Unusual marbles in a non-metamorphic succession of the SW Alps (Valdieri, Italy) due to early Oligocene hydrothermal flow

In the SW Alps, at the NE margin of the Argentera Massif, the occurrence of isolated bodies of silicate-bearing marbles (Valdieri Marble), laterally passing to non-metamorphic Mesozoic limestones of the Dauphinois succession, appears enigmatic, since their origin cannot be related to the regional metamorphism. The stratigraphic, geometric, petrographic, and geochemical features of the Valdieri Marble indicate that it originated from the upflow of hydrothermal fluids which deeply recrystallised the limestone and provoked the abundant neoblastesis of silicate minerals, at an estimated crystallisation temperature of ca. 350 °C. This paper provides for the first time an absolute U/Pb age, comprised between 30 and 31.6 Ma (early Oligocene), for the Valdieri Marble, which is affected by a syn-genetic shear foliation in turn crosscut by the main Alpine tectonic foliations. The hydrothermal fluids that formed the Valdieri Marble migrated along low-angle shear zones, mostly parallel to the sedimentary primary bedding, which were downward connected with high-angle master faults of the “Alpine SW transfer”, a main Oligocene–early Miocene transcurrent shear zone at the southern termination of the Western Alps. The localized heat flow sustaining the hydrothermal system could be related to the early Oligocene thermal event, well documented by magmatic activity in the Western and Central Alps.