Metamorphism of Lower Crust

Question 1

What is the original composition of lower crust rocks?

Answer 1

Intermediate and basic igneous, high P and T, difficult to deform e.g. xenoliths, cratonic basement

Question 2

What effect does high T have on the rocks and then subsequent cooling?

Answer 2

High T = very weak rocks, complex fold patterns

Cooling = very strong rocks, fabrics, fluids and melts absent

Question 3

What causes retrograde amphibolite facies to metamorphose?

Answer 3

Fluid influx on shear zones, late fluids enter brittle fracture causing local weakening, related to uplift

Question 4

What is a migmatite and name two kinds?

Answer 4

A mixed rock,
Peralumious granite melt (leucosome)
Restite (melanosome)

Question 5

When does melting first occurs in pelites?

Answer 5

650 degrees

Question 6

How does melting effect a migmatite?

Answer 6

Small degree partial melts stay in host rock creating very viscous melts, further melting disrupts structures, Schlieren migmatite (streak restite) and Schollen migmatite (blobs of restite) form

Question 7

Why is there a dispute if granulites form by melting or metasomatism?

Answer 7

Many leucosomes are not minimum melt compositions (qtz, plag + K-feld), normally lacks K feldspar, may be due to early loss of melt

Question 8

Describe wet and dry melting reactions

Answer 8

Wet melting reactions use up initial fluid, e.g. musc+qtz+water=Sillimanite+ melt, leaves dry restite, further melting dependent on breakdown of hydrous phases
If hydrous mineral breaks down, dry reation occurs, e.g. Biotite+Qtz=K-Feldspar+Opx+Melt

Question 9

What are the breakdown temperatures for Muscovite, Biotite and Amphibole?

Answer 9

Muscovite = 700 degrees
Biotite = 800 degrees
Amphibole = 900 degrees

Question 10

What is the role of fluid composition in the origination of granulites?

Answer 10

High T (low CO2) - dehydration by melt production
High CO2 (low temp) - CO2 infiltration promoting dehydration reactions at lower temps

Question 11

What are the 5 pieces of evidence for metasomatism and melting in granulite origin?

Answer 11

CO2 dominates fluid inclusions in granulites so CO2 present in lower crust
Temps must be high enough to cause melt
Granulites characteristically chemically depleted
18O reflects original lithological variation which argues against flushing by fluids
Small scale field evidence of CO2 rich fluid infiltration in veins

Question 12

What two main types of tectonism effects granulites?

Answer 12

Isothermal decompression - uplift immediately follows peak metamorphism
Isobaric cooling - Cooling follows peak metamorphism, granulite above chamber has clockwise P-T path, below chamber - anticlockwise P-T path