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METAMORPHIC PETROLOGY

METAMORPHISM:

Process of mineralogical and structural (textural) changes of rocks in the
solid state in response to physical and chemical conditions which differ
from those under which they originated.

      1. Mineral assemblages change.
      2. Textural Changes (foliations, cleavage, etc).

  •   Physical Conditions = Pressure & Temperature
  •   Chemical Conditions = PH2O, PCO2, fluid solutions w/dissolved solids.

ALL METAMORPHIC ROCKS WERE ONCE IGNEOUS OR SEDIMENTARY




LOWER AND UPPER LIMITS OF METAMORPHISM:
  • Low grade: diagenesis >> incipient metamorphism: 100o-150oC
  • High grade: anatexis = formation of partial melt: 750o-850oC
  • Pressure variations:
            Top of Crust 1-3 km depth (<1 Kb)
            Base of Crust 20-40 km (>10 Kb)




                                         1
FACTORS OF METAMORPHISM:

• Temperature: 100o-850o C

• Confining Pressure or “Lithostatic Pressure” (synonymous terms)
  Equals weight of overlying rock column; Same pressure on all sides.




• Directed Pressure: Related to deformation
  Sheering stresses; tectonic over pressure.




• Fluid compositions: Metamorphic pore fluid H2O and CO2 .




GOALS OF METAMORPHIC PETROLOGY:

• SETTING: Where did metamorphism occur? What was its structural and
  tectonic setting?
• PROTOLITH: Original nature of the rock before metamorphism.
• GRADE of metamorphism: How intense was the metamorphism? What were the
  physical and chemical conditions of the metamorphism?
      a. look for relict structures, fossils
      b. look at mineral assemblage and compare to experimental studies in
         mineral equilibria; estimate temp and pressure.
      c.    Calculate using mineral compositions, e.g., garnet-biotite
           geothermometer




                                               2
GEOBARIC GRADIENT = Pressure variations within Earth

• Related to amount of crust or top: approximately 285 bars/km      (upper crust)
      20 km = 5700 bars = 5.7 kb       33 km = 10 kb             1 Kb    3.3 km

Type of Crust            Thickness        Pressure at Base     Example
Normal                   35-40 km             10-12 Kb         Canada
Continental
Active Margin            60-80 km             18-25 Kb         Sierra Nevada,
                                                               Andes
Collisional Orogen       80-100 km            24-30 Kb         Himalayas
Oceanic crust             6-10 km              2-3 Kb          Atlantic ocean


Typical Pressure Range For Common Metamorphic Rocks = 2-8 Kb


GEOTHERMAL GRADIENT -- Controls Heat flow at Surface

Two Major Aspects of Geothermal Gradient:
• Conduction of heat from Mantle >> Limited effect in continents, Controlled by
  thickness of lithosphere (but note areas with thin lithosphere, e.g., Great Basin)
• Radioactive decay of U/Th/K >> important for continental crust


Type of Crust          Geothermal          Heat Flow at        Example
                        Gradient             Surface
Precambrian Shield      15-20oC/km         60 mW/m2-sec        Canada

Active Margin (Arc)    30o-35oC/km      100-120 mW/m2-sec      Sierra Nevada,
                                                               Andes
Subduction Zone          10oC/km           40 mW/m2-sec        Franciscan
(Accretionary                                                  Complex
Complex)
Collisional Orogens    25o-30oC/km      80-110 mW/m2-sec       Himalayas

Extensional Orogens    40o-50oC/km      120-150 mW/m2-sec      Great Basin

Mid-Ocean Ridge            up to        150-200 mW/m2-sec      Mid-Atlantic Ridge
                         60oC/km




                                          3
PLATE TECTONIC CONTROLS ON METAMORPHISM
• Ocean Floor metamorphism (mid-ocean ridges) = Very high T/low P.
• Subduction zone metamorphism = High P/ Low T
• Arc basement metamorphism = Low P / High T
• Continent-Continent Collision = intermediate P/T
• Continental Extension metamorphism = Intermediate P/high T


DEFINITIONS OF METAMORPHIC ROCKS

Prefixes:     Ortho-       Igneous Protolith
              Para-        Sedimentary Protolith

Examples:     orthogneiss, paragneiss

Slate, Argillite:   Low grade metamorphic rocks with partly to well-developed
                    cleavage; almost all are meta shales, metamudstones.
Phyllonite:         Higher grade than Slate, with incipient Foliation, shiny surface;
                    almost all are meta shales, metamudstones.
Schist:             Higher grade than slate, phyllonite; characterized by distinct
                    metamorphic foliation, abundant platy minerals, some
                    segregation into layers common.
Garnet-Biotite      Pelitic schist rich in garnet, biotite, & quartz.
Schist
Gneiss:             High grade metamorphic rock; varies widely in composition and
                    mineral mode; characterized by distinct compositional banding
                    in most cases.
Quartzite:          Metamorphosed quartzite or chert; very fine grained, sugary
                    texture.
Marble:             Metamorphosed limestone or dolostone. Typically unfoliated,
                    granular crystalline rocks.
Migmatite:          “Mixed Rock”, high grade gneiss in which felsic bands = granitic
                    melt, mafic areas = melting residue, or host into which melts
                    intruded.
Granulite:          Very high grade metamorphic rock; typically banded, foliated.
Granofels:          Like granulite, but lacking any preferred orientation, platy
                    minerals.




