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GEOS 444 Lecture Topics 32805

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Title: GEOS 444 Lecture Topics 32805


1
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!
2
Importance
  • Mineral Resources
  • Mountain Building Events
  • History of Continental Crust

Uncut Ruby and Sapphire
3
Metamorphic Petrology
Metamorphic rocks usually record peak
metamorphic conditions
Use specific mineral(s) to determine P-T ?
Index Minerals
(Example Garnet)
4
Metamorphism
  • Changes in rocks due to increasing P-T
    conditions and/or interaction with fluids.

5
  • Metamorphism usually involves changes in
  • mineralogy ? formation of metamorphic minerals
  • texture ? development of metamorphic fabrics

6
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

7
  • High-temperature limit grades into melting
  • Migmatites (mixed rocks) are gradational

8
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).

9
Types of Metamorphism
Two main types at tectonically active
regions (1) Contact Metamorphism (2)
Regional Metamorphism
10
Contact Metamorphism
  • thermal metamorphism due to heat of igneous
    intrusions
  • narrow zones (lt1 km wide)

11
Contact Metamorphism
thermal metamorphism at high T low P
conditions
12
Types of Metamorphism
Two main types at tectonically active
regions (1) Contact Metamorphism (2)
Regional Metamorphism
13
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

14
Regional Metamorphism
low to high P-T conditions
15
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!
16
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17
Non-foliated
Foliated
18
Common Metamorphic Fabrics
Slaty Cleavage
Schistocity
Gneissic Banding
19
Origin of Metamorphic Foliation
Produced by differential stress
Compressive
Shearing
20
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21
Granitic Gniess
Granite
22
aligned micas (muscovite)
23
Broad Compositional Categories based on
mineralogy and textures ultimately inherited from
the protolith.
Protolith rock type prior to metamorphism
24
Quartz Sandstone
25
(a) Limestone (fiossiliferous)
26
Shale
Schist
27
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.

28
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.

31
IMPORTANT CONCEPT Metamorphic assemblages are a
function of P-T and protolith chemistry ?
Different protoliths will yield different mineral
assemblages at P-T conditions
32
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.

34
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)
35
Concept of Index Minerals
Index Minerals Chlorite, biotite, garnet,
kyanite, sillimanite
Need to exist over narrow P-T range
36
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)

37
Metamorphic zones are higher resolution P-T
estimates based on a single protolith (e.g.
pelites)
38
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
39
Plate Tectonics and Metamorphism
40
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

41
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42
Variable P-T Conditions in a Convergent Plate
Setting
Low P, high T
high P and T
high P, low T
43
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44
Med P/T
Low P/T
High P/T
45
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46
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)

47
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48
Barrovian Facies Series (Medium P)
  • Chlorite zone
  • Biotite zone.
  • Garnet zone.
  • Staurolite zone.
  • Kyanite zone.
  • Sillimanite zone.

49
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50
Buchan Facies Series (Low P)
  • Chlorite zone
  • Biotite zone.
  • Cordierite zone.
  • Andalusite zone.
  • Sillimanite zone.

51
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
52
Barrovian
Buchan
53
Barrovian
Buchan
Franciscan
54
Paired Metamorphic Belts
Low P/T Series
High P/T Series
55
Low P, High T
High P, Low T
56
Low P, High T
High P, Low T
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