Regional distribution of Al-silicates and metamorphic zonation in the low-grade Verrucano metasediments from the Northern Apennines, Italy

Mineralogical and petrological studies of Triassic Verrucano metasediments of the Northern Apennines are reported. The widespread occurrence of Al-silicates allows the delineation of four metamorphic zones with increasing metamorphic grade: (1) kaolinite zone (well Perugia 2, Umbria); (2) kaolinite-pyro-phyllite zone (Monte Argentario and part of the Verrucano of the Monticiano-Roccastrada area and Monti Leoni); (3) pyrophyllite zone (Monti Pisani, Iano, Monti Leoni, the Monticiano-Roccastrada area and some wells in the Larderello region); (4) kyanite zone (Massa area and some wells in the Larderello area). The four metamorphic zones correspond to temperatures ranging from 300°C to about 450°C. On the basis of the Si content of muscovite and geological arguments, pressures of between 3 and 5 kbar are estimated. The metamorphic zones are located more or less parallel to the bent north-west-south-east trending structural zonation of the Northern Apennines, with the concave side towards the Tyhrrenian Sea. During the Alpine orogeny, the Verrucano metasediments underwent three folding phases each of which has produced an axial plane schistosity (S1, S2, S3). During the first folding phase the Verrucano sediments were buried increasingly deeply within the crust from east to west. The climax of Alpine metamorphism was attained prior to the second folding phase with crystallization of porphyroblasts of kyanite and chloritoid in a central area located between Massa and Larderello. The inferred paleo-temperature distribution pattern resembles an asymmetric thermal high defined by the kyanite zone, and surrounded by the pyrophyllite zone. A similar pattern is still present in the Tuscan crust, as indicated by a series of geothermal anomalies passing through the Northern Apennines.