Download Document

Introduction to
Metamorphic Petrology
Overview of Metamorphic Petrology
• What is metamorphism and why do we care?
• Metamorphism and tectonics
• Metamorphic textures and what they tell us about
conditions of formation
• P-T environments
• Metamorphic reactions, equilibrium
• Controls on metamorphic reactions
• Types of metamorphic rocks
Review: The Rock Cycle
• What is the rock cycle?
• Idea that a rock (igneous, sedimentary,
metamorphic) is not permanent. Igneous rocks
erode to become sedimentary; sedimentary get
subducted and melted…..
• How do metamorphic rocks form?
What is Metamorphism?
Meta = change
Morph = 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?
• Interpretation of the conditions and evolution of
metamorphic bodies, including mountain belts,
subduction zones, cratons
• Metamorphic rocks may retain enough inherited
information from their protolith to allow us to interpret
much 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?
• Use chemistry and physics to interpret textures and compositions
of minerals




field studies involve mapping, field interpretation of structure
and petrology, and collection of samples
laboratory investigations include crystal structure and
orientation studies by diffraction of x-rays, electrons, and
neutrons; texture studies by electron microscopy; and
compositional studies using electron microprobe
theory is based on chemistry and physics, particularly
thermodynamics and kinetics
experiments use high-P, high-T equipment, such as laser-heated
diamond-anvil cells to mimic Earth's interior
Some Background
• Metamorphism is continuum between
diagenesis and melting
Melting is high T end (migmatite)
 Diagenesis is low T end

Gneiss/Migmatite
•
•
•
•
High-temperature limit grades into melting
Over the melting range solids and liquids coexist
Xenoliths, restites, and other enclaves?
Migmatites (“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!!!
• What is the definition of protolith?
• What are some common protoliths?
We’ll return to this question in a lecture or 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.
Metamorphic Rocks
a
b
Metamorphic 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 Rocks