Key management practices to reduce the risk of the occurrence of Rumex obtusifolius in productive grasslands

Rumex obtusifolius (broad-leaved dock) is a problematic weed that reduces yield and nutritional value of forage in grasslands of temperate regions worldwide. We conducted an on-farm study to identify management practices and environmental factors that influence the risk of the occurrence of R. obtusifolius in high densities in permanent, productive grasslands used for forage production. Following a common protocol, a paired case–control design was implemented in Switzerland (CH), Slovenia (SI), and United Kingdom (UK) to compare parcels with high densities of R. obtusifolius (cases, ≥1 plant m−2) with nearby parcels free of or with very low densities of the species (controls, ≤4 plants 100 m−2). A total of 40, 20, and 18 pairs were recorded in CH, SI, and UK respectively. Parameters measured included data about management practices and history, vegetation cover and composition, and soil nutrients and texture. Across countries, increased vegetation cover reduced the relative risk of R. obtusifolius occurrence. By contrast, increased soil phosphorus and potassium and high soil bulk density raised the relative risk. These effects were consistent across countries, as no interactions between country and any of the factors were observed. The two indicator species for case parcels, Plantago major and Poa annua, were typical species of disturbed areas and fertile soils, while indicators for control parcels were characteristic of grasslands under medium to high management intensity (e.g., Festuca rubra, Cynosorus cristatus, Anthoxantum odoratum). We conclude that the risk for grassland infestation with R. obtusifolius can be significantly affected by management practices. Prevention measures should target phosphorus and potassium fertilisation to the forage plants' requirements, minimise soil compaction, and maintain dense swards

Zircon ages in granulite facies rocks: decoupling from geochemistry above 850 °C?

Granulite facies rocks frequently show a large spread in their zircon ages, the interpretation of which raises questions: Has the isotopic system been disturbed? By what process(es) and conditions did the alteration occur? Can the dates be regarded as real ages, reflecting several growth episodes? Furthermore, under some circumstances of (ultra-)high-temperature metamorphism, decoupling of zircon U–Pb dates from their trace element geochemistry has been reported. Understanding these processes is crucial to help interpret such dates in the context of the P–T history. Our study presents evidence for decoupling in zircon from the highest grade metapelites (> 850 °C) taken along a continuous high-temperature metamorphic field gradient in the Ivrea Zone (NW Italy). These rocks represent a well-characterised segment of Permian lower continental crust with a protracted high-temperature history. Cathodoluminescence images reveal that zircons in the mid-amphibolite facies preserve mainly detrital cores with narrow overgrowths. In the upper amphibolite and granulite facies, preserved detrital cores decrease and metamorphic zircon increases in quantity. Across all samples we document a sequence of four rim generations based on textures. U–Pb dates, Th/U ratios and Ti-in-zircon concentrations show an essentially continuous evolution with increasing metamorphic grade, except in the samples from the granulite facies, which display significant scatter in age and chemistry. We associate the observed decoupling of zircon systematics in high-grade non-metamict zircon with disturbance processes related to differences in behaviour of non-formula elements (i.e. Pb, Th, U, Ti) at high-temperature conditions, notably differences in compatibility within the crystal structure

Permian high-temperature metamorphism in the Western Alps (NW Italy)

During the late Palaeozoic, lithospheric thinning in part of the Alpine realm caused high-temperature low-to-medium pressure metamorphism and partial melting in the lower crust. Permian metamorphism and magmatism has extensively been recorded and dated in the Central, Eastern, and Southern Alps. However, Permian metamorphic ages in the Western Alps so far are constrained by very few and sparsely distributed data. The present study fills this gap. We present U/Pb ages of metamorphic zircon from several Adria-derived continental units now situated in the Western Alps, defining a range between 286 and 266 Ma. Trace element thermometry yields temperatures of 580-890°C from Ti-in-zircon and 630-850°C from Zr-in-rutile for Permian metamorphic rims. These temperature estimates, together with preserved mineral assemblages (garnet-prismatic sillimanite-biotite-plagioclase-quartz-K-feldspar-rutile), define pervasive upper-amphibolite to granulite facies conditions for Permian metamorphism. U/Pb ages from this study are similar to Permian ages reported for the Ivrea Zone in the Southern Alps and Austroalpine units in the Central and Eastern Alps. Regional comparison across the former Adriatic and European margin reveals a complex pattern of ages reported from late Palaeozoic magmatic and metamorphic rocks (and relics thereof): two late Variscan age groups (~330 and ~300 Ma) are followed seamlessly by a broad range of Permian ages (300-250 Ma). The former are associated with late-orogenic collapse; in samples from this study these are weakly represented. Clearly, dominant is the Permian group, which is related to crustal thinning, hinting to a possible initiation of continental rifting along a passive margin

New Pb-Pb single zircon age constraints on the timing of neoproterozoic glaciation and continental break-up in Namibia.

