Metamorphic Facies and Field Gradients

Transcription

1 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

2 Schematic Continental Convergent Margin Blueschist Prehnite-pumpellite Zeolite facies Greenschist facies facies High P/Moderate T Facies Series: zeolite -> prehnitepumpellyite -> glaucophene schist (blueschists). Amphibolite facies From Ernst, 1976 Adjacent to the magmatic arc see typical Barrovianstyle metamorphism: moderate P and inc. T culminating in partial melting and migmatization.

3 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

4 Facies Minerals and Protoliths

5 Facies Reactions

6 Facies Petrogenetic Grid Note rock names are facies names are often the same be careful!

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

8 Basalt -> Granulite -> Eclogite Stability Fields NaAlSi 3 O 8 = NaAlSi 2 O 6 + SiO 2 albite jadeite qtz BASALT ECLOGITE CaAl 2 Si 2 O 8 + 2(Mg,Fe)SiO 3 = Ca(Mg,Fe) 2 Al 2 Si 3 O 12 + SiO 2 anorthite opx garnet qtz Animation courtesy of Clay Hamilton and Bill Chadwick.

9 Basalt to Eclogite Garnet + Pyroxene Plagioclase + Pyroxene + Olivine Photo credits: USGS, A. Alden, Union College

10 Compatibility Diagrams Compositional Tie Lines No Solid Solution With Solid Solution Remember the Phase Rule: F = C P + 2, most systems have F >= 2

11 ACF Composition Diagrams Basalt BCR-1 A (in mol.) = Al 2 O 3 + Fe 2 O 3 - Na 2 O - K 2 O C (in mol.) = CaO P 2 O 5 - CO 2 F (in mol.) = FeO + MgO + MnO - TiO 2 - Fe 2 O 3 Developed by Eskola (1915) Compositional Tie Lines Best used for mafic and shaly limestone/dolomite protoliths

12 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.

13 Retrograde Metamorphism of Eclogite Retrograde eclogite - Fracture set (2) controlled hydrothermal fluid pathways - Retrograde vein made of chlorite + epidote + glaucophane + white mica Fresh eclogite Glaucophane belongs to the amphibole group: Na 2 (Mg,Fe) 3 Al 2 Si 8 O 22 (OH) 2. Named for its typical blue color (in Greek, it means "blue appearing"). Also seen in Blueschists.! From Best, 2003

14 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

15 Felsic Igneous Intrusion Metasomatic Skarn Highest Grade Garnet Zone (gr + di + wo) Numbers correspond to specific rxns related to index minerals within a zone: Fo -> Mont. -> Vesv. -> Gt From Burham, 1959 Crestmore, CA Lowest Grade Forsterite Zone (cal + br + clhm + sp)

16 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

17 Scottish Barrovian Zones in Pelites Conditions of metamorphism determined by experiment

18 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.

19 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

20 Hypothetical Polymetamorphic Sequence

21 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.

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