The PTt path of the gabbro...
     A gabbro crystallises at the ridge between 2 and 6 km below the surface and cools slowly as it moves away from the ridge (Sample 1). The extending oceanic lithosphere can be locally deformed ductily (=deforms without breaking) when it is still very hot. On the other hand, circulation of water is abundant, as shown by the black smokers. Consequently, all the conditions are favourable for these rocks to recrystallize and be metamorphised. At point 2, the metagabbro shows the transition, during cooling, into the amphibolite facies, then at point 3, in the GreenSchist facies (S.V.) : the rock inside the cooled plate is equilibrated under the conditions of the steady state Geotherm.
     If the (meta)gabbro is subsequently buried in a subduction zone, it will exhibit mineralogy typical of the BlueSchists facies (S.B.) (Sample 4), of the Eclogite facies (Sample 5).When the oceanic lithosphere is completely subducted, the crust of the continental margins can collide. Eclogite can be incorporated into this collision.
     At the beginning of the collision, when the continental crust is underlain by the eclogitised portion of oceanic crust, the latter is driven towards the surface while the thermal gradient evolves towards higher T. The pressure on the eclogite decreases as this one warms up: the rock (Sample 6) equilibrates under amphibolite facies conditions until the eclogite minerals disappear.
    When convergence stops, the thickened continental crust is gravitationally unstable. It thins to the normal thickness of the crust of a stable lithosphere. Deep samples go back to the surface.

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