François Guillot

The littoral zone (including estuarine environments) is considered here as a regulator of the iron flow between the land and the marine environment, observed through the Jurassic sediments of Boulonnais (Northernmost France). Certain stages of deposition during the Kimmeridgian were accompanied by synsedimentary fault movements that caused sand injections, either in sands undergoing lithification (transforming into sandstone), or in claystone levels. The early cemented sandy material and that being hosted in a low-permeability environment (claystones) were as if frozen in their diagenetic evolution and protected against most subsequent transformations. We can see there early trapped and preserved iron-bearing authigenic minerals: pyrite, glauconite and magnetite. Glauconite can take on unusual forms: bryozoan colonies epigenized into glauconite, endolithic filaments mimicking microbial structures, a film enveloping a pyrite polyframboid. All these unusual forms, as well as the majority of glauconite grains, were not preserved in sands that were not cemented or protected early on, and the iron in these minerals was released into the surrounding environment during later stages of diagenesis. Generalizing from this example, the littoral environment can be an early iron trap, favoring the formation of iron-bearing minerals that would then tend to disappear during diagenesis, releasing iron into the marine environment. Furthermore, early cemented objects, where porosity/permeability was therefore rapidly occluded, are valuable insofar as they can record diagenetic stages that are subsequently no longer traced. This is the case here for iron-bearing minerals and this is an opportunity to question the highlighting of the role of microbial activity in the formation of glauconite.

This work examines the possibility of a rapid formation of glauconite in a relatively shallow platform environment (below fair-weather wave baseline). The materials studied here are uppermost Jurassic alternations of carbonate beds and marly interbeds, namely, the Assises de Croï Formation of the Boulonnais area (northernmost France). The carbonate beds yield field evidences of an early diagenetic origin and both beds and interbeds contain glauconite, questioning the duration of formation of the glauconite, relative to that of the diagenetic carbonate beds. Carbon and oxygen stable isotope composition of the carbonate beds confirm an early diagenetic growth. Contrasted grain-size distribution patterns of glauconite and quartz grain populations (isolated after acid digestion and magnetic separation) evidenced that glauconite formed after sediment deposition. Glauconite formation allegedly requires protracted episodes of ion capture from the water column, which is no longer possible when glauconite gets trapped within authigenic carbonates. Therefore, in-situ glauconite formation preceded carbonate authigenesis. Yet, the chemical composition of grains (Fe and K) typifies glauconite as highly evolved, meaning that its formation must have lasted over times, according to conventional views. Consequently, our results challenge these conventional views and confirm that glauconite can form in relatively shallow environments (which has been already brought to light previously) and it is concluded that early diagenetic glauconite can be markedly enriched in both K and Fe, which is an unprecedented result.

