Subduction of Continental Crust to Mantle Depth: Geochemistry of Ultrahigh-Pressure Rocks. Geochemistry of Ultrahigh-Pressure Rocks

The discovery of metamorphic coesite and diamond in upper crustal gneisses provides evidence that the least dense part of the lithosphere can be subducted to mantle depths. Ultrahigh-pressure (UHP) metamorphism occurs mainly at the transition from oceanic subduction to continental collision, when a thinned continental margin enters the subduction zone. Peak metamorphic conditions in such continental crust at UHP range from 2.8. GPa, 600. °C to 5.5. GPa, 1000. °C, corresponding approximately to 90-160. km depths. These rocks are the most deeply subducted crustal slices that find their way back to Earth's surface. Small UHP units are exhumed at plate tectonic speed and cool during their ascent, whereas larger slices of subducted continental crust follow near-isothermal decompression and take 10-20. My to return to the crust.All rock types that are present at shallow crustal levels have also been observed with UHP mineralogies. Surprisingly, there is little modification of the chemical and isotopic character of these rocks during deep subduction, unless they undergo partial melting. Melting in subducted felsic continental crust is mainly related to the breakdown of the hydrous phase phengite. For deeply subducted crust, melting can occur at peak metamorphic conditions, resulting in mantle metasomatism by crustal melts. However, most UHP terrains experienced partial melting during the exhumation path, leading to intracrustal differentiation, thereby obscuring their UHP history. While UHP terrains serve as a natural laboratory to investigate interactions between different rock types and fluids at pressures that are relevant for deep subduction processes, their role in global element cycling remains uncertain.