Metamorphic history of the Alto allochthon and petrology of the adjacent Chauga belt, Ayersville Quadrangle, Georgia

The Alto allochthon is a dismembered portion of a sillimanitegrade Inner Piedmont thrust sheet which overlies the lower grade rocks of the Chauga belt. The rocks in the northwestern portion of the Alto allochthon contain aluminum silicates, whereas those in the southeastern portion lack them. Analysis in AFM space reveals that this distribution is due for the most part to differences in bulk composition and not to grade. Portions of the Alto allochthon reached the second sillimanite isograd through the reaction Ms + Ab + Qtz = Kfs + Sil + H₂O. Migmatites that are present in the Alto allochthon formed by metamorphic differentiation rather than anatexis. The Chauga belt appears to be of much lower grade than the adjacent Inner Piedmont because of the absence of aluminum silicates or staurolite. This, however, is due to bulk compositions that inhibit formation of these minerals. Garnet-biotite geothermometry in both the Alto allochthon and the Chauga belt is highly variable, with temperatures in the Allochthon at 616—720°C ± 50° and those in the Chauga belt at 516-647°C ± 50°. The wide range in temperatures from this method for both units is most likely due to the high amounts of Mn present in the garnets. Muscovite-K-feldspar geothermometry yields more consistent results, with temperatures for the Alto allochthon ranging from 641-663°C and the Chauga belt at 628°C. The Alto allochthon and the Chauga belt were metamorphosed and deformed during the same event at. 380-400 Ma. The inner portion of the Alto allochthon appears to have undergone a simple cooling path in the late stages of metamorphism. Emplacement of the allochthon occurred much later, as evidenced by the lack of superposed metamorphism.