Metamorphic Terranes and Environments

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 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.

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. Archean (3.1-3.4 Ga) Ameralik basalt dikes and host 3.8 Ga Itsaq gneiss Progressive Ductile Deformation 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 Pressure solution 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- 3.3 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 albitejadeite qtz BASALT ECLOGITE CaAl2Si2O8 + 2(Mg,Fe)SiO3=Ca(Mg,Fe)2Al2Si3O12 + SiO2 anorthite opxgarnet 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