I. Lecture Content
Introduction
Overview of Metamorphism
Metamorphism and the Rock Cycle
Agents of Metamorphism
Conditions and Types of Metamorphism
Classification of Metamorphic Rocks
Metamorphic Zones and Facies
Metamorphism and Plate Tectonics
Metamorphism and Natural Resources
Vocabulary Terms
II. Introduction
A.
The connection between metamorphism, plate tectonics and
mountain building.
·
Metamorphism is very active at plate boundaries
·
Metamorphism is especially intense at convergent plate
boundaries (magmatism + crustal thickening)
B.
The importance of metamorphism
·
Records orogenic events (preserved in the rock record)
·
Creates mineral resources
·
Turns "soft" rocks into "hard" rocks
III. Overview of Metamorphism
A.
Definitions
1.
What’s a metamorphism? Defined
2.
What’s a metamorphic rock? Defined
B. Principle Factors of Metamorphism
1. Heat
2. Pressure
3. Fluid Activity
C. Types of Metamorphism - P/T Dependent
1. Regional Metamorphism (RM) = Deep burial +
Stresses
2. Contact Metamorphism (CM)
= Contact with magma
3. Dynamic Metamorphism (DM)
= Faulting and Shearing
IV. Metamorphism - Part of the The Rock
Cycle
A.
Metamorphic Conditions
1) Pressure/Temperature/Fluid-
Dependent Processes
·
The "Zone" of metamorphism is between that of igneous
and sedimentary rock conditions.
Ø Between diagenesis and anatexis.
·
Metamorphism encompasses a wide variety of
subsurface conditions (variable
combinations
of pressure, temperature, and fluid
activity).
·
A Precursor to formation of magmas (partial melting)
2) Elevating P/T Conditions
- How do you get there?
·
Deep burial = Regional
·
Close encounter with magma = Contact
·
Tectonic stresses = Dynamic
B. Metamorphic Processes
1) Recrystallization
·
Preexisting mineral crystals are "re"-formed.
·
Mineralogy remains the same.
·
Produces noticeable changes in rock texture.
2) Neocrystallization
·
Brand new mineral(s) (crystals) are formed.
·
Mineralogy changes.
·
Produces noticeable changes in rock texture.
3) Changes
to a rock's texture and mineralogy
·
Change in texture - Ex. 1 = limestone to marble
Ex. 2 = granite to gneiss
·
Change in mineralogy - Ex = basalt to amphibolite
Ex. = mudstone = garnet
schist
4) The new
texture and/or mineralogy of any rock that has
underwent metamorphism is a result of that rock's
attempt at establishing physical and chemical
equilibrium with it’s the new metamorphic conditions.
C. Metamorphism and Mountain Building
1) Plate dynamics at convergent
boundaries produce
a
variety of unique conditions of elevated pressures,
differential
stresses, and increased heat input.
2)
Rocks can be subjected to a series of progressive
metamorphic conditions through
time in regions of
active
mountain building.
· Long-term, ever-deeper
burial
· Proximity to magmatic
intrusions
· Long-term, Vice-like
differential stresses
D. Internet Resource on Metamorphism
1) http://www.geo.wvu.edu/~lang/Geol285/Pet18MetIntro/index.htm
V. Formation of Metamorphic Rocks -
A.
Potential Source Regions (rocks) for Metamorphism
1) Mantle
2) Oceanic crust
· Spreading centers
3) Continental crust
· Base of deep depositional
basins
· Cores of active mountain
belts
· Magmatic arcs
· Subduction Zones
B.
Regions of Metamorphism - Physical and Chemical
Conditions of Metamorphism
· Source rock composition
· Temperature (Heat)
· Pressure
· Regional (tectonic) Stresses
· Fluids Activity (Amount of
H2O in rock is IMPORTANT)
VI.
Major Types of Metamorphism
A. Regional Metamorphism (RM)
1)
Characteristic of large regions of crust, called "orogenic
belts" that undergo deep burial (elevated
P+T) and
deviatoric stresses over long periods of time.
2) Major affect is three-fold, with progressive
increases in
lithostatic
and vice-like pressures and temperature.
3) The zone of metamorphism is truly regional in
extent,
with
dimensions as large as 100's km long, 10's km
wide,
and km's deep.
