U-PB STAUROLITE AGE OF METAMORPHISM IN THE WESTERN BLUE RIDGE, SOUTHERN APPALACHIANS

There are many conflicts over the accuracy of whole rock and mineral dates established for metamorphism in and around the Western Blue Ridge area of the Southern Appalachian Mountains. Some of this uncertainty originates from the method of isotopic dating, while some stems from the types of minerals or whole rocks chosen for analysis. Though there is much geological evidence (structural and fossil as well as some isotopic evidence) for established metamorphic episodes in much of the Southern Appalachians, isotopic data in the Western Blue Ridge area are not unambiguous.

Many isotopic dates from the area are obtained from low grade metamorphic minerals, which have relatively low closing temperatures and short stability fields. Hence the isotopic ages are believed to be merely cooling ages. This has caused the accuracy of the metamorphic dates to be questioned, sometimes even by the authors themselves.

Staurolite is a high-grade metamorphic mineral, which has proven itself as a reliable geochronometer. Since the mineral’s closing temperature to U and Pb diffusion is uncommonly high, at greater than 500°C, the U-Pb age should closely represent a crystallization age. The stability field for staurolite is also quite narrow, at 50°C, narrower than for most minerals used in geochronometry. This allows a metamorphic recrystallization age to be established, with a small standard uncertainty.

In the Western Blue Ridge, staurolites occur as porphyroblasts in muscovite schists. These porphyroblasts post-date the primary schistosity, but appear to have grown prior to a later crenulation. We have found the U-Pb age of staurolite clearly indicates an episode at 412(±6) Ma. This date concurs with the established Acadian Orogeny, possibly the most extensive metamorphic event in the area. Since the uncertainty on this date is only 1.5%, it is the most precise metamorphic age known for this area.