CHPTR 7 Metamorphic Rocks Flashcards
protolith
parent rock
metamorphism
transformation of rocks as the result of heat, pressure, or fluid activities
- below melting point BUT higher pressure and temp. than lithification
lithostatic pressure
uniform stress/confining pressure
uniform field of pressure experienced by most rocks beneath Earth’s surface
directed pressure (differential stress
pushes mineral grains into one direction
geothermal gradient
25-30 degrees C
the rate at which the Earth’s temperature increases with depth
what can provide heat for the metamorphic process + how
burial increases temp. - the deeper you go, the hotter it is
volcanoes - heat from intrusions and lava flow heats underlying rocks
confining pressure + example
uniform stress in all direction - lithostatic stress
lithostatic stress
pressure experienced by rocks under the Earth’s surface
pressure due to 1km of rock
25 - 30 MPa
directed pressure
differential stress - pushes mineral grains into one direction
metastomatism
metamorphism due to fluid activity
how does fluid activity affect metamorphism
accelerate chemical change which can cause new minerals to form
origins of fluids that influence metamorphic activity (4)
- trapped in pore space in rocks
- released by magma
- dehydration reactions
- hydrothermal circulation
metamorphic processes (7)
- orientation of mineral grains
- recrystallization of existing minerals
- growth of new minerals
- segregation of minerals
- baking
- shearing
- partial melting
controlling factors in metamorphism
heat, pressure, fluids
index minerals for metamorphism grade
low to high
chlorite
muscovite
biotite
garnet / kyanite
staurolite
sillimanite
3 horrible index minerals for metamorphism
andalusite, quartz, feldspar
what do andalusite, kyanite and sillimanite have in common
polymorphs
what causes kyanite vs sillimanite
+ why don’t i care about andalusite
kyanite - high pressure
sillimanite - high temperature
andalusite is a bad index mineral
foliated texture
stripes - produced by the preferred orientation of platy minerals
non-foliated texture
do not exhibit preferred orientation of minerals
burial metamorphism
low temp, low pressure, just rocks underground
contact metamorphism
high temp, low pressure
near a magma chamber and the surface
aureoles
metamorphosed zones that surround plutons or other intrusions - its from contact metamorphism
how does distance from a pluton affect metamorphic grade
- high grade metamorphism close to pluton, low grade farther away
regional metamorphism
most common metamorphism
occurs during subduction or continent-continent collision
Barrovian metamorphism protolith
shale
pelitic protolith
shale protolith
Barrovian metamorphic process
shale
slate
phyllite
shist
gneiss
what metamorphic grade does biotite and garnet signify
intermediate
what metamorphic grade does chlorite signify
low grade
economic significance of plutons/contact metamorphism
as pluton cools, hydrothermal circulation occurs by the fluids released from crystallizing rock and groundwater.
This fluid concentrate minerals and form deposits of economic minerals
slate features?
- foliated
- created through grain reorientation
phyllite features?
foliated - platy minerals cannot be seen by eye
- identified by glossy, lustrous sheen
- recrystallization; results in coarser grains
what metamorphic grade does sillimanite signify
high grade
slate foliated?
foliated
schist features + what’s unique about it at a higher grade
foliated - schistocity
>50% platy material
garnet and staurolite porphyroblasts appear at higher grade
most common metamorphic rock formed by regional metamorphism
schist
porphyroblasts
large mineral crystal that disrupts the mineral grain in metamorphic rocks
gneiss features
foliated
banding - bandss of black and white rock
more granular minerals than platy minerals
minerals present in the light colour in gneiss
quartz, feldspar
minerals present in the dark colour in gneiss
biotite, amphibole
oldest dated rock
Acasta Gneiss
migmatite
has a mixture of apparently igneous and metamorphic components. Like liquid stripes
- signifies partial melting
prograde metamorphism
increasing grade
retrograde metamorphism
decreasing grade
metamorphic zone
area with same degree of metamorphism
is marble foliated
nope
marble’s predominant minerals
calcite/dolomite
what does limestone metamorphise into
marble
what does quartz metamorphise into
quartzite
what does mafic igneous rocks metamorphise into
greenstone, amphibolite, granulite eclogite
eclogite features
- often contains pink garnet
- stable at mantle pressures
- high density
what does felsic rocks metamorphis into
gneiss
impact metamorphism
asteroid hit Earth/underground nuclear explosions
- lots of pressure
when the Earth was hit by an asteroid what rocks were formed
- shocked quartz
- coesite
- stishovite
dynamic metamorphism
formed in fault zones due to high strains
cataclasites
forms in shallow part of faults
what causes cataclastic texture
shallow faults which result in crushing and grinding of rocks
mylonites
forms in deeper part of faults (10-20km)
hydrothermal metamorphism - mid ocean ridges process
- freshly erupted basalt and gabbro are erupted from ocean volcano and still hot
- salt water percolates into the rocks. it cools into a new oceanic crust
- mineral reaction occurs: olivine into serpentine
hydrothermal metamorphism - plutons
- plutons cools, releasing water from the magma
- water invades surrounding rocks creating pegmatite
blueschist facies
low T, high P - the upper part of a subduction zone
zeolite facies
burial metamorphism
eclogite facies
high T, high P. the lower part of a subduction zone
shield
area where pre-Cambrian igneous and metamorphic rocks are exposed at the surface
platform
area where igneous-metamorphic basement overlain by sedimentary rocks
cratons
formed by a combination of shield and platform
distribution of metamorphic rock
- shields
- cores of mountain ranges - (continent continent collision)
