GEOL 2312
IGNEOUS AND METAMORPHIC
PETROLOGY
Lecture 20
Introduction to Metamorphic Petrology
March 23, 2009
DEFINITION
“Metamorphism is a subsolidus process leading to changes in mineralogy
and/or texture and often in chemical composition in a rock. These changes
are due to physical (P & T) and/or chemical conditions* that differ from
those normally occurring at the surface of planets and in zones of
cementation and diagenesis below this surface. They may coexist with
partial melting.” IUGS-SCMR
*Chemical conditions commonly involve a fluid phase
Pressure and Temperature conditions of Metamorphic Grades
Lower Limit - T >100-200°C, P not constrained; depends or
reactability of starting rock; zeolite type an accepted indicator.
Upper Limit - onset of “SIGNIFICANT” melting
EFFECTS OF INCREASING TEMPERATURE
THE PRIMARY FACTOR IN METAMORPHISM
Increasing temperature has
several effects:
1) Promotes
recrystallization 
increased grain size
2) Drive reactions
(endothermic)
3) Overcomes kinetic
barriers
EFFECTS OF PRESSURE
SECONDARY EFFECT ON MINERALOGY, MAJOR EFFECT ON TEXTURE
Pressure usually follows
“Normal”gradients, but may
be perturbed in several
ways, typically:
Low T/P geotherms in
subduction zones
High T/P geotherms in
areas of plutonic
activity or rifting
Winter (2001) Figure 21-1. Metamorphic field gradients (estimated P-T conditions
along surface traverses directly up metamorphic grade) for several metamorphic
areas. After Turner (1981). Metamorphic Petrology: Mineralogical, Field, and
Tectonic Aspects. McGraw-Hill.
LITHOSTATIC VS. DIRECTIONAL PRESSURE
Stress is an applied force acting on
a rock (over a particular crosssectional area)
Strain is the response of the rock to
an applied stress (= yielding or
deformation)
Strain Ellipsoid
Simple Shear
Pure Shear
Foliation
Lithostatic P 
Uniform Stress
Directional P 
Deviatoric Stress
Cleavage
Schistosity
Gneissic Banding
DEVIATORIC STRESS
GENERATION OF FOLIATION AND LINEATION TEXTURES
s3
s3
s2
s2
s1
s3
s2
s1 – Maximum Stress
s2 – Medial Stress
s3 – Minimal Stress
s1
s2
s3
 s1 > s2 = s3  foliation and no lineation (Flattening Strain)
 s1 = s2 > s3  lineation and no foliation
 s1 > s2 > s3  both foliation and lineation
IMPORTANCE OF FLUIDS IN METAMORPHIC REACTIONS
Evidence for the existence of a metamorphic fluid:
 Fluid inclusions
 Fluids are required for hydrous or carbonate phases
 Volatile-involving reactions occur at temperatures and
pressures that require finite fluid pressures
IMPORTANCE OF THE
PARENT ROCK
PROTOLITH
MINERALOGICAL RESPONSE TO METAMORPHISM
Minerals that form
depend on:
 T and P conditions
 Bulk composition of
the source rock
Progressive
metamorphism
of a graywacke
(dirty
sandstone)
TEXTURAL RESPONSE TO METAMORPHISM
Reflects the intensity, directionality and duration of pressure (or stress).
 Increased grain size - During prograde metamorphism or at a particular grade that is
maintained for a long period of time, minerals will tend to increase in size.
 Foliation - As new platy minerals grow, they will align themselves perpendicular to the
maximum stress direction. For clay mineral and fine-grained micas, the planar fabric
that results is referred to as a slaty cleavage. In higher grade rocks, coarser grained
mica minerals are said to impart a schistosity to the rock.
 Gneissic Banding - In very high grade rocks, the dark minerals tend to segregate from
the lighter colored minerals (feldspar and quartz) resulting in banded rock..
OTHER TEXTURAL RESPONSES
PORPHYROBLASTS
Some metamorphic minerals have strong growth habits, meaning they
will develop as large well-formed crystals called porphyroblasts
Alkali Feldspar
Staurolite
Garnet
PROGRESSIVE
METAMOPHISM
•
Prograde: increase in metamorphic
grade with time as a rock is
subjected to gradually more severe
conditions
•
Retrograde: decreasing grade as
rock cools and recovers from a
metamorphic or igneous event
•
Prograde reactions are endothermic
and easily driven by increasing T
•
Devolatilization reactions are easier
than reintroducing the volatiles
•
Geothermometry indicates that the
mineral compositions commonly
preserve the maximum temperature
TYPES OF METAMORPHISM
1. Based on principal process or agent
 Dynamic Metamorphism
 Thermal Metamorphism
 Dynamo-thermal Metamorphism
2. Based on setting
 Contact Metamorphism
 Regional Metamorphism
o Orogenic Metamorphism
o Burial Metamorphism
o Ocean Floor Metamorphism
 Hydrothermal Metamorphism
 Fault-Zone Metamorphism
 Impact or Shock Metamorphism
TYPES OF METAMORPHISM
Contact (or Thermal) metamorphism
o
o
o
Grades up to very high temperatures, low-mod. lithostatic pressures
Forms a metamorphic aureole adjacent to igneous intrusions that is best
developed in the cooler upper crust (epizone)
Commonly develops granoblastic texture creating a rock called HORNFELS
Granoblastic Texture – equant grains, 120° jcts
TYPES OF METAMORPHISM
REGIONAL METAMORPHISM
Burial Metamorphism
Follows normal geothermal gradient
with lithostatic pressure
Orogenic Metamorphism
Broad range of P-T paths with
differential pressure dominant
TYPES OF METAMORPHISM INVOLVING FLUIDS
OCEAN-FLOOR METAMORPHISM,
HYDROTHERMAL METAMORPHISM, AND
METASOMATISM
Generally low temperatures and pressures
Distinguished from other forms of metamorphism by the loss and/or gain of
material (usually transported by a fluid). Other types of metamorphism
are thought to occur as nearly closed processes (except for water loss)
Hydrothermally Altered Basalt – Spilite / Greenstone
TYPES OF METAMORPHISM
Fault Zone Metamorphism (Cataclasis)
Variable temperature, very high directed P
Typically localized to narrow zones of intense mechanical
deformation (shear zones).
Rock types formed breccia (fragmental), gouge (clay),
mylonite (strongly foliated)
Summary of
Metamorphic
Rock Types