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Metamorphism and Metamorphic Rocks • • What is metamorphism? How do temperature and pressure change rocks? Are fluids important in metamorphism? How are metamorphic rocks brought to the surface? What are the conditions of metamorphism? How are metamorphic rocks classified? What was the rock before it was metamorphosed? Where does metamorphism occur in relation to plate tectonics?
Metamorphism and Metamorphic Rocks Metamorphic rocks: • Form beneath Earth’s surface (metamorphic processes cannot be directly observed). • Are commonly found exposed in actively forming mountains, e. g. the Himalayas today. • Are always found in the centers of eroded ancient mountain belts. • Metamorphic minerals make up such economic materials as talc, graphite, marble, garnet, corundum, and coal deposits.
Metamorphism and Metamorphic Rocks Preview: Three types of metamorphic rocks: – Regional metamorphic rocks. • Occur across vast regions such as convergent plate boundaries (e. g. interior of Himalayas today) • Increased temperature and directed pressure – Contact metamorphic rocks. • Thermally driven, common along boundaries of igneous intrusions, sometimes under lava flows – Hydrothermal metamorphic rocks. • Produced by hot-fluid induced chemical changes • May be associated with one or both of the previous types
Appalachian Mountains - A belt of regional metamorphic rocks >1500 km long and 200 -300 km wide, produced by continent-continent collision.
From the Greek “meta” (change) and “morphe” (form) Metamorphic rocks – Formed under conditions between those of compaction and cementation (lithification – sedimentary rocks) and melting (igneous rocks).
What is Metamorphism? • Metamorphism describes the mineralogical, chemical, and textural changes to preexisting rocks due to increased temperature and pressure. • Metamorphic reactions occur in the solid state – no melting is involved. • Because these conditions occur at depth in the Earth, they are not directly observable. • Modern experimental petrology, however, can reproduce these conditions in the laboratory, including original rock, T, P and fluids.
Migmatite – formed by very high grade metamorphic conditions where partial melting (anatexis) occurs.
What is Metamorphism? • Rocks exhibit two types of change: • Original minerals react to form new minerals. • Rock texture is altered by changes in size, shape, and orientation of the minerals (new and/or old). • Original rock type is important: • Chemistry controls what reactions will take place, i. e. what new minerals may form. • In general the whole rock starting composition is the same, or similar to, the metamorphic rock composition. • In some cases the presence of chemically active fluids may cause substantial changes.
How do temperature and pressure change rocks? • We know: Temperature increases with depth in the Earth. • This is referred to as the geothermal gradient, typically 20 -30 o. C/km beneath continents and 60 o. C/km beneath oceans.
How do temperature and pressure change rocks?
How do temperature and pressure change rocks? • It takes ~10 -12 km burial to reach low-grade metamorphic conditions. • By contrast, lithification to form sedimentary rocks occurs at shallow depths.
How Are Higher Temperatures and Pressures Produced in Mountain Building Events? • Mountain building events involve directed pressure, e. g. the collision between India and Asia producing the Himalayas. • When one block of crust is forced over another along a fault, the lower one heats to a temperature associated with its new depth. • This is referred to as tectonic burial and produces regional metamorphic rocks.
How Are Higher Temperatures Produced by Magma Intrusion? • Intrusion of magma raises the local geothermal gradient. This causes metamorphism in rocks adjacent to the intrusion. • This produces what are called contact metamorphic rocks.
How do temperature and pressure change rocks? • Heat drives off fluids (volatiles) that may have been in the rocks. • High-temperature metamorphism causes minerals containing volatiles to lose them. Dehydration reactions = loss of water. (HEAT) KAl 3 Si 3 O 10(OH)2 + Si. O 2 (muscovite) (quartz) Al 2 Si. O 5 + KAl. Si 3 O 8 + H 2 O (sillimanite) (K-feldspar) (water)
How do temperature and pressure change rocks? • Minerals have ranges of temperature and pressure in which they are stable. – If the T and P change the minerals may transform into new minerals which are stable in the new conditions. A Contact Metamorphic Reaction: A Common Example We All Know: ~40 km
How do temperature and pressure change rocks? • Minerals have ranges of temperature and pressure in which they are stable. – If the T and P change the mineral may transform into a new mineral which is stable in the new conditions. We can use metamorphic minerals as indicators of T and P, since we know what conditions they are stable at. Phase diagrams like this have been constructed via experimental petrology. Kyanite: high P Sillimanite: high T and P Andalusite: low T and P What if we found a rock with all three minerals in it?
How do temperature and pressure change rocks? • Pressure can be simply thought of as applying stress to a rock. • Stress – The magnitude of the force divided by the area the force is applied to. • Normal stress – perpendicular to the surface. Results in change in volume and often shape. • Shear stress – force parallel to the surface. Results in change of shape, but not volume. • Strain – the deformation of a rock as a result of applied stress.
How do temperature and pressure change rocks? Directed pressure such as this is called Differential Stress.
How do temperature and pressure change rocks?
How do temperature and pressure change rocks? • Directed pressure can cause platy minerals such as micas to align perpendicular to the stress. • This produces what is called foliation and is a characteristic of regional metamorphic rocks.
How do temperature and pressure change rocks? Foliation in thin section: Defined by alignment of muscovite (brightly colored) intergrown with quartz (greys and blacks)
How do temperature and pressure change rocks? Metaconglomerate - formed from increase in temperature and directed pressure.
