Geology and geochemistry of the mafic and ultramafic intrusive rocks, Dadeville belt, Alabama

Early to Middle Ordovician back-arc basin in the southern Appalachian Blue Ridge: Characteristics, extent, and tectonic significance

“…The complex has no known basement, with the structurally lowest, oldest (based upon crosscutting relationships), thickest (structural thickness >5 km), and spatially most extensive unit (includes ~70% of the complex's nonintrusive rocks) being the Ropes Creek Amphibolite and its northwestern equivalent, the Waresville Formation/Amphibolite (Bentley and Neathery, 1970;Stow et al, 1984). The bulk of the Ropes Creek-Waresville unit was derived from mafi c and intermediate tuffs (Bentley and Neathery, 1970;Sears et al, 1981) and tholeiitic basalts and their differentiates (Neilson and Stow, 1986), and it is intercalated with intermediate (andesitic/dacitic) gneisses (Waverly Gneiss), minor pelitic metasedimentary rocks, metagraywacke (metaturbidite? ), spessartine/magnetite quartzite (metachert?…”

“…In the Dadeville complex, the Ropes Creek and Agricola units have been intruded by several suites of felsic (metatonalite and metagranite) and mafi c (metanorite, metagabbro, and metaorthopyroxenite) plutonic rocks (Bentley and Neathery, 1970;Sears et al, 1981;Stow et al, 1984;Neilson and Stow, 1986;Spell and Norrell, 1990;Steltenpohl et al, 1990), which may make up >50% of the complex. The tectonic setting of most of these plutons is unknown.…”

“…Seal and Kish (1990) reported a 462 ± 4 Ma Rb-Sr whole-rock isochron for the Franklin Gneiss, which suggests Middle Ordovician magmatism within the Dadeville complex, and which may constrain the country rocks as being Middle Ordovician or older. Neilson and Stow (1986) suggested that mafi c and ultramafi c plutonic rocks of the Dadeville complex, which occur as sills, layered intrusions, and dikes intrusive into the Ropes Creek Amphibolite, resulted from two separate periods of intrusion-the Doss Mountain and the Slaughters "suites." The Doss Mountain intrusions consist of metanorite and metaorthopyroxenite cumulates, whereas the Slaughters intrusions consist of metagabbro (Neilson and Stow, 1986).…”

“…Neilson and Stow (1986) suggested that mafi c and ultramafi c plutonic rocks of the Dadeville complex, which occur as sills, layered intrusions, and dikes intrusive into the Ropes Creek Amphibolite, resulted from two separate periods of intrusion-the Doss Mountain and the Slaughters "suites." The Doss Mountain intrusions consist of metanorite and metaorthopyroxenite cumulates, whereas the Slaughters intrusions consist of metagabbro (Neilson and Stow, 1986). The Doss Mountain is depleted in REEs, incompatible elements, TiO 2 , and alkalis, whereas the Slaughters plutons have higher REEs and TiO 2 and are rich in incompatible elements, with moderate alkali contents.…”

“…The Doss Mountain is depleted in REEs, incompatible elements, TiO 2 , and alkalis, whereas the Slaughters plutons have higher REEs and TiO 2 and are rich in incompatible elements, with moderate alkali contents. Both plutonic suites are tholeiitic, although the Slaughters intrusions have a moderately alkaline character (Neilson and Stow, 1986). Importantly, abundant geochemical evidence strongly indicates that the protoliths of both the Doss Mountain and Slaughters suites formed within a volcanic arc environment (Stow et al, 1984;Neilson and Stow, 1986).…”