Metamorphic chemical geodynamics in continental subduction zones

Chemical geodynamics is an integrated discipline that studies the geochemical structure and tectonic evolution of geospheres with the aim of linking tectonic processes to geochemical products in the Earth system. It was primarily focused on mantle geochemistry, with an emphasis on geochemical recycling in oceanic subduction zones. It has been extended to geochemical reworking and recycling under high-pressure (HP) to ultrahigh-pressure (UHP) conditions in all convergent plate margins. In particular, UHP terranes, along with UHP metamorphic minerals and rocks in continental subduction zones, represent natural laboratories for investigating geochemical transport and fluid action during subduction and exhumation of continental crust. As a result of this extension, the study of UHP terranes has significantly advanced our understanding of tectonic processes in collisional orogens. This understanding has principally benefited from the deciphering of petrological and geochemical records in deeply subducted crustal rocks that occur in different petrotectonic settings. This review focuses on the following issues in continental subduction zones: the time and duration of UHP metamorphism, the origin and action of metamorphic fluid/melt inside UHP slices, the element and isotope mobilities under HP to UHP conditions during continental collision, the origin of premetamorphic protoliths and its bearing on continental collision types, and the crustal detachment and crust–mantle interaction in subduction channels. The synthesis presented herein suggests that the nature of premetamorphic protoliths is a key to the type of collisional orogens and the size of UHP terranes. The source mixing in subduction channels is a basic mechanism responsible for the geochemical diversity of continental and oceanic basaltic rocks. Therefore, the geochemical study of HP to UHP metamorphic rocks and their derivatives has greatly facilitated our understanding of the geodynamic processes that drive the tectonic evolution of convergent plate margins from oceanic subduction to continental collision. Consequently, the study of chemical geodynamics has been developed from oceanic subduction zones to continental collision zones, and it has enabled important contributions to development of plate tectonic theory.

► Geochronological constraints on the time and duration of UHP metamorphism at mantle depths during continental collision.
► Geochemical transport and fluid action during UHP metamorphism due to subduction and exhumation of continental crust.
► Geochemical competition between thermodynamics and kinetics during continental subduction-zone metamorphism.
► Temporal and spatial relationships between protolith origin and collision type in continental subduction zones.
► Crustal detachment and crust–mantle interaction in subduction channels during continental collision.