Thematic Articles

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.

As the continental crust thickens during mountain building, it can become hot enough to start melting, leading to a profound reduction in its strength. Melt-weakened crust can flow outward or upward in response to the pressure gradients associated with mountain building, and may be transported hundreds of kilometres laterally as mid-crustal channels. In the Himalayan–Tibetan system, melting began about 30 million years ago, and widespread granite intrusion began at 20–23 Ma. Geophysical data indicate that melt is present beneath the Tibetan plateau today, and deeply eroded mountain belts preserve evidence for melt-enhanced ductile flow in the past. Flow of partially molten crust may limit the thickness and elevation of mountain belts and has influenced the deep structure of continents.

Recognising the former presence of melt in rocks which have undergone cooling and exhumation over millions of years following regional metamorphism commonly relies on the correct interpretation of grain-scale structures visible only under the microscope. The evolution of these structures during prograde melting and, later, retrograde cooling can be understood using concepts derived from experimental simulation and materials science.

Experimental studies and thermodynamic modelling have advanced our understanding of partial melting in the crust and have provided a framework for the interpretation of migmatites, residual granulites and granites. Each approach has advantages and pitfalls, and each is more appropriate than the other for investigating particular aspects of the melting process. A comparison of these two approaches may be useful because, together, they potentially give more information. A comparison of a small number of experiments with model calculations using equivalent bulk compositions shows important consistencies between the results, especially regarding the overall topologies of key melting equilibria. Despite this, several significant differences between the two approaches remain, though the sources of these differences are difficult to determine.

There is widespread evidence that ultrahigh temperatures of 900–1000 °C have been generated in the Earth’s crust repeatedly in time and space. These temperatures were associated with thickened crust in collisional mountain belts and the production of large volumes of magma. Numerical modelling indicates that a long-lived mountain plateau with high internal concentrations of heat-producing elements and low erosion rates is the most likely setting for such extreme conditions. Preferential thickening of alreadyhot back-arc basins and mechanical heating by deformation in ductile shear zones might also contribute to elevated temperatures.

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.