Topic 9 Flashcards
Define metamorphism
Transformation of one rock type into another rock type
The parent rock (prolith) turns into metamorphic rock
In this process the mineralogy, texture and sometimes chemical composition of prolith is changed
Low grade changes are slight changes while high growth are changes are substantial changes
TOPIC QUESTION - What are mineral polymorhps and how can they be used to differentiate dentists between high and low grade metamorphism
Minerals with the same chemical composition but different structure
Respond similarly to changing temperature and pressure conditions
Therefore can be used to differentiate between high and low grade metamorphism dependent on type of polymorphism present
E.g andalusite low temp low pressure indicate low grade metamorphism while siliminate is made high temp and high pressure indicate high grade metamorphism
TOPIC QUESTION What are the four agents of metamorphism
- Heat
- Confining Pressure
- Directional stress
- Chemically active fluids
TOPIC QUESTION Explain the impact of heat on metamorphism
- provides energy for chemical reactions to recrystallize existing minerals
- the newly formed mineral grains tend to be larger than the original grains
- mineralogy may or may not change
- source is geothermal gradient or rising magma
Geothermal heat - temperature increases as move downward from upper crust
- different minerals are stable at different depths (different temperatures)
- clay unstable at ~8km (200C)
- quartz and feldspar alter at higher temperatures
Rising magma - cools by releasing heat to surrounding rock
- bakes host rock
TOPIC QUESTION Explain the impact of pressure on metamorphism
Pressure increases with depth
Confining pressure is equal in all directions
- pressure increases with depth
- causes pore spaces to close
- can cause atoms in minerals to pack more closely
- can cause phase change
- does not fold or fracture rock
Differential pressure stress is greater in one direction then another
- compressional strength results in shortening in direction of greatest pressure
- occurs at convergent boundaries
- crust is shortened and thickened
- rocks are ductile in high temperature, high pressure
- minerals can be flattened when subjected to differential stress
- deform by flowing and folding
TOPIC QUESTION Explain the impact of chemically active fluids on metamorphism
- hot fluids (water and CO2) surround mineral grains and act as catalysts for ion migration
- speeds up recrystallization
- hot fluids transport matter long distances
- ion exchange between solution and hot rock
TOPIC QUESTION name and describe common non-foliated metamorphic rocks
- Marble
- medium to coarse grained
- relatively soft 3 on Mohs scale
- interlocking calcite or dolomite grains
- prolith (parent rock) limestone or dolostone - Quartzite
- moderate to high grade
- medium to coarse grained
- very hard
- massive
- fused quartz grains
- prolith (patent rick) quartz or sandstone - Hornifels
- baked by intruding magma body
- fine grained
- variable composition
- grey to black in color
- exceedingly tough and durable
- prolith (parent rock) often shale but can have any composition
TOPIC QUESTION What is the difference between foliated and non-foliated textures
Foliated - any planar (nearly flat) arrangement of mineral grains/structural features within a rock
Non-foliated - metamorphic rocks with interlocking crystals
TOPIC QUESTION name and describe common foliated rocks and order then by metamorphic grade
- slate (low grade)
- prolith (parent rock) shale, mudstone or siltstone
- very fined grains bed, has mica, chlorite, with some quartz and feldspar
- breaks in flat slabs called slaty cleave
- preexisting sedimentary bedding may still be apparent
- smooth dull surface
- can be black, green or red - phyllite (intermediate grade)
- no evidence of prolith remains
- prolith (parent rock) shale, mudstone or siltstone
- continued from slate
- mineral crystals especially micas are visible
- pronounced wavy foliation breaks along wavy surface
- shinier then slate, glossy sheen - schist (medium to high grade metamorphic rock)
- further continuation from phyllite
- parent rock or prolith - shale, slate or mudstone
- minerals are medium to coarse grained
- platy minerals dominant
- scaly foliation
- can have biotite and muscovite mica, quartz, K and Na feldspar, garnet and siliminate
- very shiny metallic appearance
- schistose foliation - gneiss (high grade metaphoric rock)
- further continuation from phyllite and schist
- prolith or parent rock - slate, granite or volcanic rocks
- minerals are medium to coarse grained
- granular and elongated minerals dominate
- quartz, K and Na feldspar, biotite and muscovite mica, amphibole, garnet and silimanite
- very distinct segregation into wide bands (gneissic foliation)!
- often resembles granite apart from banding
- much of Canadian Shield
TOPIC QUESTION What are temperature, pressure and geologic environment for contact or thermal metamorphism
By contact with magma
- rock is intruded by magma
- high temperature 600-1000c but low pressure less than 2 kilobars
- ,close to surface
- types of metamorphic rocks produced are non- foliated type depends on prolith
- many different metamorphic rocks can be produced at the same time
TOPIC QUESTION What are temperature, pressure and geologic environment for hydrothermal metamorphism
Hot fluids (geysers, hot springs)circulate through cracks in a rock causing chemical alternation by enhancing recrystallization
- can be low to high temp 0-400C
- low pressure
- close to surface
- produce non-foliated metamorphic rock
TOPIC QUESTION What are temperature, pressure and geologic environment for burial metamorphism
Huge amounts of sediment or volcanic material accumulated in a basin
- deeper in the ground ~8km
- low temp around 200C and low to medium pressure
- form non-foliated metamorphic rock
TOPIC QUESTION What are temperature, pressure and geologic environment for subduction metamorphism
Rock and sediment carries to great depths (oceanic plate)
- low temperature
- high pressure
- deep
- ,differential stress can produce foliation
TOPIC QUESTION What are temperature, pressure and geologic environment for regional metamorphism
Common, widespread type of metamorphism. Typically associated with mountain building where large portions of the crust are intensely deformed by collusion of two crystal blocks
- variable temp and pressure but highest of both core regions
- foliated ricks
- folding and faulting also common
TOPIC QUESTION define metamorphic facies and identify temperature, pressure and the tectonic environment where each is created
Metamorphic facies means metamorphic rocks formed in a very similar metamorphic environment
Metamorphic rocks that contains the same set of minerals belong to the same metamorphic facies
Subduction zone focean-continent) Eclogites facies - high temp - extremely high pressure - very deep Blueschists facies - low temperature - high pressure Hornfels facies - low to high temp - low pressure - from contact and hydrothermal metamorphism
Subduction zone (continent-continent) - dominated by regional metamorphism Zeolite facies - low temp - low pressure - burial metamorphism Greenschusr facies - med temp - med pressure Amphibiotite facies - medium temp - med to high pressure Granulite facies - high temp - high pressure
Passive margin dominated by burial metamorphism and therefore zeolite facies
TOPIC QUESTION Explain how index minerals are used to differentiate between high and low grade metamorphism
Index minerals are used to establish the metamorphic grade of a rock
- index minerals form over a specific pressure and temperature range and therefore are good indicators of the environment in which they are formed
E.g cholrite forms at low temp and pressure indicate rocks with chlorite are low grade while siliminate forms at high temp and pressure indicate high grade
