Day 2, Stop 2

Mitchell Dam Amphibolite


The Mitchell Dam Amphibolite, located west of the Coosa river, is approximately 20 meters high and 70 meters wide. The outcrop is in rather poor condition, heavily oxidized and blown up (literally). The amphibolite is composed of very fine grained hornblende, calcium plagioclase, quartz, and garnet. The rock metamorphosed from gabbros and basalts, probably at a depth of 4 to 11 kilometers and at temperatures of 550 to 760 degrees Celsius, possibly from a young oceanic crust containing carbonate sediments on the surface.

During metamorphic processes, the less competent materials, quartz and plagioclase, were deformed and twisted into fishhook nodules and ptygmatic folds, which are now oriented in an east-west direction. Quartz was also formed into oval lenses, with a rind composed of plagioclase and garnet. Small flecks of hornblende were included in the quartz matrix.

Blake examines fishhook nodules (boxed in blue) and quartz lenses (boxed in red).

Planar features on the surface of the outcrop appear to be lineations. These are actually foliations of minerals, seen in three dimensions on the corners of rock, and defined by plagioclase.


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