FERROCARPHOLITE AND MAGNESIOCARPHOLITE FROM THE MONTE-ARGENTARIO (ITALY) - FIRST EVIDENCE FOR HIGH-PRESSURE METAMORPHISM OF THE METASEDIMENTARY VERRUCANO SEQUENCE, AND SIGNIFICANCE FOR P-T PATH RECONSTRUCTION

Ferro- and magnesiocarpholite [Mg/(Mg+Mn+Fe2+) = 0.36 - 0.70, Mn/(Mg+M n+Fe2+) < 0.06] has been found in low-grade metamorphic, siliciclastic rocks of the Monte Argentario, southern Tuscany. This is the first ti me carpholite is reported from the Northern Apennines orogenic belt. T he presence of ferro- and magnesiocarpholite indicates a previously un recognised high-pressure episode during metamorphism of the Triassic m etasedimentary sequence (Verrucano) of the Monte Argentario. Thermodyn amic data of carpholite, its associated minerals (chloritoid, pyrophyl lite, chlorite) as well as its breakdown products (sudoite, kaolinite) have been used to calculate P-T conditions of successive stages of me tamorphism. The P-T path is characterised by peak conditions of at lea st 8 kbar and around 350 degrees C, followed by isothermal decompressi on to pressures below 5 kbar at about 350 degrees C, and finally low-p ressure, low-temperature (T < 300 degrees C) conditions within the kao linite + quartz stability field. The derived peak P-T conditions are c ompatible with those obtained for tectonically intercalated fragments of an ophiolitic sequence which contain mineral assemblages of the law sonite-blueschist facies.