GeoLOGY Lecture Outline

Metamorphic Rocks (Ch 8)

I. Lecture Content

Introduction

Overview of Metamorphism

Metamorphism and the Rock Cycle

Agents of Metamorphism

Conditions and Types of Metamorphism

Classification of Metamorphic Rocks

Metamorphic Zones and Facies

Metamorphism and Plate Tectonics

Metamorphism and Natural Resources

Vocabulary Terms

II. Introduction

A.   The connection between metamorphism, plate tectonics and

mountain building.

        Metamorphism is very active at plate boundaries

        Metamorphism is especially intense at convergent plate

boundaries (magmatism + crustal thickening)

 

B.   The importance of metamorphism

        Records orogenic events (preserved in the rock record)

        Creates mineral resources

        Turns "soft" rocks into "hard" rocks

III. Overview of Metamorphism

A.  Definitions

1.   Whats a metamorphism? Defined

2.   Whats a metamorphic rock? Defined

B. Principle Agents of Change of Metamorphism

1. Heat

2. Lithostatic Pressure

3. Active Pore Fluids

4. Deviatoric Stress

C. Types of Metamorphism - P/T Dependent

1. Regional Metamorphism (RM) = Deep burial + Stresses

2. Contact Metamorphism (CM) = Contact with magma

3. Dynamic Metamorphism (DM) = Faulting and Shearing

 

IV. Metamorphism - Part of the The Rock Cycle

A.  Metamorphic Conditions

1) Pressure/Temperature/Fluid- Dependent Processes

        The "Zone" of metamorphism is between that of igneous

and sedimentary rock conditions.

    Between diagenesis (sedimentary) and anatexis (igneous).

 

        Metamorphism encompasses a wide variety of

subsurface conditions (variable combinations

of pressure, temperature, and fluid activity).

 

        A Precursor to formation of magmas (partial melting)

2) Elevating P/T Conditions - How do you get there?

        Deep burial = Regional

        Close encounter with magma = Contact

        Tectonic stresses = Dynamic

 

B. Metamorphic Processes

1) Recrystallization

        Preexisting mineral crystals are "re"-formed.

        Mineralogy remains the same.

        Produces noticeable changes in rock texture.

 

2) Neocrystallization

        Brand new mineral(s) (crystals) are formed.

        Mineralogy changes.

        Produces noticeable changes in rock texture.

 

3) Changes to a rock's texture and mineralogy

        Change in texture - Ex. 1 = limestone to marble

Ex. 2 = granite to gneiss

        Change in mineralogy - Ex = basalt to amphibolite

Ex. = mudstone = garnet schist

 

4) The new texture and/or mineralogy of any rock that has

underwent metamorphism is a result of that rock's

attempt at establishing physical and chemical

equilibrium with its the new metamorphic conditions.

 

C. Metamorphism and Mountain Building

1) Plate dynamics at convergent boundaries produce

a variety of unique conditions of elevated pressures,

differential stresses, and increased heat input.

2) Rocks can be subjected to a series of progressive

metamorphic conditions through time in regions of

active mountain building.

       Long-term, ever-deeper burial

       Proximity to magmatic intrusions

       Long-term, Vice-like differential stresses

 

D. Internet Resource on Metamorphism

1) http://www.geo.wvu.edu/~lang/Geol285/Pet18MetIntro/index.htm

V. Formation of Metamorphic Rocks -

A.  Potential Source Regions (rocks) for Metamorphism

1) Mantle

2) Oceanic crust

       Spreading centers

3) Continental crust

       Base of deep depositional basins

       Cores of active mountain belts

       Magmatic arcs

       Subduction Zones

 

B.  Regions of Metamorphism - Physical and Chemical

Conditions of Metamorphism

       Source rock composition

       Temperature (Heat)

       Pressure (Depth of burial)

       Regional (tectonic) Stresses

       Fluid Activity (Amount of H2O in rock is IMPORTANT)


VI. Major Types of Metamorphism

A. Regional Metamorphism (RM)

1) Characteristic of large regions of crust, called "orogenic

belts" that undergo deep burial (elevated P+T) and

deviatoric stresses over long periods of time.

 

2) Major affect is three-fold, with progressive increases in

lithostatic and vice-like pressures and temperature.

