Pervasive crystallization of antigorite in Northern Apennine ophiolites: the example of Montecarelli peridotites (Tuscany, central Italy)

The occurrence of pervasive crystallization of antigorite associated to brittle tectonics is describedfor the first time in the External Ligurian peridotites of the Northern Apennine(Italy).Antigoritewasfound in a slide block of subcontinental peridotites included in the tectonized sedimentary mélange of the Leo Unit(western External Ligurianunits). This mélange, as those from the External Ligurian units, originatedin Santonian-early Campanian time span from a source area corresponding to the Ocean-Continent Transition Zoneatthe Adria plate margin. The antigorite wasfound within veins belonging to four systems, from V1 to V4, each characterizedby different attitude and morphology. The morphology of the veins rangesfrom massive to fibrous, with the lattercharacterized by fibers both parallel and perpendicular to the vein walls. Rare occurrence of chrysotile and calcite wereidentified within the veins. The antigorite crystallization in the peridotites cannot be interpreted as related to an orogenic high-pressure metamorphism but most likely can be regarded as developed at shallow structural level in the late stage of the rifting phase when subcontinental mantlewasprogressively exposed at the surface,intruded by gabbrobodies,and cut by basaltic dykes.