Very hot, very shallow hydrothermal dolomitization: An example from the Maritime Alps (north-west Italy-south-east France)

In the Maritime Alps (north-west Italy - south-east France), the Middle Triassic-lowermost Cretaceous platform carbonates of the Provençal Domain locally show an intense dolomitization. Dolomitized bodies, irregularly shaped and variable in size from some metres to hundreds of metres, are associated with tabular bodies of dolomite-cemented breccias, cutting the bedding at a high angle, and networks of dolomite veins. Field and petrographic observations indicate that dolomitization was a polyphase process, in which episodes of hydrofracturing and host-rock dissolution, related to episodic expulsion of overpressured fluids through faults and fracture systems, were associated with phases of host-rock dolomitization and void cementation. Fluid inclusion analysis indicates that dolomitizing fluids were relatively hot (170 to 260°C). The case study represents an outstanding example of a fossil hydrothermal system, which significantly contributes to the knowledge of such dolomitization systems in continental margin settings. The unusually favourable stratigraphic framework allows precise constraint of the timing of dolomitization (earliest Cretaceous) and, consequently, direct evaluation of the burial setting of dolomitization which, for the upper part of the dolomitized succession, was very shallow or even close to the surface. The described large-scale hydrothermal system was probably related to deep-rooted faults, and provides indirect evidence of a significant earliest Cretaceous fault activity in this part of the Alpine Tethys European palaeomargin.