DIFFERENT THERMAL HISTORIES OF THE WESTERN AND EASTERN BLUE RIDGE AND EMPLACEMENT OF THE BLUE RIDGE THRUST SHEET DOCUMENTED BY 40AR/39AR THERMOCHRONOLOGY, VA– NC–TN

The composite Blue Ridge-Piedmont thrust sheet (BRTS) consists of amphibolite-facies rocks of the eastern Blue Ridge juxtaposed above western Blue Ridge rocks. The BRTS was transected by NE-striking greenschist-facies high-strain zones and thrust above unmetamorphosed rocks of the Mountain City window. To resolve the complex metamorphic history and timing of faulting across the BRTS, rocks containing hornblende, muscovite, and K-feldspar were collected from the northwestern edge of the BRTS (Catface fault) to the eastern Blue Ridge for ⁴⁰Ar/³⁹Ar thermochronology. The samples include Mesoproterozoic gneisses and marble, muscovite-rich phyllonites, foliated metarhyolites, muscovite schists, and amphibolite across different fault zones of the BRTS, W to E: Catface fault, Fries fault and associated greenschist-facies high-strain zones, and the Gossan Lead thrust sheet (GLTS). Muscovite ⁴⁰Ar/³⁹Ar ages across the western (n = 8) and eastern Blue Ridge (n = 1) yield plateau ages of 335–347 Ma; however, hornblende and K-feldspar ages combined with petrography indicate the western Blue Ridge and GLTS experienced different thermal histories. Amphibole from marble in the western Blue Ridge yield a ⁴⁰Ar/³⁹Ar cooling age of ~950 Ma. Amphibole from the GLTS yields a plateau age of ~341 Ma. K-feldspar spectra from western Blue Ridge samples (n = 5) climb to ages >400 Ma (older than coexisting muscovite). K-feldspar from the GLTS also yield climbing spectra, but are younger than muscovite ⁴⁰Ar/³⁹Ar ages (< 300 Ma). Collectively, these data indicate that the western Blue Ridge of SW VA and adjoining NC and TN, did not attain temperatures necessary to reset Ar in hornblende or K-feldspar during the Paleozoic. From the Catface fault, ~340 Ma muscovite ⁴⁰Ar/³⁹Ar ages are growth ages corresponding to the time of regional fabric formation, and initial emplacement of the BRTS. Muscovite ⁴⁰Ar/³⁹Ar ages of ~340 Ma from the Fries high-strain zone documents cooling from garnet and biotite zones. Amphibole cooling ages from the eastern Blue Ridge indicate an earlier metamorphic event prior to 340 Ma, likely the Acadian or Neoacadian orogeny. Similar cooling ages of hornblende and muscovite in the GLTS indicate rapid cooling from above 550°C to below 350 °C during juxtaposition with the western Blue Ridge and emplacement of the BRTS at ~340 Ma.