MITCHELL DAM AMPHIBOLITES: GEOCHEMISTRY AND PETROLOGY OF CORE HOLE SAMPLES, SENIOR CLASS PROJECT

The Mitchell Dam Amphibolites represent the Southern portion of a Southwestern/Northeastern trend of amphibolite bodies located in the Coosa block of the Northern Alabama Piedmont. This trend is bounded on the southern side by the Goodwater-Enitachopco Fault and bounded on the northern side by the Hollins Line Fault.

Many of these amphibole bodies have been sampled but sparse chemical analysis has been done. There were two cores from the Mitchell Dam area, Core B was taken from the site of the parking lot for the dam and Core 2 was 10 feet away at the river’s edge. Core 2 has a total length of 86 feet. For this study, we selected 46 samples. The rocks cored from Core 2 consist of Kalk’s (1972 thesis) lithology III. Lithology III consists mostly of hornblende-plagioclase based on the petrography. Seven representative samples were chosen to perform SEM/EDS analysis of polished thin sections.

Our SEM/EDS analysis confirmed that hornblende and plagioclase were the dominant phases. The minor phases include sphenes, zircons, quartz, pyrite, apatite, ilmenite, possible aluminum silicates, potassium feldspars and what we believe to be relic pyroxenes. The Mg number of the hornblendes ranged from 65-73 and no zonation was observed. The majority of plagioclase were oligoclase ranging from An₃₅Ab₆₅ to An₁₈Ab₈₂. Rare potassium feldspars were analyzed with An₃Ab₆Or₉₁ endmembers. In a few samples, calcite veins crossed the amphibolite bodies which we did not analyze.

Forty-six samples were fused into glass beads and major element chemistry was determined on an SEM/EDS system. The results showed a subalkaline characteristic with magnesium numbers ranging from 56 to 77. A tholeiitic trend was displayed on an AFM diagram. Depth element variation diagrams show the following: Silica and titanium have minor decreases with depth. The Mg number indicates that fractionation occurred more at the top and bottom of the core with less fractionation in the middle. The geochemistry and phase chemistry of the samples suggests a tholeiitic protolith, most likely generated in a mid ocean ridge or back arc basin setting. Preliminary trace element analysis is underway.