THE MONTEFERRATO SERPENTINIZED PERIDOTITE (FIGLINE DI PRATO, PRATO) IN THE NORTHERN APENNINES, ITALY: A WITNESS OF THE LIGURIAN OCEAN MARGIN IN THE MONUMENTAL RELIGIOUS BUILDINGS OF TUSCANY

Abstract

The Monteferrato area is the source site of “Verde Prato” that, together with Carrara Marble, forms the visual duotone that typifies Renaissance buildings in Tuscany. Verde Prato is a serpentinized peridotite that crops out close to Figline di Prato. At the beginning of the 20th century, Gustav Steinmann visited and described the ophiolitic outcrops of Figline di Prato, which contributed to his recognition of the “Steinmann Trinity”, i.e., the common co-occurrence of serpentinites, pillow basalts and cherts. The Steinmann Trinity concept, in turn, formed an essential step in the development of ophiolite theories, sea-floor spreading, and plate tectonics. Because of the cultural, artistic and geologic importance of the Monteferrato peridotite, the Tuscan Region classified these quarries as historical sites in its list of the Historical Ornamental Stones Quarries. Within the Monteferrato serpentinites, large portions of the original peridotite are locally partially preserved, still presenting original microstructure of the mantle protolith. These peridotites preserve vestiges of sub-continental mantle deformation that occurred during the early stages of lithosphere extension, which opened the Ligurian Tethys in the Early Jurassic. Static crystallization of peridotitic minerals was followed by bulk serpentinization. Successive veins formed, filled by chrysotile and lizardite, with the former cutting the latter. Antigorite is also present in veins, but crosscutting relationships with the other veins are unclear. A tentative interpretation of the veining events relates the antigorite vein formation associated with the gabbro intrusions into the serpentinized peridotite, while the transition from chrysotile to lizardite relates to the activation of an extensional tectonic regime, perhaps linked to the closure of the Ligurian Tethys and bending of the subducting plate. Serpentinization may also provide the key to understand how the Monteferrato peridotites were ultimately emplaced within the Monte Morello Unit.