Anatexis

Anatexis can occur in a variety of different settings, from zones of continental collision to mid-ocean ridges. It is believed that anatexis is the process largely responsible for the formation of migmatites. Furthermore, scientists have recently discovered that partial melting plays an increasingly important role in active crustal processes, including the advancement of active deformation and the emplacement of crustal granites. As a result, active feedback between crustal shearing, melting, and granite emplacement has become largely accepted in the place of large scale, unreasonable models involving fractional melting of the mantle into granitic batholiths and plutons. Evidence for this can be seen in the physical, mineralogical, and isotopic signatures of countless granites.

Conditions for melting

Crustal anatexis is not restricted to a single tectonic setting, but rather is controlled by four primary parameters: temperature, pressure, volatile content, and rock type/composition. These parameters are highly variable and depend on depth, crustal thickness, and local variations of the Earth's geotherm. The amount and composition of partial melts likely varies locally, reflecting the heterogeneity of the Earth's crust.

Temperature

In order to induce crustal melting, the temperature must be increased past the normal geotherm. Possible sources of heat include primordial heat originating from the core of the Earth as well as the decay of radioactive elements. This heat is distributed throughout the Earth's crust by a number of different processes, including radiation, conduction, convection, and advection.

The emplacement of magmatic intrusions is also commonly associated with local increases in temperature. If the increase in temperature is sufficient, this can lead to partial melting of adjacent country rocks. If partial melting does occur, then the degree of melting is controlled by the amount of available heat in the magmatic body.

Pressure

Beneath the Earth's surface, pressure increases with depth due to the accumulation of overlying rock. At a given temperature, a decrease in pressure can result in localized melting. Melting that is caused by a drop in pressure is referred to as decompression melting. Decompression melting can occur in thickened portions of the Earth's crust and may be the result of a variety of processes, including erosion, tectonic denudation, and lithospheric thinning.

Volatile content

The amount of water available in the system plays a major role in controlling the degree of melting at a given temperature. Low water availability will suppress melting. Furthermore, the degree of water saturation of a system will affect the composition of any melt generated. Water can be derived from a variety of sources, including from surrounding country rocks (pore water) or from the decomposition of hydrous minerals (e.g. micas, amphiboles). Melting reactions involving water liberated from hydrous minerals are often referred to as dehydration melting reactions or vapour-absent reactions. Over time, dehydration melting reactions will consume all of the hydrous phases in a rock, meaning that the amount of melt generated through these reactions is controlled by the abundance and stability of specific hydrous phases. Depending on the tectonic setting, water can also be introduced to the system through the dehydration of a subducting hydrated oceanic plate or magmatic underplating.

Rock type

The composition of a parent rock has a direct effect on the composition of the resulting melt. Granitic melts are commonly classified based on the nature of their source rock. One of the more popular classification schemes for granites was first introduced by White and Chappell in 1974. This classification scheme categorizes granites based on whether they are the result of the melting of sedimentary rocks (S-type granites) or the melting of igneous rocks (I-type granites). This genetic difference is reflected in the geochemical signature of the melts themselves.