25 February 2012
One from the UTEP Reading Room
Posted by Callan Bentley
Incipient mylonitization of a granite/granitoid, as displayed in a sample on a shelf of one of the meeting rooms adjacent to the Reading Room of the Geological Sciences building at the University of Texas at El Paso.
Same side, slightly different lighting conditions:
I’d call this a protomylonite. Some of you might prefer the term “augen orthogneiss.” No systematic asymmetry of the feldspar porphyroclasts is catching my eye on either of these surfaces, so I’m going with an off-the-cuff interpretation of pure-shear-dominated deformation. Anyone notice anything telling about this rock that reveals any deeper insights?
Callan Bentley is Associate Professor of Geology at Piedmont Virginia Community College in Charlottesville, Virginia. He is a Fellow of the Geological Society of America. For his work on this blog, the National Association of Geoscience Teachers recognized him with the James Shea Award. He has also won the Outstanding Faculty Award from the State Council on Higher Education in Virginia, and the Biggs Award for Excellence in Geoscience Teaching from the Geoscience Education Division of the Geological Society of America. In previous years, Callan served as a contributing editor at EARTH magazine, President of the Geological Society of Washington and President the Geo2YC division of NAGT.
Do I see a few tiny garnets on the polished section? If so, might support a medium to high pressure & medium temperature metamorphic grade. All the other minerals appear to be typical for granitoids (k-spar, quartz, biotite). (*Gulp*–as a sedimentary guy, I’m treading on thin ice here!)
Hmmm. not sure. I’ll check the original full-res photo…
I guess “garnets” wasn’t what you were thinking of, then. There are a couple of small reddish spots on the polished slab (e.g. a little to the right and above center) that I thought might be what you were getting at with your question. Shot in the dark: missed. 🙂
having just watched the Geologic Journeys shows about the east coast yesterday, I’m wondering if the brownish spots are zircons.