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The paper presents the metamorphic trajectory recorded by metapelitic migmatites of the upper part of the Hercynian lower continental crust of the Serre (southern Calabria, Italy). The relict minerals, reaction textures and phase equilibria define a clockwise P-T path. The prograde metamorphism from temperature of about 500°C and pressure of 4-5 kbar to T < 700°C and P ∼ 8 kbar stabilized the assemblage Grt + Ky + Bt + Ms(Si/11ox=3.26-3.29) in the uppermost metapelites of the profile. Progressive heating led to H2O-fluxed and dehydration melting first of Ms, then of Bt at T< 700°C in the stability field of sillimanite. This process was followed by nearly isothermal decompression producing additional melt with a transition from Grt to a Grt + Crd stability field. Further decompression caused the formation of Crd-corona around garnet. Nearly isobaric cooling led to rehydration and retrogression across the stability field of andalusite up to the stability field of kyanite. The lowermost metapelites of the studied profile have lost most of the memory of the prograde P-T path; they record decompression and cooling. High-temperature mylonites occur in which boudinage, elongation and pull-aparts characterize the porphyroclasts. The pull-aparts in the high-T mylonites are filled with low-P minerals (Crd + Spl). The Hercynian metamorphic trajectory and the microtextures are consistent with crustal thickening and subsequent extensional regime. During extension, an important tectonic denudation probably caused the isothermal decompression. Extension also occurred in post-Hercynian times as documented by pull-aparts in sillimanite porphyroclasts filled with chlorituid within a low-grade mylonite. [ABSTRACT FROM AUTHOR]

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. In sillimanite-free xenoliths, fibrolite did not form during pyrometamorphism, because these xenoliths do not contain plagioclase. In these xenoliths, andalusite prisms are replaced by oriented fibrous corundum needles and K-feldspar. The petrographic evidence suggests that when reaction rate is high as a result of reaction temperature overstepping, sillimanite grows as fibrolite because the surface energy of {110} faces is low and their growth rate is rapid. The same explanation may hold also for the growth of acicular corundum pseudomorphing andalusite prisms in sillimanite-free xenoliths. [ABSTRACT FROM AUTHOR]