Metamorphic

Question 1

What does metamorphic mean?

Answer 1

all happened in solid state

Question 2

What does cotectic mean?

Answer 2

adding a component to a binary eutectic …

Question 3

What are the temperature limits of metamorphism?
Low-T limit

Answer 3

Weathering and diagenesis processes similar to metamorphic processes

˜100-150 °C

Question 4

What are the temperature limits of metamorphism?
High-T limit

Answer 4

Upper boundary is vague - if we heat a metamorphic rock until it melts, when is it igneous? (melt is igneous)

˜1000 °C (rule of thumb)

Question 5

What are the pressure limits of metamorphism?
Low-T limit

Answer 5

Can occur at low P if temperatures are sufficiently high

Question 6

What are the pressure limits of metamorphism?
High-T limit

Answer 6

For our purposes, the practical upper P limit is 3 GPa (˜100 km) for continental crust

Question 7

What is a fundamental assumption?

Answer 7

A rock’s mineralogy changes to achieve equilibrium in the new physical/chemical environment

Question 8

Increase in temperature/grade = ?

Answer 8

= increase in grain size

Question 9

What will minerals do under high pressure?

Answer 9

High pressure causes mineral reactions to proceed in the direction that will cause a decrease in volume

Question 10

What is metasomatism?

Answer 10

A change in bulk composition of the rock

This contrasts with changes in P, T, and stress, which do not change the bulk composition of the rock

Question 11

What is metamorphic grade?

Answer 11

The approximate “intensity” of metamorphism

• Intensity = degree of transformation of the protolith

Most commonly used to imply the maximum T (or rarely, maximum P) of metamorphism

Question 12

What are the different types of metamorphism?

Answer 12

• Contact metamorphism (km scale)
• Regional Metamorphism (10’s-100’s of km scale)
• Other types (hydrothermal, fluid, shock)

Question 13

What are 6 broad types of compositional protolith groups?

Answer 13

• Ultramafic rocks (mantle rocks and cumulates)
• Mafic rocks (gabbros, basalts, and some graywackes)
• Pelitic rocks (shales and sandstones; the most common sediments)
• Calcareous rocks (typically sedimentary limestones and dolomites, ± marls)
• Quartz rocks (cherts, quartz sandstones)
• Quartzo-feldspathic rocks (arkosic sandstones, granites and rhyolites)

Question 14

What is foliation?

Answer 14

Any planar fabric element

Like a stack of books

Question 15

What is lineation?

Answer 15

Any linear fabric element

Like a stack of arrows

Question 16

Which minerals are in the Chlorite Zone (slates and phyllites)?

Answer 16

chlorite, muscovite, quartz, albite

Question 17

Which minerals are in the Biotite Zone (phyllites and schists)?

Answer 17

biotite, chlorite, muscovite, quartz, albite

Question 18

Which minerals are in the Garnet Zone (schists)?

Answer 18

garnet (almandine), biotite, chlorite, muscovite, quartz, albite, ± oligoclase

Question 19

Which minerals are in the Staurolite Zone (schists)?

Answer 19

staurolite, biotite, muscovite, quartz, garnet, plagioclase, ± chlorite

Question 20

Which minerals are in the Kyanite Zone (schists)?

Answer 20

kyanite, biotite, muscovite, quartz, plagioclase, ± garnet, ± staurolite

Question 21

Which minerals are in the Sillimanite Zone (schists and gneisses)?

Answer 21

sillimanite, biotite, muscovite, quartz, plagioclase, garnet, ± staurolite, ± kyanite

Question 22

What are the limitations of index minerals and isograds?

Answer 22

Rocks with different bulk compositions undergoing the same P and T conditions can yield different index minerals (even if the rocks are the same type of protolith!)

–> This creates difficulties in comparing grades among different protoliths

Question 23

What is a metamorphic facies?

Answer 23

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

Question 24

What is the main difference between a facies and an isograd?

Answer 24

Facies defined by a composition of different minerals

Isograds based on occurence of one index mineral

Question 25

What is the mineral assemblage for greenschists?

