Transcript Lecture 13: Introduction to Metamorphism Rocks and Processes

Metamorphism: Fundamental Questions
• What are the subsolidus changes in fabric and
composition that occur during metamorphism?
• What transfers mass and energy to cause these
changes?
• In what tectonic settings does metamorphism
occur?
• How does the study of metamorphic rocks and
processes help us understand plate tectonics and
Earth’s evolution?
Metamorphism: Simple Definitions
• Metamorphism is defined as physical and
chemical changes that occur in pre-existing rocks
(igneous, metamorphic, or sedimentary) in the
solid state that yield in a lower free energy state as
a result changes in conditions (e.g. T and P).
– Prograde: Increasing T and P
– Retrograde: Decreasing T and P
• Diagenesis occurs at relatively low T and P and
grades into metamorphism.
• Melting or migmatization occurs at the most
intense, i.e. highest T an P limits of metamorphism
and therefore grades into igneous processes.
General P/T Conditions of Metamorphism
From Spear, 1993
Equilibration in Metamorphic Rocks
• Parent rock is called the protolith and pathway to a
new equilibrium state may result in a different
changes:
– Crystallization of new minerals with preservation of
relic textures
– Recrystallization under hydrostatic conditions yielding
a newly imposed granoblastic fabric
– Increase in grain size without changes in chemistry or
mineralogy
– Crystallization of new and new fabrics
– Recrystallization under deviatoric stress yielding
tectonite fabrics
Before and After Metamorphism: Volcanic Tuff
Relic Vitroclastic Texture
Fresh Rhyolite Tuff
Incipient Burial Metamorphism
From: Best, 2003; Wilkinson & Whetten, 1964
Simplified
Scheme for
Hydrothermal
Breakdown of
Primary Igneous
Minerals
Difficult to write
stoichiometrically
correct rxn’s because
of complexity
Liberated ions can mobile
in an aqueous fluid phase:
metasomatism
Relic Phenocrysts in Meta-andesite
Pyroxene pseudomorphically replaced by epidote
Plagioclase pseudomorphically replaced by epidote, albite, and sericite
Ostwald Ripening
Increasing time -> Increasing grain size
• Process illustrated using soap bubbles
• 120° grain boundaries mimic those found in granoblastic
textures
• Metamorphic recrystallization likely requires 105 to 106 years
• Similar process seen in volcanic bubbles during tephra eruptions
Prograde Thermal Metamorphism
Weakly
metamorphosed
Diabase Greenstone
magmatic pyx’s
replaced by
actinolites
Amphibolite well developed
granoblastic
Texture; no
remaining vestiges
of magmatic
texture
Greenstone
or fine grained
Amphibolite;
same mins;
better grain
growth
Granoblastic
Plag-pyx
Granofels; close
to gabbro solidus
but clear meta.
texture
More Textural Definitions
• Porphyblastic: similar to the porphyritic texture seen in
magmatic rocks; but larger grains, referred to as
porphyroblasts, grew under sub-solidus conditions.
• Poikiloblasts: porphyroblasts containing inclusions of
other minerals.
• Epitaxial growth: a secondary phase grows on a
crystalline substrate that has a similar atomic structure and
thus influences the orientation of the overgrowth.
• Cataclasis: Occurs when briitle rocks are broken, crushed,
and pulverized to form a dilatant, unconsolidated fault
breccia or fine-grained gouge.
• Tectonites: rocks with fabrics formed by dutile
deformation. Fabrics are strongly anisotropic
Epitaxial Growth of Secondary Minerals
Magmatic Pyroxene
Epitaxial Prismatic Amphibole
Tectonite Fabrics: Foliations and Lineations
Finite strain
ellipse: derived
from an
originally
spherical
reference
Foliation plane
is perpendicular
to the maximum
shortening
direction
Lineation is
parallel to c
or maximum
elongation
direction
Fractal Nature of Deformation
km
scale
m
scale
mm
scale
cm
scale
Development of Tectonic Fabric in Graywacke
Initial
Isotropic
sandstone
fabric
Aphanitic Phyllite
Zone; recryst. of
new grains
obliterates
orig.
sandstone
fabric
yielding well
developed foliation
Foliated
meta-graywacke
or phyllite; NB
development of
slip surfaces &
relict qtz.
Fine grained
schist; coarser
grains and
foliation
enhanced by
segration layers
of qtz + plag. &
musc. + biotite
Cleavage Formation
• Slaty cleavage: Defined by the alingment of
aphanitic platy, phyllosilicate minerals (e.g. micas
and chlorite) and graphite. Qtz lenses may remain
and locally are sub-parallel to the cleavage planes
• Crenulation cleavage: Secondary cleavage
formation that overprints and folds the primary
cleavage. Example of polymetamorphism.
• Transposition: Shearing of existing sedimentary
or compositional layers into a new oblique
orientation during ductile deformation.
Examples of Ductile Metamorphism
Archean Pillow Basalts - Yellow Knife, NWT Canada
Undeformed but recrystallized
pillow basalts
Highly deformed and transposed
Pillows (lighter colors)
From Lambert & Baragar
Recognition of Metamorphic Protoliths
• Relict Fabrics: Low grade metamorphic rocks
often retain outlines of sedimentary features (e.g.
bedding) or igneous features (e.g. pillows).
• Field Relations: Some cases allow one to trace
prograde metamorphism from the protolith
through increasing grade. Contact metamorphism
in a plutonic setting is a good example.
• Bulk chemical composition: Original chemical
composition may be retained to some degree.
Often one can use geochemical ratios of immobile
(i.e. conservative) elements.
Global Average Shale Composition
Shales are dominated
by clays (Al-rich)
and are more aluminous
than common igneous
rock types
Ca & Na are
mobile elements
In aqueous fluids.
Deposited in
Limestones.
Shales comprise
about 1/2 of all
sedimentary rocks;
Sandstones ~1/4 &
Limestones of the
rest.
End-member Protoliths
• Ultramafic: Derived from high-Mg-Fe magmatic rocks
(e.g. peridotites, pyroxinites, and dunites.
• Mafic: Derived from basalts and gabbros. High
concentrations of Mg, Fe, and Ca and Al. Usually called
metabasalts (e.g. greenstones and greenschists). Also
related are spillites (contain cordierite and anthophyllite),
derived from metasomatic alteration at the ocean ridges.
• Quartzo-feldspathic: Dominiated by qtz and fsp. And
derived from qtz-bearing meta. Rx. And lithic sandstones.
Also called psammites.
• Calc-silicate and Calcareous: Derived from “dirty” and
pure limestones and dolostones. Recrystallized carbonates
and Ca-Fe garnet, epidote, cpx, wollastonite, and tremolite
are common.
• Ferruginous: Enigmatic Fe-rich rocks including banded
iron formations and associated meta-cherts.
Controlling Factors in Metamorphism
Note that metamorphic
equilibration is also
strongly affected by
kinetic factors, which
are not illustrated.
Four Factors:
1) Temperature
2) Pressure
3) Fluid activity
4) Deviatoric stress
Schematic Continental Convergent Margin
High P/Moderate T
Facies Series:
zeolite -> prehnitepumpellyite ->
glaucophene schist
(blueschists).
From Ernst, 1976
Adjacent to the
magmatic arc see
typical Barrovianstyle metamorphism:
moderate P and inc.
T culminating in
partial melting and
migmatization.