When moving through a regional metamorphic zone, the different rocks show variable, progressive conditions, from low grade to sometimes anatexis conditions. The conditions evidenced by these rocks enable a regular evolution to be traced in the P-T diagram: it is the metamorphic gradient. The three coloured fields (HP-LT BT in french , MP-HT et HT-LP BP in french) represent the regional metamorphic evolutions most often occuring by the rocks of the regional metamorphism. These are the main metamorphic gradients. The high pressures - low temperatures (HP-LT) metamorphic gradient means that as the pressure (i.e. depth) increases, the temperature remains low. It is the reverse for the high temperatures - low pressures (HT-BP) metamorphic gradient. The pressure increase is moderate in the case of the medium pressure-high temperature metamorphic gradient (MP-HT), also known as intermediate pressure-high temperature : IP-HT). We note that the anatexis domain (in which the rocks start to melt and whose limit is shown by curve A) is only rarely reached, at very great depths, during a HP metamorphism,while it is common in the case of MP - HT and LP - HT metamorphisms. Note also that the succession of aluminosilicates is different in the case of a MP gradient, with kyanite (K in the figure), then sillimanite (S) as the temperature increases and in the case of a LP gradient with andalusite (A), then sillimanite. It is worth noting that none of these gradients coincide with the mean geotherm of a stable lithosphere (GLs). This means that these gradients were not formed under the conditions of this stable lithosphere. These 3 metamorphic gradients appear in different geodynamic contexts. The black lines define the stability fields of the 3 aluminosilicates: : kyanite (K), sillimanite (S) and andalusite (A).

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