 

3) The zone of metamorphism is truly regional in extent,

with dimensions as large as 100's km long, 10's km

wide, and km's deep.

 

4) The type of metamorphic mineral(s) that form in the

country rock depends on the following factors:

        Composition of the country rock

        Temperature

        Lithostatic pressure

        Orientation and strength of deviatoric stresses

        Length of time

 

5) The fabric (texture) of the resultant metamorphic rock

also depends on the above factors listed in #4.

        Distinctive foliation (slaty or schistose) and/or layered (gneissic)fabrics.

        Lots of platy or elongate minerals will accentuate foliated fabrics, like mica.

        Lack of platy minerals may produce rock with

no apparent foliation, e.g. quartzite or marble.

 

B. Contact Metamorphism (CM)

1) Characteristic in active magmatic belts where hot

magmas and hydrothermal fluids exist.

 

2) Major affect is a great increase in the temperature of the

country rocks adjacent to the magma intrusion.

 

3) A metamorphic aureole forms in country rock directly

adjacent to the intrusion.

 

4) The type of metamorphic minerals that form in the

country rock aureole depends on the following:

       Composition of the country rock

       Temperature of the intrusion

       Size of the intrusion

       The type/quantity of fluids in the aureole

 

5) The resultant metamorphic rocks are called hornfels.

 

6) A very active metal/sulfide ion-rich hydrothermal system

may form in some aureoles, which can be the site of

rich, metal sulfide ore formation.

       Called a Porphyry system

 

7) Metal-rich fluids curculating in carbonate-rich aureoles

can be another site of metal ore formation.

       Called Skarn deposits

C. Dynamic Metamorphism (DM)

1) Characteristic of active fault and shear zones.

2) Major affect is a great increase in deviatoric stresses of

on the country rock within or adjacent to the zone of

faulting or shearing.

 

3) The characteristic processes occurring to the rock are:

       Milling (mechanical) action

       Recrystallization

       Reorientation of minerals = foliated fabric

 

4) Mylonite is the resultant metamorphic rock

       Unique to this type of metamorphism

VII. Classification of Metamorphic Rocks

A. Primarily based mainly upon two criteria:

1) Texture -- Crystal Size and Shape; Fabric

2) Composition -- Mineralogy

B. Texture is further subdivided into two types:

1) Foliated and/or Layered Fabrics

2) Non-foliated and/or Massive

C. Foliated and Layered Fabrics

1) Foliated fabrics are defined by the preferred

orientation of platy minerals like mica.

       A flaky croissant is a good example

2) The four types of foliation is mainly a function of grain size.

       Slaty cleavage = fine-grained, tight, parallel, flat

       Phylittic - fine-medium grained, tight, but slightly wavy; sheen-like luster

       Schistose - medium-grain; wider-spaced, more wavy foliation planes

       Gneissic - medium to coarse-grained; layered fabrics are defined by compositional

layering of light- and dark-colored minerals.

D. The Major Metamorphic Rock Types -

1) Foliated and/or Layered Rocks

        Slate very fine-grained; foliated; RM

        Phyllite -- fine-grained; foliated: RM

        Schist -- medium-grained; foliated; RM

        Gneiss -- med- to coarse-grained; layered; RM

        Amphibolite - fine-med-grained; foliated; RM

        Mylonite - fine to med-grained; foliated; DM

        Migmatite - med-grained; mixed up-looking:

RM and CM.

2) Non-Foliated/Massive

        Marble -- fine- to coarse-grained; massive; RM and CM

        Quartzite - fine- to med-grained; massive; RM and CM

        MetaConglomerate - mix-grained; massive; RM and CM

        Hornfels -- fine-grained; massive; CM

        Greenstone -- fine to med-grained; variable; CM

        Skarn -- fine to coarse-grained; massive; CM

E. Compositional Classification

1) Marble - carbonate mineralogy

2) Quartzite - quartz mineralogy

3) Amphibolite - amphibole mineralogy

4) Granite gneiss - granite mineralogy

5) Garnet-Biotite schist - Dominant minerals

5) Skarn - (Ca, Mg)-silicate mineralogy

 

F. Classification Chart of Common Metamorphic Rocks

(See Table 7.1; pg. 200)

 

G. Identifying Metamorphic Rocks in Hand Samples

1) Observe the rock fabric - Foliated?, Layered?; Or No?

