The paranthurid isopod crustacean Paranthura nigropunctata (Lucas, 1846): first record from the Atlantic coast of Morocco

The littoral paranthurid isopod crustacean Paranthura nigropunctata (Lucas, 1846) is recorded for the first time from the littoral of El Jadida located on the Atlantic northwest coast of Morocco. Specimens were obtained from the invasive brown seaweed Sargassum muticum and the natives Bifurcaria bifurcata and Cystoseira tamariscifolia in January 2015. This new record further confirms a significant southward distribution of P. nigropunctata and contributes to the knowledge of the biogeography of this isopod. Heretofore, the species was only known from the western and eastern Mediterranean and some Atlantic coasts. The present finding is the first of the species from Moroccan Atlantic shores, and suggests that the species may also be present in other coastal localities from Morocco and Africa. Some data on morphology, ecology and spatial distribution of the species are provided.     

The introduction of Sargassum muticum modifies epifaunal patterns in a Moroccan seagrass meadow

Marine macrophytes sustain valuable epiphytic biodiversity. Nonindigenous macroalgae may induce changes in composition and structure of epifaunal assemblages and therefore support different assemblages from those associated with native species. In this study, differences in faunal community structure between the introduced fucoid Sargassum muticum and the native seagrass Cymodocea nodosa were tested over a year on an intertidal shallow sandy bottom at the southern introduction front, the El Jadida coastline (NW Morocco). Epifaunal community structure consistently differed between macrophytes through seasons, with more species‐rich assemblages associated with S. muticum than C. nodosa despite comparable abundances. The significantly greater epifauna diversity on S. muticum may be related to its structural complexity. However, the species contributing most to differences in assemblages between both macrophytes, such as Steromphala umbilicalis and S. pennanti, were found on both habitats with temporally varying abundances. Some species‐specific affinities were detected (Stenosoma cf. acuminatum, Elasmopus vachoni, Chauvetia brunnea). Nitrogen, dissolved oxygen, suspended matter and temperature were identified as the best explanatory variables contributing to the observed macroepifaunal patterns. This study provides evidence that S. muticum acts as a favourable and additional habitat for epifaunal species and supports a more diverse epifaunal assemblage in this Moroccan seagrass meadow.     

Exhumation of eclogite and blueschist (Cyclades,Greece): Pressure–temperature evolution determined by thermobarometry and garnet equilibrium modelling

High‐P rocks such as eclogite and blueschist are metamorphic markers of palaeo‐subduction zones, and their formation at high‐P and low‐T (HP–LT) conditions is relatively well understood since it has been the focus of numerous petrological investigations in the past 40 years. The tectonic mechanisms controlling their exhumation back to the surface are, however, diverse, complex and still actively debated. Although the Cycladic Blueschist Unit (CBU, Greece) is among the best worldwide examples for the preservation of eclogite and blueschist, the proposed P–T evolution followed by this unit within the Hellenic subduction zone is quite different from one study to another, hindering the comprehension of exhumation processes. In this study, we present an extensive petrological data set that permits refinement of the shape of the P–T trajectory for different subunits of the CBU on Syros. High‐resolution quantitative compositional mapping has been applied to support the thermobarometric investigations, which involve semi‐empirical thermobarometry, garnet equilibrium modelling and P–T isochemical phase diagrams. The thermodynamic models highlight the powerful use of reactive bulk compositions approximated from local bulk compositions. The results are also combined with Raman spectrometry of carbonaceous material (RSCM) to retrieve the metamorphic peak temperature distribution at the scale of the island. A major result of this study is the good agreement between all the independent thermobarometric methods, permitting reconstruction of the prograde and retrograde P–T trajectories. Garnet compositional zoning was used to retrieve prograde, peak and retrograde growth stages in line with the results of the P–T isochemical phase diagrams, RSCM temperature and peak‐pressure crystallization of the garnet–omphacite–phengite assemblage. Our results are consistent with previous thermobarometric estimates from other occurrences of CBU rocks (Tinos, Andros), suggesting a multistage exhumation process with (1) early syn‐orogenic exhumation within the subduction channel, (2) isobaric heating at mid‐crustal depths (~10–12 kbar) following thermal re‐equilibration of the lithosphere from a cold syn‐orogenic regime in the subduction zone to a warmer post‐orogenic regime in the back‐arc domain and (3) exhumation and cooling related to a post‐orogenic phase of extension following slab retreat. Expanding to the general aspects of subduction zones, we suggest that such metamorphic evolution of HP–LT units should be regarded as a characteristic feature of exhumation driven by slab rollback.     

Fluid–rock interactions related to metamorphic reducing fluid flow in meta-sediments: example of the Pic-de-Port-Vieux thrust (Pyrenees,Spain)

In orogens, shortening is mainly accommodated by thrusts, which constitute preferential zones for fluid–rock interactions. Fluid flow, mass transfer, and mineralogical reactions taking place along thrusts have been intensely investigated, especially in sedimentary basins for petroleum and uranium research. This study combines petrological investigations, mineralogical quantifications, and geochemical characterizations with a wide range of analytical tools with the aim of defining the fluid properties (nature, origin, temperature, and redox) and fluid–host rock interactions (mass transfers, recrystallization mechanisms, and newly formed synkinematic mineralization) in the Pic-de-Port-Vieux thrust fault zone (Pyrenees, Spain). We demonstrate that two geochemically contrasted rocks have been transformed by fluid flow under low-grade metamorphism conditions during thrusting. The hanging-wall Triassic red pelite was locally bleached, while the footwall Cretaceous dolomitic limestone was mylonitized. The results suggest that thrusting was accompanied by a dynamic calcite recrystallization in the dolomitic limestone as well as by leaching of iron via destabilization of iron oxides and phyllosilicate crystallization in the pelite. Geochemical and physical changes highlighted in this study have strong implications on the understanding of the thrust behavior (tectonic and hydraulic), and improve our knowledge of fluid–rock interactions in open fluid systems in the crust.