To provide a better picture of the active geodynamics along the Variscan suture zones during the late collisional stage (particularly regarding the evolution of the orogenic system towards HT conditions), we focused here on vaugnerites, which consist of mafic ultra-potassic magmatic rocks, intrusive into the granite-gneiss sequences of the Variscan Vosges crystalline massif. Those rocks, though subordinate in volume, are frequently associated with late-collisional granites. In the Central-Southern Vosges, they appear either as (1) pluton margin of the Southern Vosges Ballons granite complex or (2) composite dykes intrusive into migmatite and metamorphic sequences classically referred to as granite-gneiss unit (Central Vosges). Both types correspond to melanocratic rocks with prominent, Mg-rich, biotite and hornblende (20–40% vol., 64 < mg# < 78), two-feldspar and quartz. Those Vosges vaugnerites display geochemical signatures characteristic of ultra-potassic mafic to intermediate, metaluminous to slightly peraluminous rocks. Zircon U-Pb ages were obtained by Laser Ablation Inductively Coupled Plasma Mass Spectrometry. Zircon grains were extracted from a sillimanite-bearing gneiss from the granite-gneiss unit hosting the Central Vosges vaugnerites. They yielded an age at 451 ± 9 Ma, indicating a pre-Variscan Upper Ordovician protolith for the host sequence. Zircon from the four vaugnerite intrusives display U-Pb ages (± 2σ) of 340 ± 2.5 Ma (Ballons), 340 ± 25 Ma, 340 ± 7 Ma and 336 ± 10 Ma (Central Vosges). Synchronous within uncertainty, vaugnerite age data suggest a relatively early emplacement during the Late Variscan collisional history (i.e. Middle Visean times). These results are in line with previously published ages from the Southern Vosges volcano-sedimentary sequences (Oderen-Markstein) and the nearby ultra-potassic granite complexes from the Central and Southern Vosges (Ballons, Crêtes) thereby arguing for a magmatic event of regional significance. Recent petrological studies on vaugnerites suggest that they derive from partial melting of a metasomatized mantle contaminated to some different degrees by elements of continental crust. We propose here that the major ultra-potassic magmatic pulse at 340–335 Ma is a consequence of a significant change into the dynamics of the Rhenohercynian subduction system below the Central-Southern Vosges. In the light of recent thermo-mechanical modelling experiments on mature continental collision, magmatism could result from a syn-collisional lithospheric delamination mechanism involving (1) first, continental subduction evolving towards (2) the underthrusting of the Avalonian continental margin lower crust and (3) the initiation of lithospheric delamination within the supra-subduction retro-wedge (Saxothuringian-Moldanubian continental block). This delamination would drive the emplacement of an asthenospheric upwelling, initially localized along the Variscan suture zones, and gradually propagating towards the southern front of the belt during the Late Carboniferous, as the delamination front migrated at the base of the crust.

We describe a new agglutinated monothalamous foraminiferal species,Capsammina crassasp. nov., based on integrated observations of the test morphology and the chemical characteristics of materials composing the test. The new species was found at a depth of Capsammina, comprising two or more mica plates with a ring of finely agglutinated mineral grains sandwiched between them and surrounding the cell body. There is no distinct test aperture. Elemental analyses of the agglutinated grains revealed 15 different types of mineral grains of which quartz is the most abundant. The surface areas of grains exposed on fractured surfaces ranged from 1.6 to 7,700 μm²and the large plate-like grains forming the upper and lower surfaces measured about 420–2,350 μm in maximum width. The new species is morphologically similar toC. patelliformis, however, the differences in size, distribution area and depth support that these two species are distinct. This discovery is the first record of the genusCapsamminafrom the North Pacific. Therefore, it extends the biodiversity and geographical distribution of the genusCapsammina, which has been reported only from the bathyal NE Atlantic. Our finding also suggests the possibility of additional discovery of monothalamous foraminifera from around Korea.

U–Pb laser ablation inductively coupled plasma mass spectrometry was used for dating zircon grains extracted from four sedimentary and volcano sedimentary rocks of the Montagne Noire encompassing the presumed Ediacaran–Cambrian boundary interval. Magmatic zircon from two samples from the basal and middle parts of the Rivernous Formation (a rhyolitic tuff) were deposited at 542.5 ± 1 Ma and 537.1 ± 2.5 Ma, bracketing the 541 Ma age presently admitted as being at the Ediacaran–Cambrian boundary. In addition, a piece of sandstone from the underlying Rivernous Formation containing mostly euhedral zircon grains, suggesting proximal magmatic sources, yields Neoproterozoic dates ranging from 574 Ma to 1 Ga, and subsidiary older dates from 1.25 to 2.75 Ga. Another piece of sandstone from the overlying Marcory Formation yielded mostly rounded zircon grains probably issued from more remote areas, with a large spectrum dominated by Neoproterozoic dates as well as older ages up to 3.2 Ga. A comparison of both kinds of sandstone suggests a significant change in provenance, changing from a restricted source area during the Ediacaran to a much larger source domain during the Cambrian Epoch 2 that recorded contributions from different cratons of Gondwana.