Metamorphic Rocks

Metamorphism: A Process of Change

Introduction 

Metamorphic – Changed from an original “parent.” Meta = Change.  Morph = Form or shape. 

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Parent rocks are called “protoliths.” Metamorphism can occur to any protolith.

What Changes and Why? 

Protoliths undergo pronounced changes in… Texture.  Mineralogy. 

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Due to changes in… Temperature.  Pressure. 

Metamorphic Character 

Metamorphic rocks have distinctive properties. Unique minerals – Some that are only metamorphic.  Unique foliation – A planar fabric from aligned minerals. 

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Transformations can change the rock slightly to completely.

Metamorphic Processes 

Plastic deformation – Mineral grains soften and deform. Requires elevated temperatures.  Rock is squeezed or sheared.  Minerals act like plastic, changing shape without breaking. 

Causes of Metamorphism 

The agents of metamorphism are… Heat (T).  Pressure (P).  Compression and/or shear due to tectonic stresses.  Reaction with heated water. 

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Not all agents are required; they often do co-occur.

Heat (Temperature, T) 

Metamorphism occurs as the result of heat. 

Temperature (T) ranges between 200oC and 850oC.

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The upper T limit is melting. It varies based upon rock mineral composition and water content. Heat energy breaks and reforms atomic bonds.

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Sources of heat.

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The geothermal gradient.  Magmatic intrusions.  Compression. 

Pressure (P) 

P increases with depth in the crust. 250 to 300 bars per km (1 bar is almost 1 atm = 100 kPa).  Metamorphism occurs mostly in 2 to 12 kbar range. 

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T and P both change with depth. Mineral stability is highly dependent upon T and P. This stability can be graphed on a “phase diagram.”  Changes in T and P lead to changes in minerals. 

Differential Stress   

Pressure that is greater in one orientation. A commonplace result of tectonic forces. Two kinds of differential stress: Normal and shear. 

Normal Stress – Operates perpendicular to a surface. Tension – Pull-apart normal stress. Compression – Push-together normal stress.

Differential Stress 

Shear Stress – Operates sideways across a surface. Causes material to be “smeared out.”

Differential Stress 

At higher T and P, differential stress deforms rock. 

Rocks change shape slowly without breaking.


Deformation acts on minerals with specific shapes. Equant – Roughly equal in all dimensions.  Inequant – Dimensions not the same. 

Platy (pancake-like) – 1 dimension shorter. Elongate (cigar-shaped) – 1 dimension longer. 

Differential stress causes these minerals to align.


Preferred platy mineral alignment is called foliation. Foliation imparts a layered or banded appearance.  Rocks commonly break parallel to foliation planes. 

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Foliation develops perpendicular to compression. 

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Minerals flatten, recrystallize and rotate.

Inequant grains align by rotation and new growth.

Hydrothermal Fluids 

Hot water with dissolved ions and volatiles (Hydrothermal Fluids) facilitate metamorphism. Accelerate chemical reactions.  Alter rocks by adding or subtracting elements. 

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Hydrothermal alteration is called metasomatism.

Metamorphic Rock Types 

Two major subdivisions of metamorphic rocks. 

Foliated – Has a through-going planar fabric. Subjected to differential stress. Has a significant component of platy minerals. Classified by composition, grain size, and foliation type.

Metamorphic Rock Types 

Nonfoliated – No planar fabric evident. Crystallized without differential stress. Comprised of equant minerals only. Classified by mineral composition.

Metamorphic Rocks 

Slate – Fine clay, low-grade metamorphic shale. 

Has a distinct foliation called slaty cleavage. Develops by parallel alignment of platy clay minerals. Slaty cleavage oriented perpendicular to compression. Slate breaks along this foliation creating flat sheets.


Phyllite - Fine mica-rich rock. Formed by low- to medium-grade alteration of slate.  Clay minerals neocrystallize into tiny micas.  Micas reflect a satiny luster.  Phyllite is between slate and schist. 


Schist – Fine or coarse rock with larger micas. Medium- to high-grade metamorphism.  Has a distinct foliation called schistosity. 

Parallel alignment of large mica crystals. Micas are visible because they have grown at higher T. 

Schist often has other minerals due to neocrystallization. Quartz. Feldspars. Kyanite. Garnet. Staurolite. Sillimanite.

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Large non-mica minerals are called porphyroblasts.


Gneiss – Has a distinct banded foliation. Light bands of felsic minerals (quartz and feldspars).  Dark bands of mafic minerals (biotite or amphibole). 


