The tholeiites of the Valaisan domain (Versoyen, western Alps): a Carboniferous magma emplaced in a small oceanic basin

International audienceThe mafic-ultramafic assemblages of the Versoyen complex exposed in the Valaisan domain is close to the boundary between the Internal and the External domains of the western Alps. Zircons extracted from the Versoyen complex suggest an emplacement during Paleozoic times, and probably during the Visean (~337 Ma). The base of the Versoyen complex is formed of laccoliths and sills associated with black shales, while pillow basalts and tuffs predominate at the uppermost levels. Locally, basaltic dikelets intruded leucocratic gneiss. Ultramafic-mafic cumulates form the bottom of the thickest intrusions while diabases are present along the chilled margins. All these rocks have been affected by a polyphased metamorphism under eclogitic to blueschist and greenschist facies conditions. Magmatic textures have been destroyed and the igneous mineralogy is seldom preserved. The mafic rocks of the Versoyen complex show tholeiitic to alkali-transitional affinities. The pillow basalts and the sill cores have flat REE patterns characteristic of N-MORB and T-MORB. Their {varepsilon}Nd (assuming an age of 337 Ma) ratios range from + 5.7 to + 9 which suggest a mixing of N-MORB and OIB sources. The sill margins show Th, U and LREE-enrichments and negative {varepsilon}Nd ratios. These features are likely related to contamination when hot mafic magmas intruded unconsolidated sediments rich in water. The high Th, U, LREE abundances and low {varepsilon}Nd ratio of the basaltic dikelet are probably related to crustal contamination occurring during magma ascent. The geochemical characteristics of the Versoyen rocks are compatible with a tholeiitic magma emplaced into a small oceanic basin in the vicinity of a continent. The importance of pre-Mesozoic crustal thinning evidenced in one segment of the boundary between the Internal and External zones of the Alps suggests that the Pennine Front is an Alpine mega-thrust inherited from a Variscan suture

L'ÉVOLUTION MAGMATIQUE ET TECTONO-MÉTAMORPHIQUE DU SUBSTRATUM DU DOMAINE VALAISAN (COMPLEXE DU VERSOYEN, ALPES OCCIDENTALES) - IMPLICATIONS DANS L'HISTOIRE ALPINE

