I. Lecture Content
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
A. The connection between metamorphism, plate tectonics and
· 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
1. What’s a metamorphism? Defined
2. What’s a metamorphic rock? Defined
B. Principle Agents of Change of Metamorphism
2. Lithostatic Pressure
3. Active Pore Fluids
4. Deviatoric Stress
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 (sedimentary) and anatexis (igneous).
· 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
· Preexisting mineral crystals are "re"-formed.
· Mineralogy remains the same.
· Produces noticeable changes in rock texture.
· 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
V. Formation of Metamorphic Rocks -
A. Potential Source Regions (rocks) for Metamorphism
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 (Depth of burial)
· Regional (tectonic) Stresses
· Fluid 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
· 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
· Reorientation of minerals = foliated fabric
4) Mylonite is the resultant metamorphic rock
· Unique to this type of metamorphism
VII. Classification 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
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
2) The four types of foliation is mainly a function of grain size.
· Slaty cleavage = fine-grained, tight, parallel, flat
· Phylittic - fine-medium grained, tight, but slightly wavy; sheen-like luster
· Schistose - medium-grain; wider-spaced, more wavy foliation planes
· Gneissic - medium to coarse-grained; layered fabrics are defined by compositional
layering of light- and dark-colored minerals.
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.
· Marble -- fine- to coarse-grained; massive; RM and CM
· Quartzite - fine- to med-grained; massive; RM and CM
· MetaConglomerate - mix-grained; massive; RM and CM
· Hornfels -- fine-grained; massive; CM
· Greenstone -- fine to med-grained; variable; CM
· 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 7.1; pg. 200)
G. Identifying Metamorphic Rocks in Hand Samples
1) Observe the rock fabric - Foliated?, Layered?; Or No?
2) Observe the rocks grain size - Fine, medium, or coarse?
3) Observe mineralogy - ID minerals? Or No?
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
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
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
B. Contact Metamorphism - Porphyry and Skarn Systems
1) Metal Sulfides
2) Metal Oxides
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
differential pressure/ deviatoric stress
metamorphic facies series
nonfoliated/ massive texture
porphyry ore system