Metamorphic petrology

Question 1

slatey cleavage

Answer 1

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

Question 2

schistosity

Answer 2

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

Question 3

Gneissose banding

Answer 3

C grained + high grade rock

regional met

light and dark minerals separate into bands

roughly 90^0 to max pressure

may be contorted/folded

Question 4

porphyroblastic

Answer 4

large crystals grow during met

fine grained matrix

often pyrite in slate

often garnet in schist

Question 5

granoblastic

Answer 5

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

Question 6

crenulation cleavage

Answer 6

regional met
long time

several stress directions/ periods
leads to several foliations
not all same orientation

phyllite, schist, gneiss

wavy appearance

Question 7

How do new met minerals form

Answer 7

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

Question 8

What happens when stress is applied rapidly

Answer 8

E.g. fault zone

grains fracture + crushed + ground > catalysis

Produce variety of fabrics > C grained, angular clasts in fine matric

characteristics of dynamic met

Question 9

What happens when stress is applied slowly

Answer 9

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

Question 10

Pre-kinematic

see booklet for diagram

Answer 10

Metamorphic minerals appear to have been effected by later deformation

formed before met occured

Question 11

Syn- Kinematic

see booklet for diagram

Answer 11

Met minerals sow that episodes of deformation took place at the same time as mineral growth

Question 12

Post-Kinematic

see booklet for diagram

Answer 12

Minerals grew across earlier foliation after deformation occurred

Question 13

Order of events examples

Answer 13

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

Question 14

What are index minerals?

Answer 14

minerals that only form in met rocks under specific different temps and pressures

e.g. low grade (slate) =chlorite
high grade (Gneiss) =sillimanite

Question 15

List of index minerals + Rocks formed in

Answer 15

Low Med High

rock / Slate / Schist / Gneiss

index/ Chlorite Biotite Kyanite
min / Biotite Garnet silimanite
Kyanite

Chlorite>biotite>garnet>kyanite>sillimanite

Question 16

polymorph

Answer 16

Same chemical formualr
diff chemical structure

Question 17

Al2SiO5 polymorphs

See diagram in booklet

Answer 17

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

Question 18

Why are Plagioclase Feldspar and Quartz not used as index minerals

Answer 18

Stable throughout whole range of grades

can’t indicate a specific rock

Question 19

Retrograde metamorphism

Answer 19

Involves mineral Hydration, carbonation and oxidation (in prograde we loose 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

Question 20

Why do different minerals form

See graph in booklet

Answer 20

Different scenarios result in diff temps and pressures which produce Different minerals

May be asked to compare conditions