CALC-SILICATE PODS IN NORTH COLUMBUS METAMORPHIC COMPLEX AND IN JUNCTION CITY MYLONITE GNEISS OF WEST-CENTRAL GEORGIA: DIOPSIDE+GARNET METAMORPHISM FOLLOWED BY ZEOLITE CRYSTALLIZATION

Calc-silicate pods are minor but common parts of the North Columbus Metamorphic complex, a migmatitic unit that forms the central portion of the Lake Oliver Synform, the main structure of the Uchee Terrane of Georgia and Alabama. They are rare in the Junction City mylonitic orthogneiss, a possible western extension of the Carolina terrane. Amphibolite, pelitic gneiss, and metamorphosed mafic rocks are additional pod lithologies.

Though most calc-silicate pods are dominated by diopside with minor garnet, some pods contain the skarn-like assemblage of a diopside marble core surrounded by diopside, garnet, actinolite, epidote and plagioclase. Blue apatite is a common accessory. Where the marble has been dissolved, calc-silicate minerals are left behind in relief. Secondary calcite and zeolites partially fill these cavities. Stilbite, analcime, chabazite, laumantite, and mesolite have been identified by habit. In addition, blue barite is found in one cavity from Junction City.

We interpret these pods to be part of lithologically diverse terranes that were involved in migmatization during regional metamorphism (Uchee belt pods), or intruded by plutons (Junction City pods). Later, the calcite was dissolved from some pods and zeolites crystallized in the cavities probably under hydrothermal conditions.