Eclogite-facies metamorphism of continental crust at the Mesoarchean-to-Neoarchean transition

Abstract Eclogite-facies rocks record deep burial and exhumation of crust, and on the modern Earth are most commonly associated with subduction settings. The scarcity of blueschist- and eclogite-facies metamorphism from the Archean rock record indicates either that subduction was not operative or that the evidence has somehow been destroyed. We report an eclogite-facies mineral association (phengite + kyanite + rutile) preserved, with quartz, as inclusions within garnet porphyroblasts in a putative metasedimentary rock from the Lewisian Gneiss Complex in northwest Scotland, part of the North Atlantic craton. In situ Lu–Hf dating of garnet constrains eclogite-facies metamorphism to the Mesoarchean to Neoarchean transition at 2.81 ± 0.04 Ga. Phase equilibrium modeling, Zr-in-rutile thermometry, and phengite barometry constrain pressures (P) to 1.5–2.5 GPa and temperatures (T) to 580–660 °C, corresponding to thermobaric ratios (T/P) between 230 °C/GPa and 440 °C/GPa, markedly cooler than recorded by metamorphic rocks of a similar age elsewhere. The low T/P ratios may provide an early record of the transition from some pre-plate tectonic regime to one with an increasing role for stable subduction, and which ultimately became plate tectonics. This transition reflects thickening and strengthening of the lithosphere, the inevitable long-term consequence of secular cooling of the mantle.