Revised 8 / 06 (Monroe 6th ed.)

Metamorphic Rocks - Chapter 8




Factors involved in the metamorphic process

Metamorphic environments and rocks

Metamorphic terrains of limited extent

Regional metamorphism: an overview

The Rocks

Non-foliated metamorphic rocks

Foliated metamorphic rocks

Migmatites and the Formation of Granitic Magmas

The realms of dynamo-thermal metamorphism

Click here for online mineral and rock ID charts



As usual in geology, take big words apart

meta = change

morph = form

ick = tough to study

Talking about a change in mineralogy here

Considered an "iso-chemical" process

Essentially, nothing is added or lost at the elemental level

Except for a subtle to profound loss of water

Existing elements recombine into new minerals

Mineralogy ALWAYS changes in an attempt to restore equilibrium

One of the only times in geology when you can use the word "always"

Even toss the 1st Law of GeoFantasy?

Start with any rock

Subjected to different environment conditions

Commonly due to burial, or subsidence of the crust due to tectonics

Heat and pressure usually involved

Difficult process to study

Generally occurs at depth in the crust

Impossible to observe directly

Similar in this way to intrusive igneous rocks

But generally far more complex

But not too deep - usually "less than 20 kilometers"

Higher temperatures at depth lead to complete re-melting and the formation of magma

As always, this is a highly variable depth

Subject to local irregularities

Metamorphism is also considered to be a "solid-state" process

All of this happens at temperatures below the melting point of the rocks!

There are several factors which directly affect the process

Rock chemistry

Contained fluids




There are infinite variations of these factors

Results in a very complex suite of rocks!

The study of metamorphic rocks can only take place after uplift, weathering, and erosion

And long after the actual metamorphic processes have ended

Can be real tough to determine the metamorphic history of a rock

Including what it was originally!

The metamorphics are without a doubt the toughest to understand

We'll take a very broad look at them and just discuss the main categories


Factors involved in the metamorphic process

Rock chemistry

Metamorphism is an iso-chemical process

Therefore, what you start with is extremely important

The chemistry of the parent rock largely determines the composition of the resulting metamorphic rock

This should be a real no-brainer

Cook eggs and you get an omelette, not meatloaf

Unless you add a bunch of new stuff

But this is an iso-chemical process, so not much is added or lost

Limestone alters to marble, not quartzite!

Contained fluids

Generally water and carbon dioxide

Similar to how volatiles affect magmas

REVIEW: mafic to felsic

The high volatile minerals tend to react early

Release their volatile components

Two things happen:

The loose volatiles tend to act as a catalyst

The best metamorphics are commonly derived from sedimentary rocks

The resulting rock is generally decreased in the volatile components


Considered "the principle factor in the metamorphic process"

If metamorphism requires that the elemental ions migrate and recombine...

Ions diffuse easier at higher temperatures

Therefore higher temperatures tend to increase both the speed and efficiency of the metamorphic process

The increased heat directly affects the "strength" of the rock

And locally affects the Brittle-Ductile Transition Zone (REVIEW)

The resulting metamorphic rocks can be highly contorted, folded, and otherwise deformed plastically

As a general rule: the higher the metamorphic grade the greater the plastic deformation

DIGRESS TO: Metamorphic grade

Obviously, there are all possible ranges of heat (metamorphic grade)

From "just barely warm" to "just below the melting point"

But, what is the melting point?

REVIEW: Bowen's Reaction Series

The metamorphic process affects the low temperature (felsic) minerals first

This results in some VERY interesting effects at the higher grades (see below)


Heat and pressure are definitely related

Pressure leads to increased heat

In general, the increased pressure associated with the metamorphic process results in a rock with tighter packing at the atomic level

Therefore, generally higher density than the parent rock

There are several sources of pressure...

Pore-fluid pressure

Release of volatiles supplies some pressure to the overall system

Litho-static pressure (REVIEW) (Monroe; fig. 8-7, pg. 241)

The load weight of overlying rock

Equal pressure in all directions

Results in non-foliated rocks (DEFINE)

Marble, quartzite common non-foliated varieties

Directed pressure (REVIEW)

Acts in a specific direction

Generally related to tectonics

Results in foliated rocks (DEFINE)

New mineral grains grow with their long axis oriented normal to the stress (Monroe; fig. 8-10, pg. 244)

