Metamorphic petrology Flashcards
slatey cleavage
splits into thin sheets along cleavage
in fine grain + low grade
needs flat platy minerals
align 90^0 to max pressure
usually parallel to axial plane of fold
schistosity
med grade} phyllite + schist
formed by regional
flat platy minerals
align 90^0 to max pressure
light coloured mica concentrated in thin parallel bands > shiny appearance(micaceous sheen)
often garnet porphyroblasts
Gneissose banding
C grained + high grade rock
regional met
light and dark minerals separate into bands
roughly 90^0 to max pressure
may be contorted/folded
porphyroblastic
large crystals grow during met
fine grained matrix
often pyrite in slate
often garnet in schist
granoblastic
unfoliated
contact met
pressure not main factor
orientated equidimensional with few/1 mineral
e.g. hornfels, marble, metaquartzite
sometimes desc as sugary
as indistinct grain boundaries
crenulation cleavage
regional met
long time
several stress directions/ periods
leads to several foliations
not all same orientation
phyllite, schist, gneiss
wavy appearance
How do new met minerals form
Tempreature increases
increase r of diffusion of ions between minerals
still slow as solid rock
sped up by H20 > increases R of diffusion
over millions of years
changes in conditions + several periods of deformation
New minerals form
What happens when stress is applied rapidly
E.g. fault zone
grains fracture + crushed + ground > catalysis
Produce variety of fabrics > C grained, angular clasts in fine matric
characteristics of dynamic met
What happens when stress is applied slowly
Log time + low strain rate >
doesn’t fracture but deforms gradually
temp = unimportant
foliation develops due to pressure due to stress
overtime > several periods of deformation> crenulation cleavage
Pre-kinematic
see booklet for diagram
Metamorphic minerals appear to have been effected by later deformation
formed before met occured
Syn- Kinematic
see booklet for diagram
Met minerals sow that episodes of deformation took place at the same time as mineral growth
Post-Kinematic
see booklet for diagram
Minerals grew across earlier foliation after deformation occurred
Order of events examples
Porphyroblasts cut across cleavage-
cleavage formed first
porphyroblasts after (as ions move +clump to most stable)
Met mineral grow across fold-
fold first by comp forces
further met
New net minerals form
foliated mineral in cleavage that is folded-
foliation first
then folding
What are index minerals?
minerals that only form in met rocks under specific different temps and pressures
e.g. low grade (slate) =chlorite
high grade (Gneiss) =sillimanite
List of index minerals + Rocks formed in
Low Med High
rock / Slate / Schist / Gneiss
index/ Chlorite Biotite Kyanite
min / Biotite Garnet silimanite
Kyanite
Chlorite>biotite>garnet>kyanite>sillimanite
polymorph
Same chemical formualr
diff chemical structure
Al2SiO5 polymorphs
See diagram in booklet
Phase diagram
Kyanite- Top left, low temp and high pressure
Andulasite- bottom, High temp and low pressure
sillimanite- Right middle, high temp and pressure
K and S typical in Gniess
Why are Plagioclase Feldspar and Quartz not used as index minerals
Stable throughout whole range of grades
can’t indicate a specific rock
Retrograde metamorphism
Involves mineral Hydration, carbonation and oxidation (in prograde we lose H20 CO2 as this is nessacary to be stable in Earth’s surface)
as met rocks are returned to surface
chem reactions occur more slowly as temp decreases
Overall: process by which Met rocks become accustomed to low temps + change minerals
Why do different minerals form
See graph in booklet
Different scenarios result in diff temps and pressures which produce Different minerals
May be asked to compare conditions
