- Porphyroclast
A porphyroclast is a
clast ormineral fragment in ametamorphic rock , surrounded by agroundmass of finer grainedcrystal s. Porphyroclasts are fragments of the original rock beforedynamic recrystallisation produced the groundmass. This means they are older than the groundmass. They were stronger pieces of the original rock, that could not as easily deform and were therefore not or hardly affected by recrystallisation. They may have beenphenocryst s orporphyroblast s in the original rock.Porphyroclasts are often confused with porphyroblasts. The latter are also large crystals in a finer matrix, but they grew after deformation took place and after the matrix was formed. The timing of porphyroblast growth can be determined by examining the microstructure preserved (or not) within them as poikiloblasts.
In strongly deformed rocks porphyroclasts are often rotated by the
shear stress in the rock. Their shape can be used to determine the direction of the shear.
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