Metamorphic Rocks

Metamorphic rocks have undergone changes in mineralogy, texture and/or chemical composition as a result of changes in temperature and pressure

Metamorphic Reactions

ï Many reactions occur in the presence of fluids

ï Original rock may have been:

ñ Igneous

ñ Sedimentary

ñ Another metamorphic rock

Driving Forces of Metamorphism

ï The pressure and heat that drive metamorphism are a consequence of three forces:

ñ Internal heat of the Earth

ñ Weight of the overlying rock

ñ Horizontal or tectonic forces that cause the rock to deform

Contact Metamorphism

ï Occurs at high temperatures

ï Restricted to a small area around the margins of a magmatic intrusion

ï Geothermal gradients are high

Hydrothermal Metamorphism

ï Occurs along mid-ocean ridges where heated seawater percolates through hot, fractured basalt

ï Chemical reactions between heated seawater and basalt

ï Crustal rocks on continents can react with invading, hot fluids associated with igneous intrusions

Burial Metamorphism

ï Sedimentary rocks that had undergone diagenesis are buried even deeper

ï A relatively mild type of metamorphism

ï Diagenesis grades into burial metamorphism

ï Results in partial alteration of mineralogy and texture

ï Original bedding and sedimentary structures usually preserved

Regional Metamorphism

ï When temperatures & pressures increase beyond range of burial metamorphism

ï Takes place under high temperatures & pressures over large areas

ï Intense alteration of rock mineralogy and texture

ï Original sedimentary structures destroyed

ï Caused by major tectonic forces

Cataclastic Metamorphism

ï Results from the crushing and shearing of rock during tectonic movement

ï Generally localized along fault planes

ï Produces sheared, highly deformed rocks called mylonites

Metamorphic Facies

ï Describes the grouping of rocks formed under different temperatures and pressures

ï Encompass different regions of P-T space

Facies are named after certain characteristic minerals usually formed through the metamorphism of basalt

Zeolite Facies

Greenschist Facies

Amphibolite Facies

Granulite Facies

Blueschist Facies

Eclogite Facies

Hornfel Facies

Retrograde Metamorphism

Serpentine Minerals Products of Retrograde Metamorphism of Ultramafic Rocks

Common Minerals in Metamorphic Rocks

ï Quartz

ï Feldspar

ï Chlorite

ï Mica

ï Staurolite

ï Garnet

ï Pyroxene

ï Kyanite

ï Andalusite

ï Sillimanite

Foliation

ï Metamorphism of shale and clay-rich sandstone (greywacke) produces flat or wavy parallel planes called foliation

ï Produced in rocks containing platy minerals:

ñ Micas

ñ Chlorite

Foliation Can Cut at an Angle to the Original Bedding or Be Coincident With Bedding

Foliated Rocks Like Slate Split Along Foliation Planes In What Is Known As Slaty Cleavage

Metamorphism of Shale & Greywacke

ï Slate

ñ Low grade metamorphism

ñ Platy minerals too small to be seen

ñ Exhibits slaty cleavage

ï Phyllite

ñ Upper low-grade metamorphism

ñ Flakes have grown larger

ñ Increase in luster

ï Schist

ñ Upper low-grade to medium grade metamorphism

ñ Increased size of platy minerals

ñ Platy minerals clearly visible to naked eye

ï Gneiss

ñ High grade metamorphism

ñ Light and dark minerals segregated into bands, lenses and streaks

ñ Mafic minerals (biotite & amphibole) concentrated in the dark bands

ñ Felsic minerals (quartz & feldspar) concentrated in light bands

Amphibolites

ï Medium-grade metamorphism of mafic igneous rocks

ï Long, thin crystals of amphiboles are aligned in a common direction (lineation)

ï Sometimes separate light bands of feldspar are evident

Migmatite

ï Metamorphism reaches a temperature (>700^o C) where rock begins to melts

ï Felsic minerals preferentially melt

ï Mafic minerals remain solid

ï Resulting silica-rich liquid invades partially melted rock as veins and stringers

Granulites

ï High- to very-high grade metamorphism of shale, greywacke and igneous rocks

ï Display granular texture consisting of

ñ Quartz

ñ Plagioclase

ñ Pyroxene

ñ Garnet

ñ Al-silicates (sillimanite)

Isograds

The metamorphism of shale forms different minerals (index minerals) at different metamorphic grades

Index Minerals Indicate the Degree of Metamorphism Experienced by theÝ Rocks That Contain Them

ÝIsograds Represent the First Appearance of Different Index Minerals inÝ the Field

Non-Foliated Metamorphic Rocks

ï Contact Metamorphism

ï Hydrothermal Metamorphism

ï Metamorphism of rocks without platy minerals

Quartzite Forms From the Metamorphism of Quartz Sandstone

Marble Forms Through Metamorphism of Limestone

Greenstones Form Through Low-grade Metamorphism of Basalt

Sources of Metamorphic Rocks

Parent RockÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝ Metamorphic Rock

LimestoneÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝ Marble

ShaleÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝ Slate/Phyllite/Schist

SandstoneÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝ Quartzite

BasaltÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝ Schist or Greenstone

Mixed SedimentaryÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝ Gneiss

GraniteÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝ Gneiss