Friday Rocks #37: Porphyroclastic Ultramylonite

[caption id="attachment_572" align="aligncenter" width="610"]Feldspar porphyroblasts in the ultramylonite of the Pofadder Shear Zone Feldspar porphyroclasts in the ultramylonite of the Pofadder Shear Zone[/caption]

Porphyoclasts are deformed crystals found in a metamorphic rock. In a mylonite weaker minerals deform though crystalplastic processes and form a "toothpaste" texture. Stronger minerals will try to resist the deformation, but may break and stretch along with the weaker minerals. As the weak minerals recrystallize, they will flow around the remnant stronger minerals.

[caption id="attachment_573" align="aligncenter" width="610"]Feldspathic porphyclast in the ultramylonite of the Pofadder Shear Zone Feldspathic porphyclast in the ultramylonite of the Pofadder Shear Zone[/caption]

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