The concentration of magnetite in the Lower and Middle Devonian Helderberg and Onondaga carbonate formations varies in a distinct pattern along an east-west profile across New York State. Magnetite concentrations are uniformly low in the western half of the profile, increase sharply eastward to a peak value near Syracuse, and decrease farther east. The pattern strongly resembles previously reported variations in the extent of clay mineral diagenesis that reflect differences in paleotemperatures along the profile. Previous paleomagnetic studies have documented that the magnetite carries a well- defined magnetization of Pennsylvanian-Permian age, and magnetite authigenesis is therefore no younger than late Paleozoic. We attribute the correlation between high magnetite concentration and high illite content to temperature-dependent diagenesis triggered by orogenic fluids. The large proportion of secondary magnetite indicates that over most of the area, the mechanism of late Paleozoic remagnetization was principally chemical. Thermal effects played a less direct role in the remagnetization of the strata by controlling the extent of the chemical processes that resulted in overprinting of the original remanence.