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Transcript of METAMORPHISM. Fig. Ta 7.1 Definition of Metamorphism The alteration of a pre- existing igneous,...
![Page 1: METAMORPHISM. Fig. Ta 7.1 Definition of Metamorphism The alteration of a pre- existing igneous, sedimentary, or metamorphic rock (called the “parent.](https://reader030.fdocuments.us/reader030/viewer/2022032517/56649cb75503460f9497c551/html5/thumbnails/1.jpg)
METAMORPHISMMETAMORPHISM
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Fig. Ta 7.1
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Definition of MetamorphismThe alteration of a pre-existing igneous, sedimentary, or metamorphic rock (called the “parent rock” or “protolith”) by the application of heat and/or pressure. Complete melting does not occur.
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Metamorphism depends on:
Parent Rock
Equilibrium Exchange
Change in P-T-X
Geothermal Gradient
Confining/lithostatic pressure
Differential Stress: Shearing & Compression
TIME
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Confining Pressure
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CompressiveStress
Shearing
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Metamorphosed Conglomerate
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Effects of MetamorphismMineralogical: - change minerals to reflect new P-T (equilibrium) conditions.
Recrystallization: change of existing crystal size and shape to form layers (e.g., shale to schist), interlocking crystals (e.g., sandstone to quartzite).
Recombination: Recombination of elements in pre-existing minerals to form stable new ones. Addition/subtraction of ions common when fluid is present.
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Effects of Metamorphism
Textural: Re-orientation of mineral grains, especially platy (micaceous) grains in response to differential stress.
Non-foliated: random orientation – either no platy minerals or no directed stress (isotropic); Granoblastic (equigranular or isotropic) or Porphyroblastic (metamorphic porphyritic texture).
Foliated: fabric developed through directed pressure and/or shearing.
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Development of Foliation
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Non-Foliated
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Foliated
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FoliationOriented Minerals: platy or needle-like minerals growing in a plane either re-orientation or existing or growing of new.
Gives cleavage or planes of weakness.
Slate: oriented clay minerals allows the rock to be easily cleaved.
Phyllite: growth of microscopic micas to give strong foliation – rock is “shiny”.
Schist: strongly foliated with visible micas and feldspars.
Gneiss: strongly banded rock with dark bands of micas & pyroxenes, light bands of feldspars & quartz.
Migmatite: strongly veined rock as it has partially melted.
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Table 07.01
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Typical Metamorphic Minerals
Chlorite: Hydrous Fe-Mg-Al silicate
Talc: Hydrous Mg-silicate
Serpentine: Hydrous Mg silicate
Graphite: Carbon
Garnet: Ca-Mg-Fe-Al silicate
Staurolite: Fe-Al silicate
Also: Quartz, muscovite, biotite, feldspar, amphibole, calcite.
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Metamorphic GradeDegree of parent rock alteration, mostly dependent on increasing temperature for increasing grade.
Prograde: slate-phyllite-schist-gneiss-migmatite (melting).
Prograde metamorphic Reaction:
CaCO3 + SiO2 CaSiO3 + CO2
Calcite Quartz Wollastonite Fluid
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Classification
Mineralogical and textural if foliated:“garnet-mica schist”
“quartz-feldspar gneiss”
If non-foliated, named on compositional basis (and often color):
“(white) quartzite” or “(pink) marble”
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Types of Metamorphism: ContactThermal, local, around intrusions. Size of aureole depends on:Size of intrusion
Heat (composition)Fluid content of magmaFluid content of country rockCountry rock type
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Types of Metamorphism: ContactProduces non-foliated, granoblastic rocks:
Hornfels (if clastic - shale)Quartzite (if sandstone) Marble (if carbonate)
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Types of Metamorphism: RegionalAlso known as dynamothermal - produces both foliated and non-foliated metamorphic rocks.
Heat & directed pressure on rocks buried deep within the Earth - Prograde or Retrograde.
Associated with mountain belts - affects very large areas.
Prograde metamorphic reactions liberate a fluid.
Retrograde is difficult – pore spaces decrease during prograde so getting fluid back in is not easy.
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RegionalMetamorphism
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Types of Metamorphism: ShockMeteorite impact – sudden and intense deformation.
See www.meteorcrater.com
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Types of Metamorphism: ShockHigh-pressure polymorph of quartz – coesite – can form.Impact melt can form.Lots of rock fragmentation & mineral deformation.
Moon – no atmosphere so lots of meteorite impacts (micro and macro!). Produces regolith, rock flour, impact melt, breccias.
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Water in MetamorphismProvides transport mechanism and can promote reactions.Hydrothermal metamorphism: hot water streams add/remove ions. May promote ore formation.
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Metasomatism: addition of ions from external source.
Water in Metamorphism
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Water in Metamorphism
Metamorphic aureole is greater around granitic plutons than around gabbroic plutons, even though the magma temperature is lower.
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Ore-bearing veins in a mine
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Metamorphic Grade:
Degree of parent rock alteration, mostly dependent on increasing temperature for increasing grade
Prograde: SLATE -phyllite-schist-gneiss-migmatite (melting).
SLATE: oriented clay minerals allows the rock to be easily cleaved.
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Foliation can be deformed
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Prograde: slate-PHYLLITE-schist-gneiss-migmatite (melting).
PHYLLITE: growth of microscopic micas to give strong foliation – rock is
“shiny”.
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Prograde: slate-phyllite-SCHIST-gneiss-migmatite.
SCHIST: strongly foliated with visible micas & feldspars.
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Garnet Schist
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Fig. 07.14
Prograde: slate-phyllite-schist-
GNEISS-migmatite.
GNEISS: strongly banded rock with dark bands of micas
& pyroxenes and light bands of feldspars and quartz.
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Gneissose Banding
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Prograde: slate-phyllite-schist-gneiss-MIGMATITE.
MIGMATITE: strongly veined rock as it has partially melted.
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0065
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Metamorphic Facies:
Rocks having broadly similar mineral assemblages = same metamorphic facies – analogous to climatic zones.
The metamorphic facies is determined by the specific mineral assemblage present.
The facies is defined as a mineral assemblage that may only occur over a relatively narrow range of P-T conditions.
Formed under broadly similar P-T conditions.
Mineralogy dependent upon protolith composition.
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Metamorphic Facies:
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Plate Tectonics and Metamorphism
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Plate Tectonics and MetamorphismGravitational collapse & spreading – central part of mountain becomes too high & collapses. Rock forced downward.
Foliation developed parallel to collapse & spreading.
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Index MineralsForm over a restricted range of pressure & temperatures (determined in the laboratory).
When found in rocks, the P-T of formation can be reasonably estimated.
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Index MineralsKyanite, Andalusite, Sillimanite: all Al2SiO5, different crystal structures - polymorphs.Andalusite - low P & T; Kyanite - high P; Sillimanite - high T.
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0062