Dent Blanche Nappe Research Articles

Missing lawsonite and aragonite found: P\u2013T and fluid composition in meta-marls from the Combin Zone (Western Alps)

We report the first findings of several occurrences of lawsonite and metamorphic aragonite in the meta-sediments from the Combin Zone (Piemonte–Liguria ocean, Western Alps), where the early blueschist-facies episode is poorly documented. New field and metamorphic data (thermodynamic modelling and Raman spectroscopy on carbonaceous material) are used to elucidate the P–T evolution and fluid composition of the Combin Zone and investigate the lawsonite growth and breakdown reactions. Two tectonometamorphic units have been identified within the Combin Zone with distinct geometry, lithological content and P–T conditions. In the higher grade unit, metamorphic aragonite occurs as inclusions in titanite. Lawsonite and garnet were stable at peak P–T conditions (~ 16–17 kbar and 460–480 °C) at very low X(CO2) values. Lawsonite is systematically pseudomorphed, but preserves hourglass zoning or internal fabric associated with the prograde ductile deformation. The lower grade unit (~ 8 ± 1 kbar ~ 370–400 °C) is discontinuously exposed along the western base of the continental Dent Blanche nappe and records P–T conditions similar to those recorded by the Dent Blanche nappe. A metamorphic discontinuity is, therefore, documented between the largest part of the Combin Zone on the one hand, and the Dent Blanche nappe on the other hand. The discovery of lawsonite and metamorphic aragonite allows a better understanding of the large-scale metamorphic structure of the Western Alps.

Fluid and mass transfer along transient subduction interfaces in a deep paleo-accretionary wedge (Western Alps)

Focusing of fluid flow along Western Alps paleo-subduction interfaces (at the base of the Dent Blanche nappe, NW Italy), afforded by high strain rates, is testified by extensive chemical alteration accompanying previously documented shifts in O isotope compositions resulting from fluid-rock interactions at prograde to peak high-P, low-T conditions. Flushing of these structures by H2O-rich fluids led to the development of prominent metasomatic reaction zones within and among disparate rock types including the calc-schists of the Schistes Lustrés complex and m- to km-scale slivers of metabasaltic/metagabbroic and meta-ultramafic rocks.Evidence for pressure solution, to produce a strong interface-parallel cleavage, presumably reflects at least local-scale redistribution of carbonate. Related to this redistribution, some samples exhibit evidence for albite dissolution and replacement by calcite ± dolomite ± paragonite. White mica, which exhibits high-Si cores related to prograde metamorphism is variably replaced by muscovite-rich rims interpreted as related to the influx of a CO2-bearing fluid at isobaric conditions based on thermodynamic modeling. Ratios of Ba, Rb, Cs, and N to K, all largely housed in micas (for N, as NH4+), indicate up to 60% loss of these elements within a few 10 s of meters of the interface contacts, presumably reflecting the preferential partitioning of these more fluid-mobile elements into the infiltrating fluids. Whole-rock δ15Nair shows some increase approaching the interface structures, from values of +3.8‰ to near +5.0‰, a trend consistent with Rayleigh-like loss during the partitioning of N into fluids from recrystallizing white-mica.Updip fluid flow along the studied structures resulted in the deposition of carbonate and SiO2, as reflected in the abundant, variably transposed, carbonate + quartz vein generations and more pervasive carbonation (within metasedimentary rocks) and silicification (within talc-rich ultramafic rocks). The O isotope shifts proximal to the structures appear to reflect the sourcing of the infiltrating fluids at greater depths in mafic/ultramafic parts of the down-going slab or structurally complex interface region. Although models of subduction-zone volatile cycling implicitly involve the flushing of fluids along or across subduction interfaces, the permeability of these regions, among disparate rocks types and evolving in response to the localization of deformation, likely dictates the magnitude of this transfer. Field-based studies of this type can yield insight regarding mechanisms of m- to km-scale fluid mobility but, given the fragmentary nature of the metamorphic rock record, do not permit quantitative assessment of the cycling of elements such as C at any individual modern (or ancient) margin.

