Op ch08 lecture_earth3, metamorphic minerals

Chapter 8
Metamorphism: A Process of Change

LECTURE OUTLINE

earth

Portrait of a Planet

Third Edition
©2008 W. W. Norton & Company, Inc.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak

Chapter 8: Metamorphism: A Process of Change


Prepared by

Ron Parker
Earlham College Department of Geosciences
Richmond, Indiana

Introduction


Metamorphic – Changed from an original “parent.”
Meta = Change.
 Morph = Form or shape.


Parent rocks are called “protoliths.”
 Metamorphism can
occur to any protolith.




Introduction


Protoliths undergo pronounced changes in…
Texture.
 Mineralogy.




Due to change in physical or chemical conditions.
Burial.
 Tectonic stresses.
 Heating by magma.
 Fluid alteration.




Metamorphism
Metamorphism occurs in the solid state.
 It doesn’t include weathering, diagenesis, melting.
 Metamorphics often look totally unlike protoliths.




Metamorphic Character


Metamorphic rocks have distinctive properties.
Unique texture – Intergrown and interlocking grains.
 Unique minerals – Some that are only metamorphic.


Staurolite, Kyanite, Sillimanite, etc.




Unique foliation – A planar fabric from aligned minerals.

These transformations can change the rock utterly.
Fossiliferous limestone
Fossiliferous limestone

Red mudstone
Red mudstone

Garnet gneiss
Garnet gneiss


Marble
Marble


Metamorphic Processes
Metamorphic change is slow and in the solid state.
 Several processes are at work.




Recrystallization – Minerals change size and shape.



Phase change – New minerals form with…
Same chemical formula.
Different crystal structure.


Kyanite




Several processes may operate at the same time.


Neocrystallization – New minerals with P-T changes.
Initial minerals become unstable; change to new minerals.

Decomposition of the original minerals in the protolith.
Chemical reaction of the elements to form new minerals.
In this way, a shale can transform into a garnet mica schist.





Several processes may operate at the same time.


Pressure solution – Mineral grains partially dissolve.



Plastic deformation – Mineral grains soften and deform.



Causes of Metamorphism


The agents of metamorphism are…
Heat (Temperature – T).
 Pressure (P).
 Differential stress.
 Hydrothermal fluids.


Not all agents are required; they often do co-occur.
 Rocks may be overprinted by multiple events.




Heat (Temperature)


Metamorphism occurs as the result of heat.


Temperature (T) ranges between 200oC and 850oC.

The upper T limit is…melting. It varies based upon
rock mineral composition and water content.
 Heat energy breaks and reforms atomic bonds.
 Sources of heat.


The geothermal gradient.
 Magmatic intrusions.
 Compression.




Pressure (P)


P increases with depth in the crust.
270 to 300 bars per km (1 bar is almost 1 atm = 14.7 psi).
 Metamorphism occurs mostly in 2 to 12 kbar range.


T and P both change with depth.
 Mineral stability is highly dependent upon T and P.


This stability can be graphed on a “phase diagram.”
 Changes in T and P lead
to changes in minerals.




Differential Stress
Pressure that is greater in one orientation.
 A common place result of tectonic forces.
 Two kinds of differential stress: Normal and shear.




Normal stress – Operates perpendicular to a surface.
Tension – Pull-apart normal stress.
Compression – Push-together normal stress.



Differential Stress


Two kinds of differential stress: Normal and shear.


Shear stress – Operates sideways across a surface.
Causes material to be “smeared out.”



Differential Stress


At higher T and P, differential stress deforms rock.


Rocks change shape slowly without breaking.



Differential Stress


Deformation acts on minerals with specific shapes.
Equant – Roughly equal in all dimensions.
 Inequant – Dimensions not the same.


Platy (pancake-like) – 1 dimension shorter.
Elongate (cigar-shaped) – 1 dimension longer.

Differential stress causes these minerals to align.
 Alignment fabric records stress trajectory.




Differential Stress


Preferred platy mineral alignment is called foliation.
Foliation imparts a layered or banded appearance.
 Rocks commonly break parallel to foliation planes.




Foliation develops perpendicular to compression.




Minerals flatten, recrystallize, and rotate.

Inequant grains align by rotation and new growth.



Hydrothermal Fluids
Hot water with dissolved ions and volatiles.
 Hydrothermal fluids facilitate metamorphism.


Accelerate chemical reactions.
 Alter rocks by adding or subtracting elements.




Hydrothermal alteration is called metasomatism.



