9 metamorphicrocks

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Metamorphism • The transformation of rock by temperature and pressure • Metamorphic rocks are produced by transformation of: • Igneous, sedimentary and igneous rxs Thanks to CU Boulder Geology Dept for use of some of these slides Metamorphism • Metamorphism progresses from low to high grades • Rocks remain solid during metamorphism W hat causes metamorphism? • Heat • Most important agent • Heat drives recrystallization - creates new, stable minerals • Pressure (stress) • Increases with depth • Pressure can be applied equally in all directions or differentially Origin of pressure in metamorphism Confining or hydrostatic pressure: equal in all directions Directed pressure: largely in one direction or along a particular axis Main factor affecting metamorphism • Parent rock • Metamorphic rocks typically have the same chemical composition as the rock they were formed from • Different minerals, but made of the same stuff • Exception: gases (carbon dioxide, CO2) and water (H2O) may be released Progressive metamorphism of a shale Shale Progressive metamorphism of a shale Slate Progressive metamorphism of a shale Phyllite Progressive metamorphism of a shale Schist Example of slate Slate roof Common metamorphic rocks • Foliated rocks • Phyllite – Grade of metamorphism between slate and schist – Made of small platy minerals – Glossy sheen with rock cleavage – Composed mainly of muscovite and/or chlorite Phy llite (left) and Slate (right) lack v isible mineral grains Common metamorphic rocks • Foliated rocks • Schist – Medium- to coarse-grained – Comprised of platy minerals (micas) – The term schist describes the texture – To indicate composition, mineral names are used (such as mica schist) Mica Schist - note well developed foliation A mica garnet schist Common metamorphic rocks • Foliated rocks • Gneiss – Medium- to coarse-grained – Banded appearance – High-grade metamorphism – Composed of light-colored feldspar layers with bands of dark mafic minerals Gneiss display s bands of light and dark minerals Diorite to Gneiss Morph (orthogneiss - from igneous protolith) W hat are metamorphic textures? • Texture refers to the size, shape, and arrangement of mineral grains within a rock • Foliation – planar arrangement of mineral grains within a rock Outcrop of foliated gneiss Metamorphic textures • Foliation • Foliation can form in various ways: – Rotation of platy or elongated minerals – Recrystallization of minerals in a preferred orientation – Changing the shape of equidimensional grains into elongated and aligned shapes Flattened Pebble Conglomerate = flattening Dev elopment of foliation due to directed pressure
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