Metamorphic Response to Alpine Thrusting of a Crustal-scale Basement Nappe in Southern Calabria (Italy)

Abstract Structural, geophysical and age data indicate that the tilted cross-section of the Variscan continental crust exposed in the Serre of southern Calabria forms the uppermost Alpine nappe (‘Serre nappe’) of three Variscan basement slices derived from the southern European margin. This Alpine nappe stack is a fragment of the western Mediterranean Alps and rests now, after Miocene emplacement, on top of the Apennine carbonate platform. We report for the first time a P–T path for prograde Alpine metamorphism, which is restricted to the two lower nappes (Castagna and Bagni nappes) that are squeezed between cooler tectonic units, the Serre nappe above and the Apennine platform below. Therefore, we attribute their metamorphism to tectonic loading and concomitant shear heating during Eocene south-directed overthrusting of the crustal-scale Serre nappe. In the underlying Castagna nappe, Alpine metamorphism is only locally recorded, mainly by new growth of garnet, forming at the expense of retrogressed Variscan biotite dated at 43 Ma. The local existence of Alpine besides relict Variscan mineral assemblages in the strongly but heterogeneously overprinted rocks allows for characterization of metamorphic evolutions during both the Alpine and Variscan orogeneses in the former intermediate level of the Variscan crust of Calabria. The metamorphic evolutions have been reconstructed through P–T pseudosection modelling for Al-rich metasediments. In the Castagna nappe, rarely preserved Variscan garnet–sillimanite–biotite–ilmenite–plagioclase–quartz (± K-feldspar ± Si-poor white mica) assemblages formed under amphibolite-facies subsolidus conditions (650 ± 60 °C, 4·0 ± 0·5 kbar). During subsequent decompression and cooling to greenschist-facies conditions garnet was replaced by biotite–sillimanite and later by white mica–chlorite intergrowths. Retrogression of Variscan biotite is evidenced by the exsolution of ilmenite along grain boundaries and cleavages, textures that were subsequently overgrown by Alpine garnet coexisting with Si-rich white mica, rare chloritoid (in metapelites), and hornblende (in metagreywackes). Alpine garnet shows prograde zoning and is Ca-rich, and thus is distinct from unzoned and Ca-poor Variscan garnet porphyroblasts. Estimated conditions (520 ± 40 °C, 8·0 ± 1·0 kbar) record elevated pressures during Alpine lower amphibolite-facies metamorphism. In the lowermost Bagni nappe, rare prograde-zoned, Ca-rich garnet in strongly retrogressed mylonitic quartz-phyllites allows isopleth thermobarometry, which indicates lower amphibolite-facies conditions (555 ± 10 °C, 7·4 ± 0·3 kbar) resembling those for Alpine garnet growth in the Castagna nappe. The similar clockwise P–T paths for prograde Alpine metamorphism and the consistent peak pressures of 7–9 kbar in the Castagna and Bagni nappes point to a joint short-lived metamorphism during overthrusting of the crustal-scale Serre nappe within the south European margin during the north-directed subduction of the Alpine Tethys. South-directed overthrusting of the now tilted Variscan crustal section of the Serre along the up to 500 m thick mylonite horizon of the Curinga–Girifalco Line is in agreement with seismic data indicating an extended, few kilometres thick low-velocity zone (Bagni and Castagna nappes and mylonites of the Curinga–Girifalco Line) below the exposed lower crustal section of the Serre nappe. Alpine tectonic transport direction, timing and metamorphic conditions described here are consistent with those reported from the Aspromonte area in southernmost Calabria, suggesting a coeval Alpine history characterized by metamorphism owing to nappe loading and concomitant shear heating. The Alpine subduction–erosion–accretion processes inferred here for the Calabrian basement nappes resemble those proposed for the Dent Blanche nappe system in the Western Alps.

Felice Ippolito, il Comitato Nazionale per le Ricerche Nucleari e la ricerca di minerali radioattivi nel basamento cristallino delle Alpi

The National Committee for Nuclear Research (CNRN) was established in 1952 and mainly supported by Felice Ippolito, to promote and develop Italian research in nuclear science and technology. The institution was later incorporated into CNEN (1960) and ENEA (1982). CNRN created the Frascati synchrotron and the nuclear research centers of Ispra in northern Italy and Casaccia near Rome. One of its tasks was the survey for U-Th-bearing ores in Italy, run by the Geology and Mining Division chaired by Felice Ippolito. As regards the Alps, systematic field survey was carried out since 1954 by three geological teams, led by Paolo Baggio, Sergio Lorenzoni and Mario Mittempergher, whereas the scientific assistence and guidelines were entrusted to Angelo Bianchi and Giambattista Dal Piaz, with a view to providing an updated study of the pre-Triassic crystalline basement. This work was synthesized in a typed memoir titled Il Cristallino Antico delle Alpi, con particolare riguardo al versante italiano. Relazione preliminare riservata, Padova 1958, including a synthetic map at 1:1,000,000 scale. From its appearance to the beginning of the eighties, this memoir was the reference work for people addressing such issues as igneous and metamorphic features, as well as structure and overall lithology of the main basement units, all along the inner side of the Alps from Verampio dome - the core of the collisional belt - to the capping Dent Blanche nappe, from the Tauern window to the eastern Austroalpine nappes, and the Southalpine basement.

