1. 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.

2. 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.

3. Contact Metamorphic Aureole
Progressive
metamorphism of
Pelitic country rocks,
Onawa, Maine
Slate (lowest T);
fartherest from
intrusion
Spotted semihorfels
~1.5 km from intrusion
Hornfels (high T);
adjacent to the
intrusion; NB well
developed
granoblastic texture
From Best, 2003; Moore, 1960

4. 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

5. 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

6. Felsic Igneous Intrusion Metasomatic Skarn
Highest Grade
Garnet Zone (gr + di + wo)
Crestmore, CA
Lowest Grade
Forsterite Zone
(cal + br + clhm + sp)
From Burham, 1959

7. 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

8. Index Minerals and Isograds
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.

9. 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

10. Facies Reactions

11. P-T-time Paths

12. Progressive Ductile Deformation
Archean ( 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

13. Hypothetical Polymetamorphic Sequence

14. L-S Tectonite Fabric Development
Crenulation cleavage development

15. Pressure Solution and Volume Loss
removes volume
Formation of Spaced Cleavage

16. Compatibility Diagrams
Compositional Tie Lines
No Solid Solution
With Solid Solution

17. AFC Composition Diagrams
A (in mol.) = Al2O3 + Fe2O3
- Na2O - K2O
Basalt BCR-1
C (in mol.) = CaO P2O5
- CO2
F (in mol.) = FeO + MgO
+ MnO - TiO2 - Fe2O3
Compositional Tie Lines

18. AFM Projection Diagrams
Project bulk composition of average shale from ideal
muscovite + water + quartz onto AFM plane for ease of visualization
From Thompson, 1957

19. 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.

20. 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

21. 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.

22. Devolatilization and Decarbonation
Volatile bearing systems on low P/T sides of reaction boundaries