NORTHERN ALABAMA PIEDMONT GEOCHRONOLOGY: STATUS AND CHALLENGES

The geochronological framework for the northern Alabama Piedmont was established in the 1970s through U-Pb zircon, Rb-Sr whole rock, and conventional K-Ar ages by the authors and conventional K-Ar ages by other investigators. There have been only a few recent investigations utilizing modern techniques, but their contributions have been important. A GIS will document all known geochronology for the northern Alabama Piedmont in the context of the regional geologic setting.

There are several major challenges for future work. (1) The significance of Ordovician igneous activity has been emphasized recently by new zircon ages (McClellan and Miller, 2000) which confirm the original ages for the Hillabee metavolcanic rocks in the Talladega belt. The Elkahatchee Quartz Diorite (approximately 490 Ma) and the Kowaliga Gneiss (approximately 460 Ma) are two of the largest igneous rock units in the eastern Blue Ridge. The Hillabee volcanics are almost synchronous with the Kowaliga Gneiss and, as proposed originally in 1978, are interpreted as the volcanic expression of the same magmatic arc. It is inconceivable that there was no major Taconic regional metamorphism but isotopic evidence has not been found. (2) A suite of trondhjemitic plutons in the northern Alabama Piedmont is undoubtedly approximately 370-375 Ma but inherited zircons and very low Rb/Sr ratios have not allowed precise age determinations in the past. Ion microprobe analyses are critical for these rocks which have varying relationships to the major Acadian phase of regional metamorphism. (3) There is new isotopic evidence for the post-Acadian history (e.g. McDonald and others, 1999; Steltenpohl and others, 2001). This needs to be investigated in a systematic manner.

Isotope studies in the northern Alabama Piedmont have always been limited by poor exposures, intense chemical weathering, and complex but poorly understood geology. It is critical that detailed geochronologic studies utilizing multiple techniques and systematic sampling be combined with a new generation of field and petrographic studies in order to more completely understand this critical region in the development of the southern Appalachians.