porphyroclast

thumb|350px|Augen mylonite from near Røragen, Norway. This deformed megacrystic [[granite has large alkali felspar and small plagioclase feldspar porphyroclasts. Sample 18 cm x 10 cm. Many of the larger porphyroclasts have a clear σ-type geometry, consistent with top to the right shear sense.]] thumb|350px|right|A mylonite showing a number of (rotated) porphyroclasts: a clear red [[garnet left in the picture while smaller white feldspar porphyroclasts can be found all over. Location: the tectonic contact between the autochthonous Western Gneiss Region and rocks of the allochthonous Blåhø nappe on Otrøy, Caledonides, Central Norway.]]

A porphyroclast is a clast or mineral fragment in a metamorphic rock, surrounded by a groundmass of finer grained crystals. Porphyroclasts are fragments of the original rock before dynamic recrystallisation or cataclasis produced the groundmass. This means they are older than the groundmass. They were stronger pieces of the original rock, that could not deform as easily and were therefore not or minimally affected by recrystallisation. They may have been phenocrysts or porphyroblasts in the original rock.