Evaluating Peak Metamorphic Conditions of the Kyanite-Bearing Rocks at Willis Mountain, Virginia, USA

Willis Mountain, near Dillwyn in central Virginia, is the world’s largest kyanite deposit located in the Chopawamsic Fm. of the Piedmont province. However, little is known about the temperature and pressure conditions under which the kyanite deposit and its surrounding rocks formed, compared with similar-aged rocks in the Blue Ridge province to the southwest. Willis Mountain is composed of kyanite-bearing muscovite schists to kyanite quartzites which contain up to 55% kyanite. The following methods were used to evaluate the metamorphic conditions, with multiple calibrations for each: phase equilibrium modelling; trace element thermometry (titanium in quartz and zirconium in rutile); garnet-biotite thermometry; and the garnet, aluminosilicate (Al2SiO5), silica, plagioclase (GASP) barometer. Phase equilibrium modelling of two garnet-bearing samples indicates peak conditions at 580–690 °C and 8–15 kbar. Trace element thermometry for titanium in quartz ranges from 512 to 726 °C whereas zirconium in rutile temperatures range from 603 to 692 °C. Garnet-biotite thermometry indicated a range of 536–649 °C. The GASP barometer indicates pressures between 7 and 15.6 kbar. Overall, this suggests that peak metamorphic conditions at Willis Mountain were 640–665 °C and 8.5–10 kbar. These conditions correlate well with the similar-aged, kyanite-grade rocks in the Blue Ridge province, which indicates that peak metamorphism may have occurred during the Taconian orogeny, the first tectonothermal event that contributed to development of the Appalachian Mountains.