Introduction to Metamorphic Petrology

1
Introduction to Metamorphic Petrology
2
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

3
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?

4
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
5
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

6
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.
7
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

8
Some Background

• Metamorphism is continuum between diagenesis and
  melting

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

9
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

10
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.
11
(No Transcript)
12
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.
13
(No Transcript)
14
(No Transcript)
15
continental rifts (orange) crustal extension via
normal faulting leads to transport of heat to
shallow levels, followed by cooling to a normal
thermal gradient.
16
(No Transcript)
17
subduction zones (blue) rapid subduction
transports cold material into the mantle,
producing high P metamorphism.
18
(No Transcript)
19
Protolith!!!

• What is the definition of protolith?
• What are some common protoliths?
• Well return to this question in a lecture or
  two.

20
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.
21
Metamorphic Rocks
22
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.
23
Metamorphic Rocks