Metamorphic recrystallization related to the circulation of CO2-rich hydrothermal fluids: the case of the Valdieri marbles (Maritime Alps)

In the Maritime Alps, at the NE border of the Argentera crystalline massif, a kilometre-scale rock body of marbles, quarried in the past as ornamental stones (“Marmi di Valdieri”), occurs within a carbonate succession referable to the Dauphinois domain and consisting of dark marls (Entracque Marl, Middle Jurassic-Berriasian), micritic limestones with breccia beds (Lausa Limestone, Valanginian-early Aptian), dark shales and marls (Marne Nere, Aptian-Cenomanian) and marly limestones (Puriac Limestone, Turonian-Campanian). The Puriac Limestone is unconformably overlain by middle Eocene-lower Oligocene sediments of the Alpine foreland basin. Recrystallization affects the Lausa Limestone, the Marne Nere, and the lower and middle portions of the Puriac Limestone. The marbles pass gradually to poorly recrystallized rocks in a range of few tens of meters. The Valdieri marbles lower part (corresponding to Lausa Limestone) consists of pure white and grey marbles with rare mm-thick elongated and folded domains strongly enriched in muscovite, K-feldspar, albite and quartz. The upper part (corresponding to Puriac Limestone) is composed of lens-shaped, cm- to dm-thick granoblastic marbles, interlayered with mm-thick anastomosed greenish-purple levels made up of white mica (muscovite-paragonite s.s.), chlorite and epidote. The Valdieri Marbles are cut by at least two orders of tectonic foliations and deformed by folds, faults and fractures belonging to regional systems well known in the surrounding region. Although most of these meso-structural associations clearly postdate the marbles recrystallization, there are evidences of a syn-genetic shearing during the blastic event, leading to elongation of primary clasts and localized development of metamorphic foliations. The marbles are crossed by mm- to dm-thick veins filled with calcite, quartz and Fe-rich dolomite. Different types of fluid inclusions occur in the vein minerals: - early H2O-CO2 inclusions showing (at room T) LH2O ± LCO2 + VCO2 assemblage, often coexisting with CO2 vapour-rich inclusions, possibly as a result of heterogeneous entrapment; - late aqueous inclusions, generally showing LH2O + VH2O assemblage. The reported data document that the recrystallization of the Valdieri Marbles was not related to regional metamorphism but to a localized flux of CO2-rich hydrothermal fluids flowing up from the underlying basement rocks.