The figure on the left illustrates the thickening and thinning of a mountain range in the continental crust. .
The P-T-t Paths of the rocks R are represented in blue on the right. During the thickening, the rocks under the overthrust are sinking rapidly.The pressure increases steadily with depth Z. By simplifying, P = d.g.Z (d : density rocks, g : acceleration of gravity ).Because of their poor thermal conductivity, rocks are only slowly heated up. In the PT diagram, they follow paths close to the P axis. The geotherm (yellow dotted lines) moves to lower temperatures (t0). When convergence ceases, the rocks warm up and begin to rise. (P P decreases while T still increases. The geotherm evolves towards high temperatures (t1, t2, t3) and exceeds GLs, the steady state geotherm in the stable Lithosphere.As the exhumation increases, P and T decrease together.
During the prograde path (P and T increase), rocks change their mineralogy by passing through dehydration reactions of the type: H = A + V where H is an assemblage of hydrated minerals, A of anhydrous minerals and V the phase vapour (H2O). This vapour released by the rock, of low density, rises to the surface. During retrograde metamorphism, when T decreases, such reactions are crossed in reverse, but the vapour necessary for their realisation is no longer available: the higher T mineralogical associations persist. They define the metamorphic gradient (green dotted line). The intersections between the P-T-t paths and the geotherms show that the different rocks do not reach their peak temperatures at the same time.
Therefore, the metamorphic gradient, which records these 'peaks', does not exist at a given time.
The P T t Paths of the rocks are characteristic of the Geodynamic Context in which they are formed.
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