The Meso-Cainozoic marly carbonate and carbonate formations of the Umbro-Marchigiana-Sabina Series cropping out along the southern portion of the Serra valley (Martani Mts., Umbria, Central Italy) were strongly deformed during a phase of transcurrent tectonic activity initiated in Late Miocene and ended in Pliocene times. The carbonate rocks of one of such formations ("Scaglia") are particularly rich of calcite veins developed within micro- and macro-fractures formed during the deformation episode. A stable isotope study was conducted on these calcites and their host rocks in order to investigate on their genetic process. Forty two calcite and seventeen host rock samples from four sampling localities were analyzed. The delta <sup>18</sup> O and delta <sup>13</sup> C values of the "Scaglia" samples vary within the narrow range from 28.52 to 29.42 (vs. SMOW), and from 1.47 to 3.42 (vs. PDB), respectively, and are very close to those of the secondary calcite veins. Microthermometric measurements carried out on a few calcites yielded homogenization temperatures between 127 degrees and 141C and salinity of the fluid phase close to 0 per mil, suggesting that calcites crystallized from meteoric-hydrothermal solutions. It is concluded that the calcites under study originated from "pressure-solution" phenomena favored by the tectonic event which affected the sedimentary formations of the Serra valley.

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