2) Observe the rocks grain size - Fine, medium, or coarse?

3) Observe mineralogy - ID minerals? Or No?

 

VIII. Metamorphic Zone and Facies Concepts

A. Progressive Metamorphism

 

1) What is Progressive Metamorphism?

Defined: In regional metamorphism, the systematic metamorphic

evolution and equilibration of a package of rocks through ever-

increasing pressure-temperature conditions, where a rock

package's lower grade equivalents, e.g. slate systematically grade

to it's highest grade equivalents, e.g. gneiss.

 

2) Progressive metamorphism is exclusively a product of

regional metamorphism (the type of metamorphism

where conditions go through a broad elevation of both,

temperature and pressure).

 

B. Metamorphic Zones

1) What are Metamorphic Zones? Defined: In isochemical rock

systems undergoing progressive metamorphism, it is common

for the systematic appearance (neocystallization) of new minerals

with each systematic upgrade in P/T conditions. These new

metamorphic minerals are termed metamorphic index minerals

and they define metamorphic zones.

 

        See Figure 7.8, Index Minerals (page 199)

 

2) Metamorphic zones are actual rock assemblages.

 

3) The spatial association of Metamorphic Zones in the

field can be interpreted in terms of variations in peak

metamorphic conditions.

 

4) A Metamorphic Zone Illustration (Fig. 7.18; pg. 206)

 

5) What are metamorphic zones used for?

 

C. Metamorphic Facies

1) What is a Metamorphic Facies? Defined: The P-T

conditions that produce a characteristic rock type,

that has a unique, characteristic mineral assemblage

for a given parent rock.

        See Fig. 7.19; Metamorphic Facies (pg. 206)

 

2) A Metamorphic facies is not a rock, but instead, is a

specific range of Pressure/Temperature conditions.

 

3) There are a number of established metamorphic facies within

all the paired P/T conditions found in nature.

 

4) A Metamorphic Facies Diagram illustrates the different

facies (colored fields) in relation to Pressure/Temperature.

(see Figure 7.19; Metamorphic Facies (page 206)

 

5) What are metamorphic facies diagrams used for?

 

6) A Metamorphic Facies Series Diagram illustrates the

progressive P/T paths (black arrows) of each type of metamorphic facies series.

 

7) What are metamorphic facies series diagrams used for?

 

IX. Metamorphism and Plate Tectonics

A. Specific Tectonic Settings are Conducive to Specific

Types of Metamorphism

1) Divergent boundaries = Contact and Dynamic

 

2) Convergent boundaries = Regional, Contact, and

Dynamic

 

3) Mainly Dynamic

 

B. Specific Tectonic Settings are Conducive to Specific

Metamorphism Facies Series (MFS)

1) Divergent and Convergent = Contact MFS

2) Continental-Convergence = Buchan and Barrovian MFS

3) Island Arc Convergence = Sanbagawa MFS

4) Subduction Zone Accretion = Blueschist MFS

X. Metamorphism and Mineral Resources

A. Regional Metamorphism

1) Marble

2) Slate

3) Asbestos

 

B. Contact Metamorphism - Porphyry and Skarn Systems

1) Metal Sulfides

Copper

Lead

Zinc

 

2) Metal Oxides

Iron

Tin

Tungsten

XI. Concluding Thoughts

A. Metamorphic rocks form in conditions between those

where sedimentary and igneous rocks form.

1) Higher P+T then sedimentary rocks (diagenesis)

2) Lower P+T then igneous rocks (melting point)

B. Metamorphic Rocks and Processes complete the

third leg of the Earth's Rock Cycle.

1) Transforms sedimentary and other rocks.

2) Sets up host rock for partial melting.

 

C. Metamorphism is a principle process in the Building

of Mountains and the Crustal Evolution of Continents.

 

XII. Vocabulary Terms

aureole

contact metamorphism

differential pressure/ deviatoric stress

dynamic metamorphism

foliated texture

gneiss

gneissic texture

hornfels

index minerals

lithostatic pressure

marble

metamorphic facies

metamorphic facies series

metamorphic rock

metamorphic zone

metamorphism

mylonite

nonfoliated/ massive texture

porphyry ore system

quartzite

regional metamorphism

schist

schistose

slate

slaty cleavage