Il metamorfismo dei depositi clastici triassici affioranti nei Monti dell’Uccellina (Toscana meridionale)

METAMORPHISM OF TRIASSIC CLASTIC DIEPOSITS OF MONTE DEL'UCCELLINA (SOUTHERN TUSCANY, NORTHERN APENNINES, ITALY) — In this work the results of a study of the metamorphic grade of triassic clastic deposits (Verrucano) and of the overlying carbonatic formations outcropping in Monti dell'Uccellina area (Southern Tuscany) are reported. The study formations belong to four different tectonic units: Torre Cannelle, Monti dell'Uccellina, Vacchereccia and Collelungo units. The study of the metamorphic conditions of the different tectonic units has been afforded through mineral assemblages (with particular regard to Al-silicates) <<crystallinity>>, polytypism and <<b0>> of illite, as well as the crystallinity of chlorite, in the carbonate bearing rocks, the calcite-dolornite geothermometer was also applied. The metamorphic indicators point out that the Verrucano, during alpine orogenesis, was affected by an anchizonal metamorphism with an intermediate pressure character. The Verrucano of the Vacchereccia Unit characterized by kaolinite+quartz association have a lower-grade metamorphic imprint (low to middle anchizone), White K-mica presents an average crystallinity index in the range 0.30—0.40° (2), an amount of 2M1 polytype between 68-97%, b0 parameter between 9.006 and 9.008 Å and chlorite crystallinity index between 0.24 and 0.28° (2). The Verrucano of Monti dell'Uccellina Unit characterized by the kaolinite+pirophyllite+quartz association is to be referred to a slightly higher grade metamorphism (upper anchizone). Its white K-mica have average crystallinity index value of 0.29 to 0.30° (2), 2M1 polytype from 93 to 96%, chlorite crystallinity index is from 0.21 and 0.26 and b0 parameter from 9.004 to 9.007 Å, The Verrucano of Torre Cannelle Unit, in contrast with the Verrucano belonging to the other two tectonic units, does not contain Al silicates. The lacking of these or other key minerals didn’t confine the Verrucano of this unit to a well defined metarnorphic zone even though all the low-grade metamorphic indicators seem to point out a grade higher than the Verrucano of the other tectonic units. Illite and chlorite crystallinity index values, respectively from 0.25-0.27° and 0.19-0.24° (2), appear lower and the amount of 2M1 polytype higher (about 100%) than the values observed in the Verrucano of Monti dell'Uccellina and Vacchereccia units. Calcite-dolomite geothermometer applied to rocks of the Vacchereccia Unit (kaolinite+quartz zone) gives temperature of about 270°C, while for the Verrucano and Salto del Cervo formation (Monti dell’Uccellina Unit) temperatures of about 320°C were obtained.