Authigenic NH4-bearing illites in coal fractures from the Anthracite region of eastern Pennsylvania yield individual K/Ar dates of 236-253 Ma (±8 m. y.) and a composite isochron of 253 Ma (±8 m.y.). Rb/Sr data for the same samples indicate an isochron of 260 Ma (±44 m.y.), consistent with the more precise K/Ar data. The age of NH4-bearing illite authigenesis should reflect the time at which coal attained anthracite rank, because mineralogical and geochemicai evidence indicates that NH4-bearing illite formed exclusively during anthracite formation (T = 200-275 °C) from organically derived nitrogen and a kaolinite precursor.
Similar Late Permian K/Ar dates for both NH4-rich illite and K-illite in anthracite fractures and lack of correlation between percent NH4 substitution in illite and the K/Ar date suggest that NH4 does not influence significantly the closure temperature of illite and,thus, that NH4-rich illite can be used for dating the time of authigenesis. Given the uncertainties in closure temperatures for illite as well as the maximum temperatures for anthracite formation, we consider the oldest K/Ar date of 253 Ma (±8 m.y.) to be a minimum age for NH4-illite authigenesis and anthracite formation. Based on the retention of a detrital age signature in the <0.5μm size fraction of shale samples, as well as theoretical and empirical estimates of illite closure temperatures, we estimate the oldest K/Ar date of 253 Ma to be only slightly younger (no more than about 10 m.y.) than the time at which coal-bearing rocks experienced peak temperatures associated with anthracite formation, for which we suggest an age range of 265-255 Ma.
Re-examination of syn- and post-folding secondary chemical remanent magnetizations in rocks from the central Appalachians indicates a late Early to Late Permian age (260-250 Ma) for Alleghanian deformation in the vicinity of the Anthracite region. The age of folding is similar to the 265-255 Ma age range for anthracite formation interpreted from illite dates and is consistent with previous studies of vitrinite reflectance fabrics in coal, which indicate that anthracite formation occurred prior to and during folding. Thus, age data in conjunction with paleomagnetic and vitrinite reflectance data allow a time period of 25-35 m.y. for anthracite formation and Alleghanian deformation, beginning after deposition of the coal in Late Pennsylvanian or Early Permian time (ca. 290-280 Ma) and concluding by the Late Permian time (ca. 260-250 Ma), significantly longer than previous estimates of less than 15 m.y. for this thermal/tectonic event.