A two-stage exhumation of the Variscan crust: U-Pb LA-ICP-MS and Rb-Sr ages from Greater Kabylia, Maghrebides

International audienceThe significance and role of major shear zones are paramount to understanding continental deformation and the exhumation of deep crustal levels. LA‐ICPMS U–Pb dating of monazites, combined with Rb–Sr analyses of biotites, from an anatectic metapelite from Greater Kabylia (Algeria) highlights the history of shear zone development and the subsequent exhumation of deep crustal levels in the internal zones of the Maghrebides. Monazites give an age of 275.4 ± 4.1 Ma (2σ ) dating the thermal peak coeval with anatexis. This age is identical to those recorded in other crystalline terranes from south‐easternmost Europe (i.e. South Alpine and Austro‐Alpine domains) that suffered crustal thinning during the continental rifting predating the Tethys opening. Rb–Sr analyses of biotites yield a cooling age of 23.7 ± 1.1 Ma related to the exhumation of the buried Variscan crust during the Miocene as an extrusive slice, roughly coeval with the emplacement of nappes, and shortly followed by lithospheric extension leading to the opening of the Alboran sea

Constraints on the post-Variscan thermal evolution of the Ivrea crustal section (Italian-Swiss Alps) from U-Pb dating of relict rutile in middle crust amphibolites

The Ivrea crustal section from the Italian-Swiss Alps is a nearly coherent section of the pre-Alpine, lower to upper continental crust, which became part of a passive continental margin in the Middle Jurassic. Here we apply the U-Pb rutile thermochronometer to amphibolites collected along mid levels of the Ivrea section, in order to improve the understanding of the thermal evolution of extending continental margins. Relict rutile in the Monte Gambarogno amphibolite lens from the northeastern Ivrea crustal section provided a Permian age (284 ± 24 Ma). On the other hand, an Upper Triassic (215 ± 6 Ma) age was acquired for relict rutile in the Alpe Morello amphibolite lens, which is exposed along the southwestern Ivrea crustal section. We attribute these ages to U[sbnd]Pb isotope re-equilibration of rutile crystallized in pre-Permian times, in response to two distinct heating events. The first is related to the Lower Permian post-Variscan extension and is inferred to have reset the U-Pb system of rutile from middle and deep levels of the whole Ivrea crustal section. The subsequent thermal perturbation of Upper Triassic age is most likely related to the onset of the rifting process that ultimately led to crustal breakup in the Middle Jurassic. We propose that the latter heating event was not able to disturb the U[sbnd]Pb system of middle crustal rutile along the future proximal domain of the extending continental margin

From late Visean to Stephanian: pinpointing a two-stage basinal evolution in the Variscan belt. A case study from the Bosmoreau basin (French Massif Central) and its geodynamic implications

International audiencePost-convergence evolution of the Variscan belt is characterized by the development of intramontane coal-bearing basins containing volcano-sedimentary successions. In the French Massif Central, K––Ar ages on clay particles from fine-grained sediments of the Bosmoreau basin (Limousin area), help pinpoint the evolution of the basin. In the lower part of the sedimentary pile, illite in a siltstone underlying a volcanic layer previously dated at 332±4 Ma by the U––Pb method on zircon, yields a consistent K––Ar age of ca. 340 Ma. Upward in the sedimentary succession, illite yields Stephanian K––Ar ages, which can be combined to provide a mean deposition age of 296.5±3.5 Ma. The Bosmoreau basin, albeit mainly filled with Stephanian deposits, was initiated during the late Visean, i.e. ca. 30 Ma earlier than inferred from biostratigraphical constraints. During the Stephanian, the same structure was reactivated and late Visean deposits were eroded and subsequently blanketed by thick clastic sediments. These results emphasise a two-stage evolution for the Bosmoreau basin, which is closely related to extensional tectonics identified on basement country rocks, and they are used to propose a geodynamic evolution of the studied area

Are the refractory mantle peridotites from Monte Civrari (W Alps) of subcontinental or abyssal origin?

