Structural geology and petrography of the Naret region (northern Valle Maggia, N.Ticino, Switzerland)

The Naret region has a complex geological history of Alpine polyphase folding and metamorphism that affected pre-Alpine rocks of the Maggia nappe, including the Matorello group (interpreted in this study as late-Variscan intrusives), the Lebendun nappe and Mesozoic rocks of the Bedretto zone. From field observations, four main ductile deformation phases (D1 to D4) can be distinguished and, in combination with thermodynamic modelling, the tectono-metamorphic evolution for the Naret region can be reconstructed. D1 formed the initial nappe stack. During this phase, the Maggia nappe was thrust over the Lebendun nappe, at T ≤ 570°C and P ≤ 10 kbar. D2 caused isoclinal refolding of the nappe pile, at around 610-640°C and 8.5-10 kbar, and produced both the main regional foliation (S2) and a penetrative stretching lineation which is generally parallel to F2 fold axes. D2 is largely responsible for the current complicated geometry of the Lebendun nappe boundary. The main phase of porphyroblastesis occurred between D2 and D3, corresponding to a metamorphic temperature peak of ca. 640-650°C at pressures of ca. 8-9 kbar. D3 produced open folds oblique to the general Alpine trend ("crossfolding”) and locally a crenulation cleavage with a well developed crenulation lineation, at estimated temperatures of 550-610°C. The last important phase, D4, caused open backfolding of all pre-existing structures and is responsible for steepening of the main S2 foliation, to produce the Northern Steep Zone, and for a regional rotation of S3 and L3. D4 developed at T ≥ 550°C and P ≥ 3 kbar. The Lebendun nappe is a complicated structure developed as the result of non-coaxial fold interference related to D1 and D2. From tectono-stratigraphic evidence, the rocks of the Lebendun nappe are interpreted as pre-Triassic in ag

Petrology and Trace Element Budgets of High-pressure Peridotites Indicate Subduction Dehydration of Serpentinized Mantle (Cima di Gagnone, Central Alps, Switzerland)

At Cima di Gagnone, garnet peridotite and chlorite harzburgite lenses within pelitic schists and gneisses correspond to eclogite-facies breakdown products of hydrated peridotites and are suitable for studying dehydration of serpentinized mantle. Thermobarometry and pseudosection modelling yield peak temperatures of 750-850°C and pressures <3 GPa. The minimum temperature recorded by the garnet peridotite corresponds to the maximum conditions experienced by the chlorite harzburgite, suggesting that these rocks recrystallized cofacially at ∼800°C. Alternatively, they might have decoupled during subduction, as achieved in tectonically active plate interface boundaries. The major and rare earth element (REE) variability of the peridotites was mostly acquired during pre-subduction mantle evolution as a result of partial melting and reactive melt flow. The ultramafic suite is also characterized by fluid-mobile element enrichments (B, Pb, As, Sb, Cs, Li, U, Be), which confirm derivation from variably serpentinized protoliths. Similarity in the U, Pb, B, Li and Sr contents of the Gagnone peridotites to present-day oceanic serpentinites suggests that these elements were partly taken up during initial serpentinization by seawater-derived fluids. Positive Be, As and Sb anomalies suggest involvement of fluids equilibrated with crustal (metasedimentary) reservoirs during subsequent subduction metamorphism and peridotite entrainment in (meta)sediments. Fluid-mobile element enrichment characterizes all peak eclogitic minerals, implying that multiple hydration events and element influx pre-dated the eclogite-facies dehydration. Peak anhydrous minerals retain B, Li, As and Sb concentrations exceeding primitive mantle values and may introduce geochemical anomalies into the Earth's mantle. The relatively low contents of large ion lithophile elements and light REE in the Gagnone peridotites with respect to much higher enrichments shown by metasomatized garnet peridotite pods hosted in migmatites (Ulten Zone, Eastern Alps) suggest that the crustal rocks at Gagnone did not experience partial melting. The Gagnone garnet peridotite, despite showing evidence for chlorite dehydration, retains significant amounts of fluid-mobile elements documenting that no partial melting occurred upon chlorite breakdown. We propose that the Gagnone ultramafic rocks represent a prime example of multi-stage peridotite hydration and subsequent dehydration in a plate interface settin

Distribution of authigenic albites in a limestone succession of the Helvetic Domain, eastern Switzerland