                                            4
CLASSIFICATION OF METAMORPHISM BASED OF GEOLOGIC SETTING

1. Local Metamorphism:
   a. Contact Metamorphism [Adjacent igneous intrusions]
   b. Dynamic = fault zones
2. Regional = Dynamo-thermal metamorphism, effects large regions of Earth.


CONTACT METAMORPHISM

   Factors that influence development of aureole
   • Temp of magma
   • Size of the intrusion (pluton, sill, dike)
   • Chemical activity of the fluids which migrate out of intrusion: important
     transfer mechanism of heat, chemicals.
   • Character of country rocks (chemical reactivity of wall rocks) some respond
     better to metamorphism pelitic schists, limestones.
   • Mode of emplacement (forceful emplacement vs magmatic stoping)
      •      stoping develops static aureole,
      •      forceful emplacement causes deformation.

ROCKS IN CONTACT AUREOLES

   Hornfels = Massive, fine grained, sugary textured rocks, very tough;
   • textural term
   • Can have many protoliths (pelitic, felsic volcanic common)
   • No compositional meaning to hornfels
   • Can have large Xtls growing within
   • Not the exclusive rock at contact aureole

Depends on character of emplacement: Forceful intrusion produces strongly foliated
and lineated rocks, results in contact gneisses and schists.

Contact Metamorphism = Generally Isochemical, i.e., same chemical signatures of
the protolith




                                           5
SKARNS: Contact Metasomatism in Limestones
   Exception to isochemical contact metamorphism = skarn (tactite)

   • Develop along contact between granite and limestone or dolomite.
   • Coarse grained rocks
   • Consist of calc-silicate minerals (SiO2 added, CO2 lost)


Typical calc-silicate minerals:

Diopside       CaMgSi2O6                 Wollastonite      CaSiO3

Garnet         Ca3Al2Si3O12              Idocrase          Don’t Ask

Actinolite     Ca2(MgFe)5Si8O22(OH)],    Tremolite         Ca2Mg5Si8O22(OH)

Scheelite      CaWO4                     Quartz            SiO2

Calcite        Ca(CO2)3

Why do these chemical changes occur? METASOMATISM
• Two rocks with grossly different compositions.
• Chemical gradients set up and extensive chemical migration occurs.
• Skarn zone develops at contact, not continuous.




                                         6
DYNAMIC METAMORPHISM: Metamorphism associated with fault zones.

Low T, P:    Incohesive rocks; very complex (gouge, breccia).
High T, P:   Very cohesive w/strong foliations (cataclasite, mylonite).


Textural Classification of Fault Rocks:

INCOHESIVE Fault Rocks
• Fault breccias (visible fragments >30% of rock)
• Gouge (visible fragments <30% of rock)

COHESIVE
• Cataclasite (Brittle): Protocataclasite> cataclasite > ultracataclasite
• Mylonite (Ductile): Protomylonite > Mylonite > Ultramylonite

Brittle-Ductile transition ≈ 250º-350ºC




                                          7
REGIONAL METAMORPHISM

    • Found in internal parts of orogenic belts
    • Strong fabrics
    • May have evidence of multiple deformation, heating events.
    • Metamorphic history related to Tectonic Evolution of orogen.

George Barrow (1888) Scottish highlands
    • Documented progressive regional metamorphism.
    • Dalradion Series = late Precambrian to Cambrian age.
    • Diverse group of rocks = Active Compressive margin sequence: Impure
      litharenites (wackes) with mafic-felsic volcanic rocks, pelitic rocks (shales and
      mudstones); impure carbonates.
    • Thickness 10-15 km thick (some structural thickening).
    • Effected by Lower Paleozoic Caledonian Orogeny

Barrow recognized metamorphism was progressive: rocks progress from low grade
mineral assemblages to high grade mineral assemblages as temperature increases.

Barrow Focused in on pelitic rocks and mapped using INDEX MINERALS.

Index minerals for progressive metamorphism in Metapelitic Rocks:
        Chlorite            >> Slate, Phyllite
        Biotite             >> Phyllite, fine-gr Schist
        Garnet              >> Schist, Gneiss
        Staurolite          >> Schist, Gneiss
        Kyanite             >> Schist, Gneiss
        Sillimanite         >> Schist, Gneiss

This is called the “Barrovian Sequence”

C.E. Tilley referred to these lines as ISOGRADS “Same Grade”
ex. biotite isograd = first appearance of biotite in meta-shale.
• Isograds may cut across structures, bedding
• Put tick marks put to the high-Temperature side
•   Area between Isograds called “Zones”, e.g., biotite zone, garnet zone, etc.




