GO 324 Rocks and Minerals
Susan Ward Aber

www.emporia.edu/earthsci/amber/go240/metamor.htm

Emporia State University
Emporia, Kansas USA
Earth Science Department

Introduction to Metamorphic Rocks

Introduction   Texture and Structure    Composition and Facies
Methods of Metamorphism   References

Introduction

Metamorphic is taken from Greek, meta means change and morphe, meaning form. Metamorphism is the process by which preexisting rocks are transformed or altered in a solid state under relatively high pressure, temperature, and/or hot circulating fluids. Original igneous, sedimentary, or metamorphic rocks recrystallize without melting, forming minerals and fabrics that are stable under certain pressure-temperature conditions. Many minerals occur in perfect crystalline form or recrystallized using the chemical constituents of the parent rock, which can be elongated in directions parallel to the layers.

The effects of metamorphism are the formation of new minerals, changes in shape and size of mineral grains, and the development of new structures in the rock. These effects are dependent on the properties of the parent rock, type of metamorphic environment present, and the duration of the process until the new rock is finally exposed at the earth's surface.

Metamorphic Parent Rocks
Parent Rock Resulting Metamorphic Rock
Igneous rock- ferromagnesian Amphibolites, schists, gneiss
Sedimentary rock - calcareous/dolomitic Calcite & dolomite marbles, Wollastonite & Diopside skarn, Calc-silicate rock
Sedimentary - argillaceous or feldspathic Muscovite & biotite slate, schist, gneiss
Sedimentary - argillaceous or dolomitic Phlogopite marble, tremolite and actinolite schists and marbles
Sedimentary - ferruginous Specularite and hornblende schists
Sedimentary rock - carbonaceous Graphite in slates, marbles, schists

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Texture and Structure

Texture is the size, shape, and arrangement of grains with three possibilities for metamorphic: porphyroblastic, granoblastic, and crystalloblastic. Porphyroblastic are distinct crystals disseminated in a fine-grained background or matrix of crystals similar to porphyritic igneous textures. Granoblastic is a mosaic of equidimensional anhedral crystals that are roughly uniform in size, formed by recrystallization in a non-foliated metamorphic rock and similar to phaneritic igneous texture. Crystalloblastic texture means the crystals may be any size, from very fine to very large, grains meet at 120° triple junctions and all crystals are of metamorphic origin.

Metamorphic rocks are also described by their structures, which can be:

There are four types of foliation:

Metamorphic rock can also be non-foliated which is equigranular but not layered and typical of high temperature, low-pressure metamorphism.

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Composition and Facies

Typical low-grade metamorphic minerals are: talc, chlorite, epidote, glaucophane, muscovite, biotite, albite, quartz, aragonite, calcite, dolomite, etc.; while high-grade metamorphic minerals include: hornblende, pyroxenes (augite & diopside), garnet, K-feldspar, staurolite, kyanite, quartz, calcite, dolomite, etc. Some of the minerals listed in your text that are formed exclusively or mostly by metamorphism are: actinolite, garnet, chlorite, cordierite, epidote, glaucophane, graphite, kyanite, staurolite, talc, tremolite, and wollastonite.

Common Habits for Common Metamorphic Minerals
COMMON SHAPE METAMORPHIC MINERALS
Flakes muscovite, biotite, chlorite, talc, graphite
Prismatic, columnar, bladed staurolite, hornblende, andalusite, tremolite, actinolite, kyanite, sillimanite
Equant garnet, calcite, dolomite, quartz, pyrite, feldspars

The environment of the metamorphic rock is shown by the facies. A metamorphic facies is a restricted range of pressure, temperature, and other environmental variables which control the degree of metamorphism. Low-grade metamorphic rocks form under low pressure and temperature facies, while high-grade metamorphic rocks form under high pressure and temperature facies. The limits of facies are best shown on a pressure/temperature diagram and facies are named after characteristic minerals.


Image taken from: http://encarta.msn.com/
media_461547757_761578403_-1_1/Metamorphic_Rock_Facies.html

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Methods of Metamorphism

There are three common methods of metamorphism: contact, hydrothermal and dynamothermal. In addition, there are three not so common methods: burial, cataclastic, impact. Contact metamorphism (also called thermal metamorphism) is the heating of rocks surrounding an igneous intrusion which produces metamorphism because of high temperatures (hornfel facies). Hornfels, marble, skarns (calc-silicate) and metaquartzites are common contact metamorphic rocks. Hydrothermal metamorphism is the result of circulation of hot, chemically active fluids through rocks resulting in low-grade metamorphics such as serpentinite or skarn. Dynamothermal metamorphism is on a regional scale under dynamic conditions of mountain building and elevated temperatures produce blueschist, greenschist, amphibolite or eclogite facies depending upon the pressure/temperature conditions. This type of metamorphism affects large areas, at considerable depths, and operates over long spans of time. These rocks are slate, schist, gneiss, amphibolite, greenstone, marble, skarn (calc-silicate), and metaquartzite. Burial metamorphism is the result of deep burial of sediments within basins producing low-grade metamorphism with high pressures (zeolite facies). Cataclastic metamorphism occurs along active fault zones that generate frictional heat from the crushing and fracturing of rocks (mylonite). Impact metamorphism involves extraordinarily high pressure and temperature shock waves which metamorphose and melt rocks upon high-speed meteorite impacts.

Below is a simple classification table for identification of metamorphics.

Grain orientationGrain size or composition Metamorphic rock
preferred orientation banded or eyed, blocky grains gneiss
preferred orientation laminated, coarse grains schist
preferred orientation laminated, very fine, silky phyllite
preferred orientation slaty, no grains visible slate
nonoriented fine-grained hornfel
coarse, sutured mostly quartz quartzite
coarse, sutured any composition, pebble metaconglomerate
coarse, sutured calcite or dolomite marble
coarse, sutured calcium silicates skarn

If you are enrolled in this course, please email me at saber@emporia.edu, place GO 324 metamorphic points in the subject line, provide me with the URL for an educational metamorphic rock website not mentioned on this page. In the email, remind me to add one substitution point to your third test for participating and following instructions!

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References and Links

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Petrology Introduction
www.emporia.edu/earthsci/amber/go324/intro.htm 		Minerals
www.emporia.edu/earthsci/amber/go324/mineral.htm
Rocks
www.emporia.edu/earthsci/amber/go324/rock.htm 		Igneous
www.emporia.edu/earthsci/amber/go324/igneous.htm
Sedimentary Rock
www.emporia.edu/earthsci/amber/go324/sediment.htm 		Metamorphic Rock
www.emporia.edu/earthsci/amber/go324/metamor.htm
Course Field Trip
www.emporia.edu/earthsci/amber/go324/field_trip.htm 		Course Syllabus
www.emporia.edu/earthsci/amber/go324/syllabus.htm

This page originates from the Earth Science department for the use and benefit of students enrolled at Emporia State University. The curriculum is © by the author, 2001-2008. Last update 26 November 2008. For more information contact the course instructor, Dr. S. W. Aber, e-mail: saber@emporia.edu

To understand copyright, visit www.wipo.int/about-ip/en/copyright.html and lcweb.loc.gov/copyright/. All rights reserved. Susan Ward Aber.