Metamorphic rocks

Metamorphism

When pre-existing rock undergoes changes in grain size, mineral content, and/or rock texture

causes of metamorphism

heat, pressure and fluids

Protolith

the original rock from which a metamorphic rock formed

Diagenesis

Some change occurs, but PT is too low to cause significant metamorphism (does not count as metamorphism)

Migmatite

Mixed igneous and metamorphic rock.

geothermal gradient

the rate of change in temperature with depth

Effects of temperature in metamorphism

- minerals recrystallize to form larger versions

- release fluids

- Rock becomes softer -> deformation

- new minerals form (porphyroblasts)

Porphyroblasts

Large crystals of new metamorphic minerals surrounded by fine-grained matrix

bulk rock chemistry

Refers to chemical elements present in a rock

Two types of geological pressure

- confining (lithostatic) pressure: pressure increase with depth due to the weight of overlying rocks

- directional pressure (differential stress): related to tectonic forced (deform minerals)

Foliation

mineral grains are arranged in planes or bands due to pressure, usually mica

brittle behavior

Near the surface, stress causes rock to fracture

ductile behavior

Deep in the earth, high temperature, stress causes minerals to deform without fracturing

contact metamorphism

A change in the texture, structure, or chemical composition of a rock due to contact with magma.

regional metamorphism

A change in the texture, structure, or chemical composition of a rock due to changes in temperature and pressure over a large area, generally as a result of tectonic forces.

Other types of metamorphism...

Impact metamorphism, hydrothermal metamorphism

Contact aureoles

zone of metamorphism surrounding magma

Hornfels

Typical non-foliated, contact metamorphic rock

pelitic protolith

The original rock was a type of mudstone / shale

Metamorphic grade

the degree to which the parent rock changes during metamorphism (high,low,medium) (Highest PT conditions the rock has experienced)

metamorphic facies

Described by mineral assemblages that indicate a certain range of pressure and temperature conditions

Metamorphic Phase Diagrams

Describe the P-T conditions for the stability of different minerals, allow the definition of metamorphic facies

Wet melting curve

Solidus line for rocks containing hydrous minerals

Metasomatism

the process by which a rock's overall chemical composition changes during metamorphism because of reactions with hot water that bring in or remove elements

hydrothermal alteration

The process of hot water and other fluids reacting with solid rock during the formation of metasomatic rocks, such as metallic ore deposits.

Special types of metamorphism

- shock metamorphism: meteorite impact zones

- pyrometamorphism: lighting strikes

Cataclasis

The brittle fracturing and grinding down of existing rock, usually in fault zones

Mylonite

Rock formed during dynamic metamorphism and characterized by foliation that lies roughly parallel to the fault (shear zone)

What does the lithosphere consist of

crust and upper mantle

What do fluids do in metamorphisim

metasomatism, hydrothermal fluids change composition through the introduction of dissolved ions

how does stress direction influence foliation

Foliation forms perpendicular to the maximum compressive stress direction, to align in planes where stress is lowest.

Where does metasomatism occur (example)

mid ocean ridge adds mg and water to basalts

What is recrystallisation vs neocrystalisation

Recrystallisation modifies existing minerals without chemical change, while neocrystallisation forms new minerals stable under the metamorphic conditions.

What minerals are an example of recrystallisation

Sandstone → Quartzite

Quartz grains recrystallise, fuse → stronger interlocking fabric

What minerals are an example of neocrystslisation

Shale → Schist

Garnet/Staurolite/Kyanite → grow

Examples of rocks at contact metamorphism

hornfels, marble

(any non-foliated with capacity for it)

Define a metamorphic facies

stable mineral assemblages under specific P–T conditions.

What are index minerals for metamorphic rocks?

forms only under specific temperature and pressure conditions and can therefore be used to indicate the metamorphic grade of the rock.

What is an example of a metasomatic mineral

Mg + H2O = serpentine