The mafic-ultramafic igneous assemblage of the Versoyen complex exposed in the Valaisan domain mark the boundary between the internaI and the external zones of the western Alps. The geodynarnical environment of this complex remains questionable. Indeed, this complex has been interpreted as: (1) a Piemont-Ligurian klippe (hypercollision model) , (2) an ophiolitic suture (accretionary complex model), (3) a magmatic complex emplaced into a basin floored by a thin continental crust (model of structural inversion of an inherited structure). . The aim of this work was to precise the geodynarnic signification of the Versoyen complex on the basis of a muItidisciplinary approach. The main results are developed hereafter. - The ultramafic-mafic igneous rocks of the Versoyen complex display tholeiitic affinities. In the vicinity of the Petit-Saint-Bernard pass (France-Italy) and Visp (Switzerland), these tholeiites show fiat REE patterns, characteristic of N-MORB and T-MORB. Theil' ENd ratios 0 suggest the mixing of Normal -Mid Oceanic Ridge Basait (N-MORB) and Oceanic Island BasaIt (Orn) sources. - Near the Petit-Saint-Bernard pass, zircons extracted from a felsic dike of the Versoyen complex cogenetic with the mafic rocks of the complex define concordia intercept ages of 294 ± 1 Ma and 3247 ± 3 Ma, which can be interpreted to reflect dike emplacement during Paleozoic times and Archean primary zircon formation in the magma source material respectively. - The Versoyen complex has been affected by a polyphased metamorphism under eclogitic to blueschist and greenschist facies conditions. The Pressure-Temperature (P-T) conditions of the eclogitic assemblage could be related to subduction process. The Ar/Ar ages of z 33 Ma which correspond to the cooling ages have been used to define the Pressure-Temperature-time (P-T-t) path during the exhumation of the eclogites. - The Versoyen complex is affected by normal faults dipping SE. These fauIts are linked to ductile and semi-brittle shear zones and took place under greenschist facies conditions. These normal fauIts developed after 38 Ma are contemporaneous with the shortening which occurs in the external zones of the western Alps during this period. These fauIts are likely responsible for the exhumation of the eclogites and could accomodate the crustal thinning during the Alpine collision. These new results imply to reconsider the geodynarnic signification of the Versoyen complex. The emplacement of the tholeiitic rocks took place during the late Hercynian while the tectonometamorphic evolution of the complex is related to the Alpine history.Le domaine valaisan dessine la suture majeure qui marque la limite entre les zones internes et externes des Alpes occidentales et dont l'interprétation géodynamique était controversée. Cette suture est constituée d'une série de flysch (le flysch valaisan) et d'un complexe magmatique et sédimentaire (le complexe du Versoyen). Suivant les auteurs, les roches magmatiques d'affinité tholéïtique pourraient représenter: 1) une klippe d'origine piémontaise (suture d'hyper-collision), 2) une écaille ophiolitique située au front d'un prisme d'accrétion (suture océanique), 3) un complexe magmatique lié à un amincissement crustal (inversion structurale). Le but de ce travail était de trouver des arguments qui permettaient de résoudre cette controverse. Ainsi les résultats de ce travail montrent que : - Dans la région du col du Petit-Saint-Bernard (frontière franco-italienne), certaines tholéïtes sont recoupées par des filons leucocrates qui correspondent à des liquides différenciés, cogénétiques du magmatisme. Les datations UlPb sur les zircons contenus dans l'un de ces filons indiquent un âge Carbonifère supérieur- Permien inférieur pour le magmatisme du Versoyen. - Ce magmatisme présente des caractères géochimiques et isotopiques, intermédiaires entre ceux des N-MORB et des T-MORB, dans les régions du col du Petit-Saint-Bernard et de Visp (Suisse). Ces tholéïtes dériveraient de la fusion partielle d'un manteau appauvri (de type N-MORB), avec probablement la participation d'une source enrichie (de type OIB), ce qui est en accord avec une mise en place dans un domaine en cours d'océanisation. - Le complexe du Versoyen est affecté par un métamorphisme polyphasé éclogitique, puis schiste bleu et enfin schiste vert. La paragénèse éclogitique correspond à des conditions de Haute-Pression et Basse-Température (P > 13Kb, 425 < T < 475°C) qui traduisent un enfouissement à grande profondeur, lié à une subduction. Les datations Ar/Ar réalisées sur les phengites donnent des âges de refroidissement proches de 33 Ma et permettent d'établir le chemin P-T-t de ce complexe au cours de l'exhumation des éclogites. - Le complexe du Versoyen est affecté par une déformation syn-schiste vert qui correspond à un jeu normal vers le SE. La comparaison entre les données de terrain et les données sismiques ECORS suggère que les failles normales se prolongent en profondeur et s'applatissent vers 10-15 km. Cette déformation postérieure à 38 Ma explique en partie l'exhumation des éclogites. Ce jeu normal est contemporain de chevauchements dans la zone externe et pourrait accommoder un réamincissement crustal au cours de la collision alpine. Ces données montrent que l'individualisation du substratum du domaine valaisan est liée au cycle hercynien et que ses relations complexes avec le flysch sus-jacent sont liées à une inversion structurale anté-flysch, alors que son évolution tectono-métamorphique est controlée par une extension succèdant aux phases compressives

Late orogenic evolution of the Variscan lithosphere: Nd isotopic constraints from the western Alps

International audienceWe performed geochemical analyses (major- and trace-elements, Nd isotopes) of Late Variscan igneous rocks from the western Alps in order to better constrain the lithospheric evolution at the end of the Variscan orogeny. In the western Alps, the shoshonitic suite from the Croix de Fer pass and the alkaline Combeynot granite were emplaced during the Late Carboniferous. They were followed during the Early Permian by the calc-alkaline diorite porphyries from the “Zone Houillère Briançonnaise”, dated by 40Ar–39Ar, and by the calc-alkaline rhyodacites from the Guil valley. These suites postdate the Variscan orogeny and originated in a regime of lithospheric extension and thinning affecting the entire domain of the European Variscan belt. They display subduction-related geochemical features with the exception of the Combeynot granite. The Croix de Fer suite, the diorite porphyries from the Zone Houillère Briançonnaise and the felsic lavas from the Guil valley show the lowest (–7

Alpine orogenic evolution from subduction to collisional thermal overprint : the ⁴⁰Ar/³⁹Ar age constraints from the Valaisan Ocean, Central Alps

The investigated HP/LT metasedimentary units of the Valaisan and adjacent European domains occupy a key position in the Alpine belt for understanding the transition from early subduction-related HP/LT metamorphism to collision-related Barrovian overprint and the evolution of mountain belts in general. The timing of high-pressure metamorphism, subsequent retrogression and following Barrow-type overprint was studied by 40Ar/39Ar dating of biotite and several white mica generations that are well characterized in terms of mineral chemistry, texture and associated mineral assemblages. Four distinct age populations of white mica record peak pressure conditions (42–40 Ma) and several stages of subsequent retrograde metamorphic evolution (36–25 Ma). Biotite isotopic analyses yield consistent apparent ages that cluster around 18–16 Ma for the Barrow-type thermal overprint. The recorded isotopic data reveal a significant time gap in the order of some 20 Ma between subduction-related HP/LT metamorphism and collision-related Barrovian overprint, supporting the notion of a polymetamorphic evolution associated with a bimodal P-T path