Chap 14/15: Metamorphic rocks

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57 Terms

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metamorphism definition
changes in minerology and texture in the (usually) solid state due to changing external conditions (Pressure and temperature) after 200C
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how are new minerals formed in metamorphism without melting
atoms are moved one by one along grain boundaries to form a new mineral
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diagenesis meaning
chemical, physical, and biological changes that affect sediment after initial deposition up to 200C (after 200C is metamorphism)
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melting temperature occurance
600-800C, bound after metamorphism where igneous processes typically lie
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protolith meaning
starting material for a rock ie parent rock
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order of metamorphic progression with increasing temperature and pressure
* shale
* slate
* phyllite
* schist
* gneiss
* migmatite (partially melted)
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what happens along the metamorphic progression
* new or different minerals appear
* folliation appears
* increased grain size
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driving forces of metamorphism?
* heat
* pressure
* fluids
* deformation
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lithostatic pressure meaning
pressure induced from rocks

(at deep levels, pressure is equal due to flowing rock)
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hydrostatic pressure meaning
pressure induced from liquids

(pressure is equally distributed)
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typical geothermal gradient
15C/km
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regional metamorphism
metamorphism that occurs over a large scale

* heat transfer occurs when rocks heat after thrusting on top of each other which causes lower rocks to heat
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contact metamorphism
metamorphism that occurs over small region

* magma intrusion heats an area
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contact heat transfer is controlled by (3)
* size of intrusion
* temperature of magma
* fluid circulation
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heat capacity
adding the same amount of heat and energy to different materials will lead to different increases in temperature
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aureole
region of metamorphism surrounding an intrusion

* larger intrusion = larger aureole
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what affects the size of an aureole (5)
* temperature difference btwn intrusion and host
* composition of intrusion and host
* speed of emplacement
* fluid interactions (ground water and magmatic fluids)
* shape of intrusion
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amount of water as metamorphic progress occurs?
weight precent of water decreases due to dehydration reactions

* shale (12-15%) to gneiss (0-1%)
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regional deformation
rock texture will changed due to differential stress which produces folliation
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contact deformation
since the metamorphism is static, there’s no metamorphic stress and no foliation
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fabric definition
the combined effects of mineral reactions and deformation throughout the metamorphic event and the subsequent return of the rock to the terrestrial surface

* foliation is a type
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crystal settling
early formed crystals piling up at the bottom of a magma chamber
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what structure does quartz form in rocks like quartzite in thin section
triple grain bonds
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can calcite foliate?
calcite cannot since it does not have a long axis
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undulous extinction
texture resulting from a deformed crystal which causes the extinction to be wavy and irregular
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when does the reduction of grain size occur
extreme deformity that occurs when lots of heat is applied (mylonites)
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what metamorphic rock form is most useful when wanting to know about the temperature and pressure of the area
regional shales/pelites (marine origin)
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what metamorphic rock form is most useful when wanting to know about the metabasic rock
basaltic rocks
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pelite definition
alumina-rich rock that is sensitive to the metamorphic conditions

can define the general increasing metamorphic grade trend
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if a rock is on the border of having either kyanite and silimanite but one has a higher Gibbs energy and the other is lower, which form is taken?
Form with the lower gibbs energy
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pelite prograde minerals in order (7)
* chlorite
* biotite
* garnet
* staurolite
* kyanite
* silimanite
* k-feldspar
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prograde meaning
the grade changes in order from low to high temperature

* more common since energy is more available
* loss of volatiles occurs here
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metabasic rock prograde minerals in order (5)
* chlorite
* epidote
* amphibole
* garnet
* pyroxene
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retrograde meaning
the grade changes in order from high to low temperature

* needed stuff for the minerals to revert are gone such as H2O and CO2 during peak metamorphism
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what conditions make plagioclase more calcium rich?
increased temperature and pressure
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how is the kinetics of the rocks shown?
via texture
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metamorphic facies
metamorphic rocks that form under the same pressure and temperature but different protoliths
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minerals in amphibolite shale facies
quartz, biotite, muscovite, and garnet
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minerals in amphibolite limestone facies
calcite
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minerals in amphibolite basalt facies
amphibole, plagioclase, quartz
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isograds meaning
lines of constant metamorphic grade where new minerals are seen
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metamorphic zone naming
named after highest grade metamorphic mineral between the isograds
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metasomatism
fluids passing through rock that bring new elements to replace old ones, changes bulk composition of the rock
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porphyroblasts
crystal in metamorphic rock that is significantly larger than other minerals in the rock

garnet and staurolite are common
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metamorphic grade
loosely states intensity of metamorphism using the formation of minerals at each stage (ie slate, phyllite…)
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conditions needed to form greenschist
* regional metamorphism
* mid temp and pressure
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conditions needed to form blueschist
* mid to high pressure
* low temp
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conditions needed to form zeolite
* low temp
* low pressure
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conditions needed to form eclogite
* high pressure
* mid to high temp
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conditions needed to form hornfels
* low pressure
* mid to high temp
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conditions needed to form amphibolite
* mid pressure
* mid to high temp
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conditions needed to form granulite
* mid to high pressure
* high temperature
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geothermometer meaning
metamorphic reactions that are sensitive to temperature changes so can be used to determine metamorphic temperatures
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geobarometers
metamorphic reactions that are sensitive to pressure changes so can be used to determine metamorphic pressures but much less common
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skarns
calcsilicate rich rocks that can form Fe-ore deposits
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soapstone
mixture of serpentine and talc from the metamorphism of ultramafic igneous rocks
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index mineral
a mineral that forms in a metamorphized rock that indicates the minimum pressure and temperature in which the rock forms