P-T HISTORIES OF HP AND UHP ROCKS AT CONVERGENT PLATE BOUNDARIES: INSIGHT FROM 2-D NUMERICAL MODELING

Studies on HP and UHP metamorphic rocks exposed in collisional belts have shown that these rocks (1) are derived from both continental and oceanic crust, being intermingled on the length scale of typical tectonic nappes, (2) are frequently associated with hydrated peridotites, (3) reveal variable but systematically arrayed P-T-t records, with (4) narrow time constraints indicating that exhumation rates can be on the order of plate velocity. Our 2D numerical experiments (including the simulation of metamorphic P-T-time paths) suggest that one feasible model for the formation and exhumation of (U)HP metamorphic rocks involves forced flow in a self-organising subduction channel reaching down to depths of >100 km, with a geometry controlled by progressive hydration of the overriding mantle lithosphere. We propose that the internal structures of orogenic belts with nappes of HP and UHP metamorphic units showing variable P-T-t histories may in large part be due to extrusion from a subduction channel during the active continental margin stage, hence prior to collision. On the other hand, our experiments also suggest that the formation of large coherent UHP complexes in continental collision zones can result from the temporary expansion of the subduction channel during incipient continental collision, when wider and deeper (> 150 km) wedges of subducted crustal material can form. In our simulations, such a crustal wedge decays within a few million years by upward extrusion, as a consequence of necking and subsequent separation ("slab breakoff") of the subducted lithosphere and related creation of an asthenospheric window. As the shape of the P-T path of an individual HP/UHP metamorphic unit depends on its specific trajectory in the subduction channel or crustal wedge, an array of diverse, although interrelated P-T paths rather then a single characteristic trajectory can be expected for HP/UHP metamorphic complexes. The results of our simulations are consistent with the metamorphic and structural records of HP and UHP metamorphic complexes.