porphyroclast

 Aporphyroclastis aclastormineralfragment in ametamorphic rock, surrounded by agroundmassof finer grainedcrystals.Porphyroclasts are fragments of the original rock beforedynamic recrystallisationor cataclasis produced the groundmass. This means they are older than the groundmass. They werestrongerpieces of the original rock, that could not as easilydeformand were therefore not or hardly affected by recrystallisation. They may have beenphenocrystsorporphyroblastsin the original rock.

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.

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.

Porphyroclasts are often confused with porphyroblasts. The latter are also large crystals in a finer matrix, but they grew during, or after deformation took place and during or 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.

Porphyroclast systems

Where porphyroclasts have rims made of finer grained crystals, they are referred to as porphyroclast systems. The geometries of porphyroclast systems can be used to determine the sense of shear within a shear zone.

This article uses material from the Wikipedia article  Metasyntactic variable, which is released under the  Creative Commons Attribution-ShareAlike 3.0 Unported License.