Metamorphic Rocks: Formation, Types, and Facies, Study notes of Geology

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Metamorphic Rocks

Geology 200

Geology for Environmental Scientists

Regionally metamorphosed rocks shotthrough with migmatite dikes. Black

Canyon of the Gunnison, Colorado

Major Concepts

• Metamorphic rocks can be formed from any

rock type: igneous, sedimentary, or existingmetamorphic rocks.

• Involves recrystallization in the solid state,

often with little change in overall chemicalcomposition.

• Driving forces are changes in temperature,

pressure, and pore fluids.

• New minerals and new textures are formed.

Metamorphism of a Granite to a Gneiss

Two major types of metamorphism --

contact and regional

Major Concepts

• Foliated rocks - slate, phyllite, schist,

gneiss, mylonite

• Non-foliated rocks - quartzite, marble,

hornfels, greenstone, granulite

• Mineral zones are used to recognize

metamorphic facies produced by systematicpressure and temperature changes.

Fig. 6.5. Different minerals of the same composition, Al

SiO 2

, are stable at different temperatures and pressures. 5

Where does the heat come from?• Hot magma ranges from 700-

0 C.

Causes contact metamorphism.

• Deep burial - temperature increases 15-

0 C

for every kilometer of depth in the crust.Subduction and continental collision buryrocks to depths of 10’s of kms. This isregional metamorphism. Mostmetamorphic rocks form this way.

Chemically Active Fluids

• Metasomatism - fluid transport during

metamorphism can result in the gain or lossof atoms. Clays release H

O (de-watering) 2

as they change to minerals such as kyaniteor garnet.

• Hydrothermal alteration - injection of hot

water into rocks from metasomatism canproduce metallic ores such as lead and zinc.

Slate with fractures.The fractures arefilled with quartzcrystallized fromfluids (de-watering)expelled from theslate duringmetamorphism.

Textures of Metamorphic Rocks

• Foliated (Latin for leaf) - grades from slate

to phyllite to schist to gneiss. Produced byplaty minerals growing in direction of leaststress.

• Nonfoliated - typical when parent rock is

sandstone or limestone. Also occurs incontact metamorphism and extrememetamorphism (granulite facies).

• Mylonitic - formed along faults; rare.

Kinds of Metamorphic Rocks

• Slate - low grade metamorphosed shale;

very fine grained; slaty cleavage should notbe confused with bedding planes.

• Phyllite - next step up from slate, larger

mica grains give the rock a luster.

• Schist - a strongly foliated rock with large

grains of platy minerals. Named for itsminerals; e.g., chlorite schist.