ISOTOPIC ANALYSES OF CENTRAL APPALACHIAN CLAYS IN FOLDED SEDIMENTARY ROCKS OF PA, WV, AND MD REVEAL MIXING OF SURFACE AND DEEPLY-SOURCED FLUIDS AND MINERALIZATION DURING OROGENIC EXHUMATION

Radiogenic and stable isotopic studies of authigenic clay minerals from the Appalachian foreland fold-thrust belt in Pennsylvania, Maryland, and West Virginia are used to characterize the timing of secondary clay growth in folded rocks, and the sources and pathways of mineralizing fluid. 40Ar/39Ar clay encapsulation dating gives ages that range from 239 to 273 Ma that do not seemingly vary systematically across the belt and that mostly post-date the peak of the Alleghany orogeny in the area. An extended sample set of clay-rich rocks preserves δD-values of secondary illite ranging from -52 to -97‰ VSMOW, and δ¹⁸O-values ranging from 13 to 18‰ VSMOW. Using a fractionation temperature range of 120° to 160°C for clay growth, we calculate a mineralizing fluid composition in the range of -87 to -33‰ for δD and 1.7 to 8.5‰ for δ¹⁸O. The hydrogen isotopic signatures correspond with source mixing of mid-to-high elevation surface fluids with deeply-sourced metamorphic and/or highly-evolved deep basinal fluids. The oxygen isotopic signatures match previously published results from veins (Evans and Battles, 1999), but indicate significant water-rock buffering of fluid prior to illitic clay growth that obscures the surface fluid source. The timing of clay mineralization indicates late stage alteration of folded sedimentary units in the presence of mixed surface-sourced and deeply-sourced fluids, as the Appalachian orogen underwent erosion and exhumation following final collision that formed of Pangea.