Rate and time controls on metamorphic processes

Among the most important controls on metamorphism is the rate at which tectonic processes operate. The purpose of this paper is to review briefly a couple of recent advances in the field of geochronology and metamorphic petrology which have helped to elucidate these processes. Re‐equilibration at high temperatures often eradicates information that might have been recorded by rocks during their prograde and high‐temperature evolution. One of the major advances of recent years has been the recognition of the utility of garnet in preserving information on prograde thermal histories. A case study involving garnet‐bearing rocks from New England is described: In this area, garnet grew during the latter stages of prograde metamorphism and records heating through 100°C during decompression of around 2.5 kbar. Combined with chronological data on garnet cores and rims, which yield a time interval for garnet growth of 10 Ma, these data yield a heating rate of ∼ 10°C Ma‐1 and an exhumation rate of about 1 mm a‐1. Comparison with simple thermal models suggests that the heating rate is too high to be accounted for by simple conductive relaxation. At temperatures appropriate to crustal melting, even garnet re‐equilibrates such that any chronological information is lost. At these grades one approach is to use the extent of diffusional modification of growth zonation profiles to provide time constraints on the process operating. Mn distributions in garnets from the Zanskar Himalaya are used to limit the time available for high‐grade metamorphism. The times obtained of 1–2 Ma are extremely short and require cooling by emplacement of cold upper crustal rocks adiacent to migmatitic rocks.