Rôles respectifs de l''advection et de la décantation sur la ride médio-atlantique (40° à 50°N)

Coring at 45 sites in the North Atlantic permitted to determine flux velocity and chemical composition of pelagic sediments. Piston cores were used to carry out a comparative study between the post-glacial period (10,000 yr. B.P. until present) and last glacial period (75,000–10,000 yr. B.P.). Special attention has been paid to the Mid-Atlantic Ridges-Azores-Iceland area, where an enrichment of chemical elements was observed with regards to “regional ocean ground noise”, perceived on an isolated seamount of the abyssal plain.This “ground noise” characterized by the presence of Pb, Rb, U, Th, illite and chlorite, is associated to a settling vertical flux since the surface brought by surface currents and wind transport since the North American shield: sediments, here, are continental soil erosion products.The ridge is clearly enriched by Ba, Br, Fe, Ti, Mn, Cu, Ni, Co and As. Ba and Br are mostly associated with planktonic carbonates. Part of Ba may be linked to the ridge's activity. Basalt weathering on the ridge supplies a part of Fe in excess. Hydrothermal activity may account for Mn, Cu, Co, Ni and As enrichment. Most of the excess observed may be explained by intrusion of advective inputs from erosion Icelandic products (Fe, Ti, Cu, Ta, Sc and smectites), probably transported by Norwegian bottom currents.Advective flux (Icelandic-Faeroan basaltic materials) and flux linked to submarine ridge activity represent 30% of inorganic sedimentation. Vertical flux (North American continental-derived terrigenous materials) represent 70% of inorganic sedimentation. These percentages are very similar to those which were calculated for the Pacific.

Résumé

Les vitesses d'accumulation (flux) et les compositions chimiques ont été déterminées pour des sédiments pélagiques, dans 45 sites de l'océan Atlantique Nord. Les échantillons, prélevés par carottage Kullenberg, ont permis de réaliser une étude comparative de la période post-glaciaire (10.000 ans B.P. à nos jours) et du dernier glaciaire (75.000–10.000 ans B.P.). Notre attention s'est portée particulièrement sur la dorsale médioatlantique Açores-Islande où l'on observe des enrichissements en éléments chimiques par rapport au “bruit de fond océanique régional” appréhendé sur un dôme isolé dans la plaine abyssale.À ce bruit de fond, caractérisé par Pb, Rb, U, Th, illite et chlorite, est associé un flux vertical mis en place par décantation depuis la surface, apporté par le vent et les courants de surface depuis le craton nord-américain; il s'agit des produits d'érosion des sols continentaux.L'enrichissement de la dorsale est net pour Ba, Br, Fe, Ti, Mn, Cu, Ni, Co et As.Ba et Br précipite en grande partie avec les carbonates planctoniques. Une partie de Ba peut être associée à l'activité de la ride. L'altération des basaltes de la ride fournit une partie du Fe en excès. L'activité hydrothermale peut expliquer les enrichissements en Mn, Cu, Co, Ni et As.L'essentiel de l'excès observé est expliqué par un apport advectif de produits d'érosion de l'Islande (Fe, Ti, Cu, Ta, Sc et smectites). Leur vecteur serait le courant de fond de la mer de Norvège.Le flux advectif (matériaux basaltiques islando-faeroan) et le flux liéà l'activité sousmarine de la dorsale représenterait 29% de la sédimentation inorganique.Le flux vertical (matériaux continentaux terrigènes nord-américain) représenterait 71% de la sédimentation inorganique. Ces chiffres sont très proches de ceux qui ont été évalués dans le Pacifique.     

Superposition de la tectonique éburnéenne et panafricaine dans les granitoïdes de la bordure nord du craton ouest africain, boutonniére de Zenaga, Anti-Atlas central, Maroc

The Zenaga Inlier shows a comprehensive record of the Eburnian and Pan-African Orogenies. The Eburnian is characterised by high-temperature regional metamorphism and complex magmatism. The early (Azguemerzi) granodiorite has an isotopic mantle signature and was emplaced diapirically during the Eburnian Orogeny causing local thermal metamorphism. The foliation observed in this granitoid is a result of the interference between its primary syn-emplacement foliation and the regional foliation under amphibolite-facies conditions. The northern part of Zenaga has been intruded by the leucocratic granites of Tazenakht. These granites are cut by mylonites and phyllonites, corresponding to the Pan-African shear zones and accompanied with sub-greenschist-facies metamorphism during the Pan-African Orogeny. The deformation was the result of a regional sinistral transpressive event. This study in the northern part of the West African Craton shows the superposition of the Pan-African on the Eburnian Orogeny and the presence of a major fault in the Anti-Atlas.     

Découverte des roches à affinité ophiolitique dans la chaîne panafricaine au Cameroun : les talcschistes de Ngoung, Lamal Pougue et Bibodi Lamal

Outcrops of talc schists extending over >1 km have been discovered within the garnet- and muscovite-bearing mica schist of the Pan-African belt near Yaoundé (Cameroon). Mineralogical studies show that a metamorphism of the upper greenschist facies was prolonged by hydrothermal reactions. This latter led to the transformation of hornblendites into talc schists. Chemically, talc schists and relicts of hornblendite remind ultrabasic rocks, and REE patterns point to E-MORB and peridotite. It is thus suggested that the talc schists and relicts of hornblendite may correspond to slices of a dismembered Pan-African ophiolite set. To cite this article: C. Nkoumbou et al., C. R. Geoscience 338 (2006).     

Lithostratigraphie et histoire paléozoïque à paléocène des complexes métamorphiques de la région de Muteh, zone de Sanandaj–Sirjan (Iran méridional)Lithostratigraphy and Palaeozoic to Palaeocene history of some metamorphic complexes from Muteh area, Sanandaj–Sirjan zone (southern Iran)

A detailed analysis of metamorphic complexes outcropping in the Muteh area in central Iran leads to establish the regional stratigraphical column, and to propose a Palaeozoic age for the metamorphic protolith that mainly consists of volcano-sedimentary units. ⁴⁰K---⁴⁰Ar ages for minerals suggest the Mesozoic age of the metamorphic amphiboles and the Palaeocene ones for a late or even post metamorphic bimodal magmatism. To cite this article: N. Rachidnejad-Omran et al., C. R. Geoscience 334 (2002) 1185–1191.