EXAMPLE: Drop a deck of cards; gravity is the directed stress

Most common metamorphic rocks fall into this category


Some of the higher grade rocks clearly required a VERY long time to form

We can duplicate all the other factors in the lab, but not this one

This is the fatal flaw in most studies of earth processes

Click here for a discussion of geologic time and metamorphic rocks


Metamorphic environments and rocks

There are several major categories

Basically related to the size of the system

And the relative importance of heat and pressure

Local metamorphic terrains

Relatively small and isolated occurances of limited extent

Regional metamorphic terrains

Large, fully developed, and complex environments


Metamorphic terrains of limited extent

Contact metamorphism

Usually associated with increased heat

Without a corresponding increase in pressure

Litho-static or limited directed stress

Therefore commonly non-foliated

Common along the margins of small plutons (dikes, sills, etc.)

Localized heating of country rock as magma cools

Results in a thin "halo" of metamorphism

Also called a metamorphic aureole (Monroe; fig. 8-5, pg. 240)

Usually very thin (millimeters to a few centimeters)

Chill margin vs. baked zone (DESCRIBE)

Click here for a discussion of cooling history and texture

Can be larger in special cases

Hornfels: derived from shale

Dense, fine-grained, non-foliated

Skarn: derived from limestone

Skarns can be VERY important to economic geology

Calcium carbonate is highly reactive

Will extract many different elements from the cooling magma

Can result in very high grade mineral occurrences

But usually disappointingly small

Remember, they form in a contact metamorphic environment

Hydrothermal metamorphism (EXPLAIN: hydro + thermal)

Heat and chemically active solutions

Usually related to residual fluids escaping from a felsic magma chamber

Does not have to be felsic, but is probably most common

Cataclastic metamorphism

Localized near-surface fault zones (redundant?)

Rock is tectonically broken and shattered

Increases surface area

Leads to increased fluid penetration and hydrothermal metamorphism

Can also occur locally at greater depths

The added heat and pressure can accentuate the metamorphic processes

Mylonite: Greek for "mill" (Monroe; fig. 8-8, pg. 242)

Nearly complete pulverization of the rock

Leads to partial to complete recrystallization

Very tightly inter-grown minerals

Extremely hard and durable rock


Regional metamorphism: an overview

Click here for online mineral and rock ID charts

Can result in bodies of great extent

Most (but not all) are the result of directed stress environments

Also called "dynamo-thermal" metamorphic rocks

Associated with continental mountain building processes

Combined with granite, these form the cores of the continental land masses

Called cratons

Shields where exposed

Platforms where obscured by sedimentary layers

Heat, pressure, and volatiles are all important

Usually results in prominent foliation (but not always)

And very complex mineral assemblages related to local variations in rock chemistry and metamorphic grade (more later)



Click here for online mineral and rock ID charts


Non-foliated metamorphic rocks (Monroe; Table 8-2, pg. 243)

Heat and litho-static pressure predominate

Results in a recrystallization of existing material

These factors are everywhere beneath the surface

Therefore, taking a very broad view, all rocks can be considered non-foliated metamorphics to some degree

There are several common non-foliated rocks

Quartzite: derived from sandstone (Monroe; fig. 8-17, pg. 247)

Very hard and durable

Looks like sandstone

But, the rock will break through the quartz grains, not around them

Hornfels: derived from shale (usually)

Also very hard, dense, and durable

Marble: derived from limestone (Monroe; fig. 8-16, pg. 247; and "Marble," pg.234)

In most cases, the parent limestone had impurities

Add color and pattern to the marble

Can be dense and compact, but softer than quartzite or hornfels

It's made from CaCO3 like calcite and limestone

Good for carving, building stone, facing stone

Josephine County Courthouse

All three represent common marine sedimentary facies which are probably metamorphosed by the weight of overlying debris


Foliated metamorphic rocks (Monroe; Table 8-2, pg. 243)

Click here for online mineral and rock ID charts

Result of increasing heat and directed pressure

Increasing metamorphic grade generally results in a coarsening of texture

As well as a concentration of felsic and mafic constituents

Increasing grade also results in a progression specific minerals (Monroe; fig. 8-18, pg. 248)

Obviously dependent upon original rock chemistry

Called a metamorphic facies (Monroe; fig. 8-20, pg. 250) (Monroe; fig. 8-21, pg. 248)

Examples: staurolite facies, actinolite facies, greenschist facies

The same elements recombine to form different minerals at different temperature and pressure environments

Each facies indicates temperature, pressure, and fluid conditions at the time of the metamorphism

Platy minerals: mica, chlorite, graphite

Common at lower metamorphic grade

Orientation results in "foliation"

Elongate minerals: hornblende, staurolite, pyroxene

Common at higher metamorphic grade

Orientation results in "lineation"

The resulting progression of metamorphic rocks is fairly specific

With infinite gradations and variations!

