EVALUATING THE SIGNIFICANCE OF ALLEGHANIAN METAMORPHISM IN THE SOUTHERNMOST APPALACHIANS

The Appalachian Mountains expose one of the most-studied orogenic belts in the world. However, metamorphic pressure-temperature-time (P-T-t) paths for reconstructing the tectonic history are largely lacking for the southernmost end. The limited data available, indicate that 1] Alleghanian metamorphism is more widespread in the south than in the far better known New England segment of the orogeny, 2] lithostratigraphy form an arcuate - synformal outcrop pattern in Alabama with an wavelength of ~30 km which is not seen at the same scale to the north. The Alleghanian amphibolite facies metamorphic rocks in the south may reflect greater crustal thickening in the south and/or the geometry and timing of Laurentian-African collision. The large-scale arcuate outcrops suggests a tectonic style that includes large-scale crustal flow.

Neoacadian and Alleghanian amphibolite facies metamorphism have been identified in the eastern Blue Ridge in Alabama. This is documented by garnet Sm-Nd ages which range from 357±2 Ma in the north to 323-320, 330±2, and 331±4 Ma further south. The extent of the early this metamorphic belt has not been delineated. Although there are a significant number of Ar-Ar cooling ages, we lack peak metamorphic ages for amphibolite facies metamorphism in the Georgia and most of the Carolina Blue Ridge rocks. Our current data do not constrain the timing of peak metamorphism within the arcuate structure of the Alabama Inner Piedmont remains unknown.

In this contribution, we report preliminary results for a project to determine the extent of Alleghanian metamorphism and evaluate P-T paths for samples across the southernmost part of the orogeny. Metamorphic rocks are being analyzed in order to construct metamorphic P-T-t paths utilizing isochemical phase diagram sections and garnet Sm-Nd ages. Our preliminary garnet ages indicate that evidence for Taconic metamorphism is largely lacking in Alabama and much of the garnet growth in the Blue Ridge of Alabama is early Alleghanian. The P-T-t paths for these samples are compatible with crustal thickening during garnet growth. Additional ages and P-T-t paths are sorely needed in order to determine the Alleghanian tectonic history of the southernmost Appalachians.