It is important because the processes that form and exhume ultra-high-pressure (UHP) metamorphic rocks may strongly affect plate tectonics, the composition and evolution of Earth's crust.
UHP ultramafic xenoliths of mantle affinity provide information (e.g., mineralogy or deformation mechanisms) about processes active deep in Earth.
Regional metamorphic UHP terrains exposed on Earth's surface provide considerable information that is not available from xenoliths.
There is general agreement that most well-exposed and well-studied UHP terrains were produced by the burial of crustal rocks to mantle depths of >80 km during subduction.
[26] Subduction erosion also occurs beneath volcanoplutonic arcs around the world,[24] carrying continental rocks to mantle depths at least locally.
[28] The positive buoyancy of the continental slab—in opposition principally to ridge push—can then drive exhumation of the subducting crust at a rate and mode determined by plate geometry and the rheology of the crustal materials.
This type of delamination and stacking was proposed to explain exhumation of UHP rocks in the Dora Maira massif in Piedmont, Italy,[32] in the Dabie orogen,[33] and in the Himalaya.
A variant of this mechanism, in which the exhuming material undergoes folding, but not wholescale disruption, was suggested for the Dabie orogen, where exhumation-related stretching lineations and gradients in metamorphic pressure indicate rotation of the exhuming block;[36] The buoyancy of a microcontinent locally slows the rollback of and steepens the dip of subducting mafic lithosphere.
[37] If the mafic lithosphere on either side of the microcontinent continues to roll back, a buoyant portion of the microcontinent may detach, allowing the retarded portion of the mafic slab to roll quickly back, making room for the UHP continental crust to exhume and driving back-arc extension.
[50][51] Studies of numerical geodynamics suggest that both subducted sediment and crystalline rocks may rise through the mantle wedge diapirically to form UHP terranes.
[47][49][50] Diapiric rise of a much larger subducted continental body has been invoked to explain the exhumation of the Papua New Guinea UHP terrain.
[53] However, the mantle wedge above continental subduction zones is cold like cratons, which do not allow for diapirically ascending of the crustal materials.
Foundering of the gravitationally unstable portions of continental lithosphere locally carries quartzofeldspathic rocks into the mantle[54] and may be ongoing beneath the Pamir.