                                           8
Metamorphic Fabric and Texture

FABRIC:            Refers to orientation of the crystallographic lattices of the
                   minerals
TEXTURE:           Refers to mineral distributions or orientations within the
                   rock, overall structure of the rock.
Cleavage           Planar Fabric of Preferred Fracture, sub-parallel orientation,
                   non-penetrative.
Foliation:         Any Penetrative set of more or less parallel SURFACES.
Lineation:         Any Penetrative set of more or less parallel LINES.
Penetrative:       Caused by re-orientation of the crystalline fabric of the
                   minerals.


Granoblastic:      Mosaic texture of equidimensional, anhedral grains.
Lepidoblastic:     Abundant platy minerals (chlorite, biotite, etc) with strong
                   preferred orientation. Causes Foliation.
Nematoblastic:     Abundant linear minerals (actinolite, hornblende, etc) with
                   strong preferred orientation; Causes Lineation.
Poikiloblastic:    Large, metamorphic grains that enclose numerous small
                   inclusions; analogous to poikolitic igneous texture.
Porphyroblastic:   Large metamorphic crystals in a matrix of smaller grains;
                   analogous to porhyritic igneous texture.
Porphyroclastic:   Large relict grains of pre-existing mineral in finer-grained
                   matrix of recrystallized material; associated with shear
                   deformation and ductile flow.
Mylonitic:         Very fine-grained or aphanitic, anisotropic texture of
                   produced by intense ductile flow and high strain.
Flaser texture:    Mylonitic fabric with ovoid megacrysts of relict crystals in vfg
                   matrix. One type of porphyroclastic texture.




                                        9
DETERMINATION OF PROTOLITH

Six Common Types:
   1. Pelitic (shale, mudstone)
   2. Quartzo-feldspathic (sandstone, rhyolite, granite, chert)
   3. Calcareous (limestone, dolomite, marls)
   4. Basic (basalt, andesite, gabbro, diorite)
   5. Magnesian (peridotite, serpentine)
   6. Ferruginous (ironstone, umbers)


1. Pelitic Protoliths = Rocks enriched in clay minerals

• High Al2O3, K2O, lesser amounts Ca
• Micas favored because of Al content
• Also aluminosilicates: Al2SiO5 - sillimanite, andalusite, kyanite.

Kyanite:     Highest density (smallest volume) forms at higher pressures.
Andalusite: Lowest density, largest volume, forms at low pressures.
Sillimanite: Intermediate density, volume; forms at moderate T, P.

Alumino-silicate triple point = 5.5 kb at 600oC

Wet granite solidus: Shows where anatexis occurs in sillimanite zone.

Staurolite (2*Al2O5*Fe(OH)2) = Common metamorphic mineral
Need an Al and Fe-rich protolith -- This restricts occurrence




                                           10
2. Quartzo-feldspathic Protoliths: High SiO2, low Fe and Mg

•   “Psammitic”
•   Quartz-rich sandstones with varying % feldspars (“arkose”)
•   Felsic igneous rocks (rhyolites, tuffs, granites)
•   If protolith >50% quartz then probably a sandstone or chert.
•   Gneiss: Fine-grained at low grade, coarser with increasing grade.

Felsic tuffs, granite hard to tell from arkose when highly metamorphosed.


3. Calcareous Protoliths: High CaO, CO2

• Limestones and dolomite form MARBLES
• Impure limestones (with clay, silt) form Calc-silicates:
     [tremolite, diopside, wollastonite, forsterite, epidote, et cetera]



4. Basic Protoliths: Low SiO2 moderate CaO, MgO, FeO
• Basalts, andesites, gabbros - mafic igneous rocks.
• Some shale-limestone mixtures.
• Minerals depend on grade: chlorite, actinolite, hornblende, plagioclase, epidote,
   garnet.



5. Magnesian Protoliths: Very low SiO2, high MgO
• Peridotites         >> serpentine, magnesite.
• Serpentine (low T)  >> antigorite (high T serpentine), olivine.



6. Ferruginous Protoliths: High Fe2O3
• Ironstones = Precambrian iron formations (Fe-rich cherts).
• Umbers = Fe-rich cherts, shales associated with MOR.




                                           11
METAMORPHIC FACIES

Metamorphic Facies: All the rocks that have reached chemical equilibrium under a
particular set of physical conditions.

• Facies concept developed by Eskola (Norway, 1915) to compare metamorphic
  rocks from different areas.
• Look at several protoliths to determine facies.
• Facies represent specific temperature - pressure regimes.
• Named for Equivalent Mafic rock type at those conditions.

Facies of regional Metamorphism:

1) Greenschist Facies                    (includes chlorite/biotite zones of Barrow).
2) Epidote Amphibolite facies            (garnet zone).
3) Amphibolite Facies                    (staurolite/kyanite/sillimanite zones)
4) Granulite facies                      (not found in Scottish highlands)
5) Eclogite facies                       (not found in Scottish highlands)

• Names indicate specific T and P conditions and not textures or minerals ex.
  Amphibolite Facies includes calcite marbles, biotite schists and amphibolites.

