GEOLOGY OF THE DADEVILLE COMPLEX IN THE CAMP HILL AND THORNTON 7.5′ QUADRANGLES, APPALACHIAN INNER PIEDMONT OF ALABAMA

The Dadeville Complex (DC) is a large klippe located in the Alabama Inner Piedmont of the southern Appalachian orogen lying immediately above rocks of the eastern Blue Ridge and Brevard Zone, in the core of the Tallassee synform (TS). Detailed geologic mapping in two 7.5’ quadrangles (Camp Hill and Thornton) has revealed the following relationships and characteristics of the region. The DC consists of predominantly metavolcanic and metaplutonic rocks and minor metasedimentary rocks that experienced kyanite-silliminite grade metamorphism and multi-stage deformation, and was lastly folded by the open TS. Structural field data outline the overall nature of the TS in the DC. These quads contain the two major DC lithostratigraphic sequences, from SE to NW: the Ropes Creek amphibolite (RCA), a bimodal metabasalt/metatuff, metadacite at the base, overlain by the Agricola schist (AS), a metaturbidite consisting of metapelitic, metagreywacke, and minor metabasalt. These sequences are intruded by felsic plutonic rocks, with the Chattasofka Creek gneiss (CCG) (granite) intrusive into the AS, and the Camp Hill gneiss (CHG) (tonalite) intrusive into the RCA. Also intrusive into the AS is an ultramafic-mafic complex (UMC) consisting of the Doss Mountain suite (DMS) and the Slaughters Gabbro suite (SGS). The DMS is characterized by having an abundance of interlayered metaorthopyroxenite and metanorite, and their metamorphosed equivalents. The SGS consists of both olivine and non-olivine bearing gabbro. Within the UMC, there are various felsic dikes cutting through metagabbro and amphibolite, which suggest that intrusions of felsic and mafic rocks could have overlapped temporarily. Distinctive major and trace element geochemical signatures of the CCG, CHG, RCA, DMS, and SGS on tectonic discrimination diagrams all suggest formation within a volcanic arc environment. Preliminary U/Pb dating of detrital zircons in the AS and igneous zircons in the CHG yield Middle Ordovician ages, with the AS also containing a subordinate fraction of Grenville ages. These relationships suggest that the DC arc resulted from subduction of Iapetus lithosphere beneath the distended Laurentian plate, and that the DC could potentially be a key fragment of the missing Taconic arc in the southern Appalachians.