THE CRUSTAL ARCHIVE OF METAMORPHIC TEMPERATURES: A RECORD OF SECULAR CHANGE IN THE THERMAL ENVIRONMENTS OF CRUSTAL REWORKING DURING OROGENESIS

Convergent plate boundaries are characterized by one-sided subduction creating an asymmetric thermal structure with lower dT/dP in the subduction channel and higher dT/dP in the backarc/orogenic hinterland. During orogenesis these thermal environments are imprinted in the rock record as contrasting types of metamorphism, HPM–UHPM, eclogite–HP granulite metamorphism (E-HPGM) and granulite–UHT metamorphism (G-UHTM). When linked with precise ages, P–T retrieved from metamorphic belts may be inverted to yield apparent thermal gradients for peak metamorphism at particular times in Earth history, providing information about secular change in thermal regimes and geodynamics. Eoarchean–Mesoarchean crust registers uniformly low-to-moderate-P–moderate-to-high-T metamorphism. In contrast, two contrasting types of metamorphism, G-UHTM and E-HPGM, appear in the late Mesoarchean–Neoarchean rock record. This change registers the onset of one-sided subduction and the beginning of a ‘Proterozoic plate tectonics regime’ characterized by ‘hot’ collision. Results of 2-d numerical experiments show that the geodynamic regime is controlled by the degree of lithosphere weakening induced by sub-lithospheric melts. As upper mantle temperature declined to <200°C warmer than present during the late Archean, the reduced melt flux allowed stronger lithosphere stabilizing one-sided subduction. Most of the Proterozoic rock record is characterized by E-HPGM, with gradients of 350–750°C/GPa, and G-UHTM, with gradients of 750–1500°C/GPa. However, there is a dramatic change in the late Neoproterozoic with the appearance of HPM–UHPM, with gradients of 150-350°C/GPa, followed by the virtual disappearance of G-UHTM from the rock record after the Cambrian. These changes occurred as upper mantle temperature declined to <100°C warmer than present during the late Neoproterozoic, which facilitated the widespread formation and exhumation of HPM–UHPM rocks associated with ‘cold’ collision during the Phanerozoic.