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.
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. |
