Introduction to Metamorphic Rocks

Metamorphism is the solid-state transformation of pre-existing rock into texturally or mineralogically distinct new rock as the result of high temperature, high pressure, or both.

The Concept of Lithostatic Pressure

Differential Pressure

Which one is showing a banned structure?

The Concept of Geothermal Gradient

Temperature increases with depth at a rate of 20-30oC/km in the crust.

Deep

Temperature

http://www.columbia.edu/itc/ldeo/v1011x-1/jcm/Topic3/Image1aa.gif

Moho

Foliation is any penetrative planar fabric present in rocks. Foliation is common to rocks affected by regional metamorphic

compression typical of orogenic belts. Rocks exhibiting foliation include the typical sequence formed by the prograde metamorphism of mudrocks slate, phyllite, schist and gneiss.

Foliation Minerals are

Squished into

bands.

Regional Metamorphism:Large-scale deformation associated with T and P

3 Types of Metamorphism

3 Types of Metamorphism

Contact Metamorphism:The main metamorphic agent is heat not pressure.

Which rock can be found with contact metamorphic rock?

Dynamic metamorphism:Rock deformation associated with fault zones

3 Types of Metamorphism

Mylonite from shear zone in Appalachians

Metamorphic Facies and Grade

Metamorphic facies: a set of metamorphic rocks characterized byparticular mineral associations indicative of environment.

Spatial Distribution of Metamorphic Facies

Exercises –P64

Sandstone: Quartzite, Metaquartzite

Shale: Phyllite

Slate

Schist

Gneiss

Limestone: Marble

Metamorphism of Sedimentary Rocks

Protolith (Precursor)

Progression of metamorphism

Slate

Phyllite Schist

Start with a shale and then hitit with pressure and heat.

Figure 22.1. Examples of foliated metamorphic rocks. a. Slate. b. Phyllite. Note the difference in reflectance on the foliation surfaces between a and b: phyllite is characterized by a satiny sheen. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

a: a: SlateSlate

b: b: PhyllitePhyllite

Slate: compact, very fine-grained, metamorphic rock with a well-developed cleavage. Freshly cleaved surfaces are dull

Phyllite: a rock with a schistosity in which very fine phyllosilicates (sericite/phengite and/or chlorite), although rarely coarse enough to see unaided, impart a silky sheen to the foliation surface. Phyllites with both a foliation and lineation are very common.

Foliated Metamorphic Rocks

Figure 22.1c. Garnet muscovite schist. Muscovite crystals are visible and silvery, garnets occur as large dark porphyroblasts. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Schist: a metamorphic rock exhibiting a schistosity. By this definition schist is a broad term, and slates and phyllites are also types of schists. In common usage, schists are restricted to those metamorphic rocks in which the foliated minerals are coarse enough to see easily in hand specimen.

Foliated Metamorphic Rocks

Figure 22.1d. Quartzo-feldspathic gneiss with obvious layering. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Gneiss: a metamorphic rock displaying gneissose structure. Gneisses are typically layered (also called banded), generally with alternating felsic and darker mineral layers. Gneisses may also be lineated, but must also show segregations of felsic-mineral-rich and dark-mineral-rich concentrations.

Foliated Metamorphic Rocks

You end up with something that is really Gneiss!

Banned structure

The foliation in this rock is acrenulation cleavage, and is developed after the horizonalfoliation.

A foliation is any planar fabric in ametamorphic rock. In this case, thefoliation is defined by aligned sheetsof muscovite sandwiched betweenquartz grains.

This slide is indicativeof a phyllite.

This slide is indicativeof a schist.

Specific Metamorphic Rock Types

Marble: a metamorphic rock composed predominantly of calcite or dolomite. The protolith is typically limestone or dolostone.

Quartzite: a metamorphic rock composed predominantly of quartz. The protolith is typically sandstone. Some confusion may result from the use of this term in sedimentary petrology for a pure quartz sandstone.

Which one is marble/limestone?

Which one is Quartzite/Sandstone?

Metamorphism of Igneous Rocks

For most of our purposes, just put “meta” in frontof the protolith name.

Examples: metabasaltmetarhyolite

If a mafic or intermediate metamorphic rock isdominated by amphibole and feldspars:

-Amphibolite.

Summary of metamorphic rocks

Metamorphic grade VS metamorphic rocks

Composition and Temp

Pressure and Temp

How does Metamorphism Relate to Plate Tectonics?

T and P increase at collisional (convergent) plate boundaries.

In regional metamorphic terranes, the temperature and pressure regime is indicated by the distribution of metamorphic minerals across a large area.

Low metamorphic grade (low T and P; <200oC):

Slate and phyllite: chlorite, muscovite, biotite

Intermediate metamorphic grade

Schist: garnet, staurolite, kyanite

High metamorphic grade - 800 degrees C (verging on melting)

Gneiss: kyanite, sillimanite

Highest metamorphic grade: Partial melting

Migmatite: rock partially melts, but no mass loss.

Hence the mineralogy tells us the metamorphic grade of the rock.

Index minerals for Regional Metamorphism

Exercise –P67

Homework for Extra credit

1 Strike and dip model2 True and apparent dip model•Apparent dip model3 point problem4 V-rule model5 Unconformity model6-7 Fault types Normal fault model Reverse fault model Strike slip fault model Oblique normal fault model Oblique reverse fault model

8-9 Faults displacing inclined bedsNormal faults and relay rampsCross-cutting normal faultsPlunging foldsFault-propagation and fault-bend fold

Link: http://www.fault-analysis-group.ucd.ie/