Here we present mineralogical, geochemical and Nd-Pb isotope data for the ophiolitic mantle body of Monte Civrari, which is exposed west of the Lanzo Massif. The outcrops of Monte Civrari are mainly composed of antigorite serpentine schists with decameter-sized lenses of exceptionally fresh mantle peridotites locally intruded by small veins of rodingitized metagabbros. The samples of this study are medium- to coarse-grained spinel harzburgites to cpx-poor lherzolites (Cpx < 5 vol %). They generally show a protogranular texture. The spinels frequently form clusters with orthopyroxene ± clinopyroxene, possibly as products of garnet breakdown. The clinopyroxene (Mg# 90-91) show very low TiO2 (0.05-0.15 wt. %) and Na2O (< 0.1 wt. %) coupled with high Cr2O3 contents (1.2-1.5 wt%). Spinels have compositions similar to those of abyssal peridotites, with Cr# and Mg# varying between 0.32-0.34 and 0.68-0.71, respectively, and low TiO2 contents (0.05-0.10 wt%). Most samples do not show any evidence of interaction with melts and preserve highly refractory chemical compositions. The clinopyroxenes display strongly fractionated REE spectra (CeN/YbN ~ 0.001), with prominent LREE depletions (CeN/SmN = 0.004-0.005) and low HREE abundances (YbN ~ 5-6) associated with HREE fractionation (GdN/YbN = 0.4-0.5). Clinopyroxene REE compositions may be reproduced by small amounts (~5-6%) of fractional melting of a garnet lherzolite precursor followed by 10% melting in the spinel peridotite field. As a whole, Pb isotope compositions of bulk rocks fall in the fields of global MORB and plot close to the 4.53 Ga reference isochron. Nd isotopes of clinopyroxenes confirm the presence of refractory mantle domains with old ages of depletion in the Ligurian Tethys oceanic lithosphere. The origin of these mantle domains has been previously attributed to incorporation of SCLM derived from a late Variscan DMM melting event (see Mc Carthy & Muntener, 2015 and quoted references). However, geothermometry based on trivalent REE+Y exchange between clino- and orthopyroxene, yields high T estimates (TREE) of 1170-1300°C, associated with high T values obtained applying Ca-in Opx (1200-1280°C) and pyroxene solvus methods (TBKN ~ 1100°C). Thermal evolution of the Monte Civrari mantle body is thus consistent with rapid cooling and exhumation from asthenospheric conditions, similar to modern abyssal type peridotites, which apparently argues against a long residence time in the SCLM after the Permian event. McCarthy, A. & Müntener, O. (2015): Ancient depletion and mantle heterogeneity: Revisiting the Permian-Jurassic paradox of Alpine peridotites. Geology, 43, 255-258

Multi-element otolith fingerprints unravel sea-lagoon lifetime migrations of gilthead sea bream Sparus aurata

Precise knowledge of lifetime migrations is vital in exploited fish species, since all essential habitats must be protected to maintain sustainable stock levels. The present study used multi-element otolith fingerprints of the gilthead sea bream Sparus aurata (L.) to discriminate its main juvenile and adult habitats in the Languedoc-Roussillon region (Gulf of Lions, northwest Mediterranean) and infer the lifetime migrations of 12 individuals from the area (11 from the present day and 1 from the Roman era). This allowed for the first time the identification of key habitats for the successful completion of the species' life cycle in the Gulf of Lions, and the connectivity between them. Our results revealed that lagoon use by S. aurata is probably ancient (>2500 yr) and confirmed its current commonness. Yet, although most observed migration patterns were in accordance with the migratory behavior previously described for the species, strong inter-individual variations and new patterns in habitat use were detected. At the juvenile stage, a preference for shallow lagoons with low salinities was evidenced. Nevertheless, the first year of life can also be successfully completed in marine conditions. At the adult stage, lagoon use was shown to occur until at least age 4 yr, with periods of lagoon residency of up to 11 mo in a year, often including winter months. Because overwintering in the lagoons was previously thought to be impossible for S. aurata due to low temperatures, this finding has important implications for future stock management, especially since the species breeds in winter