Answer 25

Chlorite + **epidote** + **albite** ± actinolite

Question 26

What is the mineral assemblage for amphibolite?

Answer 26

>50% **hornblende** + *plagioclase* ± quartz ± sphene ± garnet

Question 27

What is the mineral assemblage for granulite?

Answer 27

**Orthopyroxene** + **clinopyroxene** + plagioclase ± garnet

Question 28

What is the mineral assemblage for eclogite?

Answer 28

Garnet (**pyrope**) + clinopyroxene (**omphacite**)

Question 29

What is the mineral assemblage for blueschists?

Answer 29

Albite + chorite + **glaucophane** + **lawsonite**

Question 30

What is the Metamorphic Field Gradient (MFG)?

Answer 30

A P-T path Any large-scale traverse **up grade** through a metamorphic terrane should cross through a **sequence of metamorphic facies**

Question 31

What is the phase rule?

Answer 31

Used to predict the number of co-existing phases a rock could produce at equilibrium

Question 32

What is Gibbs Phase Rule?

Answer 32

𝐹 = 𝐶 + 2 − Φ * 𝐹 = degrees of freedom * 𝐶 = # of components * Φ = # of phases coexisting * 2 = 𝑃 and 𝑇

Question 33

What does a compatibility diagram show?

Answer 33

How the **mineral assemblage** of a metamorphic rock in *thermodynamic equilibrium* varies with composition at a **fixed (or limited range of) temperature and pressure**

Question 34

What does the ACF Diagram (Eskola) focus on?

Answer 34

Only on plotting minerals that **appeared or disappeared** during metamorphism (i.e., only those minerals that act as indicators of metamorphic grade) i.e. at reactions A, C, and F = pseudo-components calculated on molecular basis

Question 35

Which components are ACF diagram projected from?

Answer 35

3 “component” system (A, C, F) that is projected from albite, k-feldspar, apatite, and quartz (Remember: only an assumption)

Question 36

What is the AKF Diagram (Eskola)?

Answer 36

Pelitic protoliths, high in Al2O3 and K2O, but low in CaO

Question 37

What is the A(K)FM Diagram (Thompson)?

Answer 37

Also pelitic protoliths Thompson noted that **Fe** and **Mg** are **not partitioned equally** between the various solid solution mafic minerals in most metamorphic rocks (including pelites)

Question 38

What is the key difference between A(K)FM and AFK?

Answer 38

Fe and Mg are treated as separate components in the A(K)FM diagram

Question 39

Can you describe some types of metamorphic reactions?

Answer 39

* polymorphic transformations (like Ky/Sil/An) * solid-solid net-transfer reactions (minerals, not melts) * devolatilization reactions (+/- H2O, CO2) * continuous reactions (occur over a range of P and T) * ion exchange reactions (mainly cation, e.g. Fe/Mg in OPX/CPX)

Question 40

What is the take-home point of devolatilization reactions?

Answer 40

The temperature of an isograd based on a devolatilization reaction is sensitive to the partial pressure of the volatile species involved

Question 41

What are discontinuous reactions?

Answer 41

Occur at fixed T for a given P

Question 42

What are the reactions when plotting on compatibility diagrams

Answer 42

Polymorphic (tie-lines don't change) Crossing tie-line Terminal Rotating tie-line (ion-exchange)

Question 43

What is the main take away from migmatites?

Answer 43

Leucosome in some migmatites may represent **partial melts**, whereas it may be the product of **metamorphic segregation** in others

Question 44

What are common protoliths for mafic rocks?

Answer 44

In short: **volcanic rocks and greywackes** * basaltic to andesitic *volcanic* rocks * gabbros to diorites *intrusive* rocks * mafic graywacke (sandstone) | REMEMBER: hydration needed for development of mafic metamorphic minerals

Question 45

What are 2 key mineralogical changes going from Greenschist to Amphibolite Facies?

Answer 45

* Transition from albite to oligoclase (plag becomes more Ca-rich) * Transition from actinolite to hornblende

Question 46

What have we gotten rid of in the Granulite Facies?

Answer 46

All hydrous minerals