Geodynamic Regimes and Tectonic Settings for Metamorphism: Relationship to the Supercontinent Cycle

Metamorphism associated with orogenesis provides a mineral record that may be inverted to yield ambient apparent thermal gradients. On modern Earth, tectonic settings with lower thermal gradients are characteristic of subduction zones whereas those with higher thermal gradients are characteristic of backarcs and orogenic hinterlands. The duality of thermal environments reflects the asymmetry of one-sided subduction, which is the hallmark of modern plate tectonics; a duality of metamorphic belts is the characteristic imprint of one-sided subduction in the geological record. Apparent thermal gradients derived from inversion of age-constrained metamorphic P-T data may be used to identify tectonic settings of ancient metamorphism and to evaluate Precambrian geodynamic regimes. The Neoarchean records the first occurrences of granulite facies ultra-high temperature metamorphism (G-UHTM) and medium-temperature eclogite-high-pressure granulite metamorphism (E-HPGM), signifying a change in geodynamics that generated sites of higher and lower heat flow than those implied by apparent thermal gradients recovered from the older geological record. GUHTM is dominantly a Neoarchean-Cambrian phenomenon inferred to have developed in settings analogous to backarcs and orogenic hinterlands. In addition to Proterozoic occurrences, E-HPGM is common in the Paleozoic Caledonides and Variscides, and is inferred to record subduction-to-collision orogenesis. The occurrence of G-UHTM and E-HPGM belts since the Neoarchean signifies a change to one-sided subduction of oceanic lithosphere, possibly beginning as early as the Paleoarchean, and widespread transfer of water into the upper mantle. This change registers the beginning of a 'Proterozoic plate tectonics regime' that evolved during a Neoproterozoic transition to the 'modern plate tectonics regime' characterized by colder apparent thermal gradients and deep subduction of continental crust. The age distribution of metamorphic belts is not uniform, recording amalgamation of continental lithosphere into supercratons or supercontinents.