Permian magmatism and metamorphism in the Dent Blanche nappe: constraints from field observations and geochronology

In the Dent Blanche Tectonic System, the Mont Morion biotite-bearing granite is a km-scale intrusion preserved in a low-strain volume. Zircon saturation thermometry suggests that it crystallised from a melt that reached about 800 °C. U–Pb zircon and allanite geochronology indicates crystallization of the magma in the Permian (290 ± 3 Ma; 280 ± 8 Ma, respectively). Migmatitic biotite-gneiss and amphibolite are found as xenoliths within the Mont Morion granite and constitute its country-rocks. In two samples of migmatitic biotite-gneiss zircon has metamorphic overgrowths that yield U–Pb ages of 285 ± 3 Ma and 281 ± 4 Ma, and are thus contemporaneous with the intrusion of the granite. The Mont Morion granite with its country-rocks of migmatitic biotite-bearing gneiss and amphibolite was thus emplaced at middle crustal levels while amphibolite facies metamorphism affected its country rocks. The magmatic and metamorphic record in the Mont Morion area reflects the high-temperature regime and lithospheric thinning of the Adriatic continental margin during Permian.

Provenance analysis of the Voirons Flysch (Gurnigel nappe, Haute-Savoie, France): stratigraphic and palaeogeographic implications

The Chablais Prealps (Haute-Savoie, France) represent a well-preserved accretionary wedge of the Western Alpine Tethys. They comprise a stack of sedimentary nappes related to palaeogeographic realms ranging from the Ultrahelvetic to the Southern Penninic. The provenance analysis is based on the Gazzi-Dickinson method and on QEMSCAN® for heavy-minerals. The Quartzose petrofacies is the most important of the two sources, and supplied three of the four formations of the Voirons Flysch. It is similar to the sources that fed the other flyschs from the Gurnigel nappe. It is characterised by a mature, quartz-rich assemblage and a heavy-mineral population dominated by apatite and the zircon–tourmaline–rutile mineral group. These observations suggest a Clastic wedge provenance. The Feldspathic petrofacies is derived from a feldspar-rich source associated with metamorphic clasts and a heavy-mineral population dominated by garnet. This provenance characterises only one formation of the Voirons Flysch, and is related to the axial belt provenance. This provenance analysis shows that the Middle Eocene to Early Oligocene Voirons Flysch was fed by two sources, in contrast to the other flyschs of the Gurnigel nappe, and further suggests that this flysch was not deposited in the Piemont Ocean but in the Valais domain. Based on the results and comparative provenance analysis with the other flyschs of the Gurnigel nappe, we propose a generic feeding model which involves the Sesia–Dent Blanche nappe, the sedimentary nappes incorporated in the accretionary prism, and probably the Brianconnais basement.

Polyphase greenschist-facies reactivation of the Dent Blanche Basal Thrust (Western Alps) during progressive Alpine orogeny

This study assesses the significance, geometry, and kinematics of greenschist-facies deformation along the Dent Blanche Basal Thrust (DBBT), a major tectonic contact in the Internal Western Alps of Switzerland and Italy. The DBBT separates continental units of the Dent Blanche nappe, the structurally highest unit in the Western Alps, from underlying Piemont-Ligurian ophiolites. Mylonites and deformation structures along the contact provide a record of its retrograde greenschist-facies evolution after earlier high-pressure metamorphism. A first phase of foreland-directed, reverse-sense, top-(N)W shearing (D1) occurred between ca. 43 and 39 Ma, related to exhumation of the Dent Blanche nappe from high-pressure conditions. It led to the formation of mylonitic fabrics under high- to medium-grade greenschist-facies conditions along the entire DBBT. A phase of ductile normal-sense top-SE shearing (D2) at ca. 38–37 Ma was mainly localized within underlying ophiolitic units and only partly affected the DBBT. Another phase of ductile deformation (D3) under medium- to low-grade greenschist-facies conditions at ca. 36–35 Ma occurred in response to underthrusting of European continental margin units and resulted in the updoming of the nappe stack. Especially the southeastern DBBT was characterized by bulk top-NW shearing, partly conjugate top-NW/top-SE shearing, and resulting orogen-perpendicular crustal extension. Subsequently, the DBBT was affected by a phase of orogen-perpendicular shortening (D4) and formation of folds and crenulations at ca. 34–33 Ma due to increasing compressional tectonics. Finally, a phase of semi-ductile to brittle normal-sense top-NW and conjugate shearing (D5) from ca. 32 Ma onwards particularly affected the southeastern segment and indicates exhumation of the DBBT through the ductile–brittle transition. This was followed by brittle NW–SE extensional deformation. This study suggests that the DBBT experienced a polyphase deformation and reactivation history under decreasing greenschist-facies metamorphic conditions during which different segments of this major shear zone were variably affected.

