THE NATURE AND TIMING OF DEFORMATION IN THE BLUE RIDGE PROVINCE, GREENE COUNTY, VIRGINIA

In central and northern Virginia, the Blue Ridge anticlinorium forms the hinterland edge of the late Paleozoic Appalachian fold and thrust belt. However, the timing of deformation in the Blue Ridge is poorly known. The core of the anticlinorium includes Mesoproterozoic basement rocks, overlain by a cover sequence of the Neoproterozoic Swift Run and Catoctin Formations. Many of these rocks display a NE striking and SE dipping foliation. Discrete, anastomosing zones of mylonite cut these units. The larger of these high-strain zones are up to 1 km thick, extend 10’s of kms along strike, and form a regionally extensive network. Mylonitic foliations strike NNE and dip moderately to the ESE. Elongation lineations plunge ESE and obliquely downdip. Kinematic indicators consistently indicate top-to-the northwest (reverse) sense of shear.

Basement-derived mylonites are characterized by perthitic feldspar and quartz porphyroclasts in a matrix of fine-grained recrystallized quartz, white mica, and biotite. Mylonites derived from Swift Run arkoses are composed of quartz porphyroclasts in a matrix of quartz and white mica. To the west of the field area, quartz exhibits abundant undulose extinction with little to no evidence for dynamic recrystallization. However, dynamically recrystallized quartz is common in mylonites to the east. Biotite in granitic mylonites to the west are typically altered to chlorite, whereas unaltered biotite is present in mylonites to the east. Swift Run mylonites rarely contain biotite. The transition in quartz microstructures and the overall mineralogy indicate that mylonitic rocks in the western Blue Ridge experienced temperatures of ~300 to 350° C during deformation, whereas mylonites to the east reached temperatures of ~400° C.

⁴⁰Ar/³⁹Ar mass spectrometry reveals muscovite age-spectra consistent with cooling through ~350° C during the early Alleghanian (320-260 Ma). These data do not, however, preclude earlier mylonite formation. K-feldspar age-spectra also indicate temperatures >250° C, resetting the argon ratios, in the late Paleozoic. Feldspar spectra show an argon ratio disruption in the late Triassic-early Jurassic. The total fusion ages of biotite from the samples suggest the presence of excess argon, thus making them useful as only maximum ages.