Metamorphic Rocks Flashcards
metamorphism
processes of mineralogical and textural change that occur in a rock when it is subjected to pressure, temperature, and fluid conditions different than those when the parent rock formed
parent rock
original rock before metamorphism
rock cycle and metamorphic rocks
- P and T conditions are above those of sedimentary lithification and below those of melting
- Typically between 200-850 degrees celcius and >300Mpa
- Magmas are the only thing that can’t be directly turned into a metamorphic rock -> needs to cool and become a rock first
what forces drive metamorphism?
- changes in temperature
2. changes in pressure
changes in temperature
- Average geothermal gradient 25degC/km depth
- Geothermal gradient = amount of increase in temperature with depth in the earth
- Varies depending on geologic setting (ie. Higher gradients close to mid-ocean ranges or close to igneous extrusions)
changes in pressure
- Lithostatic (equal in all directions) increase in pressure due to depth
- Differential (directed stress, which may be compressive or shear -> some areas receive more stress than others)
- Compressive differential stress produces a foliation -> needs platy minerals to form
- Minerals grow at 90 degrees to the pressure source -> ie. If the pressure is coming from top and bottom, layers will be horizontal
2 classes of metamorphic rocks
- foliated
- non-foliated
foliated rocks
Ex. Schist (metamorphosed mudstone) with strong foliation = differential pressure and platy minerals
non-foliated rocks
- Ex. Marble (metamorphosed limestone) with no foliation = lithostatic pressure (equal pressure in all directions) only OR no platy minerals
- Pure limestones only consist of carbonates, impure limestones have some clay or mud in them
changes during progressive metamorphism
- Recrystallization (coarsening)
- Formation of new minerals
- Foliage (cleavage)
- Metamorphic rock is typically denser
metamorphic process of shale
- Shale -> slate -> phyllite -> schist -> gneiss -> migmatite (right before it melts)
- As we go along, pressure and temperature increase
slate
rock takes on a platy breaking habit, >1mm planes
phyllite
fine-grained micas forming; gives a “sheen” on foliation planes
schist
individual minerals (micas, garnet) are recognizable
gneiss
medium to coarse-grained with alternating layers of mafic (dark) and felsic (light) material
migmatite
- rock begins to melt in situ = “mixed rock” with veins and patches of granitic melt preserved
- Form where high-grade metamorphic rocks exceed the melting temperature of the felsic layers but not of the mafic layers
metamorphic facies
- Particular metamorphic rocks form in particular P-T regimes/spaces
- Some facies are restricted in tectonic setting -> different metamorphic rocks form at different tectonic settings
exposure of metamorphic rocks
- Crust is like a memory-foam mattress -> thick crust with heavy mountains will sag a bit, then as mountains erode and crust gets lighter, is rises up to compensate -> rocks that were once deep in the earth are now close to the surface
- This process = Isostasy – an isostatic rift
- another analogy: Like an iceberg
where does metamorphism occur?
- shock metamorphism
- contact metamorphism
- regional metamorphism
shock metamorphism
- Impact craters
- Brief but extremely high pressure and temperature
- Effects: recrystallization of quartz to high P polymorphs, ejected blobs of molten rock called tektites
contact metamorphism
- Adjacent to intrusions in shallow crust (<5km)
- Country rock heated by conduction and hydrothermal convection
- Generally low P so non-foliated
- Often associated with hydrothermal alteration
- Results: recrystallization, new minerals, veins form
regional metamorphism
- Occurs over large areas of the crust due to increased P-T at depths >5km
- Effects: recrystallization, new minerals form, metamorphic foliation develops
index materials in regional metamorphism
- Polymorphs: forms of minerals with same composition but different arrangement of atoms
- Stable at different P and T conditions
temperature vs. depth vs. tectonic settings
- For the same depth (P), the temperature – depending on tectonic setting
- This for a given depth, different metamorphic rocks form in different settings
4 broad metamorphic settings in a convergent margin
A) Subduction zone (low T high P)
B) plate interior (normal geothermal gradient)
C) volcano-plutonic complex (near arcs – high P and T)
D) Shallow depths (contact metamorphism – low P very high T)
fluids and metamorphism
- H20 in pore spaces carries dissolved ions in solution
- Fluids increase rates at which minerals form
- Rock veins: water can get into veins and travel down near magma -> evaporates and travels up -> cools down as it travels up and precipitates gold and quartz
hydrothermal metamorphism
- Important at mid-ocean ridges
- Ocean water seeps into ocean crust – heats up and rises
- Returned to ocean via vents (eg. Black smokers) - rich in mineral sulfides
- As water moves through crust, it chemically hydrates the rocks (basalts and gabbros)
- Olivine and pyroxene get converted to hydrous minerals like amphiboles
- Rocks turn green -> “Greenstones”
- Water released from these at subduction zones promotes melting and magma generation
