Metamorphic Rocks Flashcards
What are metamorphic rocks?
- A parent rock (protolith) that changes form
- Parent rock is either igneous or sedimentary
What causes metamorphism?
- protoliths that are exposed to heating, squeezing, and hot fluids
- causes rocks to change form (size, shape, texture, colour, and/or mineralogy)
- Is a solid-state change
Over which temperatures and pressures does metamorphism occur?
Occurs over a set range of P and T conditions.
The higher the P and/or T, the higher the degree of metamorphism or metamorphic grade
Through burial, compaction, and cementation, sediments turn to stone (lithify) at
temperatures below 200C
At low pressures in the shallow crust, metamorphism occurs between
~200C to ~850C
At high pressures deeper in the crust, metamorphism can occur at
at much lower temperatures
Above ~850C, rocks melt creating
igneous rocks
What are the 3 types of metamorphism?
Heating, hot fluids, and squeezing
Heating Metamorphism
- the avg (normal) geothermal gradient in the crust is ~
25C/km - Under normal conditions, rocks at >8km depth are >200C and will metamorphose
- However, metamorphism can also happen at shallower depths
What could cause the temperature of the crust to rise above normal?
- Magma intrusions because they are unusually hot, magma is above 850C
- Tend to occur near the surface at mid-ocean ridges
- The high magma body will “bake” the surrounding country (protolith) rock resulting in contact metamorphism
What is the contact aureole?
The zone of contact metamorphism that is narrow (mm to 10s of m) and completely surrounds the magma intrusion
Water exists everywhere in the crust and in magma, therefore:
hot water is involved in nearly all metamorphic rxns, it also speeds up rxn time, end result may be the same
Hot fluids
- Hydrothermal fluids often contain dissolved elements rejected from normal silicate minerals that form as magma solidifies
- Fluids migrate into cracks in the surrounding rocks. Changes in P/T cause minerals to precipitate out of the solution.
- Gold is not a component of silicate minerals, so it often gets flushed into cracks as magma cools and solidifies.
- Pegmatite formation is likely also linked to hydrothermal fluids. Pegmatites are igneous rocks but are often found in veins and dikes.
- Hot, mineral-rich water is involved in nearly all metamorphic rxns.
Hydrothermal metamorphism
When water is the main agent of metamorphism, dominant near mid-ocean ridge magma
Squeezing
- Pressure also increases with depth due to the increasing mass of the overlying rock
- The pressure is ~equal in all directions and is called lithostatic stress
- Directed stress: stress that is higher in one direction than other
What processes can produce directed stress in the crust?
- The crust adjacent to convergent plates experiences directed stress. Compression causes crust to thicken, producing mountains.
- Directed stress results in large areas of regional metamorphism
- Directed stress also deforms rock, so regionally metamorphosed rock is often folded
The distinction b/w types of metamorphism are often blurred:
subduction zones are prone to contact, hydrothermal, and regional metamorphism
What is regional metamorphism?
occurs when rocks are buried deep in the crust. This is commonly associated with convergent plate boundaries and the formation of mountain ranges.
How does metamorphism affect composition?
- usually the same as the protolith from which it formed.
- Elements are not added or removed, just rearranged
- Exception: hot fluids can (but not always) introduce/remove elements
- Elements rearrange themselves b/c minerals that were stable at the surface of the Earth become unstable as P/T rise.
Temperature
- Some minerals are stable at low T, some at high T
- Minerals are stable at higher T when rocks are under pressure
Pressure can cause
atoms in minerals to become more closely packed
- Metamorphic minerals tend to be dense
In limestone, calcite crystals can grow:
larger, due to significant pressure over long period of time
Neocrystallization
- new minerals grow that were not present in the protolith
- At low P/T, clay minerals become unstable, break down, and reform into chlorite.
- As P/T continues to rise, the newly formed chlorite becomes unstable and breaks down to form micas, then garnet, then kyanite, then sillimanite
Index minerals only appear under specific P/T conditions. The presence of a given index mineral can tell us how much:
heat and pressure a rock experienced, or the metamorphic grade
How does metamorphism affect texture?
- Some will break down and form new minerals (neocrystallization)
- Some will grow larger and become interlocked (recrystallization)
- As a result, most metamorphic rocks have a crystalline texture
- Grains in a metamorphosed sandstone undergo recrystallization, but some remnant clastic texture may still be visible
- This is called remnant clastic texture. The rock may appear granular or sugary, but grain won’t rub off. Sedimentary structures can be preserved.
Additional metamorphic textures:
- Non-foliated (not layered)
- Foliated (layered)
When pressure is equal in all directions, minerals can grow in any orientation, developing a:
non-foliated crystalline texture, often result of deep burial or contact metamorphism
Directed stress causes platy or elongated minerals to grow:
perpendicular to main stress direction
Shale is the parent rock of most foliated rocks
In which tectonic environment do we expect to find foliated metamorphic rocks?
- At continent-continent convergent boundary
- Slate will form at relatively shallow depth
- Gneiss will form deep in the core of mountains
Metamorphism at continent-continent convergent boundary in order of least to most foliated (outermost to innermost):
Slate, phyllite, schist, gneiss
The stability of a mineral is a function of:
P, T, and fluids present
Metamorphic: if protolith is mudstone then is either
Slate, phyllite, schist, or gneiss
Naming Schists
HIGHEST-GRADE MINERAL then MOST-ABUNDANT MINERAL schist
Non-foliated metamorphic: if protolith is quartz sandstone
Quartzite
Non-foliated metamorphic: if protolith is conglomerate
Metaconglomerate
Non-foliated metamorphic: if protolith is carbonate rock
Marble
Non-foliated metamorphic: if protolith is mafic igneous rock
Amphibolite
