TIMING AND SOURCES OF FLUIDS IN THE CENTRAL APPALACHIANS OROGEN AND FORELAND; EVIDENCE FROM CLAY MINERALIZATION

Ages of authigenic, low-temperature clays in the folded Central Appalachians of Pennsylvania, Maryland and West Virginia, and extended foreland of New York show characteristic patterns. Ar encapsulation dating of clays in extended foreland samples preserve ages of 308-318 Ma, whereas folded Appalachian rocks preserve younger ages of 239-273 Ma. Regional mineralization ages in the foreland coincide with peak Alleghanian deformation in the Appalachians, while younger clays in the frontal orogen reflect low-temperature mineralization during exhumation.

Stable O-H isotopic analysis of dated clays shows that fluid-mediated growth is dominated by the activity of surface fluids in the orogen and the foreland. H isotopes of fluids (δD), calculated from clay compositions at a fractionation temperature range of 100-150C, are -50 to -60 ‰ in the foreland and -50 to -85 ‰ in the frontal orogen. These values are distinct from modern waters in the region, but match meteoric δD values for late Paleozoic Pangea. The preservation of slightly more negative values in the orogen likely reflects greater elevation during meteoric fluid infiltration.

Whereas regional mineralization ages in the foreland match the timing of Alleghanian deformation in the Appalachians, fluid signatures do not support the hypothesis of mineralization from far-traveled, orogenic fluids into the foreland. Instead, the primary foreland fluid source was infiltration of ancient surface waters. Younger clays within the Appalachian orogen show that surface fluids continued to mix with minor volumes of formation fluids and metamorphic fluids into the latest Paleozoic, as the orogen eroded and exhumed after collisional formation of Pangea.