Presentation on theme: "Metamorphic Rocks These are rocks that have changed (meta) their form (morphic). Under the influence of heat, pressure and fluids, pre- existing rocks."— Presentation transcript:
Metamorphic Rocks These 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 Rocks In 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. Shale Schist
Formation of Metamorphic Rocks WHAT CHANGES OCCUR: 1) New minerals form - even though the minerals change, most elements are provided by parent rock except water and some dissolved ions. (Same chemistry) 2) A new texture is provided – pressure often results in foliation (layering) and bent (or deformed) layers are common. 3) Crystalline texture – mineral grains grow larger THE RESULTANT ROCKS TEND TO BE LAYERED, HARDER AND MORE COLOURFUL.
Thus, 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.
Metamorphism The important factors that produce metamorphic changes are: 1) Pressure deep within the crust 2) Temperature 3) Strain (shape and volume changes as a result of stress during deformation) 4) Fluid activity (pressure due to fluids in pore spaces within the original rocks)
Where do Metamorphic Rocks form? 1) 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. 2) Fold mountain belts such as the Alps, Himalayas and Andes contain large amounts of metamorphic rocks which were deformed by folding, faulting and thrusting 3) Subduction Zones 4) Near 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 Form 1) Metamorphism 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. 2) Changes in temperature and pressure result in the formation of new minerals (those stable at the new temperatures and pressures). Aluminosilicates (Al 2 SiO 5 ) 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 kilometer Although high temperatures can bring about a significant change, pressure causes the most obvious changes.
What Happens during Metamorphism? 3) 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. 4) Changes that occur at temperatures below 200°C 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 Metamorphic SURFACE MUD 5 km DEEP SHALE (sedimentary) 10 km DEEP SLATE (low grade metamorphic) Foliation appears, minerals recrystallize 15 km DEEP SCHIST (medium grade metamorphism) Micas form, garnet grows 20 km DEEP GNEISS (high grade metamorphic) 25 km DEEP HORNFELS (Cordierite appears) The following is a very general progression from sedimentary rock to metamorphic rock based primarily on pressure.
Types of Metamorphism The three main types of metamorphism are: 1)Contact (Thermal) Metamorphism 2)Regional Metamorphism 3)Dynamic Metamorphism
Contact (Thermal) Metamorphism 1) Occurs when hot magma forms an intrusion into rock - the surrounding rock faaces high temperatures, but no increase in pressure. 2) Thus Contact Metamorphic rocks are produced in high temperature, low pressure environments with low strain (pressure) and variable fluid pressures. 3) 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. 4) 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 Metamorphism 4) These 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. 5) 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. 6) These rocks show high temperature, low pressure minerals.
Regional metamorphism This is the most common type of metamorphic rock with zones of metamorphism covering large areas of continental crust We 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 metamorphism Most 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 Metamorphism Granitic 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).
rock namemetamorphic gradenew mineralstexture Sedimentaryno metamorphism yetnonelayered, soft Slate and PhylliteLow Gradechloriteplaty cleavage SchistIntermediate Grademicaslayered, shiny GneissHigh Gradeamphibole, quartz and feldspar EclogitePartial melting (almost igneous) garnet, pyroxenebanded, partially melted Common Regional Metamorphism of Clastic Sedimentary Rocks: Shale (sedimentary) phyllite-slategarnet muscovite schistgneiss eclogite
New Mineral Growth in Regional Metamorphic Rocks
Aluminosilicates The polymorphic aluminosilicate minerals, Andalusite, Sillimanite and Kyanite (all Al 2 SiO 5 ), 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 Metamorphism Dynamic 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 Metamorphism THESE 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 Rocks rock namemetamorphic gradenew mineralstexture Basaltno metamorphism yetnoneno layering, dark ZeoliteVery Low GradezeolitesSoft, white or colourful minerals found in pore spaces of basalt GreenschistLow Gradechlorite and amphiboles layered, shiny, green to grey Amphibolite or Blueschist Intermediate Gradedark amphiboles and plagioclase Banded, green to black GranuliteHigh Gradegarnet, pyroxeneFoliation is lost! Common Regional Metamorphism of Basalt Zeolite (in Basalt) Greenschist Amphibolite Granulite
Final Important Facts 1) Size 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. 2) 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 Facts 3) Retrograde 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.