Late Dolomitization in Basinal Limestones of the Southern Apennines Fold and Thrust Belt (Italy)

The Triassic pelagic carbonates of the Lagonegro Units from the southern Apennines fold and thrust belt host discordant bodies of dolostone. These rocks show textural features typical of zebra or saddle dolomites. In this contribution, the results of field observations, petrography and geochemical analyzes performed on samples from three different outcrops are presented. Field data indicate that the peculiar zebra-type rock and brecciated fabrics were controlled by regular bedding of the micritic, pelagic limestones. The dolomitization resulted in the replacement of the host limestones and subsequent void-filling precipitation of dolomite in a dilatational stress field. Homogenization temperatures (Th) are in the range 80 - 120°C, with a clear mode of 95 ± 10°C. When corrected for maximum pressure, they indicate an upper limit for dolomite formation around 110 - 115°C. Melting temperatures of ice (Tmi) point to salinities in the range 2-6 wt% NaCl eq, with a mean of 4.2%. δ13C values overlap those of the Upper Triassic seawater, whereas the δ18O values are significantly depleted compared to the coeval seawater. 87Sr/86Sr values are higher with respect to Upper Triassic seawater, and partially overlap Middle-Upper Miocene values. The performed analyzes indicate a dolomitization process driven by warm fluids (110 - 115°C) with a salinity close to that of seawater and O-isotope ratios comprised between seawater and formation waters. It is suggested that the dolomitization was accomplished by formation waters squeezed out or from surrounding lithologies or from the underlying mélange units. Integration with available thermal data on the collisional history of the belt suggests that the fluid-flow took place after maximum burial, in the early stages of exhumation, in the assumption of a fluid in thermal equilibrium with the host rocks. According to literature data, the dolomitization event was quite widespread. It can be argued that it was part of a major fluid-flux associated with fold and thrust belt development, which possibly affected the Apulian Platform carbonates located in the foreland. The latter, now buried below the nappe stack, hosts some of the major oil fields of continental Europe.