Metamorphic Rock Types


Two major subdivisions of metamorphic rocks.


Foliated – Has a through-going planar fabric.
Subjected to differential stress.
Has a significant component of platy minerals.
Classified by composition, grain size, and foliation type.



Metamorphic Rocks


Non-foliated rocks lack a planar fabric.


Absence of foliation possible for several reasons.
Rock not subjected to differential stress.
Dominance of equant minerals.
Absence of platy minerals.



Metamorphic Rocks


Quartzite – Almost pure quartz in composition.
Forms by alteration of quartz sandstone.
 Sand grains in the protolith recrystallize and fuse.
 Like quartz, it is hard, glassy, and resistant.


Metamorphic Alteration



Metamorphic Rocks


Marble - Coarsely crystalline calcite or dolomite.
Forms from a limestone or dolostone protolith.
 Extensive recrystallization completely changes the rock.
 Original textures and fossils in the parent are obliterated.
 Used as a decorative and monument stone.
 Exhibits a variety of colors.


Metamorphic Alteration



Metamorphic Rocks


Type depends on protolith.
Minerals contribute elements.
 Some protoliths yield specific rocks.




Broad compositional classes.
Pelitic.
 Basic (or Mafic).
 Calcareous.
 Quartzo-feldspathic.




Metamorphic Classes


Pelitic – Shale protoliths.
Al-rich clay minerals yield micas.
 Rock type depends on grade.


Slate.
Phyllite.
Schist.
Gneiss.



Metamorphic Classes


Mafic – Basalt or gabbro protolith.
Rich in Fe and Mg and poor in Si, Al, Na, and K.
 Turn into biotite- and amphibole-dominated rocks.




Metamorphic Classes


Calcareous – Carbonate protolith.
Rich in Ca (limestones) and Ca and Mg (dolostones).
 Recrystallize into calcite and dolomite marbles.




Metamorphic Grade


Example: Pelitic rock.


Low grade – Shale protolith.
Clays recrystallize into larger, aligned clays to yield a slate.
Clays neocrystallize into tiny, aligned micas in a phyllite.



Intermediate grade –
Micas recrystallize and grow large to form a schist.
New minerals grow in the schist.



High grade –
Micas decompose; elements recombine into new minerals.
Neocrystallization yields quartz and feldspars in a gneiss.



Metamorphic Environments


The types (and settings) of metamorphism are...
Thermal – Heating by a plutonic intrusion.
 Burial – Increases in P and T by deep burial in a basin.
 Dynamic – Shearing in a fault zone.
 Regional – P and T alteration due to orogenesis.
 Hydrothermal – Alteration by hot water leaching.
 Subduction – High P–Low T alteration.
 Shock – Extreme high P attending a bolide impact.
 Mantle – Extreme high mantle P causes phase changes.




Contact Metamorphism
Due to heat from magma invading host rock.
 Creates zoned bands of alteration in host rock.


Called a contact (or metamorphic) aureole.
 The aureole surrounds the plutonic intrusion.


Zoned from high (near pluton) to low grade (far from pluton).



Burial Metamorphism


As sediments are buried in a sedimentary basin…
P increases because of the weight of the overburden.
 T increases because of the geothermal gradient.




Requires burial below diagenetic effects.


This is ~ 8–15 km depending on the geothermal gradient.



Regional Metamorphism
Tectonic collisions deform huge “mobile belts.”
 Directed compression thickens mountains.




Rocks caught up in mountain building are…
Heated via the geothermal gradient and plutonic intrusions.
Squeezed and heated by deep burial.
Smashed and sheared by differential stresses.



Regional Metamorphism
Regional metamorphism creates foliated rocks.
 This type of metamorphism is, by far, the most
important in terms of the amount of rock altered.




Collisional belts are often…
1000s of km long.
100s of km wide.



Hydrothermal Metamorphism
Alteration by hot, chemically aggressive water.
 A dominant process near mid-ocean ridge magma.


Cold ocean water seeps into fractured crust.
 Heated by magma, this water then reacts with mafic rock.
 The hot water rises and is ejected via black smokers.




Subduction Metamorphism


Trenches and accretionary prisms have…
A low geothermal gradient – low temperature.
 High pressures.




High P – Low T favor
glaucophane, a blue
amphibole mineral.



This concludes the
Chapter 8

LECTURE OUTLINE

earth

Portrait of a Planet

Third Edition
©2008 W. W. Norton & Company, Inc.


Op ch08 lecture_earth3, metamorphic minerals

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