Posts Tagged ‘mylonite’

To continue on last week’s theme, today’s photomicrograph is also of the quartzo-feldspathic region of the “jellybean mylonite.”

(still no scale–sometimes I wonder at my younger self 🙂 )

This picture was taken in XPL and the majority of the grains are either quartz or plagioclase, both of which are commonly black to white in XPL.   In igneous rocks, plagioclase frequently has really nice growth zoning and is then distinguishable from quartz.   In metamorphic rocks, geologists usually have to be more creative to differentiate between the two tectosilicates.   This thin section, though, shows an exception.   In the very middle of the photomicrograph is an example of plagioclase with deformation twinning.   In contrast to growth twins that develop as the mineral forms, deformation twins are due to bending & kinking the structure of the mineral after its already present.   Deformation twins are very common in calcite, plagioclase, and cordierite (though not in the samples I showed from Vermont earlier…).   The presence of deformation twins in this rock is important in understanding the deformation – heating timing of the region.   Deformation twins can form in plagioclase at a large variety of temperatures, but deformed crystals are always less stable than their almost perfect counterparts.   If enough heat is present in the system, the crystal will try to “heal” itself via a number of different processes to become more stable.   In plagioclase, deformation twins rarely survive temperatures above 400 C for very long due to this.   So, either this rock was deformed at less than 400 C or it was deformed at higher temperatures, but cooled off quickly enough to <400 C for this plagioclase to retain its deformation twins.   The presence of assymetrical garnet porphyroblasts and biotite wings tend to indicate that the deformation occurred at higher temperatures, so the latter explanation is probably more likely.

The quartz grains in the image are also showing evidence of more deformed crystals & less deformed crystals trying to “heal” themselves.   Perfect crystals have straight boundaries, but instead this photomicrograph captures a variety of jagged edges and embaying relationships.   In fact, the crystals that embaying into other grains are the more “stable” quartz crystals and they are basically trying to take over the area occupied by the “less stable” embayed crystals.   I have a good video demonstration that I use in class for this (though I don’t know where I got it… if you do, please add a comment!).


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Well, the EQ&V students did a fairly good job of identifying what kind of fault & the sense of shear on it for our last image (#4), but no one successfully ID’d the location.   A few got the state correct, but were still lacking a more precise location.   Due to that, I’m going to open up the “location” question to students currently enrolled in either EQ&V or Structural Geology here at UPJ.   If you think you know the exact location, write a comment to this post (make sure that your identity is not set at anonymous!) and I’ll award 5 pts worth of extra credit to one quiz grade.   Here’s the image again as a reminder:

As for structure #3:

The image is of a quartz (the blockier crystals both large & small) – muscovite (very thin elongate minerals that frequently cut across the blocky crystals) mylonite from the Picuri Range in New Mexico.   The large quartz grains (e.g. the big purple ones in the middle) have undulose extinction (in this view, a single grain has multiple shades of the same color) and occasional subgrains (e.g. the pale-yellow grain within the large purple grain in the left side of the image).   The smaller quartz grains that make up the matrix are elongate, but do not have aligned c-axes.   Quartz has four crystal axes: three are at 120 degrees to each other (a1, a2, a3) and the fourth is at 90 degrees to the other three (c-axis).   The orientation of the c-axis determines quite a few things about what that grain will look like both in XPL and with the accessory plate inserted.   If all of c-axes were aligned for quartz (this doesn’t work for all other minerals), with the accessory plate, all of quartz grains would be about the same color.   Since the grains have a variety of colors, the c-axes are randomly oriented.   The muscovite is present as ribbons (very thin, very elongate) and is aligned parallel to the elongation of the quartz grains.

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Because we’re currently discussing microstructural deformation in structure, I’m going to post another photomicrograph today 🙂   This one is compliments of the 2002 Microstructures class I took with Jane Gilotti at UIowa and is of a mylonite in New Mexico.   The photomicrograph was taken with the accessory plate inserted, which is a bit unusual (I’ll explain why we used it in the answer).   The structure students should:

  1. Identify the minerals present to the best of your ability
  2. Identify what the evidence of deformation is & name each type of microstructure present
  3. Explain why I took this image with the accessory plate inserted

NM-99.   (My notes don’t include how wide the field of view is… hmmm)

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