Compositional banding develops in several ways. Original layering in the protolith.  Extensive high-temperature shearing. 


Compositional banding - Solid state differentiation.

Migmatite   

Migmatite is a partially melted gneiss. It has features of igneous and metamorphic rocks. Mineralogy controls behavior. Light-colored (felsic) minerals melt at lower T.  Dark-colored (mafic) minerals melt a higher T. 

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Felsics melt first; mafics remain metamorphic.


Nonfoliated rocks lack a planar fabric. 

Absence of foliation possible for several reasons: Rock not subjected to differential stress. Absence of platy minerals like clays or micas.


Amphibolite – Dominated by amphibole minerals. Basalt or gabbro protolith.  Usually not well foliated. 

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Hornfels – Alteration by heating. Associated with plutonic intrusions.  Finely crystalline. 


Quartzite – Almost pure quartz in composition. Forms by alteration of quartz sandstone.  Sand grains in the protolith recrystallize and fuse.  Like quartz, it is hard, glassy, and resistant. 

Metamorphic Alteration


Marble - Coarsely crystalline calcite or dolomite. Forms from a limestone or dolostone protolith.  Extensive recrystallization completely changes the rock.  Original textures and fossils in the parent are obliterated.  Used as a decorative and monument stone.  Exhibits a variety of colors. 

Metamorphic Alteration


Type depends on protolith. Minerals contribute elements.  Some protoliths yield specific rocks. 

Metamorphic Intensity  

Different minerals have different T&P stabilities. Grade is a measure of metamorphic intensity. Low-grade – Slight.  High-grade – Intense. 

Metamorphic Environments  

Metamorphism occurs in different settings. The characteristics of the different settings are governed by tectonics.

Metamorphic Environments 

The types (and settings) of metamorphism are... Thermal – Heating by a plutonic intrusion.  Burial – Increases in P and T by deep burial in a basin.  Dynamic – Shearing in a fault zone.  Regional – P and T alteration due to orogenesis.  Hydrothermal – Alteration by hot-water leaching.  Subduction – High P to low T alteration.  Shock – Extremely high P attending a bolide impact. 

Contact Metamorphism 

Grades of alteration form bands around the pluton.  

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The width of each aureole zone is due to…  

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Bands range from highly altered to slightly altered. Analogous to changes in pottery with increased heating. The size of the plutonic intrusion. The degree of metasomatism.

The dominant rock is hornfels.

Burial Metamorphism 

As sediments are buried in a sedimentary basin… P increases because of the weight of the overburden.  T increases because of the geothermal gradient. 

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Requires burial below diagenetic effects. 

This is ~ 8–15 km depending on the geothermal gradient.

Dynamic Metamorphism  

Breakage of rock by shearing at a fault zone. Fault location determines type of alteration. 

Shallow crust – Upper 10–15 km. Rocks behave in a brittle fashion. Mineral grains crush-forming fault breccia.

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Deeper crust – Below 10–15 km. Rocks behave in a ductile manner. Minerals smear like taffy to form mylonite.

Regional Metamorphism  

Tectonic collisions deform huge “mobile belts.” Directed compression thickens mountains. 

Rocks caught up in mountain building are… Heated via the geothermal gradient and plutonic intrusions. Squeezed and heated by deep burial. Smashed and sheared by differential stresses.

Regional Metamorphism  

Regional metamorphism creates foliated rocks. This type of metamorphism is, by far, the most important in terms of the amount of rock altered. 

Collisional belts are often… Thousands of km long. Hundreds of km wide.

Hydrothermal Metamorphism  

Alteration by hot, chemically aggressive water. A dominant process near mid-ocean ridge magma. Cold ocean water seeps into fractured crust.  Heated by magma, this water then reacts with mafic rock.  The hot water rises and is ejected via black smokers. 

Subduction Metamorphism  

Subduction creates the unique blueschist facies. Trenches and accretionary prisms have… A low geothermal gradient – low temperature.  High pressures. 

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High P – Low T favor glaucophane, a blue amphibole mineral.

Essentials of Geology, 3rd edition, by Stephen Marshak

Chapter 7: Metamorphism: A Process of Change

Shock Metamorphism  

Rarely, Earth is struck by a comet or an asteroid. Impacts generate a compressional shock wave. Extremely high pressure.  Heat that vaporizes or partially melts large masses of rock. 

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These conditions generate high-pressure minerals.

Exposure  

How do metamorphic rocks return to the surface? Exhumation is due to uplift, collapse, and erosion.