CHARACTERIZATION OF ANCIENT APPALACHIAN GEOFLUIDS FROM O-H ISOTOPES AND 40AR/39AR DATING OF AUTHIGENIC CLAY MINERALS IN THE CENTRAL VALLEY-AND-RIDGE PROVINCE

Fluid migration from the Appalachian fold-thrust belt has been long been hypothesized as the origin of deposition of Mississippi Valley Type deposits and the remagnetization of sedimentary units in the orogen and extended foreland during late Paleozoic Alleghany orogeny. We use the O-H stable isotopic compositions of authigenic illite from folded, clay-rich sedimentary horizons in the Central Appalachian fold-thrust belt to constrain the timing and source(s) of ancient orogenic fluids.

Low δD isotopic signatures (in the range ‑60 to ‑90‰ VSMOW) preserved in authigenic illite show that infiltrating meteoric fluid represents as much as 50% of the ancient fluid volume. Oxygen isotopic signatures of clays (in the range +13 to +18‰ VSMOW) match regional vein analyses of other workers, showing that significant fluid-rock O-buffering occurred along the infiltration/migration pathway of these orogenic fluids.

40Ar/39Ar dating of illitic clays gives relatively young, Permian to Triassic ages for clay mineralization (278-236 Ma), indicating that orogenic fluid migration and low-temperature rock alteration continued for several tens of millions of years, as the Appalachian orogen eroded and was progressively exhumed.