This paper benefited from fruitful discussions with Alfredo Caggianelli and Erwin Gueguen. Many thanks to Pasquale Acquafredda (Dip. Geomineralogico, University of Bari) for the analytical facilities. Thermobarometric calculations were performed with the programs Thermobarometry by Kohn and Spear, and Thermocalc by Holland and Powell. Biotite formulas were calculated using the program Bt by Alfredo Caggianelli. Reviews by Virginia B. Sisson and Ronan Hebert are gratefully acknowledged.