Metamorphic Exam

GEOSCI_201

What is a metamorphic rock?

Any rock that has been changed into a new kind of rock through changes in mineralogy, texture, and/or composition

What are the 3 main things that define metamorphism?

Mineralogy, Texture, Composition

What volume percent of continental crust are metamorphic rocks?

60%

What are the 3 agents of metamorphism?

Temperature, Pressure, Fluid

Local Metamorphism is defined as what?

Spatially limited metamorphism

What are the 4 types of local metamorphism?

Contact, Shock, High Strain, Hydrothermal Alteration

What drives contact metamorphism?

Conduction of heat from a hot body (magma)

What is the contact aureole?

Region of country rock that is metamorphosed by contact metamorphism

What rock is typically formed through contact metamorphism?

Hornfels

How do the grains of hornfels fit together?

No crystalline form, small grains fit together like a mosaic, extremely strong

What drives shock metamorphism?

Instantaneous, extreme high pressure

What typically causes impact metamorphism?

Bolide (celestial object) impact

What does PDF stand for?

Planar Deformation Features

What drives high strain metamorphism?

Pressure and Heat (Shear stress)

Where does high strain metamorphism typically occur?

Localized at faults of shear zones

As pressure and temperature increases, the cohesion of the rock does what?

Increases

Example of no cohesion to high cohesion rock

Fault Breccia —> Pseudotachylite

What drives hydrothermal alteration metamorphism?

Heat and Fluid

What does hydrothermal alteration metamorphism form?

Zeolites

Where does hydrothermal alteration metamorphism occur?

Mid-Ocean Ridges

What type of minerals are formed from hydrothermal alteration metamorphism?

Hydrous minerals

What is regional metamorphism?

Spatially extensive metamorphism

What are the three types of regional metamorphism?

Burial, Ocean Ridge (Hydration), Orogenic

What are the two types of orogenic metamorphism?

Subduction and Alpine-Type

What drives burial metamorphism?

Pressure and Heat

What is lithostatic pressure?

During burial metamorphism, the stress (pressure) at a given depth is due to the weight of the overlying column of rock

1 bar of pressure is equal to how many Pa?

1.0 × 10⁵ Pa

1 kbar of pressure is equal to how many GPa?

0.1 GPa

1 kbar of pressure is equal to how many kilometers?

~3.3 km

1 GPa of pressure is equal to how many kilometers?

~33km

What is recrystallization?

The rearrangement/restructure of minerals in relation to one another during metamorphism

What is facilitted by the rearrangement of minerals relative to one another and the reshaping of minerals?

Change in texture

What is Ostwald Ripening?

When larger crystals grow at the expense of smaller crystals

What happens to surface energy due to Ostwald ripening?

It is minimized

Does Ostwald Ripening occur with or without shear stress?

Without

Ostwald Ripening does what to triple junctions?

Increases them

Why does granoblastic texture occur?

To minimize combined surface energy of phases in rock

What is a granoblastic texture?

The triple junction between grains at 120 degrees

Where does granoblastic texture normally take place?

Monomineralic rocks due to similar angles, polymineralic rocks can have wider/skinnier angles

Stress is strongly what?

Directional

Shear stress is a component of stress that is…?

coplanar with material cross-section

Stress is the measure of what?

Internal forces that neighboring particles of a continuous material extent on each other

Strain is the measure of what?

Deformation of material in response to stress

Shear stress is an important factor in metamorphism occuring at what boundaries?

Covergent plate

What is contact metamorphism?

Injection of hot magma into colder “country rock” (often sedimentary but not always) that results int he formation of a new mineral assemblage

P-T conditions of contact metamorphism

High Temperature, Low Pressure

(Contact Metamorphism) Open or closed system

Bulk composition can change in some cases due addition of fluid

Where is contact metamorphism most common and why?

Upper crust due to bouyancy of magmas and the upper crust is dominated by sedimentary rock

Contact metamorphism is located how deep in the crust?

~10km

What facie/s are most contact metamorphism?