4) The type of metamorphic
mineral(s) that form in the
country rock depends on the following factors:
·
Composition of the country rock
·
Temperature
·
Lithostatic pressure
·
Orientation and strength of deviatoric stresses
·
Length of time
5) The fabric (texture) of the resultant metamorphic
rock
also
depends on the above factors listed in #4.
·
Distinctive foliation (slaty or schistose)
and/or layered (gneissic)fabrics.
·
Lots of platy or elongate minerals will accentuate foliated fabrics,
like mica.
·
Lack of platy minerals may produce rock with
no
apparent foliation, e.g. quartzite or marble.
B. Contact Metamorphism (CM)
1)
Characteristic in active magmatic belts where hot
magmas and hydrothermal fluids exist.
2)
Major affect is a great increase in the temperature of the
country rocks adjacent to the magma intrusion.
3) A metamorphic aureole
forms in country rock directly
adjacent
to the intrusion.
4) The
type of metamorphic minerals that form in the
country rock aureole depends on the following:
· Composition of the country
rock
· Temperature of the intrusion
·
Size of the intrusion
·
The type/quantity of fluids in the aureole
5) The resultant metamorphic rocks are called hornfels.
6) A very active metal/sulfide ion-rich hydrothermal
system
may
form in some aureoles, which can be the site of
rich, metal sulfide ore
formation.
·
Called a Porphyry system
7) Metal-rich fluids curculating in carbonate-rich
aureoles
can
be another site of metal ore formation.
·
Called Skarn deposits
C. Dynamic Metamorphism (DM)
1)
Characteristic of active fault and shear zones.
2)
Major affect is a great increase in deviatoric stresses of
on the country rock within or adjacent to the zone of
faulting or
shearing.
3) The characteristic processes
occurring to the rock are:
· Milling (mechanical) action
· Recrystallization
· Reorientation of minerals =
foliated fabric
4) Mylonite
is the resultant metamorphic rock
·
Unique to this type of metamorphism
VII. Classification And "Id" of
Metamorphic Rocks
A. Primarily based mainly
upon two criteria:
B. Texture is further
subdivided into two types:
1) Foliated and/or Layered Fabrics
2) Non-foliated and/or Massive Fabrics
C. Foliated and Layered Fabrics
1) Foliated fabrics are defined by the preferred
orientation of
platy minerals like mica.
· A flaky croissant is a good
example
· See Figure's 7.5 to 7.8 for
photo examples
2) The style of foliation is mainly a function of grain
size.
· Slaty cleavage = fine-grain,
tight, parallel, flat
· Schistose - medium-grain; wider,
more wavy
· Phyllitic - In between that
of slaty and schistose
3) Layered fabrics are defined by compositional
layering of light-
and dark-colored minerals.
· Gneissic - layered
mineralogy; med-grain
D. The Major Metamorphic
Rock Types -
1) Foliated and/or Layered
Rocks
·
Slate – very fine-grained; foliated; RM
·
Phyllite -- fine-grained; foliated: RM
·
Schist -- medium-grained; foliated; RM
·
Gneiss -- med- to coarse-grained; layered; RM
·
Amphibolite - fine-med-grained; foliated; RM
·
Mylonite - fine to med-grained; foliated; DM
·
Migmatite - med-grained; mixed up-looking:
RM and CM.
2) Non-Foliated/Massive
·
Marble -- fine- to coarse-grained; massive; RM
·
Quartzite - fine- to med-grained; massive; RM
·
MetaConglomerate - mix-grained; massive; RM
·
Hornfels -- fine-grained; massive; CM
·
Greenstone -- fine to med-grained; variable;
·
Skarn -- fine to coarse-grained; massive; CM
E. Compositional
Classification
1) Marble - carbonate
mineralogy
2)
Quartzite - quartz mineralogy
3)
Amphibolite - amphibole mineralogy
4)
Granite gneiss - granite mineralogy
5)
Garnet-Biotite schist - Dominant minerals
5)
Skarn - (Ca, Mg)-silicate mineralogy
F. Classification Chart of Common Metamorphic Rocks
(See
Table 7.15; pg. 124)
G.
Identifying Metamorphic Rocks in Hand Samples
1) Observe the rock's texture/fabric - Foliated? or Nonfoliated?
2) Observe the rocks grain size - Fine, medium, or coarse?
