Fragments of subducted slow-spreading oceanic lithosphere are exposed continuously in the Liguro-Piemont domain of the Western Alps. By combining new and literature petrological data, interpolated ...maps of maximum temperatures, maximum Si contents of phengite as a proxy for peak pressure and thermodynamic modelling, we provide a detailed framework of the peak metamorphic conditions experienced by the distinct subduction slices. High-resolution mapping confirms the marked eastward increase in metamorphic grade throughout the domain, as well as within some slices. The compilation of lithostratigraphic, structural and radiochronological data and the estimation of sediment/mafic-ultramafic ratio for each slice allow refining the origin of these tectonometamorphic units within the former oceanic domain. The refined structural sketchmap allows to restore the geometries of the Alpine subduction at peak burial conditions. Results point to a trimodal distribution of the units with an increase in metamorphic conditions from the Upper (LPU; 320-400°C- 1.2-1.9 GPa) to the Middle (LPM; 415-475°C- 1.7-2.2 GPa) and to the Lower units (LPL; 500-580°C- 2.2-2.8 GPa). The blueschist-facies LPU and LPM units are dominated by sediments (>90%), whereas the eclogitic LPL units are far richer in mafic-ultramafic rocks (>40%). These characteristics, along with lithostratigraphic differences, reflect major differences in their initial paleogeography and/or in the mechanisms responsible for material offscraping from the downgoing slab. The peak burial depths of the LPU, LPM and LPL units are similar to those inferred for slicing and underplating in both modern and fossil subduction zones. Petrological and lithostratigraphic data suggest that the offscraping of the LPU and LPM units was mostly controlled by lithological contrasts, within pelagic shales or along contacts within the uppermost serpentinized mantle. In contrast, major dehydration reactions (such as lawsonite breakdown in sediments) likely controlled the offscraping of the LPL units at eclogite-facies conditions, possibly through high fluid pressure conditions and rocks embrittlement.
•High-resolution mapping of metamorphic grade across the Western Alps.•Trimodal distribution of tectonometamorphic units with diagnostic lithostratigraphy.•Peak burial depths of Alpine units similar to modern and fossil subduction records.•Marked lithological contrasts controlled the offscraping of blueschist-facies units.•Dehydration reactions may largely control offscraping at eclogite-facies conditions.
The Schistes Lustrés form a large and complex unit at the top of the Penninic nappe stack of the Alpine belt. Calcschists, partly of Late Cretaceous age, constitute the dominant lithology. They are ...closely associated both with blueschist facies Piemont-Ligurian ophiolites and continent-derived Mesozoic metasediments. The question of whether the Schistes Lustrés originated on continental or oceanic crust has been extensively debated among Alpine geologists and is locally still controversial. We present here new structural and stratigraphic observations, as well as Raman graphite thermometry (RSCM) data, for the Schistes Lustrés complex of the Combin zone in the Hérens, Dix and Bagnes valleys. Our observations indicate that the basal part of this Schistes Lustrés complex (defined as the Série Rousse) is systematically devoid of ophiolitic material, and rests in stratigraphic contact on the underlying Triassic - Lower Cretaceous metasediments and Paleozoic basement of the Mont Fort nappe (Prepiemont paleogeographic domain). The unconformity at the base of the Schistes Lustrés complex is interpreted as resulting from the sedimentation of the Série Rousse on a paleorelief formed by remnants of Jurassic normal fault scarps, and not as an Alpine tectonic contact, as previously proposed. The lithostratigraphic comparison with the Breccia nappe in the Prealps, as well as a foraminifer discovery, allows us to better constrain the age of the Série Rousse. It extends from the middle of the Early Cretaceous (Aptian?) to the Late Cretaceous (Campanian to earliest Maastrichtian?). In contrast, the upper contact of the Série Rousse with the ophiolite-bearing Schistes Lustrés clearly corresponds to an Alpine thrust. The thrust zone is underlined by thin and discontinuous slices of highly strained continental-margin derived Mesozoic metasediments (Frilihorn slices). RSCM data show that the recrystallization of the organic matter progressively increases on both sides towards this contact. This contact, internal to the Schistes Lustrés complex, is reinterpreted as the major tectonic contact separating the Middle Penninic Mont Fort nappe from the Upper Penninic Tsaté nappe (defined here as including only the ophiolite-bearing Schistes Lustrés and associated meta(ultra-)basites). This study clearly documents that the Schistes Lustrés consist of sediments either deposited on oceanic crust, showing locally preserved stratigraphic contacts with ophiolitic or serpentinized sub-continental mantle slivers, or sediments still resting stratigraphically on a former hyper-extended continental margin.
