THE AARON SEQUENCE: A NEW LITHOTECTONIC UNIT IN CAROLINIA, NORTH CENTRAL NORTH CAROLINA

The Carolina terrane, one of the best known subdivisions of the peri-Gondwanan crustal block Carolinia, is dichotomized into two main sequences in North Carolina: 1) the older Virgilina sequence and 2) the younger Albemarle sequence. The Virgilina sequence has traditionally been thought to be conformable and include the Neoproterozoic Hyco and Aaron formations, and interlayered Virgilina volcanics; however, recent geochronology has revealed a major time gap of approximately 37 m.y. between the Hyco and Aaron formations. Consequently, we abandon the term ‘Virgilina sequence’ and identify two new lithotectonic units within the Carolina terrane, the older Hyco arc and the younger Aaron sequence, which includes the Aaron Formation and the Virgilina volcanics as a member of the formation.

The Aaron sequence is composed of low-grade metamorphosed clastic rocks, and mafic-felsic volcanics with associated volcaniclastics. It is disposed in a large-scale overturned syncline, mainly a result of the Virgilina deformation (578 - 550 Ma), that has an axial trace trending NNE. Observations taken from detailed geologic mapping at the 1:24,000 scale indicate a disconformity between the Hyco and Aaron sequences with no deformational event prior to deposition of the younger Aaron sequence. In conjunction with field observations, high precision magnetometry was used to locate the disconformity where exposure is sparse. To better constrain the magnitude of the lacuna between the sequences, U-Pb zircon age dating was done on a sample from the uppermost Hyco formation. More than 65 zircons were picked from a felsic crystal tuff and 24 of those were selected for analysis using CA-TIMS method. Results from this analysis indicate an age of 616.52 ± 1.2 M.a. Combined with existing geochronology, the data suggest the lacuna to be on the order of 37 m.y.

The traditional conformable sequence has been used by previous workers in correlating the Carolina terrane with the northern Appalachian Avalon terrane, the two largest exotic terranes in the Appalachian orogen. However, on the basis of other geologic characteristics, this correlation has been questioned. This study contributes new data to help further evaluate this comparison.