Let's start with deep-water marine sediments and follow the process

Add heat and pressure between (and within) each step

Metamorphics are the ultimate "shades of gray" situation in geology

These are only the broadest of category names

The variations are endless


A common sedimentary rock

Very fine grain

Toss in a little sandstone and limestone and you've got your basic marine sedimentary assemblage


Little or no significant visible change (Monroe; fig. 8-11, pg. 244)

Still microscopic grains

But the mineralogy has begun to change

Usually to mica, graphite, or chlorite

Low temperature minerals with one perfect cleavage

A very hard and durable rock

Commonly used as pool table tops, roofs, and chalkboards


Begin to see mineral grains

Commonly lots of mica - gives rock a shiney look

Can be up to 50% muscovite

But can also be graphite or chlorite


A very broad category (Monroe; fig. 8-12, pg. 245)

Significant change in mineralogy, texture, and visible foliation

Well developed foliation of micaceous minerals (usually greater than 50%)

Also called schistosity

The characteristic wavy or undulating rock cleavage common to schist

May not parallel original bedding

Most primary textures and features are lost

Other minerals begin to form based on composition of original rock and new environmental conditions

Use additional minerals as modifier of name

EX: mica schist, quartz schist, hornblende schist, quartz-mica-hornblende schist, etc.


High grade metamorphic rock (Monroe; fig. 8-13, pg. 245)

Color banding of light and dark minerals

DIGRESS TO: layers vs. lenses

Lineation: orientation of prismatic minerals

Hornblende, actinolite, tourmaline, staurolite


Almost there! (Monroe; fig. 8-15, pg. 246)

Partial melting and recrystallization of felsic minerals

REVIEW: Reverse order of Bowen's Reaction Series

Results in a rock with layers of felsic igneous rock and very high grade mafic gneiss

To summarize...

Click here for online mineral and rock ID charts

Increasing grade very common in sedimentary sequences

Layers of sediment pile up deeper and deeper

Leads to lithification of the lower layers

As additional layers of sediments are added on top, the lowest portions begin to metamorphose

Followed to its logical conclusion...

Imagine an unbroken transition from unconsolidated sediments to sedimentary rock through increasing metamorphic grade to...


Migmatites and the Formation of Granitic Magmas

Migmatite - a very high temperature metamorphic rock

Because of Bowen's, the felsic constituents have reached their melting point

But the mafics still have a way to go

So we end up with a highly contorted, mixed igneous and metamorphic rock

Called "roof pendants" because they usually grade into felsic intrusives at greater depth

Several excellent examples

Kaweah River - Sierra Nevada foothills

Near south entrance to Sequoia National Park

Convict Lake area - eastern Sierra Nevada

Ashland pluton - Siskiyou Mountains, Oregon and California

Add more heat and the whole thing melts - a phase change

When I was in this class, most granitic magmas were "emplaced from below"

Usually through "forceful injection"

Kind of an ominous thought

...and where did they come from?

Direct differentiation from the upper mantle is hard to believe

In almost every case, magmas we see coming out of deep rifts in the crust are mafic

Your basic oceanic spreading ridge

They begin to purify into felsic materials where they are re-worked along the continental margins

Subduction zones and the cores of volcanic arcs

Therefore, the ultimate source of most granitic magmas must be a metamorphic process

Click here for additional information on the formation of granitic magmas

Click here for additional thoughts on the direct differentiation of granitic magmas from the upper mantle


The realms of dynamo-thermal metamorphism

No clear-cut answers, but lots of circumstantial evidence

Commonly in elongate bodies

10's to 100's of miles wide

100's to 1000's of miles long

Associated with deep- seated plutonic rocks

Batholiths like the Sierra Nevada

Form the axes of many of the world's mountain ranges

Sierra Nevada, Alps, Rocky Mountains, etc.

Intermediate to high temperatures

Intermediate to high directed pressure

Clearly long and well developed crustal tectonic environments

Time spans measured in 100's of millions of years

Moderate to great depth - but still in the crust

All this adds up to subduction complexes as the most logical location

These metamorphic suites most likely form the cores of the subduction zones


Click here for online mineral and rock ID charts



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