Class Notes - Metamorphism

Introduction

Metamorphic rocks are "changed rocks". A mineral assemblage stable at low temperatures and pressures may not be stable at elevated temperatures and pressures. As temperature and/or pressure increases "old minerals" may change (typically they increase in size) or new minerals may form. In this image a basalt flow has baked the underlying limestone.

The realm of metamorphism can be portrayed on a plot of temperature versus pressure.

Types of Metamorphism

Different types of metamorphism are displayed in the plot above:
• Contact Metamorphism - reactions driven primarily by temperature at relatively low pressures - associated with the injection or extrusion of a magma or a lava

• Dynamic Metamorphism - reactions driven primarily by pressure at relatively low temperatures

• Regional Metamorphism - no discernible source of heat (no nearby magma chamber, for example) - with increasing depth the temperature and pressure increase

• High Pressure - Low Temperature Metamorphism - pressures are higher than would be predicted from the expected geothermal gradient (field lies above the straight line in the figure given above)
At which of the following plate boundaries might you expect to find evidence of dynamic metamorphism?

divergent margin
transform margin
convergent margin


What kind of metamorphism might you expect to find taking place deep in the sediments that have accumulated in the Gulf of Mexico?

regional metamorphism
contact metamorphism
dynamic metamorphism


Metamorphic Processes

Prograde metamorphism refers to metamorphic reactions that take place with increasing temperature (and pressure). Retrograde metamorphism refers to metamorphic reactions that accompany decreasing temperature (and pressure).

During metamorphism the texture of the parent material (the protolith) may change or its mineralogy may change -- new minerals may grow at the expense of the original minerals. Metasomatism occurs IF the bulk composition of the protolith changes.

Metamorphic processes tend to cause small grains to "grow" (recrystallization) into larger grains. Recall that this is the opposite of the tendency in sedimentary processes.

New minerals may form during metamorphism:

Calcite + Quartz = Wollastonite + Carbon Dioxide:

CaCO₃ + SiO₂ = CaSiO₃+CO₂

If the CO₂ "escapes" Wollastonite will be preserved in the metamorphic rock. That is, without CO₂ the reaction will not be reversed.

Other metamorphic minerals include:
• garnet

• kyanite

• tourmaline

Certain minerals are considered to be highly "reactive" during metamorphism. Minerals like mica and amphibole contain (OH-) which may be released as water during metamorphism. Minerals like calcite and dolomite may release carbon dioxide.

Metamorphism and Plate Boundaries

We have looked at several settings in which temperatures and pressures change as geologic processes operate:
• At a divergent margin basaltic lava flows out on the relatively cold sea floor. Heat flows from the magma into the cold sea floor sediments/rocks and bakes them : contact metamorphism

• At a convergent margin cold sea floor is subducted beneath another plate. As the oceanic crust is pushed (pulled?) to greater depths it gradually heats up but remains colder than the other plate until great depths are reached : high pressure - low temperature metamorphism

• When two plates containing continental crust converge a suture zone is formed and the continental crust thickens. Temperatures and pressures at the base of the thickening crust gradually increase : regional metamorphism

• When two plates "slide" past one another the pressures (directed) can be quite high whereas the increase in temperature due to friction tend to be relatively low : dynamic metamorphism.

• At some combination of temperature and pressure the material may begin to partially melt and an Igneous process is initiated.

Classification of Metamorphic Rocks

• Banded (mineralogically layered) : Gneiss

• Not Banded (no discernible mineralogical layers) - not foliated (no discernible planes of weakness) :
  • granofels if coarse grained
    
  • hornfels if fine grained

• Not Banded (no discernible mineralogical layers) - foliated (discernible planes of weakness - typically produced by the alignment of the cleavages of clay minerals) :
  • slate - fine grained
    
  • phyllite - "surface "sparkles"
    
  • schist - coarse grained

  We have talked about a number of common Igneous and Sedimentary rocks. Each of these rocks may be subjected to metamorphic conditions. The goal of the geologist is to "see through" the effects of metamorphism and determine the parent rock type.

  • Quartzite : Quartz-rich sandstones

  • Marble : Limestone or Dolomite

  • Granite Gneiss : a Granite Granofels: Granite

  • slate / phyllite / schist : shales and mudstones

  Consider the following scenario. A granite magma invades a limestone unit which contains quartz and clay minerals. At the contact between the magma and the country rock heat flows from the magma chamber into the country rock in an attempt to being the two bodies into thermal equilibrium. In general, as the distance from the magma chamber increases, the temperature decreases. Therefore, one might observe mineralogical zones around the granite with the highest temperature minerals found closest to the granite. At some distance from the contact the country rock will show no effects of contact metamorphism.

  One would expect a temperature gradient in thickened continental crust in a convergent zone. The greater the pressure and temperature the higher the degree of metamorphism. Minerals that form during metamorphism may allow estimation of the temperatures attained. Such minerals are termed index minerals by some geologists.

  • Sillmanite - high grade conditions

  • Garnet - intermediate grade conditions

  • Chlorite - low grade conditions

Which of the following "protoliths" is least likely to exhibit a response to low to moderate grade metamorphism?

shales
mudstones
quartz arenites
carbonates


Which of the following "protoliths" is most likely the parent of an alkaii feldspar - quartz - biotite gneiss?

shale
mudstone
quartz arenite
granite


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Copyright by John C. Butler, July 29, 1995

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