K-Ar and Rb-Sr measurements on whole rocks and on separated size fractions of Pennsylvanian underclays were made in an attempt to relate the apparent ages to the degree of metamorphism, by analogy with the fixed carbon content of the associated coals. Five whole-rock samples (fixed-carbon contents ranging from 50 to 90 percent) give fairly uniform K-Ar ages between 355 and 383 m.y. The apparent ages of two samples deviate significantly from the average with values of 416 m.y. (fixed carbon, 75 percent) and 327 m.y. (fixed carbon, 95 percent). No systematic relation between coal rank and whole-rock K-Ar age is apparent.
Individual size fractions of two samples show nearly concordant but systematically decreasing K-Ar and Rb-Sr model ages (510 to 320 m.y.) with decreasing grain size. This effect cannot be explained by any simple volume diffusion model; it is most likely due to crystallization or reconstitution of 1Md-type illite. The diffusion coefficient for continuous argon diffusion is D ⩽10−28 cm2sec−1. Although the whole-rock ages are dominated by detrital illite, the sediments appear to contain a fine-grained fraction that consists of 1Md illite and mixed-layer clay and gives K-Ar and Rb-Sr ages close to the age of deposition.