GEOS 444 Lecture Topics 32805

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GEOS 444 Lecture Topics 3/28/05
Introduction to Metamorphism

• Basic Metamorphic Processes
• Metamorphic Rock Classification and Facies
• Index Minerals, Isograds, Facies Series

Read Chapter 21, 22, 25 (496-500) Bear Mtn Field
Trip, Sat. 4/2, Lot 28 800am, bring lunch
H2O Igneous Mid-term due Thursday, -5 per day
late!
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Importance

• Mineral Resources
• Mountain Building Events
• History of Continental Crust

Uncut Ruby and Sapphire
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Metamorphic Petrology
Metamorphic rocks usually record peak
metamorphic conditions
Use specific mineral(s) to determine P-T ?
Index Minerals
(Example Garnet)
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Metamorphism

• Changes in rocks due to increasing P-T
  conditions and/or interaction with fluids.

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• Metamorphism usually involves changes in
• mineralogy ? formation of metamorphic minerals
• texture ? development of metamorphic fabrics

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Metamorphic Conditions

• All changes occur in the SOLID state between
  100?C and 800 ?C

Solid State Recrystallization Metamorphism

• Metamorphic Grade refers to general P-T
  conditions

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• High-temperature limit grades into melting
• Migmatites (mixed rocks) are gradational

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Agents of Metamorphism

• Temperature
• Pressure
• lithostatic - uniform P, due to weight of
  overlying rock 1 kb (0.1 GPa) 3.3 km depth.
• differential (deviatoric) unequal P in
  different directions produces metamorphic rock
  structures.
• Fluids
• H2O-dominated CO2. Derived from metamorphic
  reactions (internal) or magmatic fluids
  (external).

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Types of Metamorphism
Two main types at tectonically active
regions (1) Contact Metamorphism (2)
Regional Metamorphism
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Contact Metamorphism

• thermal metamorphism due to heat of igneous
  intrusions

• narrow zones (lt1 km wide)

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Contact Metamorphism
thermal metamorphism at high T low P
conditions
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Types of Metamorphism
Two main types at tectonically active
regions (1) Contact Metamorphism (2)
Regional Metamorphism
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Regional Metamorphism

• Large, regional areas of crust affected
  (thousands of km2) one or more episodes of
  orogeny with combined elevated geothermal
  gradients and deformation
• Associated with mountain building processes at
  convergent plate boundaries (subduction zones
  collision zones)
• Examples Andes, Himalayas, Appalachians
• Full range of P-T metamorphic conditions
  foliated rocks are a characteristic product

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Regional Metamorphism
low to high P-T conditions
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GEOS 444 Lecture Topics 3/28/05
Introduction to Metamorphism

• Basic Metamorphic Processes
• Metamorphic Rock Classification and Facies
• Index Minerals, Isograds, Facies Series

Read Chapter 21, 22, 25 (496-500) Bear Mtn Field
Trip, Sat. 4/2, Lot 28 800am, bring lunch
H2O Igneous Mid-term due Thursday, -5 per day
late!
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Non-foliated
Foliated
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Common Metamorphic Fabrics
Slaty Cleavage
Schistocity
Gneissic Banding
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Origin of Metamorphic Foliation
Produced by differential stress
Compressive
Shearing
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Granitic Gniess
Granite
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aligned micas (muscovite)
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Broad Compositional Categories based on
mineralogy and textures ultimately inherited from
the protolith.
Protolith rock type prior to metamorphism
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Quartz Sandstone
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(a) Limestone (fiossiliferous)
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Shale
Schist
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3 Most Important Compositional Categories

• Pelites derived from Al-rich, fine-grained
  clastic sediments (shales, siltstones). Classic
  slate-phyllite-schist-gneiss sequence.
• 2. Calcareous derived from carbonate rocks
  (limestones, dolostones, shaly ls). Marbles,
  calc-silicate rocks.
• 3. Mafic and Ultramafic ultramafic to mafic
  igneous rocks. Greenstones, amphibolites,
  granulites.

