metamorphic terranes and environments ocean-ridge: hot, highly fractured rock and hydrothermal...

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