Week 6 - Metamorphic Rocks

What is metamorphism?

What are migmatites?

Metamorphism: when a pre-existing rock undergoes a change in garin size, mineral content, &/or rock texture, caused by temp &/or pressure

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Migmatites: partially melted & resolidified rock mixed into unmelted remnants

What are the limits of metamorphism?

Diagenesis: when change occurs but temp & pressure conditions are too low to cause significant metamorphism

Melting: when the solidus line for that rock is exceeded, depending on volatiles & chemical composition

How does temperature affect metamorphism?

What are mineralogical changes?

Temperature increases with depth, to stay in equlibrium, mineral recrystalize to bigger versions to become high temp minerals.

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Mineralogical changes: when new minerals form that are stable in the temp & pressure conditions of that rock, what minerals form depends on bulk chemistry of the existing rock.

What are the two types of pressure in metamorphism?

What is the geothermal gradient?

1) Lithostatic pressure: increase due to depth, equal all directions

2) Differential stress: related to plate tectonics, includes compression tension, shearing, deforms material

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Geothermal gradient: rate of increase in temperature with depth

What does pressure cause & what does it mean?

What is brittle vs ductile?

Foliation: alignment of platy material due to pressure, minerals get orientated

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Brittle: when shear stress causes rocks to fracture

Ductile: when shear stress causes deformation without fractures

What are the two main types of metamorphism?

Contact: Caused only by heat from country rock by igneous intrusion, no foliation, causes recrystallisation. High temperature, low pressure, no direct pressure, fast

Regional: Caused by heat & pressure, foliation, recrystallisation. Due to collision of plates, wide range of temp & pressure conditions, slow

What is a protolith and why does it matter in metamorphism?

Protolith: original rock the metamorphic rock came from. Depending on the protolith composition, you can tell whether the rock is regional or contact metamorphism.

What are the regional metamorphisc protoliths and their foliation types?

1) Slate - ‘slaty’ cleavage

2) Phyllite - slightly more foliation that slate

3) Schist - schistose foliation

4) Gneiss - gneissic banding

What is metamorphic grade?

What are metamorphic facies?

Metamorphic grade: refers to degree of metamorphism a rock has undergone, low to high grade

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Metamorphic facies: advanced metamorphic grade described by mineral assemblages indicating certain temp & pressure ranges

What are the minerals in the metamorphic facies?

1) Greenstone - regional, retains characteristics

2) Greenschist - regional, low grade

3) Amphibolite - regional, intermediate grade

4) Bluschist - subduction zone setting

5) Eclogite - base of crust or subducted oceanic crust, highest grade

6) Granulite - high grade

What is the wet melting curve?

What is metasomatism?

As pressure increases, water is squeezed out. As temp increases, hydrous mineral broken apart.

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Metasomatism: process where a rock’s overall chemical composition changes due to hot water adding/removing elements. Hydrothermal metamorphism

What is a skarn?

What are contact aureoles?

Skarns: crystallising plutons intrusing impure limestone, leading to release of CO₂ & it migrates to produce aureoles. Hydrothermal metamorphism

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Conact aureoles: zones in country rock arund igneous intrusion affected by heat of magma

What is shock metamorphism?

What is pyrometamorphism?

Shock metamorphism: occurs in meteroite impact zones

Pyrometamorphism: occurs due to lightning strikes

What are the two fault zone (dynamic) metamorphic rocks?

In metamorohism, what does the rock type depend on?

1) Cataclasite & mylonite

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Protolith & how and where metamorphism occurs