The Chenaillet Ophiolite is one of the best-preserved remnants of the Piemont-Liguria oceanic basin, a branch of the Central Atlantic that opened during the separation of Adria/Africa from Europe. ...Despite numerous studies of structure, petrology, geochemistry and isotope geochronology, the timing and genesis of various magmatic rocks within the Chenaillet Ophiolite are still controversial. We provide in this study integrated in situ analyses of zircon U–Pb age and O–Hf isotopes for the troctolite and albitite within the Chenaillet Ophiolite. Our new results indicate that the troctolite and albitite crystallized synchronously at ~165Ma. Zircons from the troctolite have homogeneous Hf and O isotopic compositions, with εHf(T)=+13.5±1.0 (2SD) and δ18O=5.4±0.4‰ (2SD), indicating crystallization from magmas that were derived from a depleted, MORB-like mantle. The albitite zircons give consistent εHf(T) values (+13.0 to +13.5) within errors with those of troctolite zircons, but variable δ18O values. The altered zircon domains have relatively low δ18O values of 4.7±0.6‰ (2SD) due to subsolidus hydrothermal alteration, whilst the least-altered zircon domains give δ18Ozir values of 5.1±0.4‰ (2SD), indistinguishable within errors with the troctolite zircons and the igneous zircons from the Mid-Atlantic and Southwest Indian Ridges gabbros, norites, and plagiogranites of modern oceanic crust. In situ zircon O–Hf isotopic data suggest that the troctolites and albitites are most likely cogenetic, with the albitites being formed by extreme fractional crystallization from the basaltic magma.
Based on our new age results and compilation of the reliable literature U–Pb age data, the ophiolitic gabbros from Eastern, Central and, Western Alps, Liguria and Corsica crystallized nearly synchronously at 158–166Ma, suggesting a short life span of <11m.y. for the formation of the Piemont-Liguria oceanic domain, rather than a ~30m.y. life span as previously thought. The Chenaillet ophiolite is likely a remnant of embryonic oceanic crust, rather than a piece of “mature” oceanic crust. Provided the spreading velocities of <3cm/yr full rate for magma-poor MOR sequences, the maximum width to the Piemont-Liguria oceanic floor would have been in the order of 300km.
► The troctolite and albitite from the Chenaillet Ophiolite crystallized at ~165Ma. ► Albitite was formed by extreme fractional crystallization from MORB-like magma. ► There is a short life span of <11m.y. for the Piemont-Liguria ocean formation. ► The Piemont-Liguria oceanic floor might have a maximum width of 300km. ► The Chenaillet ophiolite is likely a piece of embryonic oceanic crust.
In mid-nineteenth century Portugal, the practice of irrigation was deemed to be completely backwards in comparison with the prevailing European standards. Distant were the times of Al-Andalus, when a ...so-called green revolution occurred on the Iberian Peninsula. Within this context, Agricultural Hydraulics gained momentum as a scientific field. Although water governance had a long run, the ‘politicisation’ of water issues clearly intensified in the Regeneration political regime. From 1851 onwards, the stability required for national material progress was ensured. The country had been struggling to build roads, railways, telegraphs, ports and, therefore, the Ministry of Public Works was established in 1852 to undertake these architectural and engineering projects. This paper argues that not only due to rivers’ manipulation for agriculture, but also to the rising status of engineers, Agricultural Hydraulics was under the Ministry of Public Works and, consequently, on the sphere of engineers rather than agronomists. Focusing on the making of a military engineer as an expert on Agricultural Hydraulics by sending him abroad and fostering his literary education, this paper reveals how this knowledge was appropriated and transformed into new projects for the Tagus River. Moreover, contrary to recent review that Hydraulic Services were launched in Portugal by 1884, based on the “Plan for Organization of Hydrographic Services on the mainland of Portugal”, this paper highlights the crucial role played by the Portuguese engineer Bento Fortunato Almeida d’Eça through his work as Superintendent of the Tagus River and its Tributaries, twenty years earlier.