A new occurrence of authigenic albite was found in a Jurassic sedimentary succession of the Glarus Nappe near Walenstadt (eastern Switzerland). The euhedral shape and the chemical purity of these albites are evidence for their authigenic origin. The crystals are irregularly distributed in the sediment, highlighting the importance of the host rock composition for albite authigenesis. The crystals occur exclusively in limestones with a carbonate content higher than 80 wt-%. A diagenetic or hydrothermal origin of albite authigenesis is discussed for the studied region. Clay mineral transformation appears to be an important source of ions for albite formatio

Analytical evidence of amorphous microdomains within nitridosilicate and nitridoaluminosilicate single crystals

Single crystals of new nitridosilicates and nitridoaluminosilicates with excellent R values in X-ray investigations were analysed quantitatively using 30 to 60μm single-spot LA-ICP-MS. Significant discrepancies between expected and measured chemical composition could not be explained by the crystallographic data. High spatial resolution analysis using electron probe microanalysis (EPMA, 10μm) leads to the discovery of inhomogeneities in the crystalline material. The application of standard single-spot LA-ICP-MS with a spatial resolution of 30 to 60μm is not suitable for the analysis of these crystals as the existing inhomogeneities dominate and alter the determined concentrations. However, owing to the better detection capabilities, a scanning LA-ICP-MS procedure enables a more representative analysis of single crystals of Ca5Si2Al2N8 than single-spot LA-ICP-MS as a result of a larger sampling volume. It is highly likely that these impurities consist of amorphous, vitreous phases as powder diffraction X-ray data indicates the existence of a significant fraction of an X-ray amorphous material besides crystalline silicates. These microdomains contain less aluminium, silicon and calcium or are nearly free of aluminium, which explains the detected discrepancies in the chemical compositio

Tectonometamorphic history of the Gruf complex (Central Alps): exhumation of a granulite-migmatite complex with the Bergell pluton

We describe field occurrences of sapphirine-bearing granulites, charnockites and migmatites in the Gruf complex, Central Alps and present a new geological map and a structural analysis of the entire Gruf complex for the first time. We have carried out an accurate analysis of the relationships between granulite facies metamorphism, migmatisation and deformation within the complex, in relation to the intrusion of the Bergell pluton. Granulites and charnockites display fabrics different from those defined by the regional foliation and lineation, which are, typically for migmatites, disordered on the mesoscale. On a regional scale, strike variations are also related to the structural complexity of migmatites within which no major antiform could be identified. Irregular interfingering of sub-parallel leucosome veins and back-veining along the contact between the Gruf migmatites and the Bergell tonalite are evidence for contemporaneous emplacement and crystallisation at about 740°C and 6.5-7.5kbar in Oligocene times (ca 30Ma). Metamorphic conditions in the charnockites and granulites (>920°C for 8.5-9.5kbar) largely exceed these regional metamorphic conditions and are dated at 282-260Ma. We propose that the ascending Bergell pluton entrained the polymetamorphic, granulitic lower crust enclosed within the peripheral migmatitic Gruf comple

The role of the antigorite + brucite to olivine reaction in subducted serpentinites (Zermatt, Switzerland)

Metamorphic olivine formed by the reaction of antigorite + brucite is widespread in serpentinites that crop out in glacier-polished outcrops at the Unterer Theodulglacier, Zermatt. Olivine overgrows a relic magnetite mesh texture formed during ocean floor serpentinization. Serpentinization is associated with rodingitisation of mafic dykes. Metamorphic olivine coexists with magnetite, shows high Mg# of 94–97 and low trace element contents. A notable exception is 4 µg/g Boron (> 10 times primitive mantle), introduced during seafloor alteration and retained in metamorphic olivine. Olivine incorporated 100–140 µg/g H2O in Si-vacancies, providing evidence for low SiO2-activity imposed by brucite during olivine growth. No signs for hydrogen loss or major and minor element diffusional equilibration are observed. The occurrence of olivine in patches within the serpentinite mimics the former heterogeneous distribution of brucite, whereas the network of olivine-bearing veins and shear zones document the pathways of the escaping fluid produced by the olivine forming reaction. Relic Cr-spinels have a high Cr# of 0.5 and the serpentinites display little or no clinopyroxene, indicating that they derive from hydrated harzburgitic mantle that underwent significant melt depletion. The enrichment of Mg and depletion of Si results in the formation of brucite during seafloor alteration, a pre-requisite for later subduction-related olivine formation and fluid liberation. The comparison of calculated bulk rock brucite contents in the Zermatt-Saas with average IODP serpentinites suggests a large variation in fluid release during olivine formation. Between 3.4 and 7.2 wt% H2O is released depending on the magnetite content in fully serpentinized harzburgites (average oceanic serpentinites). Thermodynamic modelling indicates that the fluid release in Zermatt occurred between 480 °C and 550 °C at 2–2.5 GPa with the Mg# of olivine varying from 68 to 95. However, the majority of the fluid released from this reaction was produced within a narrow temperature field of < 30 °C, at higher pressures 2.5 GPa and temperatures 550–600 °C than commonly thought. Fluids derived from the antigorite + brucite reaction might thus trigger eclogite facies equilibration in associated metabasalts, meta-gabbros, meta-rodingites and meta-sediments in the area. This focused fluid release has the potential to trigger intermediate depths earthquakes at 60–80 km in subducted oceanic lithosphere. © 2020, The Author(s).ISSN:1661-8734ISSN:1661-872