Granulite:    muscovite + quartz ó sillimanite + Kspar       “2nd Sillimanite Ispgrad”
              Quartz + biotite ó Kspar + hypersthene


Eskola’s Facies for Contact Metamorphism:

1) Hornblende hornfels facies
2) Pyroxene hornfels facies

•   Do not have to be hornfels -- could be a schist.
•   Use these facies if clearly associated with a pluton.
•   Hornblende hornfels similar to amphibolite facies.
•   Pyroxene hornfels similar to granulite.




                                           12
NEW Metamorphic Facies -- Since Eskola

6. Zeolite Facies: Incipient metamorphism at low T and P

• Zeolites = Hydrated feldspars (mostly calcic).
• Temperatures 100-200oC.
• Index minerals = Laumonite, thompsonite, other zeolites, calcite, interlayered
  smectite/chlorite.
• Metabasalts: Veins and amygdule fillings
• Sandstones: Veins, interstitial pore space fillings.

7. Prehnite-Pumpellyite Facies (Sub-greenschist, low T, P)

• Index minerals = Prehnite, pumpellyite, calcite, chlorite, albite
• Temperatures = 150-250oC
• Metabasalts: Veins, amygdules, replacement of primary plagioclase, olivine,
  glass.
• Sandstones: Veins, replacement of clastic feldspars.

8. Blueschist Facies: High P at low Temperatures

•   Index minerals = Glaucophane, albite, jadeite, lawsonite, aragonite.
•   Temperatures = 250-350oC.
•   Pressures > 6-8 kb.
•   Metabasalts: Glaucophane, albite, lawsonite, sphene
•   Sandstones, shales: Jadeite, albite, quartz, lawsonite, aragonite, paragonite (Na
    white mica)




                                          13
FACIES – PROTOLITH—MINERAL ASSEMBLAGE TABLE:

Facies           Shale, Sandstone          Limestone           Basalt, Andesite
Zeolite          interlayered              calcite             Laumonite,
                 smectite/chlorite                             thompsonite, calcite,
100-200° C
                 calcite                                       interlayered
                                                               smectite/chlorite
Prehnite-        Prehnite,                 calcite             Prehnite,
   Pumpellyite   pumpellyite, calcite,                         pumpellyite, calcite,
                 chlorite, albite                              chlorite, albite
150-300° C
Greenschist      muscovite, chlorite,     calcite, dolomite,   albite, chlorite,
                 quartz, albite, biotite, quartz, epidote,     quartz, epidote,
300-450° C
                 garnet                   tremolite            actinolite, sphene
Epidote          muscovite, biotite,    calcite, quartz,       albite, epidote,
   Amphibolite   garnet, albite, quartz tremolite,             hornblende, quartz
                                        epidote,diopside
450-550° C
Amphibolite      garnet, biotite,          calcite, diopside   hornblende,
                 muscovite, quartz,        quartz,             plagioclase, garnet,
                 plagioclase,              wollastonite        quartz, sphene,
500-700° C       staurolite, kyanite or                        biotite
                 sillimanite
Granulite        garnet, Kspar,            calcite, quartz,    plagioclase, augite,
                 sillimanite or            plagioclase,        hypersthene,
                 kyanite, quartz,          diopside,           hornblende, garnet,
700-900° C       plagioclase,              hypersthene         olivine
                 hypersthene
Blueschist       Jadeite, albite,      aragonite, white        Glaucophane, albite,
                 quartz, lawsonite,    mica                    lawsonite, sphene, ±
150-350° C
                 aragonite, paragonite                         garnet
P > 5-8 Kb
Eclogite         coesite, Kspar,           aragonite, quartz, omphacite (px),
                 sillimanite,              plagioclase,       pyrope garnet
350-750° C
                 plagioclase               diopside,
P > 8-10 Kb                                hypersthene




                                          14
METAMORPHIC FACIES SERIES

Regional metamorphism: temperature/pressure vary along distinct P-T paths
(geothermal gradients) depending on tectonic setting and local variations in
heatflow.

Three general P/T types: Low, Intermediate, High

1. Low P/T facies series (ex. Ryoke Belt of Japan)
    • “Regional contact metamorphism”
    • Greenschist, Amphibolite, Granulite, Hornfels Facies
    • Characterized by andalusite ± sillimanite - Do not find kyanite

2. Intermediate P/T facies series (ex. Scottish Highlands)
    • Greenschist, Amphibolite, Granulite, Med-T Eclogite Facies
    • Same as normal “Barrovian” metamorphism
    • Sillimanite/kyanite common in pelitic rocks, andalusite rare

3. High P/T facies series (ex. Franciscan complex in California)
   • Zeolite, Prehnite-Pump, Blueschist, Low-T Eclogite Facies
   • Characterized by jadeite+lawsonite or glaucophane+lawsonite ±aragonite
      (blueshist)
   • Characterized by omphacite+garnet ± kyanite (eclogite)
   • Do not find andalusite at any grade.

