Partial melting and strain localization in metapelites at very low-pressure conditions: The northern Apennines magmatic arc on the Island of Elba, Italy

Structural and microstructural analyses and phase equilibria modeling of migmatitic amphibolite-facies metapelites from the late Carboniferous Calamita Schists, on the Island of Elba, Italy, show how the interplay between partial melting and regional (far-field) deformation assisted deformation at very shallow (P ≤ 0.2 GPa) crustal levels. Partial melting was caused by the heat supplied by an underlying late Miocene intrusion (Porto Azzurro pluton) and occurred by biotite continuous melting. The produced melt remained in situ in patches, likely experienced limited migration in stromatic migmatites, and crystallized as a K-feldspar + plagioclase + quartz assemblage. Deformation in the presence of melt occurred by melt-enhanced grain boundary sliding, producing well-foliated high-strain zones with weak evidence of subsolidus deformation at the microscale where the original melt was present. Melt crystallization caused strain hardening and forced subsolidus deformation into localized mylonitic shear zones. The localized character of retrograde deformation was likely determined by the heterogeneous distribution/ingress of fluids in the aureole that locally assisted strain localization, enhancing dislocation creep and reaction softening.