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!Schematic Continental Convergent Margin!From Ernst, 1976!High P/Moderate T!Facies Series:!zeolite -> prehnite-!pumpellyite ->!glaucophene schist !(blueschists).!Adjacent to the!magmatic arc see!typical Barrovian-!style metamorphism:!moderate P and inc.!T culminating in!partial melting and!migmatization.!Zeolite facies!Prehnite-pumpellite facies!Blueschist facies!Greenschist facies!Amphibolite facies!Facies Assemblage Definitions!The assemblages should occur together and should have formed at the same time – must be mappable like a metamorphic aureole – and also should recur in other areas.!Mineral assemblage is only affected by the chemical composition of the rock NOW regardless of any metasomatism (exchange of “volatile” or “fluid soluble” components.!There should be NO evidence of disequilibrium in the rock – that is no replacement by one mineral with another – should be no more than 2 to 6 minerals in each assemblage and 12 essential minerals in each named facies.!Additional requirements from Eskola (1920)!!- Rocks formed in same range of physical conditions (T, P)!!- Each assemblage represents a group of stable co-existing minerals !when they were formed!Facies Minerals and Protoliths!Facies Reactions!Facies Petrogenetic Grid!Note rock names are facies names are often the same – be careful!!Contact Metamorphic Aureole!From Best, 2003; Moore, 1960!Progressive!metamorphism of!Pelitic country rocks,!Onawa, Maine!Slate (lowest T);!farthest from!intrusion!Hornfels (high T);!adjacent to the!intrusion; NB well!developed !granoblastic texture!Spotted semihorfels! ~1.5 km from intrusion!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!Animation courtesy of Clay Hamilton and Bill Chadwick.Basalt to Eclogite!Photo credits: USGS, A. Alden, Union College!Garnet + Pyroxene!Plagioclase + Pyroxene + Olivine!Compatibility Diagrams!No Solid Solution! With Solid Solution!Compositional Tie Lines!Remember the Phase Rule: F = C – P + 2, most systems have F >= 2!ACF 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!Developed by Eskola (1915)!Best used for mafic and shaly limestone/dolomite protoliths !Granulite and Eclogite ACF Diagrams!Most basalts fall into dark shaded region.!Picrites (Mg-rich basalts) fall into the dotted region, !which allows orthopyroxene to become stable.!Retrograde Metamorphism of Eclogite!- Fracture set (2) controlled hydrothermal fluid pathways!Retrograde eclogite!Fresh eclogite!From Best, 2003!- Retrograde vein made of chlorite + epidote + glaucophane + white mica!Glaucophane belongs to the amphibole group: Na2(Mg,Fe)3Al2Si8O22(OH)2. Named for its typical blue color (in Greek, it means "blue appearing"). Also seen in Blueschists. !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!From Burham, 1959!Lowest Grade!Forsterite Zone!(cal + br + clhm + sp)!Highest Grade!Garnet Zone (gr + di + wo)!Crestmore, CA!Numbers correspond to specific rxns related to index minerals within a zone: Fo -> Mont. -> Vesv. -> Gt!Scottish Barrovian Zones in Pelites!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!Low Grade!High Grade!Scottish Barrovian Zones in Pelites!Conditions of metamorphism determined by experiment!Index Minerals and Isograds!Index Minerals:!specific mineral!characteristic of!a zone, e.g.!biotite & garnet;!may persist into!next zone.!Isograd:!3D surface of!constant grade;!intersection w/!horizontal is !a line.!Progressive Ductile Deformation!Archean (3.1-3.4 Ga) Ameralik basalt dikes and host 3.8 Ga Itsaq gneiss!From McGregor, 1973!Undeformed dike!in augen gneiss!Ductilely!deformed!dike and!host gneiss!Intensive!flattening!of fsp!augens;!amphibolite!boudin!formation!Hypothetical Polymetamorphic Sequence!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 (calcite -> aragonite)!– 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.!Devolatilization and Decarbonation!Volatile bearing systems on low P/T sides of reaction