How do temperature and pressure change rocks? • Increased temperature and pressure can cause recrystallization of minerals into new shapes and sizes. This process is clear in these microscope photos of a sandstone (top) and a metamorphosed sandstone (bottom) which is called quartzite. Fig 6. 9
Are fluids important in metamorphism? • Fluids can participate in two ways: • React with old minerals to from new minerals that contain the fluid (water or CO 2) • Help drive the changes of metamorphism by delivering and removing dissolved ions. Fluid makes reactions occur much faster and more easily.
Are fluids important in metamorphism? • Fluid may be present during low-grade metamorphism (and ultimately – taken away during high grade metamorphism…) • Most commonly during low-medium grade metamorphism (similar to chemical weathering reactions, but higher temperature and pressures) 2 Mg 2 Si. O 4 + 2 H 2 O + CO 2 Mg-olivine water carbon dioxide Mg 3 Si 2 O 5(OH)4 + Mg. CO 3 serpentine magnesite
Confining pressure may produce metamorphism, but only differential stress produces foliation
Cataclastic (brittle) fault zone deformation at <10 km depth Temperatures and pressures are too low for metamorphism to occur, the rock is simply shattered and broken up.
Mylonite - ductile deformation at >10 km depth. Now T and P are high enough for metamorphism, the rock may recrystallize to new minerals and plastic deformation can occur.
How Are Metamorphic Rocks Brought To The Surface? • Metamorphic rocks form many kilometers underground. • They usually get exposed due to faulting, uplift of mountains and subsequent erosion.
How Are Metamorphic Rocks Brought To The Surface? • Metamorphism of rocks occurs very slowly, it may take millions of years for a rock to convert to a metamorphic rock. • Uplift and exposure is usually relatively rapid and rocks cool quickly during uplift, this “freezes” in the metamorphic mineral assemblage. • Loss of volatiles during high grade metamorphic reactions is also a big factor. – Reactions cannot reverse easily if volatiles are gone. • Sometimes: High-grade metamorphic rock may rise to low-grade conditions and stay there for extended periods, re-metamorphosing them.
What Are The Conditions of Metamorphism? Mineral stability is experimentally calibrated in the laboratory with high temperature and pressure instruments.
What Are The Conditions of Metamorphism? Some minerals form only over a limited range of pressure and temperature – these are good index minerals.
How are metamorphic rocks classified? • Composition and texture: – We know that metamorphic processes change the minerals in a rock and often the rock texture as well. Texture and composition are thus the primary criteria for classifying metamorphic rocks. • The first characteristic is either foliated or nonfoliated. Foliated rocks are characterized by having aligned minerals, or layered minerals, or otherwise linearly arranged minerals.
How are metamorphic rocks classified? This is a foliated texture: fine-grained with minerals aligned along planes (producing rock cleavage). This rock is called slate and may contain clays, muscovite, chlorite, quartz. Parent rock is shale or tuff
How are metamorphic rocks classified? Compositional layering in outcrop - This rock is a gneiss.
How are metamorphic rocks classified? Progressive changes in rock texture and mineral content take place as the metamorphic grade increases. For foliated metamorphic rocks - the type or degree of foliation is the primary characteristic for classification. Start with a sedimentary rock – a shale….
How are metamorphic rocks classified? At low grade metamorphic conditions clay minerals begin to convert to micas and chlorite, but this process is incomplete. If differential stress is present these newly grown minerals will be aligned with the stress field (perpendicular).
How are metamorphic rocks classified? At moderate metamorphic conditions all the clay is recrystallized into mica and chlorite. Feldspars may begin to form. The crystals are now visible with the unaided eye, and the rock develops a “sheen” due to the abundance of muscovite mica.
How are metamorphic rocks classified? At medium grade metamorphic conditions the micas grow much larger, and are all aligned perpendicular to the stress. Chlorite is now gone. New higher grade metamorphic minerals such as garnet may form.
How are metamorphic rocks classified? At high grade metamorphic conditions minerals begin to segregate into layers, with light colored high Si, low Fe and Mg minerals in layers, and dark colored low Si, high Fe and Mg minerals in other layers. This is called gneissic banding, and is a type of foliation.
How are metamorphic rocks classified?
How are metamorphic rocks classified? Some metamorphic rocks do not have foliation. For these metamorphic rocks, composition (mineral content) is the primary characteristic for naming the rock.
What was the rock type before it was metamorphosed? Here are some common metamorphic changes in rock type for different starting (parent) sedimentary and igneous rock types (these are called protoliths).
Where does metamorphism occur?
Where does metamorphism occur? A contact metamorphic zone has increasing grade as you get closer to the heat source. Where do we find igneous intrusions? In convergent and divergent plate tectonic boundaries.
Contact Metamorphism by Igneous Intrusions. These are common over subduction zones.
Where does metamorphism occur? • Hydrothermal metamorphism – Involves migration and reaction of hot, geothermal fluids – Along mid-ocean ridges, divergent plate boundaries. – Above igneous intrusions, subduction zones. Diagram showing hydrothermal ore deposits at a mid-ocean ridge.
Where does metamorphism occur? Tectonic setting for regional metamorphism at a convergent plate boundary, in this case oceanic crust subducting beneath continental crust.
Where does metamorphism occur? Regional Metamorphism by Continent-Continent Collision.
Where does metamorphism occur? Progressively higher grade metamorphism of shale or mudstone may produce this sequence.
Where does metamorphism occur? There are three primary types of metamorphism—contact, hydrothermal, and regional. Contact metamorphism occurs along the margins of igneous intrusions. It is very localized and driven by increased T. Hydrothermal metamorphism is usually small scale and localized also. It occurs mostly along mid-ocean ridges. Involves hot fluids and the ions the fluids mobilize. Regional metamorphism involves increasing temperature and pressure over large volumes of crust. It is typical of converging plate boundaries.