Metamorphic Rocks 1&2

Question 1

What are the temperature limits of metamorphism?

Answer 1

Lower T-limit: 100-150 C
High T-limit: ~1000 C :
Vague, as some of the rock might be melted but some is solid

Question 2

What are the pressure limits of metamorphism?

Answer 2

Low P: No exact minimum
High P: The practical upper limit is 3 GPa (30 kbar ~100km)
Some xenoliths have P=4 GPa

Question 3

Protolith

Answer 3

Un-metamorphic rock, or the rock the meta. rock came from.

Only in equilibrium with the system that it formed in

Question 4

Why does the mineralogical composition change with P and T?

Answer 4

The mineralogy will change so it’s in equilibrium, minimising Gibbs free energy. Thereby creating new stable minerals

Question 5

What causes metamorphism?

Answer 5

Change in:
- Temperature
- Pressure
- State of stress
- Nature of metamorphic fluids (metasomatism?)

Question 6

Effects of temperature

Answer 6

Promotes recrystallization from old unstable minerals to new ones in equilibrium. Typically increases the grain size.

Question 7

Effects of pressure

Answer 7

High pressure –> mineral reactions in the direction that causes a decrease in volume.
Any phase with lower V is favored

Question 8

Deviatoric stress

Answer 8

Difference in stress (σ1≠σ2≠σ3)
Too small to influence phase equlibrium

Question 9

What do you determine metamorphic grade from?

Answer 9

Intensity of metamorphic grade is given my mineralogy and texture

Question 10

Isograd

Answer 10

A line of constant metamorphic grade. a.k.a the boundary between metamorphic zones

can be drawn as lines on a map, but are 3D on surfaces

Question 11

Barrovian zones

Answer 11

Common metamorphic zones around the world, typically due to orogeny. Specific to pelitic protoliths

Question 12

What are the typical Barrovian zones you would find? Classify in increasing metamorphic grade.

Answer 12

Chlorite zone (slates & phyllites)
Biotite zone (
Garnet zone
Staurolite zone
Kyanite zone
Sillimanite zone

Question 13

What do index minerals depend on (in addition to P and T)?

Answer 13

The Bulk Composition of the protolith, as they can undergo the same P and >T conditions and can create different index minerals (Even if protolith are the same)
This makes comparing grades among different protoliths difficult

Question 14

Metamorphic Facies

Answer 14

The range of P-T condition over which a particular common (or range of) mineral assemblage is stable

Question 15

Eskola’s Metamorphic Facies

Question 16

Difference between Isograd concept and metamorphic facies

Answer 16

Isograd: Use single index minerals to infer metamorphic grade

Facies: Use mineral assemblages to infer metamorphic grade