Transcript Metamorphic, Terranes, Facies, and Reactions

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.
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.
Contact Metamorphic Aureole
Progressive
metamorphism of
Pelitic country rocks,
Onawa, Maine
Spotted semihorfels
~1.5 km from intrusion
Slate (lowest T);
fartherest from
intrusion
Hornfels (high T);
adjacent to the
intrusion; NB well
developed
granoblastic texture
From Best, 2003; Moore, 1960
Retrograde Metamorphism of Eclogite
Retrograded eclogite
Fresh eclogite
- Fracture set (2) controlled hydrothermal fluid pathways
- Retrograde vein made of chl. + epidote + glaucophene + white mica
From Best, 2003
Epidote Vein in Granodiorite
Unaltered granodiorite
Chloritized biotites and
feldspars -> sericite and
fine grained alteration
products
Epidote vein
Ca2Fe3+Al2O(SiO4)(Si2O7)(OH)
Formed by retrograde rnx
of plagioclase + water,
likely along a cooling
fracture
Felsic Igneous Intrusion Metasomatic Skarn
Highest Grade
Garnet Zone (gr + di + wo)
Crestmore, CA
From Burham, 1959
Lowest Grade
Forsterite Zone
(cal + br + clhm + sp)
Scottish Barrovian Zones in Pelites
High Grade
Low Grade
Regional metamorphism and deformation related to the Paleozoic
Caledonide Orogeny (NA-EUR collision). 13 km thick section.
First described by Barrow (1893).
From Gillen, 1982
Index Minerals and Isograds
Index Minerals:
specific mineral
characteristic of
a zone, e.g.
biotite & garnet;
may perisist into
next zone.
Isograd:
3D surface of
constant grade;
intersection w/
horizontal is
a line.
Metamorphic Facies and Field Gradients
Metamorphic facies concept was first developed by Eskola (1914).
Numbered lines refer to specific mineral reactions commonly observed
in metamorphic rocks of that facies. Corresponding field gradients
shown in plate on right.
From Spear, 1993; Turner, 1981
Facies Reactions
P-T-time Paths
Progressive Ductile Deformation
Archean (3.1-3.4 Ga) Ameralik basalt dikes and host 3.8 Ga Itsaq gneiss
Undeformed dike
in augen gneiss
Ductilely
deformed
dike and
host gneiss
Intensive
flattening
of fsp
augens;
amphibolite
boudin
formation
From McGregor, 1973
Hypothetical Polymetamorphic Sequence
L-S Tectonite Fabric Development
Crenulation cleavage development
Pressure Solution and Volume Loss
Pressure solution
removes volume
Formation of Spaced Cleavage
Compatibility Diagrams
Compositional Tie Lines
No Solid Solution
With Solid Solution
AFC Composition Diagrams
Basalt BCR-1
A (in mol.) = Al2O3 + Fe2O3
- Na2O - K2O
C (in mol.) = CaO - 3.3 P2O5
- CO2
F (in mol.) = FeO + MgO
+ MnO - TiO2 - Fe2O3
Compositional Tie Lines
AFM Projection Diagrams
Project bulk composition of average shale from ideal
muscovite + water + quartz onto AFM plane for ease of visualization
From Thompson, 1957
Overview of Metamorphic Mineral Reactions
• Solid-solid: Involves only solid phases directly, but a fluid
phase may be involved as a catalyst.
• Solid-fluid: Release or consumption of a volatile fluid
phase. Includes redox and metasomatic reactions.
• Discontinuous reactions: Occur ideally at a single P/T
(without solid solution). Products and reactants are in
equilibrium along univariant curves.
– Polymorphic phase transitions
– Net-transfer, Heterogeneous reactions
• Continuous reactions: compositions of minerals and
modal abundance change to maintain equilibrium of a wide
range of metamorphic P/T space, e.g. ion-exchange
reactions such as Fe-Mg between garnet & cordierite.
Basalt -> Granulite -> Eclogite Stability Fields
NaAlSi3O8 = NaAlSi2O6 + SiO2
albite
jadeite
qtz
BASALT
ECLOGITE
CaAl2Si2O8 + 2(Mg,Fe)SiO3 = Ca(Mg,Fe)2Al2Si3O12 +
SiO2
anorthite
opx
garnet
qtz
Granulite and Eclogite ACF Diagrams
Most basalts fall into dark shaded region.
Picrites (Mg-rich basalts) fall into the dotted region,
allowing orthopyroxene to become stable.
Devolatilization and Decarbonation
Volatile bearing systems on low P/T sides of reaction boundaries