   Glaucophane (amphibole) has wide stability field
   Lawsonite and jadeite are more typical for defining this facies:

   A) Lawsonite (CaAl2Si2O7 (OH2) H2O) - Albite Subfacies
   • Intermediate Na,Ca-plag + H2O >> Lawsonite (Ca) + Albite (Na)
   • Lower blueschist facies

   B) Lawsonite - Jadeite Subfacies
   • Albite goes to jadeite (pyroxene) + silica
   • NaAlSi3O8 >> NaAlSi2O6+SiO2
   • Anorthite component >> Lawsonite
   • Upper blueschist facies, next go to eclogite facies




                                         15

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Metamorphic petrology

  • 1. METAMORPHIC PETROLOGY METAMORPHISM: Process of mineralogical and structural (textural) changes of rocks in the solid state in response to physical and chemical conditions which differ from those under which they originated. 1. Mineral assemblages change. 2. Textural Changes (foliations, cleavage, etc). • Physical Conditions = Pressure & Temperature • Chemical Conditions = PH2O, PCO2, fluid solutions w/dissolved solids. ALL METAMORPHIC ROCKS WERE ONCE IGNEOUS OR SEDIMENTARY LOWER AND UPPER LIMITS OF METAMORPHISM: • Low grade: diagenesis >> incipient metamorphism: 100o-150oC • High grade: anatexis = formation of partial melt: 750o-850oC • Pressure variations: Top of Crust 1-3 km depth (<1 Kb) Base of Crust 20-40 km (>10 Kb) 1
  • 2. FACTORS OF METAMORPHISM: • Temperature: 100o-850o C • Confining Pressure or “Lithostatic Pressure” (synonymous terms) Equals weight of overlying rock column; Same pressure on all sides. • Directed Pressure: Related to deformation Sheering stresses; tectonic over pressure. • Fluid compositions: Metamorphic pore fluid H2O and CO2 . GOALS OF METAMORPHIC PETROLOGY: • SETTING: Where did metamorphism occur? What was its structural and tectonic setting? • PROTOLITH: Original nature of the rock before metamorphism. • GRADE of metamorphism: How intense was the metamorphism? What were the physical and chemical conditions of the metamorphism? a. look for relict structures, fossils b. look at mineral assemblage and compare to experimental studies in mineral equilibria; estimate temp and pressure. c. Calculate using mineral compositions, e.g., garnet-biotite geothermometer 2
  • 3. GEOBARIC GRADIENT = Pressure variations within Earth • Related to amount of crust or top: approximately 285 bars/km (upper crust) 20 km = 5700 bars = 5.7 kb 33 km = 10 kb 1 Kb 3.3 km Type of Crust Thickness Pressure at Base Example Normal 35-40 km 10-12 Kb Canada Continental Active Margin 60-80 km 18-25 Kb Sierra Nevada, Andes Collisional Orogen 80-100 km 24-30 Kb Himalayas Oceanic crust 6-10 km 2-3 Kb Atlantic ocean Typical Pressure Range For Common Metamorphic Rocks = 2-8 Kb GEOTHERMAL GRADIENT -- Controls Heat flow at Surface Two Major Aspects of Geothermal Gradient: • Conduction of heat from Mantle >> Limited effect in continents, Controlled by thickness of lithosphere (but note areas with thin lithosphere, e.g., Great Basin) • Radioactive decay of U/Th/K >> important for continental crust Type of Crust Geothermal Heat Flow at Example Gradient Surface Precambrian Shield 15-20oC/km 60 mW/m2-sec Canada Active Margin (Arc) 30o-35oC/km 100-120 mW/m2-sec Sierra Nevada, Andes Subduction Zone 10oC/km 40 mW/m2-sec Franciscan (Accretionary Complex Complex) Collisional Orogens 25o-30oC/km 80-110 mW/m2-sec Himalayas Extensional Orogens 40o-50oC/km 120-150 mW/m2-sec Great Basin Mid-Ocean Ridge up to 150-200 mW/m2-sec Mid-Atlantic Ridge 60oC/km 3
  • 4. PLATE TECTONIC CONTROLS ON METAMORPHISM • Ocean Floor metamorphism (mid-ocean ridges) = Very high T/low P. • Subduction zone metamorphism = High P/ Low T • Arc basement metamorphism = Low P / High T • Continent-Continent Collision = intermediate P/T • Continental Extension metamorphism = Intermediate P/high T DEFINITIONS OF METAMORPHIC ROCKS Prefixes: Ortho- Igneous Protolith Para- Sedimentary Protolith Examples: orthogneiss, paragneiss Slate, Argillite: Low grade metamorphic rocks with partly to well-developed cleavage; almost all are meta shales, metamudstones. Phyllonite: Higher grade than Slate, with incipient Foliation, shiny surface; almost all are meta shales, metamudstones. Schist: Higher grade than slate, phyllonite; characterized by distinct metamorphic foliation, abundant platy minerals, some segregation into layers common. Garnet-Biotite Pelitic schist rich in garnet, biotite, & quartz. Schist Gneiss: High grade metamorphic rock; varies widely in composition and mineral mode; characterized by distinct compositional banding in most cases. Quartzite: Metamorphosed quartzite or chert; very fine grained, sugary texture. Marble: Metamorphosed limestone or dolostone. Typically unfoliated, granular crystalline rocks. Migmatite: “Mixed Rock”, high grade gneiss in which felsic bands = granitic melt, mafic areas = melting residue, or host into which melts intruded. Granulite: Very high grade metamorphic rock; typically banded, foliated. Granofels: Like granulite, but lacking any preferred orientation, platy minerals. 4