Kinematics of syn- and post-exhumational shear zones at Lago di Cignana (Western Alps, Italy): constraints on the exhumation of Zermatt–Saas (ultra)high-pressure rocks and deformation along the Combin Fault and Dent Blanche Basal Thrust

Kinematic analyses of shear zones at Lago di Cignana in the Italian Western Alps were used to constrain the structural evolution of units from the Piemont–Ligurian oceanic realm (Zermatt–Saas and Combin zones) and the Adriatic continental margin (Dent Blanche nappe) during Palaeogene syn- and post-exhumational deformation. Exhumation of Zermatt–Saas (U)HP rocks to approximately lower crustal levels at ca. 39 Ma occurred during normal-sense top-(S)E shearing under epidote–amphibolite-facies conditions. Juxtaposition with the overlying Combin zone along the Combin Fault at mid-crustal levels occurred during greenschist-facies normal-sense top-SE shearing at ca. 38 Ma. The scarcity of top-SE kinematic indicators in the hanging wall of the Combin Fault probably resulted from strain localization along the uppermost Zermatt–Saas zone and obliteration by subsequent deformation. A phase of dominant pure shear deformation around 35 Ma affected units in the direct footwall and hanging wall of the Combin Fault. It is interpreted to reflect NW–SE crustal elongation during updoming of the nappe stack as a result of underthrusting of European continental margin units and the onset of continental collision. This phase was partly accompanied and followed by ductile bulk top-NW shearing, especially at higher structural levels, which transitioned into semi-ductile to brittle normal-sense top-NW deformation due to Vanzone phase folding from ca. 32 Ma onwards. Our structural observations suggest that syn-exhumational deformation is partly preserved within units and shear zones exposed at Lago di Cignana but also that the Combin Fault and Dent Blanche Basal Thrust experienced significant post-exhumational deformation reworking and overprinting earlier structures.

Facies and Late Triassic fossils in the Roisan zone, Austroalpine Dent Blanche and Mt Mary-Cervino nappe system, NW Alps

The Roisan zone is a metamorphic cover unit exposed along the ductile shear zone between the Dent Blanche s.s. and Mont Mary-Cervino Upper Austroalpine outliers, Aosta Valley, north-western Italian Alps. It is characterized by the occurrence of dolostones, pure marbles, marbles with quartz, calcirudites and ophiolite-free calcschists. Locally, dolostones preserve alternances of thick massive beds and thinner levels of planar stromatolites and other sedimentary structures and textures typical of a carbonate platform. In Mt Grand Pays they contain Dasycladales and foraminifers referable to the Norian. Pure marbles and marbles with quartz grains are tentatively referred to the end of Triassic–Early Jurassic, thin-bedded marbles and calcirudites to the Early and Middle Jurassic, calcschists from Middle Jurassic to Late Cretaceous. This Roisan succession is quite similar to the one of Mt Dolin, in the Swiss part of the Dent Blanche nappe, where the same Triassic foraminifer association has been reported. There, the fossils were found only in reworked pebbles, contained in calcirudites of presumed Jurassic age. Some differences exist between the two successions: calcirudites are abundant in the Mt Dolin and sporadic in the Roisan zone, whereas calcschists are very thick in the Roisan zone. As consequence the Mt Dolin succession can be considered settled down in the proximity of the faults related to the pre-oceanic rifting of the Piedmont basin, whereas the Roisan zone could have been deposited in a more distal area.

