Chap 14/15: Metamorphic rocks

metamorphism definition

changes in minerology and texture in the (usually) solid state due to changing external conditions (Pressure and temperature) after 200C

how are new minerals formed in metamorphism without melting

atoms are moved one by one along grain boundaries to form a new mineral

diagenesis meaning

chemical, physical, and biological changes that affect sediment after initial deposition up to 200C (after 200C is metamorphism)

melting temperature occurance

600-800C, bound after metamorphism where igneous processes typically lie

protolith meaning

starting material for a rock ie parent rock

order of metamorphic progression with increasing temperature and pressure

• shale

• slate

• phyllite

• schist

• gneiss

• migmatite (partially melted)

what happens along the metamorphic progression

• new or different minerals appear

• folliation appears

• increased grain size

driving forces of metamorphism?

• heat

• pressure

• fluids

• deformation

lithostatic pressure meaning

pressure induced from rocks

(at deep levels, pressure is equal due to flowing rock)

hydrostatic pressure meaning

pressure induced from liquids

(pressure is equally distributed)

typical geothermal gradient

15C/km

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

contact metamorphism

metamorphism that occurs over small region

• magma intrusion heats an area

contact heat transfer is controlled by (3)

• size of intrusion

• temperature of magma

• fluid circulation

heat capacity

adding the same amount of heat and energy to different materials will lead to different increases in temperature

aureole

region of metamorphism surrounding an intrusion

• larger intrusion = larger aureole

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

amount of water as metamorphic progress occurs?

weight precent of water decreases due to dehydration reactions

• shale (12-15%) to gneiss (0-1%)

regional deformation

rock texture will changed due to differential stress which produces folliation

contact deformation

since the metamorphism is static, there’s no metamorphic stress and no foliation

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

crystal settling

early formed crystals piling up at the bottom of a magma chamber

what structure does quartz form in rocks like quartzite in thin section

triple grain bonds

can calcite foliate?

calcite cannot since it does not have a long axis

undulous extinction

texture resulting from a deformed crystal which causes the extinction to be wavy and irregular

when does the reduction of grain size occur

extreme deformity that occurs when lots of heat is applied (mylonites)

what metamorphic rock form is most useful when wanting to know about the temperature and pressure of the area

regional shales/pelites (marine origin)

what metamorphic rock form is most useful when wanting to know about the metabasic rock

basaltic rocks

pelite definition

alumina-rich rock that is sensitive to the metamorphic conditions

can define the general increasing metamorphic grade trend

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

pelite prograde minerals in order (7)

• chlorite

• biotite

• garnet

• staurolite

• kyanite

• silimanite

• k-feldspar

prograde meaning

the grade changes in order from low to high temperature

• more common since energy is more available

• loss of volatiles occurs here

metabasic rock prograde minerals in order (5)

• chlorite

• epidote

• amphibole

• garnet

• pyroxene

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

what conditions make plagioclase more calcium rich?

increased temperature and pressure

how is the kinetics of the rocks shown?

via texture

metamorphic facies

metamorphic rocks that form under the same pressure and temperature but different protoliths

minerals in amphibolite shale facies

quartz, biotite, muscovite, and garnet

minerals in amphibolite limestone facies

calcite

minerals in amphibolite basalt facies

amphibole, plagioclase, quartz

isograds meaning

lines of constant metamorphic grade where new minerals are seen

metamorphic zone naming

named after highest grade metamorphic mineral between the isograds

metasomatism

fluids passing through rock that bring new elements to replace old ones, changes bulk composition of the rock

porphyroblasts

crystal in metamorphic rock that is significantly larger than other minerals in the rock

garnet and staurolite are common

metamorphic grade

loosely states intensity of metamorphism using the formation of minerals at each stage (ie slate, phyllite…)

conditions needed to form greenschist

• regional metamorphism

• mid temp and pressure

conditions needed to form blueschist

• mid to high pressure

• low temp

conditions needed to form zeolite

• low temp

• low pressure

conditions needed to form eclogite

• high pressure

• mid to high temp

conditions needed to form hornfels

• low pressure

• mid to high temp

conditions needed to form amphibolite

• mid pressure

• mid to high temp

conditions needed to form granulite

• mid to high pressure

• high temperature

geothermometer meaning

metamorphic reactions that are sensitive to temperature changes so can be used to determine metamorphic temperatures

geobarometers

metamorphic reactions that are sensitive to pressure changes so can be used to determine metamorphic pressures but much less common

skarns

calcsilicate rich rocks that can form Fe-ore deposits

soapstone

mixture of serpentine and talc from the metamorphism of ultramafic igneous rocks

index mineral

a mineral that forms in a metamorphized rock that indicates the minimum pressure and temperature in which the rock forms