The geochemical signatures of sparry calcite-sealing expansion breccias, calcite veins, and host clasts were analyzed for their strontium (⁸⁷Sr/⁸⁶Sr) and oxygen and carbon (δ¹⁸O, δ¹³C) stable isotopic signatures. The breccias occur within the Lower Cretaceous Maiolica Formation. Related but different breccias are found in a few places in the Upper Cretaceous to Eocene Scaglia Rossa Formation of the Umbria-Marche Apennines fold-and-thrust belt (Italy). We propose hydraulic fracturing by fluid overpressure as a possible mechanism for generation of the breccias in these formations. Our data are compatible with the hypothesis of a hydraulically fractured breccia formed by cyclic buildup and rapid decompression of CO₂-rich fluids, with overpressures generated by entrapment of CO₂ by structural and stratigraphic seals. Strontium and oxygen isotope ratio data suggest that the CO₂-rich fluids may have originated from carbonate metasomatism of the mantle, resulting from subduction of carbonate-rich lithologies constituting the downgoing slab. This is consistent with previous conceptual models inferring that in the central part of the Northern Apennines, which is characterized by thick continental crust, CO₂-rich fluids derived from mantle metasomatism would become trapped in structural seals, creating high fluid overpressures.