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P-T Classification

• Metamorphic Facies - is a set of compatible
  mineral assemblages in metamorphic rocks that
  occur over a relatively restricted range of P-T
  conditions for a variety of protolith
  compositions.
• If we find rocks with these minerals in the
  field, then a certain facies (P-T conditions) may
  be assigned to the area.
• Facies are simply broad characterizations of the
  P-T conditions experienced in an area. They are
  represented by boxes on a P-T diagram

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• Facies are defined by specific mineral
  assemblages, dependent on P-T and protolith
  composition.
• Facies boundaries are defined by important
  mineral reactions and the appearance of
  distinctive minerals.

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IMPORTANT CONCEPT Metamorphic assemblages are a
function of P-T and protolith chemistry ?
Different protoliths will yield different mineral
assemblages at P-T conditions
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GEOS 444 Lecture Topics 3/28/05
Introduction to Metamorphism

• Basic Metamorphic Processes
• Metamorphic Rock Classification and Facies
• Index Minerals, Isograds, Facies Series

Read Chapter 21, 22, 25 (496-500) Bear Mtn Field
Trip, Sat. 4/2, Lot 28 800am, bring lunch
H2O Igneous Mid-term due Thursday, -5 per day
late!
33
Interpretation of Metamorphic P-T Conditions???

• Facies boundaries are defined by important
  mineral reactions and the appearance of
  distinctive minerals.

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Interpretation of Metamorphic P-T Conditions???
Metamorphic rocks usually record peak
metamorphic conditions
Use specific mineral(s) to determine P-T ?
Index Minerals
(Example Garnet)
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Concept of Index Minerals
Index Minerals Chlorite, biotite, garnet,
kyanite, sillimanite
Need to exist over narrow P-T range
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Geologic Mapping of Metamorphic Terranes

• Index minerals are mapped into zones with
  equivalent P-T conditions

• Boundaries between zones are called isograds
  (lines of equal P-T)

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Metamorphic zones are higher resolution P-T
estimates based on a single protolith (e.g.
pelites)
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Geothermobarometry
Precise P-T from composition of co-existing
minerals pairs
Use instrument called electron microprobe (x-rays
analysis)
Example Garnet-biotite thermometer
garnet
biotite
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Plate Tectonics and Metamorphism
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Facies Series

• A traverse up grade through a metamorphic terrane
  should follow one of several possible P-T
  trajectories and, if extensive enough, cross
  through a sequence of facies

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Variable P-T Conditions in a Convergent Plate
Setting
Low P, high T
high P and T
high P, low T
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Med P/T
Low P/T
High P/T
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Facies Series

• Miyashiro (1961) initially proposed five facies
  series, most of them named for a specific
  representative type locality The series were
• 1. Contact Facies Series (very low-P)
• 2. Buchan or Abukuma Facies Series (low-P
  regional)
• 3. Barrovian Facies Series (medium-P regional)
• 4. Sanbagawa Facies Series (high-P, moderate-T)
• 5. Franciscan Facies Series (high-P, low T)

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Barrovian Facies Series (Medium P)

• Chlorite zone
• Biotite zone.
• Garnet zone.
• Staurolite zone.
• Kyanite zone.
• Sillimanite zone.

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Buchan Facies Series (Low P)

• Chlorite zone
• Biotite zone.
• Cordierite zone.
• Andalusite zone.
• Sillimanite zone.

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The stability field of andalusite occurs at
pressures less than 0.37 GPa ( 10 km), while
kyanite ? sillimanite at the sillimanite isograd
only above this pressure
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Barrovian
Buchan
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Barrovian
Buchan
Franciscan
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Paired Metamorphic Belts
Low P/T Series
High P/T Series
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Low P, High T
High P, Low T
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Low P, High T
High P, Low T