METEORIC SOURCE OF GEOFLUIDS IN THE CENTRAL APPALACHIANS FOLD-THRUST BELT AND FORELAND CHALLENGES OROGENIC FLUID EXPULSION HYPOTHESIS; EVIDENCE FROM REGIONAL CLAY DIAGENESIS

Geofluid characterization of Appalachian rocks has relied on the isotopic signatures of secondary veins, seemingly showing deep-orogenic fluids as the dominant source. However, an orogenic fluid source for regional host rock alteration in folded units and in (unveined) foreland units is untested. The timing of clay diagenesis in the Appalachian foreland is 305-318 Ma, and 239-273 Ma for folded units to the east, based on Ar encapsulation dating. Stable isotopic analysis of diagenetic clays shows that fluid-mediated growth is dominated by the activity of surface fluids across the region. H isotopes of fluids (δD), calculated from clay compositions at a fractionation temperature range of 100-160C, are -50 to -60 ‰ in the foreland and -50 to -85 ‰ in folded rocks. These value ranges do not match less negative metamorphic sources and more negative modern waters in the region (Bowen and Revenaugh, 2003), but overlap with meteoric δD model values for Late Paleozoic Pangea (Poulsen et al., 2007). The preservation of younger, slightly more negative values in the orogen reflects greater elevation during meteoric fluid infiltration. Mineralization ages of clays in the foreland match peak Alleghanian deformation, but their surface geofluid source does not support the hypothesis of expulsion of far-traveled, hot orogenic fluids (the “squeegee” hypothesis; Oliver, 1986). Younger clays within the frontal fold-thrust belt record continued surface fluid infiltration after significant exhumation of the orogen.