Towards a Better Understanding of the Fibrolite Problem: the Effect of Reaction Overstepping and Surface Energy Anisotropy

Trachytes from the Euganean Hills District (Italy) contain metapelitic xenoliths that have been pyrometamorphosed during incorporation in the melt. In xenoliths containing sillimanite crystallized during a previous regional HT/LP metamorphism, fibrolite systematically nucleates at the grain boundaries of sillimanite prisms and within plagioclase crystals. Ternary feldspar thermometry shows that plagioclase in contact with sillimanite plots along the 750 C solvus that reflects near-equilibrium conditions of regional metamorphism. Plagioclase containing fibrolite plots closer towards the 950 C solvus, reflecting the tendency of plagioclase to re-equilibrate at high temperature during pyrometamorphism by a fibrolite-forming reaction: K -feldspar 1 plagioclase 1 ! K -feldspar 2 plagioclase 2 fibrolite: