Classifying Metamorphic Rocks

Classifying Metamorphic RocksMetamorphic RocksThese are rocks that have changed (meta) their form (morphic). Under the influence of heat, pressure and fluids, pre-existing rocks are modified in form and even in internal atomic structure to produce new rocks stable at the new conditions.

Formation of Metamorphic RocksIn every case, a metamorphic rock is produced from an another rock (parent rock). The parent rock may be sedimentary, igneous or even another metamorphic rock through heat, pressure and the movement of hot (hydrothermal) fluids.

ShaleSchistFormation of Metamorphic RocksWHAT CHANGES OCCUR:New minerals form - even though the minerals change, most elements are provided by parent rock except water and some dissolved ions. (Same chemistry)A new texture is provided pressure often results in foliation (layering) and bent (or deformed) layers are common.Crystalline texture mineral grains grow largerTHE RESULTANT ROCKS TEND TO BE LAYERED, HARDER AND MORE COLOURFUL.

Formation of Metamorphic Rocks

Formation of Metamorphic RocksThus, metamorphic rocks are pre-existing rocks whose mineralogy and/or texture has been changed by processes within the Earth.Metamorphic rocks form because of changes in temperature and pressure due to the depth of burial within the Earth without actual melting of the rocks taking place.The changes that affect these rocks occur in the solid state.

MetamorphismThe important factors that produce metamorphic changes are:

Pressure deep within the crustTemperatureStrain (shape and volume changes as a result of stress during deformation)Fluid activity (pressure due to fluids in pore spaces within the original rocks)

Where do Metamorphic Rocks form?The cratonic regions of all continents are made up almost exclusively of metamorphic rocks and most of the oldest rocks on Earth are therefore metamorphic.Fold mountain belts such as the Alps, Himalayas and Andes contain large amounts of metamorphic rocks which were deformed by folding, faulting and thrustingSubduction ZonesNear the intrusion of granitic magmas.

The fact that most metamorphic rocks are deformed indicates that metamorphism and tectonism (e.g. deformation) occur together.

New Minerals FormMetamorphism is an internal process of the Earth and occurs as a result of changes in temperature and/or pressure. Most minerals are only stable at particular temperatures and pressures.Changes in temperature and pressure result in the formation of new minerals (those stable at the new temperatures and pressures).

Aluminosilicates (Al2SiO5) are a group of minerals found commonly in metamorphic rocks. A different mineral grows depending on the pressure and temperature of surrounding rocks.What Happens during Metamorphism?The geothermal gradient:Another principle source of heat is the natural increase in temperature as the depth increases. This is known as the geothermal gradient. The typical geothermal gradient is about 30 degrees per kilometerAlthough high temperatures can bring about a significant change, pressure causes the most obvious changes.

What Happens during Metamorphism?Other than water, few other components (these include fluorine, boron) enter or leave during metamorphism. No overall chemical changes occur, the original chemical constituents are just re-adjusted during recrystallisation and the texture of the rock changes, usually becoming coarser-grained. Changes that occur at temperatures below 200C and pressures below about 300 MPa (MPa stands for Mega Pascals) are considered to be Diagenesis. Diagenesis is considered to be a sedimentary process.

What Happens during Metamorphism?Pressure affects sedimentary rocks by squeezing the mineral grains together. This eliminates the pore spaces, and expels the fluids that are present.Under increasing pressure, the mineral grains will form a tightly interlocking mosaic. If the pressure continues, the crystals may reform into fewer but larger grains.This process is known as recrystallization. The recrystallized mineral may remain the same as it was, or it may be a new, more dense mineral that is more stable at high pressures.

An example of metamorphic change: Sedimentary to MetamorphicSURFACEMUD5 km DEEPSHALE (sedimentary)10 km DEEPSLATE (low grade metamorphic)Foliation appears, minerals recrystallize15 km DEEPSCHIST (medium grade metamorphism)Micas form, garnet grows20 km DEEPGNEISS (high grade metamorphic)25 km DEEPHORNFELS (Cordierite appears)The following is a very general progression from sedimentary rock to metamorphic rock based primarily on pressure.Types of MetamorphismThe three main types of metamorphism are:1)Contact (Thermal) Metamorphism2)Regional Metamorphism3)Dynamic Metamorphism

Contact (Thermal) MetamorphismOccurs when hot magma forms an intrusion into rock - the surrounding rock faaces high temperatures, but no increase in pressure.Thus Contact Metamorphic rocks are produced in high temperature, low pressure environments with low strain (pressure) and variable fluid pressures.They are usually formed at shallow depths within the crust (usually less than 6 km). The heat sources responsible for contact metamorphism are bodies of hot magma (e.g. igneous intrusions) which raise the temperature of the surrounding rocks.The metamorphosed region is called the contact aureole. These zones contain high temperatures but low pressures and are responsible for the growth of new minerals that are stable under these conditions.Contact Metamorphism

Hornfels note the lack of foliation and the very fine-grained texture

The Contact Aureole

Well-defined contact between intrusive granite and contact metamorphic slate.

