CSIR NET June 2016 Q80: Metamorphic Petrology
HP/UHP (high-pressure/ultra-high-pressure) metamorphism requires subduction of relatively cold crustal material to great depths without melting, which is only achievable under the cooler geothermal gradients of modern-style plate tectonics.
In the Archean and earlier, mantle and crustal geothermal gradients were significantly hotter than today, so subducting slabs would heat up and melt or weaken before reaching the depths/pressures needed to generate HP/UHP mineral assemblages (e.g., coesite, diamond-bearing eclogites). Only once Earth's thermal regime cooled sufficiently in post-Archean time could subducting crust remain cool enough, long enough, at depth to preserve HP/UHP metamorphism. Option 1 is correct.
1) Correct — excessively high Archean geothermal gradients precluded the cold, deep subduction needed for HP/UHP conditions, the standard explanation in the literature. 2) Incorrect — bulk crustal composition (e.g., basaltic/felsic protoliths capable of HP/UHP assemblages) existed by the Archean; the limiting factor was thermal regime, not composition. 3) Incorrect — reaction kinetics were not the limiting control; preservation and formation depend on thermal/tectonic conditions. 4) Incorrect — continental crust did exist in the Archean, just under a different (hotter) tectonic regime.