Characterizing landscape-scale erosion using 10Be in detrital fluvial sediment: Slope-based sampling strategy detects the effect of widespread dams

Concentrations of in situ 10Be measured in detrital fluvial sediment are frequently used to estimate long-term erosion rates of drainage basins. In many regions, basin-averaged erosion rates are positively correlated with basin average slope. The slope dependence of erosion allows model-based erosion rate estimation for unsampled basins and basins where human disturbance may have biased cosmogenic nuclide concentrations in sediment. Using samples collected from southeastern North America, we demonstrate an approach that explicitly considers the relationship between average basin slope and erosion rate. Because dams and reservoirs are ubiquitous on larger channels in the field area, we selected 36 undammed headwater subbasins (average area 10.6 km2) from which we collected river sand samples and measured 10Be concentrations. We used these data to train a predictive model that relates average basin slope and 10Be-inferred erosion rate. Applying our model to 28 basins in the same region previously studied with 10Be, we find that the model successfully predicts erosion rates for basins of different sizes if they are undammed or if samples were collected \u3e25 km downstream of dams. For samples collected closer to dams, measured erosion rates exceed modeled erosion rates for two-thirds of the samples. In three of four cases where paired samples were collected upstream of reservoirs and downstream of the impounding dam, 10Be concentrations were lower downstream. This finding has implications for detrital cosmogenic studies, whether or not samples were collected directly downstream of dams, because dams obstruct most major rivers around the world, effectively trapping sediment that originated upstream

Continuity in geochemistry and time of the Tertiary Bergell intrusion (Central Alps)

The calc-alkaline Bergell intrusion is classically mapped as a main tonalite and main granodiorite unit, whose ages have been determined at 32 and 30Ma, respectively. These units are separated by a mostly thin band interpreted as magmatic "transition zone”. High precision U-Pb dating of zircons and whole rock geochemistry of transition zone rocks are combined with a compilation of available geochemical analyses and ages to characterize the emplacement and magmatic evolution of the Bergell intrusion. Detailed field work shows that the transition zone is heterogeneous, characterized by multiple intrusions of tonalites to granodiorites to quartz-monzonites containing mafic enclaves and leucocratic dykes. Commonly, magmatic flow structures such as compositional banding, schlieren and accumulation of K-feldspar megacrystals are observed. The compositions of the transition zone intrusives as well as of the geochemically fairly heterogeneous tonalite and granodiorite units span a continuous range from 49-76wt% SiO2 for mafic enclaves to leucocratic dykes. Major and trace element trends are consistent with a predominant fractional crystallization. High precision U-Pb dating of single zircon grains separated from tonalites and granodiorites of the transition zone yields crystallization ages of 31.22±0.04 and 31.13±0.10Ma, respectively. Hence, the transition zone represents an own phase of intrusion during which a range of magma types were coeval. The available ages and compositional data suggest magmatic activity over the entire range from 28 to 33Ma across the Bergell intrusion, which should not be described by two main "tonalite” and "granodiorite” stages but by a model with several magmatic main phases during which most magma types developed multiply. Geobarometry of tonalite boulders in the Como molasse (yielding a zircon crystallization age of 32.06±0.13Ma) indicate average erosion rates for the Bergell intrusion of 1.9mm/a between 32-25Ma and of 0.5mm/a since then

Structural geology and petrography of the Naret region (northern Valle Maggia, N.Ticino, Switzerland)

ISSN:1661-8734ISSN:1661-872