Biostratigraphie, géochimie et réponse des composantes microfauniques aux variations environnementales au passage Cénomanien-Turonien à Thénièt El Manchar (Monts de Bellezma, Batna, NE Algérie)

The present study deals with the biostratigraphic and geochemical analysis of the Cenomanian-Turonian deposits of the Thénièt El Manchar district in the Bellezma-Batna Range, and their vertical fluctuations. A 180 m-thick profile is described in this area. This reference section encompasses the ‘Marnes de Smail’ Formation, subdivided into four lithostratigraphic units (IA, IB, IC, and ID respectively) and dated of the Cenomanian (ammonites and coral), and the base of the ‘Dolomies de l’Oued Skhoun’ Formation (unit IIA), dated of the Lower Turonian (planktonic foraminifers). Both qualitative and quantitative analysis of foraminifers (planktonic and/or benthonic), ostracodes and of geochemical indices, lead us to reconstitute the evolution of both palaeoenvironmental and palaeobathymetrical changes during this time interval. Subsequently units IA, IB, IC and the lower part of unit ID (all Cenomanian) are displaying agglutinated benthonic foraminifers and complete ostracode shells, as evidence of a shelfal environment and weak hydrodynamism. Within these deposits microfaunal assemblages display a low-to-medium species diversity and a relatively high specimen abundance, supporting evidence of normal trophic conditions and water oxygenation. The upper part of unit ID (topmost Cenomanian), and unit IIA (basal Turonian) are documenting an environmental setting into deeper conditions, supported by the standard event succession already recorded in northern Africa, namely: the abundance of planktonic foraminifers, the occurrence of ‘filaments’ and the sudden reduction of ostracofauna. Furthermore the total carbonate isotope fluctuations (δ13C and δ18O) are documenting isotopic anomalies related to palaeoenvironmental changes. The δ13C results, coupled with those of TOC, are evidencing a low primary productivity while the δ18O data are supporting a temperature rise as the main potential drive of the onset of OAE2. These paleoenvironmental assertions are consistent with the regional paleogeographic context and are highlighting tethysian features.[fr] La présente étude livre une analyse biostratigraphique et géochimique des dépôts cénomano-turoniens de la région Thénièt El Manchar, située dans les Monts de Bellezma-Batna, et leur évolution verticale. Une coupe (TM), épaisse de 180 m, est décrite dans ce secteur. Elle correspond à la Formation des Marnes de Smail, subdivisée en quatre unités litho-stratigraphiques (IA, IB, IC, ID), datée du Cénomanien grâce à la présence d’ammonites et de corail, et à la base de la Formation des Dolomies de l’Oued Skhoun (unité, IIA), datée de la base du Turonien au moyen de foraminifères planctoniques. L’analyse qualitative et quantitative des foraminifères (planctoniques et/ou benthiques), des ostracodes et des marqueurs géochimiques, permet de reconstituer l’évolution du paléo-environnement et les variations paléo-bathymétriques au cours de l’intervalle stratigraphique concerné. Ainsi, les unités IA-IB-IC et la partie inférieure de l’unité ID (Cénomanien) sont dominées par des associations de foraminifères benthiques agglutinés et des carapaces entières d’ostracodes, témoignant d’un milieu de plate-forme et d’un faible hydrodynamisme. Dans ces dépôts, les associations montrent une diversité spécifique faible à moyenne et une abondance plus ou moins élevée, témoignant de conditions trophiques et d’oxygénation considérées comme normales. Les derniers termes de l’unité ID (sommet du Cénomanien) et l’unité IIA (base du Turonien) marquent, quant à eux, une évolution vers des conditions plus profondes, comme le montre la succession standard des événements déjà reconnus en Afrique du nord, à savoir: l’abondance de foraminifères planctoniques, la présence de ‘filaments’ et une réduction drastique de la faune d’ostracodes. En outre, les courbes isotopiques du carbone (δ13C) et de l’oxygène (δ18O) des carbonates mettent en évidence des anomalies isotopiques relatives aux modifications paléo-environnementales. Les données du δ13C et celles du COT indiquent une productivité primaire faible. Les données du δ18O, quant à elles, indiquent une augmentation des paléo-températures, cause principale du déclenchement de l’EAO 2. Ces interprétations paléo-environnementales s’accordent avec les données connues du contexte paléogéographique régional et mettent en exergue les spécificités téthysiennes

Metasomatized Mantle Xenoliths as a Record of the Lithospheric Mantle Evolution of the Northern Edge of the Ahaggar Swell, In Teria (Algeria)

International audienceMantle-derived xenoliths hosted by melilitite lavas from In Teria (Ahaggar, SE Algeria) include garnet and spinel peridotites, pyroxenite and phlogopite megacrysts. The spinel and garnet peridotites record an early deformation event, which formed porphyroclastic microstructures and olivine crystal preferred orientations, followed by static infiltration of hydrous alkaline melts. This metasomatic stage (stage 1) is characterized by the crystallization of phlogopite in the garnet and spinel peridotites, amphibole in the spinel peridotites and clinopyroxene in the garnet peridotite, which record chemical equilibration with an alkaline silicate melt. These early events were largely overprinted by carbonatitic metasomatism (stage 2), which is observed only in the spinel peridotites. Spinel peridotite major and trace element compositions, as well as the compositions of newly formed minerals, are characteristic of interaction with carbonate melt, associated with strong enrichment in incompatible trace elements in clinopyroxene. This second stage was followed by crystallization of pyroxenites (stage 3) in vein conduits, probably segregated from alkaline melts. We propose a scenario in which the different metasomatic imprints record successive stages of interaction between lithospheric mantle and sublithospheric melts throughout the Cenozoic. In Sr–Nd isotope space, the host melilitites and several xenoliths are clustered and plot close to the HIMU mantle end-member. However, some peridotite xenoliths are shifted towards more radiogenic 87Sr/86Sr values. In 207Pb/204Pb–206Pb/204Pb and 208Pb/204Pb–06Pb/204Pb space the In Teria samples define a relatively large domain characterized by high 206Pb/204Pb and 208Pb/204Pb, consistent with a contribution of an HIMU component, considered to represent a sublithospheric signature. The highest 87Sr/86Sr values are comparable with those ascribed to the EM1 mantle end-member, representing the signature of the lower continental lithosphere, and are probably inherited from the pre-metasomatic lithospheric mantle beneath In Teria. Numerical modelling of porous percolation of melt of sublithospheric origin through an EM1-like lithospheric mantle protolith reproduces the In Teria peridotite compositions, using moderately sub-chondritic Sr/Nd values for the peridotite (e.g. In Teria garnet peridotite) and moderately super-chondritic Sr/Nd values in the melt (approximately ocean island basalt values). A few spinel peridotites require a component characterized by a 143Nd/144Nd signature higher than both the EM1 end-member and the local Ahaggar basalts; the 208Pb/204Pb compositions of several samples point to a component with a depleted mid-ocean ridge basalt (MORB) mantle (DMM) signature. Thus the lithospheric mantle beneath In Teria probably did not have a uniform EM1 signature before the onset of metasomatism; it included a DMM peridotite component as well as some peridotites with elevated 143Nd/144Nd values recording long-term LREE depletion