FLUID HISTORY OF THE CENTRAL APPALACHIAN BLUE RIDGE ANTICLINORIUM

The Blue Ridge Province in the central Appalachians consists of a northwest-verging anticlinorium that is part of a crystalline thrust sheet transported during the Late Paleozoic Alleghanian orogeny. Although schistose and phyllitic units show evidence for multiple phases of deformation, the fabric of most rocks is dominated by a pervasive southeast-dipping cleavage. Fluid inclusion microthermometry was used to determine the nature of the fluids and P/T conditions throughout the deformational history from the abundant quartz veins in the area. Based on homogenization temperature (Th), eutectic temperature (Te), and salinity, five fluids were identified: I (110-160°C Th, 0-10% wt. NaCl equiv.); II (170-205°C Th, 6-12 wt. % NaCl equiv.); III (160-190°C Th, 12-22 wt. % NaCl equiv.), IV (150-180°C Th, 28-32 wt. % NaCl equiv.); and V (210-290°C Th, 8-19 wt. %NaCl equiv.). Te data suggest the fluids are primarily NaCl-CaCl₂ brines except fluid IV, which is an NaCl-CaCl₂-MgCl₂ brine. Assuming a 30°C/km geothermal gradient, isochore data from the five fluids indicate trapping temperatures of 160-280°C and depths of 4.5-9 km for hydrostatic fluid conditions and 220-430°C and 7-14 km for lithostatic conditions.

The occurrence of the different fluids is best explained by a combination of vein generation and inclusion type. Generally, pre-cleavage veins have higher Th values than syn- or post-cleavage veins. Primary inclusions generally have higher Th values than secondary inclusions. Tm values also display evidence of stratigraphic partitioning amongst the fluids. Structural position (upright or overturned limb) and proximity to fault zones do not show an obvious effect on fluid distribution.

The fluid history, P/T conditions determined from inclusion data, and fluid compositions are strikingly different from those in the adjacent Piedmont and Valley & Ridge provinces. Unlike the Piedmont, Blue Ridge inclusions contain no CO₂ and have lower Th values. Unlike the Valley and Ridge, the inclusions have no CH₄ or liquid hydrocarbons. Taken together, the fluid history of the central portion of the Blue Ridge is independent of the adjacent provinces, which argues against any orogen-wide fluid migration involving all provinces.