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 br iefly a couple of recent advances in the held of geochronology and met amorphic petrology which have helped to elucidate these processes. Re- equilibration at high temperatures often eradicates information that m ight have been recorded by rocks during their prograde and high-temper ature evolution. One of the major advances of recent years has been th e recognition of the utility of garnet in preserving information on pr ograde thermal histories. A case study involving garnet-bearing rocks from New England is described. In this area, garnet grew during the la tter stages of prograde metamorphism and records heating through 100 d egrees C during decompression of around 2.5 kbar. Combined with chrono logical data on garnet cores and rims, which yield a time interval for garnet growth of 10 Ma, these data yield a heating rate of similar to 10 degrees C Ma(-1) and an exhumation rate of about 1 mm a(-1). Compa rison with simple thermal models suggests that the heating rate is too high to be accounted for by simple conductive relaxation. At temperat ures appropriate to crustal melting, even garnet re-equilibrates such that any chronological information is lost. At these grades one approa ch is to use the extent of diffusional modification of growth zonation profiles to provide time constraints on the process operating. Mn dis tributions 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 uppe r crustal rocks adjacent to migmatitic rocks.