Metamorphic rocks

Geology 101

Metamorphic rocks

• Unlike what you may have heard, it’s not just “heat and pressure” applied to existing rocks

• Also, not due to partial melting of rocks

• What it is: “the solid-state reaction of minerals within the rock to produce new minerals and thus new rocks”

Metamorphism

• Metamorphism is a series of chemical reactions that occur to stabilize minerals in relatively high temperature and/or pressure conditions

• Notice this is not freezing minerals, like in igneous rocks

Conditions for metamorphism

Besides heat and pressure, time is needed to complete the chemical reactions and fluids (either water, or more rarely, carbon dioxide) are needed to transport ions

Recognizing metamorphic rx

• Metamorphic rocks subjected to directed pressure typically result in foliated rocks

• The rock fabric gives a sense of the pressure direction

Recognizing metamorphic rx

• Metamorphic rocks subjected to confining (or lithostatic) pressure do not show foliation

• However, because of metamorphic reactions, the rock tends to be the same hardness all the way through; cracks in a rock go through grains, rather than around them – intragrain fracture

Three types of metamorphism

Contact metamorphism• Contact

metamorphism occurs when a hot body (pluton or lava flow) cools in contact with pre-existing cold “country rock”.

• Relatively low pressures (high T, low P)

Dynamothermal metamorphism

• Also known as “dynamic” or “subduction zone” metamorphism

• Relatively low temperatures (high P, low T)

Regional metamorphism

• “Standard” metamorphic conditions - both temperature and pressure rise due to increasing depth of burial

Regional metamorphism

• Characteristic facies – an index mineral or combinations of minerals - are found that indicate a more precise maximum pressure and temperature

• For instance, garnets are geobarometers

PT diagrams

Metamorphic grade diagram

Facies diagrams

Units are °Cand kb (kilobars)where 1 bar isroughly 1atmospheric pressure

So what rocks do you find?

Sed/met rx boundary

• If there is very hot (>200°C) water flowing through rocks, minerals may be hydrothermally altered, or, in some cases, deposited

• Metasomatism creates ore deposits