Late Permian Age of Ammonium-Bearing Illite In Pennsylvania Anthracites: Temporal Constraints on Coalification in The Central Appalachians

Eric J. Daniels

K/Ar dating of authigenic NH₄-bearing illite in coal from the Anthracite region of eastern Pennsylvania yields apparent ages of 253-236 Ma (±8 m.y.). The oldest K/Ar date of 253 Ma (±8 m.y.) is a minimum age for the thermal event associated with anthracite formation and illite authigenesis. This date is probably within ~10 m.y. of the time at which coal-bearing rocks experienced peak temperatures, which suggests an age range of 265-255 Ma for this event. Age dates for authigenic NH₄-bearing illite reflect the approximate age of late-stage coalification (formation of anthracite-rank coal) because this mineral formed only during late-stage coalification (T=200-275°C) from a kaolinite (potassium-free) precursor, and the K/Ar data sugges that NH₄ does not significantly influence the closure temperature of illite.

K/Ar dates, in conjunction with paleomagnetic and vitrinite rnflectance data, constrain the thermal pulse and deformation associated with the Alleghanian orogenic episode to a time period of 25-35 m.y., beginning ir Early Permian time (~285 Ma) after coal deposition, and concluding by Late Permian time (~260-250 Ma). Syn- and post-folding secondary chemical remanent magnetizations constrain Alleghanian folding in the Central Appalachians to Late Permian time, which coincides with the postulated time range of 265-255 Ma for NH₄-bearing illite authigenesis and anthracite formation derived from the K/Ar data. The similar time range for anthracite formation and Alleghanian folding is consistent with vitrinite reflectance data that suggests anthracite formation was a syn-folding thermal event.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California