Physical Geology lecture outline - Metamorphic rocks
Definition of metamorphism: solid state change in mineral texture or content due to change in pressure, temperature or other conditions (e.g. fluids, deformation) at substantial depth below the surface.
Photograph to right of a metamorphic rock known as a migmatitic gneiss. These are commonly found in the cores of many mountain belts. Metamorphic rocks are highly varied in their character. This rock at one time was a mixture of solid metamorphic rock and layers of magma (which solidified to make the light, granitic layers here). Image courtesy of USGS: http://pubs.usgs.gov/of/1999/of99-168/of99-168-pamphlet.html
What are processes involved in metamorphism?
One way metamorphic rocks are investigated is to cut very thin rock wafers on a glass slide (thin sections) that can viewed under special microscopes. The above view is of a dunite, made mainly of olivine crystals. Note the grains have straight boundaries, and many grains are polygon in cross-section shape. This polygonal texture is very common in mono-mineralic rocks (e.g. marbles and quartiztes in addition to dunites) that have undergone solid state recrystallization. Note also that the grains are slightly longer in one direction than in other directions - this is a deformational farbic, and represents a direction of solid state stretching. Although often introduced as an igenous rock, many dunites are metamorphic in character and come from the mantle. The scale bar in the lower left is 1 mm.
This is another thin section image, this time of a phyllite from Spitsbergen, Norway. The large and colorful grains in a line are muscovite. If you look carefully, especially in the larger grain to the left, you can see the one good cleavage plane that characterizes micas. The mica grains are not only all aligned, but their ends are slightly bent in a consistent manner. This is because of solid state deformation - specifically the rock was being sheared with the top moving to the right relative to the bottom portion during metamorphism. The preferred orientation of the micas helps define the metamorphic farbic of the rock, and in hand specimen this is what gives phyllites their sheen. The surrounding grey grains are mainly quartz, and the protolith for this rock was probably a siltstone or mudstone.
What are the factors that influence metamorphism?
The realm of metamorphism is one that people can't have direct access to (since it happens deep in the earth's interior). Because it is clearly outside of common day experience it can be harder to understand. However, you can think of the transformations that happen to food during cooking and/or baking. The increase in temperature causes new compounds to form and new textures to develop, akin to metamorphism.
How does the temperature increase with depth inside the earth (the increase is known as the geothermal gradient)?
Controls on the 'pressure' (lithostatic) gradient?
Metamorphic paths (from here to down below and back again):
This is a thin section of a metamorphic rock from the deep portions of the Appalachian mountains exposed in the Piedmont of South Carolina. The large purple grain in lower center is garnet. Note the corroded moundary of the garnet and the worm-like mineral growths that surround it (a texture that makes this a symplectite). This worm-like intergrowth is a mix of hornblende (an amphibole) and plagioclase feldspar. The interpretation is that the garnet became unstable and started to break down in the solid state into hornblende and plagioclase. The metamorphic transformation was due to changing conditions during a retrograde path, and did not have the time or energy to go to completion so that a residual garnet core is still left.
Important concepts in navigating metamorphic space:
Paired metamorphic belts and tectonic settings for metamorphism;
Common metamorphic rock types and textures (most of which you will work with in lab).
Part of the marble exterior of Joslyn Art Museum in Omaha Nebraska. This is the Etowah Marble quarried in Georgia, which is famous for its pink coloration. One can also see the deformed layers and folds well displayed. A closer look would show the darker layers to have silvery mica in them. In the original limestone there were muddier layers richer in clays. When metamorphosed these clays changed into the coarse metamorphic micas. Marble is more typically white, but also can be darker. The Joslyn is full of wonderful rocks literally from around the world and makes a great geology field trip.
How can solid rocks deform so ductilely?
Image from the French Alps, from the shores of Lac du Mont Cenis of a green metamorphic mineral that grew in some marbles associated with schists. The fibrous mineral is likely tremolite (one form of asbestos), which helps constrain the conditions of metamorphism. These are amphiboles (double chain silicates), and one can note how the fibers are running more or less the same direction - i.e. there is a preferred orientation of the inequant minerals. This is because the mineral was growing as the rocks was being stretched and sheared in this direction.
Photograph above of glacial valley mountain side in NW Spitsbergen, Norway, showing lighter bands of marble folded together with darker bands of amphibolite. This style of deformation is characteristic of higher grade metamorphic rocks.
Close up of the folded marble and amphibolite layers in the unit above.
Close up of the deformation and folding at an even closer scale in these same rocks.
Two factors allow solid rocks to deform and flow in this manner - an increased temperature (typically >200° C for most metamorphic rocks, and the fact the deformation occurs slowly (low strain rate).
Reference if you would like to pursue an understanding of metamorphism farther: Con Gillen, 1982, Metamorphic Geology; Allen & Unwin, London, 144 p..
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