Stone Mountain Granite, Georgia

Stone Mountain is a granite exfoliation dome located outside of Atlanta, Georgia. It has a large bas relief memorial to Confederate generals carved on one side, a tram to the top, a railroad around the base, and a large building and a bunch of antennas on the top. The granite is fine-grained and gray.

This is on top of the Stone Mountain exfoliation dome. In the foreground are a bunch of small blackish clots (Uncle for scale) which stand out in relief. These have a different mineralogy which makes them more resistant than the surrounding granite. In this case, the black mineral clots are more numerous in a tabular zone that cuts across the exposure.

 

On close examination (sorry about the image quality) the lumps include an elongate black mineral which turns out to be tourmaline (schorl). [Width of the image is about 10 cm.]

Size and Khairallah (1989) have interpreted these as resulting from metasomatism by a boron-enriched liquid during a late sub-solidous period. They described the zoned nature of the pods with tourmaline being most common in the middle of the pods showing a light-colored biotite-free zone surrounding the pods.

I would suggest another alternative, that the tourmaline pods are mostly digested pelitic xenoliths. Reaction with the granite has altered them from their original compositions. I originally thought that the boron would have been present in the xenoliths themselves. But after discussing boron saturation in granitic melts with David London, I decided that boron would have moved from the granitic melt into the xenoliths. Tourmaline formation would be restricted in the melt by the lower Fe-Mg ratio of the melt (relative to in a pelitic xenolith).

The sheet-like concentration of these pods is the result of planar trains of xenoliths which can be generated in several fashions. Size and Khairallah (1989) suggest that Stone Mountain granite was a sheet-like intrusion. I suspect a dike-based original for the planar swarm of pods as well, with the tourmaline pod zone representing a xenolith-bearing granite dike that cuts the main granite.

Size, William B., and Khairallah, Nayla (1989) Geology of the Stone Mountain Granite and Mount Arabia Migmatite, Georgia. in W.J. Fritz, ed. Excursions in Georgia Geology, Georgia Geological Society Guidebooks, v. 9, no. 1, p. 149-177. 

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