A porphyroclast is a clast or mineral fragment in a metamorphic rock, surrounded by a groundmass of finer grained crystals.
[1] Porphyroclasts are fragments of the original rock before dynamic recrystallisation or cataclasis produced 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 been phenocrysts or porphyroblasts in the original rock.
The timing of porphyroblast growth can be determined by examining the microstructure preserved (or not) within them as poikiloblasts.