LONGSTANDING PROBLEMS REGARDING THE DISTRIBUTION AND CONDITIONS OF TACONIAN AND ALLEGHANIAN METAMORPHISM IN THE WESTERN BLUE RIDGE, WITH NEW PTT CONSTRAINTS FROM THE MURPHY SYNCLINORIUM

The Murphy synclinorium (MS) is a prominent component of the Western Blue Ridge province in northwest Georgia and adjacent North Carolina, with structural, metamorphic, and stratigraphic features that can be correlated southwest to the Talladega Belt (TB) of Alabama, and northeast through the Great Smoky Mountains to Virginia. Barrovian isograds post-date and overprint the TB and MS and extend northward to the Great Smoky Mountains. Multiple authors have presented biostratigraphic data that constrain the metamorphism of the TB to be post-Ordovician. However, many authors have considered the dominant, Barrovian metamorphism of the Western Blue Ridge to have developed during Ordovician Taconic orogeny. The present study focuses on high-alumina pelites of the Mineral Bluff Formation (=Lay Dam Formation of the TB), as sampled from three exploratory cores drilled near the center of the Murphy synclinorium near Tate, Georgia (obtained by Dr. William Fairley in the 1970s), that occur along with amphibolites and the Murphy Marble. The schists contain the assemblage mus-bio-gar±sta±kya-rut-ilm. Porphyroblasts are generally synkinematic with respect to foliation development, though some porphyroblast rotation and foliation flattening followed garnet growth. WDS elemental maps and quantitative chemical analyses of porphyroblasts and inclusions were obtained with Auburn University’s JEOL JXA-8600 electron microprobe. Major element zonation in garnet is typical of prograde growth with no indication of retrograde garnet resorption, and is consistent with rapid cooling from peak metamorphic conditions. Maps of Ti show abundant inclusions of ilmenite near garnet cores, and that ilmenite+rutile occur along garnet rims and in the sample matrix. Application of various thermobarometers to the assemblage (including GRAIL, GASP, etc.), and the Fe-Ti oxide textures, indicate the garnets grew with increasing temperature and pressure, reaching peak conditions of ca. 6.3 kb at 600 °C. These conditions are interpreted to record peak metamorphism during and immediately following the Alleghanian orogenic events that formed the Murphy synclinorium, and that at least 20 km of lithosphere overlying the present surface at the sample locale was removed by extension and erosion subsequent to Alleghanian orogeny.