Eclogitization

There is the argument that collision between two continents should slow down because of continental buoyancy, and that for convergence to continue, it should do so at a new subduction zone where oceanic crust can be consumed.

[8] Transitional granulite-eclogite facies granitoid, felsic volcanics, mafic rocks and granulites occur in the Musgrave Block of the Petermann Orogeny, central Australia.

Although limited localities are available to study, these areas provide the crucial samples to understand exhumation as well as continued subduction by continental "undertow."

Fluids, rather than pressure and temperature conditions, are the key thing that makes the process of eclogitization, and the delamination (falling away) of crustal roots, in collisional orogens (fold mountains), possible.

[9] Without H2O, reactions will not proceed to completion, leaving metamorphic rocks metastable (stuck in an incomplete state) at unexpectedly high temperatures and pressures.

Fig. 1 Metamorphic facies (bodies of rock with specific characteristics). Note the eclogite facies, which forms at the highest pressures.
Fig 2. Eclogitization Schematic showing slab detachment within mantle and area of eclogitization and densification of subducting crust, which is a possible explanation for continental "undertow"
Fig. 3 Cartoon Cross Section depicting tectonic evolution of eclogite terrain i.e. Laurentia and Baltic collision A)Early collisional phase with initial eclogitization of transitional margin between Laurentia and Baltica B)Continental Subduction C)Extension and exhumation where eclogites become exposed. Green eclogite symbols represent areas of active eclogitization and white symbols represent eclogites passing through retrograde conditions.