Dating of single zircons from low-grade metamorphosed rhyolites in the Rosh Pinah Formation of the Gariep Belt in southwestern Namibia, using the Pb evaporation technique, yielded a primary crystallization age of 741 ± 6 Ma. Both the stratigraphic position and the geochemistry of the volcanic rocks indicate an early continental rift environ- ment. The new data not only provide an age for the massive Zn-Pb-Cu sulfide mineralization associated with these volcanic rocks, but they also set a maximum age limit for Neoproterozoic continental break-up in southern Namibia. This age is statistically indistinguishable from a recently reported age of 748 + 3 Ma for stratigraphically equivalent volcanic rocks in the northern rift of the Damara Belt, suggesting that the onset of the formation of the N-S-trending Adamastor ocean and of the NE-trending Khomas ocean in central and northern Namibia occurred at the same time. The volcanic unit directly overlies, in both rift grabens, a diamictite horizon with glaciogenic features. Our new results further constrain the age of this glacial epoch, which may be correlated with the Sturtian glaciation, to around 750 M

Finite lattice distortion patterns in plastically deformed zircon grains

This study examines finite deformation patterns of zircon grains from high-temperature natural shear zones. Various zircon-bearing rocks were collected in the Western Tauern Window, eastern Alps, where they were deformed under amphibolite facies conditions, and in the Ivrea–Verbano Zone (IVZ), southern Alps, where deformation is related with granulite-facies metamorphism. Among the sampled rocks are granitic orthogneisses, metalamprophyres and paragneisses, all of which are strongly deformed. The investigated zircon grains ranging from 10 to 50 μm were studied in situ using a combination of scanning electron microscope (SEM) techniques, backscattered electron (BSE) imaging, forward-scattered electron (FSE) imaging, cathodoluminescence (CL) imaging, and crystallographic orientation mapping by electron backscatter diffraction (EBSD), as well as micro-Raman spectroscopy. Energy-dispersive X-ray spectrometry (EDS) was applied to host phases. Microstructural analysis of crystal-plastically deformed zircon grains was based on high-resolution EBSD maps. Three general types of finite lattice distortion patterns were detected: type (I) is defined by gradual bending of the zircon lattice with orientation changes of about 0.6–1.8° per micrometer without subgrain boundary formation. Cumulative grain-internal orientation variations range from 7 to 25° within single grains. Type (II) represents local gradual bending of the crystal lattice accompanied by the formation of subgrain boundaries that have concentric semicircular shapes in 2-D sections. Cumulative grain-internal orientation variations range from 15 to 40° within single grains. Type (III) is characterized by formation of subgrains separated by a well-defined subgrain boundary network, where subgrain boundaries show a characteristic angular closed contour. The cumulative orientation variation within a single grain ranges from 3 to 10°. Types (I) and (II) predominate in granulite facies rocks, whereas type (III) is restricted to the amphibolite facies rocks. The difference in distortion patterns is controlled by strain rate and by ratio between dislocation formation and dislocation motion rates, conditioned by the amount of differential stress. Investigated microstructures demonstrate that misorientation axes are usually parallel to the and crystallographic directions; dominant slips are {001}, {100} and {010}, whereas in some grains cross-slip takes place. This study demonstrates that activation of energetically preferable slip systems is facilitated if zircon grain is decoupled from the host matrix and/or hosted by a soft phase

U-Pb and Sm-Nd geochronology of the KIzIldag (Hatay, Turkey) ophiolite: Implications for the timing and duration of suprasubduction zone type oceanic crust formation in the southern Neotethys

The KIzIldag (Hatay) ophiolite in Turkey represents remnants of the southern Neotethyan ocean and is characterized by a complete ocean lithospheric section. It formed in a fore-arc setting above a N-dipping intraoceanic subduction zone, and represents the undeformed, more northerly part of the same thrust sheet that also forms the Baer-Bassit ophiolite to the south. The ophiolite was emplaced southwards from the southerly Neotethyan ocean in Maastrichtian time. U-Pb and Sm-Nd dates are used to constrain the crystallization age and duration of magmatic activity of the KIzIldag ophiolite. U-Pb dating yielded ages of 91.7 ± 1.9 Ma for a plagiogranite and 91.6 ± 3.8 Ma for a cumulate gabbro. The cumulate gabbro also yielded a Sm-Nd isochron age of 95.3 ± 6.9 Ma. The measured ages suggest that the oceanic crust of the KIzIldag ophiolite formed in a maximum time period of 6 Ma, and that the plagiogranite may have formed later than the gabbroic section. The U-Pb zircon ages from the KIzIldag ophiolite and the cooling age of a metamorphic sole beneath the Baer-Bassit ophiolite are indistinguishable within the analytical uncertainties. This indicates the presence of young and hot oceanic lithosphere at the time of intraoceanic subduction/thrusting in the southern Neotethys. The U-Pb zircon ages from the KIzIldag, the Troodos and the Semail ophiolites overlap within analytical uncertainties, suggesting that these ophiolites are contemporaneous and genetically and tectonically related within the same Late Cretaceous southern Neotethyan ocean. ©2012 Cambridge University Press