Thematic Articles

The reigning paradigm for the formation and exhumation of continental ultrahigh-pressure (UHP) terranes is the subduction of crust to mantle depths and the return of crustal slices within the subduction channel— all at plate tectonic rates. Additional processes beyond the paradigm are needed to explain the diversity of geological observations gathered from the growing study of UHP terranes—for example, variations in the size, degree of deformation, petrologic evolution, timing of UHP metamorphism, and exhumation rates. Numerical models that evaluate physical parameters in time and space have produced new insights into the formation and exhumation of UHP terranes.

Establishing the timing and duration of ultrahigh-pressure metamorphism (UHP) for crustal rocks subducted to mantle depths of over 100 km requires high-precision geochronology directly coupled with pressure-sensitive indicators. The best links between UHP conditions and an age estimate are inclusions of the UHP indicator minerals coesite and/or diamond in datable zircon or garnet. Lu–Hf and Sm–Nd garnet ages define the prograde and peak portions of the pressure–temperature path for cold (<700 ºC), fast (>1 cm/y) UHP systems. UHP metamorphism in hotter (>800 ºC) and slower (<1 cm/y) terranes is best dated by U–Pb analysis of coesite-bearing zircon domains coupled with Sm–Nd and Lu–Hf garnet analysis.><1 cm/y) terranes is best dated by U–Pb analysis of coesite-bearing zircon domains coupled with Sm–Nd and Lu–Hf garnet analysis.

Establishing the timing and duration of ultrahigh-pressure metamorphism (UHP) for crustal rocks subducted to mantle depths of over 100 km requires high-precision geochronology directly coupled with pressure-sensitive indicators. The best links between UHP conditions and an age estimate are inclusions of the UHP indicator minerals coesite and/or diamond in datable zircon or garnet. Lu–Hf and Sm–Nd garnet ages define the prograde and peak portions of the pressure–temperature path for cold (<700 ºC), fast (>1 cm/y) UHP systems. UHP metamorphism in hotter (>800 ºC) and slower (<1 cm/y) terranes is best dated by U–Pb analysis of coesite-bearing zircon domains coupled with Sm–Nd and Lu–Hf garnet analysis.><1 cm/y) terranes is best dated by U–Pb analysis of coesite-bearing zircon domains coupled with Sm–Nd and Lu–Hf garnet analysis.

Coesite and diamond in metamorphic rocks point to their very deep burial, but these minerals do not allow a precise derivation of metamorphic pressure–temperature (P–T) conditions at ultrahigh pressure (UHP). Thermodynamic calculations of mineral equilibria can accomplish this task when it is possible to assign mineral compositions to a former UHP equilibrium state. Pressure–temperature pseudosections are superior, because they often permit the construction of P–T paths to and from UHP conditions only on the basis of chemically zoned minerals such as garnet and phengite. The examples of a metapelite from Oman and an eclogite from the Erzgebirge, Germany, illustrate this method, but also demonstrate its limits. The derived paths are the basis for further geodynamic modeling and insight into tectonic processes.

Finding evidence for ultrahigh-pressure (UHP) metamorphism in crustal rocks is far from straightforward. The index minerals coesite and diamond are incredibly inconspicuous and are therefore difficult to use as UHP prospecting tools. Consequently, petrographers rely on recognizing subtle breakdown microstructures that result from pressure release during the return to the surface of the once deeply buried rock. Similarly, many other UHP minerals are first suspected on the basis of typical reaction or exsolution microstructures. Thus, the painstaking use of microscopic techniques has been fundamental to the tremendous advances in characterizing, quantifying, and understanding macroscopic-scale, deep continental subduction, rapid exhumation, and mountain-building processes.

The discovery of diamond and coesite in crustal rocks is compelling evidence that continental material has experienced pressures that can only be achieved at mantle depths. At least 20 terranes of unequivocal continental crust containing diamond or coesite are now recognized around the globe; their study constitutes a new field in petrology called ultrahigh-pressure metamorphism. The idea that continents do not subduct has given way to the notion that Earth has been sufficiently cool since the Cryogenian (~850 Ma) to allow density changes to drive continental crust into the mantle during collision. Some of this crust is exhumed to the surface, some pools at the Moho, and the rest sinks into the mantle. In this issue, microscopic observations, phase-equilibrium modeling, geochronology, and geodynamic modeling track the journey of crustal rocks to the mantle and back to Earth’s surface.