Structural and petrographic map of the Sassa gabbro complex (Dent Blanche nappe, Austroalpine tectonic system, Western Alps, Italy)

The Sassa gabbro complex outcrops in the upper Valpelline and is part of the Permian gabbros of the Collon – Matterhorn group, set in the Dent Blanche nappe of the Western Austroalpine tectonic system. This 1:2500-scale map was constructed through the synergic use of meso- and microscopic analysis of fabrics to identify the relationships of the superposed foliations with the growth of equilibrium mineral assemblages. Throughout the foliation trajectories, a mosaic of rock volumes, in which strain is differently partitioned, is highlighted on the map for all the successive deformation stages. Therefore, this mosaic shows deformation gradients spanning from unstrained or weakly strained domains, where the primary magmatic features are still well preserved, to domains in which the Alpine fabrics are pervasive and obliterate the previous structures. The pre-Alpine history is characterized by the polyphasic emplacement of the Sassa gabbro and by a coronitic growth of successive mineral assemblages during crustal thinning-related exhumation. The Alpine history is characterized by multistage heterogeneous deformation consisting of a first D1 stage developed in blueschist facies conditions followed by D2 and D3 stages developed in greenschist facies conditions. In summary, by applying this analytical method, the geologic traces of the pre-Alpine lithospheric thinning and Alpine subduction histories have been separated.

The pre-Alpine tectonic history of the Austroalpine continental basement in the Valpelline unit (Western Italian Alps)

AbstractThe Valpelline unit is a large slice of continental crust constituting the Austroalpine Dent Blanche nappe (NW Italy). The pre-Alpine evolution of this unit holds important clues about the Palaeozoic crustal structure at the northern margin of the Adria continent, about the history of rifting in the Alpine region, and thus about the thermomechanical conditions that preceded the Alpine convergent evolution. Several stages of the deformation history and of partial re-equilibration were identified, combining meso- and micro-structural analyses with thermobarometry. Reconstructed pre-AlpineP–T–t–d paths demonstrate that the Valpelline unit experienced an early stage at pressures between 4.5 and 6.5 kbar followed by migmatite formation. A subsequent stage reached amphibolite to granulite facies conditions. This stage was associated with the development of the most penetrative fabrics affecting all of the Valpelline lithotypes. The pre-Alpine evolution ended with a weak deformation associated with a local mineral-chemical re-equilibration under greenschist facies conditions at ≈ 4 kbar andT< 450°C. A Permo-Mesozoic lithospheric extension is thought to be responsible for asthenosphere upwelling, thereby causing high temperature metamorphism at medium pressure and widespread partial melting, which led to upper crustal magmatic activity.

Fluid-induced plastic deformation in the crustal Austroalpine system (Western Italian Alps): a petrologic and fluid inclusion analysis di Nadia MALASPINA, Marco SCAMBELLURI, Giorgio PENNACCHIONI & Chiara. SPAGNOLO

This work investigates the interplay between fluids and metamorphic recrystallisation in the Austroalpine Dent Blanche Nappe (Western Alps, Italy). This nappe consists of the Arolla and Valpelline Units, respectively dominated by metagranitoids and paraschists with Pre-Alpine to Alpine polymetamorphic history. In the Arolla Unit the Alpine tectonitic to mylonitic foliation anastomoses around weakly deformed bodies preserving the pre-Alpine igneous textures. In the Valpelline Unit, the Alpine structures are concentrated within a narrow shear horizon at the contact with the Arolla Unit, which parallels the pre-Alpine foliation. This contrast in the Alpine deformation between the Arolla and the Valpelline Units suggests heterogeneous fluid distribution in the Dent Blanche system during the Alpine orogeny. The relict (meta)igneous Arolla rocks record a post-magmatic, pre-Alpine stage of amphibolite-facies hydration at relatively low pressure. A deeper crustal origin of the Valpelline Unit than the Arolla Unit can be envisaged due to the presence of high temperature and pressure assemblages (clinopyroxene, garnet, plagioclase, amphibole) in the mafic rocks. In Arolla granitoids igneous plagioclase records two hydration events recorded by aggregates of: (i) pre-Alpine amphibolite-facies andesine + epidote, and (ii) Alpine greenschist-facies albite + epidote. The quartz crystals associated with the altered magmatic plagioclase contain two-phase (liquid + vapour) and three-phase (liquid + vapour + chloride daughter crystals) inclusions. Isochores of the salt-saturated inclusions plot in the andesine + epidote stability field, pointing to pre-Alpine plagioclase alteration in response to infiltration of saline aqueous solutions at amphibolite-facies conditions. Most isochores of the liquid + vapour inclusions plot in the albite + epidote stability field, suggesting entrapment during the lower temperature greenschist-facies hydration of plagioclase. Preferential fluids influx in the Arolla Unit during pre-Alpine amphibolite-facies metamorphism enhanced the reaction softening of plagioclase. Alteration of plagioclase played a major role in promoting the deformation of granitoids during Alpine tectonics.

