REACTION SOFTENING IN RHYOLITES OF THE MOUNT ROGERS FORMATION AND CARBONIFEROUS DEFORMATION ON THE BLUE RIDGE THRUST, VA–NC–TN

Field, petrographic, and argon isotopic data from mylonitic rocks along the Blue Ridge thrust in VA–NC–TN indicate that during the Middle to Late Mississippian reaction and textural softening combined to form and localize strain on folia and shear zones in porphyritic rhyolites of the Mount Rogers Formation (MRF). The Catface and Stone Mountain faults, segments of the Blue Ridge thrust, are characterized by MRF rhyolites that have been transformed to porphyroclastic phyllonites dominated by fine-grained muscovite (Ms). Away from the shear zones the primary mineralogy of porphyritic MRF rhyolites consists of randomly oriented K-feldspar (Kfs), plagioclase, and quartz (Qtz) in an aphanitic matrix. Incipient replacement of phenocryst and matrix feldspars by Ms in acidic aqueous solutions (e.g., 3Kfs + 2H⁺ à Ms + 6Qtz +2K⁺) is apparent in thin section. Replacement Ms generally forms small and randomly oriented flakes that do not form contiguous folia. In contrast, the replacement of Kfs by Ms is nearly complete in phyllonites, and here the new Ms forms contiguous monomineralic folia at the scale of the thin section. Arrested reaction textures suggest that Kfs phenocrysts react to form elongate aggregates of Ms that are particularly pure and define a strong SE lineation. Angular Qtz phenocrysts are unreacted (see above reaction) and preserved in all samples, likely because the weaker contiguous Ms folia accommodated most of the strain.

Evidence that the replacement Ms was produced in the lower greenschist facies comes from: 1) the stable occurrence of Kfs + chlorite, 2) unreset (400-450 Ma) Kfs ⁴⁰Ar/³⁹Ar ages that indicate the rocks did not exceed ~250°C, and 3) from the preservation of Grenvillian detrital muscovite ⁴⁰Ar/³⁹Ar ages in rocks of the Chilhowee Group structurally beneath the Blue Ridge thrust. Thus, the ages of retrograde Ms are interpreted to be Ms crystallization ages. In fact, ⁴⁰Ar/³⁹Ar step heating experiments on Ms from variably deformed samples of phyllonite consistently produce ages of ~330-340 Ma. These ages thus date the movement of the Blue Ridge thrust to the NW. This movement contributed to the exhumation of the Blue Ridge thrust sheet, to deformation of the foreland-fold thrust belt, and to the Mississippian clastic wedge in the Appalachian basin.