4. What is the difference between the porphyroblastic texture that is common to

Question

4. What is the difference between the porphyroblastic texture that is common to many metamorphic rocks involved in general crustal thickening and the porphyroclastic texture that is restricted to metamorphic rocks involved in crustal shear zones through granite? Both textures involve larger nuggets in a finer-grained matrix, so how are they different? 2 pts each) Choose between. (for the porphyroBlastic texture) or C (for the porphyroClastic texture) a. larger nuggets older tharn the finer grained matrix d. larger nuggets formed by grain growth b. formed by ductile shearing and recrystallization c. larger nuggets younger than e. larger nuggets of garnets and the finer grained matrix staurolites

Explanation / Answer

Porphyroblastic texture is a term used when the mineral grains are formed as a result of reaction during the metamorphic processes (i.e the mineral grain grows during the processes of metamorphism). It is involved generally in crustal thickening as during crustal thickening processes there is generally rise in temperature. For metamorphic reactions temperature is an important factor and aids in the reaction processes and growth of minerals.

Porphyroclastic texture is a term used when the mineral grains are huge grains/ porhyroclast which are already present in the rock( grains maybe formed due to igneous processes) and later are affected by metamorphic processes of directed stress etc(directed stress is an inportant factor in shear zones). which alter the shape of the porphyroclast. Porphyroclasts are restricted to metamoprphic rocks involved in shear zones through granite as in shear zones directed stress is more dominant factor than temperature. Hence the mineral reactions are slow and sluggish. Therefore the porphyroclasts are slightly altered around the periphery but dominantly their shapes are altered as a result of directed stress.

a. larger nuggets older than the finer grained matrix - porphyroclastic texture (as the grains are older than the matrix)

b.formed by ductile shearing and recrystallization - porphyroclasts (as the periphery of the clasts are usually ductle and recrystallized)

c. larger nuggets younger than the fine grained matrix - porphyroblastic (as the nuggets are younger)

d. larger nuggets formed by grain growth- porphyroblastic( as formed by grain growth)

e larger nuggets of garnets and staurolites - porphyroblastic (garnets and staurolites are typical porphyroblasts)

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