria to eliminate unreliable ages. The resulting database reveals three pulses of alkaline magmatism on the Tore-Madeira Rise at 103-80.5 Ma, at c. 68 Ma and between 30 Ma and the present. The magmatism was continuous from 103 Ma until c. 68 Ma and from c. 30 Ma until the present on the Tore-Madeira Rise, the surrounding seamounts and the Portugal coast. We suggest that the space-time distribution of this magmatism results from the interaction between a wide thermal anomaly emitting magmatic pulses and the complex motion of the Iberian plate.
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| 7. |
- Sanchez, Guillaume, et al.
(author)
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Post-spreading deformation and associated magmatism along the Iberia-Morocco Atlantic margins : Insight from submarine volcanoes of the Tore-Madeira Rise
- 2019
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In: Marine Geology. - 0025-3227 .- 1872-6151. ; 407, s. 76-93
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Journal article (peer-reviewed)abstract
- A new high-resolution bathymetric map combined with a regional Digital Elevation Model (DEM) analysis reveal the modalities of occurrence and emplacement of post-spreading magmatism along the NNE-SSW oriented, 1000 km long Tore-Madeira Rise (TMR) as well as its relationship with the activity of major fault systems including the Estremadura Fault System (ESF) and the Azores-Gibraltar Fracture Zone (AGFZ). Morphological and structural analysis of the bathymetric map were performed to map volcanic features such as eruptive cones, vents and fissures together with faults along the TMR. The new bathymetric map shows that the main NNW-SSE seamount alignment is formed by three structurally distinct volcanic massifs, the Tore, the Josephine and the Southern Volcanic Groups. The majority of the volcanoes of each group emplaced within or along specific portion of pre-existing faults (ESF and AGFZ) including splay fault, releasing bend, fault tips and interaction zones between different segments. Magmas were channelled into sub-vertical pre-existing lithospheric faults that acted as preferential pathways for the vertical magma ascent. Migration and final eruption of magma are controlled by the local stress variation induced by complex fault geometries, change in plate kinematics as well as strong shear zone anisotropy as suggested by the emplacement within localised areas of transtension. We conclude that post-spreading magma emplacement in the southern part of the Iberia margin was related to the development of a transtensional plate boundary between the Iberian and African Plate during the Late Cretaceous. More generally, our findings emphasize that the distribution of volcanism as the expression of the interaction between shallow plate tectonic and mantle processes should be included in plate kinematic reconstruction. This study also demonstrates that the accurate mapping of oceanic seafloor is pivotal to better understand tectono-magmatic evolution of volcanic seamount chains and geological processes in oceanic domains.
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