Revised 8 / 06 (Monroe 6th ed.)
Including...
Factors involved in the metamorphic process
Metamorphic environments and rocks
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
Heat
Pressure
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
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!
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
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
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
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
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
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
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
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
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...
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
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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