The Realm of Ultrahigh-Pressure Metamorphism ULTRAHIGH-PRESSURE METAMORPHISM

The concept that buoyant continents resist subduction has slowly been eroded by the recognition of coesite and diamond in crustal rocks from mainly Phanerozoic mountain belts around the world (FIG. 1). Coesite, a highpressure polymorph of SiO2, was independently discovered in two different crustal rocks almost 30 years ago. Christian Chopin (1984) reported tiny grains of coesite as inclusions in pale pink garnet in an unusual quartzite from Dora-Maira, Italy, while David Smith (1984) identifi ed coesite inclusions in clinopyroxene in an eclogite from Grytting, in the Western Gneiss Region of Norway. The identifi cation of microdiamonds as abundant inclusions in garnet from eclogites, gneisses, and schists in the Kumdy Kol unit of the Kokchetav massif, Kazakhstan (Sobolev and Shatsky 1990), followed shortly after. These metamorphic diamonds are not the gem-quality diamonds brought to the surface by kimberlite pipes and coveted by society from time immemorial; rather, they are scraggly, little skeletal crystals, commonly less than 10 microns in diameter. Microdiamond and coesite in crustal rocks (FIG. 2) are relicts of a journey that has taken surface material to mantle depths and back. Their discovery has fundamentally changed the way geologists view processes at convergent plate margins, from the Cryogenian to the present day. Coesite is commonly found in eclogite—an attractive rock that is the poster child for high-pressure (HP) metamorphism because of its easily distinguished constituents, red garnet and green omphacite (FIG. 3). Basalt and gabbro transform into eclogite at high pressure when all the plagioclase has reacted with other ferromagnesian minerals to form omphacite, a dense Na-rich clinopyroxene, and garnet. The part of pressure– temperature (P–T) space that is inhabited by eclogite has long been known as the eclogite facies, but rocks of all compositions can occupy this space. The needle-ina-haystack fi nds of diamond and coesite have extended known examples of eclogite facies metamorphism to even higher pressures and spawned a new fi eld of metamorphic petrology called ultrahighpressure (UHP) metamorphism. Mineral assemblages with pressures above the quartz-to-coesite transition are ultrahigh-pressure metamorphic rocks by defi nition (e.g. Chopin 2003; Liou et al. 2009; FIG. 4). Coesite and diamond are the index minerals of UHP metamorphism, but many other minerals and microstructures point to pressures in excess of the quartz-to-coesite transition (see Schertl and O’Brien 2013 this issue). A look at P–T space (FIG. 4) shows that eclogite facies conditions are not attained in normal continental crust but are possible when the crustal thickness doubles. Even where the crust is over 80 km thick beneath the Tibetan Plateau, the thickest crust on Earth, the rocks are still in the quartz stability fi eld. To produce coesite, rocks must travel to mantle depths. Terminology can be confusing because very high pressures are the norm deep in the Earth’s mantle and core (see Elements, June 2008); but in this issue, the term UHP metamorphism refers to crustal rocks that have experienced mantle pressures.