  • 5. CLASSIFICATION OF METAMORPHISM BASED OF GEOLOGIC SETTING 1. Local Metamorphism: a. Contact Metamorphism [Adjacent igneous intrusions] b. Dynamic = fault zones 2. Regional = Dynamo-thermal metamorphism, effects large regions of Earth. CONTACT METAMORPHISM Factors that influence development of aureole • Temp of magma • Size of the intrusion (pluton, sill, dike) • Chemical activity of the fluids which migrate out of intrusion: important transfer mechanism of heat, chemicals. • Character of country rocks (chemical reactivity of wall rocks) some respond better to metamorphism pelitic schists, limestones. • Mode of emplacement (forceful emplacement vs magmatic stoping) • stoping develops static aureole, • forceful emplacement causes deformation. ROCKS IN CONTACT AUREOLES Hornfels = Massive, fine grained, sugary textured rocks, very tough; • textural term • Can have many protoliths (pelitic, felsic volcanic common) • No compositional meaning to hornfels • Can have large Xtls growing within • Not the exclusive rock at contact aureole Depends on character of emplacement: Forceful intrusion produces strongly foliated and lineated rocks, results in contact gneisses and schists. Contact Metamorphism = Generally Isochemical, i.e., same chemical signatures of the protolith 5
  • 6. SKARNS: Contact Metasomatism in Limestones Exception to isochemical contact metamorphism = skarn (tactite) • Develop along contact between granite and limestone or dolomite. • Coarse grained rocks • Consist of calc-silicate minerals (SiO2 added, CO2 lost) Typical calc-silicate minerals: Diopside CaMgSi2O6 Wollastonite CaSiO3 Garnet Ca3Al2Si3O12 Idocrase Don’t Ask Actinolite Ca2(MgFe)5Si8O22(OH)], Tremolite Ca2Mg5Si8O22(OH) Scheelite CaWO4 Quartz SiO2 Calcite Ca(CO2)3 Why do these chemical changes occur? METASOMATISM • Two rocks with grossly different compositions. • Chemical gradients set up and extensive chemical migration occurs. • Skarn zone develops at contact, not continuous. 6
  • 7. DYNAMIC METAMORPHISM: Metamorphism associated with fault zones. Low T, P: Incohesive rocks; very complex (gouge, breccia). High T, P: Very cohesive w/strong foliations (cataclasite, mylonite). Textural Classification of Fault Rocks: INCOHESIVE Fault Rocks • Fault breccias (visible fragments >30% of rock) • Gouge (visible fragments <30% of rock) COHESIVE • Cataclasite (Brittle): Protocataclasite> cataclasite > ultracataclasite • Mylonite (Ductile): Protomylonite > Mylonite > Ultramylonite Brittle-Ductile transition ≈ 250º-350ºC 7
  • 8. REGIONAL METAMORPHISM • Found in internal parts of orogenic belts • Strong fabrics • May have evidence of multiple deformation, heating events. • Metamorphic history related to Tectonic Evolution of orogen. George Barrow (1888) Scottish highlands • Documented progressive regional metamorphism. • Dalradion Series = late Precambrian to Cambrian age. • Diverse group of rocks = Active Compressive margin sequence: Impure litharenites (wackes) with mafic-felsic volcanic rocks, pelitic rocks (shales and mudstones); impure carbonates. • Thickness 10-15 km thick (some structural thickening). • Effected by Lower Paleozoic Caledonian Orogeny Barrow recognized metamorphism was progressive: rocks progress from low grade mineral assemblages to high grade mineral assemblages as temperature increases. Barrow Focused in on pelitic rocks and mapped using INDEX MINERALS. Index minerals for progressive metamorphism in Metapelitic Rocks: Chlorite >> Slate, Phyllite Biotite >> Phyllite, fine-gr Schist Garnet >> Schist, Gneiss Staurolite >> Schist, Gneiss Kyanite >> Schist, Gneiss Sillimanite >> Schist, Gneiss This is called the “Barrovian Sequence” C.E. Tilley referred to these lines as ISOGRADS “Same Grade” ex. biotite isograd = first appearance of biotite in meta-shale. • Isograds may cut across structures, bedding • Put tick marks put to the high-Temperature side • Area between Isograds called “Zones”, e.g., biotite zone, garnet zone, etc. 8
