Garnet coalescence clogs melt extraction channels in migmatite

Here we investigate the evolution of a focused melt extraction horizon exposed in lower crustal metapelites of the Serre Massif in Calabria (southern Variscan belt, Italy) and consider the effect of melt drainage on the rheology of the orogenic lower crust. Our study shows that large (up to >10 cm long) ellipsoidal garnet aggregates associated with a small volume of leucosome in this migmatitic horizon represent clogged melt extraction channels. Using microstructural evidence, particularly electron backscatter diffraction (EBSD) data, we demonstrate that the garnet aggregates formed by coalescence of small peritectic crystals, carried along in anatectic melt migrating towards pull-apart structures, where the melt with entrained garnet pooled as it was progressively squeezed out. Phase equilibrium thermodynamic modelling of melt extraction events indicates that a minimum of 30 vol% of melt was drained out during nearly isobaric heating at average peak pressure (P) conditions of 0.97 GPa, from ∼700 °C to peak temperatures of 800–840 °C. We use numerical simulations of measured quartz crystallographic preferred orientations (CPOs) to show that melt extraction occurred under pure shear regime. Our results highlight how orogen rheology is conditioned along critical horizons where melting occurs and melt extraction is localized, leaving behind a hard residue.