Pre-foliation metamorphism in low-pressure/high-temperature terrains

Porphyroblasts in schists in some middle- to upper-crustal, low-pressure regional metamorphic terrains contain equant, random inclusions that are much smaller than the average grain size of the matrix, which may be strongly foliated, the folia typically being deflected around the porphyroblasts. These microstructures are consistent with growth of the porphyroblasts either before or during the earliest stages of development of a tectonic foliation, although previous brittle deformation may have taken place. Anatectic leucosomes may delineate bedding and/or the earliest tectonic foliations, which are folded in subsequent folding events, and new leucosomes may occur in axial surfaces of later folds. This evidence suggests that high-grade metamorphic conditions were in operation before or during the development of the earliest foliation and persisted during subsequent episodes of folding. Also consistent with this inference is the occurrence of random porphyroblasts boudinaged in the first foliation. In many middle to upper crustal, low-pressure regional metamorphic terrains, granitoids appear to be responsible for the development and localization of "regional-aureole" metamorphism, when superimposed on a more regional temperature rise. The granitoids may also induce and localize folding and the development of foliations, owing partly to weakening of the rocks by heat and fluids from the magma and especially to fluids released pervasively from prograde reactions. Such an interpretation can account for the growth of high-grade metamorphic minerals before the development of a tectonic foliation.