Fluid history of the Blue Ridge anticlinorium in the central Appalachians

Analysis of fluid inclusions in quartz veins of the Blue Ridge anticlinorium in the central Appalachians was undertaken to determine the relationship between the fluid history and structural evolution of the anticlinorium. Samples were collected from three generations of veins defined by their age relative to the Alleghanian cleavage associated with formation of the anticlinorium, and from different stratigraphic units on both limbs of the fold. All fluid inclusions observed were two-phase (L + V), aqueous inclusions. They were separated into four types based on their homogenization temperatures (Th), salinity, and salt composition. Modal Th varies from 235 °C for the oldest group to 135 °C for the youngest. Corresponding salinities decrease progressively from 19 to 6.5 wt% NaCl equivalent. Predominant salt compositions are NaCl–CaCl2 ± MgCl2 for older inclusions and NaCl for the youngest. Based on the characteristics of these inclusion types, along with the age of the vein and stratigraphic unit in which they occur, four fluids were defined that were present in different parts of the anticlinorium at different times during its development. The primary control on the occurrence of the different fluids was temporal with different fluids occurring in different age veins. A secondary control was the stratigraphy with most of the fluid flow being focused in the Catoctin and Harpers formations. Structural location, position on the fold, and location relative to major faults played minor roles. mporal and stratigraphic controls on fluid occurrence, along with an absence of CH4 and CO2, which are common in fluids in the adjacent Piedmont and Valley and Ridge provinces, indicate that the fluids in the Blue Ridge were locally derived and not part of the Late Paleozoic regional fluid migration recorded elsewhere in the Appalachians.