Hornfels + Granulite

What is a facies

Groups of minerals that occurs during certain P-T ranges

What is the equation for the conduction of heat?

(Length scale of perturbation)² / (thermal diffusion 10⁶m²/sec)

With time, the temperature from the dike does what?

Decreases

With time, the temperature of the country rock around the dike does what?

Increases

Peak temperature varies with both [what]/[what] from the dike?

Space, Distance

Peak temperature attained at [what] from the dike?

greater times further

The mineralogy preserved in aureole corresponds to what T conditions?

Peak T

Is metamorphic field gradient a geotherm?

No!

What are hornfels?

The characterisic rocks of contact metamorphism

Texture of the hornfels facies

Lack typical metamorphic textures but preserves textural or compositional evidence of original bedding in a sedimentary protolith

The mineralogy within a given hornfels facies depends on what?

Composition of protolith

How to think of lineation?

Stretching

How to think of foliation?

Contraction

What are 3 sub-facies of the Hornfels Facies

Ablite-Epidote, Hornblende, Pyroxene

Can a rock be apart of a specific facies if it lacks the index mineral?

Yes! Facies are based on PT conditions, NOT composition

Index mineral in the Albite-Epidote Hornfel Facies

Ablite-Epidote

Index mineral in the Hornblende Hornfel Facies

Hornblende

Index mineral in the Pyroxene Hornfel Facies

Pryoxene

P-T conditions of the Albite-Epidote Hornfels

350-500^oC — <0.2 GPa

P-T conditions of the Hornblende Hornfels

500-650^oC — <0.2 GPa

P-T conditions of the Pyroxene Hornfels

650-800^oC — <0.2 GPa

What mudrock is enriched in felsic elements?

Shale (Si + Al + K)

What oceanic rock is enriched in mafic elements?

Basalt (Mg + Fe)

What is a skarn?

Product of magmatic fluid interation with a calcareous country rock. (Limestone)

What is a skarn formation?

Occurs when silica-rich fluid from a magmatic intrusion interacts with carbonate country rock (limestone) to form calc-silicate minerals

What are skarn formations often associated with?

Subduction zone plutonism

What can fluids also carry when referring to skarns?

Metals like copper, lead, gold

What is an isograd?

A line on a map that corresponds to the first appearance an index minerals

What color are pure iron minerals?

Red

What color is pure magnesium (forsterite) olivine

White, and rare

What is index mineral?

A mineral that is stable over a characteristic range of pressure and temperature

What is a mineral assemblage?

A group of mineral characteristic of a certain rock type

What Temp is the smectite to illite transition?

180^oC - 250^oC

What are some bad index minerals?

Quartz and Feldspars

Cohesion does what as metamorphic grade increases?

Increases

Texture of schist

Strong foliation defined mica (+other sheet silicates)

Texture of Gneiss

Gneissocity

What is gneissocity?

Compositional banding (typically comprises quartz+plagioclase rich domains separated from domains w/ mafic minerals)

3 Rocks you’d expect in the chlorite zone

slates, phyllites, schists

2 Rocks you’d expect in the biotite zone

Phyllites, Schists

2 Rocks you’d expect in the garnet zone

Phyllites, Schists

What rock would you’d expect in the staurolite zone

Schists

What rock would you’d expect in the kyanite zone

Schists

2 Rocks you’d expect in the sillimanite zone

Schists, Gneisses

Name (atleast) 5 index minerals:

Chlorite, Biotite, Garnet, Kyanite, Sillimanite, Muscovite (quartz, feldspars)

What releasing fluid allow metamorphic rocks to do?

Equilibriate quicker and more extensively

What three minerals are in the aluminosciliate graph?

Kyanite, Andalusite, Sillimanite

Each phase is what in the aluminosciliate graph

Polymorph; same chem formula/different structure

If a point is on the line between two minerals, what does that mean for the crystals? (aluminosciliate graph)

Fully Formed

If a point goes from PHASE A to PHASE B, what does it mean for the mineral formation? (aluminosciliate graph)

Jagged A, B forming around it