Chemical and mineralogic variations during prograde regional metamorphism were studied in meta-arkoses of the Thunderhead Sandstone of the Great Smoky Mountains, North Carolina and Tennessee. Twelve localities along a profile normal to the regional isograds were sampled. Biotite, muscovite, and total rocks were chemically analyzed.
Relict quartz and K-feldspar in arkose exhibit little recrystallization until the staurolite zone, but shaly interbeds are completely recrystallized throughout the sequence and isograds are based upon mineralogic changes in these interbeds. In the garnet zone the presence of metamorphic garnet in meta-arkose, which should not be stable according to the whole rock chemistry, indicates that equilibrium was obtained only in the interstitial portions of the system.
Metal/Al ratio in meta-arkose remains constant throughout the biotite and garnet zones. At the beginning of the staurolite zone, however, numerous chemical changes are apparent in the bulk rocks and in the micas. These changes and the disappearance of relict minerals suggest the entry of a significant aqueous phase at the staurolite isograd coupled with a sufficiently high temperature for the reactions.
With the formation of staurolite from chlorite in the pelitic interbeds, sufficient water was released to provide an aqueous transporting and catalyzing medium in the arkose, allowing rapid approach to equilibrium. At the entry of this aqueous phase, the system became a macro-system; chemical equilibrium was set up over large volumes of rock rather than in the interstitial microsystems encountered at lower rank.