Abstract

The Mitchell Dam Amphibolite is the southwesternmost of a series of amphibolite bodies in the Eastern Blue Ridge of central east Alabama. Several studies have produced various types of geochemical and geochronologic data for these amphibolites, but no systematic data on a continuous section has been available.

Two test cores taken in the area where Mitchell Dam was constructed yielded a section about 138 ft thick but with 21.5 ft missing between the two cores. The lower 98 ft consist of massive hornblende-plagioclase amphibolite displaying some textural variations. The upper 32 ft consist of a metasedimentary unit that commonly shows some cataclasis. Metamorphic mineral assemblages in the rocks are typical of amphibolite facies metamorphism. Quartz, biotite, plagioclase, clinozoisite, and garnet are the common minerals in the metasedimentary units; in addition, green hornblende is added to the assemblage in metagraywackes. Hornblende and plagioclase dominate the amphibolite.

In this study, 65 samples were taken at an average spacing of 1.7 ft (range from 0.5 to 7 ft) to build profiles to show chemical variation throughout the core and determine the protolith of the amphibolites. The amphibolite has bulk compositions characteristic of subalkaline basalts. The compositions follow a tholeiitic trend on an AFM diagram. Rare earth patterns are relatively flat, and high field strength elements are notably depleted, particularly zirconium. Whole rock and trace element geochemistry suggests a tholeiitic protolith formed over a subduction zone, most likely generated in the very early stages of subduction.