  • 9. Metamorphic Fabric and Texture FABRIC: Refers to orientation of the crystallographic lattices of the minerals TEXTURE: Refers to mineral distributions or orientations within the rock, overall structure of the rock. Cleavage Planar Fabric of Preferred Fracture, sub-parallel orientation, non-penetrative. Foliation: Any Penetrative set of more or less parallel SURFACES. Lineation: Any Penetrative set of more or less parallel LINES. Penetrative: Caused by re-orientation of the crystalline fabric of the minerals. Granoblastic: Mosaic texture of equidimensional, anhedral grains. Lepidoblastic: Abundant platy minerals (chlorite, biotite, etc) with strong preferred orientation. Causes Foliation. Nematoblastic: Abundant linear minerals (actinolite, hornblende, etc) with strong preferred orientation; Causes Lineation. Poikiloblastic: Large, metamorphic grains that enclose numerous small inclusions; analogous to poikolitic igneous texture. Porphyroblastic: Large metamorphic crystals in a matrix of smaller grains; analogous to porhyritic igneous texture. Porphyroclastic: Large relict grains of pre-existing mineral in finer-grained matrix of recrystallized material; associated with shear deformation and ductile flow. Mylonitic: Very fine-grained or aphanitic, anisotropic texture of produced by intense ductile flow and high strain. Flaser texture: Mylonitic fabric with ovoid megacrysts of relict crystals in vfg matrix. One type of porphyroclastic texture. 9
  • 10. DETERMINATION OF PROTOLITH Six Common Types: 1. Pelitic (shale, mudstone) 2. Quartzo-feldspathic (sandstone, rhyolite, granite, chert) 3. Calcareous (limestone, dolomite, marls) 4. Basic (basalt, andesite, gabbro, diorite) 5. Magnesian (peridotite, serpentine) 6. Ferruginous (ironstone, umbers) 1. Pelitic Protoliths = Rocks enriched in clay minerals • High Al2O3, K2O, lesser amounts Ca • Micas favored because of Al content • Also aluminosilicates: Al2SiO5 - sillimanite, andalusite, kyanite. Kyanite: Highest density (smallest volume) forms at higher pressures. Andalusite: Lowest density, largest volume, forms at low pressures. Sillimanite: Intermediate density, volume; forms at moderate T, P. Alumino-silicate triple point = 5.5 kb at 600oC Wet granite solidus: Shows where anatexis occurs in sillimanite zone. Staurolite (2*Al2O5*Fe(OH)2) = Common metamorphic mineral Need an Al and Fe-rich protolith -- This restricts occurrence 10
  • 11. 2. Quartzo-feldspathic Protoliths: High SiO2, low Fe and Mg • “Psammitic” • Quartz-rich sandstones with varying % feldspars (“arkose”) • Felsic igneous rocks (rhyolites, tuffs, granites) • If protolith >50% quartz then probably a sandstone or chert. • Gneiss: Fine-grained at low grade, coarser with increasing grade. Felsic tuffs, granite hard to tell from arkose when highly metamorphosed. 3. Calcareous Protoliths: High CaO, CO2 • Limestones and dolomite form MARBLES • Impure limestones (with clay, silt) form Calc-silicates: [tremolite, diopside, wollastonite, forsterite, epidote, et cetera] 4. Basic Protoliths: Low SiO2 moderate CaO, MgO, FeO • Basalts, andesites, gabbros - mafic igneous rocks. • Some shale-limestone mixtures. • Minerals depend on grade: chlorite, actinolite, hornblende, plagioclase, epidote, garnet. 5. Magnesian Protoliths: Very low SiO2, high MgO • Peridotites >> serpentine, magnesite. • Serpentine (low T) >> antigorite (high T serpentine), olivine. 6. Ferruginous Protoliths: High Fe2O3 • Ironstones = Precambrian iron formations (Fe-rich cherts). • Umbers = Fe-rich cherts, shales associated with MOR. 11
  • 12. METAMORPHIC FACIES Metamorphic Facies: All the rocks that have reached chemical equilibrium under a particular set of physical conditions. • Facies concept developed by Eskola (Norway, 1915) to compare metamorphic rocks from different areas. • Look at several protoliths to determine facies. • Facies represent specific temperature - pressure regimes. • Named for Equivalent Mafic rock type at those conditions. Facies of regional Metamorphism: 1) Greenschist Facies (includes chlorite/biotite zones of Barrow). 2) Epidote Amphibolite facies (garnet zone). 3) Amphibolite Facies (staurolite/kyanite/sillimanite zones) 4) Granulite facies (not found in Scottish highlands) 5) Eclogite facies (not found in Scottish highlands) • Names indicate specific T and P conditions and not textures or minerals ex. Amphibolite Facies includes calcite marbles, biotite schists and amphibolites. Granulite: muscovite + quartz ó sillimanite + Kspar “2nd Sillimanite Ispgrad” Quartz + biotite ó Kspar + hypersthene Eskola’s Facies for Contact Metamorphism: 1) Hornblende hornfels facies 2) Pyroxene hornfels facies • Do not have to be hornfels -- could be a schist. • Use these facies if clearly associated with a pluton. • Hornblende hornfels similar to amphibolite facies. • Pyroxene hornfels similar to granulite. 12
