Author name: Michael Brown

Melt that crystallizes as granite at shallow crustal levels in orogenic belts originates from migmatite and residual granulite in the deep crust; this is the most important mass-transfer process affecting the continents. Initially melt collects in grain boundaries before migrating along structural fabrics and through discordant fractures initiated during synanatectic deformation. As this permeable porosity develops, melt flows down gradients in pressure generated by the imposed tectonic stress, moving from grain boundaries through outcrop-scale vein networks to ascent conduits. Gravity then drives melt ascent through the crust, either in dikes that fi ll ductile-to-brittle–elastic fractures or by pervasive flow in planar and linear channels in belts of steep structural fabrics. Melt may be arrested in its ascent at the ductile-to-brittle transition zone or it may be trapped en route by a developing tectonic structure.

Partial melting of the continental crust has long been of interest to petrologists as a small-scale phenomenon. Mineral assemblages in the cores of old, eroded mountain chains that formed where continents collided show that the continental crust was buried deeply enough to have melted extensively. Geochemical, experimental, petrological and geodynamic modelling now show that when the continental crust melts the consequences are crustal-scale. The combination of melting and regional deformation is critical: the presence of melt on grain boundaries weakens rocks, and weak rocks deform faster, infl uencing the way mountain belts grow and how rifts propagate. Tectonic forces also drive the movement of melt out of the lower continental crust, resulting in an irreversible chemical differentiation of the crust.