3) Observe mineralogy or other distinctive features- ID minerals?
4) Name the Metamorphic rock
5) Choose the most likely parent
rock
VIII. Metamorphic Zone and Facies Concepts
A. Progressive Metamorphism
1)
What is Progressive Metamorphism?
Defined: In regional metamorphism,
the systematic metamorphic
evolution and equilibration of a package of rocks
through ever-
increasing pressure-temperature conditions, where a
rock
package's lower grade equivalents, e.g. slate
systematically grade
to it's highest grade equivalents, e.g. gneiss.
2) Progressive metamorphism is
exclusively a product of
regional
metamorphism (the type of metamorphism
where
conditions go through a broad elevation of both,
temperature
and pressure).
B. Metamorphic Zones
1) What are Metamorphic Zones? Defined: In isochemical rock
systems undergoing
progressive metamorphism, it is common
for the systematic
appearance (neocystallization) of new minerals
with each systematic upgrade in P/T
conditions. These new
metamorphic minerals are
termed metamorphic index minerals
and they define metamorphic zones.
·
See Figure 7.8, Index Minerals (page 199)
2) Metamorphic zones are actual rock assemblages.
3)
The spatial association of Metamorphic Zones in the
field
can be interpreted in terms of variations in peak
metamorphic
conditions.
4) A Metamorphic Zone
Illustration (Fig. 7.18; pg. 206)
5) What are metamorphic
zones used for?
C. Metamorphic Facies
1) What is a Metamorphic
Facies? Defined: The P-T
conditions that produce a
characteristic rock type,
that has a unique, characteristic mineral assemblage
for a given parent rock.
·
See Fig. 7.19; Metamorphic Facies (pg. 206)
2) A Metamorphic facies is
not a rock, but instead, is a
specific range of Pressure/Temperature conditions.
3) There are a number of established metamorphic
facies within
all the
paired P/T conditions found in nature.
4) A Metamorphic Facies Diagram illustrates the
different
facies
(colored fields) in relation to Pressure/Temperature.
(see
Figure 7.19; Metamorphic Facies (page 206)
5) What are metamorphic
facies diagrams used for?
6) A Metamorphic Facies Series Diagram illustrates
the
progressive P/T paths (black arrows) of each type of
metamorphic facies series.
7) What are metamorphic facies series diagrams used
for?
IX. Metamorphism and Plate Tectonics
A. Specific Tectonic
Settings are Conducive to Specific
Types of Metamorphism
1) Divergent boundaries = Contact and Dynamic
2) Convergent boundaries = Regional, Contact, and
Dynamic
3) Mainly Dynamic
B. Specific Tectonic Settings are Conducive to
Specific
Metamorphism Facies Series (MFS)
1) Divergent and Convergent
= Contact MFS
2) Continental-Convergence =
Buchan and Barrovian MFS
3) Island Arc Convergence = Sanbagawa MFS
4) Subduction Zone Accretion =
Blueschist MFS
X. Metamorphism and Mineral Resources
A. Regional Metamorphism
1) Marble
2) Slate
3) Asbestos
B. Contact Metamorphism - Porphyry and
Skarn Systems
1) Metal Sulfides
Copper
Lead
Zinc
2) Metal Oxides
Iron
Tin
Tungsten
XI. Concluding Thoughts
A. Metamorphic rocks form in
conditions between those
where sedimentary and igneous
rocks form.
1) Higher P+T then sedimentary rocks
(diagenesis)
2) Lower P+T then igneous rocks (melting point)
B. Metamorphic Rocks and Processes
complete the
third leg of the Earth's Rock
Cycle.
1) Transforms
sedimentary and other rocks.
2) Sets up host rock
for partial melting.
C. Metamorphism is a principle process in the
Building
of
Mountains and the Crustal Evolution of Continents.
XII. Vocabulary Terms
aureole
contact
metamorphism
differential pressure/ deviatoric stress
dynamic metamorphism
foliated texture
gneiss
gneissic texture
hornfels
index minerals
lithostatic pressure
marble
metamorphic facies
metamorphic facies series
metamorphic rock
metamorphic zone
metamorphism
mylonite
nonfoliated/ massive texture
porphyry ore system
quartzite
regional metamorphism
schist
schistose
slate
slaty cleavage