Download Equilibration in Metamorphic Rocks

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 minerals 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
become mobile in an
aqueous fluid phase
causing metasomatism.
Relic Phenocrysts in Meta-andesite
Pyroxene pseudomorphically replaced by epidote Ca (Al,Fe)Al O(SiO )(Si O )(OH)
Plagioclase pseudomorphically replaced by epidote, albite, and sericite
2
2
4
2
7
(K,Na)Al
2[(OH)2AlSi3O10]
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 brittle 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 “a” or maximum
elongation
direction
Cleavage Formation
•  Slaty cleavage: Defined by the alignment 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.
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
L-S Tectonite Fabric Development
Crenulation cleavage development
Inclusions and Schistosity in in Garnet Porphyroblasts
Observation: spiral traces of
foliation within garnet
porphyroblast
Interpretation: Garnet
“rolled” or rotated during
growth – note matrix
Observation: Crenulated
schistosity within garnet
porphyroblast
Interpretation: Garnet
grew after formation of
S-C fabric
Observation: Z-shaped
schistosity within garnet
porphyroblast
Interpretation: Garnet
preserved earlier foliation –
note difference with matrix
Pressure Solution and Volume Loss
Pressure solution
removes volume
Formation of Spaced Cleavage
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
Deformation in Mixed Strength Rocks: Boudinage
Rotated boudins of amphibolite (fine
grained amph-rich meta-basalt) in granite
augen gneiss from Idaho batholith
Boudins of biotite-rich qtz-fsp schist
enclosed in granite from Idaho batholith
From Hyndman, 1985
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 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.
Metamorphic Grade
•  Prograde: Refers to a metamorphic P-T-time path
that progresses in toward a maximum final
temperature. Reactions liberate volatiles with
increasing T. –  Dehydration rxns, i.e. muscovite breakdown, liberate H2O –  Decarbonation rxns, i.e. calcite breakdown, liberate CO2
•  Retrograde: Refers to a metamorphic pathway
with decreasing T, which would be expected after
attaining peak metamorphic temperatures. Since
volatiles were liberated and migrated away during
the prograde path, retrogression is often kinetically
inhibited w/o re-introduction of water.
P-T-time Paths
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
Reaction rate = A * exp (-E/RT)
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
Fractal Nature of Deformation
km scale
m
scale
mm
scale
cm
scale
Metamorphic Terranes and Environments
•  Ocean-ridge: Hot, highly fractured rock and hydrothermal fluids
combine to alter MORBs and sediments.
•  Regional: Widespread in the roots of continental orogens. Often
involves concurrent deformation in a continent-continent collision
zone. Also called dynamothermal metamorphism.
•  Burial: Associated with thick piles of clastic and volcanic sediments
accumulated along passive margins and oceanic trenches.
•  Contact: Country rocks adjacent to igneous intrusions are subjected to
elevated T and hydrothermal fluids (induced meteoric flow and volatile
exsolution from evolving igneous melts).
•  Dynamic Shear: Formation of cataclastites and mylonites, in brittle
and ductile shear zones. Rare pseudotachylite associated with
frictional melting along fault planes.
•  Impact: Shock metamorphism and melting associated with meteorite
and asteroid impacts on Earth and other terrestrial planets.