Granulite facies ultrahigh temperature metamorphism (GUHTM) is documented in the rock record predominantly from Neoarchean to Cambrian; GUHTM facies series rocks may be inferred at depth in younger, particularly Cenozoic orogenic systems. The first occurrence of GUHTM in the rock record signifies a change in geodynamics that generated transient sites of very high heat flow. Many GUHTM belts may have developed in settings analogous to modern continental backarcs. On a warmer Earth, the cyclic formation of supercontinents and their breakup, particularly by extroversion, which involved destruction of ocean basins floored by thinner lithosphere, may have generated hotter continental backarcs than those associated with the modern Pacific rim. Mediumtemperature eclogitehighpressure granulite metamorphism (EHPGM) also is first recognized in the Neoarchean rock record and occurs at intervals throughout the Proterozoic and Paleozoic rock record. EHPGM belts are complementary to GUHTM belts and are generally inferred to record subductiontocollision orogenesis. Blueschists become evident in the Neoproterozoic rock record; they record the low thermal gradients associated with modern subduction. Lawsonite blueschists and eclogites (highpressure metamorphism, HPM) and ultrahigh pressure metamorphism (UHPM) characterized by coesite (±lawsonite) or diamond are predominantly Phanerozoic phenomena. HPMUHPM registers the low thermal gradients and deep subduction of continental crust during the early stage of the collision process in Phanerozoic subductiontocollision orogens. Although perhaps counterintuitive, many HPMUHPM belts appear to have developed by closure of small ocean basins in the process of accretion of a continental terrane during a period of supercontinent introversion (Wilson cycle ocean basin opening and closing). A duality of metamorphic belts—reflecting a duality of thermal regimes—appears in the record only since the Neoarchean Era. A duality of thermal regimes is the hallmark of modern plate tectonics and the duality of metamorphic belts is the characteristic imprint of plate tectonics in the rock record. The occurrence of both GUHTM and EHPGM belts since the Neoarchean manifests the onset of a “Proterozoic plate tectonics regime”, although the style of tectonics likely involved differences. The “Proterozoic plate tectonics regime” evolved during a Neoproterozoic transition to the “modern plate tectonics regime” characterized by colder subduction and subduction of continental crust deep into the mantle and its (partial) return from depths of up to 300 km, as chronicled by the appearance of HPMUHPM in the rock record. The age distribution of metamorphic belts that record extreme conditions of metamorphism is not uniform, and metamorphism occurs in periods that correspond to amalgamation of continental lithosphere into supercratons (e.g. Superia/Sclavia) or supercontinents (e.g. Nuna (Columbia), Rodinia, Gondwana, and Pangea).
