Geology of the Alexander City 7.5’ Quadrangle, Alabama and U-Pb Zircon Dating of the Elkahatchee Quartz Diorite, southernmost Appalachian orogen

Abstract

Geological mapping of the Alexander City Quadrangle in the eastern Blue Ridge of the Southern Appalachian orogen in Alabama reveals newly recognized units and supports multi-phase magmatism within the 880 km2 Elkahatchee Quartz Diorite batholith. Major findings from mapping include: (1) the Alexander City shear zone variably deformed rocks within the quadrangle during the Alleghenian orogeny through oblique-dextral and normal shear; (2) the earliest intrusive phase within the Elkahatchee batholith is a medium-grained biotite-rich quartz diorite, whereas later, more felsic phases are medium- to coarse-grained K-feldspar-rich granites and leucocratic plagioclase- and muscovite-rich trondhjemites and tonalites, appearing as small, lens-shaped, elongate bodies; (3) a newly recognized granitic body, referred to as Oaktasasi granite, conforms to the regional structural grain yet is undeformed to weakly deformed and corresponds to several areas previously mapped as Hackneyville Schist; (4) the ~335 Ma Sugar Creek trondhjemite intrudes the Elkahatchee Quartz Diorite and contains low angle right-lateral strike-slip shear zones with tops-southwest displacements; and (5) high-angle normal faults are documented in the northwest parts of the quadrangle. Whole rock geochemistry for the Elkahatchee Quartz Diorite suggests that it was derived from slab melt due to subduction during the Neoacadian orogeny. Rb-Hf-Ta and Rb-(Y+Nb) diagrams indicate the magmatic origin in a volcanic arc setting. Sr/Y values of ≥ 40, Al2O3 > 15, and Yb < 1.9 ppm, and enrichment in light REE and depletion in heavy REE, conform to an “adakitic” signature. Magmatic zircons from three samples of Elkahatchee Quartz Diorite were dated using U-Pb geochronology laser-ablation-sector field-inductively coupled plasma-mass spectrometry. A sample representative of the earliest, most mafic phase (SiO2 62.5%) gave a weight mean age of 378.3 ± 5.5 Ma (2σ). A younger population was found in the more felsic samples (SiO2 65.3% and 67.2%, respectively) that gave weighted mean ages of 328.6 ± 5.1 and 329.8 ± 5.3 Ma (2σ). These age data support a multi-phase magmatic history for the Elkahatchee Quartz Diorite that spanned the late Acadian-Neoacadian and early Alleghenian orogenies. The magmatic crystallization ages reported herein for felsic plutons combine with previously reported ages to support that the Wedowee-Emuckfaw boundary, on a regional scale, separates eastern Blue Ridge units intruded by granites of Devonian to Carboniferous age to the northwest from those intruded by Middle Ordovician plutons to the southeast. Such age-specific plutons could preferentially intrude different structural levels within an intact stratigraphic package or else a fault is responsible for the age disparity. The author favors that a cryptic fault may best explain this relationship.