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Metamorphism and Metamorphic Rocks

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Metamorphism and Metamorphic Rocks Processes and Products Schist Amphibolite Eclogite Gneiss * * Metamorphic Rocks: Importance and Occurrence Importance: Together ... – PowerPoint PPT presentation

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Title: Metamorphism and Metamorphic Rocks


1
Metamorphism and Metamorphic Rocks
  • Processes and Products

Schist
Amphibolite
Eclogite
Gneiss
2
Metamorphic Rocks Importance and Occurrence
  • Importance
  • Together with igneous rocks, make up gt90 of
    earths crust
  • Excellent indicator of P-T history and
    deformation history of mountain belts
  • Ore deposits (Fe, Au-Ag-Cu, Ag-Pb-Zn, U, garnet)
  • Occurrence
  • Mantle (metamorphosed ultramafic rocks)
  • Oceanic crust (alteration of ocean floor basalt)
  • Continental crust (mountain belts)
  • Specific assemblages tied to tectonic setting

3
Metamorphic Rocks Review
  • With your neighbors, discuss and answer
  • What is meant by metamorphism? Why do rocks
    undergo metamorphism?
  • What processes cause rocks to undergo
    metamorphism? List and describe several.
  • Where does metamorphism occur? Describe several
    tectonic settings in which rocks are
    metamorphosed.

Schist
?
Shale
4
The Formation of Metamorphic Rocks
  • Metamorphism
  • Mineralogic and/or textural changes in a rock in
    the solid state
  • Occurring between diagenesis and melting
  • Protolith
  • Parent rock igneous or sedimentary
  • Metamorphic agent
  • Pressure (uniform, lithostatic, 1-15 kbar)
  • Directed stress (non uniform)
  • Temperature (100-750 ºC)
  • Geothermal gradient, friction, deformation, magma
  • Chemically active fluids
  • H2O, CO2, other fluids present in most rocks
  • Temperature, pressure, or stress changes cause
    fluid migration

5
Types of Metamorphism

6
Metamorphic Minerals
  • Minerals present in metamorphic rocks reflect
  • Composition of the protolith
  • P-T conditions under which the rock formed

7
Metamorphic Rock Names
  • Metamorphic rock names are generally based
    entirely on texture OR entirely on composition
  • Specifically, rock names are commonly derived
    utilizing any one, or a combination of the
    following criterion (Yardley, 1989)
  • the nature of the parent material (bulk
    composition)
  • the metamorphic mineralogy
  • the rock's texture (grain size and fabric
    development)
  • any appropriate special name

8
Metamorphic Rock Classification by Bulk
Composition Mineralogy
9
Metamorphic Rock Classification by Texture
  • Foliated
  • Alignment of platy, acicular, or prismatic
    crystals due to directed stress and
    partial/complete neo-crystallization
  • strong tendency to split along planar surfaces
    Cleavage
  • Deformation under Ductile Conditions
  • Cataclastic
  • crushing and breaking of mineral grains with
    little/no re- or neo-crystallization
  • deformation under Brittle Conditions
  • Granoblastic
  • equidimensional re- or neo-crystallize mineral
    grains

10
Metamorphic Rock Classification by Texture
11
Special Metamorphic Rocks
12
Metamorphic Structures
  • Metamorphic Tectonites
  • Rocks with metamorphic fabric (textural and
    structural features) due to a history of
    deformation (one or more episode of directed
    stress)
  • Produced by ductile deformation with
    recrystallization
  • Characterized by oriented mineral grains
  • S- Tectonites contain a strong planar or Surface
    fabric
  • Rock cleavage (breakage along parallel surfaces)
  • L- Tectonites contain a strong Linear fabric
  • Transposition
  • Modification of original planar fabric (i.e.
    bedding) through penetrative deformation

13
Metamorphic Structures
  • Folds
  • bends in planar structures
  • Boudins
  • Sausage- (cylindrical) shaped masses usually
    formed by tension (pulling apart) of planar
    fabric in protolith
  • Joints
  • fractures (brittle) breakage of rock body no
    movement
  • Faults
  • Fractures along which movement occurs

14
Metamorphic Facies and Mineral Assemblages
Contact and Regional Metamorphism
15
Metamorphic Grade
  • One of the primary goals of metamorphic petrology
    is to interpret P-T conditions under which a rock
    (or set of rocks) formed
  • Metamorphic grade
  • relative temperature and pressure conditions
    under which metamorphic rocks form
  • Low-grade metamorphism
  • T 200 to 320ºC, relatively low pressure
  • abundant hydrous minerals
  • High-grade metamorphism
  • T gt320ºC and relatively high pressure
  • Dehydration less hydrous minerals common
  • Prograde metamorphism
  • T and/or P, grade of metamorphism increases 
  • Retrograde metamorphism
  • T and/or P, grade of metamorphism decreases 