The U–Pb ages and the trace element content of zircon U–Pb along with major and trace element whole rock data on gabbroic dikes from the Lanzo lherzolitic massif, N-Italy, have been determined to ...constrain crustal accretion in ocean–continent transition zones. Three Fe–Ti gabbros were dated from the central and the southern part of the massif providing middle Jurassic ages of 161 ± 2, 158 ± 2 and 163 ± 1 Ma, which argue for magmatic activity over few millions of years. Zircon crystals are characterized by high but variable Th/U ratios, rare earth element patterns enriched in heavy rare earths, pronounced positive Ce and negative Eu-anomalies consistent with crystallization after substantial plagioclase fractionation. The zircon trace element composition coupled with whole rock chemistry was used to reconstruct the crystallization history of the gabbros. A number of gabbros crystallized in situ, and zircon precipitated from trapped, intercumulus liquid, while other gabbros represent residual liquids that were extracted from a cumulus pile and crystallized along syn-magmatic shear zones. We propose a model in which the emplacement mechanism of gabbroic rocks in ocean–continent transition zones evolves from in situ crystallization to stratified crystallization with efficient extraction of residual liquid along syn-magmatic shear zones. Such an evolution of the crystallization history is probably related to the thermal evolution of the underlying mantle lithosphere.
Epidote-rich eclogitic metagabbro forms a small body within the Lanzada Window, upper Val Malenco, where it is associated with serpentinites and supracrustal rocks of the Lanzada–Santa Anna Zone ...(LSZ), which lies structurally beneath the Malenco unit. Conventional garnet–clinopyroxene geothermometry and garnet–clinopyroxene–phengite geobarometry indicate that the peak-metamorphic mineral assemblage (garnet + omphacite + epidote + phengite + titanite + apatite + pyrite) equilibrated at ca. 2.0 GPa and 525 °C. The bulk composition is Ca-rich (wollastonite-normative), suggesting that the rock underwent Ca-metasomatism prior to high-
P
metamorphism. The presence of eclogite within the LSZ strengthens the correlation of the LSZ with the blueschist-bearing Avers Bündnerschiefer, and confirms the former existence of a southerly-dipping subduction zone beneath the Malenco unit.
We provide geochemical and geochronological data for gabbro, diorite and albitite samples from the Montgenèvre ophiolite in the Western Alps. This well-preserved remnant of the Piemont–Ligurian ...oceanic basin shows evidence of intra-oceanic deformation and metamorphism, but has suffered minor ductile deformation and metamorphism during the Alpine orogeny. The gabbros have geochemical features and initial Nd isotopic signatures similar to that of Mid-Oceanic Ridge gabbros, indicating that they were extracted from a depleted mantle source with no evidence of continental contamination (
ε
Nd(
T)>+8). Cumulation played an important role in the genesis of these gabbros, and their rare-earth element (REE) patterns are controlled by the major cumulus phases. Modelling the REE data for the gabbros and diorites provides support for the hypothesis that the dioritic magmas have been derived from small-degree (≤5%) partial melting of the surrounding gabbros during shearing at high-temperature (high-T).
Zircons from a leucodioritic vein within sheared gabbros are poorly discordant and cluster the Concordia at 156±3 Ma, while zircons from an albitite lens within the mantle-rocks display concordant ages at 148±2 Ma. In both cases, the zircons show no evidence of inheritance, and these values are interpreted as crystallisation ages. The ∼160–150 Ma age bracket very likely records a late stage in the magmatic history of the Montgenèvre ophiolite, since the emplacement of diorite clearly post-dates the layered gabbros. These radiometric ages correlate well with the Late Bathonian to Early Kimmeridgian stratigraphic age (∼160–140 Ma) of the earliest post-ophiolitic radiolarian sediments. The whole-rock Sm–Nd system of the gabbros yielded an isochron age of 198±22 Ma (
ε
Nd(
T) of +8.8). This value is significantly older than the ∼165 Ma age of Western Alps ophiolitic gabbros published elsewhere, but is similar within error to the ∼180 Ma age of eclogitised gabbros from the Ligurides and Corsica. This early Jurassic age is interpreted as the emplacement age of the gabbroic melts into the mantle lherzolites and could be associated with the early stage of rifting between the European and Austro-Alpine continental margins.
These ages and their interpretation are consistent with the model of asymmetric mantle-denudation by an oblique detachment fault, proposed by Lemoine et al. (1987). This model implies that the newly formed lherzolite–gabbro oceanic domain probably remained close to the spreading centre and therefore experienced slow cooling and low spreading rates. This appears to be correlated by our radiometric results that suggest a life span of at least 30 Ma for the formation of the oceanic crust in this part of the Piemont–Ligurian ocean.
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