Fault-controlled saddle dolomitization during the Late Triassic Pangea breakup in the southern Adria domain (Southern Italy)

This study presents, for the first time in the southern Apennines (Southern Italy), evidence of fault-driven hydrothermal dolomitization during the late Triassic rifting of the western Adria Plate, through examination of fault-controlled saddle dolomite formation in Norian (Upper Triassic) dolomites, exposed in the western sector of the Matese Massif. This investigation focuses on dolomite breccias associated with N-S and NNW-SSE striking normal faults. These structures include layers of mature cataclasites made of clasts with angular boundaries within a highly porous matrix, crossed by veins, mosaic and chaotic breccias. The breccias consist of angular clasts of host rock dolomite, derived from the early marine replacive dolomitization of shallow-water carbonates, surrounded by coarse (ca. 500 μm) saddle dolomite cement. The saddle dolomite cement is characterized by two distinct phases. The first phase (SD1) is yellow in color and inclusion-rich, forming a rim around the clasts. The second phase (SD2) is euhedral and exhibits well-defined zoning, with a transition from cloudy to limpid crystals. The saddle dolomite cement texture and the decreasing δ18O and 87Sr/86Sr values suggest a precipitation temperature of about 100-120°C from a fluid that might have interacted with a magmatic source? with a possible magmatic component. The U-Pb dating of the dolomite cement indicates a late Triassic crystallization age of approximately 206 ± 13 Ma and 217.0 ± 6.6 Ma. Furthermore, within the ferroan dolomite cement, quartz and hydrothermal minerals, notably fluorite and apatite, occur in minor quantities. These findings suggest that the brecciation and hydrothermal saddle dolomite precipitation were related to the activity of normal faults during Pangea breakup, resulting in the separation between the SW sector of Eurasia and the western margin of the Adria Plate. These data provide the first evidence of Triassic syn-tectonic saddle dolomites in this region of the Apennine belt and highlight the important role of U-Pb dating of fracture-filling dolomite cements in unraveling the tectonic evolution of structurally complex areas.