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

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