16
Metamorphic Facies
  • set of rocks characterized by equilibrium
    mineral assemblage that reflect specific range of
    metamorphic (T-P) conditions
  • Mineral assemblage present depends on protolith
    composition and P-T conditions
  • Ex marble, metabasalt and schist all in
    amphibolite facies

17
Metamorphic Series
  • Progression or sequence of facies across a
    metamorphic terrain
  • Identified by field and experimental work

3.
2.
1.
  • Contact Series Hi T, low P contact
    metamorphism
  • Dynamic Regional Series Mod T P Barrovian
  • Static Regional Series Lo T, hi P Blueschist

18
Metamorphic Reactions and Critical Minerals
  • For a given rock composition, some reactions
    occur under specific P-T conditions
  • Critical minerals
  • minerals with limited P-T stability
  • Presence indicates certain metamorphic conditions
  • Ex
  • Kyanite
  • Andalusite
  • Sillimanite
  • Polymorphs of Al2SiO5

Al2Si4O10(OH)2 ltgt Al2SiO5 3SiO2 H2O 
Pyrophyllite       Ky or Andal   Qtz      fluid
19
Metamorphic Isograds
  • Reactions and appearance/disappearance of
    critical minerals can be used to determine P-T
    conditions
  • Isograd line to indicate same grade of
    metamorphism
  • First appearance of mineral of increasing
    metamorphic grade

20
Contact Metamorphism
  • Result of high geothermal gradient produced
    locally around intruding magma
  • Restricted to relatively shallow depths (low
    pressure)
  • Rocks generally not foliated
  • Produce fine grained rocks called hornfels
  • Contact aureole surrounding rocks metamorphosed
    by heat of intrusion
  • Size/shape of aureole depends on
  • Size, temp, cooling history of the igneous
    intrusion
  • Properties of the country rock (conductivity,
    presence of water)

21
Contact Metamorphism Hornfels
  • Mineral assemblage characterized by hornfels
    facies series
  • Albite-epidote hfls
  • Hornblende hfls
  • Pyroxene hfls
  • Sanidinite

1.
Pyroxene hornfels
22
Contact Metamorphism Skarn
  • Contact metamorphism of carbonate or siliceous
    carbonate
  • Usually involves significant metasomatism
    (chemical exchange between magmatic fluids and
    rock)
  • Can be hosts of Au, Cu, Fe, Mo, Sn, W, Zn-Pb ore
    minerals

Wollastonite-garnet skarn
23
Regional Metamorphism
  • Affects large areas of earths crust
  • Produced during orogenic (mountain-building) event

4.
3.
2.
1.
  • Foliated rocks developed under med-high T and P
  • 1. Buchan 2. Barrovian
  • Rocks developed under low-med T and high P
  • 3. Sanbagawa 4. Franciscan

24
Regional Metamorphism Barrovian
  • Normal metamorphic series recognized in
    mountain belts world-wide
  • Medium to high T low to high P
  • Moderate to high geothermal gradient
  • Critical mineral sequence
  • Kaolinite gt pyrophyllite gt kyanite gt
    sillimanite
  • Textural sequence (in pelitic rocks)
  • Slate gt phyllite gt schist gt gneiss
  • Facies sequence
  • Zeolite gt prehnite-pumpellyite gt greenschist gt
    amphibolite gt granulite

25
Regional Metamorphism Franciscan
  • Low T high P
  • Low geothermal gradient
  • Little directed stress (little deformation not
    foliated)
  • Distinguished by presence of blue, sodic
    amphiboles
  • Facies sequence
  • Zeolite gt prehnite-pumpellyite gt blueschist gt
    eclogite

26
Regional Metamorphism and Tectonics
  • Barrovian (mod-high T and P)
  • Develop in orogenic belts at convergent margins
  • Regional heat flow increased by upward movement
    of magma and migration of fluids
  • Thrust faulting thickens crust, increasing
    pressure and directed stress
  • Geothermal gradient of 20-40ºC/km
  • Blueschist (Franciscan) (low T and high P)
  • Occur world-wide in orogenic belts of Phanerozoic
    age
  • Associated with subduction zones
  • rapid subduction of oceanic crust depresses
    isotherms
  • Geothermal gradient of 10-20ºC/km

27
Regional Metamorphism and Tectonics
  • Paired metamorphic belts specific to subduction
    zones
  • Barrovian
  • Formed in rocks beneath and around the arc
  • Heat due to magmatic activity
  • Blueschist
  • formed in the mélange of the trench
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