Simplified model of the thermal and mechanical evolution of a mountain range resulting from the collision of two continents initially separated by an ocean. 
The P-T diagrams display the thermal evolution of selected samples in the different structural units. The dotted lines and associated arrows materialise the geotherm and its evolution over time. The orogenic cycle begins with the opening of the ocean. In the earliest stage of the opening of this ocean (at t1), a thinning of the continental lithosphere is accompanied by an upwelling of the warm asthenosphere towards the surface.The resulting large thermal anomaly causes a tightening of the isotherms and may be responsible for HT-LP metamorphism and the emplacement of basaltic magmas, precursors to oceanic crust, in the thinned continental lower crust. The P-T-t paths of the basic rocks and the surrounding rocks (diagrams PT at t1 ) are almost isobaric.

If the expansion continues, an ocean opens up (at t2). A gabbro, emplaced on the ridge, cools rapidly as it moves away from it.. It runs through a cooling path, isobaric, at low pressure on the P-T t2-t3 diagram ( green star t2) on the P-T t2-t3 diagram. When the oceanic lithosphere is old enough, it sinks into the mantle at a subduction zone under one of the passive margins or under the oceanic lithosphere itself (t3). The cold oceanic lithosphere is rapidly sinking (several cm per year) into the mantle. Because of the low conductivity of the rock, it warms up slowly, while the pressure increases instantaneously with depth. This sinking lithosphere is affected by a metamorphism of low-gradient, of HP-LT type. The rocks follow PT(t) paths like that of the green star at t3. If some of these samples are returned quickly to the surface while the process is still going on, they will follow almost the same route in the opposite direction (dashed retrograde path). At the base of the active margin, gabbroic magmas are emplaced by isobaric cooling ("arc" on the PT diagram at t2-t3). The following diagram shows the trajectories of two samples during the collision that followed the closure of the ocean. . The rock 1 (star) is found in the portion of the oceanic crust pinched into the suture.The latter, after having followed a HP-LT path (red star at t4) during the subduction, is heated and exhumed during the collision (at t5). The rock 2 (black dot), on the underthrusting continental crust, is affected by IP-type metamorphism. Through the interplay of movements on either side of the overthrust, the two samples end up having a common history (t6). The thick dashed line on the P-T diagrams represents the metamorphic gradient. Note that this gradient (as well as each trajectory of the 2 rocks) attains the conditions of hydrated anatexis (shown by curve A).
The dynamic mechanisms in the lithospheric mantle are poorly understood. A delamination of the latter and the crust seems to be inevitable. This probably leads to an upwelling and partial melting of the hot asthenosphere which produces basaltic magmas (in black at t7 and t8) that will intrude the base of the crust (à t8).The resulting thermal anomaly and the rapid ascent of the rocks (which do not have time to cool) are responsible for the HT-BP gradient which sometimes follows the IP gradient (diagram at t8 ) and for a voluminous anatexis at the origin of granites and the differentiation of the crust.. 
The 3 converging lines on the PT diagrams (from t4 to t8) delimit the stability fields of the three aluminosilicates : Kyanite, Sillimanite, Andalousite. Oph. : Ophiolites.