Ferro- 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
Theye, Thomas; Reinhardt, Jürgen; Goffé, Bruno; Jolivet, Laurent; Brunet, Christοphe
European Journal of Mineralogy Volume 9 Number 4 (1997), p. 859 - 874
published: Jul 23, 1997
manuscript accepted: Feb 25, 1997
manuscript received: May 2, 1996
ArtNo. ESP147050904002, Price: 29.00 €
Abstract Ferro- and magnesiocarpholite [Mg/(Mg+Mn+Fe2+) = 0.36 - 0.70, Mn/(Mg+Mn+Fe2+) < 0.06] has been found in low-grade metamorphic, siliciclastic rocks of the Monte Argentario, southern Tuscany. This is the first time carpholite is reported from the Northern Apennines orogenic belt. The presence of ferro- and magnesiocarpholite indicates a previously unrecognised high-pressure episode during metamorphism of the Triassic metasedimentary sequence (Verrucano) of the Monte Argentario. Thermodynamic data of carpholite, its associated minerals (chloritoid, pyrophyllite, chlorite) as well as its breakdown products (sudoite, kaolinite) have been used to calculate P-T conditions of successive stages of metamorphism. The P-T path is characterised by peak conditions of at least 8 kbar and around 350 °C, followed by isothermal decompression to pressures below 5 kbar at about 350 °C, and finally low-pressure, low-temperature (T < 300 °C) conditions within the kaolinite + quartz stability field. The derived peak P-T conditions are compatible with those obtained for tectonically intercalated fragments of an ophiolitic sequence which contain mineral assemblages of the lawsonite-blueschist facies.