Problems with dating metamorphic rocks
Identification of Metamorphic Rocks based on images?
Hey all, I need to identify the metamorphic rocks from their images.
Quartz is a common phenocryst in igneous rocks, but it never occurs as a porphyroblast in metamorphic rocks.
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The rock may display compositional banding and it often contains porphyroblasts (garnets in this rock). These are the constituents of eclogite which is a closely related rock type. Several less known names have been given to a specific varieties: staurotile (contains staurolite porphyroblasts), prasinite (metamorphosed mafic rock with epidote, chlorite and hornblende in equal proportions), sismondinite (chloritoid is the dominant mineral phase).
The family of schistose rocks is compositionally very diverse but most of them are derivatives of former mudstones metamorphosed to various aluminous schists (metapelites). Aluminous varieties often contain large crystals in a finer matrix.
This roughly corresponds to temperatures in the range of 300-600 °C and pressures from several to several tens of kilometers.
This means that not only composition but the formation conditions too are highly variable.
Schist is rich in flaky and soft sheet silicate minerals which makes it structurally weaker than gneiss or granite.