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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
From Best, 2003; Moore, 1960
Progressivemetamorphism ofPelitic country rocks,Onawa, Maine
Slate (lowest T);fartherest fromintrusion
Hornfels (high T);adjacent to theintrusion; NB welldeveloped granoblastic texture
Spotted semihorfels ~1.5 km from intrusion
Retrograde Metamorphism of Eclogite
- Fracture set (2) controlled hydrothermal fluid pathways
Retrograded eclogite
Fresh eclogite
From Best, 2003
- Retrograde vein made of chl. + epidote + glaucophene + white mica
Epidote Vein in Granodiorite
Unaltered granodiorite
Chloritized biotites andfeldspars -> sericite and fine grained alterationproducts
Epidote veinCa2Fe3+Al2O(SiO4)(Si2O7)(OH)Formed by retrograde rnxof plagioclase + water,likely along a coolingfracture
Felsic Igneous Intrusion Metasomatic Skarn
From Burham, 1959
Lowest GradeForsterite Zone(cal + br + clhm + sp)
Highest GradeGarnet Zone (gr + di + wo)
Crestmore, CA
Scottish Barrovian Zones in Pelites
Regional metamorphism and deformation related to the PaleozoicCaledonide Orogeny (NA-EUR collision). 13 km thick section.First described by Barrow (1893). From Gillen, 1982
Low Grade
High Grade
Index Minerals and IsogradsIndex Minerals:specific mineralcharacteristic ofa zone, e.g.biotite & garnet;may perisist intonext zone.
Isograd:3D surface ofconstant 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 gradientsshown in plate on right.
From Spear, 1993; Turner, 1981
Facies Reactions
P-T-time Paths
Progressive Ductile DeformationArchean (3.1-3.4 Ga) Ameralik basalt dikes and host 3.8 Ga Itsaq gneiss
From McGregor, 1973
Undeformed dikein augen gneiss
Ductilelydeformeddike andhost gneiss
Intensiveflatteningof fspaugens;amphiboliteboudinformation
Hypothetical Polymetamorphic Sequence
L-S Tectonite Fabric Development
Crenulation cleavage development
Pressure Solution and Volume Loss
Pressure solutionremoves volume
Formation of Spaced Cleavage
Compatibility Diagrams
No Solid Solution With Solid Solution
Compositional Tie Lines
AFC Composition Diagrams
Basalt BCR-1A (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
CaAl2Si2O8 + 2(Mg,Fe)SiO3 = Ca(Mg,Fe)2Al2Si3O12 + SiO2
anorthite opx garnet qtz
BASALT ECLOGITE
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