Variable P–T paths and HP-UHP metamorphism in a Precambrian terrane, Gridino, Russia: Petrological evidence and geodynamic implications

The Gridino Complex represents one of the oldest eclogite-facies terranes on Earth. It consists of blocks, boudins and lenses of eclogites, pyroxenites, and epidosites as well as deformed eclogitized dikes within biotite-amphibole gneisses. Detailed petrological studies of the pyroxenites and different types of eclogites reveal considerable diversity in metamorphic pressure (P) – temperature (T) conditions (from 1.3 GPa at 660 °C to 3.0 GPa at 660 °C) and fluid regimes (wet vs. dry) experienced by these rocks. Dike-related rocks escaped prograde metamorphism and reached higher pressures than the lenses and blocks that experienced considerable prograde metamorphic reworking. The variability in P–T conditions and the shapes of P–T paths are in agreement with the results of thermomechanical modeling and data from (U)HP metamorphic rocks exhumed during continent–continent collision in the Phanerozoic. The T/P ratio estimated for an eclogitized dike from Eclogitovii Island of the Gridino Complex corresponds to the gradients of < 350 °C/GPa attributed to high-pressure (HP)–ultrahigh-pressure (UHP) metamorphic belts, which are often considered as representative of modern style plate tectonics operating in the Phanerozoic Eon. The data presented in this paper suggest that occurrences of HP-UHP metamorphic terrains might be extended back towards the time of either the assembly of Columbia in the Paleoproterozoic or Kenorland in the Neoarchean.