The metamorphic record of changes in tectonothermal regime on Earth and the geodynamic implications

Contemporary metamorphism occurs in the subduction zone, characterized by low dT/dP, registered by HP-LT metamorphism, and in the arc-backarc/orogenic hinterland, characterized by high dT/dP, registered by HT-LP metamorphism. This duality is the hallmark of one-sided subduction on Earth. Granulite facies ultrahigh-temperature metamorphism (G-UHTM; T>900°C, apparent thermal gradients 800-1,300°C/GPa) occurs from the Mesoarchean–Neoarchean to the Cambrian, and is inferred at depth in Phanerozoic orogens. Medium-temperature eclogite-high-pressure granulite metamorphism (E-HPGM; P> Sill stability and Blueschists become evident in the Neoproterozoic; Lws-blueschists and LT-eclogites (HPM), and UHPM are predominantly Phanerozoic phenomena. HPM-UHPM registers deep subduction–eduction of continental lithosphere during the early stages of subduction-to-collision orogenesis; UHPM belts are generally associated with closure of short-lived ocean basins. In contrast, the Eoarchean-to-Mesoarchean generally records P-T conditions characteristic of low-P-moderate-to-high-T metamorphism. Both high-grade terranes and greenstone belts may record high apparent thermal gradients in the range 800–1,000°C/GPa, but Mesoarchean greenstones in Barberton, South Africa record lower apparent thermal gradients in the range 450-700°C/GPa that presage cooler environments.
Granulites are common, but neither extreme thermal conditions (UHTM) nor high pressures (E-HPGM) are documented before the Mesoarchean-Neoarchean. Blueschists are not found, and there is no identified imprint of subduction of continental crust to mantle conditions, although perhaps the continental crust, if subducted, was not returned but retained in the mantle. Neoarchean and Paleoproterozoic diamonds remain enigmatic regarding Precambrian UHPM. One-sided subduction may have begun locally in the Mesoarchean-Neoarchean and may have become global during the Neoarchean-to-Paleoproterozoic transition. The occurrence of UHTM and E-HPGM belts since the Neoarchean manifests the onset of a ’Proterozoic plate tectonics regime’, which evolved during a Neoproterozoic transition to the ’modern plate tectonics regime’ characterized by HPM-UHPM.