Paper No. 24
Presentation Time: 8:00 AM-12:00 PM
TECTONIC AND STRUCTURAL SETTING OF EASTERN BLUE RIDGE PLUTONIC COMPLEXES AND ASSOCIATED LITHOLOGIES, ALABAMA APPALACHIANS
Two lithotectonic metasedimentary sequences [the Wedowee (WG) and Emuckfaw (EG) Gps.] host a suite of calc-alkaline metaplutonic bodies in the Alabama eastern Blue Ridge (EBR). The tectonic origin and structural relationships between EBR plutonic bodies and their hosts is a subject of debate. The Equality, AL 7.5’ quadrangle was selected for mapping due to the diversity of units. Previous U/Pb zircon dating (by various methods) suggests that the EBR plutons range from Middle Ordovician to at least the Devonian. The apparently oldest phases (based upon geochronology) are the megacrystic (with up to 5 cm potassium feldspar augen) Kowaliga (KG) and Zana (ZG) Augen Gneisses, which are predominantly sill-like bodies. These bodies are difficult to differentiate in the field and may have originated from the same magmatic source. Each intrudes the EG, but the ZG may also intrude upper parts of the underlying WG. The largest pluton (of batholithic proportions, covering >900 km2) is the sill-like Late Devonian Elkahatchee Quartz Diorite (EQD), which is hosted entirely by the WG. It is predominantly medium- to coarse-grained I-type biotite tonalite and granodiorite in ~ equal proportions with an equigranular, hypidiomorphic gneissic (S1) texture. The EQD includes 2 distinct phases. The oldest, a hornblende-gabbro cumulate, indicates early peripheral cooling, and its spatial distribution suggests convection within the early EQD magma chamber. The youngest phase is the trondjhemitic to granitic Rockford suite (RS). Its associated dikes cross cut the dominant quartz diorite phase, but have similar U/Pb zircon ages to the EQD. Intrusive contacts of the RS show cuspate/lobate boundaries with the EQD, indicative of immiscibility between the RS magma and a partially liquid EQD during what was possibly the final stage of fracturing and cooling within the EQD. All units in the quad experienced Neoacadian-early Alleghanian regional dynamothermal metamorphism mostly above the kyanite isograd. A pervasive, coaxial, subhorizontal synmetamorphic stretching lineation is found in all units. The Alexander City fault, the southeastern boundary of the EQD, faulted the EQD against the Wedowee. This structure is a high-angle boundary that formed very late in the kinematic sequence, cutting all earlier mesoscopic and map-scale structures.