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.
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. 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 been phenocrysts or porphyroblasts in the original rock.
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.
^Passchier C.W. & Trouw R.A.J. (2005). Microtectonics. Springer Science & Business Media. pp. 132–141. ISBN 9783540293590.
A porphyroclast is a clast or mineral fragment in a metamorphic rock, surrounded by a groundmass of finer grained crystals. Porphyroclasts are fragments...
sense indicators are C/S fabrics, asymmetric porphyroclasts, vein and dike arrays, mantled porphyroclasts and mineral fibers. All of these indicators have...
occurs adjacent to a relatively large, undeformed particle, such as a porphyroclast. Pressure shadows often appear in thin sections as pairs of roughly...
metamorphic minerals relative to a phase of deformation—before deformation porphyroclast—after deformation porphyroblast. Igneous textures include such properties...
metamorphism and shearing. The core of the augen is a porphyroblast or porphyroclast of a hard, resilient mineral such as garnet. The augen grows by crystallisation...
orientation (CPO). mantled and winged porphyroclasts. Well-known examples are theta (Θ)-objects and phi (Φ)-porphyroclasts, as well as sigma (σ)- and delta...
crystal in an igneous rock. Porphyroblasts are often confused with porphyroclasts, which can also be large outstanding crystals, but which are older than...
Observatory at Depth (SAFOD) they are overarchingly composed of serpentinite porphyroclasts and sedimentary rock amongst a Magnesium-rich clay matrix. Saponite...
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