Contact MetamorphismThese thermal effects are usually restricted to the contact zones of the intrusions, hence the term contact metamorphism. However, sometimes hot fluids are released from the intrusions and penetrate the enclosing rocks along fractures and produce contact metamorphic zones. Unlike other metamorphic rocks, distinct layering and the growth of coarser grained minerals is NOT found. Since the rock is often fine-grained and not foliated, it is hard to identify contact metamorphic rocks. Usually only after observing large areas of rock surrounding an igneous intrusion can we see a contact aureole. The common rock name for contact metamorphic rock is called a hornfels.These rocks show high temperature, low pressure minerals.

Regional metamorphismThis is the most common type of metamorphic rock with zones of metamorphism covering large areas of continental crustWe will focus on these rocks in Rock Identification!Regional metamorphism is produces a wide variety of temperatures, pressures, strains and fluid flow and is normally created by tectonic processes within the Earth at a variety of depths. More simply put, this is metamorphism caused by the burial of rocks at great depth. Regional metamorphismMost metamorphic rocks occur in fold mountain belts or cratonic areas. Such rocks cover large areas of the Earth's crust and are therefore termed regional metamorphic rocks.They arise by the combined action of heat, burial pressure, differential stress, strain and fluids on pre-existing rocks.The resulting rocks are always deformed (as a result of the differential stress) and commonly exhibit folds, fractures and cleavages.Typically, grain size increases with increasing temperature and pressure.

Regional MetamorphismGranitic intrusions are also associated with regional metamorphic rocks. The most common regional metamorphic rocks are slates, schists and gneisses. Regional metamorphism covers a wide range of temperature and pressure conditions from 200 C - 750 C and 2 kbar - 10 kbar (or 5 km - 35 km depth).

Metamorphic Grades

Rock Types, Crystal Size, Foliation

Classifying Regional Metamorphic Rocks

Classifying Regional Metamorphic Rocksrock namemetamorphic gradenew mineralstextureSedimentaryno metamorphism yetnonelayered, softSlate and PhylliteLow Gradechloriteplaty cleavageSchistIntermediate Grademicaslayered, shinyGneissHigh Gradeamphibole, quartz and feldsparEclogitePartial melting (almost igneous)garnet, pyroxenebanded, partially meltedCommon Regional Metamorphism of Clastic Sedimentary Rocks:

Shale (sedimentary) phyllite-slategarnet muscovite schistgneiss eclogiteNew Mineral Growth inRegional Metamorphic Rocks

Aluminosilicates

The polymorphic aluminosilicate minerals, Andalusite, Sillimanite and Kyanite (all Al2SiO5), are common in metamorphic rocks of sedimentary origin (pelitic). Since each forms at a different pressure and temperature (see the phase diagram above) they are a great measure of the metamorphic grade. Finding these minerals is very important to geologists.

andalusite kyanite sillimanite

Dynamic MetamorphismDynamic metamorphism is the type of metamorphism that normally occurs in active fault zones and areas where the ocean crust is being subducted under continental crust. Rocks found in these zones are highly deformed and they tend to be exposed to very high pressures but only low or medium crustal temperatures.Found where ocean crust is being subducted into the Mantle!Since the original rocks being metamorphosed are formed from ocean crust, they are rich in basalt and andesite (rocks high in mafic minerals).

Dynamic MetamorphismThese minerals are not included in our collection!Low Grade Zeolite Basalt (holes in the Basalt are filled with light coloured, soft zeolite minerals.Medium grade Greenschists dark schist rocks rich in chlorite (green mica) and actinolite*(green amphibole). (* not always present)Medium High Grade Blueschist dark schist rocks rich in blue amphiboles.High Grade Eclogite hard banded rock containing garnet, pyroxene, quartz and kyanite but no plagioclase feldspar. (very close to the melting point)

Classifying Dynamic Metamorphic Rocksrock namemetamorphic gradenew mineralstextureBasaltno metamorphism yetnoneno layering, darkZeoliteVery Low GradezeolitesSoft, white or colourful minerals found in pore spaces of basalt GreenschistLow Gradechlorite and amphiboleslayered, shiny, green to greyAmphibolite or BlueschistIntermediate Gradedark amphiboles and plagioclaseBanded, green to blackGranuliteHigh Gradegarnet, pyroxeneFoliation is lost!Common Regional Metamorphism of Basalt

Zeolite (in Basalt) Greenschist Amphibolite GranuliteComparing Metamorphic Types

Comparing Metamorphic Types

Final Important FactsSize of Metamorphic zones

Contact and dynamic metamorphism are usually restricted to localized areas whereas Regional metamorphism affects large areas of the crust, sometimes over tens of thousands of square kilometres. Contact metamorphism occurs as zones a few hundred metres wide around large igneous intrusions while dynamic metamorphism is restricted to fault and thrust zones only a few tens of metres thick. All three types of metamorphism can overlap.

Metamorphic rocks and ore deposits:

Metamorphism is also strongly associated with many ore deposits. This is because metallic elements (such as lead, zinc, copper) are particularly mobile during metamorphism, especially when fluid is involved.Final Important FactsRetrograde metamorphism

Normal Increasing metamorphism is called Prograde Metamorphism

Many metamorphic rocks contain evidence of retrograde mineral changes, that is, alteration of higher grade minerals into lower grade ones. Retrograde metamorphism is normally produced by repeated regional metamorphism where a lower grade episode is superimposed on a higher grade one.

Most retrogressive events are probably just a consequence of the metamorphic system cooling down after peak metamorphism has been reached.

i.e. the system has to cool down with time and as the region undergoes uplift with time, both pressure and temperature are dramatically reduced.