40AR/39AR DATA FROM THE WESTERN BLUE RIDGE AND THEIR IMPLICATIONS FOR TIMING OF METAMORPHISM

Conventional K-Ar and 40Ar/39Ar dates on minerals from the North Carolina and Georgia western Blue Ridge (WBR) have been interpreted to reflect Taconian and Acadian metamorphism. Low-grade (chlorite zone) metamorphic rocks of the Talladega slate belt (TSB) in Georgia and Alabama, which are interpreted to correlate partly to units of the WBR, preserve rare, Middle Devonian to Early Mississippian fossils argued to indicate Acadian or Alleghanian metamorphism. 40Ar/39Ar white mica dates from the TSB are less than ~334 Ma (Cambrian[?] Heflin Phyllite) and less than ~328 Ma (Silurian-Early Mississippian, Lay Dam Formation). K-feldspar from a Grenville gneiss boulder within the Lay Dam Formation has a diffusional profile with an older age of ca. 1205 Ma stepping down to a T-minimum age of 300 Ma. Hornblende separated from a metadacitic lava unit within the Hillabee Greenstone has a complex, roughly saddle-shaped spectrum with higher temperature steps giving a preferred age of ca. 472 Ma (48.2% total 39Ar released, 2 sigma), consistent with the 470 +/-4 Ma U-Pb zircon age for extrusion (McClellan and Miller, 2000). The trough of the same hornblende spectrum has a correlation age of 361.6 +/- 3.8 Ma (30.6% 39Ar, MSWD 0.011), interpreted as the maximum age for cooling through closure. White mica separated from the same Hillabee sample has a plateau age of 320.9 +/- 1.6 Ma. White mica from the Chulafinee Schist of the Lay Dam Formation has a plateau age of 327.1 +/- 1.7 Ma. Each mineral cooling date, therefore, is compatible with metamorphism of the TSB following Early Mississippian (Kinderhookian-Tournaisian, ca. 360-350 Ma) deposition. White mica spectra become ‘more disturbed' with distance from the contact (i.e., Hollins Line thrust) with the overlying, amphibolite-facies, Eastern Blue Ridge (EBR), consistent with reheating due to hot emplacement of the EBR around 320 Ma (e.g., Kish, 1980). We explore the implications of these dates from the TSB with respect to the ‘older', Taconian and Acadian dates reported from the WBR.