Download Metamorphic Petrology

Metamorphic Petrology
Jen Parks
ESC 310, x6999
[email protected]
Definition of Metamorphism
• The IUGS-SCMR definition of metamorphism:
“Metamorphism is a subsolidus process leading to changes
in mineralogy and/or texture (for example grain size) and
often in chemical composition in a rock. These changes are
due to physical 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.”
Definition of Metamorphism
• What does that really mean?
•Changes in P, T, X than result in a mineralogical or
textural change in a rock
•Solid state transformation (no melting)
• What sort of change happens?
•Phase change: new metamorphic minerals due to
a chemical reaction
•Textural change: new textures such as alignment
of platy minerals, or progressive coarsening or fining
of pre-existing igneous/sed minerals
• What should we care?
Limits of Metamorphism
• Lower limit : Digenesis
• Upper limit : Migmatites
Agents of Metamorphism: Temperature
•Temperature can be most
important change
•Where does the heat
come from?
•Increasing pressure
with depth
•Radioactive decay
•Deformation
•Migrating magmas
Agents of Metamorphism: Temperature
• What does increasing T do?
• Adds “energy” to system, that helps to over
come kinetic barriers in the aim of driving
reactions to reach equilibrium
1. Promotes recrystallization → increased grain
size
2. consumes unstable mineral(s) and produces
new minerals that are stable
Agents of Metamorphism: Pressure
• Lithostatic pressure
• Uniform, equal in all directions
• P=ρgh
• Normal gradients (general increase with depth)
can be disturbed in certain tectonic environments
• What does changing P do?
• High T/P paths (low P) favor the formation of
low-density metamorphic minerals as
temperature rises (rifting or plutonic activity)
• Low T/P paths (high P) favor denser minerals
(subduction zone)
Agents of Metamorphism: Deformation
• Deviatoric stress: unequal in every direction
• Compression
• Extension
• Shear
• What does deformation do?
• provide energy to overcome kinetic barriers to
drive reactions
Agents of Metamorphism: Fluids
• Evidence of fluids during metamorphism:
•Fluid inclusions in meta minerals
•Formation of meta phases that require presence
of a fluid
•Meta reactions often involve dehydration /
decarbonation reactions that produce a fluid
•What can having a fluid in a system do?
•Fluid contains phases that will try to gain
equilibrium with the rock phase, which drives
reactions and results in mineralogical/textural
changes of rock phase
Agents of Metamorphism: P & T
• High grade/low grade rocks: relative term to
indicate high or low metamorphic conditions
Nature of Metamorphism
• 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
• Basically can increase and decrease in
grade, and do this multiple time
Types of Metamorphism
1.Contact metamorphism
•
Heating of country rocks during igneous intrusion or
beneath thick flows, results in growth of new metamorphic
minerals in random orientations
2.Regional metamorphism
a) Orogenic: large scale metamorphism characteristic of
mountain belts and shield areas as a result of tectonism
b) Burial: postdiagenetic, progressive changes occurring to
sedimentary rocks during burial
Types of Metamorphism
3. Hydrothermal metamorphism
• Chemical reactions as a result of circulating fluids
a) Sea floor spreading centres:
• Interaction of basalt and seawater, seawater infiltrates
fractures systems and leaches metals and silica, black
smokers
b) Porphyry systems:
• Hot fluids interaction and circulating around plutons
above subduction zones
4. Dynamic (Fault-Zone) metamorphism
•
In response to high strain, localized on along structures
5. Shock metamorphism
•
Impact of high velocity meteorites, creates high P/T minerals
at surface (diamonds!)
Metamorphic Classification/Rock Types
• Metamorphic rocks are classified on the basis of
texture and composition (either mineralogical or
chemical)
• Use prefix-type modifiers to attach to names to
stress important or unusual textural or
mineralogical aspects (ie. muscovite schist)
• Broad classification based on foliated vs.
unfoliated
Metamorphic Rocks – Foliated
Slate:
• fine grained rock with
parallel cleavage,
defined by parallel
growth of micas, due to
regional pressure
regimes, dull foliation
surface
Phyllite:
•Same as a slate,
contains very fine
grained phyllosilicates,
has a silky sheen on
foliation surface
Metamorphic Rocks – Foliated
Schist:
• preferred
orientation of fine to
medium sized
minerals and
mineral aggregates
that have been
produced by meta
processes, usually
has planar features
and is mica rich
Metamorphic Rocks – Foliated
Gneiss:
• banded rock with
alternating light and
dark mineral layers,
segregation into
layers by meta
processes
•Orthogneiss:
•Igneous in origin
•Paragneiss:
•Sedimentary in
origin
Metamorphic Rocks - Unfoliated
Rocks that are not produced by high-strain
metamorphism
Granofels: a comprehensive term for any isotropic
rock (a rock with no preferred orientation)
Hornfels is a type of granofels that is typically very
fine-grained and compact, occurs in contact
aureoles, tough, tend to splinter when broken.
Metamorphic Rocks – High Strain
•Usually results in textural change of a rock as it
reacts to accommodate stain
• Mylonite, cataclasite….
Metamorphic Rocks – High Strain
Metamorphic Rocks – High Strain
Common Metamorphic Rock Types
Marble: composed predominantly of calcite or
dolomite, protolith is typically limestone or dolostone
Quartzite: composed predominantly of quartz, protolith
is typically sandstone
Greenschist/Greenstone: a low-grade metamorphic
rock that contains chlorite, actinolite, epidote, and
albite, protolith is a mafic igneous rock or
graywacke. Greenschist (foliated), Greenstone
(unfoliated)
Amphibolite: dominated by hornblende + plagioclase,
may be foliated or non-foliated, protolith is a mafic
igneous rock or graywacke
Common Metamorphic Rock Types
Serpentinite: an ultramafic rock metamorphosed at low
grade, contains mostly serpentine
Blueschist: a blue amphibole-bearing metamorphosed
mafic igneous rock or mafic graywacke
Eclogite: a green and red metamorphic rock,contains
clinopyroxene and garnet,protolith is typically
basaltic
Skarn: contact metamorphosed and metasomatized
carbonate rock containing calc-silicate minerals
Granulite: high grade rock, composed of OH-free
minerals, no muscovite, plagioclase and
orthopyroxene are common, protolith is pelitic,
mafic, or quartzo-feldspathic