THE ALLEGHANIAN OROGENY IN THE EASTERN BLUE RIDGE: EARLIER AND HOTTER THAN PREVIOUSLY RECOGNIZED

The Alleghanian orogeny in the Eastern Blue Ridge (EBR) of North Carolina is commonly described as a middle to lower greenschist facies event that began after ~335 Ma. In contrast, in the EBR of southern Georgia and Alabama the Alleghanian is considered synchronous with cooling from amphibolite- to greenschist-facies conditions between 335-310 Ma as recorded in muscovite and hornblende ⁴⁰Ar/³⁹Ar cooling ages. We present thermochronometric ages and microstructural observations along the EBR from Georgia to southern VA, which suggest that the entire EBR records a transition from amphibolite- to greenschist-facies conditions during the earliest Alleghanian orogeny.

We use quartz and feldspar deformation fabrics, quartz EBSD data, garnet-biotite-muscovite-plagioclase geothermobarometry, monazite ages, and ⁴⁰Ar/³⁹Ar dating of hornblende and muscovite to constrain the P-T-t path of rocks in the EBR. In three locations along the EBR we observe dynamic recrystallization in quartz and feldspar consistent with at least amphibolite-, and up to granulite-facies deformation conditions. In northwestern North Carolina these fabrics are associated with deformation at 347-345 Ma, based on ⁴⁰Ar/³⁹Ar ages in hornblende and subsequent rapid cooling through muscovite closure at 335 Ma. In northern Georgia, amphibolite- to granulite-facies conditions occurred earlier than 330 Ma, and the rocks cooled through muscovite closure at ~318 Ma. Monazite ages overlap this time span, ranging from 335-310 Ma. Along the North Carolina - South Carolina border, amphibolite- to granulite-facies conditions are associated with peak P-T conditions of 675-690 ± 25° C and 7-7.4 ± 1.2 kb, and monazite ages ranging from 335-300 Ma. This similar cooling pattern along the central portion of the EBR suggests the Alleghanian orogeny can be characterized by rapid cooling from at least amphibolite- through greenschist-facies conditions. Our results blur the boundary between the Neo-Acadian and Alleghanian orogenies, and suggest the early Alleghanian in the EBR may have been much hotter (and earlier) than previously recognized. Moreover, our interpretation of a southward migration of cooling is in agreement with previous models of zipper tectonics during Alleghanian time.