Geology, Petrology, and Geochronology of Rocks in the Our Town, Alabama Quadrangle

Abstract

The area of the 1:24,000 Our Town Quadrangle of east central Alabama is underlain principally by generally undifferentiated metasedimentary units, the Emuckfaw and Wedowee Groups, which have been intruded by major southern Appalachian batholiths, the Elkahatchee Quartz Diorite and the Kowaliga Gneiss, and the less volumetric Zana Granite. Detailed field mapping provides lithologic detail for the Emuckfaw Group allowing for differentiation into multiple subunits. The subunits are defined based on the occurrence or absence of amphibolites and garnet-amphibole bearing quartzites of metavolcanic origin. In addition to this distinction, further differentiation is based on the occurrence of late fine-grained highly foliated metagranites that have intruded the Emuckfaw Group. Previous reconnaissance work indicated the presence of two texturally interesting lithologic units that commonly occur within the Kowaliga Gneiss and the Emuckfaw Group. These two lithologies can be generally described as augen gneiss and megacrystic gneiss. The megacrystic potassium feldspar grains were previously thought to have been generated by metamorphic processes acting upon the Kowaliga Gneiss or on bulk compositionally appropriate metasedimentary sections of the Emuckfaw Group. Detailed petrographic and geochemical analyses indicate that the megacrysts were not generated by a metamorphic process but rather a magmatic process. The recognition of a magmatic origin reveals exposures of the Kowaliga Gneiss that preserve the initial megacrystic crystallization texture, whereas most outcrops reflect augen gneiss derived from mylonitization of the megacrysts. Whole-rock major and trace element analyses of the megacrystic and augen varieties of the Kowaliga Gneiss indicate a strong correlation with previously reported geochemical data for the Zana Granite. U-Pb SIMS dates on zircons from the megacrystic Kowaliga Gneiss yield an age of crystallization of 441 ± 6.6 Ma with no systematic or significant variations between the cores and rims. Previously reported tentative dates for the Zana Granite imply a crystallization age of 439 Ma. Strong similarities in geochemistry and crystallization ages suggest these two intrusions, previously mapped as individual suites, should be considered one intrusive suite. In the study area, the epidote-amphibolite to amphibolite facies of the Emuckfaw Group and Kowaliga Gneiss are juxtaposed with the lower grade (greenschist to lower amphibolite facies) Jacksons Gap Group that structurally overlies them. These two terranes are separated by the Abanda fault that was previously considered a northwest-directed thrust. Detailed structural analysis of retrograde mylonites and phyllonites along the Abanda fault documents an oblique dextral-and-normal sense of movement. This oblique dextral-and-normal movement is similar to movement reported for the Alexander City fault in the northeast section of the quadrangle. In addition to constraining fault kinematics for the Abanda fault, field mapping also reveled two sets of sub-vertical brittle normal faults. Faults of the NW-SE-trending set have <2 m displacement and cut the oblique dextral-normal phyllonites of the Abanda fault. Faults of the WSW-ENE set rarely cut Abanda phyllonites and are similar to Mesozoic(?) faults known to cut the Alexander City and Towaliga mylonite zones. A 40Ar/39Ar date of ~317-315 Ma on cataclastically deformed muscovite from one of these faults implies either Pennsylvanian movement or failure of the crystals to record any record of this disturbance.