Innovative isotopic method to evaluate bioaccumulation of As and MTEs in Vitis vinifera

International audienceThe transfer of metal and metalloid trace elements (MTEs) from contaminated soil to grapevines is a major issue for grape consumption and for the associated health risks. Based on an isotopic approach, we shed light on the concept of MTE bioavailability. The bioavailable fractions are identified by using the Sr-isotope ratio as a proxy for MTEs. This allows us to differentiate three soil reservoirs: the ‘current available fraction’ in soil water, the ‘reserve available fraction’ stored in mineral phases of the soil fractions, and the ‘non-available fraction’. The reserve available fraction, representing 10 to 60% of bulk soil depending on the MTE, includes the exchangeable, carbonates, humic substance and oxides fractions. The 87Sr/86Sr isotopic signatures of grape berries and vine leaves show an additional source of MTEs, which is imported by foliar uptake and can contribute up to 10% of the MTEs in leaves. In addition, root-uptake and translocation rates show high accumulation rates of Co, Sn and Cu, and low ones for As, Sb, Zn and Cd. A daily intake between 1 and 3 kg of (dry grapes) would reach the benchmark dose level for a 0.5% (BMDL0.5). While such a daily intake of grapes is unreasonable, consumption of other local vegetables and fruit would contribute to the daily intake. Hence, a chronic arsenic exposure is of great concern for human health in mining areas. We outline the importance of geochemical tracers, such as Sr isotopes, when determining the transfer and translocation of MTEs in plants. Our method presents a high-precision evaluation of the bioavailability and bioaccumulation of MTEs, and a better understanding of these processes in plants, thus leading to a better assessment of the environmental risk on human health

Complete Alpine reworking of the northern Menderes Massif, western Turkey

This study focuses on the petrology, geochronology and thermochronology of metamorphic rocks within the northern Menderes Massif in western Turkey. Metasediments belonging to the cover series of the Massif record pervasive amphibolite-facies metamorphism culminating at ca. 625–670 °C and 7–9 kbars. U–Th–Pb in situ ages on monazite and allanite from these metapelites record crustal thickening and nappe stacking associated with the internal imbrication of the Anatolide–Taurides platform during the Eocene. In addition, new 39Ar/40Ar single muscovite grain analyses on deformed rocks were performed in three localities within the northern Menderes Massif and ages range from 19.8 to 25.5 Ma. These mylonites may be related to both well-known detachments, Simav to the north and Alaşehir to the south, which accommodate Oligo–Miocene exhumation of the Menderes core complex. U–Th–Pb data on monazite grains (22.2 ± 0.2 Ma) from migmatites emplaced within the Simav detachment confirm these ages

The Late Neoproterozoic / Early Paleozoic evolution of the West Congo belt of NW Angola : geochronological (U-Pb and Ar-Ar) and petrostructural constraints,

International audienceU-Pb zircon and Ar-Ar amphibole and biotite ages from rocks of the West Congo Belt of Angola indicate that this area underwent two main deformation events of amphibolite facies grade at c. 540 and 490 Ma, which were followed by tectonically assisted exhumation during eastward thrusting of the hinterland domain onto the foreland domain. High-grade conditions in the West Congo Belt are 20-40 Ma younger than in the Ribeira-Araçuai Belt of Brazil, its South American counterpart, or than in the westernmost Kaoko Belt of Namibia, its African correlative. In the present state of knowledge, a more appropriate counterpart to the southern part of the West Congo Belt may be restricted to the Cabo Frio Terrane in the eastern Ribeira Belt, which yields a broadly similar evolution