Ig Met Pet Lecture 24: Metamorphism of Carbonates

Contact zones

Moving magma around in the crust puts melt in contact with rocks of a different composition, driving chemical and mechanical interactions

Depth of Emplacement

The depth of plutons and contact aureoles in the crust

What does the depth of emplacement affect?

Structural and textural features of both the pluton and size of the aureole

Three primary zone of depth of emplacement

Epizone, mesozone, catazone

What depths is contact metamorphism likely to occur at, and what does it do?

At depths greater than 5 km, intrusions are likely to have thermal and chemical effects on the country rock

What is the size of an aureole a function of?

Size and temperature of intrusion, thermal conductivity, and amount of fluids

What will a larger magmatic intrusion result in?

More heat, larger contact surface, and longer time to cool

Which metamorphic facies is produced by contact metamorphism?

The hornfels series

What are the two four of carbonate protoliths?

carbonate dominant, dolomite dominant, mixed with carbonate mud, mixed with siliciclastic mud

What happens to pure and impure carbonates during metamorphism?

Pure carbonates become marbles, while impure ones form calc-silicates

If a pluton is hydrous, what happens?

Thermal effects are combined with a chemical gradient, established by silica-saturated fluids

What does a chemical gradient do to the contact aureole?

Makes it gradational

Why does fluid composition matter?

Changing the proportion of CO2 in a fluid affects the temperature at which a particular metamorphic reaction occurs

Dehydration reaction

A → B + H2O

Decabonation

A → B + CO2

Combined H2O and CO2 Reactions

A + B → C + CO2 + H2O

Prograde reactions which consume H2O and release CO2

A + B + H2O → C + CO2

When are prograde reactions which consume H2O and release CO2 suppresed?

As H2O is used up

Prograde reactions which consume CO2 and release H2O

A + CO2 → B + C + H2O

When are prograde reactions which consume CO2 and release H2O suppressed?

As CO2 is used up

If you ad head and fluids to dolostones, what happens?

The mineralogy changes

Rock-Buffered

The fluid will be used up and the overall composition will be controlled by the chemistry of the rock itself

Fluid-buffered

Rock volume will not be enough to consume all the fluid, fluid will control chemistry

Skarns

Chemical change from metasomatism at contact

Two types of skarns

Endoskarn and exoskarn

Endoskarn

Skarn like assemblages in the intrusion

Exoskarn

Replacement of carbonate country rock

Why are skarns valuable?

They are often linked to economic deposits