  • 13. NEW Metamorphic Facies -- Since Eskola 6. Zeolite Facies: Incipient metamorphism at low T and P • Zeolites = Hydrated feldspars (mostly calcic). • Temperatures 100-200oC. • Index minerals = Laumonite, thompsonite, other zeolites, calcite, interlayered smectite/chlorite. • Metabasalts: Veins and amygdule fillings • Sandstones: Veins, interstitial pore space fillings. 7. Prehnite-Pumpellyite Facies (Sub-greenschist, low T, P) • Index minerals = Prehnite, pumpellyite, calcite, chlorite, albite • Temperatures = 150-250oC • Metabasalts: Veins, amygdules, replacement of primary plagioclase, olivine, glass. • Sandstones: Veins, replacement of clastic feldspars. 8. Blueschist Facies: High P at low Temperatures • Index minerals = Glaucophane, albite, jadeite, lawsonite, aragonite. • Temperatures = 250-350oC. • Pressures > 6-8 kb. • Metabasalts: Glaucophane, albite, lawsonite, sphene • Sandstones, shales: Jadeite, albite, quartz, lawsonite, aragonite, paragonite (Na white mica) 13
  • 14. FACIES – PROTOLITH—MINERAL ASSEMBLAGE TABLE: Facies Shale, Sandstone Limestone Basalt, Andesite Zeolite interlayered calcite Laumonite, smectite/chlorite thompsonite, calcite, 100-200° C calcite interlayered smectite/chlorite Prehnite- Prehnite, calcite Prehnite, Pumpellyite pumpellyite, calcite, pumpellyite, calcite, chlorite, albite chlorite, albite 150-300° C Greenschist muscovite, chlorite, calcite, dolomite, albite, chlorite, quartz, albite, biotite, quartz, epidote, quartz, epidote, 300-450° C garnet tremolite actinolite, sphene Epidote muscovite, biotite, calcite, quartz, albite, epidote, Amphibolite garnet, albite, quartz tremolite, hornblende, quartz epidote,diopside 450-550° C Amphibolite garnet, biotite, calcite, diopside hornblende, muscovite, quartz, quartz, plagioclase, garnet, plagioclase, wollastonite quartz, sphene, 500-700° C staurolite, kyanite or biotite sillimanite Granulite garnet, Kspar, calcite, quartz, plagioclase, augite, sillimanite or plagioclase, hypersthene, kyanite, quartz, diopside, hornblende, garnet, 700-900° C plagioclase, hypersthene olivine hypersthene Blueschist Jadeite, albite, aragonite, white Glaucophane, albite, quartz, lawsonite, mica lawsonite, sphene, ± 150-350° C aragonite, paragonite garnet P > 5-8 Kb Eclogite coesite, Kspar, aragonite, quartz, omphacite (px), sillimanite, plagioclase, pyrope garnet 350-750° C plagioclase diopside, P > 8-10 Kb hypersthene 14
  • 15. METAMORPHIC FACIES SERIES Regional metamorphism: temperature/pressure vary along distinct P-T paths (geothermal gradients) depending on tectonic setting and local variations in heatflow. Three general P/T types: Low, Intermediate, High 1. Low P/T facies series (ex. Ryoke Belt of Japan) • “Regional contact metamorphism” • Greenschist, Amphibolite, Granulite, Hornfels Facies • Characterized by andalusite ± sillimanite - Do not find kyanite 2. Intermediate P/T facies series (ex. Scottish Highlands) • Greenschist, Amphibolite, Granulite, Med-T Eclogite Facies • Same as normal “Barrovian” metamorphism • Sillimanite/kyanite common in pelitic rocks, andalusite rare 3. High P/T facies series (ex. Franciscan complex in California) • Zeolite, Prehnite-Pump, Blueschist, Low-T Eclogite Facies • Characterized by jadeite+lawsonite or glaucophane+lawsonite ±aragonite (blueshist) • Characterized by omphacite+garnet ± kyanite (eclogite) • Do not find andalusite at any grade. Glaucophane (amphibole) has wide stability field Lawsonite and jadeite are more typical for defining this facies: A) Lawsonite (CaAl2Si2O7 (OH2) H2O) - Albite Subfacies • Intermediate Na,Ca-plag + H2O >> Lawsonite (Ca) + Albite (Na) • Lower blueschist facies B) Lawsonite - Jadeite Subfacies • Albite goes to jadeite (pyroxene) + silica • NaAlSi3O8 >> NaAlSi2O6+SiO2 • Anorthite component >> Lawsonite • Upper blueschist facies, next go to eclogite facies 15