Late Permian age of NH ₄ -bearing illite in anthracite from eastern Pennsylvania: temporal limits on coalification in the central Appalachians

Authigenic NH ₄ -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.). The age of NH ₄ -bearing illite authigenesis should reflect the time at which coal attained anthracite rank, because mineralogical and geochemical evidence indicates that NH ₄ -bearing illite formed exclusively during anthracite formation (T = 200-275°C) from organically derived nitrogen and a kaolinite precursor. The oldest K/Ar date of 253 Ma (±8 m.y.) is considered to be a minimum age for NH ₄ -illite authigenesis and anthracite formation. Based on the retention of a detrital age signature in the <0.5-μm size fraction of shale samples, as well as theoretical and empirical estimates of illite closure temperatures, the oldest K/Ar date of 253 Ma is estimated to be only slightly younger than the time at which coal-bearing rocks experienced peak temperatures associated with anthracite formation, for which an age range of 265-255 Ma is suggested. -from Authors