Metamorphic Petrology Flashcards
What is another word for metamorphic textures?
Metamorphic fabrics
What are the characteristics of slaty cleavage?
Splits into thin sheets along cleavage planes. Fine grained.
What are some examples of rocks with slaty cleavage?
Slate and phyllite
How is slaty cleavage formed?
By low grade metamorphism. Flat, platy minerals like clays and micas align at 90° to the direction of pressure and they’re usually parallel to the axial planes in folds
What does slaty cleavage represent?
Low grade metamorphism
How is schistosity formed?
Regional metamorphism. Alignment of flat platy minerals. 90° from direction of metamorphic pressure.
What is schistosity?
Thin parallel beds and shiny appearance where surface of micas visible (micaceous sheen)
What does schistosity indicate?
Higher grade metamorphism
What is the difference between slaty cleavage and schistosity?
Slaty cleavage: low grade. Fine grained.
Schistosity: higher grade. Coarser grained
What does gneissose banding look like?
Clearly distinguishable light and dark bands.
What makes up the light bands of gneissose banding?
Quartz and feldspar.
Usually granoblastic and equidimensional
What are the dark bands made of in gneissose banding?
Biotite and Mafic material. Foliated
Are bands in gneissose banding always straight?
No. They may be contorted or folded at 90° to maximum pressure
What are porphyroblasts?
Large metamorphic crystals within a finer matrix
What is the difference between porphyroblasts and phenocrysts?
Porphyroblasts are metamorphic.
Phenocrysts are igneous and formed by two stage cooling
How are porphyroblasts formed?
Both contact and regional metamorphism
How can porphyroblasts show the order of metamorphic events?
If they are pre, syn or post-kinematic
What are some examples of porphyroblasts?
Cubic pyrite in slate.
Garnet in schists
What is meant by granoblastic?
Unfoliated fabric formed by contact metamorphism. Randomly oriented equidimensional crystals
What is granoblsstic sometimes called?
Sugary texture
What rocks can be seen with a granoblastic texture?
Hornfels, marble, metaquartzite
What is crenulation cleavage?
Several foliations in different directions
How is crenulation cleavage formed?
Regional metamorphism. Two or more stress directions
How does crenulation cleavage make the rock look?
Gives it a wavy appearance
What rocks can have crenulation cleavage?
Phyllite, schist, and some gneisses
How do metamorphic minerals form?
As mountain buildings happen, metamorphic minerals form due to high temps. High temps increase rate ions diffuse (as a solid) between minerals.
How does water effect the formation of new metamorphic minerals?
Mineral formation takes millions of years but water speeds up this reaction as it allows ions to diffuse more rapidly
How may we identify parent rock from a newly formed metamorphic mineral?
They may grow over sedimentary structures/features
What happens if you rapidly apply stress to a rock?
Mineral grains form brittle fractures and are cursed and ground down mechanically at fault zones (cataclasis). Forms range of fabrics.
What happens when stress is applied slowly to a rock?
Results in low strain rates. Rock doesn’t fracture but gradually deform. Later stages of deformation may produce crenulation cleavage which can be used to determine stages of metamorphism
What is a pre-kinematic mineral?
Mineral was formed before deformation. Mineral will be affected by deformation
What is a syn-kinematic mineral?
Form at the same time as deformation.
What is a post-kinematic mineral?
Formed after the deformation
What are index minerals (metamorphic)?
Specific minerals that help to identify the metamorphic grade
Who discovered index minerals?
George barrow
What are the index minerals discovered by Barrow?
Chlorine (shale + phyllite)
Biotite (phyllite + schists)
Garnet (schists)
Kyanite (schists)
Sillimanite (gneiss)
Chlorine found furthest from mountain centre
How are Al2SiO5 Polymorphs formed?
Regional metamorphism
Where is Kyanite in Al2SiO5 polymorphs?
Top left in diagram.
Found in gneisses. Found in high grade metamorphism. High pressure, low temperature
Where is Sillimanite in Al2SiO5 polymorphs?
Right on the diagram.
High grade regional metamorphism. High temps and high pressure. Usually gneisses
What temperatures and pressures is sillimanite formed in Al2SiO5 polymorphs?
Maximum temp of 700°C. Maximum pressure 700MPa. Pressure exists at depths about 25km. Geothermal gradient of 28°C/km
Where is andalusite in Al2SiO5 polymorphs?
Bottom of the diagram.
Low pressure and high temps. Contact metamorphism
What is it called when all the points meet in an Al2SiO5 polymorph diagram?
Triple point
What types of metamorphism are contact, regional and dynamic?
Prograde metamorphic processes
What happens during prograde metamorphism?
Fluids such as water and CO2 are driven off. These fluids are necessary to form the hydrous minerals that are stable at earth’s surface
What is retrograde metamorphism?
Involves mineral hydration, carbonation and oxidation as metamorphic rocks are returned to the surface. Chemical reactions take place more slowly as temp decreases
