Ig Met Pet Lecture 17: Intro to Metamorphic Petrology

Metamorphic petrology

Study of metamorphic rocks

Three categories of metamorphism

Dynamic metamorphism, thermal metamorphism, dynamo-thermal metamorphism

What is the difference between the three categories of metamorphism?

Dynamic is pressure related, thermal is temperature related, and dynamo-thermal is both

What is the difference in temperature range and state of matter for igneous and metamorphic rock formation?

Igneous rocks form from the cooling of magma at 700-1200°C, while metamorphic rocks form from solid state changes at ~200-700°C

Metamorphic grade

The amount of change a rock goes through, based on the intensity of metamorphism from differing agents/conditions

What are the metamorphic grades

Low, intermediate, and high

What processes come before and after the metamorphic grades?

Lithification and partial melting of granites

Metamorphism (defined by the limits)

A process of change whereby any rock recrystallizes in response to new conditions to produce a new rock

Where do we get metamorphism?

Where plate tectonics are active, such as subduction zones, and hydrothermal vents

What tectonic process does the first path on the metamorphic facies chart correspond to?

Contact (thermal) metamorphism

What tectonic process corresponds to the second metamorphic facies path?

Volcanic arc

The third metamorphic facies path corresponds to which plate tectonic process?

Collisional mountain belt

What tectonic process matches with the fourth metamorphic facies path?

stable continent

What does the fifth metamorphic facies path correspond to tectonically?

Accretionary prism

What are the key things metamorphic petrologists hope to determine from the mineralogy and texture of a rock?

The protolith, conditions of metamorphism, and structural history

What conditions/factors contribute to metamorphism?

Temperature and pressure, presence and composition of fluid, and compositional changes to rock

Which agent/change of metamorphism is the most important

temperature

What does increasing temperature do?

Promotes recrystallization, drives endothermic reactions, and overcomes kinetic barriers

Why is fluid important for metamorphism?

Most metamorphic minerals require the presence of fluid to form

What do fluids do?

Exert control on the P and T of reactions, extent of reaction progress, and heat + chemical transport

Where are fluids commonly derived from?

Devolatilization of metamorphic rocks or from fluids expelled by magmas

Metasomatism

Chemical change associated with fluid transport

What are the six types of metamorphism?

Regional, thermal, contact, subduction, burial, shock

Burial metamorphism

The shallowest and coolest, at temps of <300°C and low pressures. Caused by burial and compression of marine sediments

Hydrothermal metamorphism

Relatively shallow, but hot, usually related to seafloor spreading and mafic rocks

Ocean floor metamorphism

Affects oceanic crust at ocean ridge spreading centers, resulting in considerable metasomatic alteration

What metasomatic alteration occurs with ocean floor metamorphism?

Loss of Ca and Si, and gain of Mg and Na from exchange between basalt and hot seawater

How are hydrothermal and ocean floor metamorphism related?

Ocean floor is a type of hydrothermal metamorphism

Contact metamorphism

Low pressure, high temperature due to location adjacent to igneous intrusion

Contact aureole

The metamorphic area generated during contact metamorphism

What are rocks closest to intrusion called in contact metamorphism?

Hornfels

Regional Metamorphism

Moderate temperature and moderate to high pressure, related to mountain building

Other kinds of metamorphism

Cataclastic and shock, caused by faulting or impact

Types of protoliths

ultramafic, mafic, pelitic, carbonates, quartz, quatzo-feldspathic

Why are protoliths important?

They dictate mineral phases in final metamorphic rock

What is the progressive nature of metamorphism?

It is assumed that rocks pass through a prograde, peak, and retrograde degree of metamorphism, and that peak conditions are preserved because retrograde metamorphism is minor

What happens during the prograde path of PTt?

Minerals formed are leftover from the original rock, and not stable at peak conditions. If they do survive, they are relict minerals

What occurs at maximum/peak conditions?

The mineral paragenesis is likely to form under these conditions

What occurs during retrograde metamorphism?

A new mineral may form, possibly replacing some of the mineral paragenesis

Index mineral

The first appearance of a new mineral as metamorphic grade increased

Isograd

A representation of the first appearance of a particular metamorphic index mineral in the field

Can index minerals still be stable in higher grade zones?

yes, isograds do no represent disappearance