Limits of Metamorphism
The temperature lower limit of metamorphism is considered to be 100 - 200 °C, to exclude diagenetic changes, due to compaction, which result in sedimentary rocks. There is no agreement on a pressure lower limit. Some workers argue that changes in atmospheric pressures are not metamorphic, but some types of metamorphism can occur at extremely low pressures (see below).
The upper boundary of metamorphic conditions is related to the onset of melting processes in the rock. The maximum temperature for metamorphism is typically 700 - 900 °C, depending on the pressure and on the composition of the rock. Migmatites are rocks formed at this upper limit, which contain pods and veins of material that has started to melt but has not fully segregated from the refractory residue. Since the 1980s it has been recognized that, rarely, rocks are dry enough and of a refractory enough composition to record without melting "ultra-high" metamorphic temperatures of 900 - 1100 °C.
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