Contrasting magma types and timing of intrusion in the Permian layered mafic complex of Mont Collon (Western Alps, Valais, Switzerland): evidence from U/Pb zircon and 40Ar/39Ar amphibole dating

We have selected and dated three contrasting rock-types representative of the magmatic activity within the Permian layered mafic complex of Mont Collon, Austroalpine Dent Blanche nappe, Western Alps. A pegmatitic gabbro associated to the main cumulus sequence yields a concordant U/Pb zircon age of 284.2 ± 0.6 Ma, whereas a pegmatitic granite dike crosscutting the latter yields a concordant age of 282.9 ± 0.6 Ma. A Fe-Ti-rich ultrabasic lamprophyre, crosscutting all other lithologies of the complex, yields an 40Ar/39Ar plateau age of 260.2 ± 0.7 Ma on a kaersutite concentrate. All ages are interpreted as magmatic. Sub-contemporaneous felsic dikes within the Mont Collon complex are ascribed to anatectic back-veining from the country-rock, related to the emplacement of the main gabbroic body in the continental crust, which is in accordance with new isotopic data. The lamprophyres have isotopic compositions typical of a depleted mantle, in contrast to those of the cumulate gabbros, close to values of the Bulk Silicate Earth. This indicates either contrasting sources for the two magma pulses – the subcontinental lithospheric mantle for the gabbros and the underlying asthenosphere for the lamprophyres – or a single depleted lithospheric source with variable degrees of crustal contamination of the gabbroic melts during their emplacement in the continental crust. The Mont Collon complex belongs to a series of Early Permian mafic massifs, which emplaced in a short time span about 285-280 Ma ago, in a limited sector of the post-Variscan continental crust now corresponding to the Austroalpine/Southern Alpine domains and Corsica. This magmatic activity was controlled in space and time by crustal-scale transtensional shear zones.

Modeling of in-situ crystallization processes in the Permian mafic layered intrusion of Mont Collon (Dent Blanche nappe, western Alps)

The Mont Collon mafic complex is one of the best preserved examples of the Early Permian magmatism in the Central Alps, related to the intra-continental collapse of the Variscan belt. It mostly consists (> 95 vol.%) of ol + hy-normative plagioclase-wehrlites, olivine- and cpx-gabbros with cumulitic structures, crosscut by acid dikes. Pegmatitic gabbros, troctolites and anorthosites outcrop locally. A well-preserved cumulative sequence is exposed in the Dents de Bertol area (center of intrusion). P– T calculations indicate that this layered magma chamber emplaced at mid-crustal levels at about 0.5 GPa and 1100 °C. The Mont Collon cumulitic rocks record little magmatic differentiation, as illustrated by the restricted range of clinopyroxene mg-number (Mg# cpx = 83–89). Whole-rock incompatible trace-element contents (e.g. Nb, Zr, Ba) vary largely and without correlation with major-element composition. These features are characteristic of an in-situ crystallization process with variable amounts of interstitial liquid L trapped between the cumulus mineral phases. LA-ICPMS measurements show that trace-element distribution in the latter is homogeneous, pointing to subsolidus re-equilibration between crystals and interstitial melts. A quantitative modeling based on Langmuir's in-situ crystallization equation successfully duplicated the REE concentrations in cumulitic minerals of all rock facies of the intrusion. The calculated amounts of interstitial liquid L vary between 0 and 35% for degrees of differentiation F of 0 to 20%, relative to the least evolved facies of the intrusion. L values are well correlated with the modal proportions of interstitial amphibole and whole-rock incompatible trace-element concentrations (e.g. Zr, Nb) of the tested samples. However, the in-situ crystallization model reaches its limitations with rock containing high modal content of REE-bearing minerals (i.e. zircon), such as pegmatitic gabbros. Dikes of anorthositic composition, locally crosscutting the layered lithologies, evidence that the Mont Collon rocks evolved in open system with mixing of intercumulus liquids of different origins and possibly contrasting compositions. The proposed model is not able to resolve these complex open systems, but migrating liquids could be partly responsible for the observed dispersion of points in some correlation diagrams. Absence of significant differentiation with recurrent lithologies in the cumulitic pile of Dents de Bertol points to an efficiently convective magma chamber, with possible periodic replenishment.

