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