Very hot, very shallow hydrothermal dolomitization: an example from the Maritime Alps (NW Italy-SE France)

In the Maritime Alps (NW Italy–SE France), the Middle Triassic and Middle Jurassic–Berriasian platform carbonates of the Provençal Domain locally show an intense dolomitization. Dolomitized bodies vary in size from some metres to hundreds of metres, and are associated with tabular bodies of dolomite-cemented breccias, cutting the bedding at a high angle, and networks of randomly oriented 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, alternated with phases of host-rock dolomitization and void cementation. In thin section, main dolomite types are finely- to medium-crystalline, planar-s replacement dolomite, and coarsely-crystalline saddle dolomite occurring both as cement and replacement phase. Fluid inclusion analysis indicates that dolomitizing fluids were relatively hot (170–260 °C), whereas the stratigraphic frame documents the very shallow burial setting of dolomitization, which occurred in the earliest Cretaceous. Collectively, these pieces of information compose the overall picture of a large-scale hydrothermal system related to deep-rooted faults, and provide indirect evidence of earliest Cretaceous fault activity in this part of the Alpine Tethys European palaeomargin. The study case represents an outstanding example of a fossil hydrothermal system, which can represent a reference for recognizing the main geometric and petrographic features of hydrothermally dolomitized rocks.