Geology and petrology of the Austroalpine Châtillon slice, Aosta valley, western Alps

Slices of continental crust pertinent to the lower Austroalpine domain of the western Alps, crop out within the ophiolitic Piemonte Zone. Among them, the Châtillon slice was studied in detail. The slice consists of orthogneiss with subordinate metabasics and very minor paraschist. The garnet-phengite-epidote-albite orthogneiss is characterised by polyphase garnet porphyroclasts. Metabasics consist of prasinite lenses and eclogite relics. Phengite-clinozoisite eclogite is characterised by small garnet idioblasts with prograde zoning; jadeite content in omphacite increases towards the rim; Si content in phengite decreases towards the rim. Garnet-glaucophane-phengiteparagonite micaschist is characterised by polymetamorphic garnet porphyroclasts, and small Alpine garnet idioblasts. A pre-Alpine amphibolite-facies metamorphism is inferred for the polymetamorphic rocks of the Châtillon slice. Paragneiss and micaschist probably derive from pre-Alpine “kinzigites”; the orthogneiss protolith was a late-Variscan porphyritic granitoid. Thermobarometry in the eclogite constrains the metamorphic peak at T ≤ 560 °C and P = 16 kbar. The HP minerals were partly retrogressed to greenschist-facies assemblages during the late Alpine tectono-metamorphic recrystallisation. The inferred Alpine P-T conditions are consistent with those for other Penninic and Austro-Alpine nappes of the northwestern internal Alps. The Châtillon slice is very similar to the Eclogitic Micaschists Complex of the Sesia-Lanzo Zone and to the other eclogite-facies Austroalpine slices of the Dent Blanche Nappe, but it could also represent a portion of the Sesia-Lanzo Zone basement, which experienced a somewhat different subduction depth. The tectonic position of the Châtillon slice within the Piemonte Zone is essential to reconstruct the geometric relationships in the Austroalpine-Piemonte nappe stack of the northwestern internal Alps.

Fold-controlled quartz textures in the Pennine Mischabel backfold near Zermatt, Switzerland

Quartz fabric analyses along the basal thrust of the Dent Blanche nappe and in the footfall Pennine units indicate south directed thrusting for the last stage of Austro-alpine/Pennine nappe emplacement. Quartz fabrics—in particular, those from the Pennine Mischabel backfold—imply complex spatial and temporal texture developments with crystallographic preferred orientations (CPO) that are commonly in disagreement with mesoscopic grain shape lineations. In consequence, kinematic directions inferred by quartz textures do not necessarily coincide with those predicted by mesoscopic structural elements such as grain shape lineations. In our particular case, orientation of grain shape lineations is rather complex in the study area, while principal axes inferred from the quartz textures remain geographically constant, and exhibit a simple geometric relationship to the Mischabel backfold. It is suggested that the grain shape lineations correspond to the total accommodated (finite) strain whereas the quartz fabrics reflect late increments of deformation. Quartz CPOs in the hinge of the Mischabel fold are similar to those along the thrust, revealing asymmetric single girdles to c-axes point maxima. Subgrain rotation is the predominant recrystallization mechanism, suggesting steady state conditions in fabric development during progressive rotational deformation. In contrast, asymmetric cross girdles dominate the upper limb and uneven small circle c-axes distributions characterize the lower limb, although recrystallization in both limbs is characterized by grain boundary migration. This clear separation of quartz textures and recrystallization (deformation) mechanism conflicts with the common view on heterogeneous deformation patterns in folds where the major component of rotational shear is expected in the fold limbs. Our study suggests that fabric development in the limbs of the recumbent Mischabel fold was reset during progressive deformation shortly prior to the transition from an active buckling to passive fold amplification, while the hinge suffered continuous bulk deformation provided by the back thrusting of the Dent Blanche nappe.

Horizontal contraction or horizontal extension? Heterogeneous Late Eocene and early Oligocene general shearing during blueschist and greenschist facies metamorphism at the Pennine-Austroalpine boundary zone in the Western Alps

Mylonitic structures related to two orogenic events are described from the upper and lower contacts of the Combin zone and the immediately overlying upper Austroalpine Dent Blanche nappe/Mont Mary klippe and the directly underlying lower Austroalpine Etirol-Levaz slice. The first event, Late Eocene in age, commenced during blueschist facies P-T conditions, but pre-dated the peak of subsequent greenschist facies overprint. The second event, Early Oligocene in age, took place during retrograde greenschist facies conditions. Most sense of shear indicators associated with the retrograde mylonites indicate top SE shearing, but subordinate top NW displacing shear sense indicators have also been mapped. Mylonitic top SE shearing appears to be restricted to the Combin zone and its upper and lower contacts. Within the Dent Blanche nappe and Mont Mary klippe and at the base of the Etirol-Levaz slice, structures were observed which developed during blueschist/greenschist facies conditions and are, in conjunction with the P-T-t history of these rocks, inferred to be older. Associated kinematic data indicate a top NW shear sense. Comparable blueschist/greenschist facies shear sense indicators have not been observed in the Combin zone. Nonetheless, the foliation in the Combin zone shows a progressive evolution from blueschist facies to greenschist facies to retrograde greenschist facies conditions. This indicates that the Combin zone and the immediately over- and underlying Austroalpine units shared a common tectono-metamorphic evolution since the Late Eocene. Finite strain data reveal oblate strain fabrics, which are thought to result from a true flattening strain geometry. Flow path modelling reveals a general non-coaxial deformation regime and corroborates significant departures from a simple shear deformation. In the study area, mylonitic top SE shearing in the Combin zone is attributed to Early Oligocene backfolding and backthrusting of the Mischabel phase. Temperature-time curves suggest slight reheating in the Monte Rosa nappe underneath and cooling in the Dent Blanche nappe above the Combin zone, hence confirming a thrust interpretation for this event. The top NW displacing structures are thought to result from Late Eocene emplacement of the Dent Blanche nappe and the Combin zone onto the Middle Pennine Barrhorn series along the Combin fault. As related structures initiated during mildly blueschist facies conditions in the Dent Blanche nappe and the underlying Combin zone and both were emplaced together onto the greenschist facial Barrhorn series, it is concluded that the structures developed as the nappes moved upward relative to the earth's surface. Thus the Combin fault is regarded as a thrust. The geometry of this structure indicates that the Combin fault is an out of sequence thrust that locally cut down section. Hence, top NW out of sequence thrusting caused local thinning of the metamorphic/structural section in association with horizontal shortening. Out of sequence thrusts cutting down section, and back-thrusts, offer the possibility of explaining the pronounced break in the grade of metamorphism across the Combin fault, i.e. the contact between the eclogite facial Zermatt-Saas zone and the overlying lower grade Combin zone, by contractional deformation.

Constraints on the late thermotectonic evolution of the western Alps: EVidence for episodic rapid uplift

Forty‐two new apatite and zircon fission track ages are presented for samples from the Western Alps in southern Switzerland, northern Italy, and southeastern France. Measured ages plotted against assumed closure temperatures yield cooling patterns for the final cooling, uplift, and exhumation of the Western Alps. Similar fission track zircon ages in the Penninic Gran Paradiso massif, Dent Blanche nappe, Sesia‐Lanzo Zone, and Ivrea Zone indicate cooling of all four units to ∼225°C by 33 Ma. Differences in apatite ages reveal differential cooling of the four blocks between 33 Ma and the present. In the Sesia‐Lanzo Zone, similarity of apatite ages regardless of elevation, together with near‐volcanic confined fission track length patterns suggest rapid cooling and uplift at ∼25 Ma compared with slow cooling of other Western Alps units around 12 Ma. Uplift is thus not continuous but episodic, often over a short time interval beyond the resolution of other methods. Such episodes of uplift, as revealed here in the Sesia‐Lanzo Zone, may be the rule rather than the exception.

Structural evolution and metamorphism of the Dent Blanche nappe and the Combin Zone west of Zermatt (Switzerland)

Structural evolution and metamorphism of the Dent Blanche nappe and the Combin Zone west of Zermatt (Switzerland)

The Alps: a case of multiple collision

Summary The numerous isotopic data accumulated during the last decade throughout the Western Alps, make it tempting to draw a picture of polyphase Alpine tectono-metamorphic history constrained by these data. Gabbroic precursors of the Mesozoic Piémont ophiolites in the Dent Blanche Nappe, point to a thinning of the continental crust by the beginning of the Triassic. Biotite cooling ages of ∼ 180 Ma, in the Valpelline-IIDK-Ivrea Unit, set an older limit to the beginning of Alpine nappe emplacement. The younger limit is given by Cretaceous ages on high-pressure paragenesis in the Sesia basement and the Piémont ophiolites. In both, this first eo-Alpine tectono-metamorphic cycle was overprinted by an Eocene-Oligocene greenschist-facies metamorphism and a recumbent schistosity. Miocene mica cooling ages, as well as apatite fission track ages, can be linked to the Miocene décollement and ‘rétrocharriage’ phase in the Western Alps, followed by isostatic re-equilibration and denudation.