PROTEROZOIC BASEMENT ROCKS OF THE SAMS GAP AND WHITE ROCK QUADRANGLES, MADISON COUNTY, NORTH CAROLINA, AND UNICOI COUNTY, TENNESSEE

Proterozoic basement units include: (1) migmatitic biotite hornblende gneiss (characterized by hornblende-bearing rock types with variable migmatization); (2) layered biotite granitic gneiss — a thick sequence of well-foliated rock (biotite granitic gneiss to quartz dioritic gneiss interlayered with amphibolite and biotite gneiss); (3) biotite granitoid gneiss — intrusive into the layered biotite granitic gneiss (massive- to thick-layered biotite granitic to dioritic gneiss); (4) Max Patch Granite (massive to weakly foliated xenolith-bearing granite); (5) Bakersville Metagabbro, dikes and sills; (6) intrusive hornblende granite. These rocks have been affected by at least four major metamorphic, igneous, and structural episodes. Protoliths were metamorphosed to granulite or near granulite grade during the Middle Proterozoic. Although later regional metamorphic episodes have largely obliterated most features and relationships, relic hypersthene and associated foliation occurs in similar units of six nearby quadrangles. The Middle- to Late Proterozoic Max Patch Granite, lacking foliation and metamorphic hypersthene, appears unaffected by the granulite grade metamorphism. Bakersville dikes and sills and Beech-like hornblende-bearing granites define a Late Proterozoic intrusive episode. An Early to Middle Paleozoic event transposes Middle Proterozoic features into a regionally pervasive foliation. Lack of pelitic sediments in the basement rocks makes determination of metamorphic grade difficult, although a small sillimanite-bearing unit on the Sams Gap quadrangle could suggest upper amphibolite facies during the Paleozoic, but this does not preclude it from being relict from the Middle Proterozoic. Late Paleozoic thrust faulting is recognized by pervasive mylonitization and greenschist recrystallization. Intensity of mylonitization varies; the most intense areas are mapped as continuous anastomosing units of sericitic mylonite. Cataclasitic mylonite in the northwest is also a product of this thrusting.