metamorphic rocks Flashcards
definition
metamorphism
solid state changes in a rock when it is subjected to conditions other than that in which it formed
factors that control
agents of metamorphism
temperature and pressure, fluids (change the minerology/composition)
prograde metamorphism
due to increasing T and P, burial
retrograde metamorphism
due to decreasing T and P, exhumation
conditions for metamorphism
- temp: between ~200°C and melting (~650°-1200°)
- time: millions of years
mineralogical changes
neocrystallization
new minerals form through solid state, chemical changes, from old
old minerals become unstable and change
mineralogical changes
recrystallization
pre-existing minerals change size and shape
- mineral identity doesnt change
- forming bigger crystals reduces the energy of the system (when heat is added)
mineralogical changes
phase transitions
structural changes in minerals (polymorphs)
metamorphic grade
intensity of metamorphism, discussed in terms of temp
textural changes
compression and shear at elevated T and P cause changes in internal texture of rocks
- minerals rotate into preferred orientations
foliation
minerals lined up in planes in metamorphic rocks, streaked or striped appearance
- direct pressure aligns minerals
- foliated rocks break along foliation planes
- foliation form by preferred orientation of minerals with an inequant habit
lineation
linear alignment of minerals within a metamorphic rock, like a bundle of drinking straws
- direct pressure aligns minerals during metamorphism
non-foliated rocks
metamorphic minerals randomly orientated
- no directed pressure during formation, equal stresses
- no preferred shape
porphyroblastic texture
large crystals, prophysoblasts, surrounded by matrix
similar to porphyritic texture in igneous rocks
special rock names
quartzite
metamorphic rock composed predominantly of quartz, recrystallized to make coarser, non-foliation
protolith typically sandstone
special rock names
marble
metamorphic rock composed predominantly of calcite or dolomite
protolith typically limestone or dolostone
special rock names
amphibolite
metamorphic rock dominated by hornblende + plagioclase, may be foliated or not
protolith typically basalt
special rock names
migmatite
composite silicate rock, commonly has dark gneissic matrix and lighter felsic portions
- high grade rock that underwent some melting
- first stages of magma
- granitite melt pods
metamorphic environments
thermal (contact) metamorphism
due to heat from magma invading country rock, creates zones of metamorphism in country rock called contact aureole
- aureole typically concentric with the pluton
- zoned from high (near pluton) to low grade (far from pluton)
- dominant rock is hornsfels
aureole zone typically < 1km in width, usually no foliated rocks
metamorphic environments
regional metamorphism
vast areas of metamorphic belts, often associated with compressional tectonics (subduction and collision)
creates foliated rocks
metasomatism
bulk composition of rock changes often due to fluids adding or removing elements
- fluids from magma can affect country stone
regional metamorphism
dynamothermal metamorphism
rocks caught up in mountain building are
- heated via geothermal gradient and plutonic intrusions
- squeezed and sheared by different stresses
creates foliated rocks
regional metamorphic environments
subduction
basalt is metamorphosed
subducting plate - low T high P
- blueschiest (only forms here)
- eclogite (high density, causes slab pull)
volcanic arc - high T high P (overriding plate)
collisional metamorphic belts
often thousands of km long, hundreds of km wide
regional metamorphic environments
sea floor metamorphism
low-med T low P
- magma from MOR heats up plate/water
- current of hot water can metamorphose rocks (hydrothermal metamorphism)
- chlorite common (green mica)
regional metamorphic environments
shock
extreme high P attending a bolide (meteorite) impact
quartz low pressure tolerance, changes to coesite (SiO)
metamorphic facies
the names of facies on a P-T diagram reflect minerals and mineral-assemblages that are stable at these conditions and provide info on the metamorphic processes that have affected the rocks
useful to interpret the history of a metamorphic rock
solid state changes in a rock when it is subjected to conditions other than that in which it formed
definition
metamorphism
temperature and pressure, fluids (change the minerology/composition)
factors that control
agents of metamorphism
due to increasing T and P, burial
prograde metamorphism
due to decreasing T and P, exhumation
retrograde metamorphism
- temp: between ~200°C and melting (~650°-1200°)
- time: millions of years
conditions for metamorphism
new minerals form through solid state, chemical changes, from old
old minerals become unstable and change
mineralogical changes
neocrystallization
pre-existing minerals change size and shape
- mineral identity doesnt change
- forming bigger crystals reduces the energy of the system (when heat is added)
mineralogical changes
recrystallization
structural changes in minerals (polymorphs)
mineralogical changes
phase transitions
intensity of metamorphism, discussed in terms of temp
metamorphic grade
compression and shear at elevated T and P cause changes in internal texture of rocks
- minerals rotate into preferred orientations
textural changes
minerals lined up in planes in metamorphic rocks, streaked or striped appearance
- direct pressure aligns minerals
- foliated rocks break along foliation planes
- foliation form by preferred orientation of minerals with an inequant habit
foliation
linear alignment of minerals within a metamorphic rock, like a bundle of drinking straws
- direct pressure aligns minerals during metamorphism
lineation
metamorphic minerals randomly orientated
- no directed pressure during formation, equal stresses
- no preferred shape
non-foliated rocks
large crystals, prophysoblasts, surrounded by matrix
similar to porphyritic texture in igneous rocks
porphyroblastic texture
metamorphic rock composed predominantly of quartz, recrystallized to make coarser, non-foliation
protolith typically sandstone
special rock names
quartzite
metamorphic rock composed predominantly of calcite or dolomite
protolith typically limestone or dolostone
special rock names
marble
metamorphic rock dominated by hornblende + plagioclase, may be foliated or not
protolith typically basalt
special rock names
amphibolite
composite silicate rock, commonly has dark gneissic matrix and lighter felsic portions
- high grade rock that underwent some melting
- first stages of magma
- granitite melt pods
special rock names
migmatite
due to heat from magma invading country rock, creates zones of metamorphism in country rock called contact aureole
- aureole typically concentric with the pluton
- zoned from high (near pluton) to low grade (far from pluton)
- dominant rock is hornsfels
aureole zone typically < 1km in width, usually no foliated rocks
metamorphic environments
thermal (contact) metamorphism
vast areas of metamorphic belts, often associated with compressional tectonics (subduction and collision)
creates foliated rocks
metamorphic environments
regional metamorphism
bulk composition of rock changes often due to fluids adding or removing elements
- fluids from magma can affect country stone
metasomatism
rocks caught up in mountain building are
- heated via geothermal gradient and plutonic intrusions
- squeezed and sheared by different stresses
creates foliated rocks
regional metamorphism
dynamothermal metamorphism
basalt is metamorphosed
subducting plate - low T high P
- blueschiest (only forms here)
- eclogite (high density, causes slab pull)
volcanic arc - high T high P (overriding plate)
regional metamorphic environments
subduction
often thousands of km long, hundreds of km wide
collisional metamorphic belts
low-med T low P
- magma from MOR heats up plate/water
- current of hot water can metamorphose rocks (hydrothermal metamorphism)
- chlorite common (green mica)
regional metamorphic environments
sea floor metamorphism
extreme high P attending a bolide (meteorite) impact
quartz low pressure tolerance, changes to coesite (SiO)
regional metamorphic environments
shock
the names of facies on a P-T diagram reflect minerals and mineral-assemblages that are stable at these conditions and provide info on the metamorphic processes that have affected the rocks
useful to interpret the history of a metamorphic rock
metamorphic facies
