Introduction to Introduction to Metamorphic PetrologyMetamorphic Petrology

Overview of Metamorphic PetrologyOverview of Metamorphic Petrology

• What is metamorphism and why do we care?What is metamorphism and why do we care?• Metamorphism and tectonicsMetamorphism and tectonics• Metamorphic textures and what they tell us about Metamorphic textures and what they tell us about

conditions of formationconditions of formation• P-T environmentsP-T environments• Metamorphic reactions, equilibriumMetamorphic reactions, equilibrium• Controls on metamorphic reactionsControls on metamorphic reactions• Types of metamorphic rocksTypes of metamorphic rocks

Review: The Rock CycleReview: The Rock Cycle

• What is the rock cycle? What is the rock cycle?

• Idea that a rock (igneous, sedimentary, Idea that a rock (igneous, sedimentary, metamorphic) is not permanent. Igneous rocks metamorphic) is not permanent. Igneous rocks erode to become sedimentary; sedimentary get erode to become sedimentary; sedimentary get subducted and melted…..subducted and melted…..

• How do metamorphic rocks form? How do metamorphic rocks form?

What is What is MetamorphismMetamorphism??

Meta = changeMorph = form

Process by which mineralogical and/or textural change occurs in the solid state as a result of a change in P, T

Why Study Metamorphism?Why Study Metamorphism?• Interpretation of the conditions and evolution of Interpretation of the conditions and evolution of

metamorphic bodies, including mountain belts, metamorphic bodies, including mountain belts, subduction zones, cratonssubduction zones, cratons

• Metamorphic rocks may retain enough inherited Metamorphic rocks may retain enough inherited information from their information from their protolithprotolith to allow us to interpret to allow us to interpret much of the pre-metamorphic history as wellmuch of the pre-metamorphic history as well

The motion of these plates is dictated largely by metamorphic petrology--principally the pressure-induced transformation of relatively low density minerals into high density minerals.

How do We Study Metamorphism?How do We Study Metamorphism?• Use chemistry and physics to interpret textures and compositions Use chemistry and physics to interpret textures and compositions

of mineralsof minerals

field studiesfield studies involve mapping, field interpretation of structure involve mapping, field interpretation of structure and petrology, and collection of samplesand petrology, and collection of samples

laboratory investigationslaboratory investigations include crystal structure and include crystal structure and orientation studies by diffraction of x-rays, electrons, and orientation studies by diffraction of x-rays, electrons, and neutrons; texture studies by electron microscopy; and neutrons; texture studies by electron microscopy; and compositional studies using electron microprobecompositional studies using electron microprobe

theorytheory is based on chemistry and physics, particularly is based on chemistry and physics, particularly thermodynamics and kineticsthermodynamics and kinetics

experimentsexperiments use high-P, high-T equipment, such as laser- use high-P, high-T equipment, such as laser-heated diamond-anvil cells to mimic Earth's interiorheated diamond-anvil cells to mimic Earth's interior

Some BackgroundSome Background

• Metamorphism is continuum between Metamorphism is continuum between diagenesis and meltingdiagenesis and melting

Melting is high T end (migmatite)Melting is high T end (migmatite) Diagenesis is low T endDiagenesis is low T end

Gneiss/MigmatiteGneiss/Migmatite

• High-temperatureHigh-temperature limit grades into limit grades into meltingmelting • Over the melting range solids and liquids coexistOver the melting range solids and liquids coexist• Xenoliths, restites, and other enclaves?Xenoliths, restites, and other enclaves?• MigmatitesMigmatites (“mixed rocks”) are gradational (“mixed rocks”) are gradational

The various plate-tectonic regimes of the Earth cause rocks to experience a broad range of pressures and temperatures, which leads to a broad range of metamorphic minerals and metamorphic rock types.

stable cratons (green): cratons are stable and relatively cold, with 'normal' thermal gradients of ~20 K/km.

magmatic arcs (red-orange): magmatic arcs are sites where heat is transported to shallow levels, producing low P metamorphism.

continental rifts (orange): crustal extension via normal faulting leads to transport of heat to shallow levels, followed by cooling to a normal thermal gradient.

subduction zones (blue): rapid subduction transports cold material into the mantle, producing high P metamorphism.

Protolith!!!Protolith!!!• What is the definition of protolith?What is the definition of protolith?

• What are some common protoliths? What are some common protoliths?

We’ll return to this question in a lecture or We’ll return to this question in a lecture or two…….two…….

A metamorphic facies is a set of metamorphic mineral assemblages each for a specific rock compositions, that form over a specific range of P and T.

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Metamorphic RocksMetamorphic Rocks

Figure 22-1c. Garnet muscovite schist. Muscovite crystals are visible and silvery, garnets occur as large dark porphyroblasts. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Metamorphic RocksMetamorphic Rocks

Metamorphic RocksMetamorphic Rocks