Experimental evidence shows that blocking temperature-relaxation time theory for single-domain magnetite can grossly underestimate laboratory thermal demagnetization temperatures of a present day viscous remanence in Devonian limestones of New York State. Thermoviscous remagnetization at moderately elevated burial temperatures during the late Paleozoic cannot therefore be readily excluded for the origin of the stable secondary component in these rocks. More generally, these results emphasize that single domain thermal activation theory cannot be assumed a priori to correctly predict temperature of thermoviscous remanence acquisition in all magnetite-bearing rocks.