U-PB ZIRCON DATING OF THE ELKAHATCHEE QUARTZ DIORITE, EASTERN BLUE RIDGE OF ALABAMA

Detailed 1:24,000 scale field mapping of eastern Blue Ridge rocks exposed in the Alexander City, Alabama Quadrangle has revealed newly recognized units as well as additional detail on multi-phase magmatism within the 880 km² Elkahatchee Quartz Diorite batholith. Within the northwest area of the quadrangle, a previously undescribed granitic body was discovered in association with schists of the Wedowee Group. This feldspar-rich, undeformed to weakly deformed, medium-coarse grained granite intrudes the schist, which resulted in gneissic textures along the contact. The granitic body is elongate and follows the general northeastward regional structural trend. Three samples from the Elkahatchee Quartz Diorite batholith were dated by zircon U-Pb geochronology (laser ablation-sector field-inductively coupled plasma-mass spectrometry). Mapping focused on distinguishing the various phases of intrusion within the batholith. Field and petrographic observations indicate that the most mafic phase, a medium-grained biotite-rich quartz diorite, appears to be the oldest intrusive phase in the area of the quadrangle, while more felsic phases of the batholith are coarse-grained K-spar rich granites and leucocratic plagioclase- and muscovite-rich trondhjemites and tonalites, appearing as small, lens-shaped, elongate intrusive bodies into the main body of the Elkahatchee. Zircons from a sample of the biotite-rich quartz diorite yielded a weighted mean of 378.3 ± 5.5 Ma (2σ). Two separate granitic plutons were sampled, and they gave ages of 328.6 ± 5.1 and 329.8 ± 5.3 Ma (2σ). The younger samples are moderately deformed and foliated whereas the older one is much more highly strained. Early Alleghanian granitic plutons are also identified in the Wedowee Group elsewhere in Alabama and by other workers in the eastern Blue Ridge of Georgia and North Carolina. This current study further documents that the Elkahatchee Quartz Diorite is an Acadian-Neoacadian pluton and that it was penetrated by magmas of early Alleghanian age. The Elkahatchee Quartz Diorite, therefore, represents a multi-phase magmatic complex whose evolution